Mackie Mk1 SRM 450 volume pot

[Mick Bailey]

The pot in these is 12mm, dual reverse audio taper. https://www.loudtechnologies.eu/shop...-3780-10000258

Looks to be out of stock everywhere - does anyone know if anyone in the UK/EU has them? At a pinch I'll have to use a stock linear taper but would prefer to get the correct part.

[bsco]

The pot in these is 12mm, dual reverse audio taper. https://www.loudtechnologies.eu/shop...-3780-10000258

Looks to be out of stock everywhere - does anyone know if anyone in the UK/EU has them? At a pinch I'll have to use a stock linear taper but would prefer to get the correct part.

Question..if you use a linear taper pot, wouldn't that cause very loud volume at the lower end of the pot?? Just curious....

[Helmholtz]

Question..if you use a linear taper pot, wouldn't that cause very loud volume at the lower end of the pot?? Just curious....

Not as much as a reverse audio taper pot.
But actual behaviour depends on the circuit and I assume it's not wired as regular vol pot (maybe in the NFB path).

[Mick Bailey]

With the reverse audio taper pot the control works fine with no bunching - it's just how the circuit is implemented in this particular powered speaker. If it's a choice between it not working at all or using a linear pot to get it back operational I'll have to go with that for the time being and put up with the poor control graduation.

[nevetslab]

For the time being, you could see about finding a dual linear 50k or higher value, and 'slug' the wiper and the one end to behave like a reverse log taper pot. Not as good as a true reverse taper pot, but better than a 10k dual linear taper. Placing a 12.7k resistor across the end and wiper will do it for 50k. Might get even better results with 100k linear pot & an 11k resistor to slug it. You can calculate the difference between the two examples in 10% increments to see which gives you the better desired taper.

[bsco]

For the time being, you could see about finding a dual linear 50k or higher value, and 'slug' the wiper and the one end to behave like a reverse log taper pot. Not as good as a true reverse taper pot, but better than a 10k dual linear taper. Placing a 12.7k resistor across the end and wiper will do it for 50k. Might get even better results with 100k linear pot & an 11k resistor to slug it. You can calculate the difference between the two examples in 10% increments to see which gives you the better desired taper.

Interesting....I will have to keep this in mind as I had to replace a pot in a Fender HD Deluxe reverb and had to install a linear pot while waiting.... for the proper one to come in.....man was the amp ever loud around 2 to 5 setting....and once I went past that, the increase in volume wasn't all that great...

[Mick Bailey]

Thanks, I'll give the resistor trick a go. I've used it a few times (mainly with pedal builds) and it's worked out pretty well.

[Helmholtz]

Thanks, I'll give the resistor trick a go. I've used it a few times (mainly with pedal builds) and it's worked out pretty well.

Don't forget that the "resistor trick" changes either input and/or output resistance. This may have an effect depending on circuit.

Can you post a schematic?

[nickb]

Thanks, I'll give the resistor trick a go. I've used it a few times (mainly with pedal builds) and it's worked out pretty well.

It could be tricky. One section is used to set the input diff amp gain and the other is wired as a conventional potential divider on the output of the diff amp. But, so long as is feels right it's OK Precision is not the name of the game.

[Mick Bailey]

The schematic is here (post #15) https://music-electronics-forum.com/...-mackie-srm450

[nickb]

Forced to use a linear pot for this myself yesterday, I thought I really should look at the rotation vs gain. Surprisingly, for 10% to 90% rotation the it's close to linear (in dB), which is what you'd want. At the bottom end it drops off quickly and I'd argue that is the feel we are used to and are happy with. On the other hand, in the last 90% to 100% the gain shoots rapidly from 4dB to 46dB. From that I'd expect the real pot taper is linear for a fairly short portion (say 10%) of the rotation then a flattening curve for the rest. Reverse audio doesn't cut it due to the hyperbolic gain vs rotation shape of the first gain stage.

In practice the linear pot feels quite nice... except for the last 10% when it gets scary!

pdf of spreadsheet attached. Plot is just for 10% to 90% rotation.


SRM450.pdf

[Mick Bailey]

I ended up using a linear pot without any resistor mods and it worked out OK because once set my customer didn't touch the volume and controlled the levels off his mixer.