Emulating Tech 21 Double Drive?

[unix-guy]

I want to try and emulate the Tech21 Double Drive pedal using the Drive blocks in the Axe Fx.

Trying to figure out a good starting point.

Some relative info on the pedal, which has a "Class A" drive and a "Class A/B" drive. The Class A drive is cascaded into the Class A/B drive.

GUIDE TO CONTROLS

DRIVE A

controls the amount of Class A power amp output stage distortion (like Vox AC30®-style amp),rich in even harmonics.The amount of harmonics is dynamically controlled by the input level.Therefore, you will influence the ratio of harmonics by how hard you pick.At minimum,the circuitry has no tonal effect on the sound and is clean.

DRIVE A/B

controls the amount of Class A/B power amp output stage distortion (like Fender®/Marshall®-style amps),rich in odd harmonics. At minimum,the circuitry has no tonal effect on the sound and is clean.

NOTEWORTHY NOTES & SUGGESTIONS
-The signal path of Driv e A runs into the signal path of Drive A/B.This results in an electronic
multiplication, rather than addition,of the amount of distortion to achieve a cascading effect. -If you have hot output pickups, you may experience a hint of distortion when the Drive
controls are at minimum.

I'm assuming that by stacking 2 Drive blocks and using the new diode controls in the Axe Fx that I can get close.

I found a schematic here:

http://www.axiomdrift.com/schematics/DD PN.gif



It's been a long time since I read schematics but it appears that Class A is Op-amp only while Class A/B with adds asymmetric clipping using 2 1N914 diodes on the negative side (I think).

Any suggestions?

 

[electronpirate]

Could you use the 'use an AMP block' as a drive in this case?

 

[unix-guy]

Could you use the 'use an AMP block' as a drive in this case?

Maybe?

To be honest, I don't know that the descriptions of Class A and A/B are really valid in regards to the distortion that is produced (relative to an Amp).

Definitely something to try... It might be interesting to try Amp (as Drive) -> Drive -> Amp

First thing I need to do is put that bad boy in the loop and then start playing around...

 

[unix-guy]

Hmm... So I tried to start into this today only to realize there is no option to have ZERO diodes on one side. You must have at least 1 diode positive and 1 negative.

Am I misreading the schematic? As far as I see there are only diodes on the negative side...

Maybe this is unique design?

 

[AlbertA]

Hmm... So I tried to start into this today only to realize there is no option to have ZERO diodes on one side. You must have at least 1 diode positive and 1 negative.

Am I misreading the schematic? As far as I see there are only diodes on the negative side...

Maybe this is unique design?

It looks like it uses a JFET as the clipping device for the + path?

 

[unix-guy]

It looks like it uses a JFET as the clipping device for the + path?

I just looked up the part TLC2262... It's an Op-amp. Or maybe you mean the TL072C? I see that is shown as a JFET Op-amp...

Mostly I'm just trying to figure out how I would similar diodes on only 1 side... Don't think I can.

 

[AlbertA]

So I would say try 2 1N914 on the - side, 1 FET type on the + side.

 

[AlbertA]

I just looked up the part TLC2262... It's an Op-amp. Or maybe you mean the TL072C? I see that is shown as a JFET Op-amp...

Mostly I'm just trying to figure out how I would similar diodes on only 1 side... Don't think I can.

Yeah the signals fed to the last TLC2262 go through JFETs (i.e. the circle with the gate and diode symbol) - should be a p-channel JFET.

 

[unix-guy]

Yeah the signals fed to the last TLC2262 go through JFETs (i.e. the circle with the gate and diode symbol) - should be a p-channel JFET.

Ok, you are probably right, I didn't follow the circuit all the way through. I'll give that a shot.

Although, if you look for the point marked as an A with a circle around it, I'm assuming that is also feeding "dry" signal through a JFET?

Also, interesting that the JFET on the A/B circuit follows the Tone pot.

I found only 3 or 4 Drives that are Op-amp based

 

[AlbertA]

Ok, you are probably right, I didn't follow the circuit all the way through. I'll give that a shot.

Although, if you look for the point marked as an A with a circle around it, I'm assuming that is also feeding "dry" signal through a JFET?

Also, interesting that the JFET on the A/B circuit follows the Tone pot.

I found only 3 or 4 Drives that are Op-amp based

Nah Never mind - I think it's just an output buffer and they must use the FETs as the switching element to implement bypass.

So looks like the first stage after the input buffer (i.e the one with Drive A control) is the one that clips the top part of the signal because it's biased through VB2. Then negative part is clipped through the second stage and the 1N914 diodes.

So back to square one, not sure how you can simulate this without having 0-diode options and for the first stage an additional option for hard clipping with some adjustable bias (like the fuzzes have)