Spectra-Sonics 701 another odd amplifier - diyAudio
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Old 25th January 2013, 11:09 PM   #1
amp_guy is offline amp_guy  United States
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Default Spectra-Sonics 701 another odd amplifier

As I was thinking about the gear that had truly blown me away with sounds I recalled the S-S 701 that crossed my bench back in the mid eighties.
I actually met the designer Mr. Bill Dilley along with Ray Kimber who later had the documentation for the S-S line of studio gear. These took the form of a circuit card, part of a rack with a common power supply unit, intended to drive a biamped or triamped speaker system. At one time I think I had a diagram of this odd circuit but If I did it is now gone. I just did a search and found this http://www.waltzingbear.com/Schemati...nics701amp.pdf
( I only post the link here because it is someone else's property)
Not entirely sure where he got it and if it is entirely correct. Seams like the boards in the rack we had there had some variations in resistor and capacitor values as though there had been some tweaking per board...but that was a long time ago and maybe a figment of my faulty memory.

Anyway Comments?
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Old 25th January 2013, 11:26 PM   #2
wahab is offline wahab  Algeria
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The VAS 47K load was surely reduced to a few K....or replaced
by a bootstrap.
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Old 26th January 2013, 01:23 AM   #3
amp_guy is offline amp_guy  United States
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More interesting perhaps is the 1n695 diode connected to the 47k, as well as the 1n5400 diode connecting the upper set of 2n6043 outputs to the lower set of 2n6043 outputs.
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Old 26th January 2013, 01:31 AM   #4
amp_guy is offline amp_guy  United States
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And you may want to compare the topology with this.
http://www.passdiy.com/pdf/classa_amp.pdf
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Old 26th January 2013, 01:35 AM   #5
amp_guy is offline amp_guy  United States
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And there are some resemblances to some servo torque motor drives I have worked with. But I assure you these did produce some very musical sounds from a pair of nice JBL L300's.

Last edited by amp_guy; 26th January 2013 at 01:37 AM.
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Old 28th January 2013, 03:16 PM   #6
amp_guy is offline amp_guy  United States
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Default single ended class a history

O.K. then I will shut up , No one is interested in the early days of single ended class A power amps.
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Old 24th February 2013, 03:04 AM   #7
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I was just talking to friend today who was a friend of Bill Dilley and Ray Kimber. He was telling me that the Spectra Sonics amplifier was a differential output topology. Unlike most power amps that are differential input these were differential output. I would love to see the actual schematic for these designs. I will ask my friend if he happens to have it, but I would doubt that he still does. He said it was and still is one of the best amplifiers with this topology,
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Old 24th February 2013, 04:04 AM   #8
amp_guy is offline amp_guy  United States
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If you could locate an accurate schematic and permission to share it that would be great! When I had one on my bench for repair back in the 80's it was the best amp I had ever heard. if I remember right it didn't give spectacular readings on my sound technology 1700b thd analyzer, but the sound it was great!
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Old 2nd February 2014, 05:55 AM   #9
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Old thread, but considering the age of the equipment, not so old after all. I used to install Spectrasonic amps. More often the larger frame (I forget the model number) that would hold eight 701 cards. But also, occasionally, the 710 frame, which had a black front plate, and held only four cards.

I found one of the 710's a few weeks ago, headed for the garbage, and rescued it. Replaced a few capacitors and one burned up resistor, and she is back in action. Woo hoo. I always LOVED these amps. They are about as close to a tube amp as solid state can get, in the way they sound.

A few notes after some of what I read . . . They did indeed have a differential output. A push/pull circuit design just really makes speakers sing compared to push only -- like so many amps are, with just positive and ground on the output.

The variations you mentioned in resistor and cap values is true. These were hand built. The amps were tuned by hand at the factory, by setting the bias to each transistor individually. Because of variations in components, this meant sometimes different values were used to bias the transistors. For resistors, I think they put a potentiometer in the circuit, adjusted to optimum levels, then replaced the pot with a fixed value of what the pot was set at. So they were all slightly different.

One of the biggest problems with these amps, was the power supplies hummed a bit. I am currently in the process of removing the internal power supply, and converting it into an external power supply, which I will relocate under my house in the crawlspace, with the +/-24vdc and transformer center tap (ground) coming in on a cable.

Enjoyed your thread, if you guys ever return here.

Grounding was also a bit of an issue with these at times, since they use a differential power supply and output a differential output. Easy to screw that up and let the smoke out. The bad part about the 710 frame over the other one who's model I can't recall, is that the 710 is unbalanced audio in, on little RCA connectors. Which works fine for my current purposes. The bigger frame used a balanced audio input that you soldered onto the backplane.
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