Hello there!
This is yet another amplifier noise thread
I guess you have seen plenty of them here and I beg your pardon to just be the next one you see...
I have been given an old Technics (National Panasonic) SU-3600 in nice state (except for some scratches and two missing knobs, but I can excuse this as it is a barn find and was for free)
I cleaned the beast and double checked everything after putting some new thermal grease and setting bias and IQ, and when I first powered it and listened some tracks I fell in love with it as it has much more depth than my other amplifiers
After further listening to it, I noticed that there is a background noise which is very distinctively composed of two components, a humming one and a hissing one... I guess the humming one would be some 150Hz and the hissing one would be some random noise, maybe thermal
One thing to note is that some of the noise is highly attenuated when I reach the middle of the master volume setting, so it seems that it is somehow related to the preamplifier rather than the power supply itself
As this is a somehow unobtainable amplifier at the moment I would like to make as little change as possible to it and get advice from other people before starting to replace all capacitors and co... they all seem fine and the sweet spot in the volume control makes me think that they may not be to blame
I'm suspecting the area around TR107 and TR207 to blame, maybe some resistors that would have gone out of range...
EDIT: to be noted that IQ and bias took quite some time to stabilize before I was able to get the specified values... trimmers have been lightly cleaned with kontact60 on a cottonstick
I attached the schematic for support, would some more experienced guys be willing to help?
This is a really nice amplifier, quite rare, which is a barn find, so it has become a duty for me to restore it to a hum-free state
Thanks in advance!
This is yet another amplifier noise thread
I guess you have seen plenty of them here and I beg your pardon to just be the next one you see...I have been given an old Technics (National Panasonic) SU-3600 in nice state (except for some scratches and two missing knobs, but I can excuse this as it is a barn find and was for free)
I cleaned the beast and double checked everything after putting some new thermal grease and setting bias and IQ, and when I first powered it and listened some tracks I fell in love with it as it has much more depth than my other amplifiers
After further listening to it, I noticed that there is a background noise which is very distinctively composed of two components, a humming one and a hissing one... I guess the humming one would be some 150Hz and the hissing one would be some random noise, maybe thermal
One thing to note is that some of the noise is highly attenuated when I reach the middle of the master volume setting, so it seems that it is somehow related to the preamplifier rather than the power supply itself
As this is a somehow unobtainable amplifier at the moment I would like to make as little change as possible to it and get advice from other people before starting to replace all capacitors and co... they all seem fine and the sweet spot in the volume control makes me think that they may not be to blame
I'm suspecting the area around TR107 and TR207 to blame, maybe some resistors that would have gone out of range...
EDIT: to be noted that IQ and bias took quite some time to stabilize before I was able to get the specified values... trimmers have been lightly cleaned with kontact60 on a cottonstick
I attached the schematic for support, would some more experienced guys be willing to help?
This is a really nice amplifier, quite rare, which is a barn find, so it has become a duty for me to restore it to a hum-free state
Thanks in advance!
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Burn in the amp for a few days and see if the hum subsides.
If not, there may be a bad electrolytic capacitor in the power supply.
If the hiss is about equal in both channels, it may be normal for that model.
Do test the preamp and power amp separately, since you can unlink their connection.
If not, there may be a bad electrolytic capacitor in the power supply.
If the hiss is about equal in both channels, it may be normal for that model.
Do test the preamp and power amp separately, since you can unlink their connection.
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Thanks a lot guys,
I'll have to retest to be sure, but I'm pretty sure the noise is there on both channels and doesn't vary with volume except for that sweet spot
I'll have a go at retesting the power amp separately,and if it confirms that the preamplifier is faulty I'll replace C231 - it was my first assumption with C126
I guess a full electrolytics recap would be a good idea if one of those has dried
The transistor replacements will also occur if it affects the noise that much
Thanks a lot for the tips guys!
I'll have to retest to be sure, but I'm pretty sure the noise is there on both channels and doesn't vary with volume except for that sweet spot
I'll have a go at retesting the power amp separately,and if it confirms that the preamplifier is faulty I'll replace C231 - it was my first assumption with C126
I guess a full electrolytics recap would be a good idea if one of those has dried
The transistor replacements will also occur if it affects the noise that much
Thanks a lot for the tips guys!
well for the hum as there are no decoupling caps at the amplifier modules I'd go with a replacement of the two rail caps as I have a pair of 8600µ Elna caps, and maybe even the diodes which are old can types, a good bridge would fit nicely there... I'll give it some time tomorrow and will report for sure
A null in hum at a certain position on the volume control can be caused by hum prior to the volume control (preamp) nulling with anti-phase hum after the volume control (power amp). This is possible because amplifiers can have several phase inversions on the way through, even if they are absolute phase correct overall from input to output. Start by separating the power amplifier section and get that hum free, then look at the preamp.
I have been able to check noise of preamplifier against noise from the amplifier,
The amplifier itself is way more quiet than the preamplifier with only a small level of hiss... when I connect the PRE OUT to AMP IN and ramp up the volume to the point that preamplifier noise reaches the same approximate level as the amplifier noise they cancel out, then the hiss continues to raise until full volume
This is encouraging, because as johnmath says, it is most probably two identical noises that are opposite phase and cancel out at some point - it encourages me to just replace the PSU electrolytics and check if this is better
If it is still noisy I'll replace all the 50V/220µ in the preamplifiers and amplifier
If there is still noise, are the 2SA666 a noisy transistor and should I replace them all, or is this only the frontend design which is favoring noise?
Thanks a lot for the help guys!
The amplifier itself is way more quiet than the preamplifier with only a small level of hiss... when I connect the PRE OUT to AMP IN and ramp up the volume to the point that preamplifier noise reaches the same approximate level as the amplifier noise they cancel out, then the hiss continues to raise until full volume
This is encouraging, because as johnmath says, it is most probably two identical noises that are opposite phase and cancel out at some point - it encourages me to just replace the PSU electrolytics and check if this is better
If it is still noisy I'll replace all the 50V/220µ in the preamplifiers and amplifier
If there is still noise, are the 2SA666 a noisy transistor and should I replace them all, or is this only the frontend design which is favoring noise?
Thanks a lot for the help guys!
This is an old transistor. In its time low noise but 16 dB is very noisy. 2SA970 is a modern low noise transistor and widely available. Swap out one channel as I suggested above. See attached datasheet.
Thanks Regenpak honestly I did not figure to look at the NF value in the datasheet, even if I had to study this in my EE courses
I guess this is the case in most of those transistors and all could be replaced, but - this is a question for future improvements - is this practical to think about replacing all of them? I have a bag of more modern pulls which would for sure have a lower noise production
I have just replaced the two 3300µ main caps with some 22000µ and the noise in the main amplifiers is still the same, maybe just a little lower...
I have then replaced the electrolytics on the amplifier boards which I cannot tell if they have dripped electrolytics or if it is flux from the soldering process (I guess this is electrolyte since other components have far less goo at the component side)
This replacement leads to a bit less hiss which is pretty good in itself
I then connected back the preamp and now the hum is quite louder at max volume and it is clearly some 100Hz
My conclusion is that the electrolytics on the amp boards were bad/dried (even if tested good at the cheap ESR tester) and now they would be working better and gain would be back up to spec
Next step is to replace the auxiliary power filter cap and recap the preamplifiers whose electrolytics show the same dripping as on the amplifier boards
If the noise is not satisfactory yet I'll replace the input transistors by more modern ones as mentioned
I guess replacing all the preamplifier transistors would make for a great improvement, but would they be gain selected? I don't want to make some bad decision there...
I guess this is the case in most of those transistors and all could be replaced, but - this is a question for future improvements - is this practical to think about replacing all of them? I have a bag of more modern pulls which would for sure have a lower noise production
I have just replaced the two 3300µ main caps with some 22000µ and the noise in the main amplifiers is still the same, maybe just a little lower...
I have then replaced the electrolytics on the amplifier boards which I cannot tell if they have dripped electrolytics or if it is flux from the soldering process (I guess this is electrolyte since other components have far less goo at the component side)
This replacement leads to a bit less hiss which is pretty good in itself
I then connected back the preamp and now the hum is quite louder at max volume and it is clearly some 100Hz
My conclusion is that the electrolytics on the amp boards were bad/dried (even if tested good at the cheap ESR tester) and now they would be working better and gain would be back up to spec
Next step is to replace the auxiliary power filter cap and recap the preamplifiers whose electrolytics show the same dripping as on the amplifier boards
If the noise is not satisfactory yet I'll replace the input transistors by more modern ones as mentioned
I guess replacing all the preamplifier transistors would make for a great improvement, but would they be gain selected? I don't want to make some bad decision there...
Why would you want to select on gain? This is a feedback controlled circuit. Higher h(FE) is better. Modern transistors have plenty of it. Just replace TR201 and then compare the two channels. Noise in L_Ch should be considerably lower and the variation in noise with the volume control signifficantly reduced.
Stereo Integrated Amplifier (1973)
Power output: 50 watts per channel into 8Ω (stereo)
Total harmonic distortion: 0.2%
Input sensitivity: 1.5mV (MM), 100mV (line)
Signal to noise ratio: 73dB (MM), 85dB (line)
National SU-3600 Stereo Integrated Amplifier Manual | HiFi Engine
Power output: 50 watts per channel into 8Ω (stereo)
Total harmonic distortion: 0.2%
Input sensitivity: 1.5mV (MM), 100mV (line)
Signal to noise ratio: 73dB (MM), 85dB (line)
National SU-3600 Stereo Integrated Amplifier Manual | HiFi Engine
The hum in the power amplifier may be caused by poor soldering e.g. a dry joint, or by some other weakness or resistance in the 0V (ground) circuitry, or a broken shield on the wiring to the 'amp in' socket, or faulty capacitors on the supply to the input side (C303, C304).
Trace out all of the ground wiring connected to 'D' and make sure it is all soldered properly. If the are 'wire-wrap' connections solder them, if there are plug-in connections treat them with contact cleaner, if there are screw connections loosen and tighten them. Short circuit the input to the power amplifier and see if that makes a difference. Get a chopstick and tap components and wires and see if you can hear an effect on the hum. Use a 'dim bulb' circuit in the power lead while you are doing this so give some protection.
If you change the main power supply capacitors I would double or quadruple their values as 3300µF is very small; put in as large as you can physically fit.
Once the power amplifier side is sorted, you can move to the preamp side.
Trace out all of the ground wiring connected to 'D' and make sure it is all soldered properly. If the are 'wire-wrap' connections solder them, if there are plug-in connections treat them with contact cleaner, if there are screw connections loosen and tighten them. Short circuit the input to the power amplifier and see if that makes a difference. Get a chopstick and tap components and wires and see if you can hear an effect on the hum. Use a 'dim bulb' circuit in the power lead while you are doing this so give some protection.
If you change the main power supply capacitors I would double or quadruple their values as 3300µF is very small; put in as large as you can physically fit.
Once the power amplifier side is sorted, you can move to the preamp side.
I have replaced the preamplifier capacitors and there is no change
When I touch the chassis most of the hum goes away so I can imagine this could be a grounding issue...
To come back to the power amplifier, I redid all the soldering but not the wires themselves, so I will do it for sure, and make sure to check if grounding the input would make some changes! this is obvious, I should have thought about it, thanks a lot
When I touch the chassis most of the hum goes away so I can imagine this could be a grounding issue...
To come back to the power amplifier, I redid all the soldering but not the wires themselves, so I will do it for sure, and make sure to check if grounding the input would make some changes! this is obvious, I should have thought about it, thanks a lot
That's concerning. I'm not sure what is the cause of that, but it should not occur. I suspect that some part of the circuit that should be at ground potential is not.
As you are working on a live circuit, have you made sure that any exposed line voltage parts are insulated, like the terminals on the power switch and auxiliary sockets, line voltage fuse connectors, etc. Gear of that vintage was often made with exposed high voltage inside as there are supposed to be "no user serviceable parts inside" and the unit should not have the lid off.
Be careful that you don't inadvertently become a current path from a live part of the circuit to ground, because the consequences can be shocking! Ideally you have a 1:1 mains isolation transformer for your own protection and a dim bulb current limiter in the supply lead to protect the amplifier you are trying to repair.
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Don't worry, I used to work on live things for years, and I have had some study and training for low to high voltage
I'm in electronics since the age of 8 and it's been nearly 30 years sinceThe chassis is quite clean and well organized, the only exposed live is around the mains tapper... the worse I can really expect is a small zap if I touch some chassis and a trimmer at the same time, the rest will not see any screwdriver when live and I take care to discharge the caps before any PCB work
I will make sure to check all grounding tomorrow