It's a trade-off, Terry. Many people do not understand about the effect of feedback. There's debate about the importance of feedback. Those who debate do not understand. If they understand, they will put the right amount of feedback into the circuit. People tried to lower the THD as low as possible, not knowing what they throw away with the bath water.
A preamp doesn't raise the gain, it only raises the input voltage. I don't know yet with the real amp because I haven't found my problem yet but on the spice file, 3vac on input only outputs 45vac on the output. I think that is a gain of 15x. Lower than any amp I've built so far.
This is a tough amp to troubleshoot with 33 transistors. They all measure good with a diode test so I must have a solder bridge somewhere. I'll stay after it. If I can't find it I'll post up a schematic with voltages for some fresh eyes. The good thing is that we know the board will work because Alex has already proven it.
Blessings, Terry
Only 45vac on the output is over 500W/4R. Replace R25, R26 and R27 with 1k instead 470R, gain will increase to 30x.
Regards
Yes, I understand it is a lot of power but 3v input is higher than most preamps will output. I changed the three 470R to 1K each and as you said I see 30vac with a 1V input.
I'm attaching the error report from Ltspice.
Blessings, Terry
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I know why it is called A40, there is 40 transistors in it, WOW. Nice.
What is max rail voltage for it ??
THX
Peter
A pre-amp that raises the input voltage has gain.
That gain adds to the system gain.
eg.
A Buffer with a gain of +0dB feeding a Power Amplifier with a gain of +26dB results in a system gain (for these two components) of +26dB
Replace the Buffer with a Pre-amp with a gain of +10dB which increases the input voltage to the Power Amplifier by about 3.1times, gives a system gain of +36dB
reduced by two transistors and now it is A38 - used Olaf´s files from earlier done A14 and A33 - so it has all rights to be called Olaf´s and accommodated by me:
(!!!IT MUST BE CHECKED!!!)
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Hi Olaf,
Can you Please post the foils for this.i would like to make a pair.
Thank you.
Rick.
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Nice works guys ,and my last layout not tested but very soon I will build a pair
Alex
Nice pcb,
Regards