Restoring a Threshold SL10

[GaryB]

There have been a few threads over the last few years on the SL10 preamp and I owe a debt of gratitude to the various people that have posted hints on how to bring this classic preamp back to like new condition. I picked an SL10 up a few months ago and finally got around to restoring it this weekend. A quick listen has been very encouraging. It sounds very, very good with a phono stage that is outstanding. The sound of vinyl with this preamp is better than any other phono stage that I've heard in my system. I like it better than my ONO clone on MM input. I haven't given the MC input a listen yet. I also like the line stage better than my FET 10/hl line stage, which is already a very good line stage. The absence of output coupling caps and the better quality power supply makes a difference. Upgrading the volume control and removing the balance control also improves the transparency.

Here are the changes that I made to the preamp.

1) I upgraded the electrolytics in the external power supply from Mallory 6800uf 60v to Cornell Dubilier 10000uf 75v capacitors, which I bought from Mouser. I also put a snubber across the rectifier bridge to tame any rectifier noise.
2) On the preamp board, I upgraded the small electrolytics (47uf 50v)with 47uf 50v Black Gate capacitors.
3) I upgraded the bigger electrolytics (470uf 16v) with 470uf 16v Black Gates
4) I changed the 47uf 20v capacitors in the servo loops with 47uf 50v bipolar Nichicon ES capacitors
5) I took the balance pot out of the circuit and replaced it with fixed 10K ohm resistors. This reduces the gain by about 4.6db.
6) I replaced the volume control with a 20K ohm SMT resistor volume control that can be bought very inexpensively on Ebay. This is similar in quality to the DACT volume controls but much cheaper.
7) Since my system is efficient, I lowered the gain of the preamp another 10db by changing the 1k ohm resistor that is in series with the volume control to a 47.5k ohm resistor. This may drop the gain too much for some systems so I will probably add a switch later that will allow the gain to be changed from the front panel. Since I no longer use the balance pot, I will mount the switch in that location.

One thing I didn't do (yet) is change the AC coupling caps for the phono inputs. There are 4.7uf 25v tantalums at the input to the MM stage and 2x 220uf 10v tantalums at the input to the MC stage. My preliminary listen to the MM stage with the original caps in place sounded quite good. How much is there to be gained by swapping these out. What would people use? Newer electroltyics? Blackgate non-polars? Organic electrolytics?

I've attached the schematic that Nelson shared a few years back since it's no longer available other than via the internet archive.

---Gary