TS808 with a full tone stack

theundeadelvis · July 07, 2011, 03:31:45 PM

Hi all! I'd like to build a TS808 clone, but I'd like the flexibility of full (treble, mid, bass) tone stack. I'm sure it's been done before, but searching the 32 million TS threads has proved to be less than fruitful so far.

Anyone have any suggestions? I'm assuming the TS has enough gain to accommodate a full tone stack, I'm just not sure the best place to insert it, and what the best tone stack to use would be without losing all of the lovely TS characteristics.

Thanks in advance!

CynicalMan · July 07, 2011, 03:54:57 PM

The best place would be after the 1k/0.22u filter. I'd throw in a FMV tone stack and then use the second op amp to make a recovery stage instead of the tone control.

One of the "lovely TS characteristics" is the fact that it has a big mid boost. The frequency response is a big hump. Why people are so crazy about this is beyond me, but I assume you'll want to preserve this response. FMV tone stacks have a mid scoop, so you'll want to get rid of that. I'd use the Duncan Tonestack Calculator and tweak values to get a tonestack with a flat response.

Another option is to use the second op amp to make an active equalizer, like this:

Just don't use the first op amp stage in that schematic. You'd also probably want to put a voltage divider before the equalizer so that it doesn't clip.

theundeadelvis · July 07, 2011, 04:08:56 PM

Thank you CynicalMan! That is great information you have provided. It's a little over my head, but I like to learn, so I'll do the homework to figure it out. I appreciate your help!

theundeadelvis · July 07, 2011, 07:05:16 PM

SO based on this schematic:

If I wanted to implement the tone stack in your image, would I replace the components circled in red with the components in the image you provided? If this is the case where would C2 tie in? Thanks again!

CynicalMan · July 07, 2011, 09:05:07 PM

Yup, put it there. C2 probably won't be necessary.

This is definitely one to breadboard first, though. I haven't tried it so I can't guarantee that it works. Also, you might want to add some attenuation on the input or change the frequency bands.

theundeadelvis · July 07, 2011, 09:32:14 PM

I think I'll build it using the tonepad layout but leave out the tone components. Then I'll build my tone stack separately (or maybe finally break down and buy that breadboard), so I can easily tweak it.

What do you mean by adding attenuation at the input? Do you mean at pin 5 of the opamp? Would this be as simple as adding a resistor? Thanks again!

theundeadelvis · July 07, 2011, 11:14:09 PM

OK, world's worst photochop, but here's what I came up with. Still not sure how to implement your attenuation suggestion, but I wanted to see if this is correct so far.

Thanks again!

CynicalMan · July 08, 2011, 09:35:01 AM

The + input should be connected to Vb. Otherwise that schematic is correct.

The diodes limit the output of the first op amp, so attenuation probably isn't necessary. The stock tone control doesn't have any, and you shouldn't have a problem as long as your preferences aren't too extreme.

I'd suggest making a few changes for guitar. The equalizer is made for headphones, so the bands are further apart than is necessary for a guitar's limited frequency range. I've played around a bit with it in LTSpice, and I'd make the following changes:

C3 - 10n
C4 - 10n
C5 - 10n
C6 - 3.3n
R7 - 100k

Also, you can more common resistor values (10k, 3.3k) instead of the weirder ones (11k, 3.6k) if you want.

theundeadelvis · July 08, 2011, 12:34:56 PM

Thanks CynicalMan for taking the time to walk me through this and running it through LTSPice. I have question. Can you explain why the input should now be connected at Vb? In the original it was connected at the junction of the 10k after Vb and 1k off of pin 1 of the first opamp stage. So I'm curious what changes using this tone stack? I'm just wanting to understand what's going on as much as I can as part of the learning process. Thanks again, your help is great appreciated!

CynicalMan · July 08, 2011, 01:05:10 PM

In short, the original tube screamer tone stage is non-inverting and the Baxandall is inverting. The original stage shifts the .22u cap and 220 ohm resistor from shunting the output of the previous stage to shunting the op amp's feedback loop. The Baxandall works by changing the impedance of the op amp's feedback loop, and by changing the input impedance of the stage. If it was non-inverting, it would have a 6dB boost, which isn't necessary or desirable. Also, an inverting amplifier can attenuate as well as boost, whereas a basic non-inverting amplifier has a minimum gain of 1.
Check this out: http://www.st-andrews.ac.uk/~www_pa/Scots_Guide/audio/part8/Page2.html

I was a bit unclear in my last post, but the + pin of the second op amp, pin 5, should be connected to Vb and not to the output of the previous stage.

theundeadelvis · July 08, 2011, 01:21:10 PM

Thanks for the explanation! So we would be looking at this then (if I understand you correctly):

CynicalMan · July 08, 2011, 03:10:20 PM

Not quite. You have the left junction of R3, R4, and R6 connected to Vb. Disconnect it from Vb and connect it to the junction of the 1k and 10k resistors.

theundeadelvis · July 08, 2011, 03:14:21 PM

Oops! So leave it like it was, but also connect Vb to pin 5. Like this?

CodeMonk · July 08, 2011, 03:35:57 PM

This has been on my "To Do List" for awhile.

theundeadelvis · July 08, 2011, 03:40:37 PM

Quote from: CodeMonk on July 08, 2011, 03:35:57 PM
This has been on my "To Do List" for awhile.

Me too. I'm hoping to get it on my "done list" soon!

I'm going to try to make a daughter board layout for the tone control that's an add-on to the Tonepad board. We'll see how it works.

CynicalMan · July 08, 2011, 04:56:53 PM

Like this:

theundeadelvis · July 08, 2011, 05:17:39 PM

Ahhhhhhhhhhhhhhhhhhhh! OK. Thanks again!

theundeadelvis · July 08, 2011, 05:26:44 PM

Well, here's what I came up with. You'd have to connect Vb to pin 5 on the TS board, then connect the appropriate pads on the daughter board to the corresponding places on the TS board. I have no idea if I have my pot orientation correct or not, and I definitely will need to double check everything.

*Just looked back at the TS board, and you'd also have to remove/cut the trace from the 1K/10K junction to pin 5 of the IC.

theundeadelvis · July 09, 2011, 04:40:48 PM

I hope to build this in the next week or two. Once I do, I'll report back with my results. Thanks again CynicalMan!

CynicalMan · July 09, 2011, 07:02:51 PM

I'm on vacation this weekend and I'm on my phone, so I can't see the pictures well, but I'll take a look on monday. Good luck.

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TS808 with a full tone stack