Ground Loop Issue 427

Okay, thanks.

Hi Spencer, everything you mentioned just now is consistent with RF problems, how the amp behaves with shorted inputs, interconnects, etc.

Some RF beads and grid stop resistors at the input of the amp might take care of it and improve the sound at the same time.

Sbank,

http://www.hamuniverse.com/rfi.html
Read past the part, "don't blame me".
There might be something in the article that may be of some help.

Ralph,

Recently, I didn't notice the hum or radio signal with other amps. So your comments make sense. However there is no trouble until the interconnects from the pre are connected to the amps. The amps with shorting plugs at their inputs are dead quiet, especially compared to other SETs that I've heard elsewhere.

Prior to changing to my solid state Vendetta Research phono stage, RF was a constant problem using a variety of excellent tube phono stages.

At this point, I'm just going to live with it, as I'm likely to be moving next year anyway. Those who've suggested moving away from the radio tower have nailed it ;-)

Spencer, low level hum is a common problem if you are putting your ear to the speaker. SETs usually use a directly-heated cathode, which is to say that the filament of the tube and the cathode are the same thing. This means that you have to have a DC supply if the power tube is not going to hum; getting the DC supply to be quiet suddenly becomes an issue!

RF issues are often caused by lack of what is known as 'grid stop' resistors in the circuit or issues in the grounding scheme of the amplifier (I'm talking about how the grounds are laid out inside the amp, not outside). If the RF is rectified, you will hear the station, and it is well-known for also causing hum.

Have you noticed the RF with any other amplifiers?

Here's an update:

After having a number of other audionuts over to listen, they all tell me that they have more noise in their rigs than I do, and that "Your hum is nothing". Well, I guess we all have different levels of tolerance. Perhaps I should just live with it.

FWIW, I ran a grounding wire from preamp to amp, and it didn't change a thing. I have heard intermittent radio signal from one speaker, when placing my ear next to the tweeter, but it disappears most of the time, and can't be heard from even a couple of feet away. Other than transforming my house into a giant copper encrusted Faraday cage, I don't have any ideas worth pursuing. Thanks to all,
Spencer

Hi Spencer, your DVM can measure Ohms. Just put it on the Ohms scale; the reading you are looking for between the RCA grounds and the chassis ground should be the same as connecting the probes of the meter together.

Most SETs have less gain than amps of larger power; the reason for this is that SETs really do need higher efficiency speakers to be useful. To keep noise down, and otherwise on account of the efficiency of the speaker, SETs simply don't need the same gain as amps intended to drive speakers that are 20 db less efficient.

If the amps have the 3rd prong of the AC cord plugged into the wall, often this will result in a rather substantial hum or buzz, depending on how the amp is grounded. So until this issue is solved, I would run the amps with some ground cheaters- the only ground in the system (for best results) should be the preamp.

Jim,

Until my new tri-wire speaker cables arrive in a week or so, I can't try anything(the old speakers were sold this week), but I will try running a grounding wire from preamp to power amp chassis.

I did already run a grounding wire from one amp chassis to the other amp chassis, per suggestion from the amp designer.

I'll post again after trying that(as well another preamp), and we'll see what progress is made. Thanks,
Spencer

11-17-08: Atmasphere
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!"

11-13-08: Sbank
Abe, according to the preamp designer, "All of the rca's on the preamp are isolated from the chassis and connected vis the 8 ga. buss. The chassis is grounded thru the charger jack only."

Atmasphere,
Is it possible the preamp is acting like a big antenna?

I still think Amandarae is on to something about the SET inputs being more sensitive than the other Amps Sbank was using.

Sbank, did you ever connect a wire from the chassis of the preamp to one of the amps chassis?

http://www.audioasylum.com/audio/general/messages/52/525608.html

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?

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?

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

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 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

Abe,

The battery charger for the preamp is 3prong, and normally is left connected. It engages when you power off the unit. I've tried it both connected and disconnected, & still get hum.

Remember that this preamp worked fine with two other pairs of amps, (Consonace push/pull EL34-based , Atma-Sphere OTL)and no hum. I am going to have a friend bring over another preamp to try here in the next few days. Thanks again.
Spencer

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.

"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."

Abe, according to the preamp designer, "All of the rca's on the preamp are isolated from the chassis and connected vis the 8 ga. buss. The chassis is grounded thru the charger jack only."

Sorry, if I don't understand, but does that make sense relating to your comment above? Does this further point to need for ISOMAX? Thanks,
Spencer

Nope, the cheapo stuff on Ebay, etc. has nothing in common with the Isomax. Again, no guarantees, but the Jensen's a very high quality item.

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

Okay here is what I've tried:

Plugging the amps in next room via extension cords, didn't change anything.

I did try ground lifters on both amps, as well as on only one; no difference

When I have just one amp(either one) connected to the pre, it still hums. Strangely, with the right amp on, removing left interconnect from left amp increases the hum heard in the right speaker slightly. Conversely, with the left amp on, removing right interconnect from right amp decreases the hum heard in the left speaker. With no interconnects or shorting plugs, there is no problem.

Isomax is a possibility. Would the cheapo similar products(e.g. $12 ground loop eliminators on ebay) give me an idea if Isomax would be effective?

As far as trying amps & pre at a friends' house away from radio tower, that might be worthwhile. Borrowing a different preamp and trying that here will be worth trying too. Thanks to all! Cheers,
Spencer

SBank
If you can take the amps to a friends house that's nowhere near the radio tower and plug them in. This will eliminate the radio tower from the equation.

Call Jensen Transformers (do a web search) and buy an "Isomax". It'll cost under $200, but also require an additional interconnect pair. They use very high quality isolation transformers. The box is about 3"x5"x1" size.

I've found the "Isomax" to sufficiently mitigate hum in my systems. When it's failed, nothing else "worked" either. I've also found that Cardas Golden Reference ICs screen hum better than any I've used, though I'm sure many others do as well - but many don't.

Good luck. I despise hum/noise.

When I lived a few blocks away from a radio station I had all sorts of RFI issues resulting in hum. Phono stages (even solid state) were especially problematic. One of my preamps gave me all sorts of fits.

Hopefully your problem is not related to the radio tower. I had to move to solve my problem.

Thanks for responses so far, guys.

I've already tried both in one ac outlet, as well as in different outlets. I have already connected a grounding wire between the amps(per manufacturer's suggestion). The speakers are not powered.

FWIW, I demoed the amps at a local dealer, and there was no problem in their system. That's part of why I think it's a ground loop, and not a repair issue with the amps. They are preowned ;-) , so sending them back is not a real option anyway.

If I can borrow another preamp, I'll try that. The long extension cords are a great idea, I'll try that too.

Does anyone thing this is an airborn RFI issue rather than an AC/ground loop problem? Thanks,
Spencer

Since you can short the amp inputs and the hum goes away, have you tried a different preamp? If the hum goes away, you know it's some sort of incompatibility between the preamp and amp. You've already tried the preamp with other amps and it was fine, so I'm guessing the hum will still be there when you switch preamps. That means your amps probably hum with everything, so either they have bad design (unlikely) or they were put together wrong, e.g. a bad solder joint, a wire misrouted, an input jack accidently shorted to chassis, something like that. Will the amp manufacturer take it back and check it out?

David

If your 2 amps are on different circuits of the house there could be a ground difference potential. Plug the amps into the same wall plug or, try a ground wire attaching the 2 power amps.

Go to another part of the house and plug it in? Or, just run a long extension cord. This will tell you if it's just a localized problem.