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Stee 8th January 2013 09:40 AM

solid state with output transformer
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I have invented a method to sound the amplifier in the solid state as a valvular, so reduce to a minimum the negative feedback with a rule called 1:100 :)
Take a simple amp

Ie without cascode in the input stage, without cascode stage voltage and with unity gain in the output stage (without Darlington)
The open-loop gain will be approximately 27dB
Set the closed-loop gain to a gain much greater 40dB
(set NFB resistor's ratio one to hundred)
This means doing the opposite of the traditional approach

You will get a sound but not distorted, but no EQ
This is because there is no longer a residual gain sufficient to correct the waveforms extensive (low frequency)
In short you are listening to the real sound of your amp
And you realize that the impedance matching the speaker load is scarce

My solution is to adopt an output transformer more powerful than that required
With very low transformation ratio (7:1)
So as not to inflict emphasis on the acoustic scene and at the same time allow the device to an internal resistance adequate

The result is excellent (tested on diagram Rotel RA-02 with toroidal transformer 230/32 +32 V 300VA)

lcsaszar 8th January 2013 11:06 AM

I would suggest to study a high school textbook on linear systems and negative feedback. You will find a formula on the first few pages how the closed loop gain can be calculated.

jan.didden 8th January 2013 11:17 AM


The feedback factor is the difference between open loop and closed loop gain.
If you wish to lower the feedback factor so that you can listen to the amp non-linearity better, you can just lower the open loop gain and save the cost of the xformer.
Loading T1 collector would be a convenient place.
You coulkd even put a pot there for continuous control of open loop gain and feedback factor.


Stee 8th January 2013 01:35 PM

closed loop gain
1 Attachment(s)
If you wish to lower the feedback factor so that you can listen to the amp non-linearity better because lose equalization ;)

the attenuation factor of the feedback(and viceversa the correction factor) does not behave as you think
is difficult to set a dime with potentiometer - better to add a resistor in parallel

wahab 8th January 2013 02:49 PM

Congratulation for the never lacking originality.....

Stee 8th January 2013 02:55 PM

I speak about equalization because the negative feedback is a system of dynamic attenuation
This system attempts (up where he space of action) to correct the deficiency by the load current increasing the voltage
This happens on the frequencies of large amplitude that the bass
where there is no output stage is really capable (large parallel end devices es.TIP35)

GoatGuy 8th January 2013 03:23 PM

Redacted by author

Osvaldo de Banfield 8th January 2013 03:47 PM

Goat, I believe you are wrong. The anode of the led is wired to GND via a 22K resistor, and the cathode to -42V, so it really has about 20 mAcurrent in it.

Although, I canīt understand how this circuit warranties or ensures an idle output voltage less than the famous 25 milliVDC though the load, and prevents temperature drift of this output voltage. (???)

GoatGuy 8th January 2013 03:50 PM

I redacted my note. You are right, Osvaldo. It is early here in San Francisco CA. The "coffee isn't working" ... is true.

Art M 8th January 2013 03:52 PM


Originally Posted by GoatGuy (

Ummm... hate to call it what it is, but "********", brother.

The "Quasi-zero feedback amplifier" in the original PDF cannot work. It has at least 4 design errors. One is significant enough to prevent operation of the long-tailed differential pair up front, completely.

Specifically, the resistor R8 is connected to ground, which prevents the current source Q1 from conduction, at all. The LED will never light, as there is no V+ path to it. Now... go find the other 3 errors ... they're not as significant, but just as important.


Please point out the other 3 errors on this proven design.

300/500W Subwoofer Power Amplifier

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