Purpose of this pot in API 536 circuit?

[JW]

Not the inspirational kind, but this potentiometer (1K) before the pan pot. I linked Boji's redraw of 536.

Is it just a final trimmer to adjust the channel level?

Which makes me ask another question. How different is a 2K2 stereo pan pot from a 10K stereo panpot in this circuit? I'm hearing the 2K2 pan seemingly appropriately. But I guess I should try 10K to see if there is a difference.

 

[mjrippe]

Looks like a level trim. Could be used to set the pan pot for -3dB at center?

 

[JW]

This isn't the slug resistor is it? Here's a similar schematic that Jeff drew up. https://www.capi-gear.com/catalog/images/gallery/pots/BI/Passive-pan-pot-network.pdf

It just looks to me like the 1K is a simple volume attenuator for final level trim, prior to the pan. Aren't the 33K the slug resistors. Which is a lot larger than Jeff has on his. (looks like a different method altogether)

 

[JW]

Duh. It has the labeling on the original schematic. 1K is indeed channel trim or "mix level" I believe it says on the schematic. Also, those 33K resistors are 3.3K on the original 536 schematic. Which explains my confusion in that regard. I believe this is what sets how far down the "middle" of the pan pot is. So, I'll try 3.3k and Jeff's value of 2.2K first.

 

[JW]

Also of note is the same arrangement in the 528 schematic, but a 1.5K pan pot and 1.8K slug resistors (not shown)

 

[FIX]

The channel trim was so you could place the fader at the optimum level for mixing with the 120mm, 600 ohm faders. The ratio was always attempted to be 10x, so the level was 1k, the pan 10k, and with the slugs, the bus resistors were 27k to 47k.

 

[ruffrecords]

The channel trim was so you could place the fader at the optimum level for mixing with the 120mm, 600 ohm faders. The ratio was always attempted to be 10x, so the level was 1k, the pan 10k, and with the slugs, the bus resistors were 27k to 47k.

Just to be clear, are you saying this 1K pot was actually a front panel control?

Cheers

Ian

 

[FIX]

It was concentric with the pan pot.

 

[ruffrecords]

It was concentric with the pan pot.

Cool. Good to know.

Cheers

Ian

 

[JW]

Ohhhhh. Okay. I didn't know it was a front panel control. I had rigged up the 20k resistor on the neg. input of the last DOA as another volume control pot. But I guess I would be boosting that way, and the 1K would be trimming down (????)

Or would it be somehow level matching 'pan off' with 'pan on' with an additional 1K (pot rolled all the way down) in series, so that turning up this pot (reducing resistance) would actually be gain above 'pan off'?

I guess my confusion is, if it is just an extra 'gain' or 'trim' control, why not put it on the common output of the pan switch so that both 'pan on' and 'pan off' can use it?

Also, I'm not seeing a path where one can send out to the main stereo ACA without going through the pan pot. Is this usual? Do API pan pots have a center detent, and signal always goes through them if sent to the stereo ACA bus?

For the viewer, the top of the pan switch (top right) is the output of the DOA, while the pan pot wipers come in on the top left, and connect to the 3.3K resistors and the bottom 'armed' position of the stereo ACA switch. Can one not send signal to the stereo ACA if pan is not armed? Weird. Maybe I'm just not seeing it somewhere.

 

[JW]

here.

 

[ruffrecords]

Ohhhhh. Okay. I didn't know it was a front panel control. I had rigged up the 20k resistor on the neg. input of the last DOA as another volume control pot. But I guess I would be boosting that way, and the 1K would be trimming down (????)

I had not expected that but from what FIX said I guessed that was the case.

Or would it be somehow level matching 'pan off' with 'pan on' with an additional 1K (pot rolled all the way down) in series, so that turning up this pot (reducing resistance) would actually be gain above 'pan off'?

I don't think anyone has ever done that in a mixer. The hard pan should match the un-panned. Hard to see how there could be any real difference

I guess my confusion is, if it is just an extra 'gain' or 'trim' control, why not put it on the common output of the pan switch so that both 'pan on' and 'pan off' can use it?

I think its use is as a gain trim to get all the main faders more or less at the same position. I am not sure how much use it is unless you have very coarse gain resolution on the mic pre gain switch

Also, I'm not seeing a path where one can send out to the main stereo ACA without going through the pan pot. Is this usual? Do API pan pots have a center detent, and signal always goes through them if sent to the stereo ACA bus?

It is standard to be able either to route a channel to single group or to a pair of groups or a stereo bus via a pan pot.. The channel signal is mono. How could you feed it to a stereo bus other than via a pan pot?

For the viewer, the top of the pan switch (top right) is the output of the DOA, while the pan pot wipers come in on the top left, and connect to the 3.3K resistors and the bottom 'armed' position of the stereo ACA switch. Can one not send signal to the stereo ACA if pan is not armed? Weird. Maybe I'm just not seeing it somewhere.

No you cannot. As I said, the channel is a mono signal. To feed it to a stereo bus the normal thing to do is use a pan pot.

Routing in different mixers varies a lot but in every classic console I know of, the channel is mono and is made stereo if required using a pan pot.

Cheers

Ian

 

[JW]

Thanks very much Ian. I'm understanding better now. I've been in Yamaha land for many years. The idea of a 'seperate' stereo bus is scary. Kidding, but here's how they did it on the Pm2000. No 1 dedicated stereo bus, but one of the 4 pair can be used.

So, when the pan switch is out here, the signal still goes to both legs of 4 pairs of buses. Also, I noticed that "pan on" matches "pan off" when "pan on's" pan pot is centered. Which makes it actually 3 dB louder hard panned, than "pan off"

(p.s. Sorry for crowding up the drawing board with too many posts on this subject. Just debating which pan circuit to go with considering I don't want to have to build out a master bus, with more rails that everthing can switch in/out of.)

 

[ruffrecords]

I agree it is unusual to have a separate stereo bus but as I said before, there are many ways to do routing.

Your Yamaha circuit has some quirks of its own. Notice is uses a 2KA and 2KC pots. Now a 2KA (log) pot is usually about 20dB down at the centre point which is way to much for a pan pot so I suspect this log pot is wired backwards which will make it about 1dB down at the centre. Then you have the 1K8 pull down resistor which is probably enough to make it 3dB. When you switch the pan out an 300 ohm resistor is switched in probably to make the level closer to the centre panned level. The figures do not quite stack up in my head so in that respect this circuit is as odd as the API one.

The common way to make a pan is to use a dual LIN pot (LIN pots are much more consistent than log or anti-log ones) with pull up resistors.

Cheers

Ian

 

[JW]

The figures do not quite stack up in my head so in that respect this circuit is as odd as the API one.

The common way to make a pan is to use a dual LIN pot (LIN pots are much more consistent than log or anti-log ones) with pull up resistors.

API's pan circuit is doing this though correct? A 10K dual linear pot with pull up resistors?

Your mentioning of 20dB down in the middle with log pots panning made something click in my head.

In my troubles figuring this out, I mistakenly, in the OTHER thread I had posted on this subject, said that I was -3dB down in the middle when I copied the API pan circuit, using 2.2k resistors and a 10K dual linear pot. Subsequent testing reveals I'm wrong. It's actually 20dB down in the middle! Don't know what I was looking at to have missed that initially, but . . .
I'm going to try another pot now. I suppose it's possible mouser sent me a wrong tapered pot. This is the one I used for the test. Says linear, but maybe a mistake was made somewhere. https://www.mouser.com/ProductDetail/652-PDB182K420K103B1

Because, this circuit shouldn't be down 20dB when panned to the center correct? (If a linear pot is used?)

 

[ruffrecords]

API's pan circuit is doing this though correct? A 10K dual linear pot with pull up resistors?

I would have hoped so but it looks to me like the have pull down resistors so that makes no sense at all.

Your mentioning of 20dB down in the middle with log pots panning made something click in my head.

In my troubles figuring this out, I mistakenly, in the OTHER thread I had posted on this subject, said that I was -3dB down in the middle when I copied the API pan circuit, using 2.2k resistors and a 10K dual linear pot. Subsequent testing reveals I'm wrong. It's actually 20dB down in the middle! Don't know what I was looking at to have missed that initially, but . . .
I'm going to try another pot now. I suppose it's possible mouser sent me a wrong tapered pot. This is the one I used for the test. Says linear, but maybe a mistake was made somewhere. https://www.mouser.com/ProductDetail/652-PDB182K420K103B1

Because, this circuit shouldn't be down 20dB when panned to the center correct? (If a linear pot is used?)

Correct. Its easy to check the pot out of circuit. Set it to about half way and you should have about 5K from the wiper to each end it it is a linear pot. If you are 20dB down it is probably because you are using a 10K pot with 2K2 pull downs

Cheers

Ian

 

[FIX]

They used audio taper for the left pan, rev audio for the right pan, and the slug resistors are to get the center point down to -3dB.

the level control was like another fader and it isn’t as deep as it seems. Just a level control to keep the fader in the best mix area.

in those days, if you didn’t like the mix you would recall is from Polaroids or drawings and remix it…

 

[Trouble Report]

I agree it is unusual to have a separate stereo bus but as I said before, there are many ways to do routing.

Your Yamaha circuit has some quirks of its own. Notice is uses a 2KA and 2KC pots. Now a 2KA (log) pot is usually about 20dB down at the centre point which is way to much for a pan pot so I suspect this log pot is wired backwards which will make it about 1dB down at the centre. Then you have the 1K8 pull down resistor which is probably enough to make it 3dB. When you switch the pan out an 300 ohm resistor is switched in probably to make the level closer to the centre panned level. The figures do not quite stack up in my head so in that respect this circuit is as odd as the API one.

The common way to make a pan is to use a dual LIN pot (LIN pots are much more consistent than log or anti-log ones) with pull up resistors.

Cheers

Ian

Considering that the pan pots feed identical left and right (odd - even) busses, I suspect that the 22kA and C designations are a typo. It’s one or the other. Or it’s one of the most bizarre panning arrangements yet.

 

[FIX]

A linear panner has 6dB of loss at each end. A aud/revaud as no loss at each end. When not using post buffers, the audio version is preferred. Linear panners don’t need pull ups.

 

[ruffrecords]

A linear panner has 6dB of loss at each end. A aud/revaud as no loss at each end. When not using post buffers, the audio version is preferred. Linear panners don’t need pull ups.

Depends what linear panner you are talking about. A simple low cost single LIN pot panner will have a 6dB loss at the extremities but one using a dual LIN pot with pull up resistors has no loss at the extremities. It also has the advantage that LIN pots are much more consistent than log and revlog pots.

Cheers

Ian