AMEK BC326 limiter attack/release mod

Hi there!
I own an AMEK BC3 (BCIII) console fitted with a pair of BC326 mono sub-groups strips with limiters.

I want to mod the limiter circuit to be able to change attack/release times as they're set for broadcast limiting (very slooooooooow release).

Ideally, I would make those switchable with timings closer to what an SSL bus-comp can do (for example: attack 0.1, 0.3, 1 ,3 ,10, 30 ms and release of 0.1, 0.3, 0.6, 0.9, 1.2 seconds).

Here is a schematic of the original AMEK circuit I got from the manual: https://imgur.com/ulhZHAA

Here's more about the limiter circuit named CB 888: https://imgur.com/ISLPdXh

I would assume the answer to my problem can be found in values of C22, C23, R64, R58, maybe more.

The rest is beyond my engineering knowledge...

Thanks for your help and input!

Yeah, C22 and C23 and the resistors in parallel with each (can't make them out) are for attack and release respectively. The transistor sinks current (and is linearized by the other in the feedback path of the op amp) and acts as a rectifier. There's a little bit of a nontrivial interaction in how they're stacked but it's not clear that it will matter to your mod as long as the attach is always a lot shorter than the release. I would change the caps to the smallest that you want and then run a shielded two conductor wire out to your switch panel. One conductor connected to + of the op amp, the other to the intersection of the two caps and the shield to ground. Then on the switch panel you switch in progressively larger caps / smaller resistors in parallel to get longer timing (you'll have to just make a little test board and send some pulses through and do some analysizing to determine the exact cap / resistor values). If you draw up a schem of the mod and post it, I think other people will be more inclined to comment. I know nothing about AMEK gear specifically.

squarewave said:

Yeah, C22 and C23 and the resistors in parallel with each (can't make them out) are for attack and release respectively. The transistor sinks current (and is linearized by the other in the feedback path of the op amp) and acts as a rectifier. There's a little bit of a nontrivial interaction in how they're stacked but it's not clear that it will matter to your mod as long as the attach is always a lot shorter than the release. I would change the caps to the smallest that you want and then run a shielded two conductor wire out to your switch panel. One conductor connected to + of the op amp, the other to the intersection of the two caps and the shield to ground. Then on the switch panel you switch in progressively larger caps / smaller resistors in parallel to get longer timing (you'll have to just make a little test board and send some pulses through and do some analysizing to determine the exact cap / resistor values). If you draw up a schem of the mod and post it, I think other people will be more inclined to comment. I know nothing about AMEK gear specifically.

Click to expand...

Thank you squarewave.
If I can find the values for the caps and resistors in a SSL bus comp attack/release circuit, would they automatically fit in THIS circuit?

furn1979 said:

Thank you squarewave.
If I can find the values for the caps and resistors in a SSL bus comp attack/release circuit, would they automatically fit in THIS circuit?

Click to expand...

No. The time depends on resistance and not just capacitance and the "stacking" of the caps / resistors makes comparison even less clear. But with some study of the two schems someone (other than me) could probably figure how what the corresponding cap / resistor values would be. Personally I would just run tone bursts through, look at the timing on a scope and come up with parts that generate appropriate times. You have to test it anyway right?

squarewave said:

I would change the caps to the smallest that you want and then run a shielded two conductor wire out to your switch panel. One conductor connected to + of the op amp, the other to the intersection of the two caps and the shield to ground. Then on the switch panel you switch in progressively larger caps / smaller resistors in parallel to get longer timing (you'll have to just make a little test board and send some pulses through and do some analysizing to determine the exact cap / resistor values).

Click to expand...

What you're suggesting here is a single control for both attack and release? Would it be possible to have independent control over the two functions in this circuit?

Thanks again! Will post a draft of a circuit soon!

No you would have separate controls.

For attack you install the largest resistor on the board and then use the control to parallel increasingly smaller resistors (but leave the slowest position open because that's defined by the resistor on the PCB). The smaller the resistor the faster the attack. I don't fully understand what the stacked cap is doing. I guess it gives a fast limited attack of some sort. If you want a more conventional attack, I think it would be ok to just leave that cap out.

For release you install the smallest capacitor / largest resistor and then use the control to parallel larger capacitors / smaller resistors to reduce the time (and again leave the slowest postion open because it's set by the cap / resistor on the board).

But I don't know if I would crap up the panel with two knobs. You might use two three position toggles to switch between three settings each for attack and release. Use an ON-ON-ON toggle so that the switch positions can be in order. That might make for a cleaner and simpler build. Get some high quality solder lug type toggles and just solder the caps / resistors directly to the lugs.

Incidentally the wires don't really have to be two conductor shielded. It could just be a bundle of three 26AWG for example.

Again, if you're not sure about the circuit, just post a schem first and I'll sanity check it.

That’s great! Will keep you posted!

My BC3 has the stereo sub group strips with a pair of two channel comps .

For some reason Imgur didnt like my computer so i couldnt see the schem's ,
Could you post the them again in the technical documents section here on Gdiy ?

Tubetec said:

My BC3 has the stereo sub group strips with a pair of two channel comps .

For some reason Imgur didnt like my computer so i couldnt see the schem's ,
Could you post the them again in the technical documents section here on Gdiy ?

Click to expand...

It seems I'm unable to upload my files directly to GroupDIY. I get an error with either jpg or pdf files (which are under 4000KB).

Tried here and in the technical documents section without success.

Oh well not to worry , I'll have another cut off Imgur tomorrow, maybe different browser or something .

Both bays of my desk came equipped EDAC's , took a while but I eventually found some of the TRS jack panels.
I still need 6 or so more of the jack back panels if you happen to have any spares you might let me know . I have a bunch of extra stereo input modules that are surplus to my needs.