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Old 13th June 2003, 08:20 PM   #1
Jennice is offline Jennice  Denmark
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Question power output stage

Hi All,

I'm new in here, but found a lot of seemingly competent brains chatting along... maybe one of you is able to give some inspitration?

I have made a voltage gain stage, with a single output (voltage), but unable to deliver a lot of current. I now need that to drive an - if possible- simple, non-feedback class A/B power stage with approx. */- 40V rails supply.

I always tend to have my output stage more complicaterd than I like it, although I know that some bias is needed along with thermal stability.

Does anyone have a good idea for a fairly simple output stage with unity gain? (I guess it's basically I power-capable voltage follower...)

Best Regards,
Jens "Jennice" Sylvest (Jens was taken as a user name)
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Old 14th June 2003, 03:28 AM   #2
paulb is offline paulb  Canada
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How about a Class A?
www.sound.au.com project 83
or
http://www.audiodesignguide.com/my/index.html#MY_FOLL
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Old 14th June 2003, 10:02 AM   #3
Jennice is offline Jennice  Denmark
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Hi Paulb,

Thanks for your reply ;-)

First, I forgot to post it in solid state section (just noticed that)


Secondly, I have two criteria which I forgot to mention:

1)
The output should be with BJT's

2)
It sould (hey, didn't I mention that? *S*) be a class A/B design, not requiring the massive bias power...

I know I'm picky...


Regards,
Jens
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Old 15th June 2003, 12:22 AM   #4
djk is offline djk
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I think the best choice would be the Threshold S300 output stage.

http://www.diyvideo.com/forums/attac...p?postid=38107

No global feedback, and you may bias it any way you want. A simplified version of this design was also licensed to Nakamichi for their receivers a while back.

I have heard both the Threshold S150 and the Nakamichi SR4A, and they are both very good.

I would use the MJE15030/31 as drivers and the MJ21193/4 as outputs. This will have enough current gain to run directly off your tube gain stage.

If you keep the bias low enough you could use only one pair of outputs on 40V, more would allow you to increase the bias.

I can recommend a DC servo design if you need help with that.
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Old 15th June 2003, 08:10 AM   #5
Jennice is offline Jennice  Denmark
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Quote:
Originally posted by djk

I can recommend a DC servo design if you need help with that. [/B]
Thank you djk for the schematic. Interesting to see. I don't know if I'll copy it directly, but it has certainly given inspiration

Looks as if the driver stage is designed for a considerable ammount of bias?

Hmm... is the circuit subject to offset problems, since you comment on the DC servo?

Thanks -
Jens
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Old 16th June 2003, 02:39 AM   #6
djk is offline djk
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"Looks as if the driver stage is designed for a considerable ammount of bias?"

Pass ran it in class A in the Threshold gear. Nakamichi ran it in a low bias AB mode.

"Hmm... is the circuit subject to offset problems, since you comment on the DC servo?"

Not really, all you need is a bias circuit and the driver and output stages. Two constant current sources, one to the positive rail, and one to the negative rail will provide the bias. Make one of these so you can trim the DC off-set to zero and you should be fine.

Hawk Audio has a 50W FET hybrid that has a real nice DC servo that adjusts both constant current sources at the same time, but they took down that page from the www.

The End Millenium from LC Audio is a no feedback amp that has a nice DC servo design too, you can download the schema on their site as a pdf.
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Old 16th June 2003, 03:25 AM   #7
AKSA is offline AKSA  Australia
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Hi Jens,

Speaking broad brush, here's my suggestions:

1. Run your voltage amplifier at around 8mA. This gives it reasonable drive. Use a 250MHz device as both voltage amp and current source.
2. Use a conventional Class AB output stage, bipolar, with fast devices, no less than 30MHz on the outputs and at least 50MHz on the drivers. Both should have working gains of around 60.
3. Use the Double emitter follower Type II from Doug Self.
4. With a combined current gain of 3,600, a healthy output current of say 6 amps will require a current injection into the drivers of 1.66mA. This is just under 5 times the stage current of the voltage amplifier, and quite acceptable notwithstanding the difficult crossover area where load alteration is variable and non-linear. However, at crossover, the load current is tiny, so this change in input impedance of the output stage is manageable.
5. Concentrate on amp stability, using the smallest lag comp cap possible. Start at 68pF and work down.
6. Try to eliminate current mirrors where possible. They guarantee diff pair imbalance, and seem to damage the sonics.

This is all conventional topology, but you'd be surprised how the sound can be tailored by scrupulous care with the device choices and working environment of the VAS and OS.

Cheers,

Hugh
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Old 16th June 2003, 05:31 AM   #8
Jennice is offline Jennice  Denmark
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Thanks to everyone for the kind support so far :-)

It was quoted:
3. Use the Double emitter follower Type II from Doug Self.
---


Isn't that basically a darlington coupling, or could anyone please post this part of the schematic, and possibly the reference to Doug Self?

Thanks -
Jens
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Old 16th June 2003, 01:31 PM   #9
AKSA is offline AKSA  Australia
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Hi Jens,

Sure, here it is:

http://www.dself.dsl.pipex.com/ampins/dipa/dipa.htm

Half way down the page, EF Type II. There are a number of output stages discussed; this is the second. I've tried them all.

Cheers,

Hugh
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Old 16th June 2003, 06:11 PM   #10
Jennice is offline Jennice  Denmark
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Hi Hugh,

Thank you very much for the link.
It seems pretty conventional, except that I don't see the need for the one or two resistors between the driver transistors.

Whst is it/they good for?

Thanks,
Jens

Btw.: I'm planning to visit eatern Australia next spring. Do you know any good resources for travel info on Sidnay & Brisbane? (spelling???)
If so, please send it directly to my home address:
jens@sylvest.net
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