Minimizing ripple in multichannel preamp - Solved

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ilya
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Minimizing ripple in multichannel preamp - Solved

Post by ilya » 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?

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JR.
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Re: Minimizing ripple in multichannel preamp

Post by JR. » Sat Jul 11, 2015 8:04 pm

I don't know what a 1073 is (nor do I want to) but your symptom sounds like a reservoir cap ripple dropping below the regulator drop out voltage.

If lucky the fix is just to use a larger cap, if not lucky you need more voltage from the transformer.

JR
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ilya
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Re: Minimizing ripple in multichannel preamp

Post by ilya » Sun Jul 12, 2015 3:31 am

Ok. Turned out I measured wrongly. Forgot to short input pins 2 and 3. Once measured properly everything was fine.
Sorry for confusion.

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JR.
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Re: Minimizing ripple in multichannel preamp

Post by JR. » Sun Jul 12, 2015 10:55 am

Glad it was not PS.

In general you want to make noise measurements with a nominal resistor termination between 2-3. Say 600 ohms down to 100 ohms.

JR
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