I am getting different response characteristics on the L and R channels after conducting some post service measurements. I'd like to understand whats influencing this and if my amp has a problem that needs fixing.
I have just completed a minor service on my Luxman M-120a power amp. I replaced a couple of small signal transistors in the input stage to fix a intermittent noise problem. I also checked and adjusted the DC offset and bias current for both channels, all within specs.
I ran a power load test into 8 ohms on both channels and all was good up to 120W.
I decided to do a frequency response measurement on both channels out of curiosity. At about 30 Watts load, i got the following:
Right Channel:
-3dB at 2 Hz and also at greater than 200 kHz! Protection circuit kicked in at about 230 kHz, at less than -1dB
Left Channel:
-3dB at 2Hz and also at around 150 kHz, rolling off gradually
I also checked responses with square wave inputs:
Right channel - slight overshoot with decaying oscillation on leading edge, starting gradually at around 8 kHz, and gradually increasing with frequency
Left Channel - slight rounding of leading edge starting at around 10 kHz, and increasing with frequency.
This is telling me my right channel is under-damped and/or the left channel may be over-damped. Is this indicative of a problem or should I just leave all alone?
Can anybody shed some light as to what may be causing this? Could it be something associated with the Duo Beta feedback, too much or not enough negative feedback? Can this be adjusted? What components/levels in what stages of this amplifier should I be checking and how?
Any advice would be much appreciated.
I have just completed a minor service on my Luxman M-120a power amp. I replaced a couple of small signal transistors in the input stage to fix a intermittent noise problem. I also checked and adjusted the DC offset and bias current for both channels, all within specs.
I ran a power load test into 8 ohms on both channels and all was good up to 120W.
I decided to do a frequency response measurement on both channels out of curiosity. At about 30 Watts load, i got the following:
Right Channel:
-3dB at 2 Hz and also at greater than 200 kHz! Protection circuit kicked in at about 230 kHz, at less than -1dB
Left Channel:
-3dB at 2Hz and also at around 150 kHz, rolling off gradually
I also checked responses with square wave inputs:
Right channel - slight overshoot with decaying oscillation on leading edge, starting gradually at around 8 kHz, and gradually increasing with frequency
Left Channel - slight rounding of leading edge starting at around 10 kHz, and increasing with frequency.
This is telling me my right channel is under-damped and/or the left channel may be over-damped. Is this indicative of a problem or should I just leave all alone?
Can anybody shed some light as to what may be causing this? Could it be something associated with the Duo Beta feedback, too much or not enough negative feedback? Can this be adjusted? What components/levels in what stages of this amplifier should I be checking and how?
Any advice would be much appreciated.
Scope pictures of the square wave response might be helpful but I don't see anything in your summary of the measurements that would concern me.
If layouts are identical this is down to small differences in active/passive components .
Like you I am a bit obsessive in getting near perfect square wave responses, it can be a combination of adjusting (increasing the value ) of the small value compensation capacitors and input slew-rate components in the right channel while watching the result on your ,scope.
The left channel would be a slight decrease.
I must emphasize we are talking very small value increases or decreases.
Like others here I have designed and built many power amps but using mainly JLH circuit variations, his capacitor compensation does not have the same effect on the slew rate as the standard position but tuning is required on all hand built amps to get a near perfect square wave.
Like you I am a bit obsessive in getting near perfect square wave responses, it can be a combination of adjusting (increasing the value ) of the small value compensation capacitors and input slew-rate components in the right channel while watching the result on your ,scope.
The left channel would be a slight decrease.
I must emphasize we are talking very small value increases or decreases.
Like others here I have designed and built many power amps but using mainly JLH circuit variations, his capacitor compensation does not have the same effect on the slew rate as the standard position but tuning is required on all hand built amps to get a near perfect square wave.