Audible channel popping

Has anyone experienced an issue of when changing channels from ultra or lead to clean, there's an audible pop(slight thump)...that gets louder as you raise the master? I've seen 2 posts on it, with no real answers. It happens slightly when nothing is being played and happens prominently when audio is pumping through (when you're playing). It's enough to where it's heard and makes channel switching to clean a deal breaker in a live pro situation.

This is not your normal relay popping on any given tube head. This is gradually increasing pop only when going from high gain or a loud drive tone on ultra and lead to clean when playing.

Help please!

This definitely is not how it should be, the amp is usually very quiet during selections.  We do have a couple of reports of it as you said but it is by no means a common problem as in a known issue in the amp.  If anyone else has experience of it and can suggest where it turned up in their own amp then I'm sure they will chime in.  (See my comment about valve changing lower down!)

This problem is usually (not exclusively) caused by having different DC levels between the two sections being switched.  The following circuit sees the step in levels as a signal and amplifies it until the new selection stabilises at the level it should.  The way around it is to put in extra components to make sure the levels stabilise to the same before switching.  There are many manufacturers out there who don't take care of details like this, even though it is cheap and easy it sometimes gets missed.  I have seen the schematics for the GM36 and they take pains to get it right!  (That's why there are so many 1-10Mohm resistors knocking about.)

If this is happening in your amp then something has changed.  I would say it is highly unlikely to be due to ageing valves unless there are other symptoms,   .  The changes in valve characteristics as they age will make for changes in the DC levels around the valve but the associated capacitors just allow the DC levels outside to adjust to compensate!  It is possible that a valve can develop a fault where the characteristics flicker slightly between two levels at random and this would set up DC differences, but that would usually mean you can hear the flickering when that valve is in circuit.

I would suspect something like one of your smaller capacitors getting a little leaky and allowing a tiny DC current to pass through it making a following circuit DC voltage level change.  The trouble is that there are so many of these areas in the GM36 where sections of circuitry are switched that it could be almost anywhere in the preamp.  I think it may take someone to chase through the amp to diagnose exactly where it is occuring.  The diagnosis is the complicated bit, the fix should be pretty easy.

A couple of questions to answer:

Has it always done this or is it just a recent development?

Is your amp more noisy than it has been on any channel, particularly the Clean, in a general sense rather than the popping?  I would expect maybe a low level of noise kind of like a shushy rumble like thunder a long way away rather than just high frequency hiss.

@bordon

It definitely existed out of the box as I've only had the amp less than a week. I have rehearsed with it once and it was very noticeable. A bandaid, workaround seems to be when I engage the noise gate on the clean channel to match the dirty channels. The pop gets really quiet and/or goes away. When I say "pop" it's not like a crackle or pop like you'd expect. It's a thump-pop similar to when you'd change wattage settings on any given amp...or the soak settings on the GM.

Also, there is a slight interference noise or crackling, very slight....when the master is turned up past 12 o'clock. On any channel. That could be power tube related. Any of this could be really.

I'm going to feel the amp out and see what happens and exchange it within the 45day window if it continues to be an issue.

Yes, I would go with you on that.  If this amp is less than a week old you definitely have something wrong there which should be corrected under warranty.  There should be no popping on channel change, the H&K circuitry is too well designed to do that.

As I said, it doesn't sound like it's the valves but you can never be absolutely certain.  You could suggest that the shop rule out the valve side of things by swapping them out for another set.  It needn't be permanent, just to test.  That will show whether there is something inside the case which can't be corrected with just a parts replacement.  Your description of the noise you get when you turn up the Master could be down to a noisy valve and it is well known that the Chinese type they fit from the factory can be bettered, but that shouldn't really be related to the popping.  On the other hand you never know, there is always more to learn so see if you can get them to try it.

The noise gate will also affect this and disguise it.  It works by monitoring the first fraction of any sound and only "opening" to allow the signal to pass when it goes above a certain level.  You pay for that by losing a little of the starting transient.  In your case, if that pop is pretty short and sharp it opens the gate with its level but has mostly passed by the time the gate is fully open.

I can say that none of us here complain about this effect and it certainly doesn't happen at all in either my own GM36 or TM36.  Go back to the store as soon as possible and get them to look into it.  At this stage, whatever you do don't be tempted to open it and fiddle!  There would be nothing to do with this that you can see anyway.  Don't give up on the amp because of this, it's a Hell of a machine once you get it working as it should and yours is a pretty unlucky rare case.  Keep us up to date here with the response you get, it all helps others.


EDIT:  Sorry, I forgot, I meant to point out that as it gets louder when you turn up the Master it cannot involve the power amp valves.  It must be coming from before them in the preamp section so at the absolute outside it can only involve V1 or V2.

The BValadez touched on something - all my patches use the noise gate on the GM, with it turned to minimum (full clockwise) - it is a wonderful noise gate, I just leave it on always.  Hmmm...

I believe there have been other posts in the past about popping between channels...  BValadez, have you checked?

BB, remember those two unexplained diodes between the inputs of that op-amp towards the output side?  I'm still feeling that those are there for switching situations...

This gets more weird. So I have 2 pedal boards.

One uses the Voodoo Lab PX8 to switch channels via midi. The other uses an HD500x to switch channels. When using the relays in the PX8 I definitely hear the pop or thump totally audible and gets louder with more master volume. Via the POD it's very minimal and does not pop loud, esp when using the noise gate. If I take everything out of the chain and plug straight into the GM, nothing in the effects loop and no midi switcher attached, there is no pop or thump. The test is...dial yourself up on a dirty patch, hit a chord and let it ring out....switch the dial to clean while chord is ringing out. NO pop/thump in that scenario.

I've also taken out all pedals, switch guitar cables...to test with only using a switcher and guitar plugged straight into the amp.

So is it possible the GM works flawlessly while only using the FSM32? That is the worst form facter design in terms of real estate on a pedalboard. I can't stand the curve design.

Another note, Maybe routing through the PX8...having to plug into it and out of it, to the amp...their relay layout doesn't play nice with the GM. Not sure.

Hmmm, I use the HD500x, connected for midi control and input side effects (not 4cm). I'll be able to try this at volume within a couple of days. Still, I've never noticed this before, and at gigs I have the Master dimed.

So is it possible the GM works flawlessly while only using the FSM32?

I have to say "No, definitely not"!  If the GM36 does not pop when controlled with the FSM432, or by hand for that manner, that proves that there is no problem with the DC levels and switching inside it.  Your problem is a DC level being imposed on the audio signal to the amp not switching control signals so the MIDI side as in the controller sending the switching signals is irrelevant.  This is caused only by something in the signal path.  It makes no difference what is causing the GM36's internal switches to change, only what they may do to the signal when they do.  If it doesn't thump when you use one controller then it won't thump when you use a different one.  If it is the amp which pops internally it will do it with any controller, if it doesn't pop with one then it's something other than the amp.  And if it is an interaction between a piece of kit attached to the input and the amp, then it's the preceding kit as the amp's input is correctly set up to prevent this.  It doesn't have a problem with any other pedal of any sort connected to its input.

For the record I will point out something here.  There are no traditional magnetic relays inside the GM36 in the signal path.  The switching is all performed with good quality solid state switches and JFETs.

When using the relays in the PX8 I definitely hear the pop or thump totally audible and gets louder with more master volume.

The answer to this then lies in the PX8.  I notice the PX8 has a "Quiet Switching" feature which seems to be a programmable delay between switching on/off and applying the signal.  I'm suspicious of this idea.  Thjis would suggest there is active circuitry in the signal path allowing it to pass or stopping it.  Do you have this set on or off?

I am sure that the problem is in the PX8 and that maybe it has an ungrounded output somewhere.  I don't mean the ground connection has become unattached, I mean that if there is a capacitor on any point which will become an output then there should always be a high value resistor between the output and ground.  Without that the DC level at the output will be undefined and will float around to whatever level it likes.  Then when the amp input is connected to it the amp's input sees the DC level as a signal and amplifies it while it does the job of grounding it and pulling the capacitor DC level to 0V.  It sounds like you could somehow have different DC levels on the outputs of the PX8's loops so when you switch between them these levels are acting as a signal and causing the thump.



EDIT:  Just had a quick flick around online and I have to say, Voodoo Labs has some reports of switching pops in its gear.  Their own response to questions about their Hex switching system was "HEX has an optional circuit (via internal jumper) you can use to get rid of switching pops."  Why have an optional circuit?  Why do you need it and why don't you just use it all the time?

And I saw a price of £341!!!!!    Surely to God that can't be right?  It's nothing more than a few relays, sockets, MIDI and a couple of buffers.

Rather than keep on editing the previous post.....

Assuming that VL have got the high value resistor thing sorted out as not every PX8 floats around, it is also possible that one of the output capacitors somewhere in the PX8 has gone leaky and is generating that DC voltage across the pulldown resistor.  That would be an easy fix.

Controversially....  I am sure that some of the advice elsewhere will be "change out all of the electrolytics and if that doesn't work then swap out all of the others."  That's the worst most useless advice you could get in any circumstance yet prople follow it because they just don't see anything else they can do!  You need to identify if there is a particular switch (or switches) which causes this.  If it is all of them then there would be a common capacitor which they all share.  It's the lowest level of dumbness to swap out correctly functioning components en masse willy nilly hoping to catch the right one.  It's on a par with me changing all of my car's numerous engine bearings every two years just because I found one is wearing!

I'm going to just deem this as the PX8 being the cause. That's obvious now. No the behavior, using an HD500x, does not mimic the PX8. Not a huge deal. I can move to another type of switcher or address the PX8 itself. Thank you all for your comments and the help.

So I continued troubleshooting/testing. I tested by using my Boss DD500 to trigger channel changes on the GM via midi. Nothing actually plugged into the DD500 and no audio routing through anything before the amp nor the effects loop. Just simply plugged directly into the amp. Just using the DD500 as a midi foot controller to test. Same "thump" when changing from any of the 3 dirty channels to clean. I also tested by using the PX8 to change channels via midi, but nothing plugged into the PX8. No audio route through it. Just guitar plugged into head, nothing in the effects loop. PX8 plugged ONLY connected to the GM via midi cable (tried 3 different midi cables) Same "thump" sound.

When I said earlier using the HD500x, where there's no thump sound...routing audio through the HD500x. It might be a case where when plugged in, through the HD500x, it somehow squashes the thump/pop. Gates it somehow.

Ugh. Annoying.

So 100%, it is not due to audio routing through a third party piece of gear. For whatever reason, using at least 2 other devices as a midi foot controller

That's a great bit of diagnostic work BV.  What you say looks like it is true though it is a new one to me with this amp, that it isn't the external device having a signal problem, it's the change process inside the GM36.  On that score, this is not a usual GM36 problem.  I certainly don't have this issue and I don't think any current members have complained of it.  There are mechanisms where it could be created within the amp of course but they would have to be seen as faults.

As you have probably seen I'm looking very closely into the circuitry of the GM elsewhere at the moment.  I've looked for areas where this issue could occur in terms of the switching as there is a shed load of it in there.  I'm amazed at how thorough the designers have been at tying this aspect down.  Like I said before, the usual mechanism is through switching between capacitors charged to different levels.  They include high value bleed resistors to equalise the voltages of everything which could be a problem.  It can also be due to fast MIDI switching between very different parameters like stage gain.  These are often controlled with JFETs.  In those cases they have been careful to slug down the switching speed of control devices to what isn't noticeable as a delay but doesn't cause any issues.  So if it is inside I would lean towards a slight problem, I only say slight because it isn't a showstopper though I know it will be extremely irritating.

You say it happens with your guitar connected direct to the amp, does it happen with no guitar connected?  You also mentioned that it occurs when switching from the 3 dirty channels to Clean.  Is that the only switching sequence which makes it happen?  You could also check the signal at the Fx loop and the preamp out into another amp to see if it still occurs there, (I would guess it will as these are late in the path but it may help if you can do it).

I would first suspect a cap going a tiny bit leaky which alters the DC level on the outer side of it.  As an engineer not a tech I would need to track it down with a scope or maybe a good meter.  A good tech who knows the GM36 on a day to day basis may have seen this before and know immediately where to go to fix it.  If this is the case, it happens in the best of equipment all the time, you have just been unlucky that it is in a place where it is so noticeable.

Bordonbert,

Thanks for being so informative. I truly appreciate it.

- It does not happen when the guitar is disconnected
- It does not happen when the guitar is connected and not playing
- It only happens with guitar connected and switching while playing
- It only happens from channels 2, 3, or 4 to 1. Only dirty to clean.

Now I'm hesitant to spend the $200 on the FSM foot controller, being it's a midi controller.

I'm guessing I'll just exchange it. Sucks because amps are all different. No 1 amp is the same (even the same model) and I get great tone from this one.

Aha! Only while you are actually playing, that's a bit different. I can only make a guess at what you are seeing here:

You play a note or chord. The amp processes the signal and at some point inside the various stages sit at a reasonable instantaneous signal voltage part of the way through a cycle of the signal. You switch from high gain to Clean at that point. The gain of some (I would guess almost all) of the stages is changed. The signal is then reduced in size very quickly. As it is part of the way through a cycle and maybe sitting at a significant instantaneous voltage this drops very rapidly because it is not amplified as much now. That fast drop is effectively a fast step signal generating very high frequency harmonics which may be made worse by the frequency shaping and clipping which is going on. The stage sees that sharp drop as a fast signal in its own right and amplifies it too. You hear that as a pop within the signal. In effect, I thin it may be the sudden change of gain within a stage, producing false signals which are amplified and intensified due to frequency response and clipping.

I have to say I don't understand why all amps don't do this, (maybe they do and I just wasn't aware of it). Anyone have any experience of this issue with other makes of amplifier? I don't know too many players who change their patch while they are actually in full flow, If I think about it I'm pretty certain I kick it in between notes or chords sort of instinctively. If this makes the amp unusable for your style of playing then I'm sorry, it doesn't look like there is anything we can do about it.