Reply by provence13

Yes, series SA are pure class A amplifier with more refinement.
It's not in the same league even if S series are very good.
You must test SA/3 or SA/3.9e.
SA/3 are not so difficult to find in second hand market for a fair price.

The musical improvement you have noticed simply confirms a well known fact: the power supply is of the highest importance in an amplifier. There is no way to get a good musicality without an excellent P.S.

Even in the high end amplifiers (and preamps), the power supply can be improved because the manufacturers always try to keep the manufacturing cost at a very low level. I do not really believe in "magic numbers": the quality also depends on the other composnents the amp is used with...

The Forte amps are not mentionned in this site because they were the "economic line" of the Threshold products, and most of the time, they cannot compete with the state of the art products such as the SA serie. But they were good compared to the vast majority of consummer amps...

Hello Phil,

I am not interested by a S/1000 because I already have a lot of amps.... Just let us know what you have bought to replace your S/1000 ?

Thanks

Hello Alessandro,

The Threshold S-500-II is not known as the best Threshold amp, and certainly not as the most transparent ...
But there are many ways to improve it, and by a very large margin !

You have to concentrate your efforts on the Power Supply, in the same way described by morrosco42 , above.

1-.Change the P.S. capacitors : this is not optional, this is absolutely required ! First, they are 20 years old, secondly they are not of a good quality! So you should ask to a technician (or do it your yourself if you know how to proceed) to change them for good capacitors, and if possible increase their value (more µF).

2-.Add small capacitors in // of the big new capacitors, like described in the Threshold T2 upgrade, here: http://www.thresholdlovers.com/articles.php?lng=en&pg=121

Adding small capacitors (like the Vishay mentionned in the article) will improve the high end frequancies, and by a large margin. Your Threshold will have a cleaner, clearer, and more precise sound in the high.

3-.Ask your technician to replace any other capacitor that would be in the signal path, by new ones, and good ones !

If you perform these 3 modifications, your S500-II will not be the same ! I do not know if it will be enouh for you, but generally this improvement brings the amp to an entire and not expected level !

4-.Regarding the fuses: my recommandation is to use fast fuses if the original fuses were fast ones. But all the fuses do not sound the same! I have asked Florent, who knows the fuses domain better than myself, to answer your question, but you should read this article: http://www.thresholdlovers.com/articles.php?lng=en&pg=158
related to the high end fuses.

I hope that these replies will help you: keep us informed !

Nounours

To: leemincy

Sorry for my late reply, but i was away from home during the Christmass period !
I'm a Newbie to this forum and am currently upgrading an s500 Series II—am far from an engineer, but mechanically capable I suppose.

OK, but be cautious in "upgrading" an amp: changing or tweaking does not always improve the sonics as many people believe, you must proceed step by stemp and be aware than some designers (amongst them Nelson PASS) know what they do when they design an amp....
First, I replaced the power caps with Epcos 33,000 mfd 100v caps, as they were pretty much the only caps I could acquire. Are the stock Mallory caps really as poor quality as you say above Nounours?

Do not mind: Epcos are good caps, and using the same brand (Mallory) is not mandatory. Modern caps are often higher quality than old ones.
What about the four filter caps installed inline with the four big power caps. I read thru the other articles that install them but cannot find these VISHAY 0.47µF/630V/85°C available anywhere. Do you happen to know the model number of these? My guess is they will help a lot, as the big 33,000 mfd caps offer great bass, but at a compromise on the higher frequencies.

You can find these Vishay caps very easily, here for example: http://fr.mouser.com/Vishay/Passive-Components/Capacitors/_/N-5g7r?P=1z0zls5Z1z0wquaZ1z0wx8w

Installing these decoupling caps will probably change the sound: this means it can be better or ...worse depending on the amp and on your taste. So when you install them, proceed in a way you can remove them if you do not like the result.
Lastly, am reading about Nelson's preference for Jfets in the front stage. Are these the same as the output stage resistor pairs? Am just wondering if these jets were stock items on the original s500 series II and should I replace these as well? What would you suggest replacing with? Any part# you recommend?

I would not recommend you to change any original transistor of the NP design unless one is broken, and even in this case I would prefer to search for the same reference. Changing caps (which are passive components) is understable and most of the time necessary after 15 years or more, but changing transistors is not changing a passive component: it is a totally different thing that I would not recommend (particularly if you are not an electronic engineer....).
I am currently playing with bypass caps for my newly rebuilt S500 Series II. The main power caps are now brand new Epcos 33,000 uf caps, which have truly taken the bass to a new level. My difficult load 4 ohm speakers have bass like never before.

Good news !
And I suppose things will continue to improve as the caps, resistors and new parts all break in a bit.

Yes
Questions:
1.) Are there any cap types (PIO, Teflon, tin foil/Poly) that are ideal for this, or any that I should stay away from? As I said, I have several and want to wire them all in // to Epcos to try them out.

I would first test the Vishay to see what the result is: you are not in the signal path (and are in an amp P.S.), so I do not see the need to go to the Auricaps.... You may try something else later.
2.) Stupid question: Is it safe to wire these various cap types directly to the big cap screws and power up? Hope so! Am assuming these are all bi-polar caps and so can be wired in either direction? How important is voltage level for this? I assume anything above the big cap voltage of 100VDC is ok?

Decoupling is decoupling, not black magic... but if you are not sure about what you do, I suggest you to ask/discuss with a technician: do not forget that you are working in a (big) amp power supply where a lot of Amperes are moving... If caps are screwed, take care to not reduced the quality of the electric contacts by adding the small caps: because the big caps deliver a lot of current and they do require a high quality and strong contact (this is one the reasons for which they have strong screws...).
3.) I read somewhere that you can experiment with .47uf to .01uf for differing levels of high frequency gain. Does anyone have any experience with this? Would love to hear what you all have tried and what works well.

despite what I have read, I am not sure that an universal rule exists regarding this question: you should try by yourself...
4.) Any suggestions for choices of caps? Anyone using Mundorf silver/gold or silver oil for this purpose? How about V-caps?

Once again you are not in the signal path but in the Power Supply area: no need to go to the exotic and ver expensive (although they are really excellent in the signal path and can largely improve the sonics when used in the signal path) caps such as V-cap for this use.

Hope this helps !