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Old 20th November 2010, 10:33 AM   #1
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Default Threshold S/500 stable at low loads?

Friend of mine listened to my setup with Acoustat 2 + 2 driven by a pair of SA/1's
He liked the sound but these amps are hard to get nowadays.
He has Acoustat Six'es that have two interfaces per channel and in parallel mode they throw a not so manageble load of 3 Ohms nominal at the connected amp. He already fried some inferior monoblocks and now drives them with PA quality amps that sound horrible.
I wonder if a Threshold S/500 could drive his Acoustat Six'es in parallel mode.
Krell always made a salespoint about their stable performance at very low loads but in the older salesliterature from Threshold a 4 Ohm rating was advised as the lowest permissible load. Some German reviews from the eighties regarding the Threshold SA/2's and S/300 resulted in blown railfuses when they try to perform some measurements with a 2 Ohm load. Even in the January 1987 Audio magazine where Leonard Feldman tested the SA/1 very favorable he stated: "Although I did perform some measurements of power output at 2 Ohms, it soon became clear that if I tried to push the amplifier to its limit with that type of load, I would blow its fuses. Then Nelson Pass supplied graphs that the SA/1 could drive those loads with a 600 Watts ouput at 1 KHz.
So I'm a little reluctant to advise a Threshold S/500 for duties on a Acoustat Six. Two of them would be better because a 6 Ohm load would have to be driven then but that would be a step to far for him.
Maybe a Krell KSA-200S or two old KMA-100's would do a better job but he liked the sound of my Thresholds on Acoustat so much and I told him a little about the designphilosophy of Nelson Pass that he found very attractive...
Any advice?

Last edited by RobertS61; 20th November 2010 at 10:36 AM. Reason: syntax
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Old 20th November 2010, 11:35 AM   #2
Zen Mod is offline Zen Mod  Serbia
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let them bleed !

at least .... after you hear what Papa will tell .......

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Old 20th November 2010, 11:41 AM   #3
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Originally Posted by Zen Mod View Post
let them bleed !

at least .... after you hear what Papa will tell .......

You mean the railfuses or the powertransistors by putting in heavier fuses?
Or just his ears...
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Old 20th November 2010, 11:45 AM   #4
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everything will bleed one day - with load like this



( I was serious only with part - wait for Papa's comment )
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Old 20th November 2010, 12:48 PM   #5
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Originally Posted by Zen Mod View Post
everything will bleed one day - with load like this



( I was serious only with part - wait for Papa's comment )
Okay, let's wait.
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Old 20th November 2010, 03:28 PM   #6
a.wayne is offline a.wayne  United States
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The standard rail fuses on the s500 will have no issues at 3 ohm . I had upgraded mine to 10 amp/ch but kept the main line fuse @ 15 ..

I'm running a Pr of S500's @ 1 ohm nominal , no issues , i just watch the temps and allow them time to cool if necessary . I did try one on a friends Maggies, 1.6 , it did drive those with no issues, barely got warm ,I'm confident in saying the 3 ohm load of the Acoustats to be of no consequence....
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Old 20th November 2010, 05:05 PM   #7
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Originally Posted by a.wayne View Post
The standard rail fuses on the s500 will have no issues at 3 ohm . I had upgraded mine to 10 amp/ch but kept the main line fuse @ 15 ..

I'm running a Pr of S500's @ 1 ohm nominal , no issues , i just watch the temps and allow them time to cool if necessary . I did try one on a friends Maggies, 1.6 , it did drive those with no issues, barely got warm ,I'm confident in saying the 3 ohm load of the Acoustats to be of no consequence....
Thanks for your reply.
I've had Magnepan Tympani's I D (best low end I ever heard with a pair of Krell KMA-200's) but they same to have a pretty resistive load of 4 Ohms I understand. Maybe the ribbon tweeter is somewhat more reactive.
I know that I can clip my Thresholds SA/1 with the Acoustat 2 + 2's.
Before that I've had a Levinson ML-3, Rowlands model 7 and KMA-200's on them and never reached their clipping points when driving the Acoustats.
The Sixes have two electrostatic elements more per channel then the 2 + 2 and are therefor 1 or 2 dB's more efficient but paralleling the two interfaces gives a nominal impedance of 3 Ohms. I know it's not a Scintilla or a Dayton Wright electrostatic but they do present a pretty tough load. The first monaural amps he tried on them literally melted away.
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Old 20th November 2010, 06:45 PM   #8
a.wayne is offline a.wayne  United States
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I cant even to begin to tell you of the pretenders out there and i did try a Krell KSA250s before, it did make the S500's sound like 50 watt amps by comparison. The Krells capture the power and the pace of live music the threshold the soul ....


Rowland 7's and a PFB600 are what's in sight ..... The search continues ............

Last edited by a.wayne; 20th November 2010 at 06:50 PM.
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Old 20th November 2010, 08:46 PM   #9
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Originally Posted by a.wayne View Post
I cant even to begin to tell you of the pretenders out there and i did try a Krell KSA250s before, it did make the S500's sound like 50 watt amps by comparison. The Krells capture the power and the pace of live music the threshold the soul ....
Isn't there an amp capturing both of these features. Krell, Rowland and Levinson always were somewhat more generous with the supplied hardware in comparison with Threshold/Passlabs.
Why not a 2 KVA toroidal in a SA/1 and S/500. Their contemporaries like the Rowland Model 7, Krell KMA-200, ML No.23 (dual mono 2 x 1.2 KVA actually), Krell KsA-100 and Rowland Model 5 all had bigger powersupplies. Nelson Pass even stressed the point of having big and stiff powersupplies in his article:
http://www.passlabs.com/pdfs/articles/powersupply.pdf

Rowland 7's and a PFB600 are what's in sight ..... The search continues ............ [/QUOTE]

I can recommend the Rowland Model 7's. Had Series II and Series IV of these beautiful ceafted, musical sounding and very powerfull amps.
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Old 22nd November 2010, 09:00 PM   #10
a.wayne is offline a.wayne  United States
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Originally Posted by BrianEno View Post
Isn't there an amp capturing both of these features. Krell, Rowland and Levinson always were somewhat more generous with the supplied hardware in comparison with Threshold/Passlabs.
Why not a 2 KVA toroidal in a SA/1 and S/500. Their contemporaries like the Rowland Model 7, Krell KMA-200, ML No.23 (dual mono 2 x 1.2 KVA actually), Krell KsA-100 and Rowland Model 5 all had bigger powersupplies. Nelson Pass even stressed the point of having big and stiff powersupplies in his article:
http://www.passlabs.com/pdfs/articles/powersupply.pdf

I can recommend the Rowland Model 7's. Had Series II and Series IV of these beautiful ceafted, musical sounding and very powerfull amps.

I have thought about modding the S500 PSU, but in the end i did not think the PSU mods alone would get me there ....

Last edited by a.wayne; 22nd November 2010 at 09:03 PM.
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