Zero Feedback Impedance Amplifiers

As requested a seperate thread on transformer based solid state amplifers.

The schematic:


... and what it looks like in practise ...


Everything to the right is power supply, and everything from the middle is amplifier.

See: for more details.

This amplifier is based on Impedance Amplification and has similar distortion characteristics to valve triode designs.

It is relativly easy to build and only has one twiddle pot to set the bias level. The pic shows a monoblock which I built nearly ten years ago and it has not needed adjustment since.

Best wishes,

P.S. Yes, it does work.
Netlist said:
Very nice approach, most Zen if you ask me.
Perfect fit for DIY.
What Class would you call it?

Thanks alot for sharing.

/Hugo :)

Hi Hugo,

Thank you.

As far as I can figure it's Class A (as I understand these definitions - but all these Class variations gets confusing very quickly) except it's biased like a Class AB amp with 100 to 200 mV.

Neither MOSFET turns off at any time (during normal operation) so there is no Class B bit, however this isn't the same as my understanding of normal Class A operation where lots of power gets disapated all the time.

My amps quiescent is low and the heatsink is correspondingly small, being barely warm to the touch when idle. Of course it warms up a little when in use. The only time it has got seriously hot was when making power measurments and driving full peak to peak sinewaves into an 8 ohm load.

An inportant thing to understand is that the output transformer's primaries swing negative as well as positive and the MOSFET for the half of the cycle which is negative is still powered but "idles" with most of the current flowing through the MOSFET going positive. However the negative arm MOSFET "WILL" regulate if something tries to overrun the main positive arm MOSFET position.

This tracking is because I am using the MOSFETs as Gate Followers, not as amplifiers. Thereforethe transformer arms follow the the gate input voltage minus the MOSFET bias level - the actual gain is a little less than unity.

So since both MOSFETS are activly working the whole time I guess it's Class A.

Thank you for your interest.

Best wishes,
jam said:
Most original Susan.

I suppose mosfet matching is mandatory. I was thinking of a differential mosfet pair to drive the outputs in place of the input transformer. (Please don't tell Steve Eddy I said that.)


Hi Jam,


mosfet matching helps to reduce the nonliniarity, but is not essential to get the amplifier working.

You could drive the outputs in that manner for an integrated amp.

Who is Steve Eddy?

Best wishes,
And the power and bias supply


Great concept and worth a try. Reminds me of Jean Hiraga's single stage mosfet amp using an output transformer. I think Hiraga used the mosfets in a gain topology though. See if I can dig up the circuit. On to more mundane stuff, can you describe your power supply and bias supply please.


Have you tried playing piano notes at high volumes?

A classic problem in that sort of circuits is transformer saturation when playing asymetric waveforms at high volumes

Another problem is bias imbalance between MOSFETs. In this design the threshold-voltage difference between both devices is applied directly to the transformer so a dozen of mV may be enough to ruin the circuit at anything but low volumes
This design is almost exactly the circuit with germanium transistors that I have in my almost 40 year old Telefunken Bajazzo portable radio. Just used it today. Sounds great! PS Dick Sequerra (hi end guru) was a consultant on that project. This portable radio runs rings around ANY portable component that I have ever heard, including Class D portable circuits.
Hi Susan,

Novel circuit - outstanding! Have you considered running a phase splitter tube for input voltage amplification? This would enable lower impedance drive to the gates, keeping input impedance high....... And would it be possible to drive the speaker from across the mosfet sources, eschewing the secondary of the transformer, using it only as a centre tapped choke?

Interesting comment from Eva on bias imbalance, and nice to hear JC likes the configuration!

I'm sure you considered these aspects, but I'd be interested in your response.




diyAudio Moderator Emeritus
2002-10-04 1:27 am
Have you tried playing piano notes at high volumes?
I have confusing problem with this design I attached. It is the similiar to Susan's design after the NJM386 (which is simply a preamp)
Bias can be adjusted independently. The output trafo is Jensen, There is also a small driver's trafo, after the NJM386. The supply is +50V single pole, going thru cap multiplier, driving OT in push-pull.

This design also have difficulty in matching with speaker. You must tweak your speaker system (passive xover, put LCR everywhere to make the response flat) to make them linear before you use this kind of power amp.
Put it with ordinary factory built speaker (like standard Dynaudio Reference)... the sound is strange.

Very long time for break-in. More than 300hours for the sound to settle. (Infact I dont know, who's breakin-in. The speakers or the power amp?)

Eva tell me that big current will make the core saturate. Even in push-pull configuration, any slight mismatch will saturate the core. So the design won't like class A. EVA seems don't suggest transformer anywhere in audio reproduction. (while she's good with transformers). She don't like the non-linearities for audio reproduction

Transformers theoritically are not linear, huge distortions for audio reproduction. But somehow, I like sound of transformers :D

This portable radio runs rings around ANY portable component that I have ever heard, including Class D portable circuits.
Is it possible that "Classical" designs are not worse than "Modern" Hifi designs?


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Looking at the pictures I see a massive output transformer, I think it may whitstand 100W at 50Hz before saturation, but it's obviously overrated since I think this amplifier does not provide such a big output swing

The circuit also uses MOSFETs as followers, providing the Vg-s variations as the only error voltage introduced in the output signal [also transformer distortion], as opposed to lumanauw's common source configuration with higher distortion and poor speaker damping [almost current drive]. Also, class A biasing avoids crossing the low current zone of the MOSFETs where they show extremely poor linearity [MOSFET Id is proportional to Vgs^2]

So this circuit may work somewhat better than lumanauw's one at the expense of higher dissipation and the requirement of a custom step-up drive transformer instead of a simple 2:1:1, 1:1:1 etc...
Susan-Parker said:

This amplifier is based on Impedance Amplification and has similar distortion characteristics to valve triode designs.

P.S. Yes, it does work.


have you got any evidence for this statement (spectral measurement).

How does this amp sound when playing fff of the philharmonic orchestra, how is the resolution and clarity.

Best regards,