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SA-8800 Phono stage problems

QSilver

QSilver

Super Member
When I connect the SA-8800 up to my turntable it plays just fine, until you want to crank up the volume. If the volume gets past the 1W mark, sometimes the 10W mark, I suddenly get a massive resonant hum. I haven't checked but it sounds like it would be around the 100-200Hz area. Reducing the volume gets rid of it but on increasing again it comes back. It reminds me of the feedback you can get with a microphone and a speaker.

What confuses me is that this is the only amp I have that's ever done this. I can put another amp in the same place on the same turntable and its fine. The subsonic filter doesn't make a difference and neither does reducing the bass on the tone controls just as a measure of ruling out feedback in some way.

It seems to be worse when using a Grado cartridge. If I use a Shure M75 you can go a little louder.

What could causes this? Is there perhaps something microphonic in the phono stage? Or is my system set up wrong?
 
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How close in proximity are the power amp and the turntable?
Does it happen with the tonearm on the rest? on a stationary record? HOVERING over a stationary record?

Magnetic feedback???
Pictures!!
 
markthefixer said:
How close in proximity are the power amp and the turntable?
Does it happen with the tonearm on the rest? on a stationary record? HOVERING over a stationary record?

Magnetic feedback???
Pictures!!
Click to expand...

They're sitting next to each other. Yes, it does do it when the arm is at rest or hovering. Just anytime. Like a microphone, just lift the volume up to a certain point and then it starts.

I compared the sound to an oscillator. Yes, its definitely around 100Hz, maybe 80-100 but that area.
 

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Try it with the speaker de-energized (use headphones or a small test speaker to detect it?).
 
Disconnecting the speakers stops the "feedback". you can see the power meters drop as soon as you set the speakers to off.

Headphones doesn't seem to cause it. I wound them up to full volume with a record playing with out problems.
 
Thinking out loud here,
Does it do it with the amp in its case?
Are the other amps cased also or open?
Can you separate them?
Can you put aluminum or copper sheet between them to isolate?
Or maybe on the speaker side of the amp, might help from feedback.
Maybe even a large cookie tin or a cookie sheet? Heck then
run a wire to ground it (the sheet/tin/al/cu)....
Which Grado?
 
The good news is that it's not magnetic. A low probability chance, but still possible.

It's acoustic, set the amp on a few pillows and if continues, it's direct acoustic - not conducted acoustic.
Try for setting on the floor too, with the pillows.
 
Good thinking Mark.

So the cartridge/TT is picking up the vibrations?
If so, then isolating the amp would work.
Interesting to see if on dresser w/pillow.
On floor with and without pillow and which makes a difference.
 
I have a SX-780 that has the same problem. It only does it on the right channel though. I have a thread that I started for it but completed. I didn’t have a turntable to test it at the time. I don’t want to hijack this thread. Should I make a new post on that thread or start a new one? TIA for advice.
 
It's related to the same problem if the QSilver doesn't mind, I don't mind post away.
You can always ask a mod to move it later. But I'm over in the SA-7800 bites the dust thread.
 
USMC Spike said:
It's related to the same problem if the QSilver doesn't mind, I don't mind post away.
You can always ask a mod to move it later. But I'm over in the SA-7800 bites the dust thread.[/QUOT
Click to expand...
While searching around some other threads I may have found the solution to my problem. The thread Pioneer SX-838 No Sound on Right Ch. for Phono 1 and Phono 2 post by member tsd71 advised cleaning (de ox) the rear inputs with a pipe cleaner or mascara brush. I did it and now have no problem. I will monitor this and hope it was the fix.
 
Ok, its not quite the same problem I think as the SA-8800 only does it when a turntable is connected to the phono input, but good to know for the future. It's best to start a new thread if you want to continue otherwise we're going to end up getting lost in each others problems without a clear process.

I'll test the amp out with pillows soon and post what I find.

The Grado I used was a Grado Blue, which, I have to say, has been a big disappointment - doesn't sound any better than any of my other cartridges and has a much lower output so I have to wind the volume even higher. This is true across a number of pre-amps. However, its not the cartridge I don't think as it does it on other cartridges I have too such as the Shure M75. Anyway will be back soon....
 
I placed the amp on top of a single duvet folded over and over to make a thick cushion.... no more horrible feedback it seems! Turned it up to output 10WpC with no issues. So that means its... conducted acoustic feedback?
 
Yup - I had suspicions about that furniture.

Just for completeness, try swapping the cushioning to the turntable.

Then, once the amp is buttoned up TIGHT, try everything again.
 
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Ok, I'll give all that a go. I thought i was odd though, that its only the SA-8800 that does this. I've had lots of other Pioneers amps in the same configuration on the same furniture and it not happen.

Still though, I'll put it all back together and insulate the turntable and have a go again.
 
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