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Author Topic: 70v distributed systems troubleshooting  (Read 22871 times)

Steve M Smith

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Re: 70v distributed systems troubleshooting
« Reply #10 on: January 28, 2015, 07:44:48 am »

All the transformer does is increase the impedance of the speaker. 

So if you have an 8 ohm speaker, you might have a transformer which makes it into an 80 ohm speaker.

Connect ten in parallel and you're back to 8 ohms again.

Crown have a good guide here: http://www.crownaudio.com/media/pdf/amps/138905-1_10-05_constant_voltage.pdf

And there is much more than you will ever want to know here: http://sound.westhost.com/articles/line-amps.html
 

Steve.
« Last Edit: January 28, 2015, 07:46:49 am by Steve M Smith »
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John Roberts {JR}

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Re: 70v distributed systems troubleshooting
« Reply #11 on: January 28, 2015, 09:27:48 am »



HOWEVER on tube 70V amps that is a different story.  You want the wattage to be close to that 90% mark.  Because a lower wattage will mean that the impedance the amp "sees" is higher, and tube amps will not last as long when driving a high impedance-while solid state amps really like high impedance loads.
While indeed solid state amps can be operated with no load, I have heard horror stories about (some) tube amps that become unstable and self-destruct when operated with no load or even switching between speakers if they become disconnected from the load momentarily.

I am aware of the 90% rule of thumb to provide headroom and account for distribution losses.

I never heard that tube amps age faster under light load while I do not know what a safe load is to insure stability (I suspect << 90% but that is just a guess).

The good news is I don't expect people will encounter any new tube install amps, so the ones out there should already be safely loaded.

JR
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Steve M Smith

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Re: 70v distributed systems troubleshooting
« Reply #12 on: January 28, 2015, 10:03:47 am »

With valve amps, My understanding is it's more a question of the load having the right inductance than an actual impedance load.

However, I don't know why I think that or where I learned it so it could be wrong!!


Steve.
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John Roberts {JR}

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Re: 70v distributed systems troubleshooting
« Reply #13 on: January 28, 2015, 11:50:21 am »

With valve amps, My understanding is it's more a question of the load having the right inductance than an actual impedance load.

However, I don't know why I think that or where I learned it so it could be wrong!!


Steve.

Power tubes have higher internal impedance than power transistors so can not drive loudspeakers directly. Output transformers are generally used in tube amps to step up the speaker load to be a better match for tubes. When negative feedback is connected around a tube amplifier, this higher source impedance will be more susceptible to phase shift from a reactive load. Capacitance will cause lag in this negative feedback that can cause oscillation when that lag causes so much phase shift that the negative feedback become positive. The resistive part of the speaker load will attenuate the negative feedback and shift the phase shift pole higher keeping the amp stable.

I suspect a well designed tube amp could be stable with no load, just like modern  solid state amps that compensate for no load, but enough of them are not stable that the general advice to not operate them without load is reasonable.   

JR
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Ivan Beaver

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Re: 70v distributed systems troubleshooting
« Reply #14 on: January 28, 2015, 12:31:51 pm »

While indeed solid state amps can be operated with no load, I have heard horror stories about (some) tube amps that become unstable and self-destruct when operated with no load or even switching between speakers if they become disconnected from the load momentarily.

I am aware of the 90% rule of thumb to provide headroom and account for distribution losses.

I never heard that tube amps age faster under light load while I do not know what a safe load is to insure stability (I suspect << 90% but that is just a guess).

The good news is I don't expect people will encounter any new tube install amps, so the ones out there should already be safely loaded.

JR
That is the reason the old Fender amps had a shorting jack for the speaker output.  The output was shorted when the speaker jack was unplugged.

I had an old Fender Champ that I put a series resistor with the speaker to drop the level so I could overdrive it hard. 

I burnt out 2 output transformers before somebody mentioned that the load to to high.

One time when the "light bulb" turned on :)

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Cailen Waddell

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Re: 70v distributed systems troubleshooting
« Reply #15 on: January 28, 2015, 03:37:56 pm »

All the transformer does is increase the impedance of the speaker. 

So if you have an 8 ohm speaker, you might have a transformer which makes it into an 80 ohm speaker.

Connect ten in parallel and you're back to 8 ohms again.

Crown have a good guide here: http://www.crownaudio.com/media/pdf/amps/138905-1_10-05_constant_voltage.pdf

And there is much more than you will ever want to know here: http://sound.westhost.com/articles/line-amps.html
 

Steve.

Thanks Steve, I had read the crown publication, but didn't quite have my head wrapped around it.  I think I do now.

lets review my math and findings....

I took one of the spare speakers and started measuring with my DMM.  The 1/2 watt tap is 514 ohms, the 1 watt tap is roughly half that, and so on. 

There are 68 speakers in the facility all tapped at 1/2 a watt.  Now this in theory is only a 34 watts.  Well within the 100w of the amp.  however, when I add the load up (1/total impedance) = (1/speaker 1) + (1/speaker 2) + (1/speaker 3), etc.  I get a 7.55 Ohm Load.  However, using the formula on Crowns website where (safe impedance) = (voltage squared)/(rated wattage), I see that a 100w amp could only support a minimum 50 ohm load. 

Is this correct logic?  If so, while I have plenty of power, the impedance is severely off, possibly indicating why magic smoke escaped the amp.  I have an old 2 channel amp that can do 600w at 8 ohms, so close enough.  If I divide the load into two channels, then I could put about a 15 ohm load on each channel, which I think would be acceptable right?  Do i need to worry about how far off the wattage is?

Sorry for all the maths but I want to make sure I understand this before I try a solution.

Thanks,

Cailen
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John Roberts {JR}

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Re: 70v distributed systems troubleshooting
« Reply #16 on: January 28, 2015, 04:35:43 pm »

Thanks Steve, I had read the crown publication, but didn't quite have my head wrapped around it.  I think I do now.

lets review my math and findings....

I took one of the spare speakers and started measuring with my DMM.  The 1/2 watt tap is 514 ohms, the 1 watt tap is roughly half that, and so on. 
Unless your DMM reads AC impedance you just measured the DC resistance. Not the same thing as AC impedance.

Hanging 514 ohms across a 70V line will draw 9.5W each...
Quote
There are 68 speakers in the facility all tapped at 1/2 a watt.  Now this in theory is only a 34 watts.  Well within the 100w of the amp.

68 x 9.5W is more than 100W  8)
Quote
however, when I add the load up (1/total impedance) = (1/speaker 1) + (1/speaker 2) + (1/speaker 3), etc.  I get a 7.55 Ohm Load.
DCR not load.
Quote
However, using the formula on Crowns website where (safe impedance) = (voltage squared)/(rated wattage), I see that a 100w amp could only support a minimum 50 ohm load. 
yup... 100W from 70V line is around 49 ohms (AC load)
Quote
Is this correct logic?  If so, while I have plenty of power, the impedance is severely off, possibly indicating why magic smoke escaped the amp.  I have an old 2 channel amp that can do 600w at 8 ohms, so close enough.  If I divide the load into two channels, then I could put about a 15 ohm load on each channel, which I think would be acceptable right?  Do i need to worry about how far off the wattage is?

Sorry for all the maths but I want to make sure I understand this before I try a solution.

Thanks,

Cailen

34W from 70V is roughly 144 ohm (AC) load.

If smoke does not come out of the 100W amp you're good...

The whole concept of constant voltage system math is to KISS so 68  1/2 W loads is 34W... job finished take a break.

JR
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Steve M Smith

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Re: 70v distributed systems troubleshooting
« Reply #17 on: January 28, 2015, 04:42:48 pm »

You really need to find the turns ratio of the transformer and from that, work out the impedance ratio (square of the turns ratio) to be sure of the load you are applying. A dc resistance doesn't tell you much.


Steve.
« Last Edit: January 28, 2015, 04:47:13 pm by Steve M Smith »
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Cailen Waddell

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Re: 70v distributed systems troubleshooting
« Reply #18 on: January 28, 2015, 05:04:08 pm »


Unless your DMM reads AC impedance you just measured the DC resistance. Not the same thing as AC impedance.

Hanging 514 ohms across a 70V line will draw 9.5W each... 68 x 9.5W is more than 100W  8)DCR not load.yup... 100W from 70V line is around 49 ohms (AC load)
34W from 70V is roughly 144 ohm (AC) load.

If smoke does not come out of the 100W amp you're good...

The whole concept of constant voltage system math is to KISS so 68  1/2 W loads is 34W... job finished take a break.

JR

Thanks JR., I'm all for KISS.  And I certainly have been known to over think things.  I guess part of my supposition was that the smoke was let out of the existing amp and perhaps we were asking too much from it. That doesn't seem to be the case.  Maybe we just drove it too hard?
Thanks though, I knew the math didn't quite make sense (or it did, it's just the wrong math)


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Hal Bissinger/COMSYSTEC

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Re: 70v distributed systems troubleshooting
« Reply #19 on: January 28, 2015, 09:14:08 pm »


People need to stop using that term.  The voltage is no more constant than it is from a 'normal' amplified system.

Steve.

Well, not exactly. The output of a constant voltage amplifier at full power will be 100, 70.7 or 25 volts regardless of amplifier wattage. So there is your constant and is why the system was named for it.

-Hal
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Re: 70v distributed systems troubleshooting
« Reply #19 on: January 28, 2015, 09:14:08 pm »


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