What is 'Good' Common Mode Rejection? - diyAudio
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Old 20th February 2014, 06:02 AM   #1
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Default What is 'Good' Common Mode Rejection?

I'm modifying my RAKK DAC to incorporate an analogue volume control as it is my only 'source' component and the addition of the volume control will be far more convenient with two mono power amps.

I'll be adding the K&K Audio RAKK DAC passive I/V module, which uses a couple of Llundahl transformers and delivers a balanced output, and will use their switched shunt attenuator too, (which can be configured for balanced or single-ended use). I could use this single ended as a passive volume control but I need to connect to the power amps over interconnects a couple of metres long so probably not a good idea and a linestage to drive the cables seems like a sensible thing to do. I don't really need gain in the linestage, somewhere in the area of 0dB to 6dB will be fine, just to drive the interconnects.

I have a pair of Welborne CATO modules that I've previously used with excellent results (purchased a few years ago before Welborne started taking money and not delivering!). The CATO is a tube/mosfet hybrid circuit. Anyway, it is possible to reconfigure the CATO module as a balanced to single-ended convertor/line stage and this is tempting, especially given the previous positive experience. However (I got there in the end but I thought context important!) the quoted Common Mode Rejection for this configuration is 50dB, which seems a bit low given that op amps and the like usually quote up around 100dB CMR. Is a figure of 50dB CMR likely to be an issue - it's not an area I have any feel for.

I guess I could suck it and see but I don't want to waste time on a no-hoper.

Alternative solutions are to reconfigure the CATO modules as single-ended linestage or I could use a Pass JFET DCB1 linestage that I have ordered from Tea-Bag, though that was destined for a DAC in a second system so would prefer to use the CATOs.

Thanks

Ray
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Old 20th February 2014, 07:01 AM   #2
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Common mode rejection for opamps is a function of frequency and 100dB is likely to only be below 1kHz. The figure is just for the chip itself - once external components are fitted to create a complete circuit, the CMRR is going to drop to the degree of matching provided by those parts (resistors typically). 0.1% resistors tend to give at best 60dB CMRR within the audio band and then only when source impedances are well matched. In this context, 50dB doesn't sound so bad does it?
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Old 20th February 2014, 07:22 PM   #3
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Thanks Abraxalito, that's really helpful. I'll give it a suck on that basis.

Ray
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Old 21st February 2014, 02:29 AM   #4
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the Gato is an Erno Borbely design from a decade before Welbourne started selling it. It was published in Glass Audio originally
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