Bose AmPlus 100 query

User / Installer and service manuals all over the place.

"Connector J4 [A8] ties one or two 10K rheostat-connected remote volume control potentiometers into Q201/221,"

Log or linear value with rotation?

Usually log for a volume control.

There is mention of a voltage controlled amplifier, a VCA.

The remote pots, might not have the actual audio signal on them (as the pots on my home made amplifier in front of me do).

This will change the noise injection characteristic, if the remote control pots are located some distance from the equipment.

Instead, the remote pots could have a DC voltage, and that feeds into the VCA logic. The VCA logic also receives a clipping-detected signal, which mutes the amp during overload.

You might guess at the transfer function, if you had the VCA chip part number. Maybe something like a four quadrant multiplier would work.

This isn't some "home project" amp, and it looks like the designers were let loose by the management. It's probably better that we don't have a schematic :-) It would likely be so complicated, you couldn't tell it was an amp.

Paul

I have a late model Fender guitar amplifier. All the pots go into a digital chip that is serially connected to a digital signal processing chip.

The only point at which the signal is analogue is right at the input, and then between the preamp and the power amp, which is class D.

Chances of fixing it if ever the digits go down is probably nil.

But on the plus size, there is no crackle on the pots at all!

Thanks to everyone who responded. I'll try a linear pot to begin with, a log pot as a fall back if required. Thanks again.

I knew of helicopter comms system that worked like that for a while - each operator's control box had a bunch of rotary knobs for the volume of various services and radios... Some bright spark in the design office noticed all the pots were linear, and so raised a change request to swap to log, but without asking the engineers who designed it!

Needless to say the pots were read in software and any log processing applied at that stage - so they ended up with a log log transfer function on the volume, which meant it went from no sound, really quiet, really quiet, really quiet, really quiet, WAY TO LOUD! To add insult to injury, this was on an old system with no proper real time design on the software - so if it got busy it could take 10 secs to get round to adjusting the volume back to something not painful.

Fortunately 10k pots of either flavour are available for a couple of pounds on eBay so getting the wrong one first won't break the bank. Next problem if the unit works ok is to get hold of a set of connectors for the various ports from a uk source at a reasonable price. Searches so far have only yielded US suppliers with ridiculous postage added.

That looks like a Phoenix Contact MSTB: