1176: attack time wrong on the manual?

[ElectronWorks]

On the 1176 manual the attack time range is rated from 20 to 800 uS
By looking at the schematic from Gyraf audio if the attack potentiometer (R77) is set on the slowest setting we get 25k series resistance after the rectifier
With C27 (220nF) the RC time constant is equal to around 5.5 mS
I really can't see how C27 is getting the extra current for 800uS at slowest attack
I then simulated the side-chain circuit in spice and the fastest attack possible is also higher at around 100uS
did the calculation again for the release and it's the same as the manual

Is the attack on the manual wrong or i'm missing something?

 

[gyraf]

your time constant is for a change in control voltage, where attack timing (which is not a well-specified parameter, no consensus about how it's best quantified) may be defined at gain changes needing much less than this?

 

[ElectronWorks]

Yes, something similar to what you're describing is happening here

Today I measured my KlarkTeknik 76-KT (not the best specimen, i know) at slowest attack + fastest release and these are the results;
Near full attenuation it is 5mS, close to the predicted attack, but using the method for measuring rc time constant (36.8% of the full signal) is closer to the manual at around 500uS.
I think the JFet non-linear cv control and the feedback topology is the main cause of this difference
But I'll test a original unit when'll get the chance

 

[Bo Deadly]

Those timing numbers are not well defined. Consider that 20 uS is 1 / 50 th of one cycle of a 1kHz sine tone. So what exactly is happening in 20 uS? The voltage deviated 10%, 50% or what?

 

[Bo Deadly]

Today I measured my KlarkTeknik 76-KT (not the best specimen, i know) at slowest attack + fastest release and these are the results;

If it's the same circuit (and I strongly suspect it is), I would wager that the KT clone is a perfectly good specimen (UREI was not known for using the highest quality parts).

Note that if you're trying to make measurements like this, an oscilloscope is preferred IMO to rule out the possibility that your audio interface isn't obscuring things (it won't capture DC offset).

And, if you're going to post plots, we really need to know the axis divisions (does that say 2mS / div?) and where your markers are and so on.

 

[ElectronWorks]

If it's the same circuit (and I strongly suspect it is), I would wager that the KT clone is a perfectly good specimen (UREI was not known for using the highest quality parts).

Note that if you're trying to make measurements like this, an oscilloscope is preferred IMO to rule out the possibility that your audio interface isn't obscuring things (it won't capture DC offset).

And, if you're going to post plots, we really need to know the axis divisions (does that say 2mS / div?) and where your markers are and so on.

Trace info is tiny but is there, no problem with DC i wasn't measuring CV but an audio impulse of the unit using REW

Today i had the time to take it apart and traced the sidechain, unfortunately it's not the same as the original schematic, it's apparent that they used components already avaiable in their stock to cut some costs, in the next days i'm going to mod it, i'll make a new post.