Sansui AU666 refurbishment with pictures / and a filter question

Hi all, just put this on the solid state part, since it is most read.

I took my amp apart, put new components on the boards and now I have to find a way to put it together. Some issues still exist, among the following.

According this (only) schematic to be found on the internet, there is a filter in between the pre amp and the main amp.
To me, it looks useless.
It looks a bit to filter out the most low frequencies (below 50Hz), and parallelled there is a filter which passes even lower frequencies like 10 Hz again....
However, the load of the filter is high impedance, and I do not know enough of AC behavior to exactly predict what it is doing precisely, looking at the values it is active below some 100Hz only.

Of course, I can GUESS its use, but if anyone KNOWS why it is there and can explain why, this would be very helpful.

Since the L+R filters are hardwired they take some space, which I could use to make a more decent loudspeaker protection using a relay, at the moment it uses a setup using a TRIAC to just shorten the output if DC appears..

Anyone knows without guessing? It is about R905 R907 R909 C905,7,9

If not sure, I guess when the amp works again, I have to measure its behavior..


Note: the left part is the end part of the pre-amp, on the right, AFTER the filter and channel selection switch, is the main power amp.
This part of the drawing as a part of the (only) drawing I can find on the internet. At least for the filter part, it is accurate (not on some other parts, it seems there is a design and/or manufacturing overlap with the au999.....)

Greetings;

This is where pics are needed. Need for experienced viewer with recognizing components before.. especially in your Global location.

binky

Binkman said:

Greetings;

This is where pics are needed. Need for experienced viewer with recognizing components before.. especially in your Global location.

binky

Click to expand...

Hi Binkman, do you mean that the picture of the part of the schematic is not visible at all, is it browser dependent? I see it here (using firefox)
As described, it is about R905 R907 R909 C905,907 and 909 in the schematic, which is a low pass and high pass filter together according this schematic.
Maybe good to mention, the pre-amp out will be rather low-impedance, and the main amp input rather high impedance, consisting of this 1 uF C801, and 10k R803, as the schematic states.

Assuming I did this right, not always a given, here's what LTSpice has to say about it. It's definitely a rumble/warp filter, but the disturbing thing is that it's having an effect at 20 Hz and slightly above. Having run these sims on the tone control circuits of various Sansui amps, I'm not too surprised. Their filter designs always seem to do a bit of unwanted shaping in the audio range. (Ignore the AU999 title in the image- I'm a bit inverted today.)

Hi Conrad.

Thanks for your effort, this helps!

So according the graph, it is a "weird" filter, indeed.

I think I have to teach myself some forgotten math stuff again, and also learn to use such simulation applications.

BTW it is of no use having the left part of the graph in such a filter, no reason to do anything with frequencies up to 5 hertz.....
Measuring the caps (they are polarized electrolytics) they are 50% more than the printed value. which would get it a little better. I wonder if Sansui design took the actual values into account, +50% tolerance was not uncommon in the past.
So the left part of your graph would be the 2,2 uF and 47k resistors, I 'd better take them out.
Maybe I take the whole filters out including the mounting, and also take the Traic loudspeaker protection out, which gives me room for mounting a better loudspeaker protector/ turn-on delay circuit.

thank you,

Gerard

finally, some progress, here.

The first picture shows I was very successful taking the amp apart

Now, taking away the protection crowbar circuit, I decided to solder some protection circuit together on a hobbyists prototyping board, having the same size as the original board.

It is based on the circuit of my Tandberg TR2075 with some changes.
However, it looks less neat than a real circuit board.
An airtight relay I found in my junkbox is mounted close to it, see the bottom of the second picture.

Note: I used all kinds of cleaning stuff, but I could not get the metal baseplate of the amp looking right....so be it.

Also the recapped plus and minus stabilizer / zobel network / emitter resistors boards for the main amp are mounted again.
Sansui did have the idea to scatter the power amp part over four circuit cards and two separate heatsinks totally....which is kind of odd.

Some pictures including schematics and some parts information.

http://members.quicknet.nl/gerard.slikker/sansuiau666.htm

I did learn the following:

- never trust an old Sansui schematic.
- I will think three times about the extent I want to take something apart.

here a pic of the finished amp

Thsi work looks amazing. Im not familiar with this amp - but I have restored many sansui amps - the 999 uses a "low end" filter which you are better off bypassing as it kills a lot of bottom end - people with these amps are recommended to bypass this filter (which I did, with great results). I am not sure if this is the same type of filter you have but search "subsonic filter" in the sansui forum to find information on it, including how to bypass (rather than remove) it. cheers

Yes, it is the same filter. This amp looks like the AU999. The main amp printed circuit board was the one of the AU999 but populated with resistor values of the 666, with exeption of two 15 ohm resisitors if I remember, the went to the base of the output transistors. they were the wrong value, because in your 999 there will be the drivers, which are extra in the 999.
Also the original crowbar protect which was in the 666 is the one I found in the schematic of the 999. It used triacs instead of transistors as the au666 schematic on the internet states. I found two different triacs on this board while it looked originally.

So thanks to the au999 schematic I looked into I was able to trace back what I got after first taking the amp totally apart and then seeing: "wtf do I have here this is not in the books?"

I did put the cable tree including pots and switches in an ultrasonic cleaner, but decided take it out again when the pots which were sort of stiff, turned easily again.
But everything was still dirty by then.
So I learned something: not to use the ultrasonic again, I also believe it can destroy the metal to carbon joints on potentiometers, but I am not sure....

The posts on AK to be found were helpful, indeed!

I will put some more pictures of changes and comments on my internet page, later.

I've got one of these on the bench right now. Compared to your photo's mine is different. For instance, the amp boards have 4 resistors less. This is compliant with the service manual. These amp boards also hold different transistors and diodes, but they are not messed with by Hobby Bob. My guess is one is a drop in replacement board, either from stock or as an exchange in the warranty period. Odd nevertheless.

The speaker protection circuit also seems to be as the service manual shows; i'm going to replace it with a relais system as this system isn't going to do much. Better to add lower value fast fuses.

It's been stored in a damp location so i'll need to do some spray painting. One of the filter boards (-) had had a catastrophic fault, blowing the transistor, resistor and capacitor. It's playing now but there is still intermittent static which is likely coming from the preamp or equalizer board.

Regards,

Bert