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Old 12th December 2006, 07:00 PM   #1
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Default Cascoded Output Stage 200W amp.

Hello All,

This is my first stab at a discrete amp. Please be kind. I have been reading about the advantages of cascoding, and I have been hankering to design my own discerete class A/B amp. So here it is. I don't have a good name for it yet.

I have not chosen all the devices yet, but for the output devices I have my eyes on the Sanken SAP-16 or SAP-15 N and P channel dralingtons. These ICs have built in thermal runaway protection and emitter resistors. This just makes this a bit easier. I may also design another output PCB for single transistors including VBE multipliers.

Anyhow, I would very much appreciate if those of you who have designed more than one of these things would take a look at what I have so far.

I have done a lot of homework already, but I would love to do more.

Some notes:

R9 is a timmer to adjust offset.

R11 is a trimmer to adjust bias.

I am hoping to use series CLDs for current sources, but I am not sure if I would run into power dissipation issues... Should I just use a standard NPN bipolar current source?

Thanks for looking!

Cheers!
Russ
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Old 12th December 2006, 07:07 PM   #2
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You sure need 4 pairs of output devices to safely deliver 200 Watts
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Old 12th December 2006, 07:10 PM   #3
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Quote:
Originally posted by ACD
You sure need 4 pairs of output devices to safely deliver 200 Watts

Undoubtedly correct. I actually figured as much, but I was trying to keep the schematic simple for now, since I know I will likely have to change a lot!

Thanks!

Cheers,
Russ
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Old 12th December 2006, 07:16 PM   #4
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You don't use the build-in temperature compensation diodes in the SAP's
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Old 12th December 2006, 07:18 PM   #5
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Quote:
Originally posted by ACD
You don't use the build-in temperature compensation diodes in the SAP's
Yes, well I thought I was, what are you seeing?

The VBE multiplier (Q12) in the VAS is simply there to get my bias voltage drop. Is there a better way?

Cheers!
Russ
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Old 12th December 2006, 07:22 PM   #6
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Use the diodes in the bias circuit like the seperate diodes used in the Leach amp
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Old 12th December 2006, 07:26 PM   #7
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Quote:
Originally posted by ACD
Use the diodes in the bias circuit like the seperate diodes used in the Leach amp

Ah yes, I understand you now. When I drew the schematic I did not assume SAP output devices, but yes, if indeed I do use them I will do exactly as you say.

In that case I would omit Q12 and simply add a trimmer I suppose.

Thanks!
Russ
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Old 12th December 2006, 07:30 PM   #8
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Both should work, and gives you much more quick bias setling and temperature compensations

I once made an simple amp using these, and the offsett after 30 min warm up never drifted more than 1 mV
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Old 12th December 2006, 07:33 PM   #9
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Quote:
Originally posted by ACD
Both should work, and gives you much more quick bias setling and temperature compensations

I once made an simple amp using these, and the offsett after 30 min warm up never drifted more than 1 mV

So with Q12 there and mounted on the heatsink, should I safely be able to use other darlingtons? or would I need more than the one VBE multiplier?
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Old 12th December 2006, 07:41 PM   #10
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What? No cascoded input devices?

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