Weird transformer

[guidoilieff]

Hi. I found an old sony "combo"like this one:




It came with a stk 411-240e (schematic: https://forum.donanimhaber.com/stk411-220e-amfi-malzemeleri--111223906 )



What most interest me is the transformer, but I don't know how these kind of transformers work. Does it have a specific name so I can make some research?

My goal is to make a guitar amplifier with it.

[merlinb]

What most interest me is the transformer, but I don't know how these kind of transformers work. Does it have a specific name so I can make some research?

It's just an ordinary power transformer.
Measure the resistance between the pins to find the different windings (it looks like it has several secondaries, and maybe a multi-tapped primary for different world voltages). Then apply mains voltage to it (you know where the mains attached to the PCB, right?) and measure the secondary voltages.

[anotherjim]

It is a conventional transformer, but the arrangement is pcb mount as well as chassis mount - you normally only see one or the other provided for.
It does have a lot of secondary taps. You can easily see the primary side in the hatched area of the pcb silkscreen and the connection jumpers for 120/220/230-240 volt AC supply use.

Old STK amp modules are all very well, but I wouldn't want to use it unless I had a spare - you can't repair them.
Funilly enough, I have an STK rack mount stereo PA amp, the power stages are discrete. Maybe it isn't the same STK?

[duck_arse]

the transformer itself appears to have a mu-metal shield [the flat grey] and a copper "turn", meant to shield from hum and short hum, so the "hi-fi" audio remains the highest fidelity. I think.

[PRR]

Show some ID, please.

The boom-box has a model number which may lead to a service manual.

The transformer has numbers on it. The part-number might never be known outside the factory. But many Japanese irons also have the Volts and Amps printed on them.

It has the copper and iron (probably not Mu) because it is TIGHT inside a boombox. High-gain audio too close to high-power transformer.

It is not a bad plan to make a g-amp, but I suspect it will favor a very low impedance, similar to all the stock speakers in parallel. This may be ~~2 Ohms. Yes, a couple 4r or a quad of 8r is in that range.

[thermionix]

I'm genuinely confused (again).  How does a power transformer favor a speaker impedance load?

[anotherjim]

Solid state amps usually have limited DC supply voltage. 12v is common for bookshelf systems (so they can use the same cheap chip amps as your car audio) while many "separates" may have 30 or 40v. With low volts, the way to develop higher powers is to supply more current into the load. Lowering speaker impedance means you squeeze more current. 4ohm is the usual minimum.
Of course, you should never overload the amp circuit or its power supply, so the minimum speaker impedance should be written on the back panel somewhere.

[PRR]

> How does a power transformer favor a speaker impedance load?

At full power, the amplifier takes some voltage and draws some current. This is nearly proportional to load impedance. For totem-pole class B OTL, the power amp "acts like" a resistor about 6X the load impedance.

Check: for *example*, 48V (+/-24V) rails will support near 32 Watts in 8 Ohms (16Vrms, 45Vpp) and suck about 1 Amp. Power input is 48W, output 32W, efficiency 66%, which isn't ideal 78% but is very real-world. Apparent resistance is 48 Ohms which is 6X the 8 ohm audio load. The ratio is well above "1" because the two halves work alternately, peak of a Sine is well above average, and there is some pi in the mix. (You could derive this exactly, but I lost that matchbook, and in practical work "6*Rl" is close enough.)

I see four speakers and these days they are often nominal 6 Ohms. This suggests a single amplifier and 1.5 Ohm loading to get all the juice out of that iron. (I'd probably run two chips and two 4r speakers; the system was designed for lightly-clipped music, not HARD-clipped.)

[guidoilieff]

Show some ID, please.

The boom-box has a model number which may lead to a service manual.

The transformer has numbers on it. The part-number might never be known outside the factory. But many Japanese irons also have the Volts and Amps printed on them.

It has the copper and iron (probably not Mu) because it is TIGHT inside a boombox. High-gain audio too close to high-power transformer.

It is not a bad plan to make a g-amp, but I suspect it will favor a very low impedance, similar to all the stock speakers in parallel. This may be ~~2 Ohms. Yes, a couple 4r or a quad of 8r is in that range.

Oh, that's why I couldn't find the model number on the front, sorry

Also, my dmm is not working properly on the AC mode, thats why the confusion also. I guess I'll have to buy another one. Maybe I'll check the fuse or some obvious damage on the inside.


Here are some more pictures

[merlinb]

Oh, that's why I couldn't find the model number on the front, sorry

Well that was easier than anticipated. See page 40:
http://diagramas.diagramasde.com/audio/Sony_HCD-GRX80_RXD8.pdf
Voltage are dotted around other pages so you have to work backwards to figure what they are at the transformer. Looks like about 10-0-10V, 0-15V, and a couple others.

[guidoilieff]

Oh, that's why I couldn't find the model number on the front, sorry

Well that was easier than anticipated. See page 40:
http://diagramas.diagramasde.com/audio/Sony_HCD-GRX80_RXD8.pdf
Voltage are dotted around other pages so you have to work backwards to figure what they are at the transformer. Looks like about 10-0-10V, 0-15V, and a couple others.

Yeah, I found it and then the power went off in my house


I cant find anywhere the voltages except for tolerances in capacitors

[merlinb]

I cant find anywhere the voltages except for tolerances in capacitors

[guidoilieff]

Thanks. We have different page numbers it seems.

Anyway. This part of the schematic shows the voltages for AC3 and AC4 but I don't understand what VH+ VH- and VL+ and VL- means on AC1 and AC2. Also don't know what VF stands for on the lowest winding on the schematic.

[PRR]

This is an ENORMOUS and tricky audio amplifier.

Aside from the housekeeping 5V 12V etc, there is The Power Amplifier, STK411 or similar. This takes *four* supply rails. Up to medium-loud it runs on +/-30V. When peaks get close to this, it switches to +/-60V. (This 60V must sag a lot, maybe to 40V at sustained full power in 2*8r.)
https://www.digchip.com/datasheets/download_datasheet.php?id=1347182&part-number=STK411-240E

Did you salvage the heatsink and enormous chip? PCB with huge caps? If not, it is going to be massive work to collect (genuine) parts to use this transformer the way it was intended.

A more practical re-purpose is to take the windings for +/-30VDC and power a LM3886 amplifier to about 40W/8r, a bit more in 4r, and you can run two for over 100W total. Something like these:
https://www.parts-express.com/yuan-jing-2-x-68w-lm3886-ne5532-audio-amplifier-board--320-6518
https://www.parts-express.com/dc-hs11118-aluminum-heat-sink-kit-1093-x-585-x-18mm--320-3280
Note the "amplifier" board also has the rectifier and big caps you need.

[anotherjim]

VH+ VH- and VL+ and VL-  are the supplies for the STK power amp module? For reasons Paul just mentioned.

VF. Historically, F or f has meant "filament". That could be valve heaters or filament lamps. Obviously not valve heaters here. Maybe it's an AC supply for panel lamps, display & backlights?

[PRR]

[PRR]

> Obviously not valve heaters here.

Trace it around page to page to page. It IS the filament for the vacuum fluorescent display.

Most of those little housekeeping supplies are of no concern to us. We can easily derive +24V, +/-15V or almost whatever from the +/-30VDC rails. Implement the +60VDC (with a much smaller fuse) and power low-voltage tube-stuff or 48V Phantom. Or feed a voltage-quadrupler for serious small-tube-werk.

[guidoilieff]

I guess the answer is no, but could it be possible to power only the main board and feed some audio level signal in the input of the stk411 (pin18/19 or pin 21/22) and expect some amplification on the outputs?

I don't mind shoving the whole main board inside a plywood cabin with a speaker





Main PWB?


Power amp


Power block

[PRR]

Yes.

EDIT!!!   Cut jumpers JW172 JW173 JW174.
EDIT!!!  Do NOT cut jumper 173!!

Feed 0.5V signal: L to JW175, Gnd to JW176, R to JW177

[guidoilieff]

Yes.

Cut jumpers JW172 JW173 JW174.

Feed 0.5V signal: L to JW175, Gnd to JW176, R to JW177

THANKS!!!!!