A Transparent Overdrive Voicing (alternative to TS-9 mid hump)

[Steben]

And now combine this with single coil
, P90 and humbucker specific characteristics.... and none of the pedals are universally perfect.

[GGBB]

If the goal is "transparent voicing" and that means no change to the tonal balance, then shouldn't the post gain shaping merely be an inversion of the pre gain shaping? We see this used in chorus/modulation circuits - kind of standard topology really:



Why don't we see this in overdrive circuits? I suppose gain controls complicate matters, and not all ODs have two stages, but I suspect nobody really likes - or at least wants - genuinely "transparent" overdrives. I think the term "transparent overdrive" is kind of silly, especially when people use it to describe pedals like the Bluesbreaker, which IMO isn't even close to being transparent and is actually very deliberately not tonally neutral.

I think it's easy to forget that controlling bass pre gain/clipping isn't about achieving tonal transparency in most designs - it's about the character of the distortion itself I think. Take for example the Bluesbreaker again. I use 33n instead of 10n for the lower feedback filter cap - not really to make it more tonally transparent by reducing bass cut, but merely to make the overdriven effect sound fatter. I like this much better than the stock Bluesbreaker. Adding back the same amount of bass _after_ clipping would not sound the same and would not be interesting to me even though it might actually be closer to being tonally neutral.

On the other hand, my RAT mod has a different goal. It tries to _not_ change the character of the distortion, but merely add low bass. And so it takes a different approach than what I do with the Bluebreaker - it raises gain of the lower frequency shelf _without_ changing the knee frequency of the feedback filter. Deeper bass frequencies are not added (except by way of the 1st order roll-off slope) - only existing frequencies are boosted in volume. The Bluesbreaker mod and most of the discussion here is around extending low frequencies _without_ changing overall gain.

So two different approaches to achieve two different goals - but both done pre gain/clipping - neither actually concerned with tonal transparency.

We think we want tonal transparency because that's more "amp like" (to use another silly misapplied term). But amps are not tonally transparent they are deliberate tone shapers. What we actually want is to not colour the tone of our amp. But the amp changes its tonal colour when gain and distortion increases, so why shouldn't a dirt pedal do the same? That would truly be more "amp-like". And that's exactly what most dirt pedals do. Forget about the quest for a "transparent overdrive" - just make/use something that sounds good however neutral or not it is.

[fryingpan]

As GGBB said, a truly transparent overdrive would be one with, essentially, pre-emphasis and de-emphasis after the distortion. It makes sense, and in fact I am designing one such circuit (for guitar and bass, where more "transparency" may be more welcome). By dialing the pre-emphasis in a way that you essentially "equalise" the frequency amplitudes incoming into the pedal to a certain degree, and then by dialing the de-emphasis to go back to the original frequency content distribution (or closer to it anyway), you ensure that you minimise IMD where it is most objectionable (in the bass frequencies) while maintaining some tonal consistency between dry and wet. But, as GGBB said, then some sort of tone shaping would probably still be welcome, which is why I will be adding a Big Muff tone control (both with a mid-scoop and a mid-hump, selectable).

[Rob Strand]

Some of these discussions are getting outside of the motivation for the thread (and also Mark's thread).   The main idea was to have a switch option to have a more transparent sound than the TS-9 voicing.   Transparent is "as deemed by the masses" based on pedals people think are transparent pedals (and those similar).   All I've done is extract an average "transparent" voicing, which is noticeably different,  then made the level sound close to the TS-9 when the network is switched over.

As GGBB said, a truly transparent overdrive would be one with, essentially, pre-emphasis and de-emphasis after the distortion.

Not really.

Why don't we see this in overdrive circuits?

Because when the clipper clips it no longer sounds transparent.   It becomes more bass heavy, woofy, indistinct - not really transparent to the ear.  This is something I spent a lot of time on in the 90's.

One reason why this for that is a guitar signal has stronger a fundamental than the harmonics.   Imagine the guitar waveform as a high level low frequency fundamental and lower level harmonics riding on top of it.   When the waveform clips, largely due to the larger fundamental,  it almost completely blocks the harmonics.   So the high frequency clarity and is lost.     No matter what you do with EQ you cannot restore that information.  (There's actually a little more to it.)

It's not just the harmonic pattern, you get problems with chords as well.   This a second effect where lower frequencies have a tendency to win in the battle to pass through the clipper.

What the pre-emphasis (or bass roll-off) network does is makes the higher frequency louder before they pass into the clipper.   This prevents the low frequency entirely blocking the high frequencies so more make it through.   When the different frequency signals pass through the clipper their original level is lost.   The de-emphasis network does not restore them back to normal.

Obviously you would expect low amounts of overdrive to approximate the transparent sound as the network responses restore to flat.    That does show up in practice.

Back the days of radio and telephone people use to research the intelligibility of clipped voice.   The signal from the vocal chords (voiced speech) contains strong low frequency information (like a guitar) which blocks the airy sounds (unvoiced speech).  If you have ever listened to heavily clipped speech you hear the frequency of the distorted vocal chords but all the airy sounds are lost and masked by distortion.  It's very muffled and indistinct - and hard to understand.   The first section of this link gives a breakdown a voice signal in the context of vocoders.
https://www.cim.mcgill.ca/~clark/nordmodularbook/nm_speech.html

Here's a simple example of blocking,



Spectrum:

1kHz is largely gone, see 900Hz and 1100Hz at low level.


1kHz largely passes if the 1kHz waveform is phase-shifted by 90 degrees: (watch-out interpreting this  )

[Steben]

What about splitting the original signal, add high pass and add it to the low pass distorted signal?

[ElectricDruid]

What about splitting the original signal, add high pass and add it to the low pass distorted signal?

Ah, the "multi-band distortion". Like most good ideas, it's been tried several times. Here's a thread discussing it:

https://www.diystompboxes.com/smfforum/index.php?topic=107155.0

As usual, Craig Anderton has been here before us:

https://paia.com/quadrafz-design/

HTH

[Steben]

What about splitting the original signal, add high pass and add it to the low pass distorted signal?

Ah, the "multi-band distortion". Like most good ideas, it's been tried several times. Here's a thread discussing it:

https://www.diystompboxes.com/smfforum/index.php?topic=107155.0

As usual, Craig Anderton has been here before us:

https://paia.com/quadrafz-design/

HTH

Yes, but only distorting a midband boosted and adding clean highpass.