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Modifying an older Sony amplifier

1. Installing an USB-DAC
2. Cooling the output stage
3. Active cooling
4. Variable subwoofer output

A recap job on a Pioneer SX-950 vintage receiver




USB DAC

This is a cheap (less than 20€) SA9027+ES9023 24Bit/96KHz asynchronous USB DAC from China, shipped in three weeks. I compared it's sound to my Musical Fidelity V-DAC II and found no substable difference. Also I did some measurements with sine waves and couldn't notice any distortions between 5Hz and 20KHz. This in addition shows the pretty good quality of the Sony TA-FB920R QS amplifier. The DAC's output leads are soldered to Tape 2 Input.

Attached on the right side panel you can see a single board whose connector has been pulled out. It is the Phono board.


2. Cooling the output stage with extra heat sinks


 Passive cooling the output stage


3. Active cooling with a PC-fan


Active cooling the output stage with a PC-fan

In my craft crate I found a small PSU (on the upper left corner of the back panel) and a 12V PC-fan - leftovers from the past. I attached the fan on top of the heat sinks with isolating foam rubber and did use super glue to combine the parts. The fan is running at about 7 Volts. Just enough for it to start and you can barely her it during operation. This may look as a bit of an overkill. Actually the amplifier is already quite hot in normal operation, although the bias current has been set correctly. That is a well-known phenomenon. This is the very cheapest active cooling mod I so far have done. I did similar mods more elegantly using a temperature controlled circuit to regulate the fan's speed but the proper DIY-kit no longer is available.


4. Variable subwoofer output

Variable Output for Subwoofer

Variable Output for Subwoofer


While listening to my Cobra-horn loudspeakers I missed deep bass. Why not hook up an unused active subwoofer (SW)? But I needed a variable output. The most elegant solution is a simple voltage divider that won't load the amp's B-out. Let's assume that bass frequencies below 50Hz are not left - right seperated in most recordings. So one channel will do the trick. The line out specs are not consistantly defined but with an output impedance of 1KΩ and a level of max. 2V you're good to go.   

The 22KΩ resistor connects LS-B(+) to line(+). LS-B(-) is directly connected to line(-) and the 1KΩ resistor connects LS-B(-) and line(-) to line(+). How does this work? Speaker selection knob (SSK) to A = SW not engaged. SSK at B = SW only. SSK at A+B = LS and SW playing. And now the math.

This circuit is designed for an 80W amp at 8Ω. We firstly will use DC-formulas for simplicity reasons. U=RxI and P=UxI. So P=RxI² and I=√P/R. √80/8=3,16. 25,29=3,16x8. Thus we need an 1KΩ to 12,6KΩ dividor. But let's have a look at my values: U=RxI - 25,29=23000xI leads to I=0,0011A. So Udrop at 22KΩ is 24,2V and line out is 1,09V. But here comes AC with a sine wave. You have to multiply the result with √2=1,41 which leads to 1,54V. It's a bit more complicated but this should give you a clue.



A recap job on a Pioneer SX-950 vintage receiver


Recap a Pioneer SX-950


Recap a Pioneer SX-950


Recap a Pioneer SX-950


Recap a Pioneer SX-950

You can find really brilliant Youtube videos that will help you to do the job on the power stage. But I wanted to display some hints how to avoid wasted time. You do not have to retwist (Does this verb exist?) the connections to the power amp section. But be careful. The leads are solid and not much flexible. They will break if you bend them too often. OK, four caps on the power board have been replaced. Now let's go to the big caps at the PSU.

Here I show you a neat replacement with KEMET capacitors, rated at 63V, 85C°. I had to add some screws and ...! It's up to you how to replace a soldered cap with a srewed one. You don't nesessarily need a schematic if you read the values and the orientation carefully but I recommend you downloading a special PDF. Just search for "service manual SX-950".