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Old 14th April 2009, 05:43 PM   #1
gni is offline gni  United States
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Default What's this Amplifier Design

This is the right channel of the Accurain Receiver sold by Radio Shack.

If someone could please take a look and describe the circuit? The amp has
about 15dB of gain without the pre section. It looks like some sort of double LTP.

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http://www.diyaudio.com/forums/showt...65#post1802165
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Old 14th April 2009, 05:50 PM   #2
Bigun is online now Bigun  Canada
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so it does, balanced all the way.

I'll let the gurus describe it, but looks to me like the signal comes in, is fed to two LTPs each one deals with one side of the signal (positive going and negative going waveform) and then a biass circuit and balanced two-stage EF output.
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Old 14th April 2009, 06:01 PM   #3
gni is offline gni  United States
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Thank for the quick reply. What are the bias transistors? (Q205,Q206)

More facts:

The amp has no bias current through the output resistors.

I ran the unit into 32 ohms bridged at 100W (56V).
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Old 14th April 2009, 06:08 PM   #4
Mr Evil is offline Mr Evil  United Kingdom
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The left hand side of the circuit is a standard symmetrical LTP and VAS. Q206/205 are the VAS transistors, just drawn upside down. I can't work out what Q207/208 are doing though. They look like a common-base driver for the output stage, but I can't see how they can possibly be biased on.
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Old 14th April 2009, 06:17 PM   #5
gni is offline gni  United States
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Symmetrical LTP and VAS. . .that's great. I could have transposed Q207/208 emitter and collector.

Would that make a difference? I'll take a look and retrace.

Q207 - C1027 npn
Q208 - A1023 pnp


Q211 is on the heatsink.

Q212 is fed by more than is shown
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Old 14th April 2009, 07:17 PM   #6
teemuk is offline teemuk  Finland
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The schematic is drawn very oddly but don't let it confuse you... This looks like a very typical amplifier topology with dual differentials feeding a dual VAS. I think you have just drawn the drivers incorrectly and mixed the base and emitter connections. The VBE multiplier should feed the bases and the emitters should feed the resistors of the output transistors. Those capacitors in the drivers are likely also drawn into a wrong place. I suggest you recheck the circuit for drawing errors.
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Old 14th April 2009, 07:48 PM   #7
teemuk is offline teemuk  Finland
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I'm almost sure it actually looks like this. (With protection circuit omitted).
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Old 14th April 2009, 08:00 PM   #8
Mr Evil is offline Mr Evil  United Kingdom
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Quote:
Originally posted by gni
...I could have transposed Q207/208 emitter and collector...
I expect you have, because if you swap them around you pretty much get the circuit that teemuk posted.


Quote:
Originally posted by gni
...Q211 is on the heatsink...
It's a Vbe multiplier, so it's on the heatsink to keep it the same temperature as the output devices, for thermal compensation.


Quote:
Originally posted by gni
...Q212 is fed by more than is shown
Q212 is part of the current limiting circuitry. It measures the current through one of the output transistors via the voltage across its emitter resistor.
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Old 15th April 2009, 02:57 AM   #9
gni is offline gni  United States
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That you all for the input . . . . sorry about the none standard drawing . . . I drew it
so it had no intersecting lines. . . still looking into the possible swap of c e b parts.
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Old 15th April 2009, 04:25 AM   #10
gni is offline gni  United States
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Version 3 Redraw
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