|TomCarlos||2/11/2018 11:32 AM|
|Crest FA901 Power Amp - One Channel Offset and Hum|
Greetings MEF members...
A friend asked me to look at his Crest FA901 Stereo Power Amp. I'm guessing this amp is "old" !! Channel B sounds fine (no hum, no dc offset at the output). Channel A has a slight "hum" (it is noticeable) and has 10mv dc offset (that is what I am measuring). There is an ever so small ripple that is barely noticeable on a scope.
As I am poking around the amp, I think it's an odd design. First, having a fan in the middle of the amp doesn't make sense to me- but oh well. Next, I see what appears to be a "Right" and "Left" set of components. When you look closer, you will see the right side is made up of preamp board connected to the large heat sink assembly that contains 6 (qty) 2SA1302 PNP transistors (the green ones). The left side is similar but there you will find the 6 (qty) 2SC3281 NPN transistors (the black ones). The schematic is a bit different for me too - in that the Channel A and Channel B are mixed onto a single drawing. I guess you have to be good to separate everything out.
Anyway... before I get way in over my head, I am wondering if the minimal offset has anything to do with the hum on Channel A.
|g1||2/11/2018 12:14 PM|
|That is not left and right but the + and - sides of the push pull.|
One channel will be the upper layer, underneath will be another layer for channel B. Just a different way of laying it out.
|TomCarlos||2/11/2018 7:53 PM|
|Geeze Louise !! The other set of Power Transistors are mounted on the bottom side of the heat sinks. It's like they are inverted for some odd reason. |
So, back to the offset and hum. I see VR1 and the LA2500 IC. I believe that is to set the bias current? Does that impact the voltage at the output?
|Jazz P Bass||2/11/2018 8:34 PM|
|I would limit any tests on that amp to power supply.|
Check for any ripple that may indicate a bad cap.
Try to discern if it is a pre or power amp issue.
Push comes to shove you can always send it to Peavey.
|TomCarlos||2/11/2018 8:44 PM|
|Thanks JPB. But if the amp has a bad cap in the power supply, I would think you'd hear the hum in Channel B as well. So there could be a filtering issue downstream (as you said... in the preamp or power amp section).|
|Jazz P Bass||2/11/2018 9:49 PM|
|I am just cautioning you.|
That is not an easy amp.
The bias is set with that LA2500 (which is basically unobtanium) (
It makes a pseudo Class A output without crossover distortion.
|The Dude||2/12/2018 7:19 PM|
|I wouldn't worry about 10mV offset. That's pretty minimal. Does the hum go away when the volume is turned down? Is the amp configured correctly? It's not in bridge mode is it?|
|TomCarlos||2/12/2018 7:21 PM|
|The power supply provides a Red and Green (power) lead to each output board. The Red supply line measures at 73.4vdc with 42mv rms ripple. It appears as a sawtooth wave. The Green supply line measures -73.4vdc with 36mv rms. The power supply also has a low voltage output of 20.8 volts sitting across C11 (1000uf) with 500mv rms ripple! I'll try to take out the P/S board and check that capacitor (probably just replace it).|
|The Dude||2/12/2018 7:27 PM|
|FWIW: I agree with your original assessment, "if the amp has a bad cap in the power supply, I would think you'd hear the hum in Channel B as well". Since the same power supply is shared by both amps, I don't think you have a power supply problem.|
|TomCarlos||2/12/2018 7:32 PM|
|I tested in the stereo mode and bridge. Now for some strange reason (as it happens on occasion), the amp seems quiet. I will retest later tonight and tomorrow. But I am still concerned regarding the ripple on C11. Thanks!|