GrandMeister 40 Popping sound when switching directly from Ultra to Clean channel

I have this issue with my GM40.

Let’s say I store a preset in the amp or app on the Ultra channel, then switch directly to the clean channel via foot switch or app, I get a loud popping sound through the speaker. This is not the relay switch sound that you can hear when switching channels. It is a popping sound that you can hear through the speaker. This popping sound only occurs when switching directly from Ultra to Clean. No other channel switching has this issue, and like I said, it is only noticeable when switching direct from Ultra to clean. Seems to get louder when power soak is decreased from 1,5,20,40. Amp is also plugged into a Furman power conditioner.

I have tried disabling the noise gate and such but, still occurs. I also reset the amp, just incase but no change.

Anyone else have this issue or know what is causing it?

Thanks

William.

Have a look here:

https://handkusers.forumotion.com/t75-popping-noise-when-changing-from-ultra-to-clean?highlight=popping

I don't have access to GM40 schematics but I do have GM36 ones and there will definitely be a family resemblance between the two. This is not an unusual problem with a lot of channel switching amps. The isolating capacitors between stages need to charge up to their working DC level and they take in current as they do. It is likely to be this current which is causing the pop in your amp. As you switch from Ultra to Clean you effectively remove a full valve stage and swap to the output of the stage before. If the DC levels do not quite match up as they should then this pop occurs as the capacitors charge up again.

H&K take great care to set things up so this does not happen. They add high value resistors to make sure that the capacitors have a supply of charging current even when their stage is not in use. As capacitors age, (not an unusually common problem despite the "swap 'em all every time the wind changes" advice you often see especially in an amp as new as your GM40), they can get a tiny bit leaky and pass a tiny DC current when there should be none. This can cause the difference in levels which leads to the pop.

Is it really bad or is it just an irritation at home? I mean, in a live band setting is it clearly audible every time you change channels? If so it may need a trip to the tech to have a few of the coupling caps in the preamp checked and replaced.

Thanks for the reply Bordonbert.
It is loud enough that you can notice it even at loud volumes. I visited my local music store and tried another GM40. It popped ever so slightly, but not nearly as loud. I will have to bring mine in and have it checked. I really don’t like dealing with this, as many times they will try and say “this is normal behavior, every amp is different” and it takes along time to get the amp back.

Yes, I know your problem with wait times to come to the top of the pile and a response of "that's normal". The trouble is that it possibly is normal.

All capacitors leak a little even from new, that shows in their specs. If I give an example, the datasheet for a Panasonic EE Series 10uF 400V electrolytic, which is in the ballpark for our GM36 coupling caps, shows for its leakage current "I < 0.06 CV +10 (µA) After 2 minutes". That means that with 200V DC across it the leakage must be less than 130uA (0.06 X 0.00001 X 200 + 10) after a 2 minute period from switch on. The time period is relevant because the insulating layer inside electrolytics begins to degrade when they are switched off, (very very very very slowly, don't read too much into that). At switch on they go through a short period of reforming and come back to their normal state. Bear in mind that current is the maximum level allowed by the Panasonic spec and the vast majority will be really below that. That is purely a statistical thing as to what yours will give. That slight leakage will cause a tiny pop in many switching actions within any high gain amp and that will also be amplified according to the Master Volume control setting if the pop is caused before it as yours is.

It's a bit of a lottery I'm afraid. If it is really noticeable then I would think it will respond to swapping out one or two of the caps. Of course there is always the chance that a new cap will be as leaky as the original but your tech should be able to advise you on how bad he thinks it is and whether it would be improved. I must admit that I never had this problem with my own GM36. It switches with the slightest of soft pops which I can only hear at high volumes when I'm not playing. The good side is that this can be tested without taking any caps out of circuit as the effect shows itself in the voltage across resistors attached to them and that can just be measured with a meter, usually with everything in place.