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Old 25th May 2012, 11:37 AM   #1
branet is offline branet  Croatia
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Default Technics SU-7100

Hi
I own amplifier Technics SU-7100K.
Inside is chip STK 0039.
I wanted to gain more power from the amplifier is ​​it possible to change the stk 0039 stk 0100, few of resistors and transformer with a higher voltage and power rating.
Here is service manuale :Technics SU-7100 | Owners Manual, Service Manual, Schematics, Free Download | HiFi Engine
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Old 25th May 2012, 12:11 PM   #2
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Forget it!

You are much better off buying a better amplifier or gutting this old technics unit and building a more powerful amp in the chassi.

To even notice a increase in volume, you pretty much have to double, triple or even quadruple the output power.

Doubling the power is only around 3dB increase in sound pressure if i remember right.
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Old 25th May 2012, 06:45 PM   #3
branet is offline branet  Croatia
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I know it all but would love to try it for fun and to learn something!
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Old 25th May 2012, 07:48 PM   #4
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Problem is, you can't replace just the power amp in this unit. It's a pre+power integrated affair that does all the voltage gain in the power amp and has the tone comtrols in the power amp's feedback loop. The volume pot is is rather on the high-impedance (read: noisy) side as well. So you'd need a new preamp section, too, which would be an opportunity to get its performance closer to state of the art. Rod Elliott has one that would work.

And then there still is the issue with the fuses in the speaker line, so you'll want some protection circuitry as well. I don't think you'll want to route the output across those contacts in the headphone socket either. If you're doing a new preamp anyway, this would be the ideal opportunity to implement a decent headphone output by modern standards, so you may want to tap off the signal between pre and power amp for a unity gain buffer using a NJM4556 with <10 ohms of output series resistance (it normally doesn't need any if you've got a decent PCB layout).

(Don't forget you'll also need a bigger heatsink.)

It is certainly doable, but expect a major project and having to learn A LOT (maybe more than you'd like).
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Old 25th May 2012, 08:53 PM   #5
branet is offline branet  Croatia
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Thank you sgrossklass!
You're right it's a bit too much for me.
Maybe you have some suggestion to get a bit more quality/power from the amplifier!?
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Old 25th May 2012, 09:15 PM   #6
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Gut out the current amplifier and drop a DX amp in there instead.
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Old 25th May 2012, 09:17 PM   #7
wahab is offline wahab  Algeria
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The STK0039K are a darlington power pack , comprising
the two drivers and power transistors as well as a VBE
multiplier.

The front end is a discrete differential as well as a VAS
with a beta enhancer.

The only valuable mod would be to add a current mirror,
although the 15pF compensation cap should be
increased in such case or even replaced by a TMC
compensation network

In this respect this amp is better than a DX or other
usual LIN even without the mods i quoted above.

Last edited by wahab; 25th May 2012 at 09:24 PM.
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Old 25th May 2012, 09:34 PM   #8
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I thought all these stk module amps were considered mediocre at best as well as less reliable and more prone to failure than full discrete amplifiers.
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Old 25th May 2012, 09:49 PM   #9
tvi is offline tvi  Australia
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Looks like some reverse engineering of the stk0039 here

Click the image to open in full size.
Click the image to open in full size.

With the rail voltages you have maybe a LM3886 might do you, more power and no obsolete parts?
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Last edited by tvi; 25th May 2012 at 09:50 PM. Reason: bad link
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Old 25th May 2012, 10:09 PM   #10
wahab is offline wahab  Algeria
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Quote:
Originally Posted by Tekko View Post
I thought all these stk module amps were considered mediocre at best as well as less reliable and more prone to failure than full discrete amplifiers.
Right that there were prone to failure but it was because
they not always had internal power transistors emitter resistor and that
a few manufacturers didnt bother to add external ones , mind you..

Since they had a built in inherently fast VBE tracker they somewhat managed
to survive in a good proportion.

This Kenwood has external power transistors emitter resistors,
wich explain that it went through time with no damage.

Be aware that the STK discussed here were just final stages
as the differential and VAS are external , built with good quality
transistors.

Other STKs can have the whole amp integrated and are often
of average quality although later parts were quite good.

Last edited by wahab; 25th May 2012 at 10:12 PM.
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