Ampeg SVT4Pro Pop problem

[mickbellinello]

Hi to all,

I noticed a problem with my brand new Ampeg SVT4Pro Head and I don't know what to do.

When I turn on/of my true bypass pedal make a loud pop. Same thing activating/deactivating my basses onboard eq.

I made some comparison test with my old Ampeg PF500 which is deadly quiet.

I tried using a buffer at the end of my pedalboard and the amp is quiet when I activate/deactivate the onboard eq, same thing when I turn on/off the stomp boxes placed before the buffer. If I remove the buffer all pops.

If I engage the -15db switch on my amp it is deadly quiet. If I use it "normal" I have pops.

I measured the dc voltage in this way: a jack connected directly to the input of the amp. using a multimeter I connected one tip of the multimeter to jack tip and the other tip to the jack sleeve.

I read 130mV when amp is in normal mode (the case when it pops). 1.5mV when I press the -15db switch and 0mV when the amp is in mute.

Do you have any ideas about the problem?

V1 bad tube or anything else?

Here's the schematic of the preamp section

Thanks

 

[JimmyM]

Are you running your pedals in the loop or in line?

 

[mickbellinello]

Are you running your pedals in the loop or in line?

Pedals in line but it pop (got 4 bass with active passive switch and all of them pops) even using only the bass connected directly.

Got no pop using a buffer just before the amp but I don't think this is normal operation.
Another strange thing is that I read 130mV when the amp is set to normal (it pops) while I read 1.5mV when I press the -15db switch (no pop).

Can you help me?

 

[JimmyM]

Pedals in line but it pop (got 4 bass with active passive switch and all of them pops) even using only the bass connected directly.

Got no pop using a buffer just before the amp but I don't think this is normal operation.
Another strange thing is that I read 130mV when the amp is set to normal (it pops) while I read 1.5mV when I press the -15db switch (no pop).

Can you help me?

Sorry Mick, wish I could. Maybe someone else can but electronics aren't my strong suit. All I can tell you is that sometimes pedals will pop when you first plug them in and turn them on, but repeated clickings can stop it. But this is a problem I've never heard of. However, I'm sure when the engineer types wake up, someone will have more of an idea than me. And of course, you're always welcome to consult with Ampeg TS.

 

[mickbellinello]

Sorry Mick, wish I could. Maybe someone else can but electronics aren't my strong suit. All I can tell you is that sometimes pedals will pop when you first plug them in and turn them on, but repeated clickings can stop it. But this is a problem I've never heard of. However, I'm sure when the engineer types wake up, someone will have more of an idea than me. And of course, you're always welcome to consult with Ampeg TS.

Don't worry. I made by myself all stomp boxes and electronics so I exclude pedals or onboard eq problem. I think that the problem is the amp, and I'm very sad because I bought it brand new.

 

[JimmyM]

Don't worry. I made by myself all stomp boxes and electronics so I exclude pedals or onboard eq problem. I think that the problem is the amp, and I'm very sad because I bought it brand new.

Does seem so since they're fine with other amps, but I never had that problem with the 4 Pro and pedals so I just don't know...

 

[agedhorse]

This is not uncommon with a tube preamp that has this type of input stage. The D.C. offset at the input is often present due to how the electrons move within the tube. If your pedal switching does not account for this in the pedal design, you will get pops.

Study up on signal switching where there's offset and the fix will become obvious.

 

[mickbellinello]

This is not uncommon with a tube preamp that has this type of input stage. The D.C. offset at the input is often present due to how the electrons move within the tube. If your pedal switching does not account for this in the pedal design, you will get pops.

Study up on signal switching where there's offset and the fix will become obvious.

thanks! What do you suggest to do? I mean, in your opinion is it suggested to send it back for an inspection or do you think the amp is ok? Changing the v1 could solve the problem?

Thanks

 

[agedhorse]

Changing V1 might help, but IMO it's the combination of the way the amp is designed (nothing wrong with it) and the way your pedals are designed.

Whenever you switch any signal that contains any DC offset, and then amplify the switched signal, that discontinuity will also be amplified, This is what you are hearing. This is not going to be unique to this amp either.

I'm assuming that you understand this since you built your own pedals, so I am suggesting that you learn about how switching noises can enter your signal path, and see how your designs handle this condition.

 

[bobdabilder]

Anything more specific? I am interested but that seems rather vague.

Changing V1 might help, but IMO it's the combination of the way the amp is designed (nothing wrong with it) and the way your pedals are designed.

Whenever you switch any signal that contains any DC offset, and then amplify the switched signal, that discontinuity will also be amplified, This is what you are hearing. This is not going to be unique to this amp either.

I'm assuming that you understand this since you built your own pedals, so I am suggesting that you learn about how switching noises can enter your signal path, and see how your designs handle this condition.

 

[rickdog]

Let me guess - you're measuring -130 mV on the input (the voltage is negative). That would be due to electrons hitting the grid and having to go to ground through grid leak resistor R9 (3.3 MOhm). Ohm's Law says that's about 40 nA. I don't find that surprising. This is just a fancy way of stating what @agedhorse said about "how electrons move within the tube."

 

[mickbellinello]

Thanks for support. Have two more question.

why if i press the -15db switch the problem disappear?

is there some test to completely exclude amp malfunctions?

thanks so much

 

[agedhorse]

Because the pad reduces the voltage on the grid due to grid leak.

It also drops the input impedance below what most passive basses sound good with.

 

[mickbellinello]

Because the pad reduces the voltage on the grid due to grid leak.

It also drops the input impedance below what most passive basses sound good with.

Thanks so so much! The amp is under warranty and when i noticed the problem i contacted the supplier. I’m waiting for a reply but in case what do you suggest me to do in case they ask me to send it back for a check? I mean, i found a nonsense sending back an amp that according to what you say is “normal operating”? Thanks REALLY for Your support!

 

[agedhorse]

I would expect that you would be directed to an authorized warranty service center, along with your pedals so that the symptoms can be reproduced.

 

[mickbellinello]

I would expect that you would be directed to an authorized warranty service center, along with your pedals so that the symptoms can be reproduced.

Unfortunely i bought it from Thomann in Germany (i live in Italy) So the only option would be to ship it back. It’s why i asked.
The problem is present also with the onboard eq of my Basses (stock preamp) but.....seriously don’t know at this point

 

[agedhorse]

If it's what I think it is, it's due to the combination of the amp's input circuit design AND the other devices that you are using. It's possible to design this out from either the amp's input or the other device's outputs, but those are not my products and therefore not my job or position to say anything more about these products.

 

[mickbellinello]

If it's what I think it is, it's due to the combination of the amp's input circuit design AND the other devices that you are using. It's possible to design this out from either the amp's input or the other device's outputs, but those are not my products and therefore not my job or position to say anything more about these products.

thanks really so much! I really appreciate Your support! Very interesting infos etc. Thanks a lot!