Hum in one channel - preamp??

[BillH2121]

This morning I noticed a hum in the left channel of my my two-channel set up when using phono input. I have a Mac C 26 preamp that I've had for 30 years that was cleaned and checked earlier this year. The hum is definately confined to one channel only as I can select stereo reverse and the hum changes to right channel. I've checked speaker and input/output connections and ground wire from Scout TT. Does this problem sound like a preamp problem? I thought it might be related to my hooking up a HT receiver yesterday that had just been serviced and sits within a couple feet of my two-channel amp/preamp set up, but when HT equipment is unplugged, I still get hum. Suggestions?

[mikebse2a3]

Looks like you have pretty well eliminated to the Preamp as the problem.

If you haven't done this yet:

Completely unhook the turntable input wires from the preamp and notice if the hum changes.

Most likely problem would be power supply capacitor failure feeding the phono section or a ground connection failure like a bad solder connection. One test you could try is to slightly tap the preamp with your hand and see if the hum changes indicating a bad connection problem.

mike

[BillH2121]

Thanks Mike - I'll give your suggestions a try. These damn Mac components - you have to get something done on them every 30 or 35 years!

[BillH2121]

Alright - did test as Mike suggested. With phono inputs unplugged, the hum is decreased but is now found in both channels, not just one as before. Also, no changes when I tap on preamp.

[mikebse2a3]

Good! now if you haven't, swap the left and right phono interconnects at the Preamp and see if the hum moves to the right channel or stays in the left(AND THAT THE LEFT STOPS HUMMING). If the hum shifts to the right then look at the left interconnects all the way to the cartridge for the problem like a bad ground. If it is easy for you to do you might even swap the wires at the back of the cartridge to see if the hum changes sides. You could have a bad cartridge if you swap the wires at the cartridge and the hum moves to the other channel.

If you play a record is the volume the same on both channels except for the hum problem?

mike

P.S. I always Keep some shorted RCA style plugs for putting on inputs to elliminate sources for problems such as this. When you unhooked both phono interconnects and thus left the inputs open its possible you will pickup noise and hum in the highgain circuits like a phono input so by using a shorted input plug you can eliminate this interferance to see if the circuit is quite on its own.

You can make your own shorting plug real easy if you have a cheap interconnect you don't mind cuting up. Just cut the interconnect close to the plug and tie the wires together thus shorting the plug. Then plug this on the phono input thats giving you the problem hum and if its gone then you know the preamp is OK and the problem is from the source like again in your case possibly a bad interconnect, broken wire or cartridge maybe.

[BillH2121]

OK - I traded left/right interconnects and the hum moved to right channel. I tried different interconnects and hum still present. I disconnected the interconnect from the tonearm to the TT and hum still there - so I reason that cartridge and cartridge wires are ok. I inserted shorting plug into phono input and and hum greatly decreased but still present in left channel when volume turned up (2:30 - 3:00 o'clock). Seems like preamp?? But why is hum greatly decreased with shorting plug?

[mikebse2a3]

OK - I traded left/right interconnects and the hum moved to right channel. I tried different interconnects and hum still present. I disconnected the interconnect from the tonearm to the TT and hum still there - so I reason that cartridge and cartridge wires are ok. I inserted shorting plug into phono input and and hum greatly decreased but still present in left channel when volume turned up (2:30 - 3:00 o'clock). Seems like preamp?? But why is hum greatly decreased with shorting plug?

The reason the hum is greatly reduced by using shorting plugs is you have reduced the sensitivity of the input " THINK LIKE IF YOU TURNED A VOLUME CONTROL ALL THE WAY DOWN" and thus any noise you hear then will be from the circuit itself and any local interferance close to the circuit created by its design provided everything is operating as designed.

In your case you can't really just unhook wires because you are dealing with a high gain circuit and if the input is left open you will get some induced noise/hum.

By the Hum being greatly decreased with shorting plug (and also moving to right side in one test) and you tried different interconnects it sounds to me like your problem is in the cartridge/cartridge wiring.

Your left channel still having some hum at a lot higher volume setting could be a seperate issue possibly.

If you E-mail me your phone number I can maybe call you and get a better idea of how to help you narrow down the problem. Just put klipsch in the subject line please so I don't accidently delete it.

mike[]

Edit: Again I would ask is the volume normal in the left channel even though you have humming when you play a record. I really believe I would swap wires at the cartridge at this point to see if the hum changes sides.

[BillH2121]

Mike - I couldn't find your email address in your profile info. Additional information I forgot to add yesterday is that there is almost no volume out of left channel when playing a record. Also, I didn't disconnect the wires at the cartridge because when I disconnected the connect from the tonearm to the TT, the hum still remained (I thought that ruled our the cartridge which is only a couple months old - am I wrong on that??). I appreciate your help - should I pm you instead with my number? Thanks, Bill

[mikebse2a3]

I've sent you a PM Bill with my e-mail address and phone numbers

Additional information I forgot to add yesterday is that there is almost no volume out of left channel when playing a record. Also, I didn't disconnect the wires at the cartridge because when I disconnected the connect from the tonearm to the TT, the hum still remained (I thought that ruled our the cartridge which is only a couple months old - am I wrong on that??)

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Ok lets simplify this so where on the same page now. You can take the right channel(THIS IS OUR KNOWN GOOD SIGNAL) from the turntable and plug it into the left channel of the preamp and if you get plenty of volume and no hum then again this tells you that the preamp is OK and that again the left channel of the turntable has the problem which again I would look at the wires connecting to the cartridge or the cartridge as the problem since you already swapped interconnects out yesterday.

If you determine the problem to be at the turntable then again swap the left and right wires hooking to the cartridge and if the problem hum and no volume changes channels then your problem is a bad cartridge. If the hum and no volume stays in the left channel then look at your wiring very carefully coming from the cartridge to your interconnect sockets of the turntable for a broken wire.

Bill I might have confused you a little on this but because of the highgain phono input circuit it really isn't the best way to just unhook the wires to trouble shoot your problems because just doing this allows this circuit to pickup noise and hum because it isn't loaded properly.

Using the shorting plugs was a trouble shooting method to see just how much Hum you had in the Phono Circuit of the Preamp without the chance of introducing hum from having no input connection on the phono inputs of the preamp.

The best trouble shooting method is what we are doing now and that is taking a known good signal(RIGHT CHANNEL FROM THE TURNTABLE) and feed it to the left channel monitoring the results which in your case if we feed a good signal into the left channel and we get good sound out of the left channel of your stereo then we know your Left channel from the Preamp is GOOD and your problem is a BAD signal from the LEFT side of your Turntable Cartridge or Wiring.

mike[]