This:
vs that:
In other words - pull out a shielded signal cable of ca 3 cm makes a substantial difference.
Seems like the world of tube amplifiers...
I will play around with cable throws during the weekend...
vs that:
In other words - pull out a shielded signal cable of ca 3 cm makes a substantial difference.
Seems like the world of tube amplifiers...
I will play around with cable throws during the weekend...
One more thing - when ground connection is provided wire-wise - should the signal cable shield be grounded from one end only or from both ends?
Academically speaking... 😉
Academically speaking... 😉
When looking at the SA-9900, some signal cables are ground connected both ends, others not, for good reason.
I haven't looked at exactly where which configuration is used but the goal is to avoid ground loops.
Hugo
I haven't looked at exactly where which configuration is used but the goal is to avoid ground loops.
Hugo
I agree, theoretically it would be best to provide wire connections for connecting boards' grounds and ground the cable shield only at one end.
At least this is what I have learned during the years.
Practice is frequently different - ground is passed via the cable shield.
But back to SA-9500 - the input stage ground (pins 6 and 21) is provided via the signal cable shield - perhaps ca 20...30 cm long.
Would it benefit - theoretically speaking - from a separate ground wire and disconnecting the shield from one end?
At least this is what I have learned during the years.
Practice is frequently different - ground is passed via the cable shield.
But back to SA-9500 - the input stage ground (pins 6 and 21) is provided via the signal cable shield - perhaps ca 20...30 cm long.
Would it benefit - theoretically speaking - from a separate ground wire and disconnecting the shield from one end?
It is my opinion that the ground schemes in these kind of amplifiers are very complex but well engineered. Altering that scheme would only be asking for problems.
Apart from one occasion in a Bryston 10B, which was humming because the case was supposed to be grounded but the powdercoating was not removed, I never changed grounds.
Hugo
Apart from one occasion in a Bryston 10B, which was humming because the case was supposed to be grounded but the powdercoating was not removed, I never changed grounds.
Hugo
I have mentioned this book before in a couple of posts.
It is extremely well written and easy to understand.
"Noise Reduction Techniques in Electronic Systems' by Henri Ott.
Hugo
It is extremely well written and easy to understand.
"Noise Reduction Techniques in Electronic Systems' by Henri Ott.
Hugo
Sorry but in this particular case there seemed to not too much sense in the initial ground wiring mess.
Starting from the fact that it had (not too common in 70-ies) 3-wire connection (N/L/PE) to the mains and - apart from the mains PE connection point to the chassis - two other star ground spots actually screwed to the chassis.
Stepping back one step - RF noise or mains pollution was probably not that level problem in 70-ies.
I agree things are being done differently these days but I must say that the SA-9900 I'm currently working on is dead silent.
I've posted scope pictures above.
@madis64 : This amp will hum, especially on the phono inputs, if the case is open and if all shields are not in place and tightly screwed to the chassis.
Hugo
I've posted scope pictures above.
@madis64 : This amp will hum, especially on the phono inputs, if the case is open and if all shields are not in place and tightly screwed to the chassis.
Hugo
I am working on the line inputs 😉
The hum problem was discovered with all covers closed.
Today's startingg point (signal cable (into the Normal/Separated switch) is the topmost grey one on the left):
To get increase in picked up noise the only thing that I need to do is to increase the horizontal run of the cable along the transformer:
Ok, horizontal run along the transforemer (actually - distance 1 or 3 cm apart does not matter) is a no-no, pull the cable straight downwards from the switch:
and clean(er) we are:
and clean(er) we are:
And why was the hum not noticed before - probably the noise of the buffer amplifiers' transistors masked it - how it looks when connecting Tone switch to ON (i.e. passing signal through tone amplifier with regulators in neutral position):
One more thing I noticed - the bottom plate and top covers do not change almost anything - noise source seems to be dinitely inside.
Problem is - the transformer is located in the middle of the amplifiear and sihnal cabling runs around it for almost 360 degrees...
One more thing I noticed - the bottom plate and top covers do not change almost anything - noise source seems to be dinitely inside.
Problem is - the transformer is located in the middle of the amplifiear and sihnal cabling runs around it for almost 360 degrees...
OK, wrapped this project up.
Some before-after comparison (RMAA measurements):
Some before-after comparison (RMAA measurements):
- step 1 is its initial state
- step 2 is after recap and cleaning the switches
- step 3 is after internal recabling.
That looks like a huge improvement on all counts. Very nice!
Do the peaks from the blue graph hide the green peaks or are there more blue peaks in the THD test?
Hugo
Do the peaks from the blue graph hide the green peaks or are there more blue peaks in the THD test?
Hugo
I would suppose that there are more peaks in the blue graph (especially above 1kHz) and they may hide some of the green graph ones.
They seem to be related to the input signal level and vol pot position. If I turn the vol pot downwards then the higher harmonics are reduced.
In its current state I was more looking at the level of the 2kHz peak (and certainly the 50Hz one that I managed to get below the level of 100Hz one).
I guess it could be still improved when jumping the control amplifier (two transistors, 22dB gain) making the input path in Tone OFF mode completely passive but that would mean attenuating the tone amplifier output signal to the same level.
They seem to be related to the input signal level and vol pot position. If I turn the vol pot downwards then the higher harmonics are reduced.
In its current state I was more looking at the level of the 2kHz peak (and certainly the 50Hz one that I managed to get below the level of 100Hz one).
I guess it could be still improved when jumping the control amplifier (two transistors, 22dB gain) making the input path in Tone OFF mode completely passive but that would mean attenuating the tone amplifier output signal to the same level.