SX-950 Low output amps

Kim Beneteau

Kim Beneteau

New Member
I've recapped all boards except for the tuner. On the bench I'm running a couple of mini JBL surround speakers to check for sound L/R operation. All seemed good, dragged it upstairs and hooked up to a pair of Infinity P363's and good clear sound. However, it seemed odd that as I turned up the volume it kind of flattened out about halfway to max. Didn't seem to have the volume or punch I would expect but still good clean sound. Did a check of the output and measured at max volume and measured 2.55 volts VRMS with a 1khz signal with an 8.2 ohm load....if I'm calculating correctly it's only putting out about 32 watts/channel where it should be pushing upwards of 110 watts/channel. I think it's in the Power Amp but it's common to both Left and Right channels...no difference between the two. Voltages at the numbered pins on the Power Amp Board are all in the ball park. Not sure where to look next?
 
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There’s a 20db mute switch on the front?
I think it should still power more than that switch limits. :dunno:
Did you check the amp settings?
Thinking on this.
 
2.55vrms would be approx. < 1Wrms at 8 ohms. where did the 32 watts rms comes from? 32 W would be approx 16Vrms continuous.

Are your probes and scope both set to 10x?

what amplitude is your 1khz sine wave at with volume turned to max? Maybe set it at 200mV with volume control at zero. Then ramp up volume to see where you stand.
 
Last edited:
HWKI said:
2.55vrms would be approx. < 1Wrms at 8 ohms. where did the 32 watts rms comes from? 32 W would be approx 16Vrms continuous.

Are your probes and scope both set to 10x?

what amplitude is your 1khz sine wave at with volume turned to max? Maybe set it at 200mV with volume control at zero. Then ramp up volume to see where you stand.
Click to expand...
I’m using my iPhone 14 with a signal generator app to generate the 1kHz sine wave out of the lightning connector to both sides of the aux input. I tried to measure the voltage coming out of the the iPhone but not sure I’m doing it correctly I only appear to be getting .2mv which doesn’t make sense.
 
Well, at least at low attenuation both the Left and Right Channels look quite stable. You have that going for you.
I've never used the phone for a freq generator, but I suspect there are limitations in doing so.
 
HWKI said:
Well, at least at low attenuation both the Left and Right Channels look quite stable. You have that going for you.
I've never used the phone for a freq generator, but I suspect there are limitations in doing so.
Click to expand...
HWKI said:
2.55vrms would be approx. < 1Wrms at 8 ohms. where did the 32 watts rms comes from? 32 W would be approx 16Vrms continuous.

Are your probes and scope both set to 10x?

what amplitude is your 1khz sine wave at with volume turned to max? Maybe set it at 200mV with volume control at zero. Then ramp up volume to see where you stand.
Click to expand...
Probes and scope both set to 10x
 
Pull the pre to amp jumpers in the back a measure the pre outs to do your testing of signal levels. Split the signal source for your L-R inputs and scope L-R at the preamp output jacks in the back
 
Kim Beneteau said:
I've recapped all boards except for the tuner. On the bench I'm running a couple of mini JBL surround speakers to check for sound L/R operation. All seemed good, dragged it upstairs and hooked up to a pair of Infinity P363's and good clear sound. However, it seemed odd that as I turned up the volume it kind of flattened out about halfway to max. Didn't seem to have the volume or punch I would expect but still good clean sound. Did a check of the output and measured at max volume and measured 2.55 volts VRMS with a 1khz signal with an 8.2 ohm load....if I'm calculating correctly it's only putting out about 32 watts/channel where it should be pushing upwards of 110 watts/channel. I think it's in the Power Amp but it's common to both Left and Right channels...no difference between the two. Voltages at the numbered pins on the Power Amp Board are all in the ball park. Not sure where to look next?
Click to expand...
Problem resolved: Previously I had trouble after I recapped the Equalizer Amp board and found that I had installed the transistors incorrectly. I thought at that time I had checked the other boards that I already been through...Flat Amp, Tone Amp and moved on. After much thought I decided to start tracing the signal from the source. Checking voltages on the Transistors on the Flat Amp I discovered some weird voltages on Q5 & Q6....was measuring from the bottom of the board so I couldn't see the position of the transistor yet. Pulled the Flat Amp board out where I could see the transistors and found that I had installed the both of them incorrectly. R&R them installing correctly this time and bingo! RMS voltage readings bounce way up.....sometimes you live and learn. Thanks for the responses and making me think :-)
 

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