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Altec 1591A attack rate: how to quicken?
Old 12th October 2013
  #1
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Analok's Avatar
Altec 1591A attack rate: how to quicken?

Okay, so I aquired an Altec 1591A in operating condition. Because of its' design circa, it has a fixed attack rate of 30-35 milliseconds...a bit slow for today's standards. Found an old posting from Mr. JW stating that it can't be increased due to limitations of the optoresistor - this I get.

What I'm contemplating is that, as the optoresistor sees more light it's resistance value increases, thus signaling the call for compression until saturated and then falls again at the rate of decreasing light. The attack rate is then dependant on when the optoresistor reacts to it's light source. Assuming that the sweep of resistance is dependant to a 0-5VDC source to the lightbulb: Couldn't one alter the voltage to the bulb so that it rests at, say...0.7VDC, thus decreasing the resistance sweep at the head of operation at the same time decreasing attack time? In essence this would also have a slight 'full time duty call' on the compression circuit, but providing the resistance is near nill it shouldn't be audible, correct?

Tell me what you think: Is this feasible? Is there a better way (such as perhaps a tiny valued 1% resistor across the optoresistor)? Any & all comments greatly appreciated. Thanks in advance, 'Slutz.

Last edited by Analok; 12th October 2013 at 11:24 PM.. Reason: technical corrections
Old 12th October 2013
  #2
Well, you could buffer and wire in parallel an LED/photodiode or phototransistor....but you would need to provide the support circuitry, as I'm reasonably sure the 1591A is voltage driven and the phototransistor is fundamentally a current driven device.

You could also experiment with exchanging the light source.... I've looked at the schematic online here, see the LDR, darlington pair close by, Schottky diode upper right from that, but too damn blurry with 5 beers in me to see what's driving the LDR....

Have a clear schematic? Jim Williams states it can't be done...but he might mean can't be SANELY done...
Old 12th October 2013
  #3
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Hi
Optoresistors resistance DECREASES with increasing light level.
You could configure an LED as well as the lamp for interesting results.
Matt S
Old 12th October 2013
  #4
Quote:
Originally Posted by Matt Syson View Post
Hi
Optoresistors resistance DECREASES with increasing light level.
You could configure an LED as well as the lamp for interesting results.
Matt S
Good catch....I didn't see he posted that.
Old 12th October 2013
  #5
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Ahhhhh, it DECREASES! Well then that puts a shelf limit on things, doesn't it? I came back in here to edit my post for two reasons (volts are DC rather than AC; thought was to add resistance in series instead of across the opto load-that's pretty much moot now). I'm not looking to delve into LED implants ATM: too touch & go compatability for the vintage of this gear. Thanks for the advice just the same

Ron C has a copy of the manual/schematic on his website (link below FYA). Could anyone speculate what affect trim pot P4 would have on the opto circuit? It's a voltage controlling 5Kohm 2W with 6.2VDC coming into it, and it threads down on into the opto lamp that seems to be idling at 0.5VDC already. There's other circuitry around it that I'm not fully comprehending, how it flows & such. Last thing I want to do is fry the opto bulb, cause these things are as rare as straight politicians!!

http://www.bayoucables.com/altec/Alt...1A-CompAmp.pdf

Again, thanks for everything thus far, guys. Proof that the GS forum still rules!!

Last edited by Analok; 12th October 2013 at 11:19 PM.. Reason: grammar
Old 13th October 2013
  #6
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wow, that thing takes time to download the schematic!
Am I getting it from someone on a phoneline connection?

ok, c6 : change to a lower value like 1uf. this should be the course timing cap to the detector (attack) . c15/c16 is the release circuit. lower is shorter time on both.

the rc branch that is R54/c13 is a timing circuit to fine tune attack based on the leading edge of the signal. lowering c13 will trim it back if it is too pumping/breathy. the resistor effects how much of a signal is passed or repress though rc time constant of the c13. if I modded and made this adjustible, it would be a c13 of .1 and the R54 at 2K trimmer. c6 would be selectable as range.
Old 13th October 2013
  #7
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but to do things by the book, altec would have to install a switch that would select different c6's and r54/c13 branches.
Old 13th October 2013
  #8
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this might explain the variables
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Altec 1591A attack rate: how to quicken?-altec-1591abreakdown.jpg  
Old 13th October 2013
  #9
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Wow, Dr.T! This all makes some sense and your mod suggestions really have the urge to heat up the ol' Weller going! Big Thanks!! All apologies for that download being so slow (it was after all the entire manual rather than just the schemo). Best to save it now before it comes down forever.

Just for giggles I went ahead & took voltage measurements at the opto bulb in rest mode - was reading low at 0.23VDC. So, I'd gone ahead and tried tweaking that P4 trim pot and sure enough it altered the voltage. Pulled it up to around a stable 0.68VDC. Attack does seem to have improved a touch, most likely because it was initially calibrated/drifted under spec. The more I toy with this boar the more I like it!! Compression you can taste

Thanks again, Doc!!
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