Ground Loop Issue 427

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

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.

Abe,
I have really enjoyed reading your responses. Especially your last one about the sensitivity of the inputs of the Amps.

I am confused though why you use the term ground loop. If the battery preamp is just hanging out there in mid air with no connection to the AC grounded system of Sbank's home, (providing at some point in Sbank's testing he had the charger unplugged from the AC power), I am having trouble finding and following the current flow.

I understand when you talked about each power supply of each component, I understand that. And I understand even with the ground lifted, on equipment connected to an AC grounded system, a ground loop hum can still exist due to leakage of the power transformers of the equipment. The equipment still shares the common AC mains.

But if the battery powered preamp is just hanging out there in mid air only connected to one Amp how can a difference of potential exist between the two units?

Thanks,
Jim

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

Hi Spencer, I'm in the camp of 'you have an RF problem'. I think, based on reading all of the above, that you have more RF susceptibility with with this amp, and that the long cables are somehow able to contribute where the other amps were not so susceptible. I don't think its a sensitivity issue.

There could also be a preamp issue that has not manifested before. If you can get your hands on a DVM, check and see if there is any continuity (less than 1 ohm) between the connectors and the chassis. Sometimes if the chassis is only grounded through the wall and the circuit isn't grounded to it you can run into trouble. That's not so bad in an amplifier but in a preamp it can result in hum, even if it worked fine with other amps!

Back to the RF issue: Have you tried different cables?

Hi Ralph,

Thanks for your interest.

I did move the preamp over near the amps, and tried multiple different 1m cables, including the highly shielded cheapo ones from a vcr(ie with the red/white/yellow all bundled together), and the hum persists, even when the preamp's power is off.

My DVM doesn't read continuity, but I'll try to borrow one that does.

So do you think that if it's RFI, that the Jensen Isomax transformer would help or not?