Volme pot predicament...

[jbo]

Hey eveybody! I have a question about potentiometers here.

On a recently completed warmoth jazz bass, I used the StewMac jazz bass wiring kit for the harness. After I got everything completed and plugged in, I noticed that the volume pots don't have a true taper in the signal...in other words, very little sound comes out until the pot is almost wide open. Apparently this is common problem with passive circuits and is fixable by using an audio taper as opposed to a linear taper pot. But when I checked the pots i used, they were audio pots.

My question is, do I have a bad batch of pots? Is there a way to fix this (without buying new pots)? Just to clarify, I wired the bass following the schematic for a 60's jazz bass off the Fender website and I used three 250k pots.

I'm truly appreciative of any help on this! This little problem has got me kind of baffled!

Thanks in advance!!
Jordan

[metalmariachi]

Sigh;
In my experience audio (log) taper pots do that. The ones found on most basses, guitars & amps are 70% taper. meaning the first 50% is from 1 to 7 and the last is from 7 to 10.


Linear tapers don't do that.

If you want a really sweet blend, wire them backward, swap the wiring to the out side lugs (reverse audio in 250k or 500k are hard to find).
you can get really subtle blending of neck and bridge that way.

MM

[jbo]

hey man, thanks for the reply! I'm not quite clear what you mean by backward though....simply just the opposite of what it is now (mirrored) or changing the position of the hot wire from the pickup.

Thanks
Jordan

[jbo]

bumpity bump!

Also, I don't understand how a Fender you buy at a music store wired the exact same way works just fine, but the one I build has a problem....sigh.

[PilbaraBass]

are you SURE that they're audio?

what's the markings on them? or did you try them by turning them while disconnected from the wiring and connected to an ohm-meter?

reason being, is audio taper IS what you're after...voltage level is not linear with hearing, hence "audio" taper...it IS entirely possibly that you have the outside lugs backward and this would be a problem...

[jbo]

I checked the tone pot out and only saw 8A1 stamped on it (besides the 250k). I went ahead and bought new Fender pots today that were labeled as "audio taper 250k" since the ones I have in the bass aren't exactly clear as to what they are. So hopefully that will help.

New question: I have the Vintage Tele knobs on the bass and was wondering why they always tighten a little crooked, do I need to adjust the split shaft?

[ReidK]

Originally Posted by jbo

I checked the tone pot out and only saw 8A1 stamped on it (besides the 250k). I went ahead and bought new Fender pots today that were labeled as "audio taper 250k" since the ones I have in the bass aren't exactly clear as to what they are. So hopefully that will help.

New question: I have the Vintage Tele knobs on the bass and was wondering why they always tighten a little crooked, do I need to adjust the split shaft?

To identify the taper of a pot, all you need is an ohmmeter:

Set the range (if necessary) on the meter to 1 meg. Measure between the two outer terminals on the pot (the setting of the pot doesn't matter for this one, but make sure the center terminal isn't touching the probe or anything else.) This number should be somewhere in the vicinity of 250K (meaning anywhere from 220-280 is fine). Now set the pot in the center of its travel (halfway). Measure from the center terminal to each of the outer ones. If these are about equal, and about half of the first measurement, it's a linear pot. If one is much larger than the other (like at least twice as large), it's a log or audio pot. It could be reverse audio, but that's not very likely.

As for the knobs... you said "tighten". Do these knobs have a set screw in the side? If so, you don't want a split shaft pot. You might get away with spreading the split shaft a little wider, but that's kind of a mickey mouse fix. Either replace the knobs or the pots if you want it to be right.

Reid

[DavidRavenMoon]

Originally Posted by PilbaraBass

are you SURE that they're audio?

If he bought the Stew-Mac kit, then they are audio taper.

[jbo]

That's what I thought about the StewMac kit. Anywho...I tried the new pots and it's still the same situation, if not worse. Is there a certain way an audio taper pot should be wired (for instance, the ground always has to be on this lug)? Should I try switching the ground and output lugs? Maybe that was already suggested to me but I missed it.

[DavidRavenMoon]

No, if you are using them in the normal "right handed" rotation, they should be fine. But that's how audio taper works. Some brands of pots might have slightly different tapers.

You might like linear taper better. With those, turning it half way up is really half way up.

[jbo]

I'm gonna try linear taper. Right now it's either off or on, no (well...hardly any) in between at all. Especially since it's gonna be fretless soon and I want to be able to have a nice blend of both pickups.

[::::BASSIST::::]

Interesting. Please update us with what happens with the new linear taper pots.

[jbo]

I put in the new pots and they do exactly what I wanted them to do!! I can solo the bridge pickup and add in just enough of the neck to add some nice bottom end. Unfortunately they were 500k rather than 250k, but I'll fix that later.

The only problem now is that I can't get the tone pot to work. I've followed both the Fender schematic and a StewMac schematic, both of which are different by the way, and with both of them, I can't get the tone pot to do anything. Any thoughts? Does the linear pot used for tone have anything to do with it? Also, I've noticed that the output is a little lower, again, is it just the linear thing, or is it the 500k resistance??

Thanks!!
Jordan

[DavidRavenMoon]

Check your wiring. Linear taper doesn't matter, and that's what tone pots usually are.

The 500K might have a better sweep than 250K, so keep that in mind if you switch them.

Stew Mac's diagrams are confusing.

Try Duncan's, it's much better.

Standard Jazz bass

[Bassamatic]

Volume pots should be audio taper because 50% voltage is not 50% volume. Our ears are logarithmic so you need a log pot. Perhaps the Stew-Mac parts were mis-marked - it can happen.

A linear pot used for volume will give you the change very fast at the end. Linear is used for tone because it is different from volume.

The tone pot is so simple - be sure that the cap is on one end and goes to ground, the center is connected to the hot lead of your output, and the other pot tab is unconnected.

[DavidRavenMoon]

Originally Posted by Gio S

Volume pots should be audio taper because 50% voltage is not 50% volume. Our ears are logarithmic so you need a log pot.

But that's all relative. What's the reference point to tell if 50% is 50%?

A 10 Watt amp at half power is not as loud as a 100 Watt amp at half power.

We also don't hear bass frequencies as well at low volume, which is why stereo preamps often have "loudness"buttons to boost the low end.

Tone controls use linear taper pots because the control at 50% IS 50% of the treble rolled off. The same is true for graphic EQ's.

EMG uses linear taper pots for the blend controls.

Perhaps the Stew-Mac parts were mis-marked - it can happen.

No they are marked right. It can be tested with a VOM.

It's probably the particular taper being used. Very few pots have real log tapers. They have two linear taper which switch as you are turning the pot.

I like some audio taper pots, and dislike others.

[PilbaraBass]

Originally Posted by DavidRavenMoon

But that's all relative. What's the reference point to tell if 50% is 50%?

A 10 Watt amp at half power is not as loud as a 100 Watt amp at half power.

We also don't hear bass frequencies as well at low volume, which is why stereo preamps often have "loudness"buttons to boost the low end.

Tone controls use linear taper pots because the control at 50% IS 50% of the treble rolled off. The same is true for graphic EQ's.

EMG uses linear taper pots for the blend controls.



No they are marked right. It can be tested with a VOM.

It's probably the particular taper being used. Very few pots have real log tapers. They have two linear taper which switch as you are turning the pot.

I like some audio taper pots, and dislike others.

i did a thread once on making a simulated audio taper using a linear taper pot and a fixed resistor shunting across the wiper and one of the legs...I'd have to look it up to find the link...

the advantage of this approach...1 (you only have to mess with linear tapers) ... 2 (you can adjust the taper to suit your own tastes ... 3 (there is no "step taper" as you just described, David)

[DavidRavenMoon]

Originally Posted by PilbaraBass

i did a thread once on making a simulated audio taper using a linear taper pot and a fixed resistor shunting across the wiper and one of the legs...I'd have to look it up to find the link...

the advantage of this approach...1 (you only have to mess with linear tapers) ... 2 (you can adjust the taper to suit your own tastes ... 3 (there is no "step taper" as you just described, David)

Yes, I remember that thread.

I've seen that info elsewhere as well. I did a little searching though my book marks and found this article at the GeoFex site, the Secret Life of Pots. It explains all the various taper. He talks about adding the series resistor to control taper. There are no actual examples though.

In that R.G. Keen writes:

If you used an audio/log taper pot for the control of the power supply we mentioned, the output voltage would increase very slowly at first, creeping up to maybe 10% of the final output at 50% of the pot rotation. It would then blast the other 90% in the last half of the rotation - very hard to control.

Which is the issue that was posted here... not much happens until the last 25% of the rotation. This is clearly an issue where the pot is using a two step linear taper to simulate a true log taper.

I think the answer is to get a higher quality audio taper pot, that uses a better taper, or get an easier to find linear taper pot, and add the resistor to change the taper.

[jbo]

I've heard of using a resistor like you guys describe, but just to clarify, this is what you're talking about right

[img]http://shop.rabtron.co.za/catalog/images/resistor1_2w.jpg[img]

if so, what resistance vale does it need to be?

[jbo]

whoops, hmmn I thought I did that right