smurfer77
Super Member
I've been lucky enough to obtain a Sansui AU-Alpha-907limited. According to the audiodatabase this was limited to 2000 units only back in 1994 at 410,000 Yen. I do question a bit the production number as there are hardly hard to come by in Japan.
We know from a previous thread that the 907limited and 907XR share service manual but to best of my knowledge the service manual is not publicly available, although we know it exists and snippets are available in various threads. A few years ago an former sansui engineer contacted me via LinkedIn and we have been penpals ever since. Let's call my penpal 'T'. A few weeks ago I told T that I've gotten interested in the alphas and explained that there are very few service manuals available. He asked "which model are you interested in?". Of course I was tempted to say all of them! But I thought that would be rude . So I explained I already have 607/707/907/MR/MRX and 907KX..... but what I really need is the 907 Limited. T says he will ask around and get back in touch. Just over a week later, I get an email with an attachment!!! As far as I understand T has scanned this for us himself from a hardcopy!
It's in Japanese but the schematic is legible (and the test points are labelled n English actually), and with the 907MR manual which has English, the 907Limited japanese adjustment procedures are clear.
At the very begining before all of this I asked T if the service manauls are secret and received the response "There are no secret because the property holder does not exist". Now, I totally understand (1) personal promises and (2) pro-tech business protection are separate issues, but as far as Sansui itself is concerned there is likely no problem at all since Sansui is no longer an entity. I asked T if I can share the 907Limited manual with everywhere and received the response "of course no problem to share with anyone who is interested the documents online. We (I and the person who found the Service Manual), are rather happy as exSansui." I lightly edited the English (although T's English is very very good), but the meaning is unchanged. I think T is not working on amplifiers these days and gains nothing from this, except sharing the joy of Sansui history. I think this passion from an ex-employee is really exemplary and this is the passion that shows in the product Sansui made! T was already very interested in the mystery 907 Extra over on Stereonet forums...
About the pdf, I've just uploaded on hifiengine; if it doesn't show in a few days give a shout and I will share via dropbox (I find the ak db doesn't let me upload for some reason so if someone could do that, please go ahead).
A few other notes:
- as noted by skippy, some panel screws are different length. Be sure to replace same length screw when you put it back together (there is a risk of shorting out the phono board on the right hand side).
- the 907Limited poweramp section might have been recommended by Sansui as power amp for "best preamp of the time", C-2302 VINTAGE (1992, 1,280,000 Yen!). The original matching power amp B-2302 was dated to (1990). gathering this info from the audio database site.
BIAS Voltage:
- 28.6mV for Limited (8.8mV for 907XR). This gives about 130mA idle current.
- Need top and bottom cover off. Left panel off also needed if doing premap DC. Probe from bottom, access VRs from top. Stand on right hand edge, so VRs for preamp near front left panel can be accessed.
- the manual refers to TP on the amp board, but you will actually find two pins, not one. The two pins connect to either side of an emitter resistor. I used a 2.5mm female plug to make life easier and just probe across those two test pins (it just skips the output coil but that is only 0.01Ohm so it hardly matters), but you could also follow the manual, but you will need to know which test pin to grab hold of (which is easy to find with DVM (the test pin you want is the one that is 0.22 Ohms from the 'out', not the one that is shorted with 'out'). Other alphas, like 607NRAII pretty much require such a 2.5mm connector plug (5-pin in the 607NRAII, which I also used on the 2-pin 907LTD (whereas the 607MR has easy to access TPs and no need for a connector/extension)).
Original bias on mine:
- if you see about 38mV at cold start up dont touch VRs just yet, it really takes some warming up to get to steady-state. Read on:
- cold start-up (Right (37.8mV, 37.8mV); Line pulling 1.49A
- Warm up took well over 60 minutes and the above settled to:
{Right (27.6 ('hot' (non-inverting), 28.6 ('cold' (inverting));
{Left: 29 (non-inverting) and 28mV (inverting)}
- at warmed up idle line pulling 1.25A
TEMPERATURE:
- On all 3 of my alphas the side with the phonoboard (right) runs 2 C hotter. The thermal situation is just non-symmetric with the left being coupled more directly to larger chunks of metal.
- My 907Limited runs about 50C at the top of the heatsinks and 53C near the bottom where the transistors are situated. That is the right side. As mentioned Left side runs slightly cooler.
OFFSET:
- for 'hot-to-cold' probe between the two ‘out’ (one for hot (non-inverting) and one for cold (inverting), which go to positive and negative speaker terminals). Just take care you negative probe isn’t grounded by your measuring device as 'cold' does not mean grounded, but in fact refers to an active output designated 'cold' (is just what they called the inverted amp in a bridged setup). ''hot-to-ground' yoy can work out and must be set simultaneously with 'hot-to-cold' as they are interlinked (one setting balance between either side's DC and other setting magnitude of one).
Power: I got 101W, both channels into 8Ohms before obvious clipping. Didn't go into detail with the distortion meters, but this thing is clearly making more than the prescribed 80W.
Side Panels:
The side panel pattern is Birds Eye Maple. I recovered mine with new vinyl as the original got dinged in shipping and it looks fantastic... I should probably post. Did think about solid wood but I thought it might look better, but less original.
Further thoughts on listening:
- I think very few people have compared the newer models with the older ones using an amp selector box and volumes set equal using SPL meter. When I did a quick comparison of the 907Limited and 607MR, without doing it properly (just changing the wires and doing volume by ear), I swore the 907Limited was bright and disappointing. I even mentioned that too a few people.
- Then next weekend I setup properly and redid comparisons. It's amazing how much the mind plays tricks. The 907Limited is in fact very very smooth and a bit warm souding, similar in sound to the 607MR and 607NRAII, on my speakers at least and driven to moderate volumes (I rarely listen loud). The differences are extremely subtle, and in fact I'm the only person on my setup who can tell the difference in blindfolded tests. I am now comparing to older amps and will report elsewhere. But summary is it's pretty useless for me to trust my own ears/brain without proper comparison setup, let alone someone else's comments with unknown conditions.
- One thing I will say now is these later Alphas all sound a lot like the AU-777A (to be fair, my AU-777A is not stock at all). Will compare some earlier alphas soon. And will compare to AU-111 (but I think for now they sound more like AU-777A/AU-70 as my memory of my AU-111 is slightly more clinical/accurate/damped).
If you have a chance please give a thumbs up or comment to thank T, as I will share this with him later.
We know from a previous thread that the 907limited and 907XR share service manual but to best of my knowledge the service manual is not publicly available, although we know it exists and snippets are available in various threads. A few years ago an former sansui engineer contacted me via LinkedIn and we have been penpals ever since. Let's call my penpal 'T'. A few weeks ago I told T that I've gotten interested in the alphas and explained that there are very few service manuals available. He asked "which model are you interested in?". Of course I was tempted to say all of them! But I thought that would be rude . So I explained I already have 607/707/907/MR/MRX and 907KX..... but what I really need is the 907 Limited. T says he will ask around and get back in touch. Just over a week later, I get an email with an attachment!!! As far as I understand T has scanned this for us himself from a hardcopy!
It's in Japanese but the schematic is legible (and the test points are labelled n English actually), and with the 907MR manual which has English, the 907Limited japanese adjustment procedures are clear.
At the very begining before all of this I asked T if the service manauls are secret and received the response "There are no secret because the property holder does not exist". Now, I totally understand (1) personal promises and (2) pro-tech business protection are separate issues, but as far as Sansui itself is concerned there is likely no problem at all since Sansui is no longer an entity. I asked T if I can share the 907Limited manual with everywhere and received the response "of course no problem to share with anyone who is interested the documents online. We (I and the person who found the Service Manual), are rather happy as exSansui." I lightly edited the English (although T's English is very very good), but the meaning is unchanged. I think T is not working on amplifiers these days and gains nothing from this, except sharing the joy of Sansui history. I think this passion from an ex-employee is really exemplary and this is the passion that shows in the product Sansui made! T was already very interested in the mystery 907 Extra over on Stereonet forums...
About the pdf, I've just uploaded on hifiengine; if it doesn't show in a few days give a shout and I will share via dropbox (I find the ak db doesn't let me upload for some reason so if someone could do that, please go ahead).
A few other notes:
- as noted by skippy, some panel screws are different length. Be sure to replace same length screw when you put it back together (there is a risk of shorting out the phono board on the right hand side).
- the 907Limited poweramp section might have been recommended by Sansui as power amp for "best preamp of the time", C-2302 VINTAGE (1992, 1,280,000 Yen!). The original matching power amp B-2302 was dated to (1990). gathering this info from the audio database site.
BIAS Voltage:
- 28.6mV for Limited (8.8mV for 907XR). This gives about 130mA idle current.
- Need top and bottom cover off. Left panel off also needed if doing premap DC. Probe from bottom, access VRs from top. Stand on right hand edge, so VRs for preamp near front left panel can be accessed.
- the manual refers to TP on the amp board, but you will actually find two pins, not one. The two pins connect to either side of an emitter resistor. I used a 2.5mm female plug to make life easier and just probe across those two test pins (it just skips the output coil but that is only 0.01Ohm so it hardly matters), but you could also follow the manual, but you will need to know which test pin to grab hold of (which is easy to find with DVM (the test pin you want is the one that is 0.22 Ohms from the 'out', not the one that is shorted with 'out'). Other alphas, like 607NRAII pretty much require such a 2.5mm connector plug (5-pin in the 607NRAII, which I also used on the 2-pin 907LTD (whereas the 607MR has easy to access TPs and no need for a connector/extension)).
Original bias on mine:
- if you see about 38mV at cold start up dont touch VRs just yet, it really takes some warming up to get to steady-state. Read on:
- cold start-up (Right (37.8mV, 37.8mV); Line pulling 1.49A
- Warm up took well over 60 minutes and the above settled to:
{Right (27.6 ('hot' (non-inverting), 28.6 ('cold' (inverting));
{Left: 29 (non-inverting) and 28mV (inverting)}
- at warmed up idle line pulling 1.25A
TEMPERATURE:
- On all 3 of my alphas the side with the phonoboard (right) runs 2 C hotter. The thermal situation is just non-symmetric with the left being coupled more directly to larger chunks of metal.
- My 907Limited runs about 50C at the top of the heatsinks and 53C near the bottom where the transistors are situated. That is the right side. As mentioned Left side runs slightly cooler.
OFFSET:
- for 'hot-to-cold' probe between the two ‘out’ (one for hot (non-inverting) and one for cold (inverting), which go to positive and negative speaker terminals). Just take care you negative probe isn’t grounded by your measuring device as 'cold' does not mean grounded, but in fact refers to an active output designated 'cold' (is just what they called the inverted amp in a bridged setup). ''hot-to-ground' yoy can work out and must be set simultaneously with 'hot-to-cold' as they are interlinked (one setting balance between either side's DC and other setting magnitude of one).
Power: I got 101W, both channels into 8Ohms before obvious clipping. Didn't go into detail with the distortion meters, but this thing is clearly making more than the prescribed 80W.
Side Panels:
The side panel pattern is Birds Eye Maple. I recovered mine with new vinyl as the original got dinged in shipping and it looks fantastic... I should probably post. Did think about solid wood but I thought it might look better, but less original.
Further thoughts on listening:
- I think very few people have compared the newer models with the older ones using an amp selector box and volumes set equal using SPL meter. When I did a quick comparison of the 907Limited and 607MR, without doing it properly (just changing the wires and doing volume by ear), I swore the 907Limited was bright and disappointing. I even mentioned that too a few people.
- Then next weekend I setup properly and redid comparisons. It's amazing how much the mind plays tricks. The 907Limited is in fact very very smooth and a bit warm souding, similar in sound to the 607MR and 607NRAII, on my speakers at least and driven to moderate volumes (I rarely listen loud). The differences are extremely subtle, and in fact I'm the only person on my setup who can tell the difference in blindfolded tests. I am now comparing to older amps and will report elsewhere. But summary is it's pretty useless for me to trust my own ears/brain without proper comparison setup, let alone someone else's comments with unknown conditions.
- One thing I will say now is these later Alphas all sound a lot like the AU-777A (to be fair, my AU-777A is not stock at all). Will compare some earlier alphas soon. And will compare to AU-111 (but I think for now they sound more like AU-777A/AU-70 as my memory of my AU-111 is slightly more clinical/accurate/damped).
If you have a chance please give a thumbs up or comment to thank T, as I will share this with him later.
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