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Tube Mono Block Ground Loop

C

calectro

New Member
Hello everyone,

I have a pair of Sherwood S-1000 integrated mono amplifiers that I am using as mono blocks for vinyl playback. I have restored both and they sound fantastic when used in stereo.

For true mono playback, a double-Y cable or an external mono switch must be used to sum the signal coming from the turntable RCA connectors to the phono inputs of each amp. When this is done, it causes a massive hum that makes true mono playback on both speakers impossible. When using a Y- cable to sum the phono signal into just one amp, it works perfectly and there is no hum. I actually don't mind and prefer listening to mono playback on one speaker, but I hate having to disconnect the phono cables and insert the Y-cable into one amp every time I want to listen to true mono. That is why I bought the external mono switch, so I could switch between mono and stereo playback with out removing the cables.

When using the mono switch, the hum is present even if just one amp is on. Both amps have a 3- prong power cable with a ground connected to the chassis. I suspect there may be a ground loop issue going on. I tried using a 3 prong-to-2 prong adapter to lift the ground on 1 of the amps and it made no difference. I only have one so I wasn't able to try it on both amps.
Any ideas on what I can do to make this work? On my other integrated stereo amps with a mono switch (Sherwood S-5000, Eico HF-81, Marantz 1060), the mono switch makes a huge difference in the sound quality when playing back mono records and would like to be able to do this with my S-1000s.
 
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I can't think of any reason why the connection you're describing would cause hum. Both amps have a 3-wire line cord, so the two chassis are connected together regardless of whether the Y-cable is used at their inputs. Is the ground wire from the turntable connected? Maybe one of the cables is defective.

Jack
 
Ground hums are frustrating. I may not know why, but I can usually troubleshoot until I get some results.
You got a hum with both the double-y cord & the mono switch? Or both are mono solutions & you went with a switch?
 
I wouldn't let the amount of hum worry you, you simply have a ground loop issue that can be resolved. I would start by trying every combination of grounding, but it seems your mono switch has introduced a problem so maybe investigate grounding that chassis to your tonearm or to each of the amps or both. You've also found one of the challenges of separate mono preamp sections, as with a stereo preamp it's relatively simple to have the equivalent of the y cable within the preamp via a switch.

I would say the convenience issue that you are describing is one reason many use two arms, one dedicate to stereo and another dedicated to mono, then a simple y cable can be used with the output of the mono arm. Not all turntables are easily adaptable though.

opnly bafld said:
@calectro

What is "true mono"? :dunno:
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I believe he is referring to signal to both amps coming from one channel of the cartridge output. Even better is output from one channel of phono preamp going to two power amp sections but that is not possible with separate mono integrated amps.
 
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Regarding what I said above, you might want to confirm that both chassis really are connected to the earth wire in the line cord.
 
sounds like classic definition of a ground loop. The chassis are tied together through the ground pin in the cord, and again through the Y splitters. I have the same issue with my two mono amps when they had grounded cords. The signal grounds came together at the preamp and in the outlet. I fixed it the wrong way by removing the ground connection so its like 1965 again. People have opinions about this, but it worked.

maybe modify your switch box so there is a 47 ohm resistor between the grounds rather than a direct connection, or use a coupling cap so there is no direct DC path.
 
I tried removind the ground connection on each amp, and it still hums extremely loud when the mono switch is engaged. There is no hum when the mono switch is off.

I'm using a Rega P2 turntable which has not ground wire, my understanding is that the tonearm ground is connected to the shield of the left channel RCA connector.

I'll try installing 47 ohm resistors between the grounds on the mono switch and see if that helps.
 
The hum you're describing definitely sounds phono-related as opposed to amp-related. You might also try non-shielded ICs from the mono switch.
 
Selmerdave said:
The hum you're describing definitely sounds phono-related as opposed to amp-related. You might also try non-shielded ICs from the mono switch.
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? Now there is a troubleshooting thought I did not have. I don't even know if I have non-shielded cables. What will this accomplish?
 
mrwood200 said:
? Now there is a troubleshooting thought I did not have. I don't even know if I have non-shielded cables. What will this accomplish?
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I don't have much more of an explanation than I have had cases where non-shielded worked better than shielded, as well as the reverse. Currently I have non-shielded from my turntable to my SUT, and then shielded from the SUT to preamp, and then ground wire from table to preamp and ground wire from SUT to preamp, but no ground wire from table to SUT, which produces significant hum. I have zero hum, but that was the result of extensive trial and error.
 
mrwood200 said:
? Now there is a troubleshooting thought I did not have. I don't even know if I have non-shielded cables. What will this accomplish?
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Just to be clear, you're saying it hums if you use a Y-adapter (2 into 1) at the turntable , a single shielded cable, then another Y-adapter (1 into 2) to drive the two phono inputs at the amps. This is all that's required to make it hum, is that correct?
 
for some reason I was thinking there was a mention of an external phono stage. I'm aware that the S-1000 is mono.
 
@gadget73, that is correct. The mono switch is in between the turntable and the Sherwood amps, connected via RCA cables.

Ok here's a breakdown of when the hum occurs and doesn't occur:

Stereo - turntable Left and Right RCA cables connected directly to the phono input of each amplifier- NO HUM

Stereo - turntable Left and Right RCA cables connected to the mono switch, with the switch DISENGAGED, then connected to the phone input of each amp via RCA cables - NO HUM.

Mono - turntable RCA cables connected to a Y-cable, connected to the phono input of ONE amp: NO HUM

Mono - turntable RCA cables connected to a Y-cable, which is inserted to the input of another Y cable (sometimes referred to as a "double-y cable" this is essentially the same as a mono switch) to split the summed mono signal across the two amps - LOTS OF HUM.

Mono - turntable RCA cables connected to the mono switch, mono switch ENGAGED, connected to the phono input of each amp - LOTS of HUM. *** this occurs regardless if the other amp is powered on/plugged in/no volume.

So, to summarize, the hum occurs when the two RCA connectors from the turntable are summed to mono and connected to the phono input of each amp. It never occurs in any of stereo scenarios. It also does not occur when a y-cable is used to sum the phono signal to mono and connected to just one amp.
 
@gadget73 , there is no seperate phono stage. I am using the integrated phono stage of the S-1000s. The mono switch is between the turntable and the phono inputs of the S-1000s
 
I think it's got to come down to how the grounds are connected in the mono switch when the switch is engaged. Are you able to open it up and see? Have you tried grounding the chassis of the switch to either or both of the amps?
 
calectro said:
Mono - turntable RCA cables connected to a Y-cable, connected to the phono input of ONE amp: NO HUM

Mono - turntable RCA cables connected to a Y-cable, which is inserted to the input of another Y cable (sometimes referred to as a "double-y cable" this is essentially the same as a mono switch) to split the summed mono signal across the two amps - LOTS OF HUM.
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This indicates to me that there might be a difference of potential between the grounds of the two amplifiers.
 
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