amz mosfet booster bandwidth control

[idy]

I have liked these mosfet boosters in the past and am experimenting with range "trimming," reducing low (and maybe high) end.

I tried smaller input caps to tame the bass (so this thing could maybe be better for acoustic guitar) and found it needed to be less that 1nf to really have a big effect. maybe 220-440pf. The need to use very small values is because of the super high input impedence, no? I could also use a smaller resistor to Vbias (maybe 1M instead of 10M)? Lower input impedence might smooth out an electric but be less than optimal for an acoustic pickup...?

What would be the best way(s) to cut some super high? There is that cap to ground on the gate, probably better to add a filter on the way out, as that would also cut any noise the thing is picking up?

[Ben N]

Have you looked at the range control on the runoffgroove Omega?

[idy]

Another question: On the breadboard this is working and sounds OK, but really distorts at maximum gain. I m playing with the bias, and replaced the two resistors (one to ground, one to +v with a 50k pot. So I can dial in a nice midpoint, maybe 4.4v on a battery that's a bit shy of 9v. But at the other end of the 10M resistor that brings that bias voltage to the gate my meter shows...like 2.2 v. Is this because my meter has about 10M impedence? If so how would I measure this voltage accurately?

I think I just answered this partly, after changing the 10M to 1M the bias measures more what I expect... But still, how does one measure these bias points with high value resistors?

[idy]

The omega uses the other part of the RC, right? it changes the input impedence so that the input cap has a higher/lower cut off? I think I am hearing some of that effect changing the resistor to 1M, I may need to try a higher value. Changing the resistor to vbias has kind of the same effect as changing a resistor to ground? Ground is the bias for a JFET gate...?

[Ben N]

Yeah, you're right. Memory playing tricks on me--what I was actually thinking of was Doug Hammond's "range" mod to Gus Smalley's NPN Boost (the official "starter project" of DIYSB). I can't find the schematic right now, especially as Doug's website is gone. But I think it had a pot panning between different value input caps on the base, or possibly bypass caps on the emitter resistor. But I'm not sure this answers your original question, so apologies.

Edit: Here is Doug's range control: https://www.aronnelson.com/gallery/main.php/v/DougH/booster/range.gif.html. It is the emitter bypass caps. I imagine the idea could be adapted to the Mosfet Boost as well.
Another one that might have something useful for you in terms of rolling off top and bottom is Gus Smalley's Overdrive, which is really a single transistor boost with some top and bottom roll-off to sound good pushing an amp or pedal. I doubt it would do much for your acoustic, but the approach to tonal control (rolling off bass with the shelving bypass cap and treble with a fixed LP filter on the output) might be useful.

[idy]

What I am becoming fond of is a smaller input cap like 220-440p and then a simple mid cut on the way out, maybe a switch or two to select. I played with the Beavis mid cut which is the same as.... the mid cut circuit on some Gibson amps... or the amz mid cut... a bridged T. This allows my acoustic (a classical sized steel string with a K&K pickup) to sound normal.
The idea is to have a full range booster that can switch to an acoustic pre with a useful preset eq. I could use one of these for voice too maybe....

[idy]

More questions.... Is there a down side to using an input cap smaller than 1nf? To retread, with the high input impedence you have to use a tiny cap to get a bass cut. Is there something wrong with this here? I have some silver mica in various small values and also film ones.

I am experimenting with bridged t filters, soldering up little "plug ins" that I can use on the breadboard... maybe end up with a half dozen on a switch in a can or something. I am not getting the math right though. Frequency is the inverse of 2pi times the square root of RRCC....?

And the twin tee filters have a much deeper notch? Is that the difference?

[duck_arse]

Another question: On the breadboard this is working and sounds OK, but really distorts at maximum gain. I m playing with the bias, and replaced the two resistors (one to ground, one to +v with a 50k pot. So I can dial in a nice midpoint, maybe 4.4v on a battery that's a bit shy of 9v. But at the other end of the 10M resistor that brings that bias voltage to the gate my meter shows...like 2.2 v. Is this because my meter has about 10M impedence? If so how would I measure this voltage accurately?

I think I just answered this partly, after changing the 10M to 1M the bias measures more what I expect... But still, how does one measure these bias points with high value resistors?

If I understand any part of mosfetting right ...... the gate current draw is zero, so there is NO voltage drop across the resistor, at any value of resistance, between the bias point and the gate itself. this would mean that the voltage measured at the bias point must be equal to the voltage at the gate pin. so measure it at the low-enough-impedance point of the bias divider, and be safe in the knowledge that the gate V is the same.

[idy]

That clarifies the thinking. Occasionally one must have faith. Trust but verify.

[idy]

This is not yet boxed, just breadboard breakout box with a strip board layout.
I notice that this is much less hum prone than when the circuit was itself breadboarded. Then I had to put the lid on the box to avoid loud hum. A compact layout helps...

What I am using now is a 120p cap for input and no pulldown resistor, because that would change input impedence and freq response.(?) And  See how that works...

On the output I added a high pass: 10n cap with a 250k pot to and 10k stopper resistor to ground. This goes from brighter and more bass-starved than I need to something pretty close to a full range boost.

Sounds good with my acoustic and its K&K pure classic(four contact heads.) Also does good things to an electric. I "always" would use it before a meatball type envelope filter and a digital delay/reverb thing.

But the way I have this now, I have an output cap on the strip board, 1uf....then I'm hooking up to the breadboard with the 10n series and pot to ground HP. I am screwing this up, no? i only need one cap here...or is the effect on the 10n only about 1%?

Interesting to see what you can do with just one transistor and the high pass(es). Both input and output are kind of "exposed" to loading... And how a pot wired as a voltage divider is a volume control, put wired as a variable resistor (with a stopper) after cap (or before?) its a bass cut control.

[duck_arse]

But the way I have this now, I have an output cap on the strip board, 1uf....then I'm hooking up to the breadboard with the 10n series and pot to ground HP. I am screwing this up, no? i only need one cap here...or is the effect on the 10n only about 1%?

can you diagram this so's I can understand, pleeze?

[idy]

        47n     250kB           10k
------()-----WWWW------WWWW------gnd
                     ^
                    out

I took the "extra" output cap off and replaced with the 47n. (Attached to the drain). This goes to a 250k pot with a 10k to ground and my output off the wiper.

I know this is super primitive, I am trying to find out how close I can get with the simplest circuit. I can work with this sound. At home at least. I find myself not distracted by the muddiness, and just playing and forgetting about it. It's OK.

I think next is to try two gain stages so I can "recover." And then monkey with James 3 band or James 2 band plus gyrator mid.... or something sort of preset. I think I will put some different stacks in a cookie tin with switching and sandwich this between two boosters for experiments. The one transistor plus high pass gives me just barely enough gain now when maxed.

Presently I find myself using three bands for my acoustic. I get good results from very different boxes, a slightly tamed version of the Wampler black '65, or a tag board layout 3 band parametric with a gain control. Treble at noon, mid all the way down, bass just a smidge. When I need to get louder I reach for the  mid knob, not the volume. Both these sound better to me than the dedicated K&K preamp. (with Bass and Treble)

Really the "ideal" seems to be: some kind of subsonic cut. So the speakers don't hop whenever I rest my hand on the strings. Then a little bass. A mid range cut. Maybe the magic is making this adjustable. Then the main body of the sound, the detail. And maybe some kind of ultra high cut to reduce harshness.

[PRR]

> a little bass. A mid range cut. Maybe the magic is making this adjustable.

7-band graphic EQ?

(Do they still sell those??)

[idy]

Why not a 7 band graphic? I guess several reasons: I want to DIY the simplest solution. I don't ever need to twiddle all those bands. I like the idea of choosing the centers of the bands that I need. Maybe the width too.

Maybe I will try a gyrator (or two, or...) after all. Maybe the dream is a box with a gain control and a single tone that compensates the fletcher munson curve.

I am also thinking that in terms of experimenting and flexibility it's neat to have a few simple filters (and filter types) in cans or boxes and play with adding gain stages until I find a combination that floats my boat. Why I build stomp boxes...

Looking at the Elliot Sound equalizer project
https://sound-au.com/project28.htm
I think about a pot choosing two input caps for Bass, two mid bands and a switch to tame some (or some more) highs on the way out.

I like my DIY 3band eq with master gain control. But I'm starting to develop a horror of too many knobs, (10) and dream of the simple life....

I also like the Wampler Black 65 but think the tone stack should work much narrower values of "boost" and cut, and that the mids should move, and I guess those primitive tonestacks interact.

Remember I want something for a very specific type of acoustic (Classsical size body, steel strings, I have one with cedar, one with spruce) with the wild shoot-for-the-moon idea that this would still be a useful booster for electric. Like the Black 65 or the 3 band.