Hexfets Vs Laterals Vs BJTs

Mosfet Devices Versus Bipolar Junction Transistors

I will try not to be biased in this article. Pun intended.  Again, no re-invention of the wheel so we will guide the newbie in the right direction.

General circuit - input choked

There’s different trains of thought when it comes to the different transistor types out there and which is suitable for what. Continue reading “Hexfets Vs Laterals Vs BJTs”

Car Audio – More on amplifiers and SMPSU

Some links to car audio circuits

If you read the previous article on how quickly we can become deaf through negligence or just plain stupidity (like this scribe) I have managed to get some gems together for the DIY types.

MTX Audio Thunder754-400w-rms-4-channel-class-ab-amplifier
            MTX Audio Thunder754 400W 4 Channel class AB amplifier

Firstly, for those wanting to repair car amplifiers, Perry Babin has done an incredible job in keeping to the facts and make his tutorials available for free or chargeable as a download.  There has been a lot of time and effort going into this so full marks to the author for doing such an amazing job.  I guess the best way to get started is from this link:  Basic Car Audio Electronics. Continue reading “Car Audio – More on amplifiers and SMPSU”

Hybrid Amps

Hybrid Amps: Some more circuits

Using the first article to get our creative juices flowing I found some more well written well designed amplifiers using tubes and semiconductors.

Nobsound MS-10D MKII
Nobsound MS-10D MKII – 2 x 25W 2 x dual Triode 6N1(P) and 2 x 6P1(P) beam tetrode. (the code without (P) is Asian, with (P) Russian.

The design link below shows various permutations to feed a high voltage swing with little distortion to an emitter follower output stage.  This circuit, titled Hi-End Hybrid Amplifier 100W uses a high end K&K solid state current generator (source) in series with a pentode voltage amplifier. The tube in this case can be a Siemens D3A or Russian 6C45.

OK, so what is our pitch here?

High safety margin, for a 100W amplifier using 4 x MJE15032 and 4 x MJE15033 output transistors in the output, the input to the current amplification circuit is through a capacitor (DC block) from the voltage amplifying pentode and an LM308 op amp for stabilising the DC offset at the output, removing the need to apply manual adjustment.

The MJE15032/MJE15033 can be obtained at a good price from Mantech (Communica advertise only the PNP at R17 each excl).

The K&K current source can be found under the two terminal current source on the K&K website. The source is a design around the depletion mode IXYS IXTP08N100D2. As these are not locally available I’d advise the reader to first open the Two TerminalCurrent Source Kit pdf file from K&K – Microsoft Word then get your nose into Rod Elliot’s article on sinks, sources and mirrors. A few minutes read up on current sources should throw some light on the subject and have you amped for your next journey.

An auto-bias heater filament Mosfet load headphone amplifier

Quite a mouthful but nice clean design pulls all the stops in hybrid design. Design found here.

Hybrid vacuum tube/solid-state audio power amplifier

Nice design which will break the bank amplifier found here – this amplifier uses an 6AN8, a pentode-triode combo. The output devices are BUZ900 and BUZ905.  The 6AN8 may not be that easily available in South Africa but the good news is that Mr. Valve lists this valve for R150.00.    Based in Pretoria, excellent service.

The BUZ900 and 905 are expensive, TO3 equivalents include the Hitachi 2SK134 and 2SJ49.  Available (no price given) at Mantech otherwise Yebo listed as ECF20N20 and ECF20P20 going for over R300 per pop. Ouch! Note that these are lateral type MOS devices and because of the TO3 can this amplifier is ideal for experimenters wishing to modify the output stage with cheaper alternatives. After all, we are looking at the voltage gain devices, chiefly pentodes driving Mosfets.





Tube Semiconductor Hybrid Amplifiers

Tube and Semiconductor Combos – the best of both worlds

An interesting project on the web is that of a 6SN7 Mosfet audio amplifier capable of delivering 50W per channel. The article, written by Simon Brown is called Amplifier War and Peace – An Hybrid Amp

Taga HTA700B V2 SE Hybrid
Taga HTA700B V2 SE Hybrid  (Note: tube pre-amplifier stage)

I am not a big believer in that tube amplifiers are so much better than that of solid state but I do believe a properly designed amplifier using both devices can address problems, both in sound reproduction and ease of build.

What I DO like about tubes is the almost ease at which one can build a pre-amp or power amp by simple point to point wiring. This type of wiring is also notoriously the best way to build these amplifiers – easy to change layout, reduce noise,  unnecessary oscillation and to modify at a later stage.

The project, found at Audio Xpress features two 6SN7s in a current source configuration driving a complementary pair of Mosfets, IRFP140 and IRFP9140.   All aspects to the design are covered but the author does warn that it’s not for inexperienced builders. Here of course we must also make mention that the V2 is running at 300V on the anode. The power supply is well thought out. The entire project can be built on prototyping board.

Another project which caught my eye and also under Audio Xpress is that written by Stephen Moore – A Hybrid Valve MOSFET SE Amp – Complete Project.  With a smidgen of Nelson  Pass in the constant current source and gate bias circuitry we, like the above circuit, have two 6SN7s or similar acting as the voltage gain stage to drive the source follower  IRFP2907Z. (using the John Broskie Aikido amplifier approach).

Both these articles are very well written with a lot of insight and explanation as to the shortcomings of Mosfet output devices, work arounds and just as important, driving these devices with high voltage swings from tube devices.

The Taga HTA700B V2 SE Hybrid amplifier boasts the following:

  • Power Output: 2x45W RMS / 4ohm; 2x35W RMS / 6ohm; 2x26W RMS / 8ohm Class A/B
  • Vacuum Tubes: 2 x 12AX7B
  • THD: Less or equal 0.1% (at rated power)  Signal/Noise ratio: =>88dB
  • Frequency Response: 20Hz – 28kHz
  • Inputs: RCA stereo CD, USB (for computer)
  • Outputs: Stereo RCA with variable volume control
  • Connectivity Wireless: Bluetooth® v4.0
  • Bluetooth® Profiles: Advanced Audio Distribution Profile A2DP
  • Headphone Impedance: 32-320Ω
  • Headphone Output Power: 1W  160Ω
  • USB Supported Data: Asynchronous 24bit / 192kHz
  • True high-speed audio processor CM6631A
  • DAC Cirrus Logic CS4344 chip
  • Included Accessories: Bluetooth® antenna, USB cable,
  • Removable IEC Power cord
  • AC Power, Power Consumption: 230-240V 50Hz, 100 W
  • Dimensions (H x W x D): 14 x 17.7 x 27 cm
  • Weight (net): 4.2 kg / pc.



Winding your own OPT

Local suppliers – South Africa

As an addendum to our previous article “Alternatives to Tube  Output Transformers” we thought it pertinent to add a bit of local flavour,  Mars Amps in Paarl to the mix.  No, I have not met the gent but Karel Mars is a well known tube guru in the country and lends his expertise to the DIY community either through workshops or on the local avforums website, found here.  (OPT winding).

Mars Amps has a variety of tube pre- and power amplifiers available in kit form, shipping both locally and internationally.

See http://www.marsamps.co.za/product/output-transformer-opt-jtm45/



Class D Chips


Class D Chips which pave the way

As mentioned in a prior article, Class D amplification is nothing new, modern switching components and topologies paving the way to resilient and high quality amplification circuits.

We can also draw a parallel between modern switched mode power supply design, H-Bridge motor control and that of Class D, all benefiting from newer ultra fast switching transistors, the bug-bear of the first class D and switching power supplies. Continue reading “Class D Chips”

Class D – The beginning of a new Era

Why Class D amplifiers are the way to go

As a young (or old) student it takes some time to get one’s head around the design idiosyncrasies of solid state amplifiers. Truth be told many of us found tube nomenclature and circuit design easier to comprehend than anything that followed “hole” theory.  Big bad things come to an end and eventually we found it easier to crib other designs.

Class D insides
Behringer 6000 4 channel Class D

Continue reading “Class D – The beginning of a new Era”

Protecting your Amplifier

Transistor Junction Temp and Derating

Protection devices for Audio Amplifiers

There is a lot of theory put into amplifier design and when one looks at these complicated circuits often the protection circuits are more sophisticated than the gain stages.  Purists believe the only true amplifier is a piece of wire with gain.  Engineers often criticise the protect circuits of creating their own problems, along with causing inferior quality reproduction.

These are the major problems come across in the design of any amplifier:

  • Heat – is there sufficient cooling for heavy duty work?
  • Can the output transistors go out of their Safe Operating Area?  (known as SOA).
  • Can the amplifier drive a 2 Ohm load when minimum load spec is 4 Ohm?
  • The dangers of mains voltage boosting and sagging?
  • Oscillations and instability.
  • Over driving and distortion.
  • Loudspeaker protection – expensive lessons to be learnt.

These are just a few of the problem areas in the design of amplifiers. Continue reading “Protecting your Amplifier”

Practical examples Differentials and Current Mirrors (Part-II)

Functional Block Diagram of LM741

A quick glimpse into the lives of differential amplifiers and current mirrors.

Here again we bring out the tried and trusted LM741.

Functional Block Diagram of LM741
Schematic 1.  Functional Block Diagram of LM741

I doubt there is anyone in the electronics field that hasn’t worked with or come across one or more of the most popular op-amps of all time, the LM741.  We peer inside schematic 1 and lo and behold, differential amplifiers and current mirrors. Q9 and Q10 are the current source, Q10 and Q11 the current sink. Continue reading “Practical examples Differentials and Current Mirrors (Part-II)”

Current Mirrors and Long Tailed Pairs

Differential Amplifiers – what they do  (Part One)

Long tail pairs or differential amplifiers have one important purpose:  To amplify the difference between two inputs or act as a switch.

Differential or Long Tailed Pair amplifier
Differential Amplifier or Long Tailed Pair amplifier

In a perfect case the voltage gain of the differential amplifier is the product of the differential gain or Ad and (Vin+  subtract Vin-). Continue reading “Current Mirrors and Long Tailed Pairs”