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Bogen MO-30 Monoblocks to AR Turntable Hum Issue

T

tube-a-lou

Addicted Member
Hi all,

So I picked up a AR XA turntable cleaned up and it works but there seems to
be a miss match problem with my Bogen MO-30 Amps bad hum, When I hook up
my Scott LK48 no hum at all, just music. I'm going to change the input RCA's to
better quality and see if that helps but if not what could it be??


Thanks
 
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whats the full signal chain from the turntable to the power amps? Exactly what has been done to the Bogen amps? Is the grounding setup of the AR stock, or has it been modified for an external ground lead? In stock form those ground through one of the RCA cables. If its got an external ground wire, it has to be modified slightly inside so that it doesn't form a ground loop.

My Thorens 160 has the same grounding setup as the AR XA. I've run that with both a Dyna PAS and my Moscode paired with my Bogen MO-100a and my Rauland-Borg mono amps. No hum issues. No ground wires or any special treatment either, just plug the RCA cables in and it all just works. Also have run my Garrard Lab 80, which does use an external ground lead. No fuss with the PAS. Haven't tried with the Moscode because I only have one phono input. I'm using an external phono preamp with the Garrard grounded to that. RCA cables from that preamp to the Moscode, no issues with that setup either. Only thing that might possibly be related, the Rauland amps do not have grounded cords, neither do the preamps or the turntable. One of the Bogens has a grounded cord, the other does not. Not for any special reason, just one had a good original 2 wire cord and the other was hacked. I only had one good 3 prong cord on hand and never got around to replacing the original cord. I don't know if grounding the second amp would introduce problems or not.
 
gadget73 said:
whats the full signal chain from the turntable to the power amps? Exactly what has been done to the Bogen amps? Is the grounding setup of the AR stock, or has it been modified for an external ground lead? In stock form those ground through one of the RCA cables. If its got an external ground wire, it has to be modified slightly inside so that it doesn't form a ground loop.

My Thorens 160 has the same grounding setup as the AR XA. I've run that with both a Dyna PAS and my Moscode paired with my Bogen MO-100a and my Rauland-Borg mono amps. No hum issues. No ground wires or any special treatment either, just plug the RCA cables in and it all just works. Also have run my Garrard Lab 80, which does use an external ground lead. No fuss with the PAS. Haven't tried with the Moscode because I only have one phono input. I'm using an external phono preamp with the Garrard grounded to that. RCA cables from that preamp to the Moscode, no issues with that setup either. Only thing that might possibly be related, the Rauland amps do not have grounded cords, neither do the preamps or the turntable. One of the Bogens has a grounded cord, the other does not. Not for any special reason, just one had a good original 2 wire cord and the other was hacked. I only had one good 3 prong cord on hand and never got around to replacing the original cord. I don't know if grounding the second amp would introduce problems or not.
Click to expand...

I restored the bogens and put in new RCA female jacks in thinking that was the problem "Not". I did hook it up to my Scott LK48 and there
was no hum, just music. I have another thread going about the AR turntable you could see what I did there. The bogens only have the none
grounded 2 plug cord, should I change them to a 3 way cord?? would that help with the hum issue??.

I do have the system set up this way,
AR turntable, PS Audio PhonoPre then the two Bogens.
 
The bogen likely has AC filaments, the HH Scott definitely has dc filaments. Check the bogen for a hum adjust pot, and tweak it for minimum hum. If there isn't one, you can add one. You should also check for shorted tubes.
 
If the hum is only there with the AR but not a different turntable I don't think hum pot adjustment or bad tubes will be the reason why. If it hums with all turntables, that might be an amp issue, but I'd expect hum with any source. Could be a preamp issue. Does the AR with the PS Audio fed into the Scott hum? have to use one of the line-level inputs.

I doubt a grounded cord would fix it, but not impossible. It could introduce more problems. If nothing else its one more variable to account for.

so the Bogens are stock? How was the original RCA jack grounded, just to chassis or was it insulated and grounded through a wire? Is the new jack set up the same as the original was?
 
gadget73 said:
If the hum is only there with the AR but not a different turntable I don't think hum pot adjustment or bad tubes will be the reason why. If it hums with all turntables, that might be an amp issue, but I'd expect hum with any source. Could be a preamp issue. Does the AR with the PS Audio fed into the Scott hum? have to use one of the line-level inputs.

I doubt a grounded cord would fix it, but not impossible. It could introduce more problems. If nothing else its one more variable to account for.

so the Bogens are stock? How was the original RCA jack grounded, just to chassis or was it insulated and grounded through a wire? Is the new jack set up the same as the original was?
Click to expand...

Hi gadget,
When the AR is used with the PS audio and the Scott there's no hum, the Bogens are not stock I replaced the Caps, few out of
tolerance resistors and the RCA's was grounded with a plain wire to the chassis, so it's the same way the other RCA's were installed.

I'm going to install the Linn tonight to see if that works better.
 
Well this is becoming more strange, I hook up the Linn tonight and it hums mostly left but in the right
as well. Now I getting depressed.
 
stock as in no changes to the circuit, just new parts of the original values, that kind of thing.

Have you ever run these amps with something that isn't a turntable? Do they hum with nothing connected to the input or with a shorting plug?
 
gadget73 said:
stock as in no changes to the circuit, just new parts of the original values, that kind of thing.

Have you ever run these amps with something that isn't a turntable? Do they hum with nothing connected to the input or with a shorting plug?
Click to expand...

Yes my CD player no hum at all. If not hooked up to anything just speakers there's
no hum at all. I made no change to the circuit just added new parts.
 

Attachments

and I guess you're using the HiZ input jack with the volume control? Nothing about the amp stands out as a reason it would hum with a turntable but not anything else. Have you ever measured voltage from the amp chassis to a outlet cover screw on the wall outlet? Wondering if maybe one of the plugs is flipped over and you have voltage on the chassis. Oriented properly you should get basically 0v from chassis to ground screw. With a cord flipped over you'll get ~100vac from chassis to ground screw.
 
I figured it out some the strange reason it's the PS audio Pre's power cord. When I moved it
around the hum would get louder or disappeared (which I like better). Why would the cord be
so sensitive?
 
tube-a-lou said:
I figured it out some the strange reason it's the PS audio Pre's power cord. When I moved it
around the hum would get louder or disappeared (which I like better). Why would the cord be
so sensitive?
Click to expand...


That’s interesting, could the ground connection be loose inside?
 
That's a cheap (hopefully) resolution! Strange problem but I'm glad you were able to figure it out. I was confused about the AR headshell issue--it was a problem with Grado plastic-bodied carts, I seem to recall.
 
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