pot values post preamp

[dj5]

Could some kind electronics whizz tell me what the disadvantages are to having a 500K pot as the volume control post preamp as opposed to the recommended 25K?
It does work, but I'm curious as to what I'm losing tone-wise (in theory anyway!)
Many thanks

[mech]

I'd use what the designer recommends.

I'm not a whizz but since no whizz replied here's my understanding of what you are asking.

All amplfier stages have a load they like to work into. Power amps work best into the load they are designed for. Too low an impedence speaker and they overheat and too high a speaker impedence and the power is low.

Preamps are basically the same but kind of backwards. Too low an impedence and it will think it's working into a short circuit (low sound/no sound), too high an impedence and it thinks it has no load to work into and may be unstable. The pot on the output of your pre is part of the load it sees.

Preamps can "work" into a wide range of loads with varying results. Some may become unstable and oscillate when the load is too high. The oscillation may be above the range of human hearing and may damage the amplifier it's connected to.

There are so many variables that your setup would need to be put on a bench and tested to see if there would be any detrimental results tonewise or otherwise.
If you can't hear a difference tonewise there may be something happening you can't hear that may not hurt anything but may not be good.

mech

[walterw]

i guess oscillation is one theoretical problem, but the obvious practical proplem is that a 500k pot post-preamp will act like a switch, giving you full-on or full-off with nothing in between.

you're going to want 25k or 50k volume pots with active stuff to get an even sweep out of the pot.

[mech]

Would depend on the taper but I didn't want to get into that......

[dj5]

Thanks for the advice guys.
This was done on the advice of a famous (but I won't name him!) luthier/repair guy in the States, in an attempt to brighten up the tone in passive mode, the pot loading the pickups as it is post preamp.
This worked, in conjunction with a change of blend pot from 250 to 500K, but I was surprised to see that the pre had been installed from new with a 250K vol pot. Either this was an attempt on the original installer to improve the tone, or a mod/mistake by Bart themselves.
I understand what you're saying about amps being designed to work into a particular load, but I've had it like this for about two years now, and as it's working still I hope any damage would have shown up by now!
The answer regarding the taper is correct, although, believe it or not, the effect is not "on/off", it just gives a more pronounced increase in the end 25% of the turn - it's quite usable.
I was surprised myself about the advice I got, but I thought "this guy is well-known - he must know his stuff" and it certainly seems to have worked. I understand that noise promlems may be increased in this set-up, but nothing too bad has occured yet, and the reason I started the thread was, as I was changing out a faulty pot anyway, would it be advisable to change the vol pot back to its correct value. However, as I said, now that I've found that the original was 250, and there WAS a significant change going from 250 to 500, I am very reluctant to change back to 25K, and have en extremely woolly passive tone.
Do you really think any damage could occur from this mi-match?
Thanks again for the advice
Mark

[mech]

....blend pot ....the pot loading the pickups as it is post preamp.

That helps....

If the Barts are passive, 25K is way too low a value for a volume pot in the preamp bypass mode and I'm surprised that it was in the design. Could be a typo on the schematic.

The preamps I build have the blend feeding the volume control which feeds the preamp. This way the PUs see what they need in either mode. Output of the pre feeds the output jack. If the pre isn't noisey it works well live but any pre noise may noticed in recording situations. You might try this setup and see how it works. The volume control should be less sensitive set up this way.

The best solution is to have a stacked pot with one section of the correct value for the output of the pre when in active and the other section the correct value for the pickups in passive. This has a couple of drawbacks in the complexity of the switching and the possibility of annoying pops when switched from one to the other without the input of the amp being muted. If passive is used only for backup in the "dead battery mode" it'd be the best of both worlds.

If your current arrangement has worked for 2 years as is, I seriously doubt you'll have any probs. Things that aren't perfect from the theory standpoint often work well.

mech

[dj5]

Thanks for that.
I've seen those stacked pots - 25K/500K, but I wouldn't have a clue how to work out the switching.
Yes the Bart has the blend pre preamp, and the vol post - strange, and I don't know how they expected the passive set-up to sound with a 25K pot.
Never mind - it works, and it sounds bearable now, so I'll leave it!
Many thanks for your thoughts on this.
Mark