Sansui AU7900 gets really hot.

I am a new owner of a AU7900. I've been listening to it for 5 hours plus to get the feel of it and fine tuning the setting in. I notice the back is really hot! Hot enough to burn your hands..:sigh: Is that normal? I never felt an amp get that hot before, so I am a bit concern mine gets hotter than other..??

Hi,

The heatsink will be hot,but u can still touch them. How about the top case?? A properly running AU 7900 wont be very hot,the way you describe looks like it need to be serviced.

Probably needs a service

Go on to hifiengine and grab the service manual: http://www.hifiengine.com/manual_library/sansui/au-7900.shtml
Find the adjustments and adjust the bias and the dc offset your gonna need a good mutimeter and a insulated screwdriver and be careful when the amp is on as it may electrocute/kill you or short the amp out. If this doesn't work you may need a service from a qualified tech

good luck!:thmbsp:

no affiliation to the following websites listed.

Mine is about the coolest running amp I've ever had. Check/set bias. There is no dc offset adjustment but you might as well check it anyways.

loop said:

I am a new owner of a AU7900. I've been listening to it for 5 hours plus to get the feel of it and fine tuning the setting in. I notice the back is really hot! Hot enough to burn your hands..:sigh: Is that normal? I never felt an amp get that before, so I am a bit concern mine gets hotter than other..??

Click to expand...

Hi,

A couple of questions that might help in getting advice.

1. What speakers are you using and are you using one or two sets of speakers

2. What was the typical position of the volume control, and bass control, and is the loudness switched in or out when you had it playing for the 5 hours

3. Where is the amp located - is it in a cabinet, or shelving, or in "free air"

4. Does it get warm/hot if left on with no music playing

Cheers

John

rasenthiran said:

Hi,

The heatsink will be hot,but u can still touch them. How about the top case?? A properly running AU 7900 wont be very hot,the way you describe looks like it need to be serviced.

Click to expand...

Top case was hot as well.

skippy124 said:

Hi,

A couple of questions that might help in getting advice.

1. What speakers are you using and are you using one or two sets of speakers

2. What was the typical position of the volume control, and bass control, and is the loudness switched in or out when you had it playing for the 5 hours

3. Where is the amp located - is it in a cabinet, or shelving, or in "free air"

4. Does it get warm/hot if left on with no music playing

Cheers

John

Click to expand...

1) Volume approx. 1/3, bass one click forward from flat. Yes loudness was on the whole time.

2) on or in shelf see picture.

3) Actually there was approximately 1 hour where it was left on with no music playing. The last hour I had to step out, when I got back I notice the amp was left "on" that was also when I discovered it was hot.

The AU-7900 does get a bit hot in my experience, but what you describe does sound like the bias needs at least checking.

I was looking at the invoice i got from the guy where he took it to have it serviced back in Oct 2014 then after that sat on the shelf. According to this bill the pots were cleaned, Caps checked "ok", bias pot placed, rebias .tst. This was done by a local licensed shop.

In that case I think Skippy124's intelligent post is very likely to hold the explanation.

loop said:

Top case was hot as well.




1) Volume approx. 1/3, bass one click forward from flat. Yes loudness was on the whole time.

Click to expand...

Speaker load can make a big difference too, depending on nominal impedance, impedance dips/swings, phase angle, etc.

whoaru99 said:

Speaker load can make a big difference too, depending on nominal impedance, impedance dips/swings, phase angle, etc.

Click to expand...

Speakers I was using a 8ohm Jbl TLX8 and yes I was told these unit don't like 4ohm speakers.

Technicians' experience and ability vary wildly. Just because it was serviced does not mean it was done properly. You should be able to hold your hand on the heat sink without discomfort when the unit is idling with no signal. Get it serviced or adjust the bias and offset yourself.

dr*audio said:

Technicians' experience and ability vary wildly. Just because it was serviced does not mean it was done properly. You should be able to hold your hand on the heat sink without discomfort when the unit is idling with no signal. Get it serviced or adjust the bias and offset yourself.

Click to expand...

Yes, that is absolutely true and correct. I would never treat what said in the invoice as a complete 100% accurate documentation. Just some information I wanted to throwing out there..

If I had to readjust the bias I would get someone qualified to do it as I have never done it and this not the time to learn on a nice piece like this.. I did some test today, I let it idled for 4 hours without signal NO heat.. Next I am going to run it again with signal at normal listen volume and check every hour to see how quickly it gets warm.

Running it at more than background volume with the loudness on does put some stress on it. Not saying that is the only explanation, I think it's definitely worth checking the bias. And it's always possible that components are on their way out that tested OK before.

EDIT: I'm not sure how many db the bass is boosted when you turn on the loudness on this amp, but they are often in the 3-6 db range. Note that increasing output by 3 db requires a DOUBLING of power. Granted you're only boosting the extreme low and high ends of the spectrum, but boosting the power into the woofer by 2x or 4x can stress the amp. A lot of people listen to music with bass boosted these days, even though that is not how music is meant to be played. If the artist and recording engineer wanted the bass to be boosted to that level, it would be that way on the recording. JMHO.

toxcrusadr said:

Running it at more than background volume with the loudness on does put some stress on it. Not saying that is the only explanation, I think it's definitely worth checking the bias. And it's always possible that components are on their way out that tested OK before.

EDIT: I'm not sure how many db the bass is boosted when you turn on the loudness on this amp, but they are often in the 3-6 db range. Note that increasing output by 3 db requires a DOUBLING of power. Granted you're only boosting the extreme low and high ends of the spectrum, but boosting the power into the woofer by 2x or 4x can stress the amp. A lot of people listen to music with bass boosted these days, even though that is not how music is meant to be played. If the artist and recording engineer wanted the bass to be boosted to that level, it would be that way on the recording. JMHO.

Click to expand...

I see your valid point. The music I listen to mostly anywhere from rock, pop, disco, but that doesn't not mean I don't listen to vocals at. Either way I have always had "loudness" on my amp, maybe it's just some I've gone accustom too. With my previous and present amps, they do not have this issue with the "loudness" being on. Well I am just going to do that run test with music on and see how quick it gets hot. If it is abnormal then I will bring it into my guy audio to have it checked out.. If it is just a case where it is just normal for this amp to get this way and everything checks out. I am just going to sell it. I have always thought this amp housing is fairly small, confining and not very well vented. That is maybe one of the contributions to the heat. :scratch2:

I was also talking to another audio tech guy today about that briefly and also talked about how it is normal for Onkyo to get hot for an example. He also said it's about the head room and the smaller amp would go under more stress at the same db than a bigger amp with bigger transformers and such. So that does fall in the lines of what toxcrusadr was talking about. Maybe this is a good reason for me to sell this one and get the bigger amp..?

Not being familiar with this particular amp or speakers, I'll let others take that question.

If you have a multimeter, you can check DC offset just by disconnecting the speakers and checking DC voltage across the outputs with the amp turned on and no input.

toxcrusadr said:

Not being familiar with this particular amp or speakers, I'll let others take that question.

If you have a multimeter, you can check DC offset just by disconnecting the speakers and checking DC voltage across the outputs with the amp turned on and no input.

Click to expand...

Ok I'll see if can dig one up as I have recently move and stuff is still packed. Or I can borrow one. I will try what you suggest and what figures or range am I looking for?

See the very top sticky thread in the Solid State forum. This will describe what and why and how. As for the actual acceptable range for your amp, it would be in the service manual for that amp, but the sticky thread will give you a generally acceptable range.

Thanks for the tip, but not sure which sticky. Not about to go through pages and pages of technical stuff. I am an audio gear fan and enthusiast, that's where it stops. Got to stick to what I am good at is computers. The most I can probably do for a unit is swapping out caps. :stupid:

Some things to consider...

When the loudness is on the curve is boosting 50Hz +10dB. The bass control has five clicks beyond the center pushing +13dB at the fifth click, so if each click is 2.6dB... then with one bass click up and the loudness on you may be +12.6dB at 50Hz... about the same as leaving the loudness off and cranking the bass tone full up. Either way would be a quadrupling of power to the low end.

In the power amp, power supply, and protection trouble shooting section of the service manual it states that the relay will fail to operate when the power transistors get hot if the posistor in the power supply protection circuit is defective (the posistor increases resistance in response to elevated temperatures from 70-100 C, that is 158-392 F). That's hot! There are no quick acting fuses on the power amplifier.

I don't see anything in the manual about adjusting DC offset.