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Sansui AU-7500 Phono Woes – sounds like a wool blanket over the speakers!

pup.biz

pup.biz

New Member
[EDIT: It is a Sansui AU-7500, not -6500 ... d'oh!]

Hello everyone,

I'm hitting my head against the wall here with my AU-6500. Would love some help or advice.

I bought the amp a few years ago and had it given a once-over (bias, recapped out-of-spec caps, cleaned) by a friend who did a good job. Didn't have a turntable at the time, but all other inputs produce clean, powerful sound. I run it through a pair of NSM 10S bookshelf speakers. The sound is exactly what I'm looking for: tonally balanced, detailed, great image. We're in a New York apartment (small-ish) so thumping bass is not a high priority, though I know the amp could accomplish it with some other speakers.

A couple months ago my partner and I purchased a turntable – a restored Technics SL-1200 MK2. Upon plugging into the phono input we found the sound was distorted in the highs, muffled (way bass-boosted, anything high was way back in the sound), and was picking up some serious interference. The amount of bass coming out of the speakers was akin to having the Loudness switch on and EQing a low pass filter. Completely different from the other inputs (Tape included). The left and right channels were also uneven and one would cut in and out.

I had the amp serviced in Brooklyn by a guy who also worked on a Pioneer receiver (SX-450) and TT for a friend of mine at the same time. Everything sounded great on return – except the Sansui phono! There were no longer cutouts, but the two channels were just evenly distorted now. It still sounds like there's a wool blanket over the speakers. Messing with the EQ to bring out the highs just reveals distortion. Lows are still far more prominent. Here's the kicker: since I had the Pioneer receiver on hand, I A/B tested the two, and it was night-and-day. The (much cheaper) Pioneer had a clean high-end and balanced tonality. So I assume it's not the cartridge or turntable. I reported this to the guy who did the repair, who took the amp back today, and after listening on his system stated "nothing's wrong" to his ears but he can recap the whole phono board if I want. I'm a little frustrated – the guy does good work and I believe him but what I'm hearing sounds so obvious I can't believe "nothing's wrong." I also heard no discernible difference when changing the impedance switch, though he reports there's some small difference.

Am I just that unused to how vinyl sounds? Am I expecting too much? Should I take it to someone else or attempt a repair myself? Do I get a separate phono pre and call it a day?

Any advice appreciated. Thanks.
 
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The next step would be to try another turntable or at least check if the cartridge and arm wiring are OK.
 
Copanel said:
The next step would be to try another turntable or at least check if the cartridge and arm wiring are OK.
Click to expand...
I would assume they are as the TT sounded good through the Pioneer receiver, but I can try to have a friend bring a TT over as well. I did try two different carts (Grado Black and Ortofon DJ) and both reproduced the problem.
 
There is an OEM RIAA phono equalization error in some Sansui models (including the AU-7500) which gives a bass emphasis.

Sansui AU-9500: Phono Stage RIAA Equalization Error

The back to back restorations of a Sansui AU-9500 and a AU-717 last year provided an opportunity to sonically compare them against each other. With a source component connected through the line stage, the amplifiers were very similar sounding, making identification of which amplifier was playing...
audiokarma.org
 
You could bring your TT to the repair shop too, so they can hear it & help troubleshoot?

I see some cartridge loading options on the rear of the AU-7500, did you try both phono inputs?
 
pup.biz said:
I would assume they are as the TT sounded good through the Pioneer receiver, but I can try to have a friend bring a TT over as well. I did try two different carts (Grado Black and Ortofon DJ) and both reproduced the problem.
Click to expand...
I missed this, sorry
 
Leestereo said:
There is an OEM RIAA phono equalization error in some Sansui models (including the AU-7500) which gives a bass emphasis.

Sansui AU-9500: Phono Stage RIAA Equalization Error

The back to back restorations of a Sansui AU-9500 and a AU-717 last year provided an opportunity to sonically compare them against each other. With a source component connected through the line stage, the amplifiers were very similar sounding, making identification of which amplifier was playing...
audiokarma.org
Click to expand...
Saw this when reviewing your thread. I will bring it up with the tech.

kwayde said:
The AU-7500 phono has two 2sc1000 xstrs in it.

Lots of good info in this thread by @Leestereo :

https://audiokarma.org/forums/index...storation-upgrade-of-a-sansui-au-7500.768097/
Click to expand...
W/r/t the 2SC1000s, the tone stage seems unlikely as the problem does not persist in non-phono inputs. The EQ/Phono stage (F-2028), however, seems like the most likely culprit. I wonder if the tech could check the EQ stage and fix the bass bump in one go. I know that the tech who did the initial inspection / tune-up a few years ago (who I won't tag unless he wants me to ;-) ) closely consulted your thread, @Leestereo. I've yet to hear whether he performed the EQ RIAA mod or not, or whether the problematic transistors were replaced. It was more of a 'leave-it-if-it-works' operation, which it did for years!
 
my 2 cents:
ask the technician if the capacitors at the input and output of the corrector have been replaced, and if so, with which ones? I had an experience when shunting an output electrolytic capacitor (in power amp) with polypropylene Panasonic caused a similar effect...
 

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Mike244 said:
my 2 cents:
ask the technician if the capacitors at the input and output of the corrector have been replaced, and if so, with which ones? I had an experience when shunting an output electrolytic capacitor (in power amp) with polypropylene Panasonic caused a similar effect...
Click to expand...
That's the AU-6500, not the AU-7500, but the coupling caps are the same.

Screenshot (733).jpg
 
Hi everyone,

Thank you all for your help! My tech located the problem and did great work.

Taking a cursory look at the phono board he noted the following:
- C601/2 (2.2uf) were replaced with 1uf
- R635/6 (27k 1/4w) were replaced with 332k 1/4w

My best guess is someone tried to mod the board to @Leestereo's spec and replaced the wrong components! In addition, they replaced those big caps at C601 and 602 with shitty off-brands. Setting aside the rest of the board's components which had aged out, this was probably doing numbers on the clarity. He ended up replacing the entire board's components to spec, and I'm glad he did. It sounds phenomenal. He demo'd a clean og press of Sublime's S/T and it really sang on his workbench setup. I took the unit home, hooked it up, and it is rocking. For the time being I'm happy without the modified RIAA curve, but always have the option to tackle that down the road. I'm more than happy with the sound, and now my attention has turned towards needing a larger pair of speakers...
 
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