1

CP4500 Stereo Bus Compressor / Re: CP4500 - Not a true Mix knob?

« on: June 07, 2021, 01:07:46 AM »

To me, it sounds different enough to tell them apart (if you know what to listen for).
I'm kinda surprised though as I was expecting much more of a difference.

I believe a lower mix setting (say 20%) would show a bigger difference.
On the traditional you'd still get the roomy brought up by the hard compressing. On the SS one, you'd just get a very mild full wet compression essentially.

It's a nice intuition what you implemented there. Unfortunately (again, IMHO) doesn't quite obtain the same effect.

2

CP4500 Stereo Bus Compressor / Re: CP4500 - Not a true Mix knob?

« on: June 03, 2021, 09:17:34 PM »

Hello,

very quick listen: I think A is the CP4500 mix knob, and B is the "DAW" mix (mix pot 100% and blend of dry signal + wet).

3

CP4500 Stereo Bus Compressor / Re: CP4500 - Not a true Mix knob?

« on: June 01, 2021, 10:19:59 PM »

Whoever has this unit can quickly verify the mix behaviour. 
Here's some calculations of what a traditional mix circuit should give as output, and what the SS circuit will give (happy to explain in detail the math behind it if needed):

How to test:

- Send 0 dBu to compressor input
- Set mix 100%
- Move threshold until you get an output of -10 dBu (do this precisely on the DAW meter, not on the analogue meter as not precise enough).
- Set mix to 50%
- Measure the output in dBu.



Given the parameters/test conditions defined above, a traditional mix circuit will give -3.63 dBu on its output.
The SS circuit will present around -5 dBu on it's output.

I don't have a CP4500 to measure. I just tried though on a unit with traditional mix circuit, and it validates the theoretical values (screenshot from audio analyser):





The best name I can think of that describes what the mix knob does on this unit is "Sidechain CV to Main VCA amount".
100% is the standard bus compressor operation.
0% forces the Main VCA to perform 0 dB gain.
50% halves the amount of gain reduction from the sidechain (e.g. -10 dB becomes -5 dB) and from the makeup (e.g. +10 dB becomes +5 dB).

4

CP4500 Stereo Bus Compressor / Re: CP4500 - Not a true Mix knob?

« on: June 01, 2021, 09:59:38 PM »

Hello, 

I am the one that raised the point about the mix knob not being a true mix knob.
Please note that I mean no disrespect or bashing of your product/brand, I only want to share my understanding of the circuit for the sake of user experience. 
I do not have a CP4500 but I've built several kits in the past, and really appreciated the care put into those items.
I did spot this issue from the launch simply by looking at the schematic. Got really surprised that after all these years no one really spotted this or even questioned the functionality as, by this implementation, a parallel compression will sound completely different than if done with a traditional mixing circuit.

Back to the issue: that circuit is not able to do mixing, period.
The only thing the mix pot is doing, in that particular position of the circuit, is attenuating the amount of control voltage sent to the main VCA. This simply translates in a modified ratio and modified makeup gain setting depending on the position of the pot.
If the pot is fully "wet", you get nominal ratio and nominal makeup gain.
If the pot is fully "dry", the VCA is fed with exactly 0V, which forces a gain of 0 dB. This doesn't mean bypassing the VCA, rather nulling the effect of the sidechain onto the main VCA.

Let's look at the issue from a different perspective: a mixer from a black box perspective is comprised of 2 inputs (dry signal and wet signal) and one output. Anything different than this, and you pretty much don't have a mixing block but something else.
For the way this pot is used in the circuit, I can only identify one input and one output, and none of these are audio signals anyway. How is this supposed to work?

I have some theoretical calculations to prove my point in trying to differentiate between the "SS" approach and a traditional mixing block, will try to post them later.
As mentioned already, I don't have a CP4500 so can't prove by measurement. But I do have a unit that has a traditional mixing block, and that validates perfectly my calculations.

it cleverly uses the compressor's own VCA instead of an additional mixer stage. 

Can you explain how? Go as technical as you wish.

The compressed signal is totally unaffected by the setting because the CP4500 is a feedforward compressor.

I don't see how this statement is relevant to explaining how the mixing operation takes place.

Even better, it doesn't add the noise and distortion of an ordinary blender

Irrelevant.



Happy to hear any comment from anyone regarding the topic.
Again, I mean no disrespect. I am only questioning the technical aspect of it.

Cheers