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NUTTTR 26th March 2005 04:46 AM

Is this possible?
Hi All,

As some may know, i have a Peavey PV-8.5c power amp...
It is soon to be relocated to sub duties and i managed to get my hands on some testing equipment :)
I got a 5kw 4 ohm resistor(!) and ran the amp using a 1khz tone directly into it.....That's bridged, so represents a 2 ohm load per channel... I got ~56v RMS out of the amp (it goes off the scale of my scope after 52v!), that's with NO clipping so because it's bridged represents about 1560wrms out of the amp... The wave was PERFECT, no flats, nothing.
What i'm wondering is this amp has "1200 w input" printed on the back of the amp... This thing was draining over 2.7kw off the mains socket (240v - i have a meter that will tell me what i'm draining too!).... The circuit breaker in the amp (8amp) clicked off after about 5 sec of this... Obviously the internal heatsinking and wiring is not up to the job of doing this a lot, but how can an amp that's got "1200w input" and also claims approx 1100w bridged output get so high? I am used to car audio equipment which is seriously limited on the power supply side - however this seems to have a MASSIVELY high output transformer compared to what i thought it did......
Is there any way i can test the transformer out for actual output? it is quite large and is very heavy (about 10kg maybe more?)..... There's a part number on it, but it looks like it's something internal to maybe peavey - however the amp has been "worked" on previously, which makes me wonder if someone has replaced the transformer with a different/more powerful one? The rails about about +-75v from memory.... Can anyone provide any info at all? Please :)

Thanks in advance


kilowattski 26th March 2005 07:00 AM

You are not deliveriing 1568 watts to your 4 ohm load. You are delivering 784 watts. Bridging means that the output voltage is doubled because the channels are shifted 180 degrees out of phase from each other and each channel is providing half of that voltage. Since each channel is driving half the load (2 ohms) at half the voltage, each channel is driving 392 watts of the 784 watt output not 784 watts of the 1568 watt output. It you do the calculations you will see it works out. The 8 amp fuse on the primary is blowing because your amp is working at a little less than 40% efficiency causing the breaker to open at about 1920 watts being drawn from the mains. You are well exceeding the recommended 1200 watts draw from the mains.

NUTTTR 26th March 2005 08:42 AM

Ok, that seems to make more sense!!
Obviously i went wrong in my calcs :)
However, what makes me wonder this is that when i put BOTH channels into a 4 ohm load i get 51v without any clipping... Per channel, meaning i am getting 650wrms per channel... so that's 1300w.... However obviously when i bridge lower ohm than this, i get lower output?
I tested with the 2 4 ohm loads as well and got 2.7kw worth of mains drain and the breaker popping after 5 sec.. (getting up to 150% of rating?)........

Wierd... i get less power into 2 ohms! (that's obviously how my calcs went wrong!)

ilimzn 26th March 2005 09:30 AM

Your scope measures peak voltages, not RMS. 51V peak sine into 4 ohms is 325W. This is because for a sinewave, RMS=0.707*Peak, and Pout = Vpeak^2 / (2*Rload)... elementary stuff, really...

NUTTTR 26th March 2005 09:54 AM


Originally posted by ilimzn
Your scope measures peak voltages, not RMS. 51V peak sine into 4 ohms is 325W. This is because for a sinewave, RMS=0.707*Peak, and Pout = Vpeak^2 / (2*Rload)... elementary stuff, really...
Nope, it definately is RMS.....
I can guarantee this scope measures RMS...... It can also measure peak if you like, but it's set on RMS..... Always has been

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