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Old 26th April 2006, 10:36 AM   #1
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Default Help with schematic

Recently I was able to get the schematic of my amp, a Pioneer A757. While I do have some basic understandig of amplifiers, I am by far not an expert, so if somebody could give me a few hints on how this schematics works, I would highly appreciate it.

I am also interested in mods/tweaks to improve the sound, if you see any oportunity in this respect. I like the sound of the amp, but it is a bit on the "dry" side for me - I'd have some more warmth, even at the cost of more distortion.

I find the schematics itself also pretty interesting, thoroughly in the style of late 80's - lots of components. I understand that the PA0016 IC is responsible with the biasing of the power stage, but I have really no clue what it really does - couldn't find a datasheet.
I also don't see a global feedback loop: there is one starting BEFORE the final stage to the diff amp, but I might oversee something there.

Later edit: there is global NF, I just missed it. The point where R243, C109, R115 get together is connected to the output of the amplifier.
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File Type: jpg pa757bw4.jpg (72.0 KB, 1252 views)
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Old 16th August 2007, 01:58 PM   #2
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Cant understand why no one replied ! Do you have the schematic for the 757 ? post it here or something so we can take a look.

What mods have you done upto now ?

I am in the process of tuning the A-400 amp, sounds pretty good upto now with just the addition of some blackgates. Biasing and more blackgates in the feedback loop next.

dennis
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Old 16th August 2007, 06:45 PM   #3
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one way to add some "warmth" is to increase the output impedance (reduce damping factor) in a controllable way. the simplest way to do this is to add a small amount (about 0.1-0.5 ohm) of resistance between the amp and the speakers. this comes at the cost of a small bit of power, and the larger the resistance the higher the wattage required for the resistor. a 5w resistor should be ok for 0.1 ohm, and a 10w resistor for 0.5 ohm. the metal cased ones that can be mounted on a heat sink are best, and the Dale ones are noninductive. 1ohm would be too much, since that would give a damping factor of 8, although you might like the sound if you say that the amp as it is is too "dry". just remember that whatever resistance you use has to dissipate it's share of the wattage. damping factor is ZL/Zo (load impedance divided by output impedance) and is a measure of how much the amp damps the effects of back EMF from the speaker (a speaker is very similar to a linear DC motor).
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Old 16th August 2007, 07:22 PM   #4
AKN is offline AKN  Sweden
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Hi bzfcocon,

Quote:
I also don't see a global feedback loop
It's hard to read the incomplete low res picture.
The feedback loop is however visible, see attached picture.
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File Type: jpg feedback.jpg (18.1 KB, 765 views)
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Old 3rd September 2007, 11:56 AM   #5
rjedb is offline rjedb  Netherlands
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I have an A757 with a problem too... And I was happy with the piece of the schematic. But some info is hard to read.

The first problem with the amp was 4 blown OPT's... I removed them...

Next problem was that one of the +/-24V psu's was dead. There was one 5 Ohm resistors open on the psu board right under the transformers. After replacement one output channel was OK.

I bought 8 (for sure) new Toshiba OPT's, the originals. After replacment there was sound with distortion... But one step closer! After some inspection AND replacing the PA0016 IC from the working channel to the bad channel the sound was better but the QCurrent was not good...(but the other PA IC is bad, that's clear)
Later I found a blown 2SC1845 just before the PA0016... But that's not the only problem...

I thinkt it's the best to buy a servicemanual for more info... I could buy new PA0016's for 15,00 and bought 2 of them... (when the amp is working I replace both of them)

But first I have to find out the problem with the Qcurrent... It's nog the PA0016. There is an unballance in the voltage over the PA0016 on pins 1,14 and 7,8. Normal it's around 2 x 3V and now it's 9V en -2.5V

More comes next...
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Old 3rd September 2007, 07:41 PM   #6
tricomp is offline tricomp  Denmark
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Default Pioneer A-757

Hi,

If you're after just the bare schematics, that's what I can help you with.
I have a reasonably good scan, at least far better than what was uploaded here.

Send me your e-mail address for a copy.

Kind regards,

/tri-comp
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Old 3rd September 2007, 09:19 PM   #7
rjedb is offline rjedb  Netherlands
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Hello Tricomp,

You can help me very much if you send me your scan of the schematic...
A problem is that I can't send you a mail because I'm a new user and in moderation... I will put my emailadres here and hope that it works that way... A mailadres what you can use is richardswebdisc@gmail.com . . . I hope it will work

Kind regards,
Richard
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Old 4th September 2007, 12:27 PM   #8
rjedb is offline rjedb  Netherlands
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Hello Tricomp,

Many Thanks for the schematic... I printed it on A3 paper, it's very good!

I send you a personal too...
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Old 29th November 2007, 02:15 PM   #9
char is offline char  Netherlands
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Default to tricomp

hi,
i also have a pioneer a757 to repair,
many resistors are blowing up,so i need
the schematics,can you the schematics
send it to me?
many thanks, char
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Old 29th November 2007, 03:22 PM   #10
rjedb is offline rjedb  Netherlands
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Hallo Char,

Do you have other questions about this amp... You can mail me in dutch!

My 757 is working great. It was one of the hardest repairs I ever did but I have no regrets! If you need a new PA0016 chip you can order it in Almere (Pioneer Holland), I did it too, it was 15,00 each... Not very expensive for a custom build chip. And check the 10 and 100 Ohm resistors too!!!
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