Trident series 65 "A LOG" pot?

[mkiijam]

Need to replace a few broken pots. The schematic calls for "A LOG." What is this? Normally I would think AUDIO LOG? or something like that or maybe "ANTI-LOG."

However, the schematic BOM also calls a few pots "INV LOG 2G" I'm guessing double ganged, but what does INV mean?

There are some marked "LOG C/D"?

Anyway, it's VR1 in the attached schematic. Maybe someone can look at the function and see if anti-log or "reverse" log makes sense in the circuit?

 

[mjrippe]

INV LOG could be Inverse Log (Reverse Log taper)
A LOG should be Audio Log
LOG C/D I am betting is your Pan Pots

 

[analogguru]

VR1 is the "MIC GAIN" and that potentiometer is in this and similar circuits most of the time a 10k C or negativ logarithmic potentiometer.

 

[mkiijam]

VR1 is the "MIC GAIN"...most of the time a 10k C or negativ logarithmic potentiometer.

...A LOG should be Audio Log...

This what I'm saying, I suppose I must measure it.

 

[Bo Deadly]

VR1 needs to be reverse log probably 5K. There are a number of designations for this but the modern expression for 5K reverse log is "C5K". But control may be wonky if the pot is not high quality or too sensitive at high gain settings if the value is too large. For best results use a high quality sealed conductive plastic C5K. But these are quite difficult to find and probably expensive. I have used the ultra-cheap alpha 9mm C5K from places like smallbear and tayda and they worked fine. The typical 16mm pot like you'll find on Ebay are ok too. But for a really high quality build it deserves to be better quality (a Bourns 51 would be very good for example but C5K is not a standard value). You could use C10K but when you get to the higher gain settings, the gain will jump at the very end of the travel. OTOH the low gain setting will not be quite as low with C5K so if the circuit calls for C10K then that would be fine (but I would still use C5K anyway). Or if you find a dual C10K you could wire both sections in parallel to make a single C5K.

 

[Khron]

But for a really high quality build it deserves to be better quality (a Bourns 51 would be very good for example but C5K is not a standard value). You could use C10K but when you get to the higher gain settings, the gain will jump at the very end of the travel. OTOH the low gain setting will not be quite as low with C5K so if the circuit calls for C10K then that would be fine (but I would still use C5K anyway). Or if you find a dual C10K you could wire both sections in parallel to make a single C5K.

Since it's wired as a variable resistor, a C10k with a 10k resistor in parallel with it should work just as well as a bespoke C5k... Shouldn't it?  ???

 

[abbey road d enfer]

Since it's wired as a variable resistor, a C10k with a 10k resistor in parallel with it should work just as well as a bespoke C5k... Shouldn't it?  ???

Not really. A typical C taper has about 10-15% at mid-rotation.
In a preamp, assuming a 50dB gain range, it will result (best case) in CCW 6dB, CW 56dB, mid-rotation 20dB. That's 14dB in the first half, 36 in the second.
With a your proposed arrangement, it would result in CCW 6dB, CW 56dB, mid-rotation 16dB.
In both cases, the range becomes cramped past 12 o'clock, but more so with your proposed arrangement.

 

[Bo Deadly]

Since it's wired as a variable resistor, a C10k with a 10k resistor in parallel with it should work just as well as a bespoke C5k... Shouldn't it?  ???

Ah, how wonderful it would be if that were true. Unfortunately it's not. If you imagine the pot in the 99% CW position, you have the small resistance of the pot in parallel with 10K which will have very little influence on the taper. That is the opposite of reverse log where you want the resistance to change very little when it's at it's smallest.

Another way to implement the gain control in this type of circuit that I think is actually the best way of all is to use a switch. I don't record much but when I have I never felt like I needed a high resolution control. You just need to find the step that keeps the thing from clipping. Or you want to push it a little in which case again, you don't particularly care. The nice thing about a switch is that you can program the gain structure however you like. So you could make the steps big at low gain and small at high gain. For example, with a 6 position switch you could do like 40, 50, 56, 60, 63, 65 dB gain or some such. This way, you do have finer control at high gain settings where it might clip.

One switch that is actually not bad for this is the Aplha SR10010F. It's a really strange piece in that it has two poles but the commons are not connected at all positions. But if you simply use it as a single pole you can wire it as a stepped pot or rheostat no problem. Its voltage rating is also a little low for most things but in this particular application, the voltage across Rg is very low so it's also no problem. The only real downside is that it's not ideal for retrofitting into an existing panel because the total travel is only 180 degrees and not the usual 300 or so. But the action is nice. The force required to switch positions is relatively low and there's virtually no "play" whereas something like a Grayhill can feel like you need a wrench and has "play" between the positions like the steering wheel on an old weathered 60's Pontiac.

 

[JohnRoberts]

This is a common problem (challenge) with this topology. You want a very slow resistance increase in the high gain hop off region, with decent total resistance for lower minimum gain and better headroom.

At Peavey we had multiple custom pots in the system with desirable tapers. As I recall it took something like three different screening steps with higher and lower resistance inks to attain the right taper. This was not very expensive in large production quantity, but not an option for low volume manufacturers.

Perhaps you can find a replacement semi-custom pot from a high volume manufacturer,,, that topology or variants on it, are widely used by numerous companies.

JR