Hum problem

Before you do send the amp back try these tricks:

1: Do you have any video components tied into the system if you have a cable box they can have different grounds and can require a ground breaker

2: Did you check your dimmers for lighting these can sometimes cause a ground hum.

3: How did you test floating the ground? At an power conditioner do you use one?

4: Have you methodically tried floating the grounds one at a time, on different components?

5: Did you test without any components connected to the amp? 

6: Have you re-introduced cables one at a time? Sometimes you can have a bad ground on an RCA cable

Hi Michael,

Have you tried changing the 6SN7s? See this thread from 2012, in which you participated. It seems that the VAC Renaissance amps can hum if heater-to-cathode leakage is excessive in the 6SN7s.

Also, I would suspect that you have two problems present, one internal to the amp that is causing the hum with the volume control at min, and a different problem which adds to the hum when the volume control is raised. If you’ve found that lifting the AC safety ground on the preamp or the amp does not affect either problem, the cause of the second problem figures to be somewhere upstream of the amp.

Also, DC offset by definition is zero Hz, and so it would not cause a hum.  And as you realize it is very unlikely to be present in the case of an amp having transformer-coupled outputs.

Best regards,
-- Al

@audiotroy 
1.  Mondial magic isolator already installed. 
2.  Will have to check dimmers but usually lights off. 
3.  Floated ground at wall and at power conditioner. 
4.  Hum is present regardless of whether other components are on or off. 
5.  Will try, thx. 
6.  I/Cs have no impact.

@almarg- @almarg- forgot about that; will swap out 6SN7s one at a time. Thx. 
 Have not tried lifting ground on preamp but the passive is a 2 prong plug so methinks that may not be it. 

My technician has been all through it and does not find a problem. Says he cannot hear it in his system, but it's quite apparent on my Horning speakers (94 dB maybe?).

See if you can hire the technician for an hour or so to come to your home and listen to the hum first hand.

Jim

Not a bad idea, Jim. I’m a regular customer and he lives in my town. If Al’s idea of swapping out the 6SN7s doesn’t work , i’ll give him a call. 

Could be induced hum (it doesn't do it in the tech's system). Is it possible to move the amplifier to another corner of the room and see if there's any change?

Did the technician put it on the scope?  If not what are the efficiency of his speakers?

Hey Swamp….

Sorry to hear this.

Maybe I’m preaching to the choir but your post did not account for:

You said “… for a while” expressly focusing on the 30/30. What was checked or swapped out upstream to put the finger right on the amp?


Using a diff preamp. Got another preamp to try?

Using a diff amp. Got another amp around there? Does this occur with any amp in the system?

As well, Got another pair of speakers? Hook them up and try it again, using the exact same spkr wires, etc. .

These ought to eliminate one aspect. Hope so, but I’m not all in on that one though it seems an area to check out and its easy enough to do I’d think.

You said your tech looked at it but could not replicate the situation in what I suspect was his shop.

Something loose inside the amp? Moved during travel?

I’d suspect he set It up as I stated above before breaking out the multi meter or scope.

Before Al chimed in I thought ‘volume control’ ckt. Or upstream is developing an issue somewhere.

Thought loose connections on spkr wires too. And if using bare wires, a single strand loose and touching the case.

Have a hard time counting on a ground suddenly arising from an IC, or when one had not been there previously eg., CATV, Dimmers, etc. save for a now failing device..

Re-reading the post, the hum is there from the jump but more noticeable once power ala tubes warm up so I’d agree on swapping out what ever tubes, one by one, could be pretty revelatory.

The mystery for me is the ‘tech’ did his investigations with the same tubes in it, I presume.

Might be as simple as the soft start relay is failing. Moving parts with make & break or vice versa contacts do pose issues with time and use. The coil in the relay could just be ready to quit. Insulation break downs, poor conductivity from pitted or dirty contacts, contacts not fully opening or closing, etc.

Depending on what the tech says elec and visually testing it, Relays often have enough info on them so a replacement can be ordered pretty easily… if you just want to see prior to making any major moves… and provided it ain’t the tubes as was said.


If bad have VAC send a new one. I’d sure look into that before packing it in and sending it to the Sunburn state for a look see.

Not to pile on because I'm not electronically very knowledgeable, but this was my experience in dealing with a phono hum: After much experimentation, I got a tip from the Tweak Shop to wrap my unshielded phono IC with aluminum foil.  Gone!  Obviously, the phono IC was picking up hum from my big Audio Valve Baldur 300s sitting within 2 feet side-by-side of my phono preamp.  Also, one of more tubes went bad in my Kondo KSL DAC a year ago causing the left channel to reduce in output.  So, tubes can cause erratic things to happen.  Good luck in finding the solution.

Thanks everyone for your help.  Alas, no cigar;-)  BTW, hum is not so bad that I cannot listen, but it is audible from 8-10  away during quite parts of songs.

@fineito- Not piling on at all.  I'll try anything I can do easily this weekend and then call my tech and VAC on Monday. 

@blindjim- upstream checks:
    1.  Hum not present when Atma S30 amp in system.
    2.  Hum present with 2 pre-amps (one active and one passive).
    3.  Hum NOT present on immediate turn on.  I hear what sounds like a gradually increasing hum after 30--40 seconds.  The relay clicks open a few seconds later and then the hum is very apparent in the speakers.  It's possible that the first hum is coming from inside the amp.  I assume that the first hum (maybe more of a buzz) is the sound of the soft start relay starting to open but I have absolutely zero knowledge of whether or not that is a reasonable assumption. 
   4.  Hum present with multiple I/Cs.  Have not checked speaker cables but seems very unlikely that the hum would be identical in both channels if it were hum induced in the speaker cables.

@almarg- I swapped in about 12 different 6SN7s in various combos.  No impact on any aspect of the problem, so I THINK I can rule that out.  Please disabuse me of that if I am wrong. 

To all- I will try swapping out 300bs but it's also hard for me to believe that all 4 were identically bad, resulting in identical hum in both channels.  Again, if I'm off base on that, please let me know.  As I've said, I have ZERO technical knowledge of how these things operate. 

Hi Michael,

My 70/70 emits a slight mechanical buzz for a few seconds before the B+ relay clicks on, which sounds like what you are referring to as the "first hum (maybe more of a buzz)." It disappears completely when the relay clicks on, and I suspect is a mechanical phenomenon involving some of the several power transformers that are in the amp. So I don’t think that initial hum/buzz is an issue.

At this point all I can suggest in addition to what you are planning to do is to reiterate the suggestion of seeing if the hum is present with nothing connected to the amp’s inputs, if you haven’t already done so. Or better yet, with shorting plugs connected to its inputs.

Best regards,
-- Al

At this point all I can suggest in addition to what you are planning to do is to reiterate the suggestion of seeing if the hum is present with nothing connected to the amp’s inputs, if you haven’t already done so. Or better yet, with shorting plugs connected to its inputs.

Al,

I was thinking the same thing. That eliminates everything ahead of the amp. I would probably go so far as to unplugging all other associated equipment from the AC mains wall outlet.

What about the sensitivity of the speakers Michael is using. Not sure which model he has. One review for Horning speakers, I quickly read over, said the sensitivity was rated at 100db.
And is there any way the speaker cables could be causing the hum? Capacitance? Inductance? I have read where some speaker cable designs can cause problems for a power amp.

Jim

Jim- The Hornings are an older model, Perikles Ultimate.  They are spec'd at 96 dB, so I guess I'd call them moderately high efficiency.  I think I've got some shorting plugs here so I'll try that out and report back later this evening. 

And is there any way the speaker cables could be causing the hum? Capacitance? Inductance? I have read where some speaker cable designs can cause problems for a power amp.

Thanks for your comment, Jim. You may be thinking of the fact that a few speaker cable types having ultra-high capacitance have been known to cause some amplifiers to oscillate, with destructive results in some cases. Goertz and the old Polk Cobra cables, for example, at least if they are used without a Zobel network connected at the output of the amp. The reason for the ultra-high capacitance in those cases being that it is a side-effect of their approach to achieving ultra-low inductance.

Such an oscillation would most likely be at a much higher frequency than is the case here, though, and would typically occur with an amp having low output impedance, considerable amounts of feedback, and particularly wide bandwidth. None of those characteristics apply to Michael’s VAC Renaissance 30/30, and in general are most likely to occur with solid state amps.

Best regards,
-- Al

@jea48
@almarg-
My speaker cables are 8' Jade Audio Vermeil.  I have no idea if they have have ultra-capacitance or ultra-low inductance.  In fact, I have no idea what inductance or capacitance are, beyond knowing that they are electrical characteristics or parameters.  JD's web site does not have any specs. 

@almarg-
Hum still present when amp turned on w shorting plugs in input RCAs.  With nothing connected to the inputs (not even shorting plugs), I get a loud buzz; much louder than hum but I believe it "on top" of the hum. 

@swampwalker,

By chance did you unplug all the other associated audio equipment from the wall AC power outlet? Audio equipment near the amp as well as the speakers?

By the way the reason you hear a loud buzz from the amp/speakers with the shorting plugs removed from the inputs is due to RFI/EMI that is allowed to enter the open input ports/jacks of the amp.

By chance do you have another pair of speaker cables you can try? If you still have the hum using different speaker cables that would rule out the cables, ime.

Edit:
Also are you using an aftermarket power cord on the amp? If so did you take the power cord with you when you took the amp to the repair shop to have it looked at?
Just for the heck of it install the factory power cord that came with the amp. Check for the hum.

Jim

Jim-
Thanks for your continued interest,
   1.  Unplugged all other equipment in vicinity from AC.  Amp w shorting plugs only... still hums.
   2.  Then changed speaker cables...still hums.
   3.  Then changed aftermarket cord to a heavy duty, OEM-type AC cord w molded on male and female connectors  (no idea if I even got an AC cord when I bought amp used)...still hums. 

Jim/Al- I'm guessing that we have now isolated this problem to the amp, no?

BTW, RFI was most likely from powered wireless connection to internet for my music server.  No change in hum pretty much rules out the server and ancillaries, I imagine. 

BTW, RFI was most likely from powered wireless connection to internet for my music server

Did you kill that thing when you were isolating things to find out what might be causing the hum?

Has this amp always had the hum issue? If not, can you think of anything you may have changed or added that maybe now is causing the hum?

One more thing to try, although it may be grasping at straws:  With the amp turned off, rotate the feedback control through all of its positions a few times, to wipe off or at least reduce oxidation that may be on its contacts.

The condition of the various contacts on that switch are one thing that comes to mind that could affect both channels in a similar manner.  And the age of the amp makes their condition somewhat suspect.

Good luck.  Best,
-- Al
 

Jim- yep I had everything off and double checked after reading your post

Al - will try that but if that’s what it is I am going to strangle my tech. The feedback control was kind of funky in its operation and the main reason I brought it in. The switch operates better know mechanically and he SAID he fixed it and also spent 3-4 hours giving it a complete going over. 

Jetter- I guess I’ll have to dig up those old 300 bs. 

swampwalker

1. Hum not present when Atma S30 amp in system.
For me, interchanging in and out amps would be enough for me to point a finger right at where the issue resides. All else being equal and it appears so.

That move alone should resolve any suspicion of wires or speakers, or upstream gear, or even household effects as culpable items of interest.

My long ago EXP in fixing electronics points me towards the practical common sense sied of things as the approach I used back in the stone age, as a rule.

Guessing out loud as its all I can do here….. I’d say whatever is wrong is wrong in the VAC amp and quite immediately. ‘why’ it becomes more and more pronounced is due to the amp coming into a more stable operating posture so some devices are in play right off and if failing or failed, one would think the amp would either not work at all or work poorly.
Capacitors have terribly quick rise times individually. A bank of them even in parallel takes a tick longer to come up fully but no more essentially.

https://www.electronicproducts.com/Passive_Components/Capacitors/Failure_modes_in_capacitors.aspx

Caps don’t always create an open ckt. Preventing current or voltage transference 100% and hence, a lack of system wide operation. Caps don’t usually become the culprits either…. But nothing lasts forever man. Even caps.. a failing cap might be in play here.

I’d have ?? take a closer look at them IMO.

Also, to lessen or possibly prevent shipping it out, in the DIY vein, depending on just how exciting you want this step to become you can either unplug the amp from power, or leave it plugged in.

Tift the cover from the amp and leaving all tubes in place, as systematically as I could use a bottle of compressed air to clean it out. Or a vacuum cleaner with a thin tipped accessory or merely some pretty good small paint brushes to ensure nothing has gotten into the amp causing the issue. Just a thought. Dust + humidity + heat & repeat causes issues indeed.

Beyond a step by step front to back visual inspection of connections, boards, for issues with attention to excessive heat areas, where insulation or insulators may be compromised, or merely loosened, I’d say something is telling you it is sick and about to quit… but wants to surprise you with the timing of its exodus. Kind of a thoughtful thing if you think about it.

Although, an abrupt failure to operate would probably be less frustrating.

Deep regrets. Best of all possible speedy solutions.

@almarg -
Rotating the feedback switch back and forth reduced the volume of the hum as the feedback was increased and then of course an increase as feedback reduced back to zero.  I believe that this is expected but of course I totally lack the knowledge to understand why (something about changing the impedance maybe...of course I can say those words without knowing what that means, either).  Any harm in trying some good old contact cleaner on the switch?  Would I need to remove the bottom plate on the amp to do that?  If so, will that expose me to danger from high voltages? 

@swampwalker Michael, yes, increasing the amount of feedback lowers the gain of the amp, and therefore a corresponding reduction in the hum level is not surprising.

The one possibility of harm if you were to spray contact cleaner on the switch would be if you were to inadvertently physically damage something during the process, which is a real possibility given the bulkiness and weight of this beast. And yes, you would have to remove the bottom plate, and lethal voltages might be present at various circuit points under the chassis if a considerable amount of time has not elapsed since the amp was turned off. So it’s probably not worth bothering to do that unless your experiments suggest that marginal contact integrity in the switch is contributing to the problem. If wiggling the switch slightly without changing its setting does not affect the hum level it would very likely be an indication that the switch is not contributing to the problem.

Best regards,
-- Al

The problem is not the feedback switch. The hum changes because with increased feedback, some of the hum is wiped out.

The hum is independent of tubes, so right now its sounding like you have filter capacitors in the power supply that are failing. It could also be a bad ground somewhere that relies on a mechanical connection, but at any rate it sounds like you're going to have to send it out for repairs.

@Atmosphere > “…. capacitors in the power supply that are failing. It could also be a bad ground somewhere that relies on a mechanical connection, but at any rate it sounds like you're going to have to send it out for repairs.”

Blindjim > As I alluded to these two items earlier(caps & grounds), and you have been more specific as to which caps, wouldn’t those devices which have physical movement associated with their functionality be more suspect for abnormal operation of the amp? Eg., relays, power sw, etc., generally speaking?

Just wondering out loud. Thanks.

@swampwalker
I reckon its just a matter of time and well, you know, now. My deepest condolences and best wishes.
I dig a thorough systematic eval to ensure the or those, physical components which have failed, and or are failing to be found out and replaced is in order.

As I alluded to these two items earlier(caps & grounds), and you have been more specific as to which caps, wouldn’t those devices which have physical movement associated with their functionality be more suspect for abnormal operation of the amp? Eg., relays, power sw, etc., generally speaking?

Not really, and not on a hum problem like this. Initially- the unit is better, gets worse as it warms up. That's not a mechanical issue.

@swampwalker here's a test for you- run the amp with shorted inputs and wait for it to hum. Then turn the unit off for 5 seconds and then back on again. Did the hum die instantaneously with the switch?

If yes that's a power supply problem.

Did the hum not come back as loud when you turned it back on?

This is a bit of a trick- failing filter caps will sometimes 'heal' for a bit with an off/on surge, but the hum will come back. So if the answer is 'yes' its just about certain its a filter cap, but if it was no, filter caps are still at the top of the list regardless.

@atmasphere- Thank you for taking the time to think about this and to reply.  It appears that I was a little unclear in describing the problem.  The amp is dead silent (inputs shorted, no other items plugged in to nearby receptacles) until the soft-start relay starts to open.  For a few seconds I get a buzz which I attribute to mechanical noise as the relay opens up.  Almarg confirms that his Ren 70/70 does the same.  When the relay fully opens, the buzz goes away and then there is a hum from the speakers that is constant in volume and does not change over time (or at least not significantly).  When the amp is inserted into the system, the hum remains the same volume from full attenuation, until reaching around 9 or 10 o'clock. 

I tried shutting the amp down and then immediately restarting as you suggested.  The soft-start circuit kicks in, so we are back to square one.  Quiet, a buzz (only as the relay is opening), and then the hum.  

Hi Michael,

To clarify, on my 70/70 there is a slight mechanical buzz (which I suspect to be transformer-related) for a few seconds just **before** the B+ relay closes, about 20 seconds or so after the amp is turned on. (I assume the relay is closing, i.e., completing a circuit path, rather than opening when it presumably applies the high voltage B+ to the tubes at that time). Also, the relay closure itself is essentially instantaneous on my amp, at least since VAC worked the amp over a few years ago when I sent it in for an unrelated repair. Prior to that the relay would clatter for a couple of seconds when closing, which it should not do if in top condition.

Best regards,
-- Al

Thanks- sounds like the relay is a secondary problem. The hum is nothing to do with the relay though. Its a process of the amplifier's operation. If you've already played with the tubes, then the filter caps are still on the table.

Thanks Ralph. Good to know.

I have  a few questions.

If your amp did not hum at the techs shop then it's not the amp.
1. Is your amp in a rack or free standing on the floor.
    ( if in a rack take it out of the rack  / unplug everything except the speakers )
2. You lifted ground, how did you do that . Switch on the amp or removing the ground leg off of the power cord. You need to lift the electrical ground of the power cable to the amp.Using a 3 to 2 adapter  does the trick put you have to pull off any metal loop that would touch the center screw that holds the wall plate on the wall outlet plug.
Take your power DODD transformer out,plug straight into wall jack.
3. Have you tried other speakers to make sure it's not your speakers crossovers.
If the ground is till there then it's the amp for sure.
 If you are clean then plug in the Preamp  with nothing in the input section,turn it on. ( do not put the frame of the preamp in contact with the amp frame ).
4. LED lights of any kind on the audio AC wall socket all the way to the circuit breakers ( notoriously buzzy and it goes down the ground leg)
Same applies to dimmers of any kind.
5. If you put a meter on your AC wall jack ,what is the reading between the + leg to ground ( should be 120V ac ) then - to ground ( that should be 0 ac volts ).
If you have dimmers on that AC line turn them low to midway and then check your ground voltage again.If it goes higher than previous test with them off you might have found your problem.Even completely off LED light circuits still burn power that is  leaked by the dimmers ( hence LED light glow when in the off position )

It makes no sense that the amp is quiet with another system and installed in your set up it starts buzzing.
Anyhow a lot of good thoughts in the upper threads but there is no mention on how your systems is installed in the room ( rack or no rack ) Components touching other components. I fight those issues daily with amp racks more so in permanent installation steel frames and rails.

Good luck.

I had this with an Audible Illusions modulus 3a pre. The tech fellow I hired could find no explanation. I used a DAC direct to power amp for about 3 Mos. One day I tried the preamp and the hum was gone. My neighbor was big into ham radio. He had moved. So rf? 

Couple of tips:
1) Try using only 1 outlet for all of your components. Obviously using a multiple outlet strip that can accommodate everything and is rated at 20 amps. Insure that the outlet is dedicated if you can.
2) Experiment floating the grounds on the power amp. If no difference, then try the preamplifier.
3) Ultimately it may come down to new shielded Interconnects.
Best of luck!

Addendum to my previous post above.
**Important consideration when using a cheater plug to float the ground on any component.
Many cheater plugs are polarized. You must eliminate the polarized lugs so you can change the orientation of the plug. One way may not change the hum but rotating it (which you would not be able to do prior to eliminating the polarized lugs) may do the trick.
Then using a volt/ohm meter with everything plugged in but off. Measure from the ground lug on the preamplifier to the chassis ground on the amplifier. Obtain the lowest resistance reading by changing the power plug orientation.