ANOTHER ST70 hum issue

What was a minor annoyance with my modern full range speakers has become unbearable with my highly efficient Altec VOTTs.

I’ve run through all the Curcio document as well as the sticky here. The 60hz hum does disappear with shorted RCA inputs. BUT, it is there with every other electrical device in the room unplugged, with or without inputs connected, lifted ground or not (irrelevant with nothing else attached). I even pulled the whole room circuit breaker and ran my bucking transformer off a UPS temporarily. Simply impossible for it to be generated externally.

Best I can tell all wiring and wire twisting rules have been applied (it is a factory-built Triode USA version). It’s on both channels equally so swapping driver tubes had no effect.

Any ideas where to look next?

Thanks,
Jeff

With the inputs shorted you don't hear hum? Then the source of the hum IS NOT the amp! The hum is being fed from the upstream components to the amp.

Plus the very efficient speakers make the hum more audible.

The amp is not the cause/issue.

Jim McShane wrote:With the inputs shorted you don't hear hum? Then the source of the hum IS NOT the amp! The hum is being fed from the upstream components to the amp.

Plus the very efficient speakers make the hum more audible.

The amp is not the cause/issue.

Thanks for responding, Jim.

If the hum exists with no other device connected besides the speakers, and all other electrical devices within 30 feet unplugged, what possible source could there be “upstream”? Also, the hum diminishes slightly by lifting the ground. With no other devices connected, how can this be a ground loop?

I am not trying to be argumentative. Just trying to find a way to enjoy an amp that otherwise appears to be a great match for my Altecs. But I am out of ideas. The amp is factory built Triode USA, stock with the exception of the “yellow sheet” diode mod, an inrush current limiter (my house current is 120-122 so I also use a bucking transformer at about 116) and 16 ohm output enabled for the Altecs. I am very pleased with the sound with the exception of the hum.

I don’t want to arbitrarily replace tubes without some hint that it might make a difference. I have checked and double-checked wiring, twisting is done where appropriate. The only assembly recommendation I have read about not implemented appears to be the “star ground” concept. Still trying to evaluate whether that is a practical mod in this amp.

Thanks again,
Jeff

Jeff,

You wrote "The 60hz hum does disappear with shorted RCA inputs". Shorting the amp inputs means you have zero signal coming in to the amp. If the hum goes away because you shorted the inputs then by process of elimination the hum has to be coming in through those inputs!

Nobody said it was a ground loop. Other things besides ground loops can cause hum.

You also wrote "what possible source could there be “upstream”?"

The interconnect cables could be picking it up, especially if you have unshielded cables. Also whatever electronics (preamp, disc player, etc.) you normally have plugged into the amp could be the cause.

And there is some possibility that the RCA input jacks aren't properly grounded and that when you short them you provide a needed ground path; or the internal wiring is picking up hum. But with shorted inputs that's unlikely although they are worth investigating.

With super high efficiency speakers like you have now getting rid of hum can be a very difficult process!

So to proceed further with Jim’s comments. The interconnect cables used to go back to the next piece of equipment must be checked. Try shorting them at the far end. If no hum then the next piece of equipment (preamp?) is plugged in. Test. Then the next. Somewhere the hum will remain. That will be where the problem originates from or is contributing to your issue.

Apparently I am not making the circumstances clear.

This amp is generating 60/120hz hum when NO interconnect cables are attached. The only things touching this amp are the mains power cord coming through a ground-lifted bucking transformer providing approximately 117vac, and speaker cables to a pair of unpowered Altec Lansing A5s.

No CD, no receiver, no preamp, no inputs in fact they are unplugged and unpowered. Every electrical device in the room has been unplugged from the walls including lamps, clocks, overhead lights, etc. Nothing powered by house current is within 30 feet of this amp. When the amp is turned on, it is silent for 15-20 seconds at which point the hum is very audible. If at that point shorted RCA plugs are inserted into the inputs, the amp is silent. The hum is louder if I reintroduce ground through the power plug.

The hum also disappears if the amp is powered on with no power tubes. The hum is unchanged with the changing of the 4 power tube positions. The amp functions very well and sounds great otherwise and in fact brief frequency response tests show excellent flatness throughout the audible range.

Besides looking for some advice as to where I might begin to troubleshoot without arbitrarily replacing tubes, I post this in the interest of advising that the conventional wisdom that if hum disappears with shorted inputs, it MUST be coming from outside the amp; this is simply not 100% true. THERE IS NO 100/120vac 60hz electrical device within 30 feet of this amp and it hums. The hum is not worse when my audio devices are connected and music is played, unless I remove the ground lift at which point it is much worse (ground loop).

Hope I have made the circumstances clearer.

Thanks for any input.
Jeff

The amp was quiet with shorting plug ? Then the issue is outside the amp.
Open inputs can pickup noise from the environent.

Well if there is truly nothing wrong then with this amp, it is truly unfortunate. I’ll have to be back in the market for a different tube amp, I guess because this level of hum is unbearable. Hard to believe that
ST70s are not suitable for driving high-efficiency speakers.

Neither of my SS receivers exhibit any hum with open inputs operating in the exact same environment as the ST70, but they struggle to drive these Altecs sufficiently.

Thanks all for the feedback.
Jeff

I think you are quitting too early.
The difference between you SS gear and ST70 is that the st70 has a much higher input impedance,
it will pick up electrical noise easier.
Connect your cables to the st70, short them at the far side. Does it hum ? Then your cables
might be your source of hum.

If it hums while connected to your preamp, then you have a ground loop that tries to use the
cables grounding.
Use a coarse wire and connect to the st870 chassies and the pre chassie. Does the hum
go away or reduce ? There is your problem.

jeffjmr wrote:Apparently I am not making the circumstances clear.

This amp is generating 60/120hz hum when NO interconnect cables are attached.

As Peter posted - having nothing connected to the amp inputs is not the same thing as shorting the amp inputs! This might seem counterintuitive at first, but trust me - it is 100% true and correct. His explanation about the higher input impedance being more likely to pick up noise/hum is also spot on.

I think you'd be wise to follow the advice you received here. It'll help you narrow down the cause of the hum.

I built a VTA ST-70 and its driving the horns on a pair of Klipsch Jubilee's. The system has some noise, but I have to stick my head inside one of the horns to hear it. So I'd say sensitivity is not the problem.

I think no one has explained the shorting issue quite right.

If your amp is on and connected to speakers, with nothing connected to the RCA inputs, it is completely normal for an amp to buzz. It may not but is certainly may. Yours is buzzing, but it may not be the same buzz you're hearing with cables and components connected to the inputs. I think that's what others were trying to tell you.

Now you short the inputs and the amp gets quiet. That tells you the amp doesn't have a grounding issue. MAYBE.

The ST-70 amp's input RCA's are CHASSIS GROUNDED. If they are not grounded properly the amp will act like nothing is connected to it and buzz even with cables and components connected to the inputs and music may even play.

Shorting the inputs might simply be providing a ground point to the input that is not grounded properly, thereby eliminating the buzz.

Now, not all ST-70's have chassis grounded inputs. You will have to determine this. If they are, remount them, making sure they have good contact with the chassis and that no one left insulators on the input connectors that are blocking them from making good chassis contact, also make sure the chassis is properly grounded to circuit boards and transformers.

While it is hard for me to get my head around the concept of this much 60hz energy entering the empty RCA jacks from devices in other rooms, I will admit that my interconnects could be improved upon.

I have carefully checked all grounds including the jacks and besides the aforementioned lack of the “star ground” concept, all seems in order.

I can see the potential advantages of some wire that uses double shielding. There is a Sommer brand that has copper braid and a carbon wrap that I have read about with great reviews but I don’t see it available anymore. Several Beldon samples appear to have very similar specs. So what are some recommendations? I’m perfectly happy to assemble myself.

Thanks,
Jeff

Here are reasonably priced cables.

https://www.bluejeanscable.com/store/audio/index.htm

Hum problem accidentally solved.

On a lark I purchased a used JK Audio Pureformer isolation transformer a few weeks ago, not really expecting any significant effects. Yesterday just before I hit the “place order” button for the BlueJeans cables, I decided to plug the iso transformer in.

Hum is completely gone. And as an added bonus, bass response was dramatically increased. Using the same cheap interconnects so I am even more baffled by what is going on here than I was. I reintroduced the ground to the ST70 and still no hum.

I will order the BlueJeans cables anyway as I know these particular cables I have are not shielded and have high capacitance.

Perhaps those of you who have graciously offered advice can help explain not just how the transformer has eliminated the hum but also how the bass response which was something I was going to work on after solving the hum issue has suddenly improved. Sonically things are great but if there is some hidden downside to line iso transformers please advise.

Thanks again to all for sharing your knowledge, and your patience.

Jeff

Hi Jeff, sorry I missed your original post back in Jan, but here is my assessment.

The most important clue you have is that the hum goes away when you ground the input RCA jacks. When you ground the inputs this ensures that there is absolutely no signal at all going into the amplifier as others have said, but this does not necessarily mean the source of hum is not from your amp. Let me explain.

You have been turning off other electronics in the room thinking these could be the source of 60/120 Hz hum going into your amp. They are not. The source is more likely the ST70 itself.

The input RCA jacks are connected to the amplifier circuit board by wires, and these wires act like little receiving antennas to pick up stray fields inside the chassis (Typically 120 Hz rectifier noise but could get some 60 Hz as well). When you amplify these induced signals you get the hum you hear.

When you ground the input jacks these signals go to zero. So rather than stoping "outside" interference from "getting in your RCA jacks" what you are more likely doing by shorting the inputs is stopping these internal wires from acting as antennas and picking up stray fields from your ST70 power supply. The circuit board traces and components themselves can also act as pickup points, and signals from these too would get dumped to ground when you short the inputs.

So in short, the source of the hum is likely inside your ST70 and is being picked up by the input wiring and circuitry prior to the first tube, and when you ground the RCA jacks what you are really doing is dumping this induced hum signal to ground.

The fact that the hum went away with the isolation transformer means that you reduced the source of the hum (the ST70 power supply somehow fighting with your mains and chassis ground?). The floating input circuit is still just as sensitive to hum as ever, but there is now much less of it to pick up.

This is my best guess based on my experiences.

Thanks rjp.

You described my sense of things. I am not qualified to argue with the experts, which I am not.

But the stance that shorted inputs absolutely eliminates the amp as the source of the hum never made sense to me. 99% of the time? Maybe. But isn’t it possible that internally generated hum is getting shorted to ground with shorted inputs and therefore silenced?

Just hovering an oscilloscope probe around the circuitry reveals plenty of internal sources for hum.

The hum is gone with the Inline IT, even with the original very cheap interconnects. I don’t know exactly why, but I suspect it may have something to do with impedance.

End result is the ST70 is a great match for the Altecs. The 60+ year old drivers and dividing network have been silent for over 30 years and seem to improve with each playing.

Again, my thanks to all who have offered advice. Wish I were more qualified to return in kind.
Jeff

jeffjmr wrote:Hum problem accidentally solved.

On a lark I purchased a used JK Audio Pureformer isolation transformer a few weeks ago, not really expecting any significant effects. Yesterday just before I hit the “place order” button for the BlueJeans cables, I decided to plug the iso transformer in.

Hum is completely gone. And as an added bonus, bass response was dramatically increased. Using the same cheap interconnects so I am even more baffled by what is going on here than I was. I reintroduced the ground to the ST70 and still no hum.

I will order the BlueJeans cables anyway as I know these particular cables I have are not shielded and have high capacitance.

Perhaps those of you who have graciously offered advice can help explain not just how the transformer has eliminated the hum but also how the bass response which was something I was going to work on after solving the hum issue has suddenly improved. Sonically things are great but if there is some hidden downside to line iso transformers please advise.

Thanks again to all for sharing your knowledge, and your patience.

Jeff

The isolation transformer has "isolated" your grounds and signal levels with respect to ground and eliminated the cause of the hum. This is your solution.

I should clarify for others who may experience a similar problem.

The isolation transformer that solved the problem is NOT a mains power isolation transformer which I had already tried to no avail.

This is a stereo input isolation transformer. Pic attached. Completely eliminated the hum. Previous attempts at ground lifting, powering off and unplugging all other devices in the same room and on the same circuit did nothing.

Jeff

Since the hum is present when nothing is plugged in I would look for a bad ground connection inside the amp, especially the connection from the input board to ground and the shield of the input jacks to the input board as well. Could be a bad/broken solder joint. The shield of the inputs to the ST70 are NOT at chassis ground but connected to ground through a 10 ohm resistor. There may be a failure here as well. The audio isolation transformer does the same thing as shorting the input plug for DC voltages and provides a ground reference for the signal which is missing.

Because the shield of the ST70 input is not at chassis "ground" I would still keep the audio isolation transformer in line especially if you drive the ST70 with non Dynaco source to keep the signal "grounds" isolated.

On another note many amps, especially guitar amps ground the inputs when nothing is plugged in to prevent hum and noise.