Biasing the Dual AMZ Mosfet Booster

[gaussmarkov]

the prophecysound.com schem for a dual amz mosfet booster puts a trim pot in place of R2 (62k) in jack orman's amz mosfet boost schem.  jack briefly explains some of the issues:

Since the mosfet is an enhancement mode device, the biasing required is not quite as simple as that used for jfet circuits.

and

The dc voltage measured from point A on the schematic to ground should be 4.5v to 5.5v with the circuit idling and no input. The value of R2 should be tweaked until the point A voltage is correct.

i am interested in running the dual booster at both 9v and something higher (for more headroom).  i will experiment but i would also like to understand what to expect.  is it correct to expect that the biasing for 9v will yield ball-park biasing for 12v or 18v?  

i'm interested in what different voltages do for this double boost in my rig.  maybe i will prefer a hotter output with less distortion than 9v can deliver when driving the second stage into clipping.  maybe i can achieve satisfactory results with just 9v.

has anyone had any experience with this sort of thing already?  any observations or thoughts are appreciated.  thanks in advance.

[petemoore]

Since I have time...
 Maybe measure the GSD pins of the transistors and they should double in ratio.
 There's some biasing that takes up a swing room, which would throw the figures off,..I'd think alot less that 10% anyway, you have nothing to lose as long as everythings rated for 18v, trying it out and listening/measureing to see what the GSD voltage ratios do when PS V is doubled.
 I'm typing only, I should go reread how to bias mosfets.

[Threefish]

gaussmarkov - did you end up trying the AMZ mosfet booster at 12v? I'd like to do it, but i'm guessing there will be some component changes (the 9.1v zener?).
If anyone else has done it, or can explain how to, please share.

Thanks

[amz-fx]

gaussmarkov - did you end up trying the AMZ mosfet booster at 12v? I'd like to do it, but i'm guessing there will be some component changes (the 9.1v zener?).
If anyone else has done it, or can explain how to, please share.

No changes necessary for 12v operation.

-Jack

[gaussmarkov]

gaussmarkov - did you end up trying the AMZ mosfet booster at 12v? I'd like to do it, but i'm guessing there will be some component changes (the 9.1v zener?).
If anyone else has done it, or can explain how to, please share.

Thanks

no, i haven't done this yet.  but it's still on my list of things to do.  i've been on the road for the last month or so--so it hasn't been possible.  it's been hard to just login here and check on things.    if you try it, let us know what you think.

[Threefish]

Look at that - I ask on the forum, and email the Great Jack Orman, and he replies to both. WHAT A GREAT GUY! Thanks Jack.

Armed with this, I intend to throw one together tomorrow night. I'll report on any differences I can hear between the 9v and 12v versions. I'm guessing there will be none to little - let's see gaussmarkov.

[gaussmarkov]

Look at that - I ask on the forum, and email the Great Jack Orman, and he replies to both. WHAT A GREAT GUY! Thanks Jack.

Armed with this, I intend to throw one together tomorrow night. I'll report on any differences I can hear between the 9v and 12v versions. I'm guessing there will be none to little - let's see gaussmarkov.

just to be clear, the obvious difference will be headroom.  if you don't push the 9v version into clipping, then you won't hear any difference for the same signal into 9v and 12v.  as i understand it, there is no question that there will be a difference;  a 9v setup cannnot swing as wide as a 12v setup.  so there is a range of amplifudes in which one will clip and the other will not.

thanks for taking this on, threefish!  i am really interested in what you find.

[Threefish]

I hope we're still kicking the ball in the same direction here gaussmarkov - I've managed to lose the schem for the dual booster, and it's in series you want to run them? Running one AMZ MB into another is something I'd been wanting to try for a while, and then this thread turned up. I've sort-of paralleled two of them in the past, but i'll get to that later.
Anyway, I've put together two individual boosters, so could fiddle with them in series or parallel, with one running at 9v and the other at 12v.
In short, through the full range of clean to distorted sounds you get out of this circuit, I couldn't discerne any difference in the 9v or 12v versions, at any setting or any configuration. I don't have a 18v supply on hand, so I couldn't test them that far. When I'd finished the first, I biased it on 9v, and then plugged 12v into it with no adjustments, and got no change in sound or volume. Biasing stayed spot on. I did twiddle the trimpot (I used a 100k trimpot in place of the 62k resistor) just out of curiousity, but came back to the exact spot I marked on the trimpot case.
Just as an aside here, my experience with doing this circuit four times in the past has always been that you get to a point with the biasing and it's like flicking a switch - it doesn't work, then it does.
Moving on - I then finished the second booster and plugged 9v into one, and 12v into the first I did. I then started testing one boosting the other, and then swapping them, again testing both of them through the whole range of gain. I then rigged them up parallel with a DPDT switch to test if going from one to the other (or just running the two at different voltages adjacent to each other) caused anything unusual. Nope. Through all of this, the only thing worth noting was that the final output (after the second stage) with either the first or second stage (or both) at higher gain settings was a distinctly darker sound, though with all that gain, that should be expected.
I don't know if I'll box this up. It sounds great, but in that heavier-sounding way that I've got in a few of my other pedals already. With the stages split, and some out and in sockets between the two stages, could be useful in a box as a clean pre+post effects router in front of my amp.
I'll find a use forthem in any case. It's a damn fine circuit. Thanks again Jack Orman.

Just to go a bit off topic, one way I've implemented the mosfet booster was with a DPDT switch after the 100uF cap (C5 on the AMZ schem) and two 5k gain pots hanging off either side of the switch, so I could switch between two gain settings. The only drawback about this arrangement was the volume difference between the two settings. It's really handy though.

[amz-fx]

I've managed to lose the schem for the dual booster, and it's in series you want to run them?

Yes, in series.  No mods necessary.  You can also include a dpdt to bypass the second one to give you the option of one or two boosters.

regards, Jack

[gaussmarkov]

I've managed to lose the schem for the dual booster, and it's in series you want to run them?

Yes, in series.  No mods necessary.  You can also include a dpdt to bypass the second one to give you the option of one or two boosters.

regards, Jack

ditto:  in series.  my plan is to box the two in one enclosure with the option of using them seperately.

threefish, thanks for all this.  so from what i understood, putting the 12v version second in the series did not deliver a cleaner sound when the 9v was turned up than doing the reverse (12v first and hot, 9v second).  that's where i expect you to find some useful differences, because the 12v version should be able to handle greater amplitude (without clipping) in the input signal than the 9v.

[Threefish]

Hello again Jack - thanks. In light of what I found with my experiments, does this all sound right to you?? Love to hear any thoughts you have on all this, or gaussmarkov's thoughts towards 18v experiments.

And good morning gaussmarkov.
I suppose the difference between 9v and 12v isn't a lot, though it is a big proportional increase. If not volume or headroom difference I was at least expecting some slight sound difference or, I dunno, something. My hearing's still pretty good, but I'm only human.
So will you have a crack at an 18v? You may hear something there, though in light of my experiments, it wouldn't surprise me if there was still none. Remember to use all 18v rated stuff........... I just checked the 2N7000/BS170 data sheet. 18v is within safe voltage.

I suppose an upshot of all of this is that I now know that if I can't find the 9v adaptor for my AMZ booster (the first one I built still lives at the front of my chain), I'll just plug in that spare 12v I have.

[gaussmarkov]

And good morning gaussmarkov.
I suppose the difference between 9v and 12v isn't a lot, though it is a big proportional increase. If not volume or headroom difference I was at least expecting some slight sound difference or, I dunno, something. My hearing's still pretty good, but I'm only human.
So will you have a crack at an 18v? You may hear something there, though in light of my experiments, it wouldn't surprise me if there was still none. Remember to use all 18v rated stuff........... I just checked the 2N7000/BS170 data sheet. 18v is within safe voltage.

I suppose an upshot of all of this is that I now know that if I can't find the 9v adaptor for my AMZ booster (the first one I built still lives at the front of my chain), I'll just plug in that spare 12v I have.

threefish, i bet i will have the same reaction as you.  and i will still try it so that i can return the favor.  also two things are peaking my interest.  first, in the blender box discussions that i have read there is repeated concern about not having enough voltage for hot input signals.  so it seems like that issue should come up here.  second, zvex's super-duper is a similar circuit that has received a lot of praise.  i'd like to know what that's about first hand.  but all of this has to wait until i get home in february. 

thanks again!

[amz-fx]

Love to hear any thoughts you have on all this, or gaussmarkov's thoughts towards 18v experiments.

It can make a lot of difference with a single booster. With two in series, the first booster is amplifying the signal almost to clipping and the second will overdrive, even on 18v.  Almost any setting you can get with 2 boosters on 18v, you should be able to duplicate at 9v

My preference is a Mosfet Booster driving a Mini-Booster.

regards, Jack

[gaussmarkov]

Love to hear any thoughts you have on all this, or gaussmarkov's thoughts towards 18v experiments.

It can make a lot of difference with a single booster. With two in series, the first booster is amplifying the signal almost to clipping and the second will overdrive, even on 18v.  Almost any setting you can get with 2 boosters on 18v, you should be able to duplicate at 9v

My preference is a Mosfet Booster driving a Mini-Booster.

regards, Jack

this is very helpful to know.  jack, thanks very much!!!

[gaussmarkov]

one more word, which i was reminded about when i reread jack's note about 9v vs 18v making a big difference for a single booster.  i also had in mind what happens when you are sending a hot signal into the first booster.  in that case, isn't the situation similar to a single booster?  you may not be able to keep from overdriving the first 9v  booster.  i was going to suggest that threefish try running a red llama or similar into the two boosters.  or maybe even a maxed out tubescreamer.  but i mistakenly thought something interesting would come out of the 2-booster experiments he was doing.