[sdiy] Noise source in Bryston 10B crossover

[cheater cheater]

There is no better crossover that you can buy that I know of

The schematics are up in the top post, but the discrete op amps aren't
detailed. I bet they're all exactly the same, that's what looking at
the PCB from the top indicates - I bet if I look under, the layout's
just replicated 9 times per board.

On Mon, May 10, 2021 at 5:34 AM Mike Bryant <mbryant at futurehorizons.com> wrote:
>
> TBH without full schematics and layout you may be onto a hiding for nothing.   You'll spend a huge amount of time possibly getting nowhere as even if you find the noise source it may be hard to fix it.
>
> How much is a better crossover ?
>
>
> -----Original Message-----
> From: cheater cheater [mailto:cheater00social at gmail.com]
> Sent: 09 May 2021 22:02
> To: Mike Bryant
> Cc: synth-diy at synth-diy.org
> Subject: Re: [sdiy] Noise source in Bryston 10B crossover
>
> No layout picture per se, only the photos you see on imgur. I can try and disassemble it further later on to get the underside of the board, that could make it easier to see what's going on.
>
> On Sun, May 9, 2021 at 9:50 PM Mike Bryant <mbryant at futurehorizons.com> wrote:
> >
> > Oh I assumed the op-amps were discrete modules.  Do you have a layout picture ?
> >
> > -----Original Message-----
> > From: cheater cheater [mailto:cheater00social at gmail.com]
> > Sent: 09 May 2021 18:46
> > To: Mike Bryant
> > Cc: synth-diy at synth-diy.org
> > Subject: Re: [sdiy] Noise source in Bryston 10B crossover
> >
> > Thanks a lot for your email Mike.
> >
> > On Sun, May 9, 2021 at 6:15 PM Mike Bryant <mbryant at futurehorizons.com> wrote:
> > >
> > >
> > > If you convert it to stereo mode, is the noise still there, and the same on both channels ?
> >
> > Can't do that, it's hardwired, it's a special version made for PMC.
> >
> > > Does the noise remain if you short the input to ground.
> >
> > Yes
> >
> > > 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.
> >
> > Not sure how I'd do that. The op amps are discrete. Would you like to to look at the photos and help me figure this out?
> >
> > > TBH with 48V supply you've so much headroom the noise should be indiscernable.
> >
> > The specified noise is -90 dB. I would like to get to somewhere closer to -110 dB.
> >
> > What's the best way to measure the noise output without spending too much on measurement equipment? I was wondering if it's out of spec...
> >
> > I have some scopes but getting them out right now would be a huge hassle.
> >
> > > -----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.p
> > > df
> > >
> > > Photos: https://imgur.com/gallery/CNcXtZn
> > >
> > > Discussion on DIY Audio:
> > > https://www.diyaudio.com/forums/analog-line-level/372026-noise-outpu
> > > t- pmc-main-monitors-due-bryston-crossover.html
> > > _______________________________________________
> > > Synth-diy mailing list
> > > Synth-diy at synth-diy.org
> > > http://synth-diy.org/mailman/listinfo/synth-diy
> > > Selling or trading? Use marketplace at synth-diy.org
> > >
> >
>