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Old 1st March 2010, 12:56 AM   #1
GaryB is offline GaryB  United States
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Default Restoring a Threshold SL10

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
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Old 12th March 2013, 01:06 AM   #2
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You have performed a fantastic up grade in your LS-10 preamp. I have a suggestion or two, to offer.

#1) connventional power supply solid states present a horrendous level of stored energy at charging current cut-off 120/second. This can be seen with a 40 mhz oscilloscope at at very component in the power supply including the 120 Volt AC line cord. It is impossible to remove this trash; snubers do very little.You can achieve vacuum tube rectifier quality using Fast Recovery Rectifiers which do not store any energy__ Stone-Cold Clean.

#2) Have you ever wondered "what a well engineering Power Cord" can do? Consider this__ "All of your Audio and Digital Components Gossip To Each Other Via the Power Cords." There is also considerable noise in the 120 Volt AC Power coming into you house panel from the Power Provider.

Of course their are many Fat Wire Fakes; however, MIT CVT Inc., and Transparant Audio Inc., offer power cords that have considerable attenuation starting just above 60 Hz extending well beyond 20,000 Hz to capture all this interfernce.

#3)Tantalum capacitors used in Threshold Preamps have a reputation of short circuit failure __A high quality capacitor should present a series resistance exceeding 200,000 ohms measured with forward & reversed ohm meter polarities using a Simpson Model 260 Multi-Meter on the 10,000 X ohm scale__this is not the case with the Green Gum DropThreshold Tantalums; The same goes for military grade tantalums.

If you look at the rate of charge with a high impedance setting of the ohm meter, Thresshold tantalums react like electrolytic capacitors, ie, the charging rate is extremely fast in one polarity and extremely slow in reversed polarity of the ohm meter.
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Old 12th March 2013, 06:51 AM   #3
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[QUOTE=Robert the cat;3406831]The subjectof power cords contnued....Of course their are many Fat Wire Fakes; however, MIT CVT Inc. power cords offer considerable attenuation starting just above 60 Hz extending well beyond 20,000 Hz to capture all this interfernce.... [quote/]

MIT CVT Inc. power cords present a RESISTIVE SHUNTof FIVE OHMS across the 120 volt AC power line in a spectrum extending from slightly above 60hz to well beyond 20,000 Hz on the AC power line.

Application of a series 10mh inductor preceding this five ohm shunt power cord results in a power conditioner capable of providing extremely significant attenuation of power line interference.
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Old 12th March 2013, 08:35 AM   #4
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Default AC Noise Filters

MIT__Music Interface Technologies, Inc offers an inexpensive Power Line Conditioner__the "Duplex 20 AC NOISE FILTER"__which provides a RESISTIVE SHUNTof FIVE OHMS across the 120 volt AC power line in a spectrum extending from slightly above 60hz to well beyond 20,000 Hz on the AC power line.

Application of a series 10mh inductor preceding this five ohm shunt filter results in extremely significant attenuation of power line interference. You will hear a new world of audio sound.
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Old 12th March 2013, 03:31 PM   #5
GaryB is offline GaryB  United States
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Reviving a 3 year old thread to discuss commercial power cables???
You must be kidding.
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Old 13th March 2013, 03:24 AM   #6
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Default CORRECTION IN POST#3

Quote:
Originally Posted by Robert the cat View Post
Post#3 Incorrectly Reads: "MIT CVT Inc. power cords present a RESISTIVE SHUNTof FIVE OHMS across the 120 volt AC power line ....
Post # 3 should read:"The MIT CVT Inc. "Duplex 20 AC NOISE FILTER" presents a RESISTIVE SHUNTof FIVE OHMS across the 120 volt AC power line in a spectrum extending from slightly above 60hz to well beyond 20,000 Hz on the AC power line....[Quote/]

Application of a series 10mh inductor preceding this five ohm shunt filter results in extremely significant attenuation of power line interference. You will hear a new world of audio sound.

Last edited by Robert the cat; 13th March 2013 at 03:30 AM.
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Old 13th March 2013, 10:18 AM   #7
Zen Mod is offline Zen Mod  Serbia
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