Volume before preamp, or after?

[ctmullins]

Hi guys - I've had this question rattling around in my head for a while now, and I can't find any definitive answers, so I'm turning to you experts.

I've seen lots of preamp wiring diagrams. Some have a 50k volume pot wired between the preamp and the output jack. Some have a 250k/500k volume pot wired between the pickup and the preamp. Some have both as an option.

Are there any tonal considerations between these two options? I'm going to be wiring up some new basses soon, and I could do it either way, but I don't know if one method is preferred over the other, and if so, why?

Thanks!

[line6man]

If you want an active/passive switch, the volume must go before the preamp, otherwise the volume pot will load the high impedance passive signal. Otherwise, it's generally preferable for the volume pot to be after the preamp.

[ctmullins]

Thanks. Here's a bit more info - I do plan on wiring in some kind of active/passive switch, but the intent is to use it only as a failsafe. As such, I was thinking of putting it on a push/pull, and perhaps routing just the neck pickup straight to the jack. You know, an "emergency" mode that would have the fewest electronic components possible.

I could accomplish that using a 500k push/pull before the preamp, or a 50k push/pull after it. Would either of those be preferable over the other for any reason?

[Smilodon]

I take it we are talking about passive pickups here, in which case you'll need a 500K (250K to 1M) before the preamp.

You could do a 50K after the preamp, but you then can't use the volume control in passive mode.

[Steve Dallman]

The advantage to having a post preamp volume is that whenever you turn the volume down, you turn down any hiss or noise the preamp contributes. A volume after the preamp also causes less tonal change of the pickup. I'll take improved signal to noise ratio over passive operation.

I've put active preamps in my basses since the mid 70's and have never had one fail, or lost a battery during a gig.

[ctmullins]

Originally Posted by Smilodon

I take it we are talking about passive pickups here...

Yep.

... in which case you'll need a 500K (250K to 1M) before the preamp.

You could do a 50K after the preamp, but you then can't use the volume control in passive mode.

I'm okay with that. As a failsafe, running the pickup straight to the jack seems reasonable to me.

Originally Posted by Steve Dallman

The advantage to having a post preamp volume is that whenever you turn the volume down, you turn down any hiss or noise the preamp contributes. A volume after the preamp also causes less tonal change of the pickup. I'll take improved signal to noise ratio over passive operation.

Thanks - this is the sort of info I'm after!

I've put active preamps in my basses since the mid 70's and have never had one fail, or lost a battery during a gig.

I used to be dogmatically pro-passive, for purely ideological reasons. After I heard enough people say just what you said above, I began to re-think things. Now my stance is completely reversed! But I still like the idea of having a failsafe mode (kind of like a "limp home mode" on today's electronically-controlled automobiles).

[line6man]

Originally Posted by ctmullins

I do plan on wiring in some kind of active/passive switch

Then you don't get a choice. If the volume pot goes after the preamp, it will have to be of the appropriate value for the low impedance signal it will be controlling at that point in the signal chain. In passive mode, the output impedance will increase significantly, but the load from the pot will remain the same, so you will kill most of the output.

[ctmullins]

... but, as mentioned, I'm proposing bypassing both the preamp and the volume pot when in passive mode ...

[line6man]

Originally Posted by ctmullins

... but, as mentioned, I'm proposing bypassing both the preamp and the volume pot when in passive mode ...

Nevermind, I should pay attention sometimes.
You might consider a fixed resistor across the output, to simulate the load of a pot in passive mode. The pickup will be brighter and hotter without it.

If you're bypassing volume pots, definitely place the volume pot after the preamp.

[ctmullins]

Thanks! Interesting idea about the resistor - I'll probably try it both ways.

[bassbenj]

Originally Posted by line6man

Then you don't get a choice. If the volume pot goes after the preamp, it will have to be of the appropriate value for the low impedance signal it will be controlling at that point in the signal chain. In passive mode, the output impedance will increase significantly, but the load from the pot will remain the same, so you will kill most of the output.

Actually, you can put a 250k or 500k pot after the preamp. I've got a couple of basses like that. What you gain is you can have an "after" volume and still switch between active and passive. What you lose is that high resistance pot will isolate the active cable driver from the cable. Hence, unless the volume control is full on, you'll find that high frequency cable roll-off that is cured by an active preamp, will once again be there in both active and passive modes. (note a 250k pot after the preamp may require a 50k fixed resistor on the preamp output in active mode. )

Life is compromise.