BYOC Mosfet Boost - increasing input impedance >470K

[box_maker_1]

Aiiight folks -

I've got a BYOC Mosfet boost kit on order. While going over the instructions, I noticed that the input impedance is 470K Ohm.
Now.... there is vigorous debate on the internet about whether 470K Ohm input impedance is sufficiently high for passive pickup instruments. But hey, the Zvex SHO, on which this pedal is based, advertises a 5M Ohm input impedance.

So, the question: How easy would it be to mod the BYOC Mosfet Boost pedal to increase its input impedance to something closer to the 1M+ Ohm range?  Would it involved just swapping out a few components or are we talking a redesign of the circuit.

As always, thanks for the wisdom around here!

[Steben]

To be honest, 470k is indeed quite enough unless you are planning to run several humbuckers in series.
That 470k seems to be mainly the input resistor to ground. So you can swap that one out for any other.
But no, do not expect a major tonal change (if any).

[box_maker_1]

Thanks Steben!

I agree that it seems unlikely to make an audible difference. 

But *if* I did want to play around with input impedance, would it be as simple as changing out the 470K Resistor at R1 with.... say a 1M Ohm resistor, like this one at Mouser: Carbon film, 1M Ohm, 250mW:

https://www.mouser.com/ProductDetail/TE-Connectivity-Holsworthy/CFR16J1M0?qs=sGAEpiMZZMsPqMdJzcrNwkUE5oQyOPzF8GwgfeijOUw%3D

[Vivek]

Welcome to our group !



The input impedance would be total effective resistance of many resistors in parallel

Maybe like this

First resistance across guitar, currently 470k
Parallel to a reverse biased diode
Parallel to a reverse biased diode
Parallel to 10Megs
Parallel to 10Megs
Parallel to MOSFET input impedance at gate


Let's roughly say input impedance is equal to

First resistance across guitar, currently 470k
Parallel to 4Megs

Ie around 420k

(Here the resistances in parallel are huge and don't have too massive an impact on the total impedance, but it is good practice to always see the hidden paths and know if they should be considered or not)


And I wouldn't expect a noticeable tonal difference by increasing the impedance further


PS it's a good idea to post the schematic in the first post

[idy]

That first resistor is what they call a "pull down" resistor. It is there only to keep the pedal from "popping" when you hit the footswitch. It allows the input capacitor to discharge when the pedal is bypassed, otherwise the outside end of that cap is "hanging" and has no path to ground.
1M is a common value for these, *any* 1m resistor will do. Sometimes a bigger value will work, will still drain the cap in bypass. Sometimes we try values and pick the largest that still produces no pop, if we are obsessed with high input impedance.

[PRR]

.... there is vigorous debate on the internet about whether 470K Ohm input impedance is sufficiently high ...

The internet argues everything.

[Vivek]

That first resistor is what they call a "pull down" resistor.  Sometimes we try values and pick the largest that still produces no pop, if we are obsessed with high input impedance.

There are noise considerations too,

But I don't know anything about noise calculations.

[box_maker_1]

@PRR - The internet is indeed made for arguing! And, we all know that audio discussions online are *at least* 85% voodoo.

@Vivek + @idy: Thanks for the replies.  I've still got a lot to learn here.  If I'm reading your responses correctly, changing the input impedance of this design is more complicated than just replacing the R1 resistor for a higher value, because the R1 resistor is there to give the C1 Cap a path to ground while the pedal is in bypass mode.  Yeah?

For those of you following along at home, here is a link to the schematic we are discussing:
http://byocelectronics.com/mosfetboostschematic.pdf

[Steben]

single coils are 5k to 10k ish in pure resistance. Humbuckers are a bit like ... you know ... double that 
So for the sake of pure resistance, 470k or 1M won't dot a lot of difference.

The real deal is the capacitance (of the cable) and the inductance of the coils. Especially inductance. Combined with the input impedance of the circuit we have a low pass filter.
Single coils are 2 to 3 ish Henries. Humbuckers once again double that.

with corner freq -3dB point = R / (2piL) and Zi = 470k (it is actually a bit lower than that, see other posts)
we have with average single coil 30kHz and humbucker 15kHz. This is quite high and has very little influence.

With Zi = 250k, this becomes 16Khz and 8kHz. Still acceptable but a bit of audible influence with the humbucker.
Lower than that gets a bit annoying.
But any low pass filter with a higher freq than 10kHz is rather ok.

Yet...The volume control pot of the guitar is parallel to the input impedance. This will lower the 470k. It's why humbucker guitars usually have 500k to 1M pots instead of the single coil ones that have 250k to 500k.

[antonis]

You can wire your 3PDT switch with IN grounded for bypass mode, get rid of R1 and enjoy an input impedance of 10M in parallel with 10M/(A+1), where A = stage gain (R4/VR1)..

[fowl]

The internet argues everything.

That's not true!

[box_maker_1]

@Antonis - now you're talking!

So, I know just enough to be dangerous here.  I see the components you are referencing in the schematic, but I'm not sure what connections to bridge in order to achieve what you are describing.  I'm thinking it would be something in relation to point 5 in SW1A, yeah?

Also, what are other implications for that mod (a big POP! when going to bypass mode?)

Can you explain further?  Much appreciated!

[PRR]

You have not even got the kit yet, but already you are assuming the designer/supplier is a fool and you can do better.

[antonis]

I was ready to post some switch wirings but I realized your booster is already OK..
Switch lug 5 goes to lug 6 when effect is bypassed, which lug 6 is then grounded 'cause lug 2 goes to lug 3..

No popping issues 'cause it's the same effect as if R1 was of a very low value..

P.S.
POP occurs when effect is engaged (with or without ring..  ) 'cause input cap is place inside effect,,

[box_maker_1]

@anotonis - Ahh!  I see exactly what you're talking about, thank you.

Well, it seems like playing with different values for R1 at the very least won't break anything. Although the consensus seems to be that it won't have an audible impact. I'll give it a shot. I'm here to learn!

[idy]

What they are saying is that R1 is unnecessary. Leave it out. By grounding the input during bypass, that input cap has the best possible route (short circuit) to ground and should not charge up or pop when engaged.

Then if you still want to play with it, try putting a resistor in. And people are betting you won't hear any difference.. What you imagine you hear no one can vouch...

The "circuit grounded in bypass" is a real common trick, I do it without thinking. You can read some about bypass switching and pops... the history of bypass....start here:
http://www.geofex.com... I can't seem to link to the sub-page...look under "technology of the..." and then chose "bypass switching." Absolutely riveting. A ripping read.

[box_maker_1]

@idy - I'm slow, but I get there eventually. Thanks. I'll leave R1 out of the circuit and then try putting it back in - we'll see how active my audio imagination is that day in terms of there being any real difference.

I see that riving article. I'll go through sometime when I'm having insomnia   

BTW: here is the direct link: http://www.geofex.com/Article_Folders/bypass/bypass.htm

[idy]

Note that the "technology of..." was written way back before 3pdt switches were cheap. And it maybe doesn't explain the popping part....I can't find the one I'm looking for...

here it is:
https://www.mrblackpedals.com/blogs/straight-jive/6629778-what-really-causes-switch-pop?_pos=1&_sid=c52fc8d30&_ss=r

[box_maker_1]

With the very real possibility of beating a dead horse...

Reading the article from Mr. Black... it would leave me to believe the the R1 resistor (before C1) and VR2 (after C2) are *necessary* to limit what Mr. Black refers to as "Switch de-bounce and the eternal exchange of noise and energy".

UNLESS - R2 does that same job (even though it comes *after* C1).  And obviously VR2 is staying put.

Oh man, that horse really is dead. 

[idy]

I don't think so. The third schematic on his page shows pull-downs on the in and out of the circuit (both places pretty much have to have caps there.) Mr Black (DeVille) casts some shade on this. "Guys say....trust me I'm a builder..." Because its not always necessary, and doesn't always fix the popping problem. It is cheap easy answer that has a downside (impedance) and isn't a panacea.

In the fourth schematic he shows the first (input) pulldown is gone...and instead, the input of the circuit is grounded when bypassed, which should really pull that cap down! This is the wiring your BYOC schematic shows. Switch terminals 4,5,and 6 accomplish this. The R1 is, as Jack would say, moot.

But yeah, its a dead horse. You should build it and then see if you like it. Leave R1 out. Oh, and watch out. An unfortunate characteristic of BYOC is that they kind of force you to build the circuit and complete it in the box. Much wiser is to get everything working (use the method they show for getting the pots lined up right, but don't tighten them, leave the stomp switch wiring for later and  maybe leave the 9v jack off, just tack solder a battery or something and then and only then but it inside the box. Because it is a royal PITA to have to take the whole thing apart to correct some silly mistake.

The DC jacks they use have the nuts inside the box, and they can't be removed without cutting the wire or desoldering.

A lot of us use breadboard or a "break out box" to test circuits. Jacks and power and a tested and working bypass set up in an easy roomy box. breadboard or screw terminals to hook up wires from the circuit to the jacks etc.