The Sansui AU-317 power amp design/schematics (50W/8Ω)
It's been a favorite amongst (vintage) commercial amps and the original design is from 1976.
I'd like to ask your thoughts on this design and how this amp could sound if rebuild with more modern parts (now every original semiconductor is obsolete), if it has room for further "improvements" (mods/tweaks) and a comparison with a DIY project in the similar range (50-75W) class AB amplifiers.
Here, we have the power amplifier section:
And below, the power supply:
Full schematics in GREAT resolution > here <
Power output: 50W into 8Ω / 72W into 4Ω
THD: 0.03%
Slew rate: 40V/μS
Freq. resp.: DC to 200,000Hz
Input Sens./Impedance: 150mV, 47kΩ
It's been a favorite amongst (vintage) commercial amps and the original design is from 1976.
I'd like to ask your thoughts on this design and how this amp could sound if rebuild with more modern parts (now every original semiconductor is obsolete), if it has room for further "improvements" (mods/tweaks) and a comparison with a DIY project in the similar range (50-75W) class AB amplifiers.
Here, we have the power amplifier section:
An externally hosted image should be here but it was not working when we last tested it.
And below, the power supply:
An externally hosted image should be here but it was not working when we last tested it.
Full schematics in GREAT resolution > here <
Power output: 50W into 8Ω / 72W into 4Ω
THD: 0.03%
Slew rate: 40V/μS
Freq. resp.: DC to 200,000Hz
Input Sens./Impedance: 150mV, 47kΩ
An externally hosted image should be here but it was not working when we last tested it.
Another picture with a few waveforms:
An externally hosted image should be here but it was not working when we last tested it.
Thanks, sreten.
This means that it is an awesome amp.
Yes , as it is about >100db pssr , 10ppm 1k and @ 50ppm 20K.
Simplicity vs performance is VERY good.
Driving an EF3 and embellished with a full symmetric drive , 2ppm/20ppm
is possible.
Member Vzaichenko uses this basic topology to derive solid state performance
from a Valve hybrid.
OS
Hi OS,
I agree 100% with you! This amplifier does sound very good. Aside from replacing some of the old smaller electrolytics with good new ones I can't suggest much else that won't get you into trouble.
Hi Karl,
If you have to replace the trimmer controls (for bias and DC offset), replace with the same general type (single turn), size and value. Then you will have to set the controls, especially bias, to where the old ones were. Even then, you'll have to read the bias current and adjust it close to the required values. Do it quickly for each channel, adjust more closely afterwards. Of course you will not have speakers connected. Never connect speakers to an amplifier for the first power-up. If you don't have the meters or experience, do have a good technician familiar with this to do it for you. It's easy to blow up an amplifier doing this.
-Chris
I agree 100% with you! This amplifier does sound very good. Aside from replacing some of the old smaller electrolytics with good new ones I can't suggest much else that won't get you into trouble.
Hi Karl,
If you have to replace the trimmer controls (for bias and DC offset), replace with the same general type (single turn), size and value. Then you will have to set the controls, especially bias, to where the old ones were. Even then, you'll have to read the bias current and adjust it close to the required values. Do it quickly for each channel, adjust more closely afterwards. Of course you will not have speakers connected. Never connect speakers to an amplifier for the first power-up. If you don't have the meters or experience, do have a good technician familiar with this to do it for you. It's easy to blow up an amplifier doing this.
-Chris
Karl, a few simple things that you could try:
One. The “Black Flag” small-value capacitors that Sansei (and Kenwood) used in the mid-70s to stabilise their circuits are notorious for deteriorating over time, and can cause circuit failure when they go. Replacing them with Mica or NPO ceramic is recommended.
Two. The biasing trimmer VR05 tends to be a weak point, so make sure that it is in good condition, or replace it if you wish to be sure.
Three would be to insert a 240ohm resistor between ZD604 and C09, which will filter the zener noise with a 20Hz LPF. Increasing the value of C09 up to 100uF could also be attempted.
Four would be to do the same thing for C03 and ZD603.
Five would be to change R31 to a metal film type with a tight temperature coefficient. The NFB resistor sees a fair amount of voltage and power across it, which can modulate the value and lead to distortion. Using a resistor with a 25ppm/C temperature coefficient, for example Vishay Dale RN70E2202FB14 (available at Mouser), can minimise this issue for not a lot of money. Or, the resistance could be doubled and two pieces used in parallel.
hth, jonathan
One. The “Black Flag” small-value capacitors that Sansei (and Kenwood) used in the mid-70s to stabilise their circuits are notorious for deteriorating over time, and can cause circuit failure when they go. Replacing them with Mica or NPO ceramic is recommended.
Two. The biasing trimmer VR05 tends to be a weak point, so make sure that it is in good condition, or replace it if you wish to be sure.
Three would be to insert a 240ohm resistor between ZD604 and C09, which will filter the zener noise with a 20Hz LPF. Increasing the value of C09 up to 100uF could also be attempted.
Four would be to do the same thing for C03 and ZD603.
Five would be to change R31 to a metal film type with a tight temperature coefficient. The NFB resistor sees a fair amount of voltage and power across it, which can modulate the value and lead to distortion. Using a resistor with a 25ppm/C temperature coefficient, for example Vishay Dale RN70E2202FB14 (available at Mouser), can minimise this issue for not a lot of money. Or, the resistance could be doubled and two pieces used in parallel.
hth, jonathan
Thank you all!
I have a second AU-317 with a some small signal transistors faulty (can't remember which one now as the amp was closed in a box in another room for months until now), and the amp is no in good condition anyway with few PCB traces broken, so I thought it might be worth trying to do some "experiments" in this specific unit by changing some of the 11 transistors and maybe the 2SK109 input IC in the power amp section.
ostripper, thank you for the numbers on performance. Just thinking on the "GLA and kypton V" mods from your Z3900.
anatech, thank you for the suggestion on the trimpots. I've seen people (including myself once) replacing all the old pots with Bourns or Vishay multi-turn ones. So the choice of single turn here is interesting for this unit.
jcarr, very interesting suggestions. Yes I was thinking to replace the Zeners, too, other than all lytics. Many people say to replace the Zeners in the AU amp series. This unit has a few fuse resistors that drift values in time, going from original value of 150Ω up to 1k and beyond. There are 4 of them (R35, R36, R45, R46).
Lazy Cat, we are waiting to know the five changes in the front-end VFA topology, man...
I have a second AU-317 with a some small signal transistors faulty (can't remember which one now as the amp was closed in a box in another room for months until now), and the amp is no in good condition anyway with few PCB traces broken, so I thought it might be worth trying to do some "experiments" in this specific unit by changing some of the 11 transistors and maybe the 2SK109 input IC in the power amp section.
ostripper, thank you for the numbers on performance. Just thinking on the "GLA and kypton V" mods from your Z3900.
anatech, thank you for the suggestion on the trimpots. I've seen people (including myself once) replacing all the old pots with Bourns or Vishay multi-turn ones. So the choice of single turn here is interesting for this unit.
jcarr said:
jcarr, very interesting suggestions. Yes I was thinking to replace the Zeners, too, other than all lytics. Many people say to replace the Zeners in the AU amp series. This unit has a few fuse resistors that drift values in time, going from original value of 150Ω up to 1k and beyond. There are 4 of them (R35, R36, R45, R46).
Lazy Cat, we are waiting to know the five changes in the front-end VFA topology, man...
Hi Karl,
If some folks feel that the control is too touchy (it isn't in this case), then it is a simple matter to restrict the range of the control by reducing the value and adding a resistor to make up the difference on the other side (lower, opposite R41). You may find that to find the correct range, R41 needs to increase in value and the new resistor dropped in value. This make adjusting the bias current much easier and helps to prevent you from increasing the bias current to destructive errors. That bit is called design work. I still don't like seeing multi-turn controls in these circuits, but if you can find one that is rated for more then the tail current for the diff pair, you could get a 100R one and replace VR03 (?) with it and pull VR01 completely. That eliminates the course and fine trimmer adjustments. Me? I'd probably leave it stock. You might want to match R23 and R27. If you can, matching TR01 and TR03 would reduce distortion. The entire point of IC01 is that the pair is matched and thermally connected. If they drift out of balance, replacement would restore the good sound as the match there is also critical to the distortion levels. The point about increasing the power rating and type to metal film for R31 is good. I also do this to good amplifiers quite frequently. The upgrade is lost with low quality amplifiers.
The smaller electrolytic capacitors should be replaced. There are only 5 per channel to deal with, and the smaller ones have less electrolyte and "dry out" faster than the much larger capacitors do.
-Chris
It is only about using the parts that fit into the design parameters. You don't need multi-turn controls for this for starters. Look at the current capacity of the wiper between these two control types. The multi-turn controls are designed for potentiometer use only. That means no wiper current. All the current in that leg of the circuit must pass through the wiper and it was not designed for that purpose.
If some folks feel that the control is too touchy (it isn't in this case), then it is a simple matter to restrict the range of the control by reducing the value and adding a resistor to make up the difference on the other side (lower, opposite R41). You may find that to find the correct range, R41 needs to increase in value and the new resistor dropped in value. This make adjusting the bias current much easier and helps to prevent you from increasing the bias current to destructive errors. That bit is called design work. I still don't like seeing multi-turn controls in these circuits, but if you can find one that is rated for more then the tail current for the diff pair, you could get a 100R one and replace VR03 (?) with it and pull VR01 completely. That eliminates the course and fine trimmer adjustments. Me? I'd probably leave it stock. You might want to match R23 and R27. If you can, matching TR01 and TR03 would reduce distortion. The entire point of IC01 is that the pair is matched and thermally connected. If they drift out of balance, replacement would restore the good sound as the match there is also critical to the distortion levels. The point about increasing the power rating and type to metal film for R31 is good. I also do this to good amplifiers quite frequently. The upgrade is lost with low quality amplifiers.
The smaller electrolytic capacitors should be replaced. There are only 5 per channel to deal with, and the smaller ones have less electrolyte and "dry out" faster than the much larger capacitors do.
-Chris
It may be worthwhile to read Section 1.3 on the following page by diyAudio member Bonsai.
Ovation e-Amp: A 180 Watt Class AB VFA Featuring Ultra Low Distortion
The current-source filtered zener mod would take the form of the right-most configuration in the LTP schematic.
BTW, Audio Karma has a good forum on old Sansui amplifiers. There may be a few threads on the AU-317 model.
hth, jonathan
There are several fusible resistors in the circuit which tend to drift over time and cause problems. These should be replaced. There are some diodes that heat up nearby elco capacitors, causing leakeage and failure. Inspect all components visually. Semis do not need replacement if are working fine.
This amp is a classic. Keep it.
This amp is a classic. Keep it.
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