volume control

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Old 02-16-03, 07:54 PM
jim millett's Avatar
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Question volume control

at our church we have an amp that puts out 150 watts i have been tasks to put a pair of 35 watt speakers in the foyar and a jensen 12 inch speaker ,possible model number C12RSB, watts unknown,in the nursery they want me to put a volume control at both locations . i have bought two metal boxes to install two pots my question is what size pots do i need for each location it has been years since i have done anything like this . figureing out the pot size could use all the help i can get
 
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Old 02-21-03, 12:30 PM
Teratum
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Your problem seems simple, but in fact a tough one. If anyone can figure a perfect solution for it, he/she will put the high power speaker manufacturers out of business.

Anyway, what you are trying to do is to use the 35W speakers with 150W amplify without blowing it up. You will not be able to find a pot big enough and economical enough for this application. Your pot will have to withstand 150W which is big,heavy and impractical.

I want to suggest a "not so perfect" solution but economical for you and your church. Try using 2
4-ohm 20W reistors in series (becomes 8 ohms 40W) and connect in series with the 35W speaker. You will notice that you have to crank the volume higher to get the same loudness from the speaker. But in this case the speaker (assume 8 ohm impedance) will always see only half the power come in from the amplify which prevent it from being damaged by the overwheming power.

By changing into this configuration, your load for the amplify will no longer be 8 ohms but 16 ohms. The power delivered out from the amplify will also reduced to half, 75W into 16 ohms. The whole system is safe to use without potential damage, but ,of course, you have to sacrify for less sound volume through your speaker, which you may not like it.

I hope it works.
 
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Old 02-21-03, 03:44 PM
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since the amp is up stairs and the other two rooms down stairs i have a slight problem . they want to control the volume in the rooms down stairs i have two metal boxes to put the resistors in and can do all the wireing in the box . wires from amp in /wires to speakers out. so with those resistors can i now put a pot for volume control in the metal box.
 
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Old 02-22-03, 07:52 AM
RickJ6956
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This is a tough one. But not becaue of amp power.

In actuality, most sound companies use amps that are overpowered for the speakers it will be driving. Too much power is not responsible for blown speakers. The major culprit is "clipping" the amp, which sends a distorted signal to the speakers. That distorted signal acts like DC voltage to the speaker. In fact, an underpowered amp can easily blow speakers that are designed to handle more than the amp's rated power.

Even so, a "master setting" on the amp -- lowering its sensitivity or volume control -- can reduce the power sent to the speakers in the first place.

The important factor to consider here is the total load applied to the amp. Impedance matching plays into this and L-pads add additional variables to the equation. Let's keep the L-pads out of it for now:

-- Three 8-ohm speakers wired in parallel = 2.67 ohms. A 150 watt amp (into 8 ohms) will produce nearly 450 watts into 2.67 ohms (unless internally limited). Not a good situation.

-- Three 8-ohm speakers wired in series = 24 ohms. The amp will output roughly 50 watts into this load, but it still isn't good. Plus, L-pads will be interactive. Reducing the level in the nursery will also reduce the level in the foyer, regardless of the foyer L-pad's setting.

-- Run the two 35-watt speakers in series. Run that line in parallel with the nursery speaker. 5.3 ohms. Output power = over 225 watts. Turn the amp down and this might work for you. Make sure the amp is capable of running into a load of less than 8 ohms. (Most are.)

The proper way to do this is to put an autoformer on the amp's output and run a 25- or 70-volt system. The amp "sees" the transformer as its 8-ohm load. Each speaker would get a step-down transformer and each area would get its own 25- or 70-volt volume control. (Depending on local codes, 70-volt systems may have to be run in conduit.) Your amp may already have taps for this type of system. (What make & model is it?)

This doesn't have to be expensive. Every day, sound companies pull useable but outdated components out of commercial establishments. Call them to see if they'll sell you used parts. Many will donate old, still-working components to churches. (Maybe a blurb in the church's newsletter will turn up someone who knows someone in the business?)

Good luck. Let us know how it turns out.
 
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