Super simple phase inverter for fuzz boxes?

[Zork]

Hi, I'm running a dual amp setup with a clean signal in a bass amp and the other signal through a pedalboard in a guitar amp. I'm having phase issues with my two main fuzzes, a Tone Bender Mk3 and a Ibanez Standard Fuzz. Both invert the phase when not in bypass and steal my low end, so I would like to put a phase inverter in both right before the bypass switch. Can you folks point me to a simple circuit that runs on 9V and doesn't mess up their sound?

[goroth]

Build the AMZ op amp buffer. It will invert your output.

http://www.muzique.com/lab/buffers.htm

You can get a board direct from Jack, or there are probably kits available at lots of diy sites.

[qersty]

lpb-1 is the simplest you can get

[Zork]

LPB-1 looks perfect with a few tweaks for less amplification. In the tone bender, I could replace the tranny with a pnp type, say a GT308, reverse the polarity of the power and be set for positive ground operation, right?

[Zork]

http://beavisaudio.com/techpages/HIW/hiw1.gif

...so if I understand this correctly I change the values for R3 and R4 and ditch the variable R5. Any ideas on values? I really don't need any additional amplification. And I wouldn't need to change R1 and R2, would I?

[goroth]

Yeah, the LPB is fixed gain and then you are just dumping the signal to ground with the pot/R5. Increasing R3 will lower gain, or decreasing R4. If my shitty understanding of electronics is correct unity should have R3 and R4 roughly at similar values.

LPB-1 has quite low input impedance, which may or may not be a problem. They sound rad, but they aren't very transparent.

[Zork]

I've got one of the newer reissues so I'll give it a try how they work with each other. To my ears the lpb1 beefs up the mids slightly, so this is actually quite cool for the fuzzes.

[goroth]

Yeah, if you've already got one just turn the volume down to unity and see if it does the trick, for sure.

[Zork]

Yep, there more I think about, the more I lean torwards simply replacing the pot with a trimmer to adjust everything to my needs. Like so: http://tagboardeffects.blogspot.com/201 ... r.html?m=1

One more question, though: for the positive ground version I need a pnp transistor. I've got some GT308 with a hfe of roughly 60-70 and a few AC126 and AC125 with hfe of about 110. Which one would you choose? To my understanding, the GT308 are much more stable and less sound shaping than the AC125/126 but the hfe value is crucial to the amount of amplification in the LPB-1. The 2n5088s or 2n5133s commonly used in the LPB1 seem to have a hfe higher than 300 typically.

[crochambeau]

If my shitty understanding of electronics is correct unity should have R3 and R4 roughly at similar values.

Close, but not quite.. making R3 & R4 identical will simply limit the available output swing the transistor can produce to about one half of the rail voltage (internal equivalent resistance of the transistor in saturation/full on will slightly change the absolute value of the "voltage divider" comprised of R3 & R4). The Beta, or gain, of the transistor can still take a lower level signal and push it up to that point.

I like to settle on a value for R4 (which affects bias (which affects input impedance)) and reduce R3 (within reason) to reduce gain. You can also set up a voltage divider/set and forget volume control/trim pot after C2 to normalize levels, as sometimes toying with R3/etc. too much enters the realm of sounding like shit. A lot of classic designs also put the level management at R3 by turning that into two resistors and tapping a divided output signal from there

My advice would be to get your stage sounding the way you want, and then reduce level as needed with some R5 equivalent (it does not have to have a knob) or the R3 divider trick.

[crochambeau]

I've got some GT308 with a hfe of roughly 60-70 and a few AC126 and AC125 with hfe of about 110. Which one would you choose? To my understanding, the GT308 are much more stable and less sound shaping than the AC125/126 but the hfe value is crucial to the amount of amplification in the LPB-1. The 2n5088s or 2n5133s commonly used in the LPB1 seem to have a hfe higher than 300 typically.

It sounds like amplification factor is secondary to getting your polarity inverted, so I would try some from each batch, select what sounds best to you, and then reduce level (if needed "?") on that.

[goroth]

If my shitty understanding of electronics is correct unity should have R3 and R4 roughly at similar values.

Close, but not quite.. making R3 & R4 identical will simply limit the available output swing the transistor can produce to about one half of the rail voltage (internal equivalent resistance of the transistor in saturation/full on will slightly change the absolute value of the "voltage divider" comprised of R3 & R4). The Beta, or gain, of the transistor can still take a lower level signal and push it up to that point.

Insert Maxwell Smart missed by that much dot gif.

Thanks for the great explanation buddy. That was kinda obvious in retrospect.

[crochambeau]

Identical values of negative feedback resistor and input resistor in an opamp stage will result in (very close to) unity gain, so the matching resistor rule does have a precedent in another topology.

[Zork]

Yeah, if you've already got one just turn the volume down to unity and see if it does the trick, for sure.

So, yeah, I tried it: Sounds pretty bad, unfortunately. It obviously shifts the amps back in phase, but the fuzz sound gets very harsh and brittle. It's actually much, much worse than the out of phase sound.

[Zork]

If my shitty understanding of electronics is correct unity should have R3 and R4 roughly at similar values.

Close, but not quite.. making R3 & R4 identical will simply limit the available output swing the transistor can produce to about one half of the rail voltage (internal equivalent resistance of the transistor in saturation/full on will slightly change the absolute value of the "voltage divider" comprised of R3 & R4). The Beta, or gain, of the transistor can still take a lower level signal and push it up to that point.

I like to settle on a value for R4 (which affects bias (which affects input impedance)) and reduce R3 (within reason) to reduce gain. You can also set up a voltage divider/set and forget volume control/trim pot after C2 to normalize levels, as sometimes toying with R3/etc. too much enters the realm of sounding like shit. A lot of classic designs also put the level management at R3 by turning that into two resistors and tapping a divided output signal from there

My advice would be to get your stage sounding the way you want, and then reduce level as needed with some R5 equivalent (it does not have to have a knob) or the R3 divider trick.

Thak you for the great explanation. I already learned a ton from this thread.