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Old 9th January 2002, 06:46 AM   #1
ergo is offline ergo  Estonia
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Default a bit different amp front end layout

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I have been playing around with this basic idea. In theory (at least in my head) the dif amps connected this way would have good current balance without using current mirrors on their collector loads nor the resistors that are not equal. The thing works in SPICE simulation, but I also constructed it in real life and seem to have a lot of problems. The currents are not equal even if I remove the VA stage transistors.....

My question mostly is, if anyone has tried such layout or what thoughts does it provoke in your head. For some reason the idea is attractive to me but as I don't have that much of experience, might be I'm totally overlooking some possible problems..

Ergo
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Old 9th January 2002, 07:07 AM   #2
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Doesn't look like the differential amplifiers can work as such,
since they can't share a common current source. What you
appear to have is an independent current source for each
transistor, so no 'balancing' action can occur. In fact, it looks
sort of like a current mirror, but on the wrong side.

Self and Sloan in their books give some examples of rather
more complex differential amplifiers, some using complementary
feedback pairs, but I found them difficult to understand.

I don't really understand negative feedback either...I just take
it on faith that it works.
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Old 9th January 2002, 10:15 AM   #3
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You've rediscovered the phono stage used in the Hafler DH101 preamp, were the feedback network is the RIAA and the output transistors are darlingtons. The current sources were just resistors to the rails.
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Old 9th January 2002, 10:17 AM   #4
Geoff is offline Geoff  United Kingdom
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Ergo

Have you seen my message to you in the 'Everything Else' section of the forum regarding my attempts to reply to your email?

Geoff
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Old 9th January 2002, 02:02 PM   #5
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Lightbulb complementary schematic

Hi Ergo,
Please have a look at the GAS Thaedra or Thoebe schematic in the link. They used the same idea for there line amp and Ampzilla and Son of Ampzilla poweramp. It is more than 26 years old!
The scheme is also used to some extent by Elantic and Harris(now Intersil) in some of there IC's
http://home.earthlink.net/~narf1/thopho.jpg
I am a happy owner of a Ampzilla and I can tell you the circuit works best if <B>all four</B> inputtransistors are matched. You need a lot of them to find four equal!
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Old 9th January 2002, 05:13 PM   #6
mlloyd1 is online now mlloyd1  United States
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Elso:
Thanks for posting that schematic. I'm curious; is there a reason that the current sink/source resistors for the input diff pairs are returned to the emitter of the output device instead of the rails? It appears to be some sort of feedback. Otherwise, it looks very much like the Lin configuration that Borbely likes to use.

mlloyd1
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Old 9th January 2002, 05:41 PM   #7
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Default crircuit detail attributed to James Bongiorno

Hi mloyd1,
I have no idea it seems to be a James Bongiorno speciality. He does it all the time in all his circuits.
BTW the HaflerDH200 also uses this scheme but without the detail you described. No wonder as JB worked for Hafler and SAE . SAE also uses the scheme in a lot of there amplfiers.
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Old 19th January 2002, 02:05 PM   #8
tvi is offline tvi  Australia
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Old 24th March 2009, 03:32 AM   #9
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Default gas ampzilla son of

Elso i have thaedra preamp and son of ampzilla i just love them
i know you love your ampzilla
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Old 24th March 2009, 03:53 AM   #10
PH104 is offline PH104  United States
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posted by mlloyd1
I'm curious; is there a reason that the current sink/source resistors for the input diff pairs are returned to the emitter of the output device instead of the rails?



I believe that increases the compliance of the current source. Curcio does the vacuum tube equivalent with his Dyna mods.
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