FETs as diodes

[jrc4558]

How would you describe the change in tone they produced i na tubescreamer circuit?
I am currently using two 1n914s reversed-parrallel (stock configuration), although have tried every popular mod, but involving only signal diodes and rectifier deiodes. None of them produce quite the dynamic responce I want. I will certainly give the mosfets a try.

Also, there has been a design by Joe Davisson, called Amber Boost (?). I beleive it had 2N7000's as clippers in NFL, yet they were not followed by any other diode then themselves. Would this mean that they both could be clipping "like" regular diodes (because the setup seems similar to WGTP suggestion, but with two mosfets)?

[bancika]

can other FETs be used for this? like BS170, MPF102 or J201?
Tnx

[WGTP]

Yes, see above.  I thought this was a great thread, which is why I keep posting places.   

[johngreene]

How would you describe the change in tone they produced i na tubescreamer circuit?

That particular picture is of a tubescreamer clone that I put MOSFETs in as diodes. The date on the PC board looks like 12/97 (dang, nine years....where did the time go?). They cause the bass reposne to be significantly increased giving a much darker sound. The distortion is softer and most people that tried it didn't like it as they were more used to the harder distortion of silicon diodes. It gives a pretty transparent overdrive sound that crunches nicely on 5ths but never seemed to have enough 'cut' to work for leads. Which is why I changed everything to this design:
http://www.geocities.com/greene_pedals/images/glasspak.jpg

I am currently using two 1n914s reversed-parrallel (stock configuration), although have tried every popular mod, but involving only signal diodes and rectifier deiodes. None of them produce quite the dynamic responce I want. I will certainly give the mosfets a try.

I think there is a lot to be said for using higher-threshold devices in the tubescreamer type circuit. Try different LEDs. Every MOSFET sounds different, trying to find the one that sounds good is a long and lengthy process. The threshold voltage and transconductance variation from one model MOSFET to the other makes huge differences. The higher the gain of the MOSFET, the more like a silicon diode it will sound.

Also, there has been a design by Joe Davisson, called Amber Boost (?). I beleive it had 2N7000's as clippers in NFL, yet they were not followed by any other diode then themselves. Would this mean that they both could be clipping "like" regular diodes (because the setup seems similar to WGTP suggestion, but with two mosfets)?

A lot of people like the sound of the MOSFET body diode. It clips much harder than the 'MOSFET wired as a diode' but not as hard as a 1n914.

--john

[jrc4558]

A lot of people like the sound of the MOSFET body diode. It clips much harder than the 'MOSFET wired as a diode' but not as hard as a 1n914.

--john

Thanks for the info! Could you possibly explain about this MOSFET body diode, (for the slow group) ? How should I connect the MOSFET transistor to get this diode?
thanks again.

[WGTP]

See above.   

[grolschie]

How about using just one mosfet without the series diode as the clipping device. (and I mean just one mosfet replacing the whole "standard" two diode back to back thing) Would it give asymmetrical clipping where the tops of the waveform would be clipped very softly and the bottoms would be clipped much harder?

If it works like this the harder clipping would happen a earlier than the softer, but it could probably be biased towards the softer side.

I might have understood it all wrong though...

Edit: Based on the other MOSFET-clipper thread it seems that it should work as I thought.


yes, it would work just as you described.

 Try it!

--john

Replying to an old thread. Did anyone give this a go? If so, what were the results please?

cheers,
grol

[WGTP]

I have tried it a number of times and it is another option in the large number of possible clipping combinations.

Using the typical BS170 or 2N7000 Mosfets, the "body diode" clips around .6v and the actual mosfet around 2v.  This creates very asymmetrical clipping and may be to your liking YMMV.  It is similar to using a std. 1N914 diode and an LED in clipping threshold.

As John Greene mentioned, and I have noticed on some "boutique" pedals, the body diode is the one being used, not the mosfet.  Also, as John has pointed out, there are a number of different mosfets that will work, each with a slightly different sound.  The lower the Transconductance, the better.  Supertex has a few that work and some will have a mosfet clipping threshold lower than 2v. making the combination of the body diode and mosfet more symmetrical.     

Here is a switch that allows either method or neither to be used.   

http://www.aronnelson.com/gallery/main.php/v/WGTP/Mosfet_Switching.jpg.html?g2_imageViewsIndex=1

[twangquack]

Using the typical BS170 or 2N7000 Mosfets, the "body diode" clips around .6v and the actual mosfet around 2v.  This creates very asymmetrical clipping and may be to your liking YMMV.  It is similar to using a std. 1N914 diode and an LED in clipping threshold. As John Greene mentioned, and I have noticed on some "boutique" pedals, the body diode is the one being used, not the mosfet.

I really like Mosfets as clippers and always use the body diode. I first used IRF510 andIRF520 Mosfets with 1N34A diodes as "traffic cop," then started using BS170 Mosfets ... but found (as previously mentioned) that due to the approx. .001uF of capacitance of the IRF5** and the ultra-low capacitance of the BS170, I was soon adding various values of caps (often between .0005uF and .001uF, sometimes a bit higher) parallel to the clippers to give that somewhat warmer clipping tone. It usually has to be adjusted for the particular circuit. I think I added higher capacitance values to the Mosfet clippers (body diode) in an OCD clone than the BS170s in several TS clones.

In a Tubescreamer circuit, Mosfets sound great as clippers but sometimes, at least to my ear, the more "in your face" tone of the symmetrical 1N4148/1N914 can sometimes sound better. It may just gel with that design. But then again, a BS170/1N34A opposite a red LED is a pretty cool sound, and sometimes the Mosfets (and a more asym arrangement) seems to sound better. These days, there seems to be no less than three clipping options in pedals I build, and Mosfets (body diode) always figure into at least one of those. I also like 1N34A diodes in series ... but because they can be a bit darker, I never add capacitance to those, to soften that arrangement. They sure don't need "softening" but I really like 'em, too.

In the OCD clone I made, the Mosfets were a must (obviously) ... although I ended up switching in various clipping thresholds, using various combinations (in series) of both 1N34A and 1N270, with the BS170 Mosfets. I really like using a BS170+1N34A x BS170+1N270 as clippers ... then add a small amount of capacitance (tuned for each circuit) as needed. Even with the Ge diodes added to the BS170 Mosfets (to use the Mosfet's body diode), at least a little bit of added capacitance is usually in order. But that's a matter of taste and will vary.

I just worked up a Big Muff variant that sounds wonderful with the BS170+1N34A x BS170+1N270 combination as one of the clipping options (but, in this case, with no capacitance added parallel to the clippers).

[grolschie]

Thanks for the info guys. 

I was wondering if the order of clippers makes a difference? e.g. with a 2 diodes per side arrangement, would there be any difference between:
  ge->si  (on both sides),   versus,   si->ge   (on both sides).

I was wondering about that specifically in the MOSFET body diode clipping arrangement. If the ge diode clips at a lower threshold than the MOSFET and is also before the MOSFET in the circuit, whether the majority of the clipping heard was just the ge diode.

Also, would one hear any difference between a 1N60 and a 1N34/a as clippers? Thanks in advance.

[grolschie]

I suppose that instead of: 

D─┬──────               D─┬──────
G─┘                  or       G─┘
S────|◄──               S────►|──

Could you do this??:

D─┬──►|──              D─┬──|◄──
G─┘                  or      G─┘
S────────              S────────

[WGTP]

The important part is the direction of the "traffic cop" or "routing" diode.  It can be before or after the Mosfet.  You don't need a routing diode for the mosfet body diode because of the lower threshold the signal will naturally go in that direction.  It is only needed to force the signal thru the higher clipping threshold of the actual Mosfet.  Using a GE and body diode in series raises the threshold a little and insures that the clipping is soft.

If I understand correctly, diodes in series will produce the clipping of the "softest" diode at the threshold of the series value.  In other words, if you put a SI 1N914 (.7v) and a GE 1N34A (.3v) in series the result will be the softer clipping of the GE at 1.0v.  What I don't know, is if the clipping of the mosfet body diode is softer than a GE.  It's use in several circuits implies than it is.   

[grolschie]

Thanks for that. I would've presumed that the majority of the clipping heard, would be from the Ge which has a much lower threshold - thus thinking all this MOSFET clipping could perhaps be only slightly different to plain Ge diode clipping. I guess the test would be to use an LED or silicon diode with the MOSFET?

[WGTP]

On the first page R.G. suggests using "schottky diodes like the 1N5819" in place of a GE diode.  It has a low threshold and a Sharpe turn on, so it impacts the other "diodes" less.  I think that means you will hear the clipping coming from the Mosfet better.  GE's are also used for their low threshold and "just to be safe" soft clipping.  SI's can also be used, but they raise the already high clipping threshold of the Mosfet.  You can also use the body diodes of each Mosfet as the "routing" diode for the other Mosfet as seen at AMZ and here.   

http://www.aronnelson.com/gallery/main.php/v/WGTP/Moster.jpg.html?g2_imageViewsIndex=1

[grolschie]

Thanks for that. 

[grolschie]

Just a quick question:

When using MOSFET clipping (BS170s + 1N60's) in a TS-type pedal feedback loop (using them as MOSFETs, not the body diode method), which would be louder: said arrangement, or 3mm RED LEDs? Should one be much louder than the other? Thanks in advance.

[WGTP]

In my experience, Red LED's measure around 1.8v.  2N7000's measure slightly higher and BS170's are slightly higher yet, over 2.0v.  Then if you add in a GE diode for proper "routing" your at around 2.3v.  So, the Mosfet TS will be slightly louder.  With the softer Mosfet clipping, it will also sound less distorted.  Should work well as a boost with the higher/cleaner output.   

Having said that, a Supertex TN0702 has a lower threshold more like a standard SI diode around .7v. 

http://www.supertex.com/pdf/misc/n_e_mode_mosfets_SG_device.pdf

[grolschie]

Thanks for that. I wired up both options, but one is considerably louder than the other. I haven't debugged yet, but am hoping I don't have a short somewhere. :-)

[KHStudio]

I have some info & questions concerning the OCD & the various clipping setups I've found in my own pedals.. I thought it fit this thread & wanted to share the link:

http://www.diystompboxes.com/smfforum/index.php?topic=102882.0