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High noise output from PMC Main Monitors due to Bryston Crossover
High noise output from PMC Main Monitors due to Bryston Crossover
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Old 7th May 2021, 05:55 PM   #1
cheater is online now cheater  United States
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Unhappy High noise output from PMC Main Monitors due to Bryston Crossover

Hi all,
I have set up my main monitors and I am in a bit of a bind as to the noise output. I think the culprit is the line-level analog crossovers, but I'm baffled by that, because those speakers and their electronics are supposed to be used in high-end studios, and that's definitely too much noise. The noise is white noise and it's mostly constant but fluctuates in loudness every now and then which would indicate to me either RF/EM pickup, or bad power supply capacitors. However, the thing has a grounded, steel, 1mm sheet box, so I don't necessarily think it's RF/EM pickup. The noise is put out via all transducers. It's the same loudness on both sides. I don't have an SPL meter capable of measuring this noise, but it's much too loud for room-level listening, to where it already gets tiring mid-day. I am a little more than 2m away from the speakers. Turning the power amp sensitivity on the rear panel down from 1V to 2V helps alleviate the issue (predictably the noise is 6dB lower), but it's still present. I perform main output attenuation in digital, before the DAC.


Do you think that based on my testing I am right to point to the crossovers as the culprit?

Is this sort of noise output to be expected from this level of equipment? (I list everything below)

How can I improve this without compromising monitoring quality?

A visual inspection of the insides of one (right) crossover revealed no obviously leaking or blown up capacitors; everything seems to be in order; the PCBs were signed off on 3.4.09, so I wouldn't expect there to be an issue here. Everything is very clean, but there's clearly a bunch of botching done at the factory due to the customization done for PMC.

I tested that the issue is at the crossover by doing the following:

1. Turn off left crossover - noise gone from left channel.
2. Turn on left crossover - noise back in left channel.
3. Turn off right crossover, disconnect cable from DAC going to crossover, short left channel pins 2 and 3 on the DAC end, and move the DAC and its associated cabling far away from the crossovers and amps - noise still present in left channel, no noise in right channel.
4. Restore the connections to their normal state; turn on both crossovers; unplug the cable feeding left HF from the left crossover to the HF stereo amp, unplugging it at the crossover - noise gone from left channel.
5. Plug the cable back in - noise back in left channel
6. The same tests provide the same results on the right channel


My electronics are as follows:

1. PMC MB2 XBD quad-amped main monitors, stereo
2. the standard electronics supplied by PMC for that, which are a customized version of Bryston SST2 electronics:
- PMC 10B crossovers. They are not the same as Bryston 10B. PMC 10B have main and LFE input and HF, MF, and LF output and all I/O is balanced XLR and there is no (front-panel) frequency adjustment; Bryston 10B have only line input and only split to HP and LP, have single-ended I/O, and allow frequency adjustment for each band using front panel rotary switches. Also note that the Bryston 10B is a stereo crossover (one box for both channels); the PMC 10B are mono crossovers, so you have to have two of them.
- PMC-branded Bryston 3B SST2 (stereo, 250W/channel), 4B SST2 (stereo, 500W/channel), and a pair of 7B SST2 amps (mono, 900W/channel). As far as I know those are exactly the same as Bryston, just with PMC branding. The 3B goes to HF, 4B to MF, and one 7B feeds LF and XLF to the left channel while another 7B feeds LF and XLF to the right channel.
- The XLR cables are 1m store bought ones, they're not amazing but they're fine. Two conductors + shield. I opened all the male plugs and cut the connection between the shield and pin 1 and insulated it with heat shrink, so now the shield is only connected on the female side.
- The crossovers are at the top of the stack, so they're pretty warm, but it's not like they're burning, they're merely warm to the touch.

Thanks
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Old 7th May 2021, 08:30 PM   #2
johnmath is offline johnmath  Australia
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You are working very near field to the monitors and from your description of fault finding you are possibly hearing the noise floor of the crossover.

Solution: Turn down the gain of the power amplifiers until you can no longer hear the hiss, could be as much as -20dB. The crossovers will still have enough output to drive the amplifier to clipping with headroom to spare. This is the normal method for optimising signal to noise for professional audio

Given the vintage of the 10B it is possible that the op amps used internally could be up spec'd to quieter modern ones.

You could also be hearing the noise floor of components upstream to the crossover. It the system noisey with no inputs connected to the 10B?

There are settings in the 10B to increase or decrease input gain. You made need to make some adjustments for optimal signal levels.

Last edited by johnmath; 7th May 2021 at 08:55 PM.
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Old 7th May 2021, 09:20 PM   #3
Mark Tillotson is offline Mark Tillotson
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The specs I found for the 10B crossovers say S/N ratio 90dB.

A good figure for an active crossover would be more like 115 to 120dB S/N ratio, if designed carefully (according to Doug Self).

So it sound like these crossovers might be rather poor performance.

90dB S/N at 0dBV is about 32V rms, or 224nV/√Hz. Most good audio opamp stages are going to be somewhere in the 5 to 15nV/√Hz range for noise.

That 90 figure is possibly limited partially by mains hum of course, but still its rather indicative that the circuitry is not designed for low noise, which matches your experience.
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Old 7th May 2021, 10:59 PM   #4
cheater is online now cheater  United States
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Quote:
Originally Posted by Mark Tillotson View Post
The specs I found for the 10B crossovers say S/N ratio 90dB.

A good figure for an active crossover would be more like 115 to 120dB S/N ratio, if designed carefully (according to Doug Self).

So it sound like these crossovers might be rather poor performance.

90dB S/N at 0dBV is about 32V rms, or 224nV/√Hz. Most good audio opamp stages are going to be somewhere in the 5 to 15nV/√Hz range for noise.

That 90 figure is possibly limited partially by mains hum of course, but still its rather indicative that the circuitry is not designed for low noise, which matches your experience.
Yeah but it shouldn't be bad. It's meant for mastering, and it's used at the top studios in the world, so why would it be noisy like that? I feel like it's performing less than it should, but I don't know really. I can't find any numbers for it. Did you find a 90dB figure somewhere?
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Old 8th May 2021, 12:43 AM   #5
johnmath is offline johnmath  Australia
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The Bryston 10B is a very old design; it changed from discreet transistors to op amps during its production life. Things have moved on since then. 30 years ago when I ran a studio we never had ran power amps at full gain. As I said previously -20dB was a pretty standard gain setting for amplifiers to minimise noise. The 10B has +/- 24 volt rails so is probably capable of outputting +26dB into a 10kΩ load. Maybe the max output level is in the specs Mark found. There's some basic spec on Stereophile: Bryston 10B electronic crossover Specifications | Stereophile.com
The measured performance underwhelming by todays standards, at least for HiFi.

Last edited by johnmath; 8th May 2021 at 12:59 AM.
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Old 8th May 2021, 12:55 AM   #6
cheater is online now cheater  United States
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I really like your idea of possibly upgrading the semiconductors. When I had a look inside I noticed some 5536's and what seemed to be discrete transistors, but they might as well have been voltage regulators. There might be some more things, there were daughter boards obscuring what's under them. But first I'd like to find out if my unit is just out of spec.

You mention attenuation. What do you suggest would be the best way to attenuate here? voltage divider on both hot and cold inside the xlr plugs? In-line attenuator like this hooked up on the output of the crossover?

JTS MA-123 – Thomann UK
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Old 8th May 2021, 02:47 AM   #7
johnmath is offline johnmath  Australia
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To optimise your signal gain structure you might need to add or remove gain and/or attenuate signals at each interface so each process is working at an optimal level for noise and headroom. The output of the crossover is apparently too hot to drive amplifiers with inputs rated at 1 or 2V, which is unsurprising if it can drive +26dB (~15 volts).

I think it unlikely to be out of spec. The noise specification isn't particularly good, you are driving amps at full gain (amplifying the noise floor of the crossover) and you are listening close to the speakers. What you hear is what I would expect you to hear.

Something between white and pink noise will be generated by all active stages. You say you have noise across all eight outputs. A single faulty/noisy op amp or transistor is not going to affect all outputs.

It is not impossible for a faulty power supply to inject noise, but I would expect that kind of fault would manifest differently in the different outputs because of the different circuitry for each section of the crossover.

Given the relationship between Bryston and PMC I am assuming that the PMC 10B is really a Bryson 10B with fixed frequencies to suit the PMC speakers.

The Bryston 10B has internal settings for gain structure. If the same is true for your 10B then the first thing is to check the most appropriate settings are selected for your signal path. Do you have any manuals or documentation from PMC for your crossover?

If there is no input attenuation available on the power amplifiers other than the 1V/2V switch you would need to make balanced attenuators to plug into the amplifier inputs. We used to make these to fit into a male / female XLR adapter. They are probably available off the shelf these days.

Attenuation at the last possible position in the signal chain attenuates all signal impurities collected along the chain up to the point of attenuation and at the amplifier is usually is the most appropriate place to put it. You just need to be sure that upstream gain settings are not amplifying noise unnecessarily before determining how much attenuation is needed at the amplifier.

Last edited by johnmath; 8th May 2021 at 02:49 AM.
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Old 8th May 2021, 02:55 AM   #8
johnmath is offline johnmath  Australia
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The JTS attenuator might be OK. I like the switchable levels. the spec 20Hz - 22kHz bothers me. A simple attenuator should be DC - blue light, or at least 0.1Hz - 1MHz.
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Old 8th May 2021, 03:00 AM   #9
johnmath is offline johnmath  Australia
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I can't vouch for this company, but they seem to have dealers everywhere and they are not priced like hifi voodoo signal improvers.

Attenuators

If you used 10dB attenuators, you also have the 6dB available on the amps. 16dB should drop the noise to virtually inaudible and still allow 10dB of headroom in the crossover output drive capability.
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Old 8th May 2021, 08:29 AM   #10
cheater is online now cheater  United States
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Bryston erased all mention of the 10B from their website, but luckily they have archived their old website and the docs are here: 10B-STD Electronic Crossover - Bryston

My best guess is that internally the PMC 10B is a Bryston 10B LR, which is put in the "Three Way Mono" mode. You can read about it in the manual on that page. The brochure lists -90dB noise, but that's for the SST version, and mine is SST2, so I don't know if the spec might have changed.

I think the attenuators are 20-22k because that's what they measured for, not because there's a high pass filter inside. But if there is, then it looks like this thing can be opened up and gutted, and I have a soldering iron, so it shouldn't be much of a problem. Maybe I should buy one first and then go on with the rest.


Additional noise across all outputs could be added by one of two things:

1. noisy power regulation

2. bear in mind that each of those things is a series of low and high pass Linkwitz-Riley filters, so if the first gain stage or two are noisy, then everything is noisy.
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