external volume control between power amp and speaker

[missinglink]

Hi everyone. Newbie here. I just want to ask if it's possible to put an external/additional volume control/potentiometer between the power amp and the speaker. I understand that the power amp outputs not only audio signal but electricity. I'm just wondering if there's a special kind of pot to do the job. Thanks in advance and looking forward to your reply.

 

[Nigel Goodwin]

Hi everyone. Newbie here. I just want to ask if it's possible to put an external/additional volume control/potentiometer between the power amp and the speaker. I understand that the power amp outputs not only audio signal but electricity. I'm just wondering if there's a special kind of pot to do the job. Thanks in advance and looking forward to your reply.

You need a special VERY high power wirewound potentiometer, it has to withstand the full output power of the amplifier.

It's a pretty poor solution, and only used normally in 100V line PA systems, where each speaker is only relatively low power.

 

[Diver300]

It generally isn't practical.

When you said "I understand that the power amp outputs not only audio signal but electricity", that is sort of correct, but all signals that travel in wires are electricity. The power levels at the output of a power amplifier are much larger than the power levels for signal lines, such as the input of a power amplifier.

The power levels on signal lines, such as the input of a power amplifier, are so low that there is usually no damage done if they are shorted out. Also, with those levels, a potentiometer can be used to reduce the level without the potentiometer producing much heat.

If you try to put a potentiometer on the output of a power amplifier, it will overload the amplifier, or add too much impedance or both. It will also have to be physically big to handle all that power. Also, because everyone else puts the potentiometers at the input to the power amplifier, high power potentionmeters are rare and expensive.

The only application where the volume control is between the power amplifier and the speakers is on the in-ear headphones where the power level is tiny.

Tell us more about what you are trying to do.

 

[canadaelk]

For proper loading between amp and speaker a L or T pad is needed.

But, with any pad, the damping provided by the amp to the speaker will deteriorate the lower the level on the pad is set. E

 

[audioguru]

Stores that used to sell audio equipment sold a tapped transformer to mount in each room of a home or restaurant. It adjusted the volume or switched off the speaker, was efficient and provided a low impedance to damp resonances of the speaker. Then many of these transformers were connected in parallel to the output of a fairly high power amplifier. The transformers were pretty small (to fit inside an electrical box) so had poor bass response.
The amplifier did not have a stepup transformer on its output.

 

[missinglink]

Thanks for the quick reply everyone. Here is my situation, I'm helping my brother in-law to setup his sound system in his new spa. He doesn't want everyone to have access to the power amps that's why he kept it in a secure place. He wants volume control for each room that's why I really need an additional volume control other than the ones provided on the existing equipment. I suggested to put a distribution amplifier/splitter so we can put volume controls for each output of the distribution amp before the power amp but I think he's tight on budget and don't want to put out more money for another equipment. That's why I'm thinking if there are other ways to put external individual volume controls for each speaker.Do you guys know any ways to accomplish this? Thanks again in advance.

 

[Nigel Goodwin]

As I've already said, big wirewound pots - and preferably using a 100V line system, you can (or at least could) buy speakers with the 100V line speakers (with transformer), and a pot already wired correctly.

But for any hopes of quality, use separate power amps and low level volume controls.

 

[crutschow]

As Uncle $crooge noted, typically an impedance matching volume control is used for that purpose.

 

[Nigel Goodwin]

As Uncle $crooge noted, typically an impedance matching volume control is used for that purpose.

No impedance matching required, only valve amps required impedance matching.

On a transistor amp it's fine feeding a higher impedance, in fact it's BETTER. You simply wire the pot as a potentiometer feeding the speaker, something like a 50 ohm pot for an 8 ohm speaker - a BIG wirewound one.

 

[crutschow]

No impedance matching required, only valve amps required impedance matching.

On a transistor amp it's fine feeding a higher impedance, in fact it's BETTER. You simply wire the pot as a potentiometer feeding the speaker, something like a 50 ohm pot for an 8 ohm speaker - a BIG wirewound one.

The impedance matching has nothing to do with valve (tube to us Yanks) amps. It's for having more than one speaker attached to the amp.

But even if you have only one speaker connected, most impedance matching controls have a setting of 1X for only one speaker. The advantage of this type of control is that it is transformer based, and is thus more efficient and has better speaker damping characteristics than a big old resistive pot (a dual pot for stereo).

 

[missinglink]

thanks for all your replies!!

 

[YaranAyr]

Well, I have seen these Model 1275 PA Monitor speakers from Fender. They are 200-Watt and have a volume control. I looked around A LITTLE BIT for any more schematic info - nothing yet. I, or you, can try Fender themselves - I think they are supposed to be Pretty good about schematics. Or maybe look for replacement parts.

 

[Mr RB]

You don't need impedance matching on solid state amps provided you don't exceed the total load. I think Nigel said that too?

I've done this in practice with a rotary switch and a few 10W resistors, as you put the resistors in series it doesn't waste as much heat as you first expect because the resistors are usually pretty large (to reduce the volume enough) like 50 ohms, 100 ohms, 220 ohms etc and the total current is quite low when the resistors are in series.

 

[Nigel Goodwin]

Well, I have seen these Model 1275 PA Monitor speakers from Fender. They are 200-Watt and have a volume control. I looked around A LITTLE BIT for any more schematic info - nothing yet. I, or you, can try Fender themselves - I think they are supposed to be Pretty good about schematics. Or maybe look for replacement parts.

It's just a big pot, as I've already mentioned in posts #3,#7 and #9.

I doubt there will be a schematic, as there's nothing really to have a schematic of?, and looking on Fenders site they don't weem to mention it.

However, is it actually a volume control?, more likely it's just a level control for the tweeter only (which is a MUCH easier problem, requiring a MUCH smaller pot).

 

[gary350]

If you have 400 watts of output to the speaker then you will need a 400 watt volume control.

 

[Mr RB]

Those Fender speakers might have a power amp built in, it's very common with music and PA equipment these days.

Also if it is a rheostat type variable resistor they normally start about 15 ohms (loudest) and go to maybe 150 ohms (quietest). So it it was 100W into 8 ohms, with the control on loudest (series +15 ohms total 23 ohms) it would be about 34W total with 12W at the speaker and about 22W lost as heat in the pot. Then it gets better as the pot is turned "quieter" so by the time the pot is 150 ohms you get 5W total with 0.25W at the speaker and 4.75W lost on the pot.

 

[YaranAyr]

Totally Passive AND Have Total Overall Level Controls.

So It looks like these are:
Totally Passive AND Have Total Overall Level Controls.

 

[Nigel Goodwin]

So It looks like these are:
Totally Passive AND Have Total Overall Level Controls.

Looks to be so:

CAUTION! When operating with large amplifiers
(greater than 150 watts at 4 ohms) always set the
level control between the 3 o’clock and 5 o’clock
positions. Failure to do so may lead to overheating
and damage the level control.

However, subject to restrictions as above.

Like I've said repeatedly for doing this, crude HUGE potentiometer - obviously in this case, not HUGE enough