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Help for my hybird zero feedback AMP

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Deal All,

I had already finished this no NFB power AMP no conspicuous bad sound. However, observed from CRO the input against output signal have phase shift start from 500 Hz. Measured it will have 40 degree shift when input signal set to 25k Hz. Although, the output signal frequency no change corresponding to input frequency.

When input waveform change to square wave its output waveform change to wave shaped. Hence, the following questions want to get your help. Thanks.

1) When we use No NFB design will turn out above problem ?

2) I know this design first stage is voltage gain -> Common source as current gain so that out of phase 180 output happened. But I don't understanding what the function of resistor 47 ohm shown in attached diagram ? For my experience I can't to follow this value because the zero DC output can't be achieved until I replaced it with 650 ohm value.

3) Will the sound get better if set higher DC bias on 10k VR ?

4) At input part I added the thick line connect them , not sure it is exactly connection? Or just sticky follow the schematic showed ? Is that somewhat a tricky thing ?

Much appreciate your answer.

CK
 

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  • nofeedback_amp7.zip
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PRR

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Joined 2003
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Zero DC output is set by adjusting R37 R38. Changing the 47Ω seems to change output bias current. R37 R38 also set bias current. This is very much a DIY bias scheme; my head hurts to look at it, and setting the bias for specific devices may need a lot of heavy understanding and experimentation. There were no set-up intructions with this plan?

> 40 degree shift when input signal set to 25k Hz. ... square wave its output waveform change to wave shaped.

Of course. The tubes are a fairly high impedance. The MOSFET inputs are big capacitors. It is going to have a high frequency droop, and detectable phase shift before obvious roll-off. While some folks will disagree, I think 40 degrees at 25KHz is "nothing". (Put a mike right against a tweeter and measure 20KHz phase.....)

> When we use No NFB design will turn out above problem?

NFB just trades excess gain for lower overall gain/phase error. In either approach, if 40 deg at 25KHz is "bad", you need a better amplifier. The "no-NFB!" man will use a bigger (lower resistance) input tube. I can't read what you have, but try 2A3 (big-side of 6EM7 is similar and a lot cheaper). Or several in parallel. Four 2A3/6EM7 should shift your 40 deg phase point up to like 100KHz or 1MHz.

> At input part I added the thick line

I don't see why it was drawn with 4 tubes but only one of them connected. I think you are right.

Why did you ZIP a JPG? The file-size change is small, the hassle to open-open it is mildly annoying.
 
Hello PRR,

Nice to see your reply. I agree with your thinking.

On input side where use 2x 5751 tubes. Still I do not find 2A3/6EM7 can be directly substituted.

A question is the original design of this AMP since 1992 produced from a famous company. Somewhat reports source from a magizne can see a good square wave output tested in a 10kHz as input.

An accurate frequency respones at range 20-30kHz is my base-line requirement to the audio AMP. Not sure can you help me to solve it ?

Some people tell me to increase the value of coupling cap, now it used 0.3u per each. Any comment on it ?

Not sure the original JPG file size 88kB can't be upload to site but the ZIP file can success. Sorry to this inconvience cause.
 
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