Author Topic: CP4500 - Not a true Mix knob?

May 22, 2021, 07:03:59 PM
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tomheavybeats

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Hi
I just bought the CP4500 to use on drum bus and one of the key features for me was the mix knob for parallel processing. On the ‘500 Love’ FB group someone was saying how it is not a true mix knob. To quote:


“…It has a mix knob, but it doesn't work as a dry/wet control. Rather as a "ratio scaler", from full nominal ratio to none. Which is absolutely NOT the equivalent of mixing dry and wet…

Just analyse the schematic, you'll see that mix knob is simply a voltage divider on the CV (Control Voltage) to the main VCA, meaning you go from full CV effect to no CV effect.
This translates to lowering ratio and makeup, definitely not mixing dry and wet.
Imagine doing drums parallel compression, which is mixing dry to heavily compressed. This circuit just doesn't do that”


Please can you comment on whether this is true?
Cheers!
Tom
« Last Edit: May 24, 2021, 10:50:05 AM by tomheavybeats »

May 24, 2021, 09:33:30 AM
Reply #1

JPK

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This statement is totally false. Our mix knob does exactly what it says, a mix between the input signal and the compressed signal. To do this, it cleverly uses the compressor's own VCA instead of an additional mixer stage. The compressed signal is totally unaffected by the setting because the CP4500 is a feedforward compressor. Even better, it doesn't add the noise and distortion of an ordinary blender
JPK

May 24, 2021, 10:43:36 AM
Reply #2

tomheavybeats

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Many thanks for clarifying, I will post your reply to the FB group too so it can assure other users

Cheers
Tom
« Last Edit: May 24, 2021, 10:49:22 AM by tomheavybeats »

June 01, 2021, 09:59:38 PM
Reply #3

NewWaveHesperian

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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

June 01, 2021, 10:19:59 PM
Reply #4

NewWaveHesperian

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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).



June 03, 2021, 04:23:56 PM
Reply #5

JPK

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Quote
Please note that I mean no disrespect or bashing of your product/brand
No issue here, you are welcome.
Quote
This simply translates in a modified ratio and modified makeup gain setting depending on the position of the pot.
This is exactly what also does an external mix between a compressed and dry siganl.

What you say is true but the fact is, it works.
During the product development I started by a classical blender with a double pot. Then I had this intuition that this solution could work: After all, the pot goes linearly from a 100% dry to 100% compressed signal. What happens in between? So I tried it and it worked. There is a difference though in the pot law which is different from a traditional blender. In particular it doesn't adds the 3 to 5dB in the mid position.

Quote
a parallel compression will sound completely different than if done with a traditional mixing circuit.
Please listen :
One is a mix 50/50 of the dry signal and compressed signal (mix pot on 100%).
The other is the same compressed signal with the mix pot at 50%.
Can you tell which one is wrong?
https://www.soundskulptor.com/audio/drums-mix50-a.mp3
https://www.soundskulptor.com/audio/drums-mix50-b.mp3

Dry drums: 
https://www.soundskulptor.com/audio/drums-dry.mp3
Compressed drums with mix knob @100%:
https://www.soundskulptor.com/audio/drums-compressed.mp3


JPK

June 03, 2021, 09:17:34 PM
Reply #6

NewWaveHesperian

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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).





June 05, 2021, 03:51:24 PM
Reply #7

JPK

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Yes A is CP4500 mix.
Does it sound completely different than if done with a traditional mixing circuit?
JPK

June 07, 2021, 01:07:46 AM
Reply #8

NewWaveHesperian

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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.