Pot value question

[Nooblett]

I'm trying to make a simple volume cut pedal, and i'm wondering what value pot i should use to be able to turn down the volume about halfway (max).

thanks!

[line6man]

It would depend on the impedance of the signal you're adjusting the volume of.
What's feeding into it?

[Nooblett]

A passive electric upright, should I measure the impedance of that and then from that get an appropriate pot value?

[line6man]

Originally Posted by Nooblett

A passive electric upright, should I measure the impedance of that and then from that get an appropriate pot value?

If it's passive, I would assume it has medium-ish impedance pickups, but I know absolutely nothing about upright pickups.

Try a 500K pot, see how well that works.

FWIW, pickups have a complex impedance that is not easy to measure, given that impedance is frequency-dependent.

[Nooblett]

I'm going over phasors and ac circuit analysis in my circuit theory class, i'm not doing as well as i'd like to be but couldn't you just put a source ac voltage into it and then measure the current then do Z=V/I? I doubt i'd need to do that but if i do i have access to a bunch of nice equipment i could try that with.

and thanks, I'll probably give 500k a shot, i've looked around a bit for the right number online for the pickups (http://www.fishman.com/products/details.asp?id=17 these things) but i've had trouble finding it.

[line6man]

Originally Posted by Nooblett

I'm going over phasors and ac circuit analysis in my circuit theory class, was hoping that i could ask my prof or something :$.

Thanks, I'll probably give 500k a shot, i've looked around a bit for the right number online for the pickups (http://www.fishman.com/products/details.asp?id=17 these things) but i've had trouble finding it.

Wonderful! This stuff is a lot of fun when you get into it.

That looks like a piezo pickup, in which case, it's likely to have a very high output impedance,(And if it interests you, the reactive component of the impedance is largely capacitive.) so you'll want a much higher pot value, otherwise you'll just kill all your treble/output. 10M ohms, perhaps? I have no idea what's common with piezos, you usually use a buffer to lower their impedance, but you said the bass was passive.

[Shoewreck]

Originally Posted by line6man

you'll want a much higher pot value, otherwise you'll just kill all your treble/output. 10M ohms, perhaps?

Combined with lower value pots piezos loose lows, not highs. Losing lows is probably even less desireable, but 10M pot is no good to the rest of circuit and would result in lost highs (forming a simple RC lowpass filter with cable capacitance) and raised noise levels.
Going active (at least have a buffer at the volume pedal's input, then use a lower value (10K-25K) pot) would get you rid of a whole lot of troubles.

[Nooblett]

Hmm, yeah the fact that the pickups are capacitive would cause a decent bit of trouble. So i suppose this is more complex then i thought then. Would it be near impossible to maintain a true-bypass circuit while trying to do this? I was thinking at first it would just be a pot in series with the signal and i could use that to tweak it but the idea that the pickups are capacitive does change everything.

[Nooblett]

Is it viable to just stick an inductor in there (after measuring the impedance) to keep the impedance purely resistive? or would that do something crazy that i don't want.

edit: nvm that, the frequency varies based on what note you're playing so that's pretty much impossible to do. right? sorry i'm pretty new to this so i'm semi-clueless

[delta7fred]

Originally Posted by Nooblett

Is it viable to just stick an inductor in there (after measuring the impedance) to keep the impedance purely resistive? or would that do something crazy that i don't want.

edit: nvm that, the frequency varies based on what note you're playing so that's pretty much impossible to do. right? sorry i'm pretty new to this so i'm semi-clueless

Strictly speaking you need a charge amplifier (charge to voltage converter) before your volume pot. Piezo pickups are completely different to magnetic ones.

If you have any good text books (or Google) look up charge amplifier and gen up on the theory of them. Not many people who use them know much about them so now is the best time to learn. I saw a schematic for a nice simple one to build a few days ago, but as usual I cannot find it now. I will post it if I do.

I have never tried sticking an inductor in to drop the voltage of a piezo and have no idea what the outcome would be. You could try it but I suspect you have nailed it with it 'doing something crazy'.

[delta7fred]

Found the link

http://www.cafewalter.com/cafewalter/fetpre/index.htm

If you are handy with a soldering iron you could soon build one of these, put it in a box with a pot on the output and away you go.

Edit
It is powered by 2 batteries so needs 18volts

[Shoewreck]

Here's my favourite piezo input stage:

You may set the gain by changing C2. Lower capacitance = higher gain.

[Nooblett]

Originally Posted by delta7fred

Found the link

http://www.cafewalter.com/cafewalter/fetpre/index.htm

If you are handy with a soldering iron you could soon build one of these, put it in a box with a pot on the output and away you go.

Edit
It is powered by 2 batteries so needs 18volts

all right i think i'll go ahead and build this thing, my only question is what is the resistor labeled R8? it looks like a resistor with 2 settings? one that has 1-2 at 0 ohms and the second that has 3-2 at 100k? i should probably know what that is without asking.

and then what value pot should i use at the output of this buffer?

thanks for all the help guys it's really appreciated!

[delta7fred]

Originally Posted by Nooblett

all right i think i'll go ahead and build this thing, my only question is what is the resistor labeled R8? it looks like a resistor with 2 settings? one that has 1-2 at 0 ohms and the second that has 3-2 at 100k? i should probably know what that is without asking.

and then what value pot should i use at the output of this buffer?

thanks for all the help guys it's really appreciated!

R8 is a 100k preset, a pot with usually a screwdriver slot. You could replace it with a pot and then you would have a your volume control, but.........

It looks overly complicated to me, I would just take the -ve side of C5 to the top (clockwise end) of a 100k pot, the bottom (counter clockwise end to the ground, same as the preset), and the wiper as your output. Hope you follow that.

Why not build the one Shoewreck has posted, it is simpler, only requires one battery, has a volume control, and is tried and tested?

[Nooblett]

yeah you're right, i'll go ahead and build the one shoewreck posted. it does look much simpler .

you think i could use a variable capacitor (c2) to set the volume, something with a range somewhere around 1n Farads?

[delta7fred]

Originally Posted by Nooblett

you think i could use a variable capacitor (c2) to set the volume, something with a range somewhere around 1n Farads?

R4 is the volume pot.

There is no advantage having a variable capacitor for the gain, if your pickup overloads the FET then alter c2 until it doesn't. A 1nF variable will probably be HUGE.

[bassbenj]

Originally Posted by delta7fred

R4 is the volume pot.

There is no advantage having a variable capacitor for the gain, if your pickup overloads the FET then alter c2 until it doesn't. A 1nF variable will probably be HUGE.

You can't really have a cap for a volume control. I've never used this circuit so I can't say things for sure about it, but I have a couple of comments. One is that input impedance for a bass preamp should be 10 megs. 1 meg inputs is more for guitar. Note R3 is 1 meg. If the input isn't very high Z you tend to lose the deep bass. I'm worried about the capacitive divider used to control gain. The way this is set up it takes the inverted signal from R2 and feeds it back to the input. Generally this will tend to lower the input impedance even more! It may be worth a try but I am suspicious.

Compared to the PZP-1 while it may take 18 volts it probably also works on 9 volts (one battery) it has a 10 meg input (R2) and doesn't have a lot of gain like the other one (it's a buffer not a preamp). Hence it is capable to driving most anything although the "volume" pot being 100k does raise the impedance for cable driving. No problem driving an EQ circuit, however. I think that one has a much greater chance of working well on a bass.

[Nooblett]

Originally Posted by delta7fred

R4 is the volume pot.

There is no advantage having a variable capacitor for the gain, if your pickup overloads the FET then alter c2 until it doesn't. A 1nF variable will probably be HUGE.

ah, right on then, thanks for everything! I'm gonna go ahead and build this circuit and i'll update this letting you know how it goes.

[Shoewreck]

Originally Posted by bassbenj

You can't really have a cap for a volume control. I've never used this circuit so I can't say things for sure about it, but I have a couple of comments. One is that input impedance for a bass preamp should be 10 megs. 1 meg inputs is more for guitar. Note R3 is 1 meg. If the input isn't very high Z you tend to lose the deep bass. I'm worried about the capacitive divider used to control gain. The way this is set up it takes the inverted signal from R2 and feeds it back to the input. Generally this will tend to lower the input impedance even more! It may be worth a try but I am suspicious.

I get good results with the circuit. It really has less than 1 Meg input impedance, but the impedance is capacitive in audible range, just like a piezo pickup is, thus there is no loss of bass frequencies. It's just an ordinary inverting amplifier circuit, except it has a common source stage instead of an opamp and gain setting capacitors (It's safe to assume that piezo impedance is capacitive) instead of gain setting resistors.
A good buffer with high input impedance works just as good, but it doesn't have the gain presetting convenience.