Stompoxes modified for line level?

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Blue Jinn

Blue Jinn

Rider of the ARPocalypse
I've thought about doing this w/ a few of the tonepad boards. My thought is to lower the input impedance from 1M down to 50k - 100k, adding somekind of buffer after the output pot or better yet eliminating that pot and putting a simple opamp circuit to have a volume control w/ low output impedance, and upping the PSU from +9v to +/- 15v. (Or +12 to +18v if not opamp based)

Criticism invited.
 
Why do you want line level outputs Jinn?
What is the goal?
IIRC most op amps could handle 12 - 18 volt psi!!!!! IIRC.
You should easily be able to get enough gain out of a simple op amp circuit. I tried it with a voltage divider on the positive input.
Of course input anti pop resistor is optional.
Probably won't help with a low impedance thing but you could use it after you pedal of choice.
I might be off beam but I'm still unsure of the goal.
Sorry if this doesn't help.
 
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I'm looking to modify a stompbox circuit for line level in and out, so use as an insert or effect send on the board, and not as a guitar only effect. Add it to the rack.

I want to get e.g. a cheap compressor, using the tonepad Ross or DOD 280 clones. I want the headroom I'd get with a dual psu, (they usually just use a single supply on the opamp) and (I think this is true) lower noise with an input impedance down around 50-100k. Also, most stompboxes have a 100k or so pot on the end for "volume" which would be too high for a typical line level input. I've looked at those specifically, and can cut a few traces on the board to make those modifications. (I don't want to etch my own boards)

The Anderton compressor has a simple opamp circuit on the output side as volume/makeup gain, with the added benefit of low impedance output. That part is simple enough to put on one of those Radio Shack ic boards. The DOD circuit is similar to the Anderton, (optical) with a different side chain. I have drawn out the 280 circuit with basically that Anderton output. I *think* this would work, but am looking for input.
 
Blue Jinn said:
I'm looking to modify a stompbox circuit for line level in and out, so use as an insert or effect send on the board, and not as a guitar only effect. Add it to the rack.
Click to expand...

Stupid question maybe...
Would a DI box and a reamp box not do?
You get DI boxes with line out, right?
 
I f you wanted to keep to 9volts single ended there are a couple of wrinkles...

Use a dual op amp in balanced mode to deliver 6dB more signal volts. My back of env' math tells me that would give you about +20dBu max out?

Better would be a good low Z drive chip such as the NE5532, balanced drive mode again but into a 1:2 audio traff.

Then there are chips designed for low voltage work and can deliver near rail to rail outputs. The numbers escape me atmo but they can be pricey!

Dave.
 
@kip4 the link seems to validate the concept; tonepad has two circuits (that I've looked at so far) a Ross clone and a DOD 280 clone. I am not keen going point to point and I don't want to etch my own boards, so I'm looking for something "off the shelf" that I can mod. I have boards for the PRR Varimu (which I'll get around to) and the D-AOC, but the latter is fairly complex and I don't want to embark on that just yet.

@steenamaroo I would think that would work using a stompbox unmodified, but that is not what I'm looking to do here. FYI, there are boards available for the transformer/FET "Bo Hansen" DI.

@Dave: I don't want to stay single sided 9v, but use the stombox circuit, with a +/- 15v bipolar power supply. AFAIK, that increases the headroom, by giving the opamp more room to swing. Also, I want to stick as much as possible with the existing circuit to use an existing PCB. Two of the tonepad pc boards I've found so far I can cut traces to do just that. Also, AFAIK, it would be better from a noise standpoint (please correct me if I'm wrong there) to drop the 1M input resistor down to around 100k or so, and then buffer the output.

However, the transformer output sounds intriguing.
 
Blue Jinn said:
@kip4 the link seems to validate the concept; tonepad has two circuits (that I've looked at so far) a Ross clone and a DOD 280 clone. I am not keen going point to point and I don't want to etch my own boards, so I'm looking for something "off the shelf" that I can mod. I have boards for the PRR Varimu (which I'll get around to) and the D-AOC, but the latter is fairly complex and I don't want to embark on that just yet.

@steenamaroo I would think that would work using a stompbox unmodified, but that is not what I'm looking to do here. FYI, there are boards available for the transformer/FET "Bo Hansen" DI.

@Dave: I don't want to stay single sided 9v, but use the stombox circuit, with a +/- 15v bipolar power supply. AFAIK, that increases the headroom, by giving the opamp more room to swing. Also, I want to stick as much as possible with the existing circuit to use an existing PCB. Two of the tonepad pc boards I've found so far I can cut traces to do just that. Also, AFAIK, it would be better from a noise standpoint (please correct me if I'm wrong there) to drop the 1M input resistor down to around 100k or so, and then buffer the output.

However, the transformer output sounds intriguing.
Click to expand...

Lowering resistors will lower noise (by the sqr root of the ratio) but I don't have a link to the exact circuit?

Power supplies: Don't go above 15+15V, the gain in headroom for most applications is trivial for 18V (about 2dB IIRC) and at 2x 15V the chips will never fail!

If you go for a 1 to 2 stepup traff remember that the load reflected back to the IC is divided by 4 and you need a high current driver like the NEs, Tl072s might not cut it.

Dave.
 
Couple things.

1) AFAIK, there isn't a whole lot of current coming through the 1M bias resistor, and therefore not a whole lot of noise injected to begin with. Changing it doesn't affect the total noise floor very much. Series resistors are a bigger issue, including those in any feedback loop that might be involved. I'm not currently looking at a schematic, but I think this thing with the in-Z is a red herring for you.

2) +/- 15 V is 30V altogether. Assuming that all of the caps in the circuit are rated to handle 30V (most won't be if it's a kit intended for 9V), you can usually just plug in a +30VDC supply and go.

3) On the out-Z, the V pot will of course only affect it when it's turned down. That probably doesn't help. You can try to put something smaller in there, or stick a buffer after it. Buffers are easy, especially with opamps.

Hope some of that helps.
 
"1) AFAIK, there isn't a whole lot of current coming through the 1M bias resistor, and therefore not a whole lot of noise injected to begin with. Changing it doesn't affect the total noise floor very much. Series resistors are a bigger issue, including those in any feedback loop that might be involved. I'm not currently looking at a schematic, but I think this thing with the in-Z is a red herring for you.

Yes. (schematic PLEASE!) if it is just a bias resistor and thus shunted by a lower source Z the noise is virtually shorted out.
Noise due to current flow is called Shot Noise and is very much lower for the feeble currents in audio circuits compared to the normal Johnson noise.

2) +/- 15 V is 30V altogether. Assuming that all of the caps in the circuit are rated to handle 30V (most won't be if it's a kit intended for 9V), you can usually just plug in a +30VDC supply and go.

Again, without a circuit I can't really know but you cannot just slap 30V in place of a centre zero supply. There is a stunt you can pull with resistors or better with an op amp to generate a center zero.

3) On the out-Z, the V pot will of course only affect it when it's turned down. That probably doesn't help. You can try to put something smaller in there, or stick a buffer after it. Buffers are easy, especially with opamps."

Yes.

Dave.
 
ecc83 said:
2) +/- 15 V is 30V altogether. Assuming that all of the caps in the circuit are rated to handle 30V (most won't be if it's a kit intended for 9V), you can usually just plug in a +30VDC supply and go.

Again, without a circuit I can't really know but you cannot just slap 30V in place of a centre zero supply. There is a stunt you can pull with resistors or better with an op amp to generate a center zero.
Click to expand...
We're talking about converting a stompbox which is already set up for a single-ended 9V supply. The "stunt" with the resistors will presumably have already been accomplished. I'm saying that rather than convert it back to a center 0 (+/- 15 as has been mentioned) supply, he could just use a single-ended supply with the same total V.
 
Sorry folks, I was looking around for this. Linked is hte tonepad version. I am assuming that the feedback resistor at 1M is what you are talking about? I have to double check but I think that vactrol is eithe 1M or 500k? Adjust that for lower noise?

Also, why +30v instead of +/- 15? changing to dual supply on this one isn't that hard, and easier to get built (for me) than a 30v supply.

tonepad -- file information

The pdf was too big to attach...I will scour around for another version. There are a couple floating around with subtle differences. This was the one I intend(ed) to use.
 
Honestly not sure that it would be worth it for the noise. The GGG scheme I'm looking at has 3M9 in the feedback loop parallel to the vactrol. I don't know for sure what changing the balance of those two components will do, but if you can find a vac with smaller dark resistance, it should be a matter of scaling the R proportionately...but you also have to scale the R and pot on the ground leg of the feedback loop. You'd also have to scale the cap in that ground leg, except that it could probably just fuck off if you're going bipolar supply.

You might get more mileage more easily reducing all those 10K Rs in series with the signal. GGG calls the R1 and R6, then R7 has to match R6. I'm no expert by any means, but I cant imagine why those would need to be so big.

I really just recommended the single ended supply to save a little effort converting the circuit, but you're right that it should be pretty easy in this case. If the bipolar supply is at least as much easier as the "difficulty" of the conversion, then... Well, I guess there's also the thing that a bipolar supply requires a special connector. If you leave it single ended, you could use just about any DC supply, or even a battery in pinch. Maybe you don't need that kind of flexibility.

I'm kind of worried that a higher supply voltage is going to change the way the LED works. Seems like it'll come on sooner and be brighter for a given input...

I'm honestly not completely convinced that this is even worth it. The idea of adding headroom to a compressor circuit seems strange to me somehow. With 9V supply, the opamps will be able to put out somewhere around 7.5V p2p, which will light up your VU meters somewhere above 0VU. Do you really need more than that? You'd probably need to adjust the size of that pot we talked about earlier in order to be able to get a higher threshold for higher input Vs... It really might just need maybe an attenuator on the input and a good buffer (you could add gain here, if you really want more output) after after the V control and work just fine. Heck, it'd probably work just fine as is as long as you're a little careful with your levels. Don't suppose you have access to one of the pedals just to try?
 
Thanks. I already have a +/- 15v supply built :-) I'm not worried about connectors, as I'll hard wire the PSU and this (and some other units I'm looking at, including the Anderton that I've already built) will be racked. I've not had a chance to try out the Anderton, need to change a part on it, and wire it up. At the moment I'm still unburying from some construction, so this is all *theoretical* at this point. :-D

I looked at the other scheme too. I don't get the 3M vs 1M resistors with the same Vactrol, and I'm not sure whose clone is "correct."

I'll take a look at the suggestions w/ the 10k resistors. You are more competent that I am here!

Also, as to the LED, I am not 100% sure of myself, but I had (and I don't have my redraw here) changing the resistor after the LED to keep the same current at 15v. (Now, that is assuming I'm correct on that and that my Ohm's Law math is correct.)

I need to also compare this to the Anderton, and bounce against the Vactrol he uses. His is a line level or guitar level with a resistor change. His and the 280 seem to have similar design features.


I'll compare and look at my re-draw and post back. Thank you for all the input so far.
 
have you tried this with just an insert cable? since inserts are unbal hi-z connections (typically) or is the noise floor too high?

on another note, would be interesting to find out how a 5532 would work in that circuit compared to lm358.

for transformer coupling to/from unbalanced impedance equipment (used on inserts) to balanced line: 600:10K input 10K:600 out.
on a side note, Neve uses 10K to 10K transformers on the send and return jacks in the design of his boards.
 
drtechno said:
have you tried this with just an insert cable? since inserts are unbal hi-z connections (typically) or is the noise floor too high?

on another note, would be interesting to find out how a 5532 would work in that circuit compared to lm358.

for transformer coupling to/from unbalanced impedance equipment (used on inserts) to balanced line: 600:10K input 10K:600 out.
on a side note, Neve uses 10K to 10K transformers on the send and return jacks in the design of his boards.
Click to expand...

The 5532 is superior in almost every way ON PAPER! Especially for noise*
where it is specc'ed at about one tenth the noise of the LM. Slew rate is vastly better (but doesn't matter) but the NE draws about twice the current so watch that if you need a lot of them or battery life is an issue.

*This assume however that the IC is the predominate noise source, it rarely is. Often (as has been alluded to) it is big value resistors that make all the row!
In any case, once the box is connected to the "outside world" internal noise is often swamped.

Dave.
 
Output Pots even necessary?

I wanted to revisit this a bit. For this use, there is probably always going to be a fader preceding the circuit, so is it really even necessary to have a pot on the output as a "volume control" ?

E.g. PRR's Variable-mu does not, the "volume" is set by the signal being sent to it. I would also think the output impedances would already be low enough or a simple FET/transistor buffer, would suffice.
 
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