Finally attacking TSL100 channel switching latency

[jamesmafyew]

I've seen posts on several different boards with people complaining about the "volume ducking" or "switching latency" or "reverb fade in/out" on the DSL and TSL series of amps. No one seems to have found a definitive solution for this though, that I know of. If they have, please disregard the rest of this post and tell me where to go read!

I have heard people say that it's a problem with the logic circuits in the footswitch. Well... I've looked at the schematics of the footswitch and can't seem to find a reason why that would be. Furthermore, when I use the buttons on the front of the amp, it still has the latency. I understand that Marshall designed this in as a "feature", and from what I can tell it's not built into the footswitch.

One person modded his DSL 401 to get rid of the ducking by clipping two transistors. He had a website up here which doesn't seem to be working anymore. I do remember though that he clipped T15 and T16, which are clearly labeled as "volume ducking" in the schematic (see attached images).

Essentially, I'm trying to figure out which transistors are analogous to these two in my TSL 100. I have figured out that TR3 on the TSL's main board is analogous to T15 on the DSL schematic, but I can't figure out what would correspond to T16. Could it be TR1? Is there something else I'm missing? (see attached image)

Would there be harm in taking TR1 and TR3 out to see what happens?

Full TSL 100 schematic is here, DSL 401 is here.

I'd appreciate any help in getting this figured out.

[Gregg]

Most of the amps especially higher gain ones these days have a dedicated mute circuit that will ground the signal for a short time while the realys are switching to avoid the usual clicks and pops they are producing.

Full TSL 100 schematic is here, DSL 401 is here.

This is not the ful TSL100 schematic. Get all the files from DrTube and on one of the PDFs - tl10-65-02.pdf you'll see two J174s - FET1 and FET2. They are part of the mute circuit.
Usually ou should look for P-channel FETs as J174 or J175 in parallel with the signal somewhere around the master volumes, before the FX loop, or after that.
Also the NSL32 is not very fast switching LDR and there's plenty of them in the schematic.
Be prepared however to "get" some pops after you "cut" the transistors in question.

[jamesmafyew]

I'm not looking to disable the mute circuit entirely, more just get rid of the annoying fade/swell when changing from the clean to either of the OD channels and vice versa... and so trying to replicate the aforementioned DSL 401 mod on my TSL. From what I can tell with my ears and looking at these two schematics, doing that would only require changes around the reverb return, or near CON7 on the TSL mainboard schematic.

It looks to me now like TR3 may be doing the job of both T15 and T16, since the TSL's FX loop and reverb structure is different from the DSL 401. I think one of the transistors in the DSL is for the reverb, the other for the effects loop. The TSL has them arranged differently, and appears to send the reverb signal out the FX send (I know this from using a slave amp hooked up to one of the FX sends, not from the schematic). So one transistor would mute/fade them both at their return point...?

However, given that my issue only happens going to or from the clean channel, these two transistors you're pointing out look like they're worth some attention. I may just have to try several different options and see what works.

Thanks for the pointer!

[jamesmafyew]

To follow up on this:

I took out all three transistors in question --- TR3 (main board) and FET1 and FET2 (clean board). As far as I can tell, the change has had almost no effect at all. I took them out one at a time and listened to the relative change... but there's not much difference. No new pops or noise were added. If anything, the switching is more stable/predictable, but still essentially the same problem.

I've always had a small pop when changing from clean to OD1 or OD2, but that change is always instantaneous. The only change that is NOT instantaneous is the change from either OD1 or OD2 BACK to the clean channel. It has an annoying gap in the sound and then the volume fades back up over about 200ms to the full clean volume. The problem gets less severe the longer the amp has been on.

All three of the removed transistors had 22uF caps on their gates, which led me to believe that they were collectively somehow involved in the "fading in" effect -- but that's apparently not the case. Could this "fade-in" effect just be a limitation of the NSL32 opto-isolators? If so, why would all switches except OD1/OD2 -> Clean be instantaneous?

Then again, there are two more P-channel FETs that I haven't touched -- TR1 and TR2 on the main board (see 3rd pic in original post). Could either/both of these be responsible?