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Old 9th October 2012, 11:43 PM   #11
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Sorry there should be 1 more wire coming out of the transformer (V-CT-V).

Edit: replaced diagram

Last edited by Fusion916; 9th October 2012 at 11:45 PM.
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Old 10th October 2012, 12:01 AM   #12
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is it one commen PSU for 2 ch amp?
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Old 10th October 2012, 12:13 AM   #13
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Its the class A, running dual supplies thats the problem.
Reference should be -ve rail, opamp is compounding the issue.

Edit: just remembered you are using half wave rectification. not good.
what idle current are you using. ?

Last edited by whizgeek; 10th October 2012 at 12:19 AM.
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Old 10th October 2012, 12:37 AM   #14
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Quote:
Originally Posted by AudioSan View Post
is it one commen PSU for 2 ch amp?
It's only 1 channel (so far). Just building this amp to learn the design principles.
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Old 10th October 2012, 12:40 AM   #15
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Quote:
Originally Posted by whizgeek View Post
Its the class A, running dual supplies thats the problem.
Reference should be -ve rail, opamp is compounding the issue.

Edit: just remembered you are using half wave rectification. not good.
what idle current are you using. ?
Why is running dual supplies a problem for class A? And why would -ve be the reference? And how is the opamp compounding the issue? What that is bad about using half wave rectification other than larger caps are needed?

Bias current is about 400mA. As I said in the op if I lower the bias current the hum does decrease substantial.
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Old 10th October 2012, 07:10 AM   #16
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I think whizgeek has the answer. When you had posted your first diagram (post #1) it looked like an AC coupled amp on single rail but its not

The problem as whizgeek says is that ripple (yes its 60hz for half wave) is getting into the amp. The base bias network for the first transistor needs to be very very clean and referenced to "ground" or "zero" volts point.


Dug I think mentioned the opamp based current sink. That could be an issue too for the same reasons.

These are great circuits for learning...
one idea might be to replace the current sink with a simple two transistor type circuit.
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Old 10th October 2012, 07:23 AM   #17
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Something like this. The values would have to be re-calculated.
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Old 10th October 2012, 07:52 AM   #18
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What does the top resistor need to be split in two and a signal ground cap (im assuming what this is) needed on the first stage bias network? What value cap (or cutoff freq) do I need?

I've replaced the opamp with a current mirror (not the wilson you posted, a simple diode connected device on the right) but the problem persist but it persist in a different way. It seems then the bias resistor (connected to the diode device) is cold the output is dead slient, then slow the output starts to hum again (maybe about 3-4 seconds after the power is turned on).
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Old 10th October 2012, 01:03 PM   #19
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Quote:
Originally Posted by Fusion916 View Post
What does the top resistor need to be split in two and a signal ground cap (im assuming what this is) needed on the first stage bias network? What value cap (or cutoff freq) do I need?
I've made a basic error in my quick drawing... I'll come back to you. The principles correct the implementation not so
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Old 10th October 2012, 02:14 PM   #20
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Here you go then. The error was in the bias network polarity.

I don't what values of resistor you have used but these are typical. The current sink of around 400ma is a limiting factor in how far negative the output can swing. This is nearly at the point of clipping. I used two individual transistors to make a darlington... not sure if that is what you have and I used a non darlington for the current sink.

I think the circuit could be rehashed significantly while keeping the basic topology but hopefully this will help with what you have.
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