[sdiy] Noise source in Bryston 10B crossover

[Mike Bryant]

If you convert it to stereo mode, is the noise still there, and the same on both channels ?   

Does the noise remain if you short the input to ground.

After that, I'd go through with a 100ohm resistor and short each op-amp input to ground and see if any place reduces the noise.   Then tell us where that is and we can suggest where to look next.

TBH with 48V supply you've so much headroom the noise should be indiscernable.


-----Original Message-----
From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of cheater cheater via Synth-diy
Sent: 09 May 2021 16:17
To: synth-diy
Subject: [sdiy] Noise source in Bryston 10B crossover

Hi all,
I am using a Linkwitz-Riley based three-way crossover from Bryston. It turns out to be the major source of noise in my setup and I am trying to see if anything can be improved on the circuit in order to lower its inherent noise, which is standard at -90dB. It uses linear voltage regulators (7824C, 7924C) as well as what I guess are discrete transistor based op amps. My main issue is with noise in the HF output. I think it is the first output from the subsequent filter stages, which should make it easier to analyze the schematic... but I can't really make much sense out of it.

My crossover is a PMC 10B, which is a rebranded Bryston 10B hardwired in three-way mono mode, and is balanced.

1. First of all I am trying to understand where the noise comes from.
Are the op amps adding this noise? Is it the voltage rail noise being compounded by several stages of op amps?

2. If I wanted to improve noise on the voltage rails, what would I do?
The 7824C has noise specd at 10 uV/V_O which I assume means 240 uV at 24V, and the 7924C is specd at 170 uV at 24V. That gives us 410 uV / 48V = -101.36 dB - can this be improved upon? There isn't a huge selection of linear 24V 1A regulators on Mouser. The only other is from ROHM, and (I'll spare you translating the Japanese data sheet) their +24V regulator is at 170uV, which isn't that much better than 7824C.

3. What could I do to improve the filters themselves? Can I replace the transistors used with something that provides less noise? Could I replicate the boards with the same topology but with modern low noise op amps instead of the discrete transistor based ones used by Bryston?
There are op amps that go as low as 1nV/sqrt(Hz) nowadays... I wonder how hard it would be to repurpose the schematic to a low noise integrated op amp? Note that this device runs at +/-24V rails - I'm not sure how one would use modern low level op amps like this - any suggestions here?

4. The noise is constant, white-ish, and every now and then it will fluctuate and become quieter for a second or so. What causes this fluctuation?

BTW I know I can get rid of a bunch of the noise using gain staging, but I would like to eke out more dynamic range out of this set up, as the following Bryston electronics easily hit -113 dB noise, so it sucks to lose 24 dB headroom to noise.

Schematics: https://archive.org/download/bryston_10B_SCHEMATICS/10B_SCHEMATICS.pdf

Photos: https://imgur.com/gallery/CNcXtZn

Discussion on DIY Audio:
https://www.diyaudio.com/forums/analog-line-level/372026-noise-output-pmc-main-monitors-due-bryston-crossover.html
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