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Old 22nd April 2009, 12:08 AM   #11
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Let's see,

Dan D'Agostino was only involved with Threshold by virtue of using
his offices at Dayton Wright to help with distribution in the early
years '75 to about '79. By the time the silver amps were designed,
he was busy with his own silver amps at Krell.

The magazine photo was in Stereo Sound and the living room shot
was Rene's, and the pic was published in '81.

By the way I do have a CD chock full of this stuff.

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Old 22nd April 2009, 08:32 AM   #12
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I reckoned it was Rene's because in interviews you aways seemed to have setups with the smaller Thresholds (probably having more efficient speakers)
and Rene the bigger amps.
I saw some references at a www.passlabs.com/ np or /threshold dir at this forum but they were all deadlinks now you can't keep them forever of course. Together with a friend I'm in the progress of developing a highendclassics site with American audio equipment that defined the highend scene.
So I'm really interested in that CD.

Jon Soderbergh filled in the gap between the Series I and II versions and stated that the update dealed - apart from a rearrangement of the connectors at the back for the S/1000 - mainly with the audio-board to get a better grip with biasdrift and thermal runaway problems, that were definetely settled with the optical bias versions. I think all the designers of poweramps had their problems in the seventies and eighties to keep the bias at a stable level.
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Old 30th April 2009, 02:37 AM   #13
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Default Biasproblems

Again, this is the second pair in Germany I encountered that has been modified from a Bridged configuration (a S/1000 setup) to a massive parallel configuration (like a SA/1) One of the audioboards has been taken out and the other one has been replaced by a goldcolored board. Railvoltage is 62.3 Volt good for a 170 Watts at 8 Ohm instead of the bridged 500 Watts rating of a S/1000.
The bias I measured over several emmitorresistors was very low somewhere between 4 and 6 mV. The coolingfins did not even become lukewarm.
Of course I tried to correct that but the blue Bourns oneturn trimmer for that purpose was a pain in the ***. Only a really small fraction of a turn gave a increase to 200 mV and the toroid started to hum immediately. I managed to get a value for both amps at approx. 90 mV (over a 1.3 to 1.6 Ohm resistor) resulting in 60 mA biascurrent multiply that by 40 (transitors)gives 2.4 A = 46 Watts Class A at 8 Ohms. The coolingfins became pretty warm but certainly not to hot to touch and you could leave them there for more then ten seconds without real discomfort.
The I put the amps back together, top- and backplate turned them on again and the became only luke warm. I took off the lid again measured the bias and found out that it had sunk to 35 mV. I led the toplate of and slowly it started to rise again to 60 mV.
The next day I coldstarted them and and they both warmed up within 30 minutes with both plates on they became pretty warm/hot. Opened the topplate again and measured 100 mV.
Ergo I see no way to stabilize the amp temperature/biascurrent because I don't no what to expect and even with only the backplate of and a setting of 90 mV obtained within half hour warming up time, then switching of and as quick as possible attaching the backplate in 5 minutes and switch the amp back on again, sees a sharp drop in temperature again as stated above. Should I start with 130 - 150 mV and hope that after putting both plates on again the settles for 90 mV?
To get the specified 160 Watts Class A I would need 4.5 Amperes of biasing 112 mA per transistor. The hardware and the configuration is the same as a SA/1 so I pressume it should be possible.

Hope that anybody can give me a advice how to proceed with this matter.
It sounds by the way very nice on my set.

Thanks in advance
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Old 30th April 2009, 06:40 PM   #14
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It's a little difficult to say, given that the amplifier may have a new
unknown front end board.

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Old 30th April 2009, 08:02 PM   #15
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It's definitely a Thresholdboard with the number:
540305A probably an A at the end to indicate a revision.
I think Threshold supplied a couple of these boards to update a S/1000 (Series II) to a SA/1 for people that wanted the latest technology with these amps.
It's basically the same hardware rearranged.
The two larger boards with the 40 powertransistors on them has the same type coppertraces and lettertype to designate them. These start with 52xxxxx.
Jon Soderbergh advised me to switch the oneturn 5kOhm trimmer with a multiturn version from Bourns or Spectrol.
I deduce when the top- and backplate are of the chassis, a sensor detects a temperature fall and compensates this with a biascurrent increase. This consistent behaviour, the only problem is that I have to take the backplate of to reach the biastrimmers and it's a kind of guess at which position I have to turn the trimmer to get a satisfactory level when the amp is closed and has settled down.
I never experienced this sensisitive behaviour with other highend amplifiers when adjusting the bias.
From the center of the audioboard, a component with two black leads soldered to it, protrudes into a hole through the silver metal chassis (where the 20 transistors are mounted on) towards the coolingblocks. It is covered with a transparent kind of plastic. I reckon it's a temperature sensor to monitor a possible overheating situation or it has something to do with the biascircuit, or maybe even both.
Can you sir (or anybody else) relate to the mods or this kind of behaviour which I have pictured as precisely as possible.
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Old 30th April 2009, 08:59 PM   #16
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I can only tell you that the thing with two black leads and a clear case
is a thermal cutout, which opens when the amp overheats. It is not
intended as a bias control, although I suppose you could interpret
shutting the channel down as a method of controlling bias.

Who did the work modifying the amplifier?

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Old 30th April 2009, 09:57 PM   #17
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WBS Akustic Systems, Stephenshausen Germany your former distributor/importer for Threshold.
Got a letter from Russel A. Carnes dated feb 1988 that they were your authorized source in Germany in those days.
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