Another diode bridge compressor
- Thread starter Heikki
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Help Support GroupDIY Audio Forum:
In a compressor we don't want control voltage on the output. That's why when diodes or tubes are used as a variable gain element they are in a push-pull or bridge circuit. Here we are changing the current flowing through the diode bridge with the control voltage which results in a common mode voltage. Often this common-mode voltage is called common-mode thump. Transformer or differential amplifier needs to be used to reject this common-mode voltage. Since the diode bridge isn't perfectly balanced the control voltage doesn't end up being perfect common-mode voltage and some of it bleeds through to the output which can result in an audible "thump" sound.
It's a noise that happens when the compressor starts to compress. The side-chain sends a voltage that changes gain. It also is superimposed to the signal. Since it's applied symetrically to both phases of the signal, it can be eliminated by using a differential amp or a transformer. In practice, elimination is not perfect.
Thank you so much for your explanation.....
Luckily i think i didn t noticed on my proto
i ll do some measurement on REW....
so if i understand a solution is to add a sort of 2 stage inverting follower from sidechian to diodeBridge... right?
always thanks for your patience and attention
best
Luckily i think i didn t noticed on my proto
i ll do some measurement on REW....
so if i understand a solution is to add a sort of 2 stage inverting follower from sidechian to diodeBridge... right?
always thanks for your patience and attention
best
I'm not understanding what you are trying to solve?
Your compressor has a 3 op amp instrumentation amplifier after the diode bridge that takes care of the common-mode voltage.
Note that CMRR in opamp specs has very little to do with CMRR in a balanced connection.
Now i see the difference between the 2 projects and why one uses a transformer and the other doesn't. Thanks!
Take a look at the THAT corporation literature on this.
So this referred to the transformer and diff amps already in the schematics ? (Which are there to control/tame/reduce thump from being fed back into the SC, where it might trigger non-program material derived 'over'-compression.)
Cos like richiyobs I too first had thought it referred to the output of the SC before CV hits the diodes, and that driving those diodes differentially (BD140 --nah, DRV134--expensive) could somehow be beneficial, because... those diodes are fed in parallel so one side might draw a tiny tad more than the other.
Or instead of hooking diodes to ground, hook that side to CV-inverted, as in driving Ec+ and Ec- of a VCA ?
Thanks i ll check!
Best
That's a suggestion I made a couple of posts ago, however I don't think it's that useful in practice. Note that it's what the Siemens U273 does, albeit with a different bridge arrangement.
Do you have the "simplified" schemo. What did you simulated it with?
If LTspice, can you post the .asc file?
https://sound-au.com/articles/vca-techniques.htmlHere is the link... It was my starting point for dbComp too
Have to find the asc... in any case
It s few components
Have to find the asc... in any case
It s few components
Did you simulate the one on fitg 10 or in fig 10b?
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