Pan Pot Characteristics

[Geoff_T]

Originally posted by The Buzz

2] Constant Power - this is where the Left (or Right) output of the panpot circuit will be 3dB down when the control is varied from hard Left to Centre (or hard Right to Centre).

Hi

That's the way all vintage Neve consoles were wired, if that's anything to go by.

[le purp]

are you using log and anti log tapers or
slugging linear pots?

Neve slugged linear pots.

Linears are cheaper and if don't like the way you slugged them you can change it, and change the center down point.



le purp

[Peter Simonsen]

Hi Tim,

I´d go with # 2..

2] Constant Power - this is where the Left (or Right) output of the panpot circuit will be 3dB down when the control is varied from hard Left to Centre (or hard Right to Centre).

Kind regards

Peter

[fleximan5]

Hi Guys
I recently rebuilt a Trident Fleximix console and replaced the original Omeg pan pots with Clarostat's. Both are 10K Log / anti-log.
The Fleximix manual calls out for 3.9K slug resistors for -3db at center. I tried this with the Clarostats and the middle is too quiet (noticed some centered vocals printed too hot on a mix). There really doesn't seem to be a coherent image at center either.

So I am guessing the tapers must be different?

I tried the Clarostat w/o slug resistors and the center position was definitely hotter than L or R.

Can anyone help me determine what value to use? What should I listen for?

I've tried a few values from 4.75K to 10K (more values on order) but I am not sure how I'll know if it is dialed in properly?

It seems to me that If I have the wrong value resistor I can't hear the recorded sound's image sound pan coherently from left to right. I get the sensation that center just sounds like equal volume from left and right speakers...the image just blows out/seems too wide?
I didn't have this problem with the Omegs and the slug resistors varied from 2K to 14K!
I'd appreciate any suggestions.
All the Best
F5

[Jim Williams]

Constant power is used in most designs, otherwise there would be a drop in level at the center postition.

The best I've heard/used is Soundcraft's "active pan" designs. Those use a dual 10k linear pan pot and an opamp buffer with negative AND positive feedback to create a pure sine/cosine response. Nothing else is as smooth. It's a patented design so you probably can't use it for a few more years. You can see examples on the Delta, Venue, Sapphyre, 3200, 6000, etc.

[Matt Syson]

Hi
Most of the desks manufactured for broadcasting (at least the UK) were -3 at centre (constant power).
However, the actual 'law' you have will depend on the pot and any 'slugging' done with pull up / down resistors.
I favour log / reverse log with pull downs as it helps maintain decent ( if not sparkling) cutoff at the 'ends'. Low value pull 'ups' require very low end resistance of pots.
Either Doug Self or Snurdcruft hold the patent for the quite cunning 'active panpot' design.
Matt S

[fleximan5]

Very interesting Jim and Matt.

Jim - I read an article on how you mod the Soundcraft consoles and make them sound incredible (very low cross talk if I remember as well).

Matt - So it would seem slugging resistors can be used to tweak Log/anti-log pots as well. Am I correct to assume that one of those active pan pot piece o kits might not fit in a Trident Fleximix input strip?

I am a NOOB so if you guys don't mind giving some advice/opinions, I have socketed one of my pan pots in order to try different resistors but I have no clue what range of values and in what resistivity increments I should be trying to get the best response?

If Trident recommends 2.2K for -6db and 3.9K for -3db on log/anti-log pots, what value resistor starts to pull down the center insignificantly?

I will be happy to post what I find.

Thanks Again for the notes guys.

[Matt Syson]

Hi
The 'active panpot' design needs certain other requirements as it involves amplifiers in a certain configuration so may not be applicable to other desks without considerable work.
Slugging resistors are determined by the whole circuitrry around the pan pot not just it's value and even when you have worked this out, pots are usually 20 percent tolerance so the value you find may not be correct for all channels or even both 'halves' of the same pot.
Put a preset or another pot in temporarily then measure it, or spend some time with a calculator and still be up to 20 percent out.
Slugging of 'simple' pots is possible but to do a similar trick with a fader, for example P+G or ALPS may not work as they require light loading of the wiper (typically 100K Ohms).
Matt S

[Jim Williams]

The active pan design requires the use of an opamp buffer before the pan pot. Trident used buffers in the 80 series as a straight voltage follower circuit. The Fleximix has no pan buffers, it's a passive pan pot design with slug resistors and insertion losses. You will just need to emperically select them.

I believe that patent was issued in the early 1990's, it may be available to use commercially as that patent may have expired by now. If so, I would use it.

[fleximan5]

Thanks very much Gents
I really appreciate the info......now to make sure everything else in the audio chain is top notch before I start my new adventure.
I did check the Clarostats with a voltmeter and the resistivity is very consistent from gang to gang and pot to pot (IE 9.9K/9.8K gang to gang). They are conductive plastic and sound very nice.
I will report what resistor value I find works best.

[fleximan5]

OK .... Very interesting.
So I tried 11 different values of resistors and 5.11K Ohms was the only value that made the movement of the image (male vocal) detectably pan from left to right and seem to sit detectably in the center.

Other values made the sound sound hard panned L or R.

4.99K and 5.6K resistors didn't work.

I will be trying other values I can get between 5K to 5.6K to see if I can get any improvement.

All the Best
F5

[Jim Williams]

If you pick up a couple cheap resistor decade boxes you can emperically find the right working slug resistors in a few minutes.
MCM Electronics: Home and Pro Audio/Video, Security and Test Equipment

If you measure the tapers on the original pan pots, you may find the tapers not log/antilog but "S" tapers. If that's the case, you may be best to order custom Clarostat pots with that option.

[fleximan5]

Thanks Jim
Much appreciated. I never knew such a box existed (saw one for capacitance as well).
The schematics call out for log/rev log but it wouldn't be the first time I found an error in the Flexi schematics. The bottom gang of the original Radiohm pot says Log B on it and a C is stamped on the top gang.

Am I correct that this is log / rev log?

Anyway just a couple of $ worth of resistors to try now.

I am surprised how specific the slug resistors have to be to get the desired effect though.

All the Best
F5

[fleximan5]

OK
So the final results are in and 5.23K ohm is the best.
I used Dale Vishay CMF55 resistors.
5.11Ks are also pretty good but the 5.23 value seems to allow better perception of the image moving around the center and more of a perceivable exact center.
For mixing, do you guys think some mixing and matching of the two values could be adviseable (I.E. low frequency signals vs higher frequency signals per channel)?
Any other thoughts?
All the Best
F5

[JohnRoberts]

Keep in mind the bulk resistance tolerance for most screened carbon element pots is on the order of 20%. Tweaking you resistors that finely may need to be repeated for each pot...

JR

[fleximan5]

Thanks John
I did have that thought....I'll go slow/one at a time (16 inputs, 6 aux's).
Lucky for me the clarostats are conductive plastic and I measured each gang to see how close they matched. They are all pretty tight.

[ruffrecords]

Originally Posted by fleximan5

Thanks John
I did have that thought....I'll go slow/one at a time (16 inputs, 6 aux's).
Lucky for me the clarostats are conductive plastic and I measured each gang to see how close they matched. They are all pretty tight.

What was the actual total resistance of each pot? I assume they are nominally 10K?

Cheers

Ian