(Electric) Guitar "boost" pedals, how can we objectively compare these?

[johnnygeorgia]

Anyone else involved in the guitar community? There's definitely some interesting measurements that could be made to determine the quality of their equipment, but there is little to no actual data of anyone doing so!

In that subculture, most equipment is described very non-succinctly, like a famous pedal called the Ibanez Tube Screamer is "a great overdrive that has a mids hump". I'm left wondering "what frequency range defines 'mids'? what is the more specific shape of this hump? what happens to the bass and trebles?" Being so nonspecific, I'm wondering how we can get better answers on characterizing guitar equipment.

For example, I think the most straightforward item to measure would be a "boost" pedal, which simply amplifies a guitar's signal. I'm wondering what sort of measurements are relevant for this: I can definitely think of at least two items: frequency response at different gain ("boost") levels, and SNR. In one of the pedals I list below, there's mention of THD+n, but I'm not sure why that specifically is relevant.

Research of trying to find the "most transparent" boost online leads me to three separate products by different manufacturers.
1. The Pete Cornish NB-3 provides some data (but not any graphs). This company has a legendary reputation.
2. The Creation Audio Labs MK 4.23 posts some data themselves and a guitar magazine published a frequency response graph
3. The L.A. Sound Design BB-01 provides no data whatsoever (but the company has an outstanding reputation in guitar history)

I don't necessarily trust any of these as they are not third-party measurements (except perhaps the bass magazine one). Even if I did, they each provide different data, which makes comparing the products a headache.

So I guess my question to you guys is: what measurements are relevant in this case? And how could I obtain such measurements? Electrically, a guitar is a high-output-impedance driver that can only put out small signals, in the range of millivolts, if that matters.

Thanks so much!

 

[LTig]

For a simple boost panel meaduring S/N, FR, THD and IMD should be sufficient since it should be transparent. IMV S/N is much more important because FR is changed by tone controls anywhere later in the chain, as is intentional added distortion of the input stage, the only reason to apply a boost pedal in the first place.

 

[RayDunzl]

So I guess my question to you guys is: what measurements are relevant in this case? And how could I obtain such measurements? Electrically, a guitar is a high-output-impedance driver that can only put out small signals, in the range of millivolts, if that matters.

For a basic test, feed the gear under test a millivolt test signal from REW or similar and see what happened to what comes out...

 

[johnnygeorgia]

For a simple boost panel meaduring S/N, FR, THD and IMD should be sufficient since it should be transparent. IMV S/N is much more important because FR is changed by tone controls anywhere later in the chain, as is intentional added distortion of the input stage, the only reason to apply a boost pedal in the first place.

Thanks so much for the quick response!! My specific use case for a boost pedal is a bit different actually: I'm feeding a clean signal into the boost, which goes into a compressor. The idea here is to turn the compressor into effectively a leveling amplifier by boosting its input signal. This is to increase sustain. That logic checks out right?

Would FR be worth measuring at different "boost" levels? I imagine especially at high gain settings, there may be some wonkiness there.

For a basic test, feed the gear under test a millivolt test signal from REW or similar and see what happened to what comes out...

Thanks the response! Would the output impedance of the REW signal cause any discrepancy in performance between the REW signal and a real guitar signal?

 

[DVDdoug]

There's definitely some interesting measurements that could be made to determine the quality of their equipment

"Quality" is mostly subjective when you are intentionally trying to alter the signal in a creative or artistic way, except you could measure noise. But even noise is tricky because the guitar itself picks-up noise and any gain will boost it.

And as you may know, guitar amplifiers are not supposed to be high-fidelity. They all have their own sound characteristics and most guitar players have their favorite amp as well as their favorite guitar.

I read a story about a guy who repaired a guitar amp. Besides repairing the failure he found a "flaw" in the circuit design and "fixed it". But the guitar player said it didn't sound right when he got it back so the technician had to put it back the way it was originally built.

A boost pedal could certainly be clean amplification but you might overdrive and distort the amp's input.

 

[johnnygeorgia]

"Quality" is mostly subjective when you are intentionally trying to alter the signal in a creative or artistic way, except you could measure noise. But even noise is tricky because the guitar itself picks-up noise and any gain will boost it.

And as you may know, guitar amplifiers are not supposed to be high-fidelity. They all have their own sound characteristics and most guitar players have their favorite amp as well as their favorite guitar.

I read a story about a guy who repaired a guitar amp. Besides repairing the failure he found a "flaw" in the circuit design and "fixed it". But the guitar player said it didn't sound right when he got it back so the technician had to put it back the way it was originally built.

A boost pedal could certainly be clean amplification but you might overdrive and distort the amp's input.

Haha that totally sounds like something a guitar player would say! Thanks for the response. It's a great point that the amp is the be-all-end-all of the FR curve at the end of the day. With my particular use case, I'm inclined to think that it's still worthwhile to avoid coloring the signal where possible, though (but please correct me if you disagree). Do you think this use case makes sense? I'm feeding the boost signal into a compressor to effectively create a leveling amplifier.

I'm in awe of how quickly you all have been responding to me and am really grateful for the advice.

 

[RayDunzl]

Would the output impedance of the REW signal cause any discrepancy in performance between the REW signal and a real guitar signal?

Could...

Make figuring that out Step 2.

You'll find that even the cleanest signal from a guitar will be difficult to quantize.

Single note (electrical signal) of a bass guitar, softly plucked so the second harmonic wouldn't exceed the level of the fundamental:

Red is the peak level, black a little lower as the live signal decays.

The harmonic content will differ wildly depending on how the string is plucked.

 

[kemmler3D]

I think for a boost pedal, gain would be the first measure, then noise. Noise could be important for any gear, since if you add a lot of pedals to your setup, at some point you risk having really annoying hiss while you're up on stage.

However, as @DVDdoug says, I don't know if there's a lot of purpose in measuring THD for guitar effects.

 

[OnLyTNT]

"Quality" is mostly subjective when you are intentionally trying to alter the signal in a creative or artistic way, except you could measure noise. But even noise is tricky because the guitar itself picks-up noise and any gain will boost it.

And as you may know, guitar amplifiers are not supposed to be high-fidelity. They all have their own sound characteristics and most guitar players have their favorite amp as well as their favorite guitar.

I read a story about a guy who repaired a guitar amp. Besides repairing the failure he found a "flaw" in the circuit design and "fixed it". But the guitar player said it didn't sound right when he got it back so the technician had to put it back the way it was originally built.

A boost pedal could certainly be clean amplification but you might overdrive and distort the amp's input.

I agree. A simple FET booster should be sufficient for clean boost when needed a few db higher. Well, I mostly play high gain so what do I know

@johnnygeorgia, by the way a compressor is actually a kind of booster itself, and something +30 db into a compressor would be overkill in your case in my opinion, try out a simple FET booster, even may be a FET buffer would do the job may be?

 

[LTig]

Thanks so much for the quick response!! My specific use case for a boost pedal is a bit different actually: I'm feeding a clean signal into the boost, which goes into a compressor. The idea here is to turn the compressor into effectively a leveling amplifier by boosting its input signal. This is to increase sustain. That logic checks out right?

Yep.

Would FR be worth measuring at different "boost" levels? I imagine especially at high gain settings, there may be some wonkiness there.

I would measure it.

Would the output impedance of the REW signal cause any discrepancy in performance between the REW signal and a real guitar signal?

REW has no output impedance but the output of your sound interface. I would expect though this is much lower than a typical guitar pickup so no problem.