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Old 27th October 2002, 08:29 PM   #1
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Default no working amplifier

I've built this amplifier and it didn't worked!
I had about 500mV DC at the output, the bias current run from 0 to 1A and i coldn't make it lower. Injecting a signal the sound was too low, in the teorical max input voltage.

I have no idea of what it could be and how to fix it?
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Old 27th October 2002, 08:39 PM   #2
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Have you calculated all DC levels?

Have you calculated max input and output signal? The the current mirror feed the next stage properly?

How much open-loop gain do you have (or want)?
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Old 27th October 2002, 09:41 PM   #3
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Diogo,

Can you measure the DC voltage (without signal, respect to ground) at every pin of all the transistors of your amp, and copy this on the schematic diagram (or in a text file) ?

Regards, Pierre Lacombe.
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Old 28th October 2002, 03:04 AM   #4
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I have calculated the max inputand output signals.
And I believe, based on my project, that the current mirror feeds the next stage, but since i'm onlya rookie I can be mistaken.
I have something like 1100dB open loop gain, and I belive it is enouth, but again I can be making a huge mistake.

This amp is based on a Single output pair that I built and is working fine, this one has two pairs in the output and I removed the Vbe multiplier, the rest is the same , with only a few resistor values changed!

I'm open for anything you fellows have to say about it!
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Old 28th October 2002, 03:14 AM   #5
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Real cute of you to post a sideways image. We all get to crane our necks as a reward for helping?
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Old 28th October 2002, 04:41 AM   #6
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Quote:
Originally posted by Bill Fitzpatrick
Real cute of you to post a sideways image. We all get to crane our necks as a reward for helping?
Always with the helpful comments eh Bill.

dave
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Old 28th October 2002, 05:47 AM   #7
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For sure.
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Old 28th October 2002, 07:35 AM   #8
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Quote:
Originally posted by Bill Fitzpatrick
For sure.
You can't attach an image wider than 800 pixels. With no way to shrink it he just did the best he could. Chill out.

dave
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Old 28th October 2002, 08:24 AM   #9
mirlo is offline mirlo  United States
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Default Is the bias voltage high enough?

The output devices have threshold voltages that could be as high as 4.0 V. Your bias circuit maxes out at about 5.6 V for both sides.

However, even if the bias is too low, you should still get icky class-B performance, not nothing!

So that's probably not the problem ... hmmm...

I am curious: what is the purpose of the 1n4007 diodes?

-- mirlo
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Old 28th October 2002, 08:35 AM   #10
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Quote:
Originally posted by Diogo
I have calculated the max inputand output signals.
And I believe, based on my project, that the current mirror feeds the next stage, but since i'm onlya rookie I can be mistaken.
I have something like 1100dB open loop gain, and I belive it is enouth, but again I can be making a huge mistake.

This amp is based on a Single output pair that I built and is working fine, this one has two pairs in the output and I removed the Vbe multiplier, the rest is the same , with only a few resistor values changed!
You mean 110 dB? This is slightly too much if you don't have full control over the poles (corner frequency) in each stage. 60-100 dB is a normal value. I think your main problem is around the current mirror and the MOSFET. How much can the gate voltage "move" (up and down)?
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