It is not a grounding issue. You are working on an open amplifier with its bottom plate removed. Replace it and your hum stays away.
Ok so I'll be replacing the shielding and bottom plate and see where it goes...
Hiss is still there though...
Hiss is still there though...
Update...
The cover and shield are indeed removing most of the hum, this is good
Grounding the amplifier inputs effectively completely nulls the hiss
Grounding the preamplifier input has no effect on hiss
I guess if I want to tell which half of the preamp is hissing (before or after tone control & selectors) I will have to play with alligator clips to ground inputs at (101) and (104) - I could also completely bypass the selectors to see if this kludge of switches is picking all that noise, or put a RC filter somewhere in between?
Also, I have trouble finding the NFB signal path which someone mentioned at the beginning, would someone be able to give me pointers? It may also pick noise in my opinion
The cover and shield are indeed removing most of the hum, this is good
Grounding the amplifier inputs effectively completely nulls the hiss
Grounding the preamplifier input has no effect on hiss
I guess if I want to tell which half of the preamp is hissing (before or after tone control & selectors) I will have to play with alligator clips to ground inputs at (101) and (104) - I could also completely bypass the selectors to see if this kludge of switches is picking all that noise, or put a RC filter somewhere in between?
Also, I have trouble finding the NFB signal path which someone mentioned at the beginning, would someone be able to give me pointers? It may also pick noise in my opinion
With the bottom cover on and no hum in the power amplifier, there is now no volume control null point for hum with the preamp connected?
If the hiss in the power amp section drops when you short the input, then it is the Johnson noise of the 470kΩ // 100kΩ input impedance of the amplifier. You don't need to worry about this, because this hiss source is "shorted" by the low output impedance of the preamplifier stage when connected. If there is hum with the input shorted, that is a different problem related to supply current circulation paths usually in the ground circuit.
For all inputs except phono the only active stage in the preamp is the tone circuit, which consists of three transistors Tr201,3,5 (LHC) and Tr202,4,6 (RHC). If the the tuner / aux / tape inputs are hissing, these transistors are implicated.
If there is hum on these inputs, the supply regulator circuit, particularly C201 and Tr207 are indicated.
If only the phono input is noisy the the phono stage is where you need to look. Phono inputs are always noisy with on cartridge connected because of their high input impedance and high gain. Shorting the input usually makes a huge difference, which is to be expected.
If there is hum on the phono input with it shorted, then the phono supply regulator is implicated, Tr107, C125, C126, or it may just be poor separation / screening from the supply transformer. Hum induced by magnetic fields with be at 50/60Hz, whereas hum from power supply ripple will be at 100/120Hz. Sometimes a mu-metal shield can be fashion around the phono circuit to reduce mains hum induction. Occasionally the screening of the input cable to the phono preamp is inadequate. If there is hum with the connector shorted, but the hum goes when shorted at the circuit board end, it is the screening of the cable. I it makes no difference and there is no ripple on the supply (see above), then it could be an earth loop / common earth problem within the actual amplifier layout.
I often double or quadruple the supply capacitors to reduce ripple and hence hum at 2 x mains frequency. Each doubling will in theory drop the voltage ripple across the capacitor by 6dB. At the time of design the values are often chosen based on the target price for the design, i.e. they are as small as can be got away with with the product still saleable at its price point.
If the hiss in the power amp section drops when you short the input, then it is the Johnson noise of the 470kΩ // 100kΩ input impedance of the amplifier. You don't need to worry about this, because this hiss source is "shorted" by the low output impedance of the preamplifier stage when connected. If there is hum with the input shorted, that is a different problem related to supply current circulation paths usually in the ground circuit.
For all inputs except phono the only active stage in the preamp is the tone circuit, which consists of three transistors Tr201,3,5 (LHC) and Tr202,4,6 (RHC). If the the tuner / aux / tape inputs are hissing, these transistors are implicated.
If there is hum on these inputs, the supply regulator circuit, particularly C201 and Tr207 are indicated.
If only the phono input is noisy the the phono stage is where you need to look. Phono inputs are always noisy with on cartridge connected because of their high input impedance and high gain. Shorting the input usually makes a huge difference, which is to be expected.
If there is hum on the phono input with it shorted, then the phono supply regulator is implicated, Tr107, C125, C126, or it may just be poor separation / screening from the supply transformer. Hum induced by magnetic fields with be at 50/60Hz, whereas hum from power supply ripple will be at 100/120Hz. Sometimes a mu-metal shield can be fashion around the phono circuit to reduce mains hum induction. Occasionally the screening of the input cable to the phono preamp is inadequate. If there is hum with the connector shorted, but the hum goes when shorted at the circuit board end, it is the screening of the cable. I it makes no difference and there is no ripple on the supply (see above), then it could be an earth loop / common earth problem within the actual amplifier layout.
I often double or quadruple the supply capacitors to reduce ripple and hence hum at 2 x mains frequency. Each doubling will in theory drop the voltage ripple across the capacitor by 6dB. At the time of design the values are often chosen based on the target price for the design, i.e. they are as small as can be got away with with the product still saleable at its price point.
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Thanks a lot, it is converging with my impression
The volume control null point still occurs (it was with hiss) at middle of the potentiometer
As this is a dual inverted volume control just between the phono preamp and the tone preamp, would it be possible that noise is coupled from the phono part to the tone part and it nulls out at middle position because both noise parts are equal? I know the reasoning will look poor, mostly because of a lack of knowledge from my part, but at least I'm trying 🙂
One thing to note is that the wiring, shielding and co are all top quality, and I replaced C32 with a 4700µ and C33 & C34 to 22000µF, so mains noise would not be present in my opinion especially since the amps are dead quiet when inputs are grounded
I will try to replace the regulator transistors, the capacitors at least are good enough according to the tester and there is no difference when using another one instead.. this may be improved later if I like but at this point my main concern is hiss
I think it may also be related to the input transistors TR201/202 feeding the amplifier with excessive noise... if it was induced by the NFB being sensitive to noise wouldn't if be stable over the volume range (except null point where two noise cancel out)? Or is the center tap of the potentiometer reducing noise gain at this position due to some filtering effect?
Thanks a lot for the help, its great support from the community here and always very enjoyable to learn in those conditions... I just hope I don't go in too many directions at once regarding my questions 😉
The volume control null point still occurs (it was with hiss) at middle of the potentiometer
As this is a dual inverted volume control just between the phono preamp and the tone preamp, would it be possible that noise is coupled from the phono part to the tone part and it nulls out at middle position because both noise parts are equal? I know the reasoning will look poor, mostly because of a lack of knowledge from my part, but at least I'm trying 🙂
One thing to note is that the wiring, shielding and co are all top quality, and I replaced C32 with a 4700µ and C33 & C34 to 22000µF, so mains noise would not be present in my opinion especially since the amps are dead quiet when inputs are grounded
I will try to replace the regulator transistors, the capacitors at least are good enough according to the tester and there is no difference when using another one instead.. this may be improved later if I like but at this point my main concern is hiss
I think it may also be related to the input transistors TR201/202 feeding the amplifier with excessive noise... if it was induced by the NFB being sensitive to noise wouldn't if be stable over the volume range (except null point where two noise cancel out)? Or is the center tap of the potentiometer reducing noise gain at this position due to some filtering effect?
Thanks a lot for the help, its great support from the community here and always very enjoyable to learn in those conditions... I just hope I don't go in too many directions at once regarding my questions 😉
I am not sure what Panasonic were up to with this unusual arrangement of stacked control. I think they believed it capable of smoother control than the typical audio taper potentiometer of the day.
I wonder whether replacing the VC with a single 50kΩ audio taper could reduce hiss; you'll need to find one a quality one with the loudness tap, e.g. Alps RH2702 50k.
If the hiss is a function of the circuit topology, there may not be a solution other than to replace the circuitry with a different design.
I wonder whether replacing the VC with a single 50kΩ audio taper could reduce hiss; you'll need to find one a quality one with the loudness tap, e.g. Alps RH2702 50k.
If the hiss is a function of the circuit topology, there may not be a solution other than to replace the circuitry with a different design.
Just another clue, at the physical halfway on the VC, the wiper of the second section is over the loudness filter link, which to the wiper looks like a high pass filter connected to ground, which would attenuate the hiss.
I think I would eliminate the VR1 part and replace VR2 with an Alps RH2702 50k.
I think I would eliminate the VR1 part and replace VR2 with an Alps RH2702 50k.
I do not think it would be as simple because according to online info those potentiometers were custom made for the amplifier and the resistivity is not standard along the course of action, plus they are one single know with two pairs of pots back to back, which makes for a too drastic change in my opinion
To work with modern sources with a high signal level (CD, DVD, DAС), you will need an additional pot at the amplifier input or a change in the NFB.
I don't think so, a soundcard output at the aux input was totally fine and there is an input level pot at AUX1
I have grounded the input of the second preamplifier stage and dead silent... I'm starting to fear that kludge of selectors is what is picking gall that noise and I will have to dive deep into it
First thing to do will be to open that front panel and redo the soldering there... lucky me, it has a collapsible panel and it shouldn't be too hard to attain...
I have grounded the input of the second preamplifier stage and dead silent... I'm starting to fear that kludge of selectors is what is picking gall that noise and I will have to dive deep into it
First thing to do will be to open that front panel and redo the soldering there... lucky me, it has a collapsible panel and it shouldn't be too hard to attain...
National didn't have their own laws of physics then, or now. What they did relates to what parts were available back in the '70s. You can accept the performance of the amplifier that is designed in, or change the design. The selector switches will not (in fact cannot) add hiss.
This is not what I was trying to say, just that there is a quadruple pot to replace with a center tap at a custom position, and that it would be a pain to replace by some modern equivalent just because of the mess of wires, in case the noise comes from that part
I thought that since I grounded the inputs of the preamplifier it could have been that the noise comes from before that zone, especially since when I ground the input RCAs the amp still has noise (between those two points there are only 4 selectors and a dual pot per channel)
Maybe it is juste because of TR101 and TR201 bases tied to ground via the AC path?
Honestly I don't think replacing the pot with another one will get rid of hiss, so now that I think about it I would bet on the transistors
I thought that since I grounded the inputs of the preamplifier it could have been that the noise comes from before that zone, especially since when I ground the input RCAs the amp still has noise (between those two points there are only 4 selectors and a dual pot per channel)
Maybe it is juste because of TR101 and TR201 bases tied to ground via the AC path?
Honestly I don't think replacing the pot with another one will get rid of hiss, so now that I think about it I would bet on the transistors
I'm suspecting the input devices of each preamp section to create noise
When I select the phono source, there is some huge noise which is normal as the gain is higher, and with input grounded there is no noise
Same with the other preamp side, when I ground the input the noise disappears
My first guess will be to replace regulator transistors first, because it could be that noise generated from TR107 would be coupled somehow to the input of the second preamp stage, and with an odd number of transistors be of inverse phase from the noise generated by TR207... each part of the volume pot being wired opposite the only area where the signal would not be attenuated would be at the middle point, hence canceling of noise at that point, two antiphase noise canceling each other
It could also be C231 and C126 which I haven't tried replacing yet, even if I doubt it since the other capacitors are intact
It may be simplistic but this is the reasoning I can do after all those advice... I'll keep reporting 🙂
When I select the phono source, there is some huge noise which is normal as the gain is higher, and with input grounded there is no noise
Same with the other preamp side, when I ground the input the noise disappears
My first guess will be to replace regulator transistors first, because it could be that noise generated from TR107 would be coupled somehow to the input of the second preamp stage, and with an odd number of transistors be of inverse phase from the noise generated by TR207... each part of the volume pot being wired opposite the only area where the signal would not be attenuated would be at the middle point, hence canceling of noise at that point, two antiphase noise canceling each other
It could also be C231 and C126 which I haven't tried replacing yet, even if I doubt it since the other capacitors are intact
It may be simplistic but this is the reasoning I can do after all those advice... I'll keep reporting 🙂
It is normal for preamps to have increased noise with open inputs.
This is not a sign of a circuit defect, but is simply from abnormal operation with open inputs.
Replacing parts will not change this. The thermal noise of a typical low impedance source
is far less than the noise from the high impedance which results without proper input termination.
This is not a sign of a circuit defect, but is simply from abnormal operation with open inputs.
Replacing parts will not change this. The thermal noise of a typical low impedance source
is far less than the noise from the high impedance which results without proper input termination.
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It is not the transistors, or shorting the input would not reduce the hiss. It is the high impedance terminating the input of the amplifier or preamplifier that is the source of the hiss, as Rayma posted above. Lowering the source impedance reduces the hiss.
A modern pot will have better tracking and lower noise than the arrangement National have used. Replacing the pot reduces the mess of wires and simplifies the schematic, and the lower 50kΩ impedance I suggest reduces the thermal noise generated at the input to the amplifier; see the modified volume circuit below:
A modern pot will have better tracking and lower noise than the arrangement National have used. Replacing the pot reduces the mess of wires and simplifies the schematic, and the lower 50kΩ impedance I suggest reduces the thermal noise generated at the input to the amplifier; see the modified volume circuit below:
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OK I'll have a go at finding a dual pot with tap, but I'm still worried that the RC filter would be impacted by this change and the tone would be altered... does this affect the filter at all? This is the loudness filter and I like it a lot, it complements my speakers nicely making for no need of subwoofer...
I realize now, having read here and there, that the highest value the highest thermal noise...
I will have a look at your previously recommended model and make a mouser order... thanks a lot! If you see other components to replace, don't hesitate to tell
I realize now, having read here and there, that the highest value the highest thermal noise...
I will have a look at your previously recommended model and make a mouser order... thanks a lot! If you see other components to replace, don't hesitate to tell
For an explanation of the source of the noise in your SU-3600 see here:
Resistor Noise
Resistor Noise
"In every amplifier circuit, the input resistor is critical. Any noise at the input signal will be amplified by the full gain. It is therefore of high importance to choose a low-noise resistor at the first stage, as well as a low resistance value."
The loudness circuit will not be affected but simplifying the stacked potentiometers, however the values of C1 and R27 may need to change to match the value of the new pot. I'll post again when I've checked.