Zvex SHO: bump highs?

[Plexi]

Hi forum!


Looking for some alternative to my warhorse's LPB, I'm back to this ultra used-talked-builded-dissected-analyzed-etc SHO.

How could I add some highs to this?

I remember that reducing the input cap, due the high amount of input impedance I guess, doesn't make any difference.
Should I'll try something ridiculously low, like 100pF there?

[thermionix]

At some point it has to make a difference.  I'd imagine 100 pF would be way too small.  The Rangmaster has 4700 pF and that might still be too small, but maybe a good place to start.

Maybe reduce 47K to lower output impedance?  Just an idea.

[jubal81]

Add a small cap from the Source of Mosfet to ground. Start with 1n and work your way up to try them out.

[d95err]

Add a small cap from the Source of Mosfet to ground. Start with 1n and work your way up to try them out.

That would work for low gain settings and be less and less effective as you turn the gain up (zero effect at max gain).

You could reduce the 1M bias resistors to something like 100k and then use a smaller input cap.

Neither solution would increase highs, but rather reduce lows.

[Plexi]

Thanks everyone for your sugestions.
I'm breadboarding, and I'll back with some update soon

[anotherjim]

You could just be making a MOSFET Rangemaster.
Anything less than 2.2nF input cap will cut some bass.
Any cap across the gain pot will favour treble gain. Cap could be from that pots wiper to ground or, for different approach, to the source. Try from around 100nF & lower.

[GGBB]

You could also reduce the output cap. At 10u, there's a lot more room to play around than with the input cap.

[Plexi]

Yesterday I've tried to enter, and seems that the forum was down.
Seems that my comment didn't reach.


Some update:
Reducing the in cap is the key, as like the LPB and many others.
Reducing out cap didn't do any difference, due the amount of impedance that comes from the mosfet, I guess.
Didn't tweaked any other value, only placed a 10k pot where the voltage input resistor (5k1) is.

I like to place a fixed resistor or trimpot from S to ground..and for some customer's this is a very pleaseable "mod" to their SHO's.
Just a value where the Mosfet didn't reach that ugly distortion, and place a100k pot as master volume, with 10k resistor to ground.

[GGBB]

Reducing out cap didn't do any difference, due the amount of impedance that comes from the mosfet, I guess.

You'd probably have to go down to at least 100n to hear even a small difference. 22n - 47n is probably in the right ballpark.

[Plexi]

Of course... I did.
Reach the 100nf, lower than 47n loss all the "body", and get a bit of distort.
I forget to mention that I've used 22n to te in cap, leave the out cap with 10uf.
I'll try to place there 1uF and leave the 22n.

[Gus]

That circuit you posted does not have high input resistance.

As the gain is increased the input resistance is reduced by the drain to gate resistor feedback.
You posted with two 1 megs not 10 megs.

[Plexi]

That circuit you posted does not have high input resistance.

As the gain is increased the input resistance is reduced by the drain to gate resistor feedback.
You posted with two 1 megs not 10 megs.

What about this?

[anotherjim]

Have to say I prefer the sound of MOSFETs with independent gate bias. Needs a trim pot to set the bias to suit individual parts. Set it for drain voltage at half of supply voltage, then further adjust for best sound.  Input impedance is then down to the gate bias resistor, which can be as large as you need.

Input protection diodes should have a current limit resistor in series, usually between input cap and the diodes. Can be fairly large value. You see 10k to 33k often.

[marcelomd]

Have to say I prefer the sound of MOSFETs with independent gate bias. Needs a trim pot to set the bias to suit individual parts. Set it for drain voltage at half of supply voltage, then further adjust for best sound.  Input impedance is then down to the gate bias resistor, which can be as large as you need.

I like this idea, but I'm having a hard time to visualize it. Do you have a schematic? Thanks!

[amz-fx]

I like this idea, but I'm having a hard time to visualize it. Do you have a schematic? Thanks!

http://www.muzique.com/schem/mosfet.htm

[amptramp]

The SHO uses feedback from collector to gate via the upper 1 megohm gate resistor.  As the input goes positive, the drain goes negative and makes this upper resistor "look" much smaller than 1 megohm.  When the input goes negative, the drain goes positive and this still has the same effect - the input impedance looks lower from the guitar side.  If you have a low input impedance, this interacts with the inductance of the pickup to limit the high frequencies.  One you turn the volume control on the guitar down, this adds series resistance so you get the effect of the volume control turning down the lows more than the highs.  The independent gate bias does not have this effect.  The feedback is used to stabilize the drain current.

[anotherjim]

More reading...
http://www.geofex.com/article_folders/mosboost/mosboost.htm
...which contains this...

Ideally, the Vbias adjustment pot range is set by the upper and lower resistors. If you can't be bothered working out what value those resistors should be, they can be wire. This will make it harder to tweak the pot position, but not impossible. I throw anything from 20k to 100k trimmer pots in, depending what I have and don't bother with the fixed resistors.
Rsource needs to be fixed. If you want more AC gain, bypass it to 0v with a cap or a cap in series with a variable resistance.

[marcelomd]

Jack, Ron, and Jim: got it! I was thinking about something way more complex... Thanks a lot!