Loudspeakers Local volume controls on PA amplifier

[garyvp]

We use an inexpensive Radio Shack 40 amp PA amplifier to monitor the performance space. We have five circuits and 6 speakers (control booth, dressing room, etc) branching off the output, each with a good quality volume control on the speaker. Most speakers are small, cheap, and from 6-8 ohms impedance. The system has worked fine for years.

However, we have fried several amps in the past two years and I cannot figure out why. Some facts:
• If I meter (I use a Fluke) each speaker I get the name plate impedance at the speaker and on the input site to the volume control.
• The home runs to the amp are anywhere from 50 to 150 feet. Wire varies from 16 to 20 AWG.
• All lines have been tested for continuity, shorts or loose connections
• If I measure resistance at each circuit at the amp, there is a drop. A speaker with a reading of 8 ohms reads 4 ohms right before the amp (when disconnected from the amp). This pattern I consistent with single and double speaker set ups (I expected this for two 8 ohm speakers, but not for singles).
• The resistance of all of the speakers wire together is 3.8 ohms.
• The amp has out puts for 4,8,16 ohms.
• We are using the 4 ohm connection

I was considering placing an impedance matching volume control between the amp and the junction for all the speakers coming in and getting it up to 8 ohms and then use the 8 ohm output.

Any suggestions?

 

[BillESC]

Is it a stereo amp? You have three speakers on each side?

 

[garyvp]

No..just mono.

 

[museav]

Resistance is just one half of impedance, reactance is the other half and is frequency dependent (the impedance values reported for speakers are simply nominal values closer to the minimum values and the actual impedance values may vary greatly at some frequencies). So measuring the DC resistance is not the same as measuring impedance, to measure impedance you typically use an impedance bridge or dedicated measurement device to measure impedance, something like this ZM Series Impedance Meter or this Dayton Audio - WT3 Woofer Tester used with a laptop or this VIBOX - Voltage and Current Measurement Interface used with an analyzer.

Given that it would be a bit unusual to wire a system like this for direct operation and that the little Radio Shack 40W PA amps have a 70V output, I wonder if you might possibly have what is supposed to be a 70V system. Do the speakers have transformers on them? What are the volume controls, do you have a model number?

 

[WooferHound]

you have mentioned that the system has worked fine for years, are you having a problem with it now, if so can you describe the symptoms.

Generaly, when you hook up more than about 3 speakers to an amplifier, it is called a distributed system and is wired differently than than smaller systems.
You would use an amplifier with a 70volt speaker output and then using impedance matching transformers on all your 8 ohm speakers. The Matching Transformers will have Taps on them so you can control the relative volume of each individual speaker.

It is possible to give a regular a 70volt output by reversing a Matching Transformer on the output, but, that transformer must be able to handle the total output from the amp you are using.

What are you doing using Radio Shack gear anyway ?!

 

[garyvp]

It is a community theatre and they had used these cheap amps for 30 years. One of them failed about 3 years ago. The newer ones fail more quickly. The symptoms are that the volume and mic gain are set high, works just fine for 6 months, and then dies.

My brother is a EE and he noted your concerns on the subject of impedance.

Note that you can buy a kit from Radio Shack with four 8 ohm speakers that get wired the same way with 18 wires. Their speakers are not powered.

Also, we don't need much volume from these, but the amp is almost on full to power them.

I also have several impedance matching controls (which I could put on all the speakers).

What is a better choice for a PA amp? Assuming I want to spend under 300$.


I will also rewire everything to a minimum of 16 awg,

I will rewire everythi

 

[WooferHound]

You need to learn about 70v distributed systems, this is the correct method to do what you need. Using amplifiers with 4-8-16 ohm outputs will never work correctly.
70v Speakers Systems - 25v 70v 100v Commercial Audio Systems
70-Volt Speaker System
Shavano Music Online - Speaker Wiring - 70 Volt Transformer systems
Internet Sound Institute - 70-Volt Speaker System

you can use any amplifier with a 70 Volt Output . . .

 

[museav]

You need to learn about 70v distributed systems, this is the correct method to do what you need. Using amplifiers with 4-8-16 ohm outputs will never work correctly.

It can work and is actually fairly common in some applications, especially where the losses and low frequency response of most transformers are not acceptable. However, this does indeed seem like a good application for a 70V (actually 70.7V, people have gotten lazy) system.

The problem with direct wiring is that you have to wire the individual loads in specific series and/or parallel combinations to keep the overall load correct. Not only do you have to do this initially, but any change in the loads, such as adding or deleting a speaker, will change the signal going to the other speakers and may require the wiring to be revised. This is made more difficult by the fact that while speakers may have a nominal single impedance value, the actual impedance may vary greatly with frequency. When you start mixing multiple speakers with different impedance response curves in a series-parallel system you can get resulting loads that vary all over the place. In comparison, a constant voltage (25V, 70.7V, 100V, etc.) system allows you to simply parallel loads as long as the total power draw does not exceed the output of the amplifier, so if properly planned you can add or delete speakers without any change to the other speakers.

Constant voltage systems also tend to reduce line losses much in the same way that high voltage lines are used for long runs by the power company, even the general concept with step up transformers at the start and step down transformers at the end is similar. Thus constant voltage systems are common where there are very long runs such as airports, convention centers, raceways and so on.

Constant voltage system also allow you to control the maximum and relative volume at each speaker. Each speaker has a transformer that typically has multiple 'tap' values, say 1W, 2W, 4W and 8W or something similar and the tap used determines the maximum output to the speaker. By selecting the taps for each speaker you can adjust their relative volume and maximum volume.

Anyways, the point is that if you have a series/parallel type direct driven system then the impedance of each line and how they are wired together is critical. If anything changes it could change the load the amp sees in such a way as to cause the amp to fail. The 'impedance matching' volume controls might help as they could present a more consistent load to the amp but you would still have the issue of any of them being modified or any being deleted or added affecting the system. That is one of the reasons why this type of application is commonly approached as a constant voltage system.

 

[garyvp]

Well, you guys have certainly given me an education - as well as a sip out of a fire hose.

Will explore the 70v, ahh, sorry, 70.7v option. Can each speaker be controlled (A must) in these systems?

Thanks

Gary

 

[FMEng]

Well, you guys have certainly given me an education - as well as a sip out of a fire hose.

Will explore the 70v, ahh, sorry, 70.7v option. Can each speaker be controlled (A must) in these systems?

Thanks

Gary

You can adjust the volume of one or several, 70 V speakers using an autoformer volume control. Here is an example of one:
Atlas Sound - AT10D

 

[garyvp]

Just realized the amp has a 70.7v port. And I found the manual online - supports the thread nicely. I am under the 4ohm impedance. I think that I can reconfigure the speakers to get 6-7ohms and move all connections to the 8 ohm port. Otherwise, I'll go the transformer route.

Also have a legacy 5.1 channel amp that may also work.

 

[Chris15]

If your existing speakers are satisfactory and you like their volume controls, consider the option of installing a transformer to feed each. That way you save on the cost of new speakers but leverage the advantages of a constant voltage system...

 

[garyvp]

Re: Local volume controls on PA amplifier - NYC code?

As a temporary measure, I put in an impedance matching transformer right after the amp and that at least got the resistance between 8-10 ohms.

I intend to go the 70v route but as I am in NYC and this is 70v, what restrictions are there on the wiring? Can I do this with standard 14/16 awg speaker wire?

 

[museav]

Re: Local volume controls on PA amplifier - NYC code?

As a temporary measure, I put in an impedance matching transformer right after the amp and that at least got the resistance between 8-10 ohms.

I intend to go the 70v route but as I am in NYC and this is 70v, what restrictions are there on the wiring? Can I do this with standard 14/16 awg speaker wire?

You can actually usually get away with lighter gage wiring for 70V than you can for 4/8 Ohm operation, 14 or 16 AWG is usually fine unless maybe in situations where you are driving extremely long line like a stadium or airport or where you have a number of higher power, high fidelity speaker/transformer devices like in some ballrooms, convention centers, etc.

I can't speak specifically to NYC code but 70V systems are usually not treated any different than any other speaker system. Then again, look at many higher output amp/speaker combinations used for concerts, large installs, etc. and you'll find potential voltage and/or current for speaker lines that can equal or exceed a typical AC power circuit.

 

[garyvp]

Re: Local volume controls on PA amplifier - NYC code?

Thanks for all the replies. Here is where I am. I have the tranies, and am running either 70 or 25v - will experiment. I am running 7 speakers and 5w each seems fine, some less - these are dressing room and green room house speakers. The amp is 40w (will run at about 30W). Sound quality is not a big issue. Each exisiting speaker had a proper volume attenuator and these still work fine after the tranny. Also reqires with 18AWG speaker plenum grade wire - better fire rating. That resovled some problems as well. I wonder why I waited so long to discover this - all the pluses have been stated above and all over the internet - no worries about speaker matching or pairing (none of mine are the same), easy to adjust the relative power, and simple wiring. Overall, it was well worth the 150$ in parts and wire. I am now among the faithful.

 

[headcrab]

Re: Local volume controls on PA amplifier - NYC code?

I have seen high voltage speakers connected with CAT5e, but I don't like that due to the voltage rating of the cable.