EMI/RFI suppression in tube amps

[GainFreak]

Hi,

I noticed that in Marshall AVT series amps extra measures are taken for EMI/RFI suppression at least compared to other amps. Not only at the input but on all line outputs as well. Similar arrangement can be found also in some hybrid distortion pedals like the HT-Dual for example. In tube amps usually the 68k grid stopper is supposed to take care of that. In some amps a single ferrite bead from input to the first tube grid is used.
In the AVT series input arrangement we have a common mode choke and a 3 terminal capacitor taking care not only for the active input but for the ground as well. I noticed that two legs of the second capacitor are actually shorted.
As I'm not very much into theory could anyone please shed more light on the subject and in case one decides to take care similar care for the EMI/RFI what to look for when choosing a common mode choke for the input and ferrite beads for outputs?
Thanks in advance.

AVT50 input section:

http://web.archive.org/web/200710151...vt50_60_02.pdf

[Steve Conner]

I believe these are there for two reasons.

1: Solid-state preamps are better than tube ones at rectifying RF and turning it into unwanted audio, so they need a more effective filter than the old 68k stopper resistor.

2: Leo Fender's amps didn't need to pass the IEC 54321whatever radiated immunity test.

3: Nor did they need to pass the ESD test, but they'd have passed it anyway: when was the last time you saw a tube destroyed by a static discharge? The input filters will help to take the edge off ESD transients.

When choosing a common-mode choke, identify the parts used in the AVT50 and order the same ones. Remember that any filter capacitors you put in the differential mode add to the capacitive loading on the guitar pickups.

I never really saw the point in ferrite beads for tube amps, they are about a factor of 50 less effective than the 68k stopper. I think the little ones that combine two beads with a three-terminal ceramic cap are potentially better, though: they're a 2-pole filter, so they roll off at twice the dB/octave of the old grid stopper.

[Enzo]

Yep, like anything else, amplifiers came through history.

Not only what we have heard, but Leo's grid stopper was also half of an input pad. That less sensitive input was 6db down.

[GainFreak]

Remember that any filter capacitors you put in the differential mode add to the capacitive loading on the guitar pickups.

Aren't pickups already capacitively loaded enough by the guitar cable which is 100-150pf per meter?

I think the little ones that combine two beads with a three-terminal ceramic cap are potentially better, though: they're a 2-pole filter, so they roll off at twice the dB/octave of the old grid stopper.

The 68k grid stopper maybe a good solution but it's known to add a lot of noise especially in high gain amps. So if one would like to omit it will the common mode choke or a single bead on the active end + this kind of 3 terminal capacitor do the same job or only either of them will be enough?

[Steve Conner]

Aren't pickups already capacitively loaded enough by the guitar cable which is 100-150pf per meter?

Yes, but those little 3 terminal caps come in sizes up to 10,000pF. Don't be installing one of those and wondering where your guitar signal went.

I think the 3 terminal cap is the most important part. (It already includes two ferrite beads, so you don't need a separate one.)

The common mode choke is for isolating the amp's two groundplanes from each other, as far as I know. It prevents RF from getting in through the change of ground reference. If your preamp is already grounded to chassis through the input jack, you could leave it out.

[GainFreak]

Yes, but those little 3 terminal caps come in sizes up to 10,000pF.

The ones used by Marshall are 47pf and without the built in beads.

The common mode choke is for isolating the amp's two groundplanes from each other, as far as I know. It prevents RF from getting in through the change of ground reference. If your preamp is already grounded to chassis through the input jack, you could leave it out.

I usually use plastic jacks and don't ground them at the input so I guess the 3 terminal with the beads will be OK.

[Steve Conner]

OK, if you have an ungrounded plastic jack, then you ought to use two of the 3-terminal caps as shown in the Marshall schematic. Feed the signal through one, and the ground pin of the jack through the other. Connect their ground terminals to the chassis near the input jack.

This will divert any incoming RFI onto the chassis before it gets into the preamp.

[GainFreak]

OK. Thanks.

[GainFreak]

It's may be a little bit off topic but since more and more adapters are the SMPS type now is it necessary to take any additional measures not to let high frequency garbage into the sound path when using them with stompboxes and hybrid/tube designs?

[Steve Conner]

Yes. The simple answer is don't use the SMPS ones.

Many SMPS have noise output in the audio band, which RFI filters do nothing to remove. The RFI output from them is already suppressed to meet EMC regulations, or they couldn't legally be sold.

[GainFreak]

The simple answer is don't use the SMPS ones.

That's what I do.

Many SMPS have noise output in the audio band, which RFI filters do nothing to remove.

If you can hear it it's good. AFAIK they work at around 100kHz which is between the audio band and RF. I guess there isn't much you can do about that noise.

[kg]

op amp first thing after the input jack?

icky!

[Enzo]

I think we shouldn;t confuse potential for noise of an SMPS with an actual noise problem. AFter all, plain old vanilla linear wall warts can add tons of hum and noise too. Find out if your SMPS is making noise into the system before worrying about the noise.