Tech21 double drive - clipping curiosity?

[Govmnt_Lacky]

Would definitely reconsider the 2.2uF as film. Although they might be attainable.... not as easy as 2.2uF electros. Or, possibly make the lead spacing for back-to-back 1uF electros a'la the way they do it in some LFO circuits 

[chi_boy]

Would definitely reconsider the 2.2uF as film. Although they might be attainable.... not as easy as 2.2uF electros. Or, possibly make the lead spacing for back-to-back 1uF electros a'la the way they do it in some LFO circuits 

Could add an extra pad so it is builders choice. 

Could C6 and C8 be subbed with 1uF?  Any thoughts?

[midwayfair]

Would definitely reconsider the 2.2uF as film. Although they might be attainable.... not as easy as 2.2uF electros. Or, possibly make the lead spacing for back-to-back 1uF electros a'la the way they do it in some LFO circuits 

Could add an extra pad so it is builders choice. 

Could C6 and C8 be subbed with 1uF?  Any thoughts?

Make the level pot 250K and 1uF will be virtually identical.

[Govmnt_Lacky]

I smell a PCB layout a'comin!!!! 

[slacker]

Are 2u2 hard to get in film? I can get them just as easily as electros, more expensive but no harder to get, I'd have thought anywhere that will sell you a TLC2262 would sell suitable film caps.

You could probably get away with C6 as 0.47uF might roll off just a little low end, or parallel two of them to get about 1uF that should work fine.

[Pojo]

I haven't seen any film caps bigger than 1uf myself...and even 1uf are physically about as big as what I'd want to put in a stompbox....so I would prefer going with the 250k level pot and a 1uf....or use a 2u2 electro and determine the correct polarity.

"Would it be possible to achieve the asymmetrical hard/soft clipping using discrete components? Or any other ideas to accomplish that general idea?

I was thinking about using the cascaded jfets which gives a more pleasant hard clipping to my ears, and biasing them so that they clip only one side. I imagine I would have to alternate the drain voltages on each fet because each stage inverts the signal. For example, a biasing scheme using 3 stages with a 9v supply that would have the drains at something like 7v, 2v, 7v respectively. Not sure how I would go about the soft clipped side...maybe just copy the opamp scheme used here. Thoughts?"

^^ Bumping what I wrote earlier in hopes somebody has something to add here. I'm not a huge fan of opamp clipping. 

[chi_boy]

I smell a PCB layout a'comin!!!! 

Does it smell like Osh Park or is it more like a home brew kind of aroma?

[chi_boy]

Would definitely reconsider the 2.2uF as film. Although they might be attainable.... not as easy as 2.2uF electros. Or, possibly make the lead spacing for back-to-back 1uF electros a'la the way they do it in some LFO circuits 

Could add an extra pad so it is builders choice. 

Could C6 and C8 be subbed with 1uF?  Any thoughts?

Make the level pot 250K and 1uF will be virtually identical.

So then to use a 1uF for C8, would R12 need to be about 220k?

And what does the math look like for the level pot?  I'm not seeing it, and not sure what to look for.

[Govmnt_Lacky]

Does it smell like Osh Park or is it more like a home brew kind of aroma?

It has the strong smell of FR4 dust with a slight hint of fabbed PCB mixed in. Why not both? 

[duck_arse]

seeing as the tlc2262 is a rail-to-rail cmos type, will replacing it with a crummy old tl072 have any impact on the clipping section?

[thelonious]

Re the 2.2uF film caps - I've had good success with the 10mm and 5mm Kemet 2.2u film caps from Mouser. They are big, but if you want all film they're one of the better options out there. The 10mm is a longer form factor and the 5mm is a taller form factor.

[DrBoogey]

Hi all!
I took my double drive apart some years ago and found two 3.3V zeners in the clipping section.
So i think the schematic in the fist post is not correct in the third opamp section with the two 1N914. There should be two 3.3V zeners there as found in the tech21 character series (there are some schematics around in thenet).
I had the circuit on the breadboard then and the sound was identical to the original. I used all TLC2262 opams.
I also found two aditional resistors in the power supply section. One in the positiv feed and one in the return line to GND pin, as i remember, both were 330 Ohm.

cheers

[slacker]

Usful info, cheers for sharing.

[thelonious]

two 3.3V zeners in the clipping section.

So would that clip one side of the wave at ~1V or so just like the 1N914s in series, and clip the other side of the wave at 6.6V?

[chi_boy]

Re the 2.2uF film caps - I've had good success with the 10mm and 5mm Kemet 2.2u film caps from Mouser. They are big, but if you want all film they're one of the better options out there. The 10mm is a longer form factor and the 5mm is a taller form factor.

These look like they would work.  $0.78ea.  Not cheap, but not a deal breaker either.

Mouser Part No. 80-MMK5225K63J06L4

http://www.mouser.com/ProductDetail/Kemet/MMK5225K63J06L4BULK/?qs=sGAEpiMZZMv1cc3ydrPrF3QBXSIrGKcEvnngee94CYg%3d

[slacker]

I use these http://uk.mouser.com/Search/m_ProductDetail.aspx?WIMA/MKS2B042201F00KSSD/&qs=sGAEpiMZZMv1cc3ydrPrFwgzP98AO9idQxtZeVFQ89g= 5mm lead spacing, they're about the same size as say a 100uF electrolytic cap. Don't think I pay as much as mouser wants though.

[chi_boy]

I saw those too, but the Kemet's were lower cost.  The Wima's would fit too.

So this is the latest version based on all the input.  Highlights include TLC2262 for both IC's and space for film caps with 5mm spacing.  The board is big, fat and lazy but it will fit in a 125B.  I used the outline and control layout from a different project, so that part should be fine.  And it's double sided, so this is the Osh Park flavor.

[midwayfair]

two 3.3V zeners in the clipping section.

So would that clip one side of the wave at ~1V or so just like the 1N914s in series, and clip the other side of the wave at 6.6V?

No; put them opposed and in series.

A > C > C > A

When the wave swings one way, the current "sees" 3.3V from the first Zener and the normal .7v from the Fv of the second. When it swings back, the 3.3V of the second Zener is used now and the first Zener's Fv of .7v is used. Result is 4V Fv in both directions!

This is a VERY high threshold, symmetrical clipping arrangement. FWIW, you can use MOSFETs the same way and get high threshold soft clipping.

The two 1N914s as shown will only clip one side of the wave, but the two Zeners running at 9V might not add any audible clipping with certain op amps. At 9v supply, you're likely to hit the power rails on many common op amps on at least one side before you get to 4V.

I'm pretty sure DrBoogey is right; I can't see a manufacturer using the arrangement shown with 1N914s. It's just weird. Using Zeners like that, though, is a known trick.

Chi_Boy: The calculation for the volume pot and decoupling cap is a normal RC high pass filter. http://sim.okawa-denshi.jp/en/CRhikeisan.htm

[DrBoogey]

two 3.3V zeners in the clipping section.

So would that clip one side of the wave at ~1V or so just like the 1N914s in series, and clip the other side of the wave at 6.6V?

No; put them opposed and in series.

A > C > C > A

When the wave swings one way, the current "sees" 3.3V from the first Zener and the normal .7v from the Fv of the second. When it swings back, the 3.3V of the second Zener is used now and the first Zener's Fv of .7v is used. Result is 4V Fv in both directions!

Yes, you are right in theory, but the threshold voltage is current dependent, as there is little current in the feedback of the opamp, you get about 3V clipping in each direction.
As I can say again, in the original double drive, there are 2x 3.3V zeners instead of 2x 1N914 wired as midwayfair described.

[Pojo]

I thought the whole point was to only have half the waveform being clipped in that stage? With hard opamp clipping in the previous stage taking care of the other half of the wave?