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Old 8th October 2004, 06:48 PM   #1
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Default Zero Feedback Impedance Amplifiers

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

The schematic:

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

Click the image to open in full size.

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

See: http://www.susan-parker.co.uk/zeus.htm 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,
Susan.

P.S. Yes, it does work.
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Old 8th October 2004, 09:13 PM   #2
Netlist is offline Netlist  Belgium
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Very nice approach, most Zen if you ask me.
Perfect fit for DIY.
What Class would you call it?

Thanks alot for sharing.

/Hugo
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Old 8th October 2004, 09:47 PM   #3
jam is offline jam  United States
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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.)

Regards,
Jam
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Old 8th October 2004, 09:59 PM   #4
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Quote:
Originally posted by Netlist
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,
Susan.
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Old 8th October 2004, 10:08 PM   #5
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Quote:
Originally posted by jam
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.)

Regards,
Jam
Hi Jam,

Thanks.

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,
Susan.
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Old 8th October 2004, 10:12 PM   #6
jam is offline jam  United States
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Steve Eddy is Mr.Tansformer. He has one on every circuit he has posted.

Could you please give us more details about the transformers you have used ?

Regards,
Jam
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Old 8th October 2004, 10:51 PM   #7
Ejam is offline Ejam  Australia
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Default And the power and bias supply

Susan

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.

Regards

Ejam
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Old 8th October 2004, 11:04 PM   #8
Ejam is offline Ejam  Australia
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Default Hiraga Amp

Susan

The Hiraga amp was the Nememis PP posted at:
http://www.bonavolta.ch/hobby/en/audio/audioel.htm
The circuit however uses a differential input instead of your transformer.

Regards

Ejam
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Old 8th October 2004, 11:27 PM   #9
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Interesting.
What did you use for the transformers?
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Old 8th October 2004, 11:51 PM   #10
Eva is offline Eva  Spain
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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
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