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.




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”

Top Links for the DIY Audio Nut

Learning about Audio

Although SEO is all about well written material sometimes we do need to veer off this patch of oasis and not re-invent the wheel.  Audio circuitry, your own equipment, testing, listening and boasting is all part of the job. Here we list some awesome websites which should get your own creative juices flowing.

Illustrated History of High End Audio V1 - Robert Harley
Illustrated History of High End Audio V1 – Robert Harley

But first, some words from Popular Mechanics and some home grown products…  (16 September 2013), which sort of sums it all up. Continue reading “Top Links for the DIY Audio Nut”

Sony home theatre – Reworking the surround amplifier for stereo use

Rear Surround Module - Sony KM7

Making the most of 7.1 or 7.2 with the extra surround channels in a Sony

Most home theatre amplifiers have 5 active channels and one dedicated low frequency pre-amplified output for driving an active sub-woofer.  Sony amplifiers for the consumer often has extra channels which are on a daughter board designed to drive up to 100W into an 8 Ohm load. These are the ones the DIYer should be on the lookout for. In a previous column I mentioned that these can often be picked up for about R100.00 in faulty condition. Continue reading “Sony home theatre – Reworking the surround amplifier for stereo use”

Amplifiers & Light Control

Autona 125W 4 Ohm

Mixing loud music with a light show

Pink Floyd and Electric Light Orchestra, music enhanced by the control of light.  What is great powerful music without a visual experience.  Not Ozzy Osbourne biting a bat or dove’s head off of course but something more appealing, sound to light, chasing, strobing and laser. Of course it’s every young inventor’s dream to build their own dazzling light show but where to begin?

Autona 125W 4 Ohm
Autona 125W 4 Ohm

Sound amplification:

Sound or audio engineering is not just a profession but an art….. exactly the same applies to an engineer who spends his life building top quality audio amplifiers.

40W Audio Amplifier - Quasi Complimentary
40W Audio Amplifier – Quasi Complimentary – More Information below

Audio amplification is split into two parts (1) the voltage amplifier or pre-amp. This amplifier has a number of pre-requisites:  being able to amplify a tiny signal like that from a microphone or turntable (with RIAA equalisation),  being able to boost the bass or treble or equalise the signal for different room parameters (stage, hall etc) and (2) the power amplifier, which takes this signal and amplifies it further to feed a loudspeaker.  All pretty simple, right? Continue reading “Amplifiers & Light Control”

Total Harmonic Distortion meter

Hantek DSO

THD Meters – how they work

Many years back the must haves for anyone building audio equipment would have been the multimeter (DVM), oscilloscope and a function generator. With specialised audio test equipment falling in price over the last twenty years two other pieces of equipment are now also found on the test bench, the THD meter and spectrum analyser.  Indeed your laptop or desktop could be used for many of these functions but it can be cumbersome and/or inaccurate because of sound-card bandwidth limitations.

Hantek DSO
Hantek DSO – 1202B

The THD Meter is definitely a must have, even something for impressing one’s audiophile friends.  Consisting of an oscillator, internal or external, a notch filter to attenuate the fundamental to as little as possible and a voltage / instrumentation amplifier to drive the resultant combination of harmonics and noise to the output meter or video graphics display.

Block Diagram of THD Meter using Wien Bridge Notch Filter
Block Diagram of THD Meter using Wien Bridge Notch Filter

Interesting DIY angle to THD metering from “Wensan” – DIY Audio – Simple THD Meter

Robert Cordell,  American (as in USA) electrical engineer and expert in the audio field kindly publishes his high end THD Analyzer circuit on his website, Cordell Audio.

Analysing the Waveform – skirting the FFT

The analysis of signal waveforms is rather mystical to most of us and becomes a highly complicated mathematical subject when breaking a signal or rather batch of frequencies down to a function of time. FT, or Fourier Transform converts the waveform data in the time domain into the frequency domain each containing individual signals of phase, amplitude (magnitude) and frequency.

Modern computer software makes this previously time consuming and highly mathematical task accessible to the home experimenter using what is now commonly known as FFT or Fast Fourier Transform.

Digital Signal Processing

Digital signal processing in the modern computer has made decomposition of signals in audio a common discussion point on the forums, possibly not always in our best interests because of the complexity and hence confusion.

FFT and Nyquist are of interest to the engineer, especially now in the DSP domain where bit rates and sampling frequencies have become an integral part of our lives.

Our modern (and even older) hardware can run software for analysing and changing the audio signal – and sometimes it’s free. See Audacity and it’s features.

Shit in Shit Out

Technically speaking it is not probable that an ill designed amplifier using inferior components will sound good. 🙂

It is probable that with the assistance of application notes, spec sheets and white papers from most manufacturers to build a quality product with the minimum of fuss.

Hantek  – USB oscilloscope, multi-function

I have four oscilloscopes, three of them analogue. I like the ease of triggering and set up. Coming from the analogue era certainly assists – the digital scopes don’t appeal from a GUI perspective until one needs to record an event or compare functionality to price. They become indispensible. Just look at the hand-helds.

The digital scope I use is the Hantek 3062. It has a logic and spectrum analyser, frequency counter and of course it’s a scope.  It cost about R4K five years back from Kmeasure, based in Pretoria. Very good service. I mention this as an aside, there are always sensible, cost effective solutions in our pinched South African market.

Lastly, of course we need a good quality function generator.  I find Instek to be of exceptional quality and reliability. Available at Mantech Electronics. (Cape Town, Johannesburg, Durban).

Further Reading:

Cordell Audio:  Robert Cordell Build your own THD Meter

KMeasure – Professional measurement systems (based in Pretoria)

Signal Lab – SigView Signal Analysis software

The VCF or Voltage Controlled Filter



The Golden Ear vs X-Ray Vision

Fact versus Fiction – our hearing is pretty hairy

There is a gem of a website call Audio Check which allows you to test your sound system, DACs, headphones as well as hearing. I recommend all those interested in audio to pop over there, light a cigar, grab a cup of tea or your favourite brew and do some reading.

Another great read is “Testing Audiophile claims and myths“.

To sum it up:

Look, it has all but gotten pretty boring over the years.  When in my early 20s and in the 80s I had a sound system which I would rate as pretty powerful, about 100W per channel into a 4  Ohm load. When it came to parties and the venue owner’s system ran out of steam we’d plug in this baby and crank the volume. The quality of the sound was proportional to the amount of alcohol.  It probably deserved to have a rating of about 2/10 compared to modern lower powered systems.

What I have learnt through the ages is i) to take your time to build an amplifier ii) use the best components possible in the power supply, cooling, consumer controls, jacks and plugs iii) we now have a much wider access to and better quality patch cables and connectors iv) electronic consumer goods have become much more reliable generating some pretty exotic powers. Oh, this list goes on.

Good looks until it opens it’s mouth

A good looking amplifier doesn’t mean it was well built. It’s under the hood that counts. Also, knobs and good quality pots cost good money.

Don’t be hood-winked into buying an amplifier or any component for that matter without doing your homework. In South Africa, unless it’s very high end audio, the retail store is not going to do a delivery and then a collection again after you have done a test over the week-end.

Brands we know and trust

Most of the better known amplifiers (and pre-amps) which would include Yamaha, NAD, Harmon-Cardon, Denon, Onkyo, Rotel are well designed, are great for normal listening and more.  Get something which likes your speakers and the speakers like the amplifier. Reading up about ‘which’ loudspeaker is crucial and is always the best bet – it’s going to cost.  Be warned again, a set of loudspeakers may sound good with one amp but not the other. In South Africa Mission is often partnered with NAD.  I don’t know about this match but there’s thousands of people out there raving about the dimension, the colour, the warmth, the depth, the ooh, ooh, the orgasm of things.


Headphones are a touch more personal – each one to their own of course. I don’t believe most of the marketing hype.  A set of LG headphones which I received as a gift about 15 years back astonished me in the quality (excellent). I like Senheisser. That does not mean you have to.

Bitrates and Sampling

Too many articles pointing to the hazards of dubious marketing – our hearing is only 20Hz. to 20kHz. Mine is maybe 30 Hz to 14 kHz. 24 Bit / 192kHz may not sound better than 44.1kHz at 16 Bits. Maybe 8, but not 16.

Milking the snake of all it’s oils

What does all of this mean of course?  Have you seen the advertisements adorning Hi-Fi mags promoting audio and power cables?  R 1 000/m or more. In a magazine years back the fundis tested numerous cables and the one they thought sounded best was telephone cable. (yep ma’am, that cable running to your house, not to your telephone).  Now we get told how good CAT-5 cable can sound, both on the input and output. Kettle plugs?  Just make sure the L is Live. Some aren’t and don’t ask me where they come from.


Spectacles may improve one’s eyesight but a high powered amplifier is not going to improve your hearing. Most of what we want doesn’t always follow the rule of logic. Do read “Testing Audiophile claims and myths” and Audio Check!