Bence Szalai
New Member
Hi! I'm first time here, so please let me know if I'm doing something wrong!
I'm looking for some help on an issue my Pioneer SX-750 has developed recently. The left channel has a problem audible, while the right channel seems to be fine.
The issue I can hear on the left channel I can describe as:
- input signals and parts of the input signal lower than 45% of the standard input volume I've been using (from my computer) are lower in overall volume, approx 25% of the right channel,
- input signals and parts of the input signal that reach above 45% of the normal input volume are somewhat louder, but still peaking at around half of that of the right, and heavily distorted by a cutting sounding like insufficient power to drive the signal to it's full extent, and
- the issue doesn't seem to be related to any specific frequency range as far as I can hear.
Please see the attached drawing representing the relation of the Input and Output of my Amp - based on my ears.
I've indeed tried to switch the inputs and the speakers to see if the issue is really with the Amp, but in both swaps the issue stayed on the Left channel, so it's not related to my input source or the speakers. I've also switched the speakers to output B from my usually used A, but the issue remained, so it's not the Power Amp part either. It affects signals coming from Aux source and FM Radio both. (Did not try AM or Phono.)
The issue is not affected by Tone, High Filter, Loudness or Duplicate switches, however the issue is far less significant if I switch Mode from Stereo to Mono. Now being curious how come this is happening, I've tested it also with input signal only on Right and Left side in Mono and Stereo mode, here are my results as {No.) Input - Mode - Results} format:
1.) Right input only - Stereo - Right speaker OK, Left speaker Silent
2.) Right input only - Mono - Right speaker OK, Left speaker OK
3.) Left input only - Stereo - Right speaker Silent, Left speaker heavily distorted
4.) Left input only - Mono - Right speaker somewhat distorted, Left speaker somewhat distorted
So the strange part is that even when there's input only applied to the Left channel the overall distortion is lighter in Mono mode than in Stereo, but impacting both channels, which results in contradicting suggestions to me:
- The fact that the issue is impacting both channels in case 4. and none in case 2. suggests that the issue is happening before the Mono Mode switch.
- However the fact that the distortion is lighter in case 4. than case 3. suggests that the mono switch does two things actually: average the input for both channels, run them over both preamp channels, and than average them again, otherwise if the issue was really (only) before the Mono Mode switch the distortion should be the same strength in case 4 as in case 3, but that's not the case.
As I have doubt that the mono switch couples the channels at two separate points in the signal flow, my assumption is that this may somehow be related to the power supply of the Preamp, which can explain how the problem can be coupled between the two channels depending on which are driven in the first place.
Also I've noted that if input applied to both channels, turning Mode to Mono reduces the audible distortion to almost unnoticeable for both channels, however listening carefully it's only a result of the relative small peak signal strength of the malfunctioning Left channel to that of the Right, so when they are coupled the distortion significant on it's own for the left channel gets shadowed by the much stronger signal of the Right channel. This could explain why it sounds okay in case 2. too.
So basically this is all I know about the issue at the moment. I've had to replace some caps on the Power Amp Power Supply cca. a year ago, but the PreAmp Power Supply was not touched for at least 15 years since I own the Amp. I'm not sure about the service history before. I've bought it from a service; they said they had fixed the Power Amp as one of the channels were probably short circuited or so, but no other issues were mentioned.
I'm looking for some recommendations on what to check and what I might need to replace if you could recognise the issue by description maybe. Any help would be appreciated!
Thanks in advance!
I'm looking for some help on an issue my Pioneer SX-750 has developed recently. The left channel has a problem audible, while the right channel seems to be fine.
The issue I can hear on the left channel I can describe as:
- input signals and parts of the input signal lower than 45% of the standard input volume I've been using (from my computer) are lower in overall volume, approx 25% of the right channel,
- input signals and parts of the input signal that reach above 45% of the normal input volume are somewhat louder, but still peaking at around half of that of the right, and heavily distorted by a cutting sounding like insufficient power to drive the signal to it's full extent, and
- the issue doesn't seem to be related to any specific frequency range as far as I can hear.
Please see the attached drawing representing the relation of the Input and Output of my Amp - based on my ears.
I've indeed tried to switch the inputs and the speakers to see if the issue is really with the Amp, but in both swaps the issue stayed on the Left channel, so it's not related to my input source or the speakers. I've also switched the speakers to output B from my usually used A, but the issue remained, so it's not the Power Amp part either. It affects signals coming from Aux source and FM Radio both. (Did not try AM or Phono.)
The issue is not affected by Tone, High Filter, Loudness or Duplicate switches, however the issue is far less significant if I switch Mode from Stereo to Mono. Now being curious how come this is happening, I've tested it also with input signal only on Right and Left side in Mono and Stereo mode, here are my results as {No.) Input - Mode - Results} format:
1.) Right input only - Stereo - Right speaker OK, Left speaker Silent
2.) Right input only - Mono - Right speaker OK, Left speaker OK
3.) Left input only - Stereo - Right speaker Silent, Left speaker heavily distorted
4.) Left input only - Mono - Right speaker somewhat distorted, Left speaker somewhat distorted
So the strange part is that even when there's input only applied to the Left channel the overall distortion is lighter in Mono mode than in Stereo, but impacting both channels, which results in contradicting suggestions to me:
- The fact that the issue is impacting both channels in case 4. and none in case 2. suggests that the issue is happening before the Mono Mode switch.
- However the fact that the distortion is lighter in case 4. than case 3. suggests that the mono switch does two things actually: average the input for both channels, run them over both preamp channels, and than average them again, otherwise if the issue was really (only) before the Mono Mode switch the distortion should be the same strength in case 4 as in case 3, but that's not the case.
As I have doubt that the mono switch couples the channels at two separate points in the signal flow, my assumption is that this may somehow be related to the power supply of the Preamp, which can explain how the problem can be coupled between the two channels depending on which are driven in the first place.
Also I've noted that if input applied to both channels, turning Mode to Mono reduces the audible distortion to almost unnoticeable for both channels, however listening carefully it's only a result of the relative small peak signal strength of the malfunctioning Left channel to that of the Right, so when they are coupled the distortion significant on it's own for the left channel gets shadowed by the much stronger signal of the Right channel. This could explain why it sounds okay in case 2. too.
So basically this is all I know about the issue at the moment. I've had to replace some caps on the Power Amp Power Supply cca. a year ago, but the PreAmp Power Supply was not touched for at least 15 years since I own the Amp. I'm not sure about the service history before. I've bought it from a service; they said they had fixed the Power Amp as one of the channels were probably short circuited or so, but no other issues were mentioned.
I'm looking for some recommendations on what to check and what I might need to replace if you could recognise the issue by description maybe. Any help would be appreciated!
Thanks in advance!
