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Threshold S/500 stable at low loads?
Threshold S/500 stable at low loads?
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Old 22nd November 2010, 09:25 PM   #11
Nelson Pass is offline Nelson Pass  United States
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Threshold S/500 stable at low loads?
Then you need to buy a Krell.

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Old 22nd November 2010, 09:31 PM   #12
a.wayne is offline a.wayne  United States
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................................ All temporary until i save up for the x600's

Last edited by a.wayne; 22nd November 2010 at 09:34 PM.
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Old 22nd November 2010, 10:07 PM   #13
RobertS61 is offline RobertS61  Netherlands
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Quote:
Originally Posted by a.wayne View Post
I have thought about modding the S500 PSU, but in the end i did not think the PSU mods alone would get me there ....
Where?

The Heinemann On/Off rockerswitch (actually a AC Circuitbreaker) does not permit almost any size bigger as the already supplied 1000VA.
Toroids tend the get wider not higher when the VA rating increases.

I've seen the space that the 2.5 kVA toroids took in the chassis of the KMA-200 and Model 7 and the're huge.

I think with plenty of powertransistors (2 x 20 in your case) 4 big Mallory's a
2 kVA toroid could gave you the full power at 2 Ohms.
Ideally your S/500 should deliver 250 Watts perchannel @ 8 Ohms, 500 @ 4 Ohms and 1000 @ 2 Ohms. The 4 Ohm rating would be the RMS max because you have a 1kVA toroid that can deliver 2 X 500 Watts @ 4 Ohm (according to the salesliterature of Threshold the toroid could deliver two times it's nominal rating four a couple of minutes though).
With a 2 kVA toroid you would extend your low load drivingcapacity to 2 Ohms.
Bear in mind that each Threshold heatsink section of those days is good for dissepating 40 Watts contineously so on that basis alone 1000 Watts contineously would definitely overheat the amp. In real life though you will never need that kind of perfomance. You primarily need those heatsinks to waste of the Class A portion of 25 Watts RMS per channel for the S/500.

In a Threshold Technical Paper called: 'High-performance amplifier concepts' dated somewhere in the mid eighties (the paper gives Sacramento as the company homebase and in 1987 they were already moved to Auburn), Nelson Pass states under the Section: 'Power supplies, Are capacitors really the Boulder Dam?' that and I quote:
"Unlike the graphic images that have been portrayed by some manufacturers wherein power supplies are a giant reservoir that can be "drained" by a particularly large transient signal, power supplies maintain relatively stable equilibrium when properly designed. (....)
The power supply of the Threshold S/500 contains 120.000 uF of capacitance and uses a 1000 watt power transformer. With both channels driving up to 800 watt peaks into a 6 ohm loads, we see the power supply drop from a maximum of 80 volts to a minimum of about 76 volts, or about +0%, -5%)"
So maybe the powersupply of your Threshold is stiff enough.

Strange thing though is that in the article about power supplies mentioned before:
http://www.passlabs.com/pdfs/articles/powersupply.pdf
Nelson Pass starts in the second alinea with:
"The entire power circuitry of an amplifier can be seen as a community water system. The sun, driving the weather cycle, deposits water on the landscape, and it collects in a lake behind a dam."
I call that powerfull graphic imagery!
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Old 22nd November 2010, 11:49 PM   #14
a.wayne is offline a.wayne  United States
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Been there thought that !!!! ....

The trannies wont fit inside the case and you would want to lower the operating voltage for the lower loads a bit. This would require an outboard PSU and mod, mod, mod.. better to move on to bigger with the necessary sonics and then mod..

Papa suggested a pr of x600's so far ..... $$$$$ weak .......


Quote:
Originally Posted by BrianEno View Post
Where?

The Heinemann On/Off rockerswitch (actually a AC Circuitbreaker) does not permit almost any size bigger as the already supplied 1000VA.
Toroids tend the get wider not higher when the VA rating increases.

I've seen the space that the 2.5 kVA toroids took in the chassis of the KMA-200 and Model 7 and the're huge.

I think with plenty of powertransistors (2 x 20 in your case) 4 big Mallory's a
2 kVA toroid could gave you the full power at 2 Ohms.
Ideally your S/500 should deliver 250 Watts perchannel @ 8 Ohms, 500 @ 4 Ohms and 1000 @ 2 Ohms. The 4 Ohm rating would be the RMS max because you have a 1kVA toroid that can deliver 2 X 500 Watts @ 4 Ohm (according to the salesliterature of Threshold the toroid could deliver two times it's nominal rating four a couple of minutes though).
With a 2 kVA toroid you would extend your low load drivingcapacity to 2 Ohms.
Bear in mind that each Threshold heatsink section of those days is good for dissepating 40 Watts contineously so on that basis alone 1000 Watts contineously would definitely overheat the amp. In real life though you will never need that kind of perfomance. You primarily need those heatsinks to waste of the Class A portion of 25 Watts RMS per channel for the S/500.

In a Threshold Technical Paper called: 'High-performance amplifier concepts' dated somewhere in the mid eighties (the paper gives Sacramento as the company homebase and in 1987 they were already moved to Auburn), Nelson Pass states under the Section: 'Power supplies, Are capacitors really the Boulder Dam?' that and I quote:
"Unlike the graphic images that have been portrayed by some manufacturers wherein power supplies are a giant reservoir that can be "drained" by a particularly large transient signal, power supplies maintain relatively stable equilibrium when properly designed. (....)
The power supply of the Threshold S/500 contains 120.000 uF of capacitance and uses a 1000 watt power transformer. With both channels driving up to 800 watt peaks into a 6 ohm loads, we see the power supply drop from a maximum of 80 volts to a minimum of about 76 volts, or about +0%, -5%)"
So maybe the powersupply of your Threshold is stiff enough.

Strange thing though is that in the article about power supplies mentioned before:
http://www.passlabs.com/pdfs/articles/powersupply.pdf
Nelson Pass starts in the second alinea with:
"The entire power circuitry of an amplifier can be seen as a community water system. The sun, driving the weather cycle, deposits water on the landscape, and it collects in a lake behind a dam."
I call that powerfull graphic imagery!

Last edited by a.wayne; 22nd November 2010 at 11:52 PM.
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Old 23rd November 2010, 07:23 AM   #15
RobertS61 is offline RobertS61  Netherlands
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Quote:
Originally Posted by a.wayne View Post
Been there thought that !!!! ....

The trannies wont fit inside the case and you would want to lower the operating voltage for the lower loads a bit. This would require an outboard PSU and mod, mod, mod.. better to move on to bigger with the necessary sonics and then mod..

Papa suggested a pr of x600's so far ..... $$$$$ weak .......
Strange, for driving a low load the XA-160.5 would be much better, same hardware lower railvoltage better performance at low loads.
Threshold did the same with S/1000 and SA/1, S/1600 and SA/12.
S/1000 and the S/1600 were the bridged versions of the SA/1 and SA/12

[QUOTE]Then you need to buy a Krell.QUOTE]
The initial question was if a Threshold S/500 could drive a pair of Acoustats Sixes in parallel mode. I know that a Krell can probably do that.
Rougly the same question would be if a X350.5 could do that.
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Old 23rd November 2010, 04:03 PM   #16
a.wayne is offline a.wayne  United States
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The s500 will drive the acoustats ..
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Old 23rd November 2010, 08:36 PM   #17
RobertS61 is offline RobertS61  Netherlands
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Quote:
Originally Posted by a.wayne View Post
The s500 will drive the acoustats ..
Thanks Wayne.
I was already reassured that you stated in an earlier post that you use your S/500's to drive a one ohm loan (Apogee Scintilla's?)
I reacted on a post from NP: "Then you need to buy a Krell" because that indicated to me that Nelson lost track, due to our conversation about the size of powersupplies in US highend amps, what the initial question was.
As I indicated before I had a diverse scala of US amps driving my Acoustats and sonically the SA/1's came out on top of them with the JRDG model 7's as a close second. I already invested significantly in them by replacing all the original large can Mallory's with new ones from CDE and a faulty powertransistor and the complete Stasis section in one channel. In both amps I connected a MS35 Series Inrush Current Limiter from Ametherm to relieve the stress on the onboard AC Circuitbreaker and finally calibrated the bias for an heatsinktemperature of approx. 53 degrees Celcius.
I'm very satisfied with them.
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Old 23rd November 2010, 10:11 PM   #18
a.wayne is offline a.wayne  United States
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The SA1's are great amplifiers and beautiful .. congrats ....
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Old 23rd November 2010, 10:50 PM   #19
RobertS61 is offline RobertS61  Netherlands
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Quote:
Originally Posted by a.wayne View Post
The SA1's are great amplifiers and beautiful .. congrats ....
Thanks.
Sad thing is that on the market the're asking ludicrous amounts of money for them many times even above the 6000 USD new price.
These amps are 20 to 25 years old so there's a great chance that they need some refurbishing labor to get them on spec again.
Jon Soderbergh advises to replace the large can capacitors after 20 years of service and these are not exactly cheap.
It's basically a parallel mono version of the S/500 with a lowered railvoltage and a increased biassetting. Maybe it's the fascia with the meter that is attractive (Just like on the Stasis 1). When that's out of style again they become cheaper again
Have to replace burned out lightbulbs regulary so when I'm out of those tiny lightbulbs, I sell them....
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