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Hayden 10th December 2004 02:05 PM

Sansui au-7900
Ok I have the transformer and caps out of a Sansui au-7900 and I would like the schematics for the power supply circuit.
I also want to know what amp rating is this thing and what wires are what can any one please help.

purplepeople 10th December 2004 11:41 PM

This is the only site I know of with good Sansui info. No output specs for the AU-7900, but you might be able to extrapolate based on the other specs. I would guess it to be around 70W.


destroyer X 11th December 2004 02:21 PM

Take a look at your back panel, and see the consumption.
Your amplifier may have a maximum consumption around 400 Watts.

Normally, efficiency is not more than 65 percent, and you must consider that those watts take into account the Leds, lamps and all things that make energy consumption.

If you took 65 percent as energy transformed in audio energy, and the rest 35 lost as will calculate 260 watts of real audio....130 each channel at 4 ohms and 65 each channel at 8 ohms.

Can be a little better, lets say 140 X 2 to 4 ohms and 70 x 2 to 8 ohms.... a perfect, and enougth power, considering 4 ohms.

Other way to check is to find output fuses (if they exist)

3 amperes each channel means power not bigger than 36 watts each channel at 4 ohms.

4 amperes each channel means power not bigger than 64 watts each channel at 4 ohms.

5 amperes each channel means power not bigger than 100 watts each channel at 4 ohms.

6 amperes each channel means power not bigger than 144 watts each channel at 4 ohms.

7 amperes each channel means power not bigger than 196 watts each channel at 4 ohms.

8 amperes each channel means power not bigger than 256 watts each channel at 4 ohms.

9 amperes each channel means power not bigger than 324 watts
each channel at 4 ohms.

10 amperes each channel means power not bigger..... 400 watts
each channel at 4 ohms

And 10 amperes is very difficult to reach....may be some fuze over sized.... always think in lower than the values i told you... and this is some easy method to obtain with 25 percent maximum error the power value.

Other method to evaluate is to measure the heatsinks, all fins and the long rail.... multiplying side by side, each 9 squared inches of surface (consider one fin of 9 squared inches as 10 watts..... do not calculate both surfaces..... so, 3 X 3 inches is 9 square inches (not 18!)... and this is 10 watts power dissipated)

do not need to reduce to 65 percent..... the value is the maximum power, divide by two to obtain each channel at 4 ohms, and divide by two again to obtain at 8 ohms.

If you have internal heatsinks, reduce the result in 25 percent, as ventilation no good....if heatsinks is outside, use the trick, if the sinks have some aid blower from factory.... increase power multiplying by other words.... calculating 100 watts, if some factory fan, 300 hundred watts is the best value.

Those tricks will help you to evaluate.

Also use some multimeter..... put some resistor, big one, as load at the output, better if not an inductive coiled big one.... can associate a lot of big carbon resistors..... a lot of 1K in parallell to reach around 8 ohms..... inside some can with dry sand or oil...can be some Milk can.

Also you can buy some industrial load....8 ohms made by one straight bar, with fins or smaller full of oil inside. Put your multimeter and inject 60 hertz from your finger in the phono input.... use the magnetic one.....and , the multimeter in AC scale bigger than 50 volts AC, it will measure with good precision in that frequency...50 hertz or 60 hertz, magnetic field, captured by your body, as you have mains wire near you sending to your liquid body magnetic are the generator.

Put one 100 ohms resistor in series with the positive terminal, and the load connected from output plus to minus.... conect one speaker, a big one, as the frequency will be 60 hertz, from the free 100 ohms resistor lead to the, energy will pass inside the 100 ohms resistor to reach the speaker, only a small fraction of the entire power...12 times less than the power that will go to the 8 ohms "hot" load (charge).

Increase the volume till you see some increase of volume in the speaker, and it will start to sound harshy....noise and loud as guitar distorters....go reducing till the sound sinusoidal return to be agreable, as sinus is very good to human ears... see the voltage at you Multimeter....lets see 30 volts!

Multiply this 30 volts by itself and divide by the load 8 ohms, that's your channel (one single channel working produce more power than two channels working).... this is your maximum power, and error will be less than 10 percent.

In this case 30 X 30 = 900...............900 divided by 8 ohms=112,5 watts rms over 8 ohms, one single channel can expect 225 watts over 4 ohms.

This is not common...values from 17 to 25 are more common to find.

This detailed explanation, is directed to diy beginner, young boys studying, of course, you PHD, may hate read those foolishes.

But thank you all the attention.

I have not any intention of precision, as i do not care about that, in this case, is HOW TO DISCOVER POWER, with some not precise methods.



destroyer X 11th December 2004 02:53 PM

Other method to discover power.
Switch your supply on, and measure the voltage...lets see, for instance plus 45 and minus 45.

Take the half of the negative (same as positive) rail voltage..... 22.5

This 22.5 will be the minimum indistorted swing you may expect from a normal amplifier..... 22.5 Volts Rms....

Multiply 22.5 X 22.5= 63 watts of power can be achieved...... but if you use two channels driving at same time...hard driving.... the supply voltage will reduce to something around 39 volts...beeing very good aproach.

39 divided by two= 19,5

Now multiply 19.5 X19.5 = 380..... divide 380 by 8 ohms = 47 Watts rms under charge.

This supply, may be enougth (if big to transfer many amperes), to 47 Watts x 2 over 8 ohms with hard use, and the peaks can reach 63 watts (dinamic, short time power output)



DigitalJunkie 11th December 2004 05:19 PM

I have an AU-7900 here,Great amp! I really like it.. (for SS,anyways ;) )

I think it's (conservativly,IMO) rated at something like 70W per channel..So,figure you'll have about ~140W total output to play with from your new amp.

The primary side probably has a bunch of wires coming out,The tranny has dual primaries,with taps for adjusting to your local line voltage.. (0-100V-110Vac (x2) or something similar)

Gee,it's been so long since I had mine open,I forget which wires are which.. I think there are a white and a grey wire,that get soldered to GND (The CT"s of the secondary windings...) uhmm.. I think theres a couple orange wires,which supply the pre-amp stuff,phono amp,and whatnot. (probably something like 20vac?)
And I think the main windings are purple?
I dunno,Like I said it's been a long time,but if you cant figure it out,lemme know and I'll unplug the ol' Sansui,open it up and find out which is which for ya.

Aww... what the heck,I'm bored anyways....
(If you'll please hold for a moment,I'll go open up the amp.)
(A few minutes later...) Oh dear lord... I forgot how heavy it is to lift off the stack of equipment.. *catches breath*.

Exterior Tag says..
100/120/220/240Vac 176W consumption

Grey/white are CT's to GND.
Purple (+grey CT) wires are your main outputs (each lead has an 8A fuse.)
Orange (+white CT) are the secondary outputs (for preamp,etc.Each fused at 1A.)

Then theres the bundle of blue/red/green/yellow/brown/orange wires.. These go to Sansui's odd voltage selector plug. I've no idea of the pinout of the plug,so I dunno if I can help you here...
On second thought,I DO have a schematic of a similar Sansui amp (AU6600)

OK,looks as if blue/green/yellow are primary #1
Orange red,and brown are primary #2

blue/orange : 117V
green/red : 100V (taps)
Yellow/brown : 0V

There ya go! :D
Ohh,also,for more Sansui info there is a Yahoo groups "ClassicSansui" I think.

Hayden 12th December 2004 01:25 PM

Awesome thanks allot but sorry I just have the transformer and the two caps, I donít have the amp.
I was going to use them to make a power supply for another amp I made.
Thanks again.

destroyer X 12th December 2004 02:07 PM

hehe, you was clear enougth... i really did not read correctly.
I the way, someone can make a good use with the informs.

Never mind...lets be happy.


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