"Balance" control

JohnB

For the bass I am building I am planning to use a balance pot to mix the pickups, with a no selector switch and  single volume and tone control.  Any thoughts as to the type of pot that might be used? I am leaning towards a 1 - 5 Kohm linear.   Otherwise I could just use a separate volume for each pickup.

Build thread 

http://www.thefretboard.co.uk/discussion/45812/i-m-all-about-that-bass#latest

mart

I've used this one with success before: http://www.thomann.de/gb/goeldo_balance_poti.htm

JohnB

Interesting @mart - that looks like a duplex volume ? Is it two log pots, tapered in opposite directions?

ecc83

Such pots were common on hi fi and other audio systems and intended to correct for less than symmetrical speaker placement (or sometimes different speakers!).

Yes, one pot is a regular log track the other anti log. Note that a 500k pot will insert a serious amount of "tone sucking" resistance into the circuit but maybe HF loss is no so much an issue with a bass?

Dave.

ICBM

There's a neat way of doing it which avoids the problems of dual-gang balance pots, although it does mean the blended sound is different - put the pickups in series and use a single Linear pot.

That gives a uniform volume across the whole blend range with no tone suck. I'm not sure why it isn't more commonly used…

If you're going to do that, you ideally need both the blend pot and the volume pot to be double the value they would normally be for the same pickups to maintain the same total loading, so for typical Jazz Bass pickups, a 500K Lin for the blend and a 500K Log for the volume.

If that sounds interesting I'll write up how to connect it .

mart

The pot I linked is essentially what Warwick use in most of their basses, which is what led me to it initially - copying and modding their circuitry. 

My understanding is that the main benefit is that when both pickups are on (ie in the centre position), you get the full volume from both with no attenuation.

@ecc83 I'm curious about the tone loss, and how to avoid that - using a 1meg pot instead?

And @ICBM I'm interested in the series circuitry - I'd love to see a circuit and hear the pros and cons.

ecc83

I don't know about the Warwick circuitry but in the general application the balance control inserts about 6dB loss per channel because there is a contribution from two speakers. (see Duggie Self's Audio Circuits for the full SP on pan pot laws and balance controls.)

I could be wrong but I don't see how this can work in a passive circuit and give zero loss and a low output Z?

Dave.

ICBM

OK .

It's actually very simple, as long as at least one of the pickups doesn't have a single-core shielded cable. If they have single wires (like most J pickups) then it's easy.

Connect the ground end of the neck pickup to the ground (anticlockwise) terminal of the blend pot and to the overall ground.

Connect the hot end of the neck pickup and the ground end of the bridge pickup to the middle terminal of the blend pot.

Connect the hot end of the bridge pickup to the top (clockwise) terminal of the blend pot and to the top terminal of the volume pot.

That will give you the 'obvious' direction of rotation with 'forward' being full neck pickup and 'back' being full bridge pickup.

(The tone pot is connected as normal to the top end of the volume pot and the jack as normal to the middle terminal of the volume pot.)

Any shielding in the pickup cavities should still be connected directly to the overall ground, not to the pickups.

mart

ecc83 said:

I don't know about the Warwick circuitry but in the general application the balance control inserts about 6dB loss per channel because there is a contribution from two speakers. (see Duggie Self's Audio Circuits for the full SP on pan pot laws and balance controls.)

I could be wrong but I don't see how this can work in a passive circuit and give zero loss and a low output Z?

Dave.

Oh dear, I'm beginning to think I'm completely out of my depth here. That 6dB loss because of contribution from two speakers - what is that relative to? Is that 6dB lower than if, say, you just connected the pickups in parallel with no volume control?

Actually, I just remembered that the Warwick pots (made by MEC, and available from them at stupid prices) aren't simple log pots. Each track seemed to be 2-piece linear, so for the first half of its travel on one track it would do nothing, leaving virtually no resistance between the wiper and one tag, and then in the second half it travelled the full range so that, at the other extreme, there was no resistance between the wiper and the other tag. And the other track would do the opposite - sweeping through the full range in half the travel, and doing nothing for the second half of travel.

So, in the middle position, both pickups are in parallel and connected to the output with no resistance between them and it, and connected with 2 500k resistances in parallel with them (the two tracks on the blend pot). Is that still going to give the 6dB drop? Or is the 6dB drop a result of using log pots instead of these 2-piece linear tracks?

mart

ICBM said:

OK .

It's actually very simple, ...

Ok, I understand the circuit, and it's as I'd guessed. I suppose that when you switch from one end, and one pickup, through the centre position with both pickups in series, you get quite a lift in volume, dropping away again as you go through to the other end. Is that right, or have I missed something?

ICBM

mart said:

Ok, I understand the circuit, and it's as I'd guessed. I suppose that when you switch from one end, and one pickup, through the centre position with both pickups in series, you get quite a lift in volume, dropping away again as you go through to the other end. Is that right, or have I missed something?

The volume stays consistent throughout the turn because the volume on one pickup falls while the other rises. When it's at the centre, each pickup is at half volume. (Assuming the pickups are adjusted so they're same volume individually!)

You're correct about the Warwick pot if I remember, that's how it works. You will get a small dip in volume at the centre caused by the natural phase cancellation between the harmonics, but not much.

mart

ICBM said:

The volume stays consistent throughout the turn because the volume on one pickup falls while the other rises. When it's at the centre, each pickup is at half volume. (Assuming the pickups are adjusted so they're same volume individually!)

...

Ah yes, the resistance that's in parallel with the pickup gradually attenuates it as it goes round. I've never been good at understanding parallel wiring. I've got the hang of what happens if you put zero resistance in parallel with a pickup, and what happens if you put a large enough resistance in parallel, but I get vague about what happens in between - is it just a linear interpolation, or something more exotic?

JohnB

I was thinking to simply use 1 linear pot with one pickup connected to each end of the track and the wiper going to the volume and tone circuit. Could just use separate volume controls like my Aria bass 

ICBM

JohnB said:

I was thinking to simply use 1 linear pot with one pickup connected to each end of the track and the wiper going to the volume and tone circuit.

That won't work at all well - it won't turn down the unused pickup very much, even with a very high pot value. For a volume control to work properly it needs to be a potential divider, not just a variable resistance.

JohnB said:

Could just use separate volume controls like my Aria bass 

That will work, but I find a blend control easier to use and a more 'elegant' solution. If you're going to fit two pots you might as well make one of them a blend if that's what you want.

JohnB

Thanks @ICBM I should have thought of the potential divider point

ThePrettyDamned

ICBM;760440" said:

There's a neat way of doing it which avoids the problems of dual-gang balance pots, although it does mean the blended sound is different - put the pickups in series and use a single Linear pot.

That gives a uniform volume across the whole blend range with no tone suck. I'm not sure why it isn't more commonly used…

If you're going to do that, you ideally need both the blend pot and the volume pot to be double the value they would normally be for the same pickups to maintain the same total loading, so for typical Jazz Bass pickups, a 500K Lin for the blend and a 500K Log for the volume.

If that sounds interesting I'll write up how to connect it .

I think on my prs alike, I wired the pickups to a single linear reverse pot so as you turn it one way it blends to one pickup and the other, the other.

Works great cheaper and easier to repair too.