Phono amp damage?

[jacklegjosh]

Hello all,

I recently picked up a really nice Sansui 5000x. Got it home, everything sounds good except the phono inputs. I start with my turntable, and it just sounds really staticky and awful. So then I check with a few other inputs. Still distorted. Did some cleaning with Deoxit, still sounds bad. All told, I probably ran the phonos for a max of 10 minutes at low to moderately-low volume, just to check it. Might have been less, but definitely not more. In any case, I just decide to call it a day, and I'll check on it tomorrow. Maybe that Deoxit will seep in and everything will work better tomorrow.

I get up and check it, no such luck. Go to work, and it hits me; I am an idiot. My turntable has a preamp in it. The input I was using it on before wasn't strong enough so I had it switched on. The Phono stage on the Sansui has its own preamp. The sound I've been hearing is distortion, not static. And all along, I've been jamming things into the Phono jacks, trying to test this problem with the reckless abandon of the frustrated.

So I go home for lunch and switch off the preamp on the TT and immediately the signal is clean. It could be my imagination after all of the frustration, but it does, however, sound just a little... off. Not particularly clear, not terrible, just not the "wow" that one would hope for listening to vinyl thru such a legendary(-ish) amplifier.

Now I'm sad that I may have done a little damage to the phono section of my nice, new (to me) vintage friend! So maybe you fine folks can help me: How can I tell if my unit has been damaged and it's not all in my head? Realistically, what should I expect to have been damaged? *IF* it has been damaged, what can be done? If work must be done on it, how difficult/expensive is said work?

Your collective input would be greatly appreciated. Thanks!

 

[Pio1980]

Probably not, you'd hear it if so.

 

[Watthour]

There appears to be a 68K resistor between the Phono 1 and Phono 2 inputs in each channel to chassis ground. This may be to limit noise and to limit input transients, but if a higher level signal (2V P-P) is applied to the phono inputs that resistor MAY be damaged.



The rest of the inputs appear to route through the SAME preamp stages so if the tuner and other inputs sound clean the preamps themselves may be undamaged.

 

[jacklegjosh]

Great, thanks ya'll! So if something WERE to be damaged, it would probably be the resistors in the path? For the record, I'm an enthusiast and I'm certainly open to learning, but I couldn't find my way through a schematic to save my life.

I listened to it again last night and the Aux sounds great, and the Phono sounds good. I think I'm just convincing myself that with the compression and light static of the vinyl that something is off because, quite honestly, I probably haven't heard it so unadulterated before, so it's probably all in my head.

 

[chazix]

2V P-P across a 68K resistor will dissipate about 10 microwatts, if I did the math correctly. I think we can be very confident that those resistors weren't damaged.

To me, the interesting resistor is the 1K in series with the preamp input. That resistor will prevent any harmful level of current due to an input overload, so I don't see any way for a line-level input to do any electrical damage at all. At least, not if everything was hunky-dory to begin with.

Cheers,

chazix

 

[MacNoob]

compression and light static of the vinyl

If you're hearing compression and static on your vinyl you DO have a problem somewhere - but it's not likely a problem with the phono input - more likely to do with table setup and / or record cleaning.

 

[jacklegjosh]

@MacNoob It's pretty faint. I think I'm trying to talk myself into a problem that doesn't really exist. I think what I'm hearing is the "Loudness" setting on at low volumes. I think it sounds a little weird if you don't get it above a certain level. When lots of things are happening at once on a recording, it sounds a little fuzzy. I am wondering if it's not in part the RCA cables I have hooked up. I've got about 12' of basic RCA cables running to it (although, I don't really know who legitimately makes a "good" RCA cable, except maybe Monster, to whom I will never give any of my money).

Everything else sounds great, so I think it's all in my head, or a combination of small, overcome-able things.

@chazix Thanks for your input, that makes me feel a lot better!

 

[MacNoob]

You have 12' of RCA from the table to the receiver?

That's OK if you have the pre amp turned on at the phono, and connect to a line level input like aux-in at your receiver.

It's not OK if you have the pre amp turned off at the phono, and connect to the phono input at the receiver.

The phono output is very low level and sensitive to the cable and distance. There's a reason that tables with fixed RCA cables have cables that are only around 3' long.

I don't think you've mentioned anywhere what table you're using, but if you want to use the pre amp in the receiver you need very short, low capacitance cables from the table.

Two of the video conductors from a short "component video" cable used to hook up a VCR or DVD player is a decent option. Video cables tend to be lower capacitance than audio.