Film capacitor vs tantalum for ADHD (OCD)

[eh la bas ma]

Hello,

I am building an OCD clone, I am not sure on which version of the unit it is based, I read the v.2 on the pedalpcb forum.

https://www.pedalpcb.com/docs/ADHD.pdf

The kit from musikding includes only film caps, for all unpolarized caps in the circuit,  and I would like to know if it is a good choice ? Maybe there is one or two caps that better should be tantalum, for a better sound quality ?

I wonder if a diode switch mod makes sense on this circuit (symetrical Led, for exemple) and, if it is, where should it be ? I am guessing D6 but i read on this forum "The type of mosfet has nearly zero influence on the sound, only the internal protection diode is used for clipping." on this thread https://www.diystompboxes.com/smfforum/index.php?topic=53004.40  (I don't know which version they were talking about).

Thank you for your help.

[Bunkey]

Hi,

As far as audio reproduction goes; film caps offer better sound quality than tantalums. Tantalums generally have more distortion and are considered inferior to film in HiFi applications, whether they impart a particular character to the OCD pedal that's favourable I don't know - something to experiment with.

AFAIK tantalum was originally used where high capacitance was required in small packages, as an alternative to electrolytic. These days electrolytic is generally favoured over tantalum where audio quality is a concern.  It may be that tantalums have a particular sound but if your idea of better sound quality is transparency then film capacitors are the better cap.

Maybe someone who's built an OCD with tantalums can be more specific here.

[Bunkey]

PS. Diode D6 looks like a zener for biasing the base of Q5.

The actual clipping is, as far as I understand from the brief exchange between John and Aron in the thread you linked, a product of the internal diodes in MOSFETS Q2 & Q3.

You could switch between this mosfet clipping pair and diode clipping if you wanted to make the pedal more like a Tube Screamer.

[idy]

The way your schematic shows, the MOSFETs are acting just like two silicon diodes. One of those funny things, why would a smart designer use a component in such a way that adds nothing to the design but..mojo. Some models of OCD "fix" this.

The original I think played with different combinations with G and S together.

If you put D and G together and keep S separate, and then put them in series (either way, S to S or the other four legs together) you get in each direction a body diode + a MOSFET. You could also get this by putting them in parallel with regular diodes "pointing away" form the S's. But why waste two diodes?

I have two favorite asymmetric uses: just use one, D AND G together, S alone. Now you get diode in one direction, MOSFET in the other. I also put a diode with the band away from the source. Now I get MOSFET + diode in one direction and no clipping (or opamp clipping) in the other. Easy to put both (and a few others!) on a toggle and experiment.

[eh la bas ma]

Thank you for your replies,

"AFAIK tantalum was originally used where high capacitance was required in small packages, as an alternative to electrolytic. These days electrolytic is generally favoured over tantalum where audio quality is a concern.  It may be that tantalums have a particular sound but if your idea of better sound quality is transparency then film capacitors are the better cap."

Ok, so I have my answer on this question, thank you Bunkey.

"The actual clipping is, as far as I understand from the brief exchange between John and Aron in the thread you linked, a product of the internal diodes in MOSFETS Q2 & Q3.

You could switch between this mosfet clipping pair and diode clipping if you wanted to make the pedal more like a Tube Screamer."

Thanks, but i already have one, with 3 mods. I think I'd like to keep the mosfet clipping and try to add Leds or ge diodes before or after these mosfets, if this makes any sense ? I even have a roto-tone to make it easy with the testing: https://guitarpcb.com/wp-content/uploads/2018/06/The-AMAZING-ROTO-TONE-DELUXE-Final.pdf
I just don't really know how to make good use of it in this circuit.

The way your schematic shows, the MOSFETs are acting just like two silicon diodes. One of those funny things, why would a smart designer use a component in such a way that adds nothing to the design but..mojo. Some models of OCD "fix" this.

The original I think played with different combinations with G and S together.

If you put D and G together and keep S separate, and then put them in series (either way, S to S or the other four legs together) you get in each direction a body diode + a MOSFET. You could also get this by putting them in parallel with regular diodes "pointing away" form the S's. But why waste two diodes?

I have two favorite asymmetric uses: just use one, D AND G together, S alone. Now you get diode in one direction, MOSFET in the other. I also put a diode with the band away from the source. Now I get MOSFET + diode in one direction and no clipping (or opamp clipping) in the other. Easy to put both (and a few others!) on a toggle and experiment.

There isn't any ge diodes in my build, I would be curious to hear what it sounds like, and I've got a few spare diodes available (ge, bat41, Leds...) . If I understand you correctly, I should connect a switch  with different kind of diodes somewhere ? like on this exemple :


If D6 zener is biasing something, I should put the switch on D1 ?
By "D" do you mean Led ? By "away from the source" do you mean D1 output ?

[Rob Strand]

"AFAIK tantalum was originally used where high capacitance was required in small packages, as an alternative to electrolytic. These days electrolytic is generally favoured over tantalum where audio quality is a concern.  It may be that tantalums have a particular sound but if your idea of better sound quality is transparency then film capacitors are the better cap."

Ok, so I have my answer on this question, thank you Bunkey.

One major difference is tantalums tend to last a lot longer than Electrolytics since they don't dry out.   One caveat being tantalums across power rails can fail over time (say 10 to 25 years).    They also fail short which is a real pain to debug when you have 10+ caps across the power rails on a large board; when one fails the others will fail soon, so you just replace the lot.

Another difference is tantalums have lower leakage than electrolytics.  Not such a big deal for audio but for averaging circuits in test instruments the leakage can introduce measurement errors.    You can get low leakage electros.

[eh la bas ma]

Ok, so there isn't any interest to choose tantalum over unpolarized film caps. Only with electrolytics...

[Rob Strand]

tantalum over unpolarized film caps.

I'd try to choose films over tants but when you get to larger values the size and cost of films definitely pushes you to choose something else.

For SMDs in the range 1uF to 10uF you have the option of ceramics.  In this case you have to be aware
that small ceramics can suffer from severe voltage dependency.

[idy]

Here are two MOSFETS set up to give diode+MOSFET in each direction, symmetry at a level maybe higher that LED...




            D G   S            S   G D

_______I_I     I______I     I_I________

Another good one you get a marked asymmetry; Mosfet +diode vs opamp clipping

           D G      S                 diode

————I_I        I——————->I—————————-

Then add a switch shorting out diode; now you get MOSFET vs body diode.

I should try to use DIY Layout Creator so this doesn't look like trash....

[POTL]

I will add fuel to the fire. if you think that capacitors will greatly affect the sound, look at the photos of the insides of a real pedal there are ceramic capacitors installed, which means the film will no longer sound like the original

[DIY Bass]

I just finished one of these.  I used a 1N34A diode in series with one of the FETs the same as the original OCD.  Sounds nice.  From memory I measured about .6V voltage drop across a single FET and just over .8V drop across the FET/diode combination, so they are around the same as a silicon diode but just a bit under/over. 

[eh la bas ma]

I just finished one of these.  I used a 1N34A diode in series with one of the FETs the same as the original OCD.  Sounds nice.  From memory I measured about .6V voltage drop across a single FET and just over .8V drop across the FET/diode combination, so they are around the same as a silicon diode but just a bit under/over.

I would like to understand how you managed to do that. Could you please post a schematic and/or a picture of the inside of your build, showing this modification ?

I understand clipping is about cutting off the "top half, bottom half or both halves" of the signal waveform. Is there some audible difference between different kind of diodes with the same voltage drop ? Does the signal interacts with diodes depending on materials they're made of, if we experiment with the same voltage drop ?

[Bunkey]

I just finished one of these.  I used a 1N34A diode in series with one of the FETs the same as the original OCD.  Sounds nice.  From memory I measured about .6V voltage drop across a single FET and just over .8V drop across the FET/diode combination, so they are around the same as a silicon diode but just a bit under/over.

To add to this, the 1N34A is a Ge; if you used an LED in series with the MOSFETS as you suggested originally you'd end up with a >3vF and not much clipping at all.

If D6 zener is biasing something, I should put the switch on D1 ?
By "D" do you mean Led ? By "away from the source" do you mean D1 output ?

D1 in this circuit is a polarity protection diode on the supply and neither it nor D6 are involved in clipping here, they should not be touched.

If you were to add switchable diodes they would either replace the MOSFETS Q2 & Q3 or they would go in series with these, between the IC and ground (VREF in this case)

The 'D' 'G' and 'S' mentioned refers to the 'Drain', 'Gate' and 'Source' terminals on the MOSFETs.

When Idy said "I also put a diode with the band away from the source", he meant that he put a diode in series with the MOSFET source terminal and orientated it so the anode of the diode was connected to the MOSFET source and the diode cathode (shown by the band on the casing) was facing away from the MOSFET (and I assume going to ground).

I would like to understand how you managed to do that. Could you please post a schematic and/or a picture of the inside of your build, showing this modification ?

I understand clipping is about cutting off the "top half, bottom half or both halves" of the signal waveform. Is there some audible difference between different kind of diodes with the same voltage drop ? Does the signal interacts with diodes depending on materials they're made of, if we experiment with the same voltage drop ?

The diodes are added between the MOSFETS Q2/Q3 and VREF on the schematic you're using.


If I were you and I was wanting to try different diodes, I would first replace the MOSFETS entirely with whichever diodes you're interested in trying, as this is much simpler to do and will give you a better idea of how they sound.
Ge's have a gentler 'knee' to their clipping, they breakup sooner and transition into cutoff more gradually than silicon, so it produces a softer clipping. They also add a lot more harmonic content than Si's. Every diode has its own character so comparing two different diodes of the same vF will sound different. It's something you have to experiment with.

I really suggest first trying them without the MOSFETS, in this case.


Edit: I would actually just build the kit as it's supposed to be built, enjoy it and get a feel for how it works.
I would then put together a similar diode clipping circuit on a solderless breadboard and experiment with different diodes combinations there because it's a lot easier to do than trying to experiment by modifying an existing PCB-based pedal.

[eh la bas ma]

Thanks again for all your clarifications. I will proceed as suggested in your reply.