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Old 30th March 2009, 12:51 AM   #1
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Default My first discrete amp

Hello!

This is my first post here and my first discrete amplifier also.
I have built a couple of Gainclones in the past but now I thought it
was time for a "real" amplifier .

The constant current sources are set to 5mA but I'm not sure if that's ok?

Thoughts, comments, questions, all feedback will be appreiciated.

Thanks.
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Old 30th March 2009, 02:48 AM   #2
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looks pretty simple and straightforward.....
you have a good opportunity to thermally couple the input transistors. put a sil-pad between them and bolt them together.

you might want to run the VAS current source at 10 or 20ma, maybe higher

you might want to beef up those drivers (Q11, Q12) though... with a beta of 25 on the output devices, you'll be pushing those drivers hard......
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Old 30th March 2009, 04:23 PM   #3
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I changed the drivers to pre-drivers instead and increased the VAS constant current to 25mA. I also added an extra 10R in parallel with the output inductor and a couple of protective diodes in parallel with the output drivers.

Feel free to give me more feedback, I'll start working on the PSU for now.
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Old 30th March 2009, 04:35 PM   #4
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What were you're reasons for adopting medium power devices in the voltage amplifier stages rather than small signal devices?
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Old 30th March 2009, 05:13 PM   #5
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Default same

Quote:
Originally posted by AndrewT
What were you're reasons for adopting medium power devices in the voltage amplifier stages rather than small signal devices?

question from me
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Old 30th March 2009, 07:42 PM   #6
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I just like to overdimension things, use powerful devices even if it's not really necessary. The power rails will only be at about +-30V so this amp will never come near its full power potential.

I'm also planning on fitting this thing in a rather small chassis with 4 surface mounted toroid transformers at a total of 60VA.

New schematic, rev 0.2:
http://hem.bredband.net/b1438270/Amp.../Amplifier.pdf

Chassis I'm planning to use:
http://hem.bredband.net/b1438270/Amplifier/Chassis.pdf
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Old 30th March 2009, 07:51 PM   #7
Mr Evil is offline Mr Evil  United Kingdom
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I don't think you will gain any reliability from using larger transistors in the first two stages because it's rare for any failure to occur there anyway. However you do suffer from reduced bandwidth and reduced gain, and therefore increased distortion.
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Old 30th March 2009, 08:17 PM   #8
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Default Re: My first discrete amp

Quote:
Originally posted by danfo098
Hello!
Hello

Quote:
Thoughts, comments, questions, all feedback will be appreiciated.
Thanks.
Looks like a reasonable circuit to start with. I'd like to add to what has been suggested. It is a good idea for the dc resistance seen by the bases of Q3 and Q4 to be equal. This is because as the temperatures of Q3 and Q4 drift, so will their betas and their base currents. If the dc resistance is different on each base, then the base voltages will drift apart causing the output offset voltage to drift. Does this make sense?

IOW, the value of R1 and R4 should be equal. If you make R4=10k you can make R5=330 for a similar closed loop gain.

Regarding C2. The base currents of Q3 and Q4 will cause a small voltage drop across R1 and R4 which will cause their bases to be at a slightly negative voltage. So if you use a polarized cap for C2, the positive pin of C2 should go to GND. BTW, my experience is that a non-polarized electrolytic will sound a little better here, if you have one. 10uF is ok but a little on the low side - 50uF would be better.

Brian
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Old 30th March 2009, 09:26 PM   #9
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This could be the one, revision 0.3:

http://hem.bredband.net/b1438270/Amp...ifierRev03.pdf

Thanks for all the feedback.
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Old 30th March 2009, 09:51 PM   #10
Mr Evil is offline Mr Evil  United Kingdom
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The addition of the Vas buffer will improve performance, but I would recommend keeping things simple since this is your first attempt.
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