Zendrive explanation

[fuzzo]

Hi guys,

I would like to understand some little things about the zendrive schematic and the mosfet clipping.



why the output cap is so low compared to regular overdrive like tube screamer (1uF) it's a conceptor choise or there're some reasons to put this value here ? maybe because of the voice control that plays with gain and EQ in the same time ?


What does the 1K resitor befor the volume pot ?

Why all schematic using Mosfet in clipper has Ge diode with them ? what does the Ge diode with Mosfet ? give a softer or smoother sound ? I can only put the mosfet in the feedback loop ?

Thanks !

[liquids]

Its 470n = .47uF. 1uF = 1000n, so 470n is about half that, i.e. not that small.  100n wouldn't be unusual, or smaller.....maybe you are thinking it's 470pF?

[Sir H C]

What does the 1K resitor befor the volume pot ?

I guess to give some isolation if you accidently short the output.

[/quote]
Why all schematic using Mosfet in clipper has Ge diode with them ? what does the Ge diode with Mosfet ? give a softer or smoother sound ? I can only put the mosfet in the feedback loop ?
[/quote]

The MOSFET has an internal diode, the "body diode" that otherwise would kick in at a voltage below the MOSFET diode-connected device threshold.  So the Ge diodes keep this diode from turning on with minimal extra signal level.  MOSFETs are really four terminal devices with the fourth terminal connected to the third.

[fuzzo]

Its 470n = .47uF. 1uF = 1000n, so 470n is about half that, i.e. not that small.

Yeah but why this particular value ? to cut the bass a litle bit the bass produced by the voice control ? I mean with this 470n the frequency corner is different, bassless in this case I guess. I ask that 'cause all ovedrive schemos, I saw, have a higer value for this output cap .

The MOSFET has an internal diode, the "body diode" that otherwise would kick in at a voltage below the MOSFET diode-connected device threshold.  So the Ge diodes keep this diode from turning on with minimal extra signal level.  MOSFETs are really four terminal devices with the fourth terminal connected to the third.

Ok so nothing constrain me to keep this additional diodes ? I can remove them if I want but the signal coming to the Mosfet will be different and so the clipping too.

[Sir H C]

 Remove them and you will no longer get true MOSFET clipping but standard Silicon diode clipping.  The cool thing about the diode connected MOSFET is that they have a different sound to a standard diode when clipping, but since the threshold voltage and required overdrive for the devices is much higher than the body diode of the device pointing the other way, the other device would clip first removing the whole reason for putting them in.

[grolschie]

But, this arrangement is (MOSFET body) diode clipping, not actual MOSFET clipping, right? One needs to reverse the polarity of the diodes to get the MOSFET to work right?

[Sir H C]

I was looking at that and having a hard time telling if those are PMOS or NMOS devices, if PMOS you are right.  I always put the line on the source leg, I hate the body picture.

[grolschie]

Really? I thought it was for N-channel MOSFETs like BS170 and IRF520.

ref: http://www.diystompboxes.com/smfforum/index.php?topic=38581.0

[WGTP]

You can get 2 significantly different sounds by reversing the GE diodes and using a switch.   

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

[Sir H C]

I will have to think more but I think the diodes are backwards for MOSFET clipping.  IIRC:

1.  The body contact of an NMOS device is P+. 
2.  The body is connected to the Source, which should always be the most negative potential for an NMOS device.
3.  In this configuration, the P+ on the source and the N+ of the drain connection act as a diode, with the source the + and the drain the -. 
4.  So you are getting the diode of the device working in this configuration.

So, diodes are backwards for a PMOS diode connected setup.

[glen2410]

Im having some trouble with this circuit.
1.Its very noisy,it has a loud humming noise when the volume is up. When i turn the vol down it becomes quieter but the signal is weak.
2.The pots do not seem to be working or changing tone in anyway.Not entirely sure that the section above the IC is working.
This is my first build,however I'm used to working with circuits and soldering and stuff.
Could anyone here help me find out what is wrong? Appreciate it

[bluebunny]

Welcome!  Check out the sticky "DEBUGGING" thread at the top of this forum.

[Cozybuilder]

Your symptoms make me think there is a ground problem- I'd start with verifying those connections. Chip power connected?

[duck_arse]

are you powering with battery or ac adaptor?

[glen2410]

Im using an Adaptor. However I rebuilt the circuit and this time got it right. So the pedals functional
BUT now When i shifted the circuit to an enclosure-when the pot comes in contact with the metal enclosure,there is this nasty noise that takes over. Im not sure what I'm supposed to do here on.

[duck_arse]

well, now you know you have a short somewhere. do you have a multimeter? you'll need to find what where is shorting, and insulate it.

can you take some nice clear photos of what you are working on, and post them here?

[Transmogrifox]

why the output cap is so low compared to regular overdrive like tube screamer (1uF) it's a conceptor choise or there're some reasons to put this value here ? maybe because of the voice control that plays with gain and EQ in the same time ?

As answered above, it is not as small as you would think.  3dB low cutoff is 3.2 Hz against the 100k output pot, which generally will dominate unless you're going into a low impedance input.  This is more than large enough.  Even a 47n (10x smaller) would not cut into typical guitar frequency range.

What does the 1K resitor befor the volume pot ?

Ensures the op amp is stable even when connected to long (capacitive) cables.

Why all schematic using Mosfet in clipper has Ge diode with them ? what does the Ge diode with Mosfet ? give a softer or smoother sound ? I can only put the mosfet in the feedback loop ?

As mentioned above, the configuration shown is only using the MOSFET body diode for clipping.  If you want the softer turn-on then you need to reverse the diodes.  As it's drawn it's hard for me to imagine it sounds much different than having 2 diodes in series.

You can put the MOSFET without the ge diodes (or make a switch to short one or both of them).  Without the Ge diodes you will clip the MOSFET body diodes fine.  You need the ge diodes if you want to swap connections and use MOSFET clipping.