Hi everyone,
I'm finishing rebuilding a good old Sansui AU-111 from the early 70s, and now that's it has been thoroughly recapped, retubed and biased, i'm fine tuning it, hence I need some help.
Sorry for the long post but I wanted to provide as much information as possible, so the questions are a bit scattered through the post (the questions are in italic)
First thing I've done was to change the more than unbalanced volume pot by a 250K cms commuted one (with center tap), now both channels are exactly the same.
The old 250K pot measured 150K on both channels (IN to GND) and CT at 95k (from IN to CT)
Speaking of this, witch value should really be the center tap? 75K or 125K? (as it's a commuted pot I can choose my CT point).
-I also shielded the power switch zone witch reduced induced hum significantly, still i'm getting audible hum with my efficient Heresy III at listening position.
When disconnecting Preamp stage from the main amp stage, I do not have any hum, perfectly quiet.
Where should I look at to hunt down this 50hz hum (frequency tested on scope)
-Last problem I have is related to the treble pot unbalance pointed by smurfer77 on AK forums. In fact my outputs are unbalanced and frequency response all other the place. All the tests I've done where made with an Analog discovery 2 using both Waveforms and Thatstuffmade audiosuite.
The balance pot was set at the middle position, treble and bass switch to "defeat".
This is the preamp stage where trouble happens.
First thing I've done was to check if my balance pot was balanced by injecting signal directly at the volume pot input.
perfect balance on both channels, phase rotation is probably due to the fact that I'm using both generator outputs (I do not have this phase shift when inject mono signal on both channels)
When injecting from the treble pot center tap here's what I got:
clean
Then when injecting on the Tape monitor input (bypassing the phono stage)
Unbalanced output.
Now comes the frequency response curves:
When injecting from Treble Center tap:
Volume pot at 90°
Volume pot at 180°
Volume pot at 270°
The channels are well balanced, the response is rolled of at the low frequency (I'm bypassing the feedback stage) and changes with volume.
Now when injecting from the Tape monitor input:
Volume pot at 90°
Volume pot at 180°
Volume pot at 270°
Here the frequency response are full but varies with volume and the channels are not balanced anymore.
So my problem lies on the feedback circuit on V2/V3 second triode witch leads to the Treble pot witch sets the feedback loop resistance.
Here are the pot measurements on different points (I made the measurements with the pots in circuit)
I have a significant difference in resistance between the two pots, the left one having more resistance, so I'm assuming I need to add resistance to the right one, but where should I put this resistance?
1. between the pot and the input (from v2/V3 first triode Cathode)
2. between pot Center tap and V2/V3 second triode Grid)
2. between pot and Feedback
I hope someone might help me on those issues.
I'm finishing rebuilding a good old Sansui AU-111 from the early 70s, and now that's it has been thoroughly recapped, retubed and biased, i'm fine tuning it, hence I need some help.
Sorry for the long post but I wanted to provide as much information as possible, so the questions are a bit scattered through the post (the questions are in italic)
First thing I've done was to change the more than unbalanced volume pot by a 250K cms commuted one (with center tap), now both channels are exactly the same.
The old 250K pot measured 150K on both channels (IN to GND) and CT at 95k (from IN to CT)
Speaking of this, witch value should really be the center tap? 75K or 125K? (as it's a commuted pot I can choose my CT point).
-I also shielded the power switch zone witch reduced induced hum significantly, still i'm getting audible hum with my efficient Heresy III at listening position.
When disconnecting Preamp stage from the main amp stage, I do not have any hum, perfectly quiet.
Where should I look at to hunt down this 50hz hum (frequency tested on scope)
-Last problem I have is related to the treble pot unbalance pointed by smurfer77 on AK forums. In fact my outputs are unbalanced and frequency response all other the place. All the tests I've done where made with an Analog discovery 2 using both Waveforms and Thatstuffmade audiosuite.
The balance pot was set at the middle position, treble and bass switch to "defeat".
This is the preamp stage where trouble happens.
First thing I've done was to check if my balance pot was balanced by injecting signal directly at the volume pot input.
perfect balance on both channels, phase rotation is probably due to the fact that I'm using both generator outputs (I do not have this phase shift when inject mono signal on both channels)
When injecting from the treble pot center tap here's what I got:
clean
Then when injecting on the Tape monitor input (bypassing the phono stage)
Unbalanced output.
Now comes the frequency response curves:
When injecting from Treble Center tap:
Volume pot at 90°
Volume pot at 180°
Volume pot at 270°
The channels are well balanced, the response is rolled of at the low frequency (I'm bypassing the feedback stage) and changes with volume.
Now when injecting from the Tape monitor input:
Volume pot at 90°
Volume pot at 180°
Volume pot at 270°
Here the frequency response are full but varies with volume and the channels are not balanced anymore.
So my problem lies on the feedback circuit on V2/V3 second triode witch leads to the Treble pot witch sets the feedback loop resistance.
Here are the pot measurements on different points (I made the measurements with the pots in circuit)
I have a significant difference in resistance between the two pots, the left one having more resistance, so I'm assuming I need to add resistance to the right one, but where should I put this resistance?
1. between the pot and the input (from v2/V3 first triode Cathode)
2. between pot Center tap and V2/V3 second triode Grid)
2. between pot and Feedback
I hope someone might help me on those issues.
Center tap are just that, at the center position of either ends as you turn. On a linear pots it is 50% of the R from the pot spec. On a logarithmic pots (ie typ volume control) it is about 10-15% depending on make. See link. Potentiometers (Beginners' Guide to Pots)
Sound like you have a ground loop in your chasis ground and house ground. Try using a different ground point on your RCA input connector. See link. Ground loop (electricity - Wikipedia)
Most tone control circuit expect a certain source and load impedance to work properly. Injecting signal in the middle of the circuit will yield only headaches (for me at least).
Sound like you have a ground loop in your chasis ground and house ground. Try using a different ground point on your RCA input connector. See link. Ground loop (electricity - Wikipedia)
Most tone control circuit expect a certain source and load impedance to work properly. Injecting signal in the middle of the circuit will yield only headaches (for me at least).
Hi cowneko,
All tests should be made with the signal injected in a high level input - like AUX.
Don't expect perfection with those old units, they will be far from perfect.
You can't compensate a pot that measures differently from it's mate by adding resistance anywhere.
The only things you can do with audio equipment is replace the resistors and capacitors with the correct types that will withstand the voltages you actually put across them. That and use good brands of tubes. For a unit such as this, it is also a restoration. Don't try and make it into something it isn't. I would pull the stepped volume control and use an Alps or Noble with the same shaft type as the original. It's a classic amplifier, don't mess with it.
The main filter caps are done by now. Replace with similar capacitors. If you used radial or axial capacitors, start over and do it right.
I just did a complete rebuild and upgrade on one of these beauties. It turned out wonderfully musical to listen to. Distortion was lower than many tube amps, higher than some. The tone and volume controls are all original still and the customer loves it - knowing it is not a measuring instrument. I did have to undo a messy "rebuild" someone else made, and replace some broken tube socket pins.
If you want perfection, build another tube amp using new transformers, but do not expect these wonderful older products to perform like that.
I also rebuilt a Sansui PM-2020. Historically important, it has been rebuilt with the original controls and the closest filter capacitors available. Even the shielded signal wires needed replacement! But the important things are that :
1.) It sounds great
2.) It is still a Sansui PM-2020
Although I might have to sell it if the owner doesn't pick it up.
So do good work and enjoy that classic for what it was. Trying to do the things you are doing only messes it up and you no longer have an AU-111. They are worth too much money to ruin like that.
By the way, there were two main production runs on that model. Mine was the second. Are you sure yours was also the second run?
-Chris
All tests should be made with the signal injected in a high level input - like AUX.
Don't expect perfection with those old units, they will be far from perfect.
You can't compensate a pot that measures differently from it's mate by adding resistance anywhere.
The only things you can do with audio equipment is replace the resistors and capacitors with the correct types that will withstand the voltages you actually put across them. That and use good brands of tubes. For a unit such as this, it is also a restoration. Don't try and make it into something it isn't. I would pull the stepped volume control and use an Alps or Noble with the same shaft type as the original. It's a classic amplifier, don't mess with it.
The main filter caps are done by now. Replace with similar capacitors. If you used radial or axial capacitors, start over and do it right.
I just did a complete rebuild and upgrade on one of these beauties. It turned out wonderfully musical to listen to. Distortion was lower than many tube amps, higher than some. The tone and volume controls are all original still and the customer loves it - knowing it is not a measuring instrument. I did have to undo a messy "rebuild" someone else made, and replace some broken tube socket pins.
If you want perfection, build another tube amp using new transformers, but do not expect these wonderful older products to perform like that.
I also rebuilt a Sansui PM-2020. Historically important, it has been rebuilt with the original controls and the closest filter capacitors available. Even the shielded signal wires needed replacement! But the important things are that :
1.) It sounds great
2.) It is still a Sansui PM-2020
Although I might have to sell it if the owner doesn't pick it up.
So do good work and enjoy that classic for what it was. Trying to do the things you are doing only messes it up and you no longer have an AU-111. They are worth too much money to ruin like that.
By the way, there were two main production runs on that model. Mine was the second. Are you sure yours was also the second run?
-Chris
Alllensoncanon:
Thank you, I know about that, but I was asking because there no clear consensus about witch value it is on this pot, some says 75K other mid point (although as it's a log pot, should be 37/38k) and as mine was just so out of spec I did not have a definite reference.
anatech: thank you for you opinion, but I think you're a little to quick on your judgement as you don't even know how I restored this amp.
Just for pointers, this is how it looked like when I got it
And this is how it looks now
the amp was previously mistreated (they maybe tried to remove the anodization with an acidic bath of some sort, don't have a clue, but nonetheless, the front plate was in no way fixable (except by doing a re-anodization and silkscreen), so I redesigned a front plate (i'm graphist) and have it made by Schaeffer.
Concerning the electronic restoration I agree with you on that we shall not try making something the amp is not to begin with.
I checked every single caps in the amp for value, esr and leakage
85% of the caps where swollen or/and leaky, some where just starting to come apart (I even had one diode bypass cap witch had exploded), I only kept 4 mica (on the power amp stage), 6 ceramics, and two psu cans (the 4x 20uf and 2x40uf witch measured as new), all resistors are original (they tested great)
Every caps has been changed like for like (elna oil for russian K40y-9 pio, elna MP with polyester or russian ptfe, mica for mica, axial elco for axial elco)
I did not change any values (taking into account the small changes due to the use of normalized values)
The Vol pot has been changed because it was just busted and completely out of specs
I also changed the two impedance and phase switches on the back plate for brand new ones as they where also busted.
everything else is as original as it can be, I got a complete set of original tubes (with Hitachi 6l6GC Blackplates) and a brand new one with a quartet of TAD 6L6WGC blacplates, so no problems concerning the quality of tubes.
In my opinion this is how you're respecting an amp and the engineers behind it, being a vintage gear does not mean that it has to perform poolry, I'm not trying to transform this amp into something it is not, but I just try to make it perform as it was intended to perform, no more, no less.
Also, leaving broken/bad/out of spec components in for the sake of keeping it fully original is plain nonsense all the most if you want the amp to sing and not just being a collector item (this 111 is my own amp).
Lastly I completely agree with you that it's a spectacular amp, even more now that I found where to put that resistor (between feedback and treble pot), result, perfect balance and steady Frequency response. When listening, larger scene, better definition.
Thank you, I know about that, but I was asking because there no clear consensus about witch value it is on this pot, some says 75K other mid point (although as it's a log pot, should be 37/38k) and as mine was just so out of spec I did not have a definite reference.
anatech: thank you for you opinion, but I think you're a little to quick on your judgement as you don't even know how I restored this amp.
Just for pointers, this is how it looked like when I got it
And this is how it looks now
the amp was previously mistreated (they maybe tried to remove the anodization with an acidic bath of some sort, don't have a clue, but nonetheless, the front plate was in no way fixable (except by doing a re-anodization and silkscreen), so I redesigned a front plate (i'm graphist) and have it made by Schaeffer.
Concerning the electronic restoration I agree with you on that we shall not try making something the amp is not to begin with.
I checked every single caps in the amp for value, esr and leakage
85% of the caps where swollen or/and leaky, some where just starting to come apart (I even had one diode bypass cap witch had exploded), I only kept 4 mica (on the power amp stage), 6 ceramics, and two psu cans (the 4x 20uf and 2x40uf witch measured as new), all resistors are original (they tested great)
Every caps has been changed like for like (elna oil for russian K40y-9 pio, elna MP with polyester or russian ptfe, mica for mica, axial elco for axial elco)
I did not change any values (taking into account the small changes due to the use of normalized values)
The Vol pot has been changed because it was just busted and completely out of specs
I also changed the two impedance and phase switches on the back plate for brand new ones as they where also busted.
everything else is as original as it can be, I got a complete set of original tubes (with Hitachi 6l6GC Blackplates) and a brand new one with a quartet of TAD 6L6WGC blacplates, so no problems concerning the quality of tubes.
In my opinion this is how you're respecting an amp and the engineers behind it, being a vintage gear does not mean that it has to perform poolry, I'm not trying to transform this amp into something it is not, but I just try to make it perform as it was intended to perform, no more, no less.
Also, leaving broken/bad/out of spec components in for the sake of keeping it fully original is plain nonsense all the most if you want the amp to sing and not just being a collector item (this 111 is my own amp).
Lastly I completely agree with you that it's a spectacular amp, even more now that I found where to put that resistor (between feedback and treble pot), result, perfect balance and steady Frequency response. When listening, larger scene, better definition.
Of course, this as been done, but as the problem did not lie on the phono stage, out of the picture.
Of course the goal is not to make it perfect, it as it's flaws by design and that's not the point.
But wrong, adding a compensating resistor to an unbalanced feedback loop is working (measure and listening proves that), i'm not trying to re-balance the pot themselves but only the center tap value witch is fixed
Already answered those comments on my previous post.
Wrong, two of them measure perfectly (4x20uf and 2x40uf) the two 200uf where leaky.
Again, that's not the point, I just wanted this amp to perform as it's supposed to perform, no more.
That's what I always try to do and I enjoy the amp greatly, that's a wonderful piece of gear. I do not have the impression of having messed with it, but I think I just put it out of it's original misery and make a AU-111 sing again.
Mine is a second production unit from 71, four bias pots.
Not sure I'd have kept any of the electrolytic capacitors. It is a bit like replacing only one windscreen wiper because the other one has OK rubber. You can guarantee you will not be able to see out of the windscreen in just a few weeks.
These things all seem to have some form of built in obsolescence, and there is not a lot that can be done to stop it.
However, it is a beautiful unit!
These things all seem to have some form of built in obsolescence, and there is not a lot that can be done to stop it.
However, it is a beautiful unit!
Sure if I had any doubts about those caps I would have changed them right away, however they are absolutely not leaky at all (at least for now) and value/esr are spot on (I woke the amp on variac/lightbulb after recapping). Still, I already have modern replacements parts for them (also multi cans) in case those where to go wrong (or maybe I'll restuff the old cans with new caps, i'll see when that time comes)
I found out why I had too much hum on output, two solder joints where weak on the psu side, re flowed them, now I only have 8mV peak to peak of residual hum, virtually inaudible 🙂
Thank you, I too am very happy with the outcome of the restoration and it is surprisingly pristine on the inside (was already like that when received) witch is quite rare.
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Just for demonstrating the effect of adding a 22k (in my case) compensating resistor on the feedback loop, here are the measurements:
Channel balance, now spot on
Frequency response, close enough between channels
THD, first Harmonics at -70db....spectacular
THD vs Frequency
So the effect is real and it can be heard!
Channel balance, now spot on
Frequency response, close enough between channels
THD, first Harmonics at -70db....spectacular
THD vs Frequency
So the effect is real and it can be heard!