Minimizing ripple in multichannel preamp - Solved
Posted: Sat Jul 11, 2015 6:27 am
I'm building a 2-channel preamp based on 1073 circuit. Currently I have the 24V regulators on each preamp board, so I feed the unregulated DC from a PSU board (just a general bridge into the cap). Everything is fine when I have only 1 channel working. However, when I connect the second channel to the PSU, I get "dirty" output from that channel that sounds like ripple (and I'm 99% sure it's ripple).
I tried all grounding variations I could think of. All of them gave me different results, but none of them were ripple free for the second channel. The most noisy was this star ground scheme: unreg PSU common to star GND and modules GNDs (I tried different spots, almost no difference) to star GND. The least amount of ripple I get is in this configuration: unreg PSU to 1st module, 1st module reg GND to chassis, 2nd module daisy chain pwr/gnd from the 1st module.
I can't say that the ripple is huge. The first module is completely hum-free and has a noise floor @ -100dB (relatively speaking, looking at my DAW spectrum analyzer). The second module has different harmonics (100, 150, 200, 250 etc up until approximately 1K) that are at -90dB. And it's audible. By audible I mean if I crank the gain on the module up to maximum (+75dB) I can hear it. It's completely gone if I power both modules from 1 regulator, but the reg runs quite hot - 60 C on the heatsink.
Are there any ways of working around that issue? Or is it inherent to designs with individual regulators?
I tried all grounding variations I could think of. All of them gave me different results, but none of them were ripple free for the second channel. The most noisy was this star ground scheme: unreg PSU common to star GND and modules GNDs (I tried different spots, almost no difference) to star GND. The least amount of ripple I get is in this configuration: unreg PSU to 1st module, 1st module reg GND to chassis, 2nd module daisy chain pwr/gnd from the 1st module.
I can't say that the ripple is huge. The first module is completely hum-free and has a noise floor @ -100dB (relatively speaking, looking at my DAW spectrum analyzer). The second module has different harmonics (100, 150, 200, 250 etc up until approximately 1K) that are at -90dB. And it's audible. By audible I mean if I crank the gain on the module up to maximum (+75dB) I can hear it. It's completely gone if I power both modules from 1 regulator, but the reg runs quite hot - 60 C on the heatsink.
Are there any ways of working around that issue? Or is it inherent to designs with individual regulators?