freestompboxes.org

Once again, hello and happy new year.
I am already finishing exploring analog effects technology, only the VCA compressor and noise maker remained (I will ask about this later) and urei1176.
I liked the Keeley compressor pro and Bondi FX 2026 pedals, they have a full set of adjustments and look pretty versatile, and their sound is very transparent (this is good and bad at the same time).
I read a lot of information, but I didn’t find a detailed answer about the Attack and Release adjustments.
There are 3 schemes (maybe more, but I found only 3).
The first 2 are presented on the THAT website and are powered by transistors (circuit 1) or the original THAT chip (circuit 2), but in general they look almost identical. There is also an informal scheme using dual-channel Op Amp(circuit 3).
Someone had experience of assembly or is familiar with the operation of these schemes? What's better?
The third scheme looks very simple, but it seems to me that attack and release will affect each other (if this is true) and the time of attack and release are identical, which is pretty bad, can it be fixed.
The second circuit is simple, but the original chip consisting of four transistors is quite expensive, can it be replaced with 4 matched transistors? If so, which parameter to look at? HFE?
The first scheme is the most difficult, but attack and release are independent, the price is low, but some components are not so easy to find (18M resistor).
Tell us which scheme will be the most correct?

Hi POTL,
I can't tell you which is the most correct, but I can help you understand them

for the 2 transistor based circuits, they are exactly the same concept but with differences in implementation. they are both effectively 2 constant current sources that are switched into a capacitor. the current is controlled by pots, and they are switched on and off by comparators.
in the simple circuit, the current in the current mirror is controlled by an emitter degeneration resistor, which degrades Vbe of the output transistor, while in the more complicated one, it degrades the Vbe of the controlling transistor.

the mirrors with 3 transistors are called "emitter follower augmented" mirrors. they have lower error due to base current, which is probably significant when you are setting the mirror current with an 18Meg resistor. depending on your requirements, you could increase the mirror current and the capacitor to get the same slew rate, since i=Cdv/dt.

in both circuits, the matching will have more of an effect on change over temperature than anything else since you are trimming over a reasonably wide range. how much do you care about temperature variations? how much do you care about the extremes of the range being identical for attack and release?

note that for the quad transistor package, you probably only care about matching within the current mirrors. any mismatch between pairs could be taken care of by trimming 680k current setting resistor.

for the op-amp based one, the attack and release are separate because of the diodes. the first op-amp is open loop most of the time, so is acting like a comparator and hitting the rails. when it does so, the diodes block one or other of the pots from conducting, and the other one completes the circuit depending on whether the output is slewing up or down.

the op-amp circuit is a voltage follower, while the other 2 will just slope up or down untill the cap charges up to the maximum headroom. this means if you need a controlled output voltage after the slewing, you may prefer this circuit.

the op-amp circuit will also not charge linearly, because its basically a squarewave into an RC circuit. if your input signal is significantly less than the power rails it will seem relatively linear, but if you get too close to the rails it will tail off exponentially.
from the values, and the comment that maximum slew = 500ms, it looks like this circuit was designed for about 0 to 1.5V input signal with +/-15V rails

what are your requirements for this circuit? what are your priorities (small / cheap / linear / repeatable / etc)

good day
I understand you, then we will not take into account the circuit with operational amplifiers.
Although a superficial inspection of the keeley compressor pro says that this option is used in this pedal.
For me, affordability and stability are important.
In the documents THAT does not indicate that the transistors require matching, but at the same time, they offer their own chip with matched transistors, perhaps the matching is not needed.
I also read in the documentation that most of the resistors in the circuit with bull transistors are used to give the correct characteristics to the pot.
However, in the circuit with their chip there is no diode bridge.
The circuit with the chip looks attractive and there are no rare nominal components in it, I wonder if it can be assembled without the original chip and if the transistors need to be matched by what parameter.