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Which Pot for Active EMG?

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Which Pot for Active EMG?

Postby Gordon Bellerose » Fri Mar 03, 2017 12:51 am

I have been reading about EMG active pickups, and they recommend a 25 k pot.
Is that the correct value, or a misprint?
I need your help. I can't possibly make all the mistakes myself!
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Re: Which Pot for Active EMG?

Postby Beate Ritzert » Fri Mar 03, 2017 6:54 am

That's a correct value; the output impedance buffer of an active pickup is designed for such small loads. The biggest advantage of such a design is the lack of audible treble loss when the volume is turned down.
Even smaller values might become difficult because that might draw too much current from the buffer - we do not know the details of the buffer design. Larger is of course possible, but You would lose the biggest advantage of active pickups - the lack of treble loss when the volume is turned down. The more the larger the value of the pot is. Aside of that the value is uncritical - if You don't have a 25k pot at hand, but some 50k or 100k You might give these a try - that just reduces the usable length of the cord a bit (in terms of treble loss). With pots of 250k or 500k the guitar would work similarly to a guitar with passive pickups. Not quite, because the treble resonance of the pickup will remain decoupled from the circuitry after the buffer.
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Re: Which Pot for Active EMG?

Postby David King » Fri Mar 03, 2017 6:32 pm

Beate,
That's a great answer! Thanks.
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Re: Which Pot for Active EMG?

Postby Gordon Bellerose » Sat Mar 04, 2017 3:07 am

I just saw your answer Beaten. Very well explained. Thanks.
I need your help. I can't possibly make all the mistakes myself!
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Re: Which Pot for Active EMG?

Postby Brian Evans » Sat Mar 04, 2017 11:17 am

Beate, why do you feel a high resistance pot will bleed trebles audibly when driven by a solid state pre-amp? I have always felt that the high frequency bleed with a passive magnetic pickup was due to the reactance of the pickup coil, which has a decent inductance from the coil and capacitance from the winding. I would have assumed that the output impedance of an active EMG pickup was pretty purely resistive in nature. FWIW, I swapped an active emg into a guitar and didn't change the pots, which were 500K. It worked fine, the volume control worked, the tone control worked but the output was extremely high. I believe that the EMG design has a 10K output impedance (as mentioned I assume it's mainly resistive) and the 25K volume pot is there to act as a voltage divider to reduce the output at full volume. I have read that a typical passive pickup will have an output impedance (not DC resistance, which is usually around 6K ohms to 10K ohms) of around 100K ohms at 3KHz to 5Khz, and that is what makes the pot a serious high frequency bleed. My electronics is a long time ago and a galaxy far far away, though... :)

Edit: I recall reading about how the 250K pot for single coil and 500K pot for humbucker tradition came about. Leo Fender put 1 Meg pots in his first Broadcasters, and the trebles were too shrill. Not enough treble bleed. He went to 250K pots to tame the high end by increasing treble bleed. Humbuckers were naturally less shrill, and needed less taming, so they tend to get 500k pots. You imply that high value pots will have greater treble bleed with active pickups - I would love to understand why?
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Re: Which Pot for Active EMG?

Postby David King » Sat Mar 04, 2017 6:14 pm

Brian, In a passive circuit the cable capacitance probably overwhelms the coil's self capacitance. An active pickup with a lowish impedance output should be able to drive a 100 feet of cable without noticeable treble loss. A passive instrument will start getting bogged down in a 10-20 food cable. The good answer is to buy a better cable but that's easier said than done.
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Re: Which Pot for Active EMG?

Postby Beate Ritzert » Sun Mar 05, 2017 7:12 pm

Sorry for reacting slowly, i am currently having severe trouble in my business; it looks a if i need to close my shop for a year or so. And that takes up thinking capacity...

Brian Evans wrote:Beate, why do you feel a high resistance pot will bleed trebles audibly when driven by a solid state pre-amp?


The upper resistor branch of a vol pot (upper resistor of that voltage divider) and the following capacitances (cable capacitance and input capacitance of the amp) form a low pass with a corner frequency dependent of the pot position. It will become smaller when the resistor is large.

The output impedance of the circuit - regardless of active buffer or passive pickup - can usually or at least qualitatively be neglected because it is small compared to the (full!) value of the pot.


On the other hand, the cable capacitance also has an effect on the resonance of a passive pickup, and it matters a lot if the pot is far open or if its far closed. That effect does not occur in an active setup, with a buffer on the pot. So a value of, say, 250 k or 500 k will lead to a treble loss in an active setup as well, but the sound will be affected differently when the volume is turned down. In order to avoid this You might consider a setup of
"buffer" -> "pot" -> "buffer".
As there is usually a very small capacitance of the input of a buffer, the value of the pot does not affect the sound to much.
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Re: Which Pot for Active EMG?

Postby Brian Evans » Mon Mar 06, 2017 11:46 am

"The output impedance of the circuit - regardless of active buffer or passive pickup - can usually or at least qualitatively be neglected because it is small compared to the (full!) value of the pot."

I had a different view of the impedance of a passive pickup, obviously. The research linked below shows that a typical pickup (p-90, for example) has an impedance of up to 900K ohms peaking between 5Khz and 8Khz. At those frequencies, the pickup impedance would seem to be quite material. Layer that on top of the other influences, and the non-linearity of human hearing (massive peak around 5Khz, down almost 20db around 100 hz) and who knows how to predict what you will actually hear! I always have to step back and remind myself that this is not my old Hi-Fi system - guitars produce sound, they don't reproduce it. The result is what matters, not the path.

BTW, the research linked also measures the circuit parameters including a volume pot, a cable, etc, showing how all the things link together. It's quite interesting. I'm sorry to hear you are having business issues - you wish they would go away so you can focus on fun stuff.

https://courses.physics.illinois.edu/ph ... ements.pdf
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Re: Which Pot for Active EMG?

Postby Beate Ritzert » Mon Mar 06, 2017 12:41 pm

Brian, that's right, but it is not the whole story. It is indeed true that the pickup resonance needs to be damped by the load impedance (it is usually a bit smaller than in Your example, but Your point remains true, of course) - without that damping a guitar pickup sounds terrible. You need such a resistive load and also the capacitive load equivalent of the cable in it he input of a buffer as well.

But outside the resonance the load impedance is always large compared to the load. Regardless of that the lowpass effect i described is real, and it will occur also with high Z pots after some buffer stage. And it will be noticeable. Another example where this can be seen is the Miller effect in connection with large grid stoppers of a tube stage, which forms a very similar low pass if You look at the equivalent networks.
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Re: Which Pot for Active EMG?

Postby David Schwab » Thu Mar 30, 2017 10:59 am

I have found that the older EMG pickups (not the X series) actually sound better with 100k pots. They had more presence. The reason is that the output impedance of the original series of pickups is a bit on the high side (about 150k). So you get less loading with the higher value pot.

On a passive instrument, the reason why something like a 1MEG volume control will sound duller when turned down is that the volume control is a voltage divider. So when the pot is on 10, you have the full resistance between the signal and ground, and no resistance with the signal in series. This is will give the brightest tone, and the pickup will have a higher resonant peak. But if you turn that pot half way down (assuming it's a linear taper), you now have 500k to ground, but also 500k in series with the pickup. This increases the impedance and dulls the high end.

In the end, using a buffer right after the pickup is the only way to retain the full frequency response.
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