Ground Loop Issue 427

Hello,

When you connect the shielded cable, is the cable shield connected on both ends? Have you tried shielded cables where the shield is only connected at the amp side? preamp side?

I think that what you are experiencing is common to SET amp where the input ground is directly connected to the chassis as soon as it enters the amp. If your preamp is not configured the same, there is always a loop everytime you connect the cable. In this case, the only solution, if you do not want to re-configure your preamp, is to have a Ground Loop Isolator because it seems that the ground plane of your "battery powered" preamp is of different potential as "seen" by the amp. You can also try connecting a wire to a ground point in the preamp chassis in series with a 10 ohm resistor (two watts rating or more)and see if by connecting it to the amp grounding point, the hum will be less or there is a change in magnitude. If that is the case, then you can confirm that the ground potential on both components are indeed not the same. In any case, you might want to try a ground loop Isolator.

Hope this makes sense.

regards,

Abe

Hello Spencer,

I see. On normal usage, is the charger connected directly to the preamp all the time or you have to disconnect after charging?

If it is the latter, then it should be quiet because it uses the main amp ground, through the negative pins of the output RCA's as the ground point. This is akin to having a star ground connection that goes back to the ground of the main amp power cord ground. If it is the former, then I would like to ask you if the charger have a three prong or a two prong ac plug? Have you tried with or without the charger, if possible?

If after trying all those plus the ones you already tried, the hum still persist, the only think you need to try is to have a ground loop isolator between the amp and the preamp before resulting to troubleshooting the connections for hum loops inside the preamp. I believe the preamp is your problem.

To understand how the isolator works, you can read it here http://www.jensen-transformers.com/an/an004.pdf

I hope this helps.

Hello Spencer,

Yes, I believe you that the preamp works on the other two amps. But the thing is, what is the input sensitivity of the two amps compared to the 845? It is possible that the ground loop is there already but not audible with the two amps while when using the 845 with higher sensitivity on the inputs, the hum becomes more pronounce.

For example, I have two amps running 2A3 output tubes. The first one uses 6SL7 as the input driver and needs 1 vrms for full ouput. The other one uses a type 76 tube as the first stage and 6SN7 as the second stage before the output tube. In this amp, it only needs 0.3 volts for full output. So what does that tells us, that if you compare the two, a 10 mV AC hum for example going to the first amp will be seen as 3.33 times higher as seen by the second amp. That could be the reason why the hum was not audible on the other two amps.

It is unlikely that you will hear a hum coming out of the preamp when you are using a Push Pull amp because it cancels at the output stage. Another point to consider is that both the EL34 and 6AS7G(OTL)are Indirectly Heated Triode (IDHT), while the 845 is a Directly Heated Triode (DHT) which can be more suceptible to hum since the cathode and the filament are one.

You mentioned that when the input to the 845 amp is shorted, there is no hum. This is the standard way of measuring the hum figures for SET amps, and is the prefered way to "null" out the hum if you have humbucking pots on the filament supply. If by shorting the inputs you do not hear a hum, then the amp is not the culprit. When you plug an input cable to the amp inputs, the ground of the preamp becomes a part of the overall ground path. This is where the hum loop can exist.

regards,

Abe

Hello Jim,

I understand what you meant and I am having a hard time to explain it and maybe I am getting confuse as well but I was talking about signal ground not PSU ground.

Don't know how to elaborate it any further really since it is easier for me to see it if I feed section by section from a freq generator, the way my mentor taught me, and look at the output to determine the loop. This simply means that the signal groundpath has to travel one direction: input-----> output and if it is not the case, a scope will display noisy output from the stage under test which verifies the ground loop that can manifest itself as a loud hum.

Apologies Jim, I am out of words to explain what I really wanted to.

regards,

Abe