MXR FET Driver (and a minor concern)

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MXR FET Driver (and a minor concern)

Postby imaradiostar » 03 Oct 2014, 07:46

So I bought one of these because no one has traced it yet and I was curious how it's built and how compares to my DIY tube driver. It'd be nice to have that sound in a more compact format, right?

First off- it uses LND150's at +-15 volts. I found that interesting. I haven't tried to trace it but at a quick look it's mostly the traditional tube driver circuit built with the mosfets in place of the triodes and a little high frequency switcher generating the positive and negative rails with some tiny inductors. My pedal (the tan one) uses a tiny SMD op-amp, slightly different form the Bonamassa version pic elsewhere on the forum.

It doesn't clean up as nice as my tube driver, and that's a disappointment. But I think there might be an issue. The drains are sitting around 10 volts. The sources are around -14. When I build a FET pedal I usually tweak to have the drains around half of the available voltage. That is not the case here.

There are trimpots but I'm afraid to tweak, which is silly but true.

Whadya think? Go for it? What can it hurt, right?

Jamie
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Re: MXR FET Driver (and a minor concern)

Postby deltafred » 03 Oct 2014, 08:48

Go for it.

Take a photo of the trimpots before you start tweaking.
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Re: MXR FET Driver (and a minor concern)

Postby sinner » 03 Oct 2014, 09:07

15v on drain? What a waste of LND150's

Give them a proper v+
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Re: MXR FET Driver (and a minor concern)

Postby teknoman2 » 03 Oct 2014, 10:55

I am waiting for my jb fet driver to get it on my hands.
When I got it I will trace it also and compare it with yours.
From some pics I ve seen there isn't too much difference between the fet and the jb fet driver.
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Re: MXR FET Driver (and a minor concern)

Postby imaradiostar » 03 Oct 2014, 13:44

sinner wrote:15v on drain? What a waste of LND150's


Totally. I just don't get it. Why use a 500 volt part where a much lower value part can be made to work similarly? And if you've got a 500 volt part and you're already using a boost converter, why not run the things at "real" plate voltages? Maybe the high voltage part distorts in a more pleasing manner because of the high "on" resistance or something.

The other thing- I've always thought jfets sound more like triodes and mosfets sound more like pentodes. I wonder why the dang thing doesn't just use jfets? Perhaps because some of the good ones have just gone out of production? Or maybe I'm wrong and the mosfet actually does sound better in this application.

Of course I have no idea what tricks he might be doing to get the LND150's to act more "tube like." That remains to be seen.

The drain pots are multiturn trimmers. I guess I'll measure before tweaking.

I bought the pedal on ebay so who knows what someone might have done inside it. It is odd, though, that both drains were within a fraction of a volt.

Jamie
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Re: MXR FET Driver (and a minor concern)

Postby sinner » 03 Oct 2014, 14:11

I don't think LND150 act more valve alike, it acts like ideal valve. Can be used as a direct triode replacement and afaik tests of it was done (by someone on sloclone). What I think is this mosfet is completely wasted in this application. This is another muppet idea next to use valves as a clipping diodes in fx pedals.

Im not suprised id does not clear out
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Re: MXR FET Driver (and a minor concern)

Postby imaradiostar » 07 Oct 2014, 05:17

OK, been busy. Fiddled with the trimpots a bit. I can only adjust the drains as low as 7 volts with the trimmers so that's a little odd. It didn't seem to make much of a difference anyway. Still sounds just so-so compared to the real deal. I wanted to love this thing- I really did. James Santiago makes it sound good but I have a feeling he can do that with anything!

Other odd stuff- gates are at -12.9 volts, sources are at -13.9. Doesn't much matter though- I think the thing kinda sounds like terd. One of these days I'll open it up and really see what's going on but it appears that the gate and drain are held at those positions for a reason, which seems counterproductive to me. I would expect the gate to be more negative than the drain on a self-biased common source mosfet but that must not be how they're being used.

Some day in the not too distant future I'll put it on the scope and see what's going on in there.

JT
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Re: MXR FET Driver (and a minor concern)

Postby imaradiostar » 17 Feb 2015, 06:35

Just an update if anyone cares. I don't hate it as much as I did but I still believe it's a total waste of a good idea. I believe I'm either going to sell it or put a mini dual triode in place of the battery and make it a "real" 4 knob tube driver in a small enclosure.

I haven't traced it yet but from how the tone controls act I don't think they used the same circuit as the real deal. Again, seems like a shame when so many people like them for what they do.

Either way I'm going to breadboard some solid state alternatives to see what I come up with. In the past a solid state varidrive went really well. I'm hoping (for the way I used a tube driver) a pair of jfets and careful level tweaking will get me where I want to go. For me it's almost more of a mid boost than anything else.

Good luck to anyone that buys one, I hope it sounds better than mine!

Jamie
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MXR FET Driver

Postby bc108 » 04 May 2018, 02:05

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Re: MXR FET Driver

Postby dan.schumaker » 08 May 2018, 21:54

Looks like Bugg already did a version of it....
http://www.pedalpcb.com/product/mosfetdriver/

I also have a 1590a version available...
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Re: MXR FET Driver

Postby bc108 » 09 May 2018, 00:40

dan.schumaker wrote:Looks like Bugg already did a version of it....
http://www.pedalpcb.com/product/mosfetdriver/

I also have a 1590a version available...


Thanks there..!
Schematic here: http://www.pedalpcb.com/docs/MOSFETDriver.pdf
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Re: MXR FET Driver (and a minor concern)

Postby nickthemen » 14 May 2018, 12:23

i miss the high cut in the schematic, any idees?
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