MXR Phase 100 - output level [SOLVED]

[DjPsychic]

Hola,

Just got an older block MXR Phase 100. Love it, but I feel there's a little bit of volume loss when engaged. I did TB it so that may be culprit...

Is there a resistor I can swap out that would provide a little more output level?

[The G]

According to these guys, there seems to be an impedance issue and using a buffer (or a buffered pedal) after the phaser will cure it.

Off-topic: Please do make an effort to post into the proper forum.

[stolen]

Hi!

If it's like the ggg schematic, the output impedance should be reasonably low at around 3k. You could try to locate those output mixing resistors (R18/R19) and see if they actually have those values in case a buffered pedal remedies the issue.

Other than that, the input gain is 1+R3/R13, so you can adjust those values. You might not wanna decrease R13 too much, so replacing R3 with a 20k-50k trimmer would be the simplest solution.

Note that the resistors might be labelled differently in your version, so you might want to verify where the input buffer is located by simply tracing from the input pad, the feedback resistors should be nearby.

Hope this helps!

Edit: If it's an old unit, it could be very well dried out electrolytes too - particularily a retired C17 would result in quite the gain loss, among other things. You might wanna swap all of them if they look fishy.

[DjPsychic]

According to these guys, there seems to be an impedance issue and using a buffer (or a buffered pedal) after the phaser will cure it.

Off-topic: Please do make an effort to post into the proper forum.

Oh man I’m sorry for the misplaced post, my apologies!

[DjPsychic]

Hi!

If it's like the ggg schematic, the output impedance should be reasonably low at around 3k. You could try to locate those output mixing resistors (R18/R19) and see if they actually have those values in case a buffered pedal remedies the issue.

Other than that, the input gain is 1+R3/R13, so you can adjust those values. You might not wanna decrease R13 too much, so replacing R3 with a 20k-50k trimmer would be the simplest solution.

Note that the resistors might be labelled differently in your version, so you might want to verify where the input buffer is located by simply tracing from the input pad, the feedback resistors should be nearby.

Hope this helps!

Edit: If it's an old unit, it could be very well dried out electrolytes too - particularily a retired C17 would result in quite the gain loss, among other things. You might wanna swap all of them if they look fishy.

What I’m experiencing is only a very slight volume drop, so I don’t think anything is failing. Probably more likely a symptom of the filtering of certain frequencies. But still would like to see if a small boost in level is at all possible.

With the “intensity” on the last setting (4th click to the right) it’s less noticeable most likely due to the wider sweep.

If I were to replace R13 with a pot as suggested, do I also need to replace the value of R3?

Thanks for the informative response!

[stolen]

If I were to replace R13 with a pot as suggested, do I also need to replace the value of R3?

Nope, just one of them is perfectly sufficient . But we'd prefer R3 as a pot and keep R13 as is to not load the bias voltage too much.

[DjPsychic]

If I were to replace R13 with a pot as suggested, do I also need to replace the value of R3?

Nope, just one of them is perfectly sufficient . But we'd prefer R3 as a pot and keep R13 as is to not load the bias voltage too much.

Oops misread that my bad.

Thank you!

Slightly off topic - is there a thread on converting the "Intensity" to a normal pot? I think I've seen threads on a depth pot possibly?

[stolen]

Slightly off topic - is there a thread on converting the "Intensity" to a normal pot? I think I've seen threads on a depth pot possibly?

Sure. There's 3 things going on that the intensity switch changes:

- Resonance:
The side of the switch that connects R1 can simply be replaced by a mono pot by using the cw and wiper lugs. Maybe C100k or so to get an okay taper? If you only have log pots to play around with you can wire it up in reverse to find a value you like, of course.

- Modulation Bias:
There's already a trimmer in there, you can just make that externally accessible and restrict the range to useful values by messing with R46/R48

- Modulation Depth:
Actually the switch doesn't do a whole lot there from what we can see. Without changing the circuit a bunch it's not trivial to make that adjustable without changing modulation bias since it supplies a constant current on top of the lfo signal. As a hack, using another NPN follower from the bias voltage and blending with the Q4 output with a 10kB pot or so would probably give decent results with a reasonably low parts count.

This is 3 additional pots instead of only 1, but it probably would be worth it. Modulation bias is often overlooked but massively changes the sound of a phaser.

[DjPsychic]

Slightly off topic - is there a thread on converting the "Intensity" to a normal pot? I think I've seen threads on a depth pot possibly?

Sure. There's 3 things going on that the intensity switch changes:

- Resonance:
The side of the switch that connects R1 can simply be replaced by a mono pot by using the cw and wiper lugs. Maybe C100k or so to get an okay taper? If you only have log pots to play around with you can wire it up in reverse to find a value you like, of course.

- Modulation Bias:
There's already a trimmer in there, you can just make that externally accessible and restrict the range to useful values by messing with R46/R48

- Modulation Depth:
Actually the switch doesn't do a whole lot there from what we can see. Without changing the circuit a bunch it's not trivial to make that adjustable without changing modulation bias since it supplies a constant current on top of the lfo signal. As a hack, using another NPN follower from the bias voltage and blending with the Q4 output with a 10kB pot or so would probably give decent results with a reasonably low parts count.

This is 3 additional pots instead of only 1, but it probably would be worth it. Modulation bias is often overlooked but massively changes the sound of a phaser.

Dude thank you!

Just installed the 50k works great!

[DjPsychic]

- Resonance:
The side of the switch that connects R1 can simply be replaced by a mono pot by using the cw and wiper lugs. Maybe C100k or so to get an okay taper? If you only have log pots to play around with you can wire it up in reverse to find a value you like, of course.

Question about the Res pot: Are you replacing R1 with a C100k?Or wiring it in series with R1 and switch.

BTW I'm Using the GGG layout - it seems to match my 80's Block P100 ( http://www.generalguitargadgets.com/pdf/ggg_p100_lo.pdf )

[stolen]

BTW I'm Using the GGG layout - it seems to match my 80's Block P100 ( http://www.generalguitargadgets.com/pdf/ggg_p100_lo.pdf )

Oh that's useful info !

Question about the Res pot: Are you replacing R1 with a C100k?Or wiring it in series with R1 and switch.

R1 should remain, wire the pot in series. You can reduce R1 to increase the maximum resonance, but there should never be a short circuit. Also at high resonance the circuit starts self oscillating, which might be interesting for you if you're into noise music, but if you don't want that to happen if you accidentally kick the pot live on stage you might wanna tiptoe around that limit. A trimmer is a viable option here if you want to squeeze out the most resonance without oscillation.

We're not perfectly sure if C100k is the ideal taper to be honest, C50k might be a better fit, you'll need to try that on your own, not gonna throw that at a simulator today .

[DjPsychic]

R1 should remain, wire the pot in series. You can reduce R1 to increase the maximum resonance, but there should never be a short circuit. Also at high resonance the circuit starts self oscillating, which might be interesting for you if you're into noise music, but if you don't want that to happen if you accidentally kick the pot live on stage you might wanna tiptoe around that limit. A trimmer is a viable option here if you want to squeeze out the most resonance without oscillation.

We're not perfectly sure if C100k is the ideal taper to be honest, C50k might be a better fit, you'll need to try that on your own, not gonna throw that at a simulator today .

Sorry for this next question, but what position is "in series" in the circuit? I'm new to the more complex phase circuit, I'm really only familiar with OD/fuzzes at this point



In between R8/R1? Or between R1/switch?

Thank you again!

[stolen]

Hi, don't worry! It can be either - if two parts are in series and nothing is connected to their common node the order doesn't matter .

[DjPsychic]

Hi, don't worry! It can be either - if two parts are in series and nothing is connected to their common node the order doesn't matter .

Thank you so much. I'm testing a A100k by lifting one end of R1 Connecting pot then back into circuit board ("after" R1 towards rotary switch).

There is no noticeable difference, not sure what I'm doing wrong. Will keep troubleshooting

[stolen]

Interesting - R1 should be small enough for sufficient feedback. For testing you could just short R1 so that the potentiometer can go arbitrarily low; this should definitely be audible in the extreme settings. Are you sure the rotary switch is bypassed correctly?

[DjPsychic]

Interesting - R1 should be small enough for sufficient feedback. For testing you could just short R1 so that the potentiometer can go arbitrarily low; this should definitely be audible in the extreme settings. Are you sure the rotary switch is bypassed correctly?

Yes, I'm pretty sure it's been bypassed correctly. When I added TB switch everything sounded as it should.

I ended up skipping the resonance mod, and used the hole I drilled for the "gain/level" pot.

I also put in a dry/wet pot, replacing R18/R19 with a 10k pot. Per: https://www.diystompboxes.com/smfforum/ ... ic=74008.0

It worked for a while but now I've lost some of the phase stages, if that makes sense. Doesn't sound as full or lush.

Oddly though, if the circuit board gets shorted out while I'm testing (as in accidentally touches a switch lug.) the full phase seems to appear.

Looks like I've got myself buried in issues now

Needed to step away

[dallek1]

Interesting - R1 should be small enough for sufficient feedback. For testing you could just short R1 so that the potentiometer can go arbitrarily low; this should definitely be audible in the extreme settings. Are you sure the rotary switch is bypassed correctly?

Yes, I'm pretty sure it's been bypassed correctly. When I added TB switch everything sounded as it should.

I ended up skipping the resonance mod, and used the hole I drilled for the "gain/level" pot.

I also put in a dry/wet pot, replacing R18/R19 with a 10k pot. Per: https://www.diystompboxes.com/smfforum/ ... ic=74008.0

It worked for a while but now I've lost some of the phase stages, if that makes sense. Doesn't sound as full or lush.

Oddly though, if the circuit board gets shorted out while I'm testing (as in accidentally touches a switch lug.) the full phase seems to appear.

Looks like I've got myself buried in issues now

Needed to step away

Hello
I have a script; I installed the 50k pot per this information. Thank you everyone. Please can you point me to a layout with the part numbers please? When you say “if the circuit board gets shorted out while I'm testing “what do you mean? Because it at least sounds like you have an intermittent? One more thing do you have or know where the calibration procedure can be found for this unit, please? It is likely I have all of this but I'm getting old.
Thank you
ns