MOSFET amplifiers – the quick and the dead

Mosfet Amplifer 100W - 150W

MOSFET amplifiers – design criteria

Most audio amplifier enthusiasts will have built one Mosfet amplifier in their lifetime and hopefully if it didn’t self destruct quite a few more. I have been relatively lucky in this aspect because mostly I stick to the tried and trusted circuits and heed the designer’s advice like glue.


The first amplifier I built was the Maplin ETI 100-150W 2SK134/2SJ49 version and this thing was a freaking dream. Considering it was sometime around the 80s I vaguely recollect the only company in SA selling the Hitachi output devices was Candi’s in Durban.  I do NOT know whether the original Candi’s is the one at this address. I do recall the service was incredible (good) and living down in small town in the Western Cape took faster than modern couriers through the local GPO.

Mosfet Amplifer 100W - 150W
The legendary Mosfet Amplifer 100W – 150W – (please let us know whether copyright protected @ webmaster)

Bose to the rescue or was it the MOS amplifier kit?

The amplifier was a copycat of the one sold currently by except for the older Hitachi transistors used in my kit.  All testing was done with a CD input through a NAD 1020 and compared to three amplifiers I had in my arsenol, a Kenwood, Sansui and Akai. All of them had outputs of about 30 – 50W p.c. Listening to one CD with tones set to flat the NAD, which I never particularly liked, definitely had a far more distinctive sound. Some said bright (I never thought the 1020 was bright), fatter – ummm, yes, the NAD preamp wasn’t that bad after all and possibly, definitely, maybe…. was it the clarity? Suddenly the preamp didn’t sound that bad, the speakers which were home built using Fane 12″ drivers and some off the shelf high frequency drivers. Swapping this out for a friend’s Bose 901s I realised that Bose was possibly king of speakerdom.  But in reality it was the MOSFET amplifiers which made all that much difference.

Why? It’s all in the design and layout

Now to the big question, why?  A lot about the quality of an audio amplifier is put down to design. Some of the simpler designs are best and let’s be honest here, in a class A amplifier tubes and MOSFETs rule in many ways.  We use less active components to get the required output currents and voltages. Higher gain stages. Lateral MOSFETs such as the K134 and J49 were used in what was loosely termed, bomb-proof amplifiers but this could never be so because strangely enough most complaints came about due to oscillation and self destruction. This is the silent death and the quick and the dead mentioned previously.  The power bandwidth of the MOSFET amplifier designed by ETI I do believe is extra-ordinary. Oh, yes, I “built” an amplifier once which used what we commonly termed Hybrid blocks (all encapsulated) – possibly manufactured by Hybrid, which could rock anyone’s socks.  Looking at resulting output figures based on equipment loaned to us by the CSIR which included power bandwidth, slew rates, frequency response and distortion, MOSFETs were the in thing. But…

Gain is good but read the instructions

MOS amplifier circuits, like sensitive BJT amplifiers with tons of gain need to be very carefully laid out.  Go ahead and design your board at your own peril. The ETI board is one such amplifier – you need oodles of confidence and even more oodles of knowledge to know that you are sitting on an oscillator of sorts.  Best write up I think on this and board design is on Rod Elliott’s website, project 101.


One thing we need to make quite clear here is that lateral MOS devices are very expensive, over R100.00 per device as opposed to the MJ15003 selling for about R60.00 each.  (ex VAT).   So assuming you are to build an amplifier of about 200W per channel you would be looking at about double the price for MOS devices.  And will there be that much difference in audio signature – depends what you want.  Too many design engineers have foregone Mosfet for BJTs and the reasons are given all over the net. Most importantly one needs to stick with what you know.


Buy that kit, you’ll love it.

Yebo Kit

Please support Rod Elliott as well – the boards are 22AU$ each. Transistors about R160.00 each at Mantech.

Autona AL125

Autona AL125 - VAS and current drive

As promised – the Autona AL125

The Autona AL125 was the workhorse of the 70s and possibly even early 80s in the DJ arena.  The amplifiers compared to what we see today were well made but I would not be surprised if the engineers pushed the SOA (Safe Operating Area) to the limits, the DJs even more so.  As promised in our previous article on the 2N3055 here are some pictures I have of three of these delightful little amplifiers.

Article as promised from the Silicon Bubble – the 2N3055

autona al125
The famous “little” Autona AL125

Note the one above on the RHS which has an interesting tale to tell. See below.

autona disco amp the AL125
Autona AL125

The amplifier on the RHS used to be used at a hotel for the PA system.  Using the amplifiers without an additional aluminium heatsink was just inviting trouble as can be seen by the very burnt fibreglass board where the 2N3055s are seated.

autona 125
A better looked after module
heatsink supplied with AL125
Autona AL125 with supplied heatsink

AL125 heatsinking

The alloy heatsink supplied with these modules were supposedly good for 50W into 8 Ohms. As a rush job this may well have sufficed but looking at the image above,  continuous playing into a load very much higher than 50W was a big no no.  The overheated one drove an entire floor of PA speakers, perhaps about 10 x 8″  8 Ohm drivers through a line transformer with no additional heatsinking – for more than 12 years.

Most teenagers during the 70s and 80s bought, built, modified or blew up replicas of these amplifiers. Not having a directly coupled output stage had its merits.

Autona AL125 - VAS and current drive
Autona AL125 – VAS and current drive

Further Reading:  The Quasi Complimentary Power Amplifier Autona AL125

The silicon audio bubble – the 2N3055

2N3055 Amplifier schematic

This slow old dinosaur, the 2N3055, outlives many new devices

Imagine building an amplifier in the late 60s which didn’t break the bank but could outperform most tube amplifiers in terms of raw power.  Rand per Watt silicon has no equal in the audio industry.  Audio meaning reproduction of a  music source of course,  pressurised air or steam is in a different league.

2N3055 Amplifier schematic
50W 8 Ohm Amplifier (let us know if this is copyright – webmaster)

There were many ‘transistorised’ amplifiers in the 60s, mostly low power and Germanium. Germanium was the runaway king, a little bit too temperamental for DIYers and of course they could be very expensive.  The common configurations used the AC128/AC127 and AD161/AD162 transistors. Philips made some pretty good audio equipment with these devices but it wasn’t until the birth of the Silicon wonder, the 2N3055 that things started to take shape both in terms of reliability and power.

Gramps’s Gramaphone

In the 60s most homes would at maximum have a sound system of a few watts, perhaps 5 to 6, powered from a single tube output stage in what was known as a Gramophone, the radio-record player combo. You know the type granny and grandpa had that you were not allowed to touch.  Ball and Claw, magnificent finish. If the reader here is under 25 I doubt you know this or possibly have ever seen one.  Even reel tape recorders were not that common unless you were a real geek and here we would be looking at 300Hz to 7kHz for an entry level model.

RCA – the 2N3055

The 2N3055 was an RCA invent, introduced in the 60s but really only seeing a wide usage in the early and middle 70s. The ratings, as I recall, which I won’t state from the book spec was 115W, 15A, 60V, Hfe 20 at 4A and a cut off Ft of about 800kHz. These were well suited to linear regulators and inverters. They had to run cool, 100W quickly became 60W at about 70 degrees C or even lower.  Catastrophic failure was usually caused by inadequate cooling,  short circuits and my personal favourite – bad seating of the transistors on the aluminium and a common problem in these earlier devices, “secondary breakdown”. (just watch your +Vcc friend).

The all time best seller in South Africa was the legendary Autona 125W 4 Ohm (with 4 transistors in the output stage) running on a single rail 80V supply. Coupling to loudspeaker was through a 2200 uFd electrolytic which alone was enough to either blow the transistors on switch-on or take the tweeters out. These “disco’ amplifiers were sold at the legendary Hamrads in 1976/77 for about R36.00 each.  The DJ fraternity stuck to Fane and then Novik 12” speakers, mostly 8 Ohm.  (audio out 80W into 8 Ohms).  Compare this to the bullies of today.

In time these amplifiers made way for split rail power supplies and the complimentary 2N3055/2N2955. Were any of these actually any good? Almost definitely – bear in mind that we listened to loud and not quality. Quality wise, well one shouldn’t forget that NAD 3020 used these transistors as well. In time these transistors were made more reliable, for instance the 2N3055H had a higher operating voltage and of course manufacturers were looking at higher cut off frequencies due to changes in design technology.  (hometaxial to epitaxial). The 2N3055 is also a highly counterfeited commodity which nearly always renders them useless in any circuit – I always looked for the Toshiba range. They were also more reliable, along with RCA (if they were not the counterfeit ones).

See picture here of a 2 x 50W into 8 Ohms amplifier.

So where does one find ourselves modern days?  The 2N3773 became my favourite for reliability and then of course as we moved to the insane amps of the era, we had the Carver 400 and 700 Watters.   Here most technicians are using MJ15003s/complimentary pairs. They aren’t cheap but when one is building an amplifier to power a 300W sub these speakers are not cheap either. 4 transistors per leg is not uncommon and at rails sitting at about +/-50V to +/- 80V things start becoming complicated if care is not taken.

The 2N3055 may be old, but it certainly hasn’t been forgotten.

I’ll remember to post a picture of the 125W Autona modules this weekend coming 18/19 Feb 2017.  These units could run 50W with the aluminium heatsink supplied into 8 Ohms at 50V DC.

[Editor’s note: in the forums many newbies are put off building a 2N3055 audio power amplifier by the “experts” because of the age of the device. I find this rather disappointing, there were thousands of amplifiers around then and now too, using these transistors. Yes, there are many replacements which may or will sound better at a price but for anyone wanting to learn, let them do so at their own pace, their own pocket and at proper instruction – not just “don’t do it!”]

Changing the argument in modern audio – the digital sound

Asus Xonar U7

The DAC and digital sound – big quality at half the expense

Coming from an analog website it may sound as if we are doing a 180 here but the truth remains, we still have the analog versus digital argument.  Something which will never quite be put to bed, the digital war continues.

My first CD player was the Sony CDP1, a small, heavyweight, state of the art digital ‘icon’ to the audio world. In fact at the time I was not even aware that it was first commercial release, rather just wanting to get my hands on a CD player, a Sony, to impress my friends.

The vinyl years

Two things stood out at the time, one without doubt the easy way to play the media – damn but vinyl was finicky, and then you had the problem of friends playing disks which were so bad that it would damage the stylus (was I the only guy that had a freakin’ sound system) and the big one,  no crackles, pops, buzzing, clicks, jumping, feedback and of course,  the big one – lifting the arm to change tracks.

As time went by CD players dropped in price, had more functions and became quite a part of our lives. Everyone had one by the 1990s. Turntables were packed away or trashed, vinyl was dead. Or was it?

The CD years

I stopped listening to vinyl for about fifteen years, it was always cassette tape or CD and then DVD.  I gave away both my Sony turntables which I DJ’d with in the early 80s. The marketing machine was fine tuning us for the silver disk and they did a remarkable job except for one thing – multimedia computers came on the scene in the mid-90s.  (PCs and Windows, Apple was always the front-runner).  We suddenly could compress our 40MB file to 4MB in the form of MP3s. We became used to this,  piracy was no longer something we found on the high seas, it was in everyone’s home.  iTunes suddenly kicked in and we could buy individual tracks not the whole CD where the artist possibly had only one good track. CD prices started falling and before we knew it the audiophiles started reminded us of a couple of things that we had forgotten about.  CD quality is 20 to 20, vinyl is so much more.  In the last decade suddenly we found vinyl on the shelves again.

The shortest route between two points is the straight wire

The digital versus analogue argument is all about the shortest path, sampling frequency and DAC quality. Vinyl has two paths, de-emphasis at the RIAA amplifier (where disk cutting excursions on the lower registers uses a lot of space the engineers de-emphasise these excursions and boost the treble). On playback the reverse is applied. The second path is the amplifier itself.  Simply put, cut out the unnecessary and only amplify the necessary.  Add tone controls if you wish but make sure the amplification is flat, as close to dammit from 0Hz to 100kHz. Wishful thinking but yes, technology can allow this.  Makes one think how the NAD 3020 became so popular with an output stage using transistors from the Ark.   But let it be said that these amplifiers were very popular and indeed, more because of vinyl than anything else.

Vinyl supposedly reduces fatigue, is non-sterile, not compressed to hell and back and also, importantly, does not have the 20 to 20 limitation.  A guru even went so far as to say that music is something we also sense outside the audio spectrum.  He is a very good musician, who am I to argue. So what has made that incredible CD player now near redundant?  The DAC and the internet of course.  Breaking up an analog signal into parts and then re-assembling  these parts or as in a CD, taking 1s and 0s and then re-assembling to get a ‘resemblance’ of analog is just not on.

Technology has allowed us to push the clocking of a digital circuit into the hundreds of Megahertz, the Digital to audio converter now even runs at 384kHz or higher.  CD players, or rather the media is sampled at 44.1 kHz. Using the Nyquist–Shannon thereom, sampling frequency must be a minimum double the sample rate – 44.1kHz also encompasses anti-aliasing filters which reduce the bandwidth.  Lots more of that on wiki.  Bottom line is that all CD players run at 44.1kHz – man decided that our uppermost frequnecy limit is 20kHz so let it be. But DACs have improved in leaps and bounds over the years, high end DACs costing upwards of R30 000.00.  Ouch. But here’s the thing.

We take an analog signal, slice and dice so it can fit nicely onto your computer and then rebuild this signal through the DAC to reproduce a nice sliced and diced analog signal. Why not just copy your vinyl onto a high end tape recorder, preferably an R2R where the media can sit for 10 to 20 years and hopefully not be subject to degrading.  Oh, yes this happens.  The Audio Technica AT-LP120-USB Turntable sells for about R 7 500.00, throw in a RTR for about R 10 000.00 and Bob’s your uncle, or aunt.

Asus Xonar U7
Asus Xonar U7 – 192kHz A-weighted 114dB front at 24bit/96kHz freq response 10Hz to 46kHz

But no, not to be, not to be.  Just have a look at how long it takes to set up your reel to reel – your computer boots up in 20 seconds and then you have the Xonar U7 as I have. Exceptional quality and in only 60 seconds you have YouTube, access to free downloads, very good quality audio. And that’s the thing – if you want a no frills, high quality audio setup go the digital route. If you are looking at the pseudo-acoustic route go analog. Real analogue. In the real world a high quality setup of either can be expensive. For Sunday afternoon listening pleasure I opt for the analogue.  Day to day listening, digital. It’s all in the head any way.

The Internet of Everything

We have all heard this phrase and it’s not one which will go away quickly – the IoT or Internet of Things is going to change the way we react with the net and the universe.  At home you will have a menu listing trillions of songs and movies. You won’t need to keep a library – you will need to pay royalties but as the companies supplying this service become more popular these royalties will be measured in terms of cents.  It’s happening now already.  We live in a quick fix world – digital streaming media is the way things are happening.


Peavey Classic 50/50 – EL84 screamer

ART Pro MPA2 Digital with Peavey Classic 50/50

The classic Classic Peavey 50/50

Thanks to Paul in Kenilworth, CT,  I picked up this real Classic at a great price. They realistically go for even up to R5 000 in good nick and then one still has the overseas shipping charges to contend with and possible re-tubing. I do believe there are many of these power amplifiers floating about in South Africa so keep your eyes open.

What is so marvellous about this amplifier is that it can be easily modded, can be set up for home use and best of all these are stereo units or more in pro circles, two channel 50W RMS. And yes, of course – 50W per channel for tube amplifiers at a near giveaway price is the main attraction. Bear in mind that a 35W per channel EL34 aplifier is going to empty your wallet of some R20 000.00.  But keep this a secret please!

Power output of the Peavey Classic 50/50

Sceptics are often quoted as being surprised at the output performance of these amplifiers as they use the baby brother of the EL34, the EL84.  Well not really. The EL84 is a high gain pentode which requires very little drive to bring it into saturation. Aha!  This explains the popularity amongst the overdrivers. The 6BQ5 is the USA equivalent. The 6V6 is less sensitive. The EL84 used in guitar amplifiers brings out a distinctive and prominent treble tone. Do not confuse this with lacking bass – EL84s when in the right configuration make exceptional quality amplifiers.

The manufacturers of guitar amplifiers regularly configured the output stages of these EL84s to run at a plate (anode) voltage of between 400V and 425V – over 100V that of the manufacturer spec. Before jumping to conclusions remember that tube specs are given for max plate current and voltage. Reducing the plate current and supplying high voltages is not uncommon. One just cannot do both – the EL84 can dissipate up to 14W in a controlled environment – controlled meaning reducing screen grid voltage as well. These amplifiers are known to run sometimes for up to 15 years+ without a tube change. Coming from a radio telecommunications background it was not uncommon for SSB transmitters to never have their output tubes replaced. In class AB1 an SSB transmitter is also running at about 50% efficient, 3kW in for 1.5kW out in marine use.  The bigger issue was power supplies – often over 100kg to power these beasts. Tubes are survivors and are many times more resilient than the older RF transistors (even in audio use).  So tube amplifiers designed around the manufacturer spec is often downplayed for many years of reliability. Even at 400V applied to the plates of an EL84 does not mean it will fall over in 3 months to a year as many doom and gloom harbingers decided.

The one thing to be careful of though is driving any tube amplifier without a load.  Especially when pushing a tube plate voltage to the max such as the Classic series. This will cause breakdown in the primary windings of the output transformers. Note the two diodes across the EL84s. Wonder what they are for?

One channel Classic 4040 Power Amplifier
One channel Classic 5050 Power Amplifier

Looking at the schematic it’s no wonder that these were so popular. Besides the design being super simple tube rollers had fun because the tubes are very easy to change.

Classic 5050 EL84 Output Octet
Classic 5050 EL84 Output Octet

These amplifiers rely heavily on the added fan cooling often not seen in low power tube audio. As the tubes are pushing their maximum ratings within a very compressed space however it’s not just a good idea, it’s essential.  These Peavey amplifiers are also known for their compact size and weight, a 2U chassis makes it easily transportable, in rack or out.

Troubleshooting these Peaveys

Trawling through the net one often is surprised not to see skilled technical folk not warning wannabe repairmen to ensure that whatever voltmeter, DMM etc they will be using is indeed high impedance. Years back one would have a VTVM (Vacuum Tube Voltmeter) which was designed for this and measurement of ultra low voltages.  The cheap low Ohmic meters of the day, chiefly analog of course, would play havoc on tube bias.  Unsuspecting DIYers were often surprised at the rosy glow coming from the anode when measuring control grid to ground.  So, please be careful.

There are four main checks when it comes to tube performance: Plate or anode voltage, screen grid,  control grid bias and heater voltage.   If you are not sure, have not worked on tube gear before then now’s the time to call in skilled help.  Tube voltages kill!

All voltages and currents should be in the service manual, but not always.  The general rule of thumb is to remove the tubes with power off and capacitors discharged. Clean the socket properly – tube heaters draw a fair whack and if not seating properly there will be a very large degrade in performance. I mention this as well because an amplifier I repaired years back lost bias through poor socket connections with the tubes going into saturation and showing a healthy red glow from the anode. So just be extra cautious, socket pins can cause problems, especially in road gear.

Heater filaments very seldom burn out. I have found that because of the current draw on bigger amplifiers (and some smaller) if there is a fuse-holder make sure that the fuse is seating/connecting properly.  This was a big problem with portable TV sets where the last thing the tech looks at is the voltage drop across the fuse. Even older fuses cause a drop but don’t now make this a fetish – just make sure the heaters sit at spec value.

Control grid voltages: always in audio negatively biased to Cathode. Below is a brief description and also gives transformer secondary voltages.

Bias supply
Bias Supply – control grid bias and 12V relay coil for stereo / mono use.
V-Relay – the relay coil is grounded in “Mono” mode.
Tube Bias
Tube Bias

Mains transformer 110V or 220V primary


  • sec 1       300VAC @ .318 amps  =  425V feed to anode/plate  /  100 Ohm 5W dropper to screen grids
  • sec 2      41.5VAC CT @ .318 amps =  – 58V to control grids
  • sec 3      6.19VAC @ 5.16 amps = heater supplies
ART Pro MPA2 Digital with Peavey Classic 50/50
ART Pro MPA2 Digital with Peavey Classic 50/50

Next up will be the test, tube pre-amp to tube power amplifier.

The Harman/Kardon AVR161S

Harmon/kardon AVR161 review

Home Theatre AVR161S 5.1 harman/kardon

Being pretty frugal when it comes to buying audio equipment I felt it time to replace my tattered old 2004 JVC with a receiver which would not break the bank. Having four options at hand,  Yamaha RXV379, Harman/Kardon AVR161S and the new Yamaha RXV381 or HTR4068 I was stumped for choice. Knowing someone years back with a harman/kardon repair job who was a first class arsehole left me dodging the 161 like a Bryan Habana. But and this is a very big BUT, many folk have raved about the reliability of this brand and of course the quality. As the retailer was selling this unit for at least R500.00 cheaper than competitors I felt it was money well spent for a known good brand with a switched mode power supply. Not that I have anything against either Yamaha or linear supplies.

Harmon/kardon AVR161 review
Harmon/kardon AVR161

The audio output is nothing to write home about, 85W RMS x 2 channels driven.  I needed the HDMI more than anything which was 5 inputs, more than I needed in any event. Optical and coaxial inputs, 2 x analog RCA inputs and composite video output make this HT receiver as barren as the Sahara but this is what appealed to me.  And the setup is ridiculously easy. Neat and tidy. An omni-directional microphone is part of the package and we were up and running within ten minutes.

AVR161S - rear connectors
AVR161S – rear connectors

Not going into what it can and cannot do, it’s all in the manual, it must be emphasised that this little receiver has excellent reproduction quality, it is quick to set up and the power is more than ample.  The on screen display is simple to understand and all inputs are easily allocated – in fact I would say it’s the easiest installation I have ever done. But then again this is what HK is all about.

This is not a multi zone receiver like the 171 – I really don’t know of many people using this function either properly or on a practical level.

I did not like the volume control illumination but I did like the fact that the dimmer on the remote reduces this to a pleasing level.

For a little home theatre amplifier albeit big brand coming in at just under R 8 000 on the South African market is a rare steal.  Thumbs up to Govan Mani in Durban.

harmon/kardon AVR161S
The AVR161S all set up and in action

I’d like to say this was an elementary review but when one purchases something for their own entertainment and nobody else it may come across as being biased justifying the purchase. In essence – just Iove it.

[Editor’s Note:  Harman International Industries is headed up by Dinesh C. Paliwal as CEO. In November 2016 the company revealed it was to be acquired by Samsung. I wonder what Mr. Trump thinks of this?

Brands include: Harman Kardon, Becker, JBL, Crown Audio, dbx, Martin Professional, AKG Acoustics, Lexicon, Infinity, Mark Levinson, Revel, Soundcraft and Studer]

The Famous L20 – sold on eBay

L20 - 350W audio amplifier

There is one amplifier that seems to get more attention than most and that’s the L20. I have no have experience of the L28 which has a built in speaker protection circuit.

L20 - 350W audio amplifier
L20 200W 8 Ohms – 350W 4 Ohms at +/- 60V – advertised on eBay

Straight talk, these are mono amplifiers, there is no bias preset neither speaker protection so make sure you get one for each amplifier or the Vellemann K4700 dual channel protector.

Communica sell the K4700. I purchased mine from Velleman USA. In 2014 R300 incl. import. These are for two channels.

K4700 - dual channel speaker protector
K4700 Velleman dual channel speaker protector

Pro-Tecc in Durban sell their own brand. Last pricing was R46.00. Mono only. Spade or terminal posts – easily modified.

Mono speaker protector
Speaker protector – mono – from Protec Durban

The Kit

Easy to assemble, no schematic, painting by numbers. One can build one amplifier in an hour easily. Be careful of polarity. You must have a DMM to actually measure the resistances before populating – sometimes the colour coding can be a bit daunting especially in low light or when you are no longer a spring chicken. Board quality from most of these Eastern suppliers is very good although some of the manufacturers do complain of counterfeiting, poor quality boards and dodgy transistors.  Make sure you have PayPal protection and the supplier is authentic.

The power supply

Power supplies are obtainable from most of the exporters or kit suppliers but be warned, exporting of Toroidals or even EI transformers are very expensive due to the weight. Support local.  Swiftheat is one. See below.

Transformers for these amplifiers was supplied courtesy of a Sony audio guru in Cape Town. The STR500 series (plus others) carries a 220 : 42-0-42 transformer which seems to be adequate for one channel. I very happily became the proud owner of 4 of these.  Why? They are prone to overheating when these amplifiers are slammed hard, the heat protection sensor in these transformers goes open.  With a bit of digging one can remove the sensor and bypass.  Once full wave rectified and smoothed out you should be looking at about 60V DC per leg or +/-60V.

Smoothing caps were 2 x 10 000uFd per leg, obtained from Yebo in Bellville. Code ELC108, currently priced at just under R100.00 each.

Note:  The original power supply which I still have was made up of 6 x 400~500VA 7-0-7 (14V) UPS transformers.  I loaded the power supply so that there was a constant draw of about 25A per leg at 52V (3 in series * 1.414) without any sign of a dodgy heating pattern amongst these transformers over about 8 hours.  The problem is that the power supply weighed in at 24kg.

A really great alternative is to try Swiftheat in Johannesburg – their pricing for a 1000VA toroidal used to be about R 2 200.00 exclusive VAT.  This is a very fair price based on what it would cost for an import or through the DIY channel. This included washers and dipping for audio use.  Note that this pricing was April 2015.  Include 7 days after payment for manufacture.

Heatsinking was obtained from Mantech, code A0M0097. Purchased two 8 3m lengths. Each amplifier used a 40cm run.

L20 Sound quality

Exceptionally good. I was worried about bias, this is factory and designed and obviously we would have concerns about transistor mismatches etc. Not to be, all four amplifiers I purchased were rock steady and from what I can recall about 20-30mA.  The amplifiers were tested into 2 * Dixon 600W 4 Ohm (you know the one – 18″ bass driver and millions of piezo high frequency drivers) plus 2 x Dixon 15″ (which sound pretty shitty but for clubbing, who cares).

The quality of reproduction is amongst the best I have ever heard.  To be frank I did not trust the Sony transformers but they held out.  The original PSU bank of UPS transformers laughed off the current draw, bearing in mind that when using a program source such as music, proper music, Floyd, Deep Purple, Black Sabbath and Metallica 🙂 the current drain is really minimal compared to potential. And that potential one could nearly weld with.

Some little things to remember:

The heatsinking at full power is not really adequate once boxed. Use a fan blowing onto the output transistors ensuring each transistor gets the same treatment. You do not want to have thermal issues causing the output transistors to start current grabbing.

The original PSU using UPS transformers had sensing on each secondary rail. Because each transformer was separately fused primary and secondary this would shut off power if only one fuse blew preventing one rail of the PSU output to remain hot.

The speaker protection circuits, whilst great for peace of mind will not prevent voice coils burning in the event of a relay contact being burnt shut.  These relay contacts are not designed for professional and high powered use. Bear this in mind.

In conclusion

I still have the L20 x 4 channel amplifier which was going to be used for home theatre. I subsequently purchased the Behringer NU6000 4 channel amplifier. The L20 is in Cape Town, I am in Durban otherwise I would have posted photos. I am looking for McIntosh style VU meters though. Like most, these really add a professional finish to any home built equipment.

McIntosh Amplifier
McIntosh Amplifier – damned gorgeous

The JVC RX-5042 mods

RX-5042 Amplifier blocks

Why the RX-5042 for modification?

In our previous article we rant about the cheap RX-5042 which can be picked up for next to nothing.  The problem is that it is a home theatre amplifier, along with the poor tone controls and lack of MM or MC cartridge inputs.  To keep the audio circuitry strictly short path analog and of course ensure that power amplifier speaker protectors remain operative we add minor changes – a separate stereo preamplifier plus RIAA turntable input.

Relays configure the routing of the left and right channels, the center and surround inputs are muted (grounded) and an LED shows when analog with Baxandall tone controls have been selected.

RX-5042 Power amplifier
RX-5042 Power amplifier – click to get bigger image.
RX-5042 Amplifier blocks
RX-5042 amplifier showing modules – click for larger image

In our next article we will drill down to the changes showing stereo preamplifiers with tone controls and switching. This will include LFE out.

Should Class D Amplification be classified as digital?

Inside the Behringer 6000 4 channel Class D

Class D Amplification – digital or analog?

Here we look at two things:

  • Is class D digital or analog?
  • How do they really sound?

The conundrum of audio, the one that all audiophiles abhor, Class D.  There are many beliefs behind this one of course, one being chiefly that Class D is digital because the output devices are running in one of two states, on or off, 1 or 0.

Is Class D didgital or switching
Nuking the Speakers with Class D

The theory is not so simple however.  1s and 0s are binary which is not only fundamental to all computer languages but is set at a reference predefined voltage e.g. 5V and frequency (clock).  The class D amplifier utilises sampling, modulation and switching as a basis of operation. The switched output before filtering will show pulses of varying mark space ratios – the 1:1 or equal ratio has no audible output.

Check this out dude/dudess:

The marketing dilemma

One thing that always struck me as distinctly odd was how the marketing machine would advertise a specific brand of car audio as digital or switching amplifiers. This can be seen on the Hi-Fi scene as well as in Pro-audio.  The car audio amplifiers used switched mode power supplies with class AB audio, the rest, well digital amplifiers known to us as class D.  A throw-around of quick catching and nice sounding phrases which never described the equipment adequately.  An amplifier with a DSP component could be called a digital amplifier I suppose but it doesn’t fully describe the amplification technique does it. DSP can be added to the input of an audio amplifier running class A and then be called digital. Class D however is not digital, sorry. It’s a switching amplifier. Period.

And more marketing

Along with rail shifting amplifiers with all their glorious classes, the fantastic class T switching amp, digital or switching class D the public must by now be thoroughly confused. I think even professional designers must be confused as hell – as soon as one class becomes relevant another one takes shape. Years back we had class AB1, 2 etc.  Class C was for RF (yep, Morse code was one use) and the top end amplifiers were supposedly all class A.  In fact, us, the general user should only know three things – does the output stage amplify the entire signal (class A), are the positive and negative excursions driven separately with minimum distortion (class AB)  or does the output contain varying mark space pulses (class D).  Over the last fifty years all three have been in use and still retain the same definition. Whether the amplifier uses +150V at high power or only +50V below 20W is really irrelevant. Whether it amplifies the entire signal or only part of it on purpose by design is relevant.

Can we hear the difference between the classes?

Many people can hear things I cannot.  We have listening tests to prove or disprove which amplifiers are best. Theoretically the best audio amplifier should be the Class A beast. Heavy in power supply and cooling (as it is the least efficient), very expensive, it amplifies the entire input signal as it should – in absolute mirror but for amplitude. The pedantic will cry for no phase shift either.

Truth be told Class A amplifiers do have a good reputation but usually when built by reputed companies or if self built, there is no expense spared for power supply and cooling.

Class AB amplifiers – we call them AB because in pure B we would have switching distortion which sounds horrific so a little bias is applied to the output stage to push them into their linear region (ahhh, slowly going back to class A).  Class AB can sound just as good as A if designed properly but there is always that niggling suspicion that we have that damned crossover distortion and the output stage is not linear. But over a few watts of drive into the speaker they do come into their own.

Class D. Try as I might I always think, believe, hear whatever that AB has better reproduction at the high frequencies. Having said then we need to be looking at class A, possibly bi-amped.

And what does my 40 years of experience tell me? Build your own. Play around a bit. Have fun. That’s how the best engineers started.  Your home built amplifier will always sound best.

One of the best designs (although I never used the commercial PCB but regret it now – this will come next though) was Rod Elliott’s 60-100W project:  I used a NAD pre-amp (1020A) to drive these modules.  OK, so I never used a PCB and yes, I only built these because I was interested to get back into electronics as a hobby – I was really very, very impressed.  The noiseless monster. In the late 70s I built a kit which used an MJ802 and 4502 which knocked the socks off the competition – well this one was better. I really think where this project stood out was that it was better than the 3020A (remember that the 1020A is a 3020A without the power amplifier).  I always thought that the 3020A sounded too flat.

Going to purchase the project PCBs at the end of the month. (31st January 2017). Let me know whether you need so we can consolidate costs.  webmaster at analogian dot co dot za


So what makes the perfect amplifier?


So what makes the perfect amplifier

Your ears, really!

Many years back we were told that a radar is designed around the magnetron. Of course this is true because microwave energy falls within a specific band or bands. I feel the same about loudspeakers and amplifiers.

What makes the perfect amplifier
Behringer NU4-6000 and Pioneer A-5

Enclosures are designed around the drivers used and power amplifiers around the power supply. Pre-amplifiers or voltage amplifiers around the program source.

Topology is also important, class A, AB or D? There are many topologies of course and a lot of design work goes into supply rail shifting techniques.

What about solid state or tube/valves? Mosfet or bipolar?

It’s rather an ambitious task to be the manufacturer of the world’s perfect amplifier because truth be told, although we are at critical mass we still have ways to go and yes, they’re damned expensive.

Boulder, Macintosh, Conrad-Johnson, Emotiva, Dynaco ST-70, take your pick. What is relevant though as your tastes go up, so does the price. Speaker prices easily go upwards of a million Rand and buying a top end amplifier you don’t sit with pine wood boxes.

What makes the perfect amplifier
Behringer NU4-6000 and Pioneer A-5

How many times have you sat in an acoustically treated room and hearing a tiring lifeless display of sound from a 20 000 Rand system.  Often not set up properly, often the acoustics dampen the high end flattering the low end. Always best at home and set up to hear what you want to hear and not the sales pitch.

This is a question posed by novices and experts and truth be told to dive into the mysteries of sonic performance of an amplifier one would need to analyse the input vs output signal through pretty sophisticated audio test equipment.

In many ways NAD is voted king of the entry to mid level manufacturer amplifier but I find with incorrect speakers they sound, almost, and dare I repeat, lifeless. I love Sansui, Pioneer and Kenwood which I would put in the same price range. Yet NAD is often rated the better, the 3020 is often even mentioned in the top 20 listings by many audiophiles yet the figures are not exceptional by today’s standards.

An amplifier which has near zero imperfection under electronic testing should yield very good sonic performance and often this is the case. The program material through the standard onboard audio DAC on your computer or notebook may not be high performance and will not do your power amplifier justice. There is a vast difference, say, in using a Xonar U7 which is what I use over the onboard, which in this case is a Realtek High Definition device. I have used the Focusrite Scarlett 18i20 which to my ears is better than the Asus and this can make a dull amplifier come to life.

Pioneer A5 modifications for line drive
Behringer NU4-6000 and Pioneer A-5 rear – note the modified 10K:1K XLR outputs to drive the class-D amplifier (also note the fan outlets, rusting – possibly too close to the Durban air).

Sticking to analog rules then we need to ensure that with turntable use there is no hum and noise. Going the high gain path of any pre-amplifier is where we start picking up issues, more often than not, man made. Entry level turntables should be avoided – often they are the mass marketed USB variety. So we then move on to reel tape recorders, supposed with exceptional quality heads and a great frequency range. The tape recorder head reproduction is also amplified with a great deal of gain, a good starting point for noise if the wiring is not of professional standard i.e. shielded, earthed properly etc. A good quality tape recorder’s reproduction can be frighteningly close to the real thing so possibly this is the best audio source.

What I have found and this is obviously opinion based, the amplifiers which have created the most impression are all rated at over 100W (continuous) per channel class AB which means big power supplies – they will always be heavy. There will be a lot of aluminium and copper for cooling and the transistors paralleled to handle 4 Ohm or even 2 Ohm loads for a continuous rating.  Power amplifiers do not need to have millions of buttons – in fact it can cheapen the look. Class A amplifiers of 50W or more you would need a bulldozer to bring to your front door.

I have an NU4-6000 which is used to drive Eltax Millennium 500 series speakers (R2 000.00 2nd hand). These speakers are not highly rated by the audio community but the noise makers praise them endlessly. The Behringer amplifier is actually an exceptional match for these loudspeakers, easily driven to about 300W continuous, nice balance between highs and bass, which for rockers is real kick arse stuff. These speakers are made to be driven hard. On a 50W Sony amplifier I found my setup efforts rather a mishmash where Castle Knight 1s and 2s excel.

So looking at the amplifier, place first emphasis on the loudspeakers and program source. The general rule of thumb is to buy the most expensive loudspeaker you can afford.  The amplifier is going to be your choice – always purchase a known brand and do research.

In conclusion:   Many home enthusiasts are purchasing professional series amplifiers which are often more affordable than that designed for the so called consumer market. The preamplifier can be your choice or do what I have done, used an old Pioneer amplifier modified for line out (R500.00 Cash Converters).

By the way, here’s an exceptional article on the Behringer Class D NU6000 3kW x 2 and the KAM KXD7200 3.6kW x 2 

Class D amplifiers are scorned or loved but one thing is for sure, they are getting better and of course, cheaper as well. I find they make excellent bass drivers.

To get a really good system going one should read up about crossovers, bi- and tri- amped systems. More about this later.