FMV Tonestack help please!

[Willybomb]

Gudday all.  I tried to make a Tonemender based on a vero layout I saw kicking around, but it's been consigned to the fail bucket - for some reason I had tubescreamer levels of dirt going on underneath everything, and some weird ossilation on the pots when they were turned...  The volume pot just increased the dirt when it was turned down.  I couldn't find the problem so I just ripped off the tonestack schematic and found a few values from around the place to make my own.

It'll go in a box with some sort of boost afterwards to bring the volume back up but currently it seems to have some strange behaviors in that the pots act more like volumes than EQs per se (although I understand that the FMV tonestack is a bit counter intuitive in how it works).

Anyway, can someone have a look at this for me and tell where, if anywhere, I'm going wrong.

Thanks,
Willy.

[RobA]

The tone stack is really influenced by the input and output impedance that it sees. That could be part of the issue, especially if you are feeding it into a low impedance stage after.

I don't understand the 100k resistor to ground on the right of your schematic. That isn't in the "standard" FMV  type tone stacks that I know of.

[Willybomb]

Well, it's there because it was in the Tonemender schematic!

I'm having a play with the Duncan tonestack calculator at the moment and the graph seems to mirror what I'm hearing.  *shrug* I dunno.  Anyway, here's the updated vero + schematic - is there anything fundamentally wrong?

[RobA]

It all depends on what you are looking for tone wise. But, with the values you've got in there the mid cut is sitting at about 170Hz and the bass control is mainly working below 80Hz. Overall, that's going to make the majority of your pots' action working as a bass doing very little, mid being a volume knob, and the treble working as a treble boost when pegged. And that's before you look at what happens with your input and output impedances. 

What are you running this into for the next stage? The Tonemender circuit depends on the input buffer and the input impedance of the op amp amplifier after the TC.

In the Duncan calculator, you can change the Zsrc and the R(load) and see what happens with these.

I'd start with the Fender values that are the defaults and then tweak from there. The Marshall TC is fairly useless in my opinion and I say that as someone who prefers Marshalls over Fenders.

[RobA]

One question on what you described from the Tonemender when you had the whole thing together is which op amp did you use? The give a few choices, but the TLC2272 is going to give you a much better chance at getting high gain with a clean signal than the other two. I'd kinda expect the 5532 and 072 to distort at high gain in that circuit. I haven't actually built it, so I don't know for sure, but it does look like it would to me.

[Willybomb]

I had a TL072 in there at first, but there was all sorts of horrible click/popping from the pots, presumably.  Chaning to a 4558 fixed that but I still had this underlying overdrive type sound going on underneath.

Edit:  BTW, yeah, I'm going to move a swap the 47 and 100 caps around and change it to the fender tonestack, looks much more usable.

[RobA]

The 4558 is going to distort at gain for sure. It'd probably be a nice distortion, but it's going to distort, especially if you've got strong pickups and/or some gain from pedals in front of it. A rail-to-rail output op amp like the TLC2272 or a TS922 would really help.

Along with swapping the caps around, you should consider moving to the pot values and laws that the Fender TS uses. They have a huge impact on the sound and the feel of the controls.

[Willybomb]

Yeah, it wasn't a case of distortion at gain AFAIS, I was just running a guitar straight into it, nor was I running it that hard, and it was also present with the tl072.  Anyway, it's the first thing to go into my new "box of fail".

However - the stockstack circuit is working as expected now.  Keeping in mind what you were saying about impedence, I plugged it into my SHO clone before the powered PA speaker instead of directly in.  I also swapped the 100n and 47n around (making it more similar to the fender stack) and and suddenly it's working as advertised.

What was interesting was that before using the boost, turning the treble pot to the right attenuated pretty much everything.  It was more like a volume control and I had pins 1 and 3 swapped to bring the volume up when turning it to the right (as it was dropping the volume).  Hooked it up to the boost and I had to unswap 1+3 so that turning to the right increased the treble as you'd expect.

To this end I put together the AMZ miniboost from here: http://tagboardeffects.blogspot.com.au/2013/01/amz-mini-booster.html to put behind the tonestack, and promptly found it wouldn't work.  Dunno why, I've checked and tripple checked my vero layout.  Maybe I fried the jfets - I'm replacing them tomorrow and we'll see what happens.

[Willybomb]

I've got a working Klone buffer kicking around.  If it's just a case of having a high impedence on the output of the tonestack, would putting the buffer there work (instead of the boost circuit)?

[RobA]

The TL072 is going to be nearly as bad as the 4558 at distorting in this circuit. Still, it sounds like there may have been something else wrong too.

Do you mean the Mini-Booster didn't work at all or just not with the tone stack? I don't know why the AMZ Mini-Booster wouldn't work with the TS if it is working on its own.

The tone stack will cause some drop in gain that you probably want to make up for with some sort of boost after, but just getting the frequency response to work right only needs a high input impedance buffer stage. The other thing ti consider is that it also needs a low output impedance from whatever comes before it. That's why in the Tonemender they have the buffer before and the boost after. The other thing the pre-buffer does is bring the signal up to Vref and decouples the TS from any capacitance that would be present in whatever comes before the Tonemender. You don't have to do it that way, but it can help in some situations.

[Willybomb]

Nah, the mini-boost wouldn't work at all.  I didn't get to pair it up with anything, but that's an issue for another day.

Thanks though, I was going to put the buffer last in this build, just before the master volume pot, but it makes more sense to put the buffer before the tonestack, and then follow that with a boost.  The cab sim I've build has a volume control but I want to make the cab sim switchable out of the signal chain, so I'll need a separate volume pot.

Anyway, the origianal issue is solved.  Anything else after that is gravy, and probably needs it's own thread.