Hopefully someone will have some suggestions on what my amps issue is. I have had it bench tested and 1 channell runs at 124 Watts and the other about 130 watts. I bought it a couple years ago and it always sounded great but never seemed to have a lot of power. I know why now after a bench test by a technician friend of mine. Has anyone ever seen something like this before, my friend doesnt have a lot of experience with an amp like this. Any ideas of where to look will be appreciated
Thanks Jeff
Thanks Jeff
It could be as simple as your not driving it hard enough.
Some amps are more sensitive than others.
Some amps are more sensitive than others.
The amp is rated 250/channel @ 8 ohms with a gain of 29dB (28) I predict the power supply rails around +/- 90 volts and the output should get pretty close to the rails when it clips. Are the power supplies at the correct value? (get a service manual and don't take my guess as fact) To reach 180 Vp-p unloaded you may need to drive it 6.4 Vp-p or 2.25V rms as the previous poster stated. It's very important that the line voltage is correct during the test as a 5% drop in line voltage causes a 10% drop in the power. I would expect your power supply rails to sag from around 90 to 75 under load - and those load resistors are getting pretty hot! If the amp is not reaching 130V p-p under load at clipping with 4.64 V p-p drive, it's in need of repair.
G²
The rail voltages are suppose to be +-81v and they are there not sure what is next. any more ideas??
Jeff
Jeff
Sure. Now that you know what the rails are supposed to be you can monitor one (or both if you have 2 meters) while driving the amp into a load. 250 Watts is 126.49 Volts p-p so the rails have to be at least 65 volts (that's a better than average power supply) and the input drives at 4.51 Volts p-p (gain 28). 130 Watts is 91 Volts p-p so several things may be happening. Line voltage low, load resistor actually less than you think (unlikely and easily verified), insufficient drive or an amplifier fault but 2 channels with the same fault making partial power and not simply burned out is VERY unusual. So hook it up and tell us more. A scope photo at clipping would be good.
G²
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does anyone know for sure what the input sensitivity is on my Adcocm 5800. What input voltage signal do I need to drive this amp to full ouput. Thanks jeff
I am just wondering about your measurement methode. I use 1khz sign wave and oscilioscope to see clipping point to measure peak to peak voltage to calculate wattage. Did you use scope and signal generators to get this reading?
I didn't make those numbers up. The Adcom owners manual say the gain is 29dB which translates to 28. 130 Volts p-p (264 Watts @ 8 ohms) divided by 28 is 4.64 Volts p-p. Show the schematic and we can tell you exactly the gain. This value is perfectly normal compared to other high Wattage amps.
G²
Thanks for the help the only reason i was asking was i emailed adcom and they told me that it was 1.5 v, I didnt buy this because the math doesnt work. My friend who is working on my amp will be doing some testing today.
I will let you know what he comes up with tomorrow
Thanks Jeff
I will let you know what he comes up with tomorrow
Thanks Jeff
1.5 Volt rms is 4.24 Volts p-p so we're not terribly different. I just like p-p Volts as that is what you see on a scope.
G²
I spoke to my friend and he has been able to get the amp to output its full 250 watts, The input was too low and when it was increased he was sending a clipped input signal. The amp requires a much greater input than my onkyo av reciever will output for sure. He is going to adjust the bias urrents to balance the outputs and it should ok.
I guess I am now looking for a good line driver to increase the output of my preamp, any suggestions on that.
Thanks to all who contributed info it was greatly appreciated.
Jeff
I guess I am now looking for a good line driver to increase the output of my preamp, any suggestions on that.
Thanks to all who contributed info it was greatly appreciated.
Jeff
You might consider building a 'booster' into the Onkyo. One dual opamp, 10 or 12 resistors and some bypass caps and you're in business. I would NOT put it into the power amp as the power supplies can get quite 'dirty' when delivering power. I STRONGLY recommend NOT changing the gain of the Adcom to increase its sensitivity. My guess is you'll need an increase of 2 to 3 times to make it work.
G²
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