Pot in-line with output of amp

[Cheeseweasel]

I'm trying to find a suitable pot to replace the volume control on a lecture-room AV installation.

 

The installation uses a combined audio switcher/amplifier that is locked away in a cupboard, connected to a remote control panel mounted on the lectern. Also on the lectern is a volume control which is connected in series between the output of the amp and the two passive speakers in the lecture room (i.e. when it's turned down it's getting the full output of the amp into it).

 

Just to be clear, for the LH channel of the amp, the -ve terminal is connected straight through to the -ve terminal of the speaker. The +ve terminal of the amp is connected to the +ve terminal of the speaker, with the pot in between.

 

I'm not an electronics expert by any means, but instinctively this seems like a bit of a poor solution. I'm guessing that the reason the pot has gone crackly and is fully "open" at both ends of its travel is that the high-level output of the amp has damaged it. I can't find the model of the pot (it seems like a cheap and cheerful one, just says "Ameg" on the back - a name that doesn't return anything on google).

 

So, I need to find a suitable dual logarithmic pot to replace it. Let's say the amp is 50W into 8ohms (that's a pretty liberal estimate, it's probably not so powerful). At this kind of level should I be worried about the power rating of the pot? I'm looking at a pot which is rated for 30Vac, power rating 0.05W (RS #236-9604). Would this do the job?

 

Thanks in advance.

[Bobbsy]

Instead of re-inventing the wheel and building something, how about something like THIS?

 

Bob

[themadhippy]

? I'm looking at a pot which is rated for 30Vac, power rating 0.05w

unless you want a free smoke machine then no.

is it a standard low impedance system or a 100v line system, if its a low impoedance (4, 8 or maybe 16 ohm) do a seartch for an L pad,you want the power rating to equal or be greater than the amps raiting and the impedance the same as the amp and speakers.

[Andrew C]

Something like this devicefrom CPC would be a better way to approach the problem. This is a cheap solution, there are probably better devices out there!

[Cheeseweasel]

Instead of re-inventing the wheel and building something, how about something like THIS?

 

Bob

 

Yeh, that looks like a much neater solution. I was hoping to get away with just changing the pot though, as it would be a two-minute soldering job as opposed to having to jigsaw a big chunk out of the lectern to fit a new plate. Technically, this kind of AV work is outsourced to an external contractor and I shouldn't be spending loads of time on it. I just want to get it sorted for when the university term starts and the room needs to be used for lectures again.

 

? I'm looking at a pot which is rated for 30Vac, power rating 0.05w

unless you want a free smoke machine then no.

is it a standard low impedance system or a 100v line system, if its a low impoedance (4, 8 or maybe 16 ohm) do a seartch for an L pad,you want the power rating to equal or be greater than the amps raiting and the impedance the same as the amp and speakers.

 

It's a low impedance system. Small, passive hifi-type speakers, as you say 4-16 ohms.

 

I should maybe have mentioned that the pot is 50kR. So, if the amp gives 50W @8R it will give 0.008W @50kR, no? I'm not all that sure about calculating powers etc so let me know if I'm talking rubbish or making incorrect assumptions.

 

RS do sell pots with higher power ratings btw, I just chose that one because it is a good quality one that is particularly designed for audio signals. And at the end of the day, the one that's broken is a pretty bog-standard pot and it didn't catch fire, so I guess I can get away with just replacing it - if it breaks again I'll buy a proper volume control like those suggested.

 

Something like this devicefrom CPC would be a better way to approach the problem. This is a cheap solution, there are probably better devices out there!

 

I agree - if I was designing the system myself I would have done it differently. However, changing the broken pot is just one job on a long list of things I need to do, and ideally I'd like to leave it at that.

[adamharman]

I should maybe have mentioned that the pot is 50kR. So, if the amp gives 50W @8R it will give 0.008W @50kR, no? I'm not all that sure about calculating powers etc so let me know if I'm talking rubbish or making incorrect assumptions.

 

With the pot at one end of it's travel, yes. As it approaches the other end, you're putting the 8R speaker in series with a very small part of the 50k. Eventually the pot will be the smaller part of the resistance so will be carrying almost the full output of the amp. On the end stop, it may be ok. Slightly off it, I'd expect smoke. Any further and the sound would be almost inaudible. Assuming you've got your facts right, it's a miracle it ever worked in the first place!

 

I suspect the amp is a much lower power, and the pot is rated at somewhat more than 0.05w. I think I've seen 5w and 10w pots which weren't tooooo much bigger than a "bog standard pot"

[Cheeseweasel]

Thanks everyone, I'm going to order an L-Pad, as madhippy suggested.

 

I just took the existing pot apart and cleaned the crud off the tracks and wipers - it works better, but hums badly when it's not fully turned up. Also, the HF end of the sound only comes up when the pot is fully turned up - I guess there is some sort of capacitative loss going on. Bottom line is, the pot that was installed seems like the wrong tool for the job.