What makes a "touch sensitive" pedal touch sensitive?

[sjturbo]

I would like to understand what type and what in the circuit makes it touch sensitive? I have built a ROG umble and would like to improve the touch sensitivity of it? If that is not possible suggestions on a low gain touch sensitive circuit would be apprecitated!!

[phatt]

Hello sjturbo,
that comes close to the most inteligent Q you could ask.

Yes correct today with so much focus on more and more gain, dist, OD, wanker drive.
Then there is no chance of getting touch response.

Reason is actually quite simple Whether you use a circuit that runs on 9Volts or a Valve rig with a garzillion Volts the principal is the same. Send a signal bigger that the power supply and it will no doubt clip/compress or limit the swing. Which is simply distortion of the original waveform.

In the case of Valve Amps just send a bigger waveform and it will compress the output and due to the High voltage design (plenty of headroom) it's not that hard to make it dynamic. That is one reason why tube amps are so desired.

Though Once you introduce more and more preamp gain more often than not the signal is slamed long before it reaches the output stages where nice things used to happen with Valve amps.

SS Poweramps are almost always devoid of any compression so it all has to be designed in a different manner.

When it comes to SS circuits or pedal power (i.e. runs on 9 volts) you have to be very carefull with design. It's actually debateable as to whether using diodes to clip or just letting the signal slam into the supply rails is any better that each other. (Example; Teck21 Sansamp just slams into supply rails (no Doides) and it was a huge success,, go figure)

Yes with carefull design you can indeed make opamps (even at 9 volts) work in a similar way to the most revered Tube amps of yesteryear.

It's a mugs game when it comes to low voltage circuits as you do need to consider the Headroom you are working in. for starters,,You just have to stop hanging 1 meg resistors off everything in sight assuming mojo will be forthcoming.

Rather that writing stuff that will quite likely end up causing arguements with teckno geeks who know more maths than me,, just go to SS Guitar and searh for some of my Circuits. Search for *phatt* schematics or just search for PhAbbTone, DDC,, Maxiverb.

My DDC circuit by itself is nothing stunning but used in conjunction with the matching tone circuit is likely to be very rewarding if you want touch from an all SS circuit.

The magic trick is to get each stage working at just the right amount of swing so that by the time it gets to the output it's all just waiting for a little more attack from you the player and it goes into clipping.
If you are chsaing that SRV type clean compression then YES my designs will certsainly come close to that kind of dynamic response.

I build Valve stuff as well so I do know what works and what is just plain wank.
Cheers Phil.

note; If there is some interest I'll repost some schematics here as well ,,,
but as it's a part time hobby it's hard to keep up with a lot of different forums.
Phil.

[fosnal1950]

When it comes to headroom , the new TC electronic pedals claim to been an output 3 to 4 times the voltage a regular pedal has thus increasing the headroom.

Doesn't a touchsensitive pedal have most of its distortion ( clipping ) going on in the last stage ? Or is this a misconception ?

[sjturbo]

A big fat Thanks goes to phatt for a good explanation to what was apparently a good question. Based on all of the feedback I guess I was the only one in the dark.

[RnFR]

yeah, I think Phil was right when he said it was basically a balancing game of trying not to make each stage do too much. it's really an additive thing, where you hope to end up with the right combination of of gain in the correct frequencies. you can't slam it all to death with gain, whether it's 1M feedback resistors, or 100uF bypass caps. you can see where I attempted something along those lines here(albeit with a discrete platform).
http://apocalypseaudio.blogspot.com/201 ... drive.html

[MoonWatcher]

...basically a balancing game...

Yep - it all comes down to balance. Touch sensitive is mainly the ability to stay linear with smaller signals, and just clip appropriately when you breach that barrier with more heavily picked notes or chords. It has to be balanced to continually transition back and forth between the two states.

Designers love to rag on clipping diodes, but they serve a critical purpose for many of these designs since they can clamp the signal adequately over a range that's hard to approximate with only transistors, especially when you realize that guitars with all sorts of differing pickups (higher or lower output) will be plugged into it. Diodes also provide an opportunity to tweak the compression effect to taste (by the pedal designer).

More voltage before the circuit slaps the rails isn't always the answer. With something like jFET's you can end up dealing with transconductance issues even if carefully biased to run at 18VDC. I've also noticed that many jFET based circuits tend to run those dual low pass filters on the output - lots of upper order harmonics that have to be attenuated or you get that "chewing on tin foil" effect.

I also think that if you exaggerate that high freq rolloff effect in spite of whatever preamp topology you choose AND provide enough of a voltage gain that you can get the right treble balance in spite of the unorthodox approach.

Since solid state devices will never behave like tubes/valves, it requires an approach that works within the confines of their physical operation.