Vintage Technics SU-C01 gain/sensitivity mod?

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Folks, I would love your help in perfecting my vintage pre-amp:

I recently completed a refurb of a Technics SU-C01 preamplifier (caps, resistors, wiring, earthing conversion). It sounds lovely and is a real pleasure to own (partnered with the tuner and power amp/supply from the same line) - I love its build quality and compact dimensions.

The sources attached to the preamp are an ipod dock in the Aux input and a Technics ST-C03 tuner into the Tuner input. The modification I would like to make is to reduce the gain/sensitivity, so that I have more travel on the volume control. Currently, I can only use the first 30% of the volume control travel until it hits high volume levels (i.e. ~3 steps out of 10).

I know that I could buy an in-line attenuator for the RCA cables, because the sources are likely a higher voltage output than the design allowed for (it was late 1970's after all), but I would really prefer a soldered solution on the PCB if possible, so I've attached the schematics below. The closeup schematic shows the part of the circuit around the volume pot and then the tone filters which i think is the focus for any mod. There is also an IC SVIM5213L, which I believe is an amplifier/buffer stage (see the block diagram below - tried but failed to find a schematic for this old chip).

Some options I thought about, but have little knowledge on their impact:
1. modify an existing voltage divider within the circuit - e.g. R303, R305 (L ch.)
2. add a load resistor, bridging the volume pot (VR301-3/4) Ground pin to circuit Grnd.
3. remove SVIM5213L from the circuit... (probably too complicated?)
4. change the value of R501, R502

I never use the balance control, so if removing the 250k balance pot from the circuit helps, then that would be fine. In addition, I have a compact TKD pot (20k), which i would happily fit in place of the existing volume pot - likely better quality and would fit easily, but this is a very different value to the current 100k pot.

Any suggestions for an elegant solution?!



  • SU-C01 schematic.png
    SU-C01 schematic.png
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  • SU-C01 schem closeup.png
    SU-C01 schem closeup.png
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  • SU-C01 svim5213L block.png
    SU-C01 svim5213L block.png
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Thankyou JonSnell for the suggestion - it worked and it is a neat mod on the board. I had a pair of 56K Kiwame resistors in my stock which I substituted for the 100k you suggested, and used a pair of 5k1 in the R501/2 position. There has been a drop in white noise hiss at the speakers, which although was not a great problem before, is also welcome. Now I have full travel on the volume, which is great! I do think that the dynamics of the music have dropped-off slightly - the sound seems a little flatter - any idea if that is that linked with my choice of resistor values?
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.