[KCLam]

Triamped  the PMC MB2/XBD, crudely hooked up with whatever cables available.

First impression, the bass is muddy (the amp driving the bass was the used unit I bought recently, perhaps needs running in?).
There is a huge improvement in the mids and highs and the dynamics and spaciousness is significant. However the process is painful one including moving the speakers, and there is still a lot of tweaking to do.

Two of the 3 Brystons 14B ST is totally silent when turned on or off, but one gives a pop everytime when the switch is turned on or off.

Is there a problem with that unit or is the problem loose connection or something else?

Any feedback and advice is appreciated.

Thank you.

[Mike-48]

I have a B14 ST that makes a noticeable "tick" through the speakers when turned off.  It also makes a slightly quieter tick when turned on.

It started doing that when a year old or so.  I corresponded with B. service about it but never sent it back as the problem didn't seem severe enough to me to warrant the ~$100 freight cost, plus being with no amp for weeks.

One of these days, I should send it back -- but what would I listen to then?

[YoungDave]

Generally, an engineer would put .1mfd of capacitance across the switch to prevent the spike you are hearing.  This is really desirable when the switch directly controls large currents, but some amp designs use the power switch solely to operate another circuit, such as a soft-start, that in turn switches on the amp much more quietly.  In those cases, capacitance across the switch is still a nice touch, but not really mandatory.  A 25-cent ceramic disk capacitor will do this job very nicely.

I don't know what the Bryston circuit looks like, but it sounds to me like their design may not include this trivial and commonplace capacitor.  That leaves the silence or noisiness of the switching operation entirely up to the quality of the switch - and switches are notable for 2 characteristics:
1.  Really, really cheap construction
2.  Indifferent quality control - one switch may be much quieter than another identical switch from the same manufacturer.

Surely Bryston includes schematics of their products in the owner's manual?  Check to see what the switching circuitry looks like.

The poor quality of switches is one of several reasons for  designing the circuit so that the switch does not directly turn on the amp, and for putting capacitance in parallel with the switch contacts.

Anyway, this noise is not likely to be symptomatic of problems in the amp such as an incipient failure of signal processing or amplification stages- although it might indicate a somewhat less than optimal design of the power supply switching circuitry.

One last thought - this noise could possibly have enough amplitude to destroy a tweeter, as it may include a lot of energy at or above the limits of human hearing.  Many tweeters can take such a spike OK, but I still wouldn't want to subject them to it.  That is why it is a good idea to use some kind of muting circuit in an amplifier design, so that the speakers receive no power from the amp until it has stabilized - and not at the instant of turn-on.

Cheers

[Mike-48]

Dave: Thanks for the info.  The Bryston does have a muting circuit, but perhaps it's not working.  Your comments on tweeters have motivated me to look into sending it back sooner rather than later.  Mike

[YoungDave]

Hi Mike!

Funny, I never can organize my thoughts well enough to completely cover a topic...but it's fun to try!

Your muting circuit is probably working just fine.  Muting circuits are best able to mute the output in 2 cases:

1.  When first turned on, they don't allow any output until the circuit has stabilized, thereby avoiding the loud "thump" caused by a hefty DC on the output (~5 VDC) as the amp & power supply sections stabilize.  Big DC applied to speakers can make a woofer voice coil into a projectile - not good for the woofer!

2.  If a malfunction in the output section, such as a failure causing a short in an output device, allows some or all of the ~50 VDC rail voltage to appear at the output, the muting circuit will very quickly disconnect the speakers, therby saving them from certain destruction.

I don't think a muting circuit could help much on shutdown, though.  In this case, the amp is stabilized, the speakers are getting a normal signal, (the amp is not muted), and you switch it off.  A muting circuit would have to be clever enough to anticipate you switching off, because that switching spike is of such a short duration that it is gone by the time the muting senses it.  The shutdown spike is best ameleorated by a better switching setup, such as the capacitance discussed previously, or a really nice switch (I rarely see good quiet switches, even in aerospace systems).  

The pop is caused by arcing across the contacts (very mild arcing will not cause pitting of the contacts, but is audible, especially when boosted 25-30 dB by the amplifier).  Some man-rated aerospce switches are designed not to arc (pretty important consideration in 100% oxygen), but they are expensive and ugly compared to what you have.  Most aerospace switches, such as used in aircraft, are designed for long service life, not freedom from switching noise transients.

The trouble with sending an amp back is, the manufacturer might only make sure that it is performing as specified.  If the design permits such transients, then the amp is already performing as specified and it might be making the round trip for nought.  It might be a good idea to talk to an actual person at B's service dept., and see if they are interested in taking real steps, even if it requires the addition of a capacitor or two across the power switch (or some other circuit modification), that is not a part of the original design.  Of course, they may already be aware of this problem, and have a suitable fix just waiting for the perceptive customer to call them.   Also, I make no claim to be a Bryston expert, and the appropriate circuitry might already be there, and just not working right.

If the amp is not under warranty, and if you know of a local proper audiophile serviceman, you might be able to get personalized service there.  I sure wouldn't take a fine amplifier to a TV repair shop, though...

Cheers

[Mike-48]

The trouble with sending an amp back is, the manufacturer might only make sure that it is performing as specified. If the design permits such transients, then the amp is already performing as specified and it might be making the round trip for nought. It might be a good idea to talk to an actual person at B's service dept., and see if they are interested in taking real steps, even if it requires the addition of a capacitor or two across the power switch (or some other circuit modification), that is not a part of the original design.

Dave: I think your comment were clear & I did understand what you were saying. The amp did not make this noise when it was new, and an email exchange with Bryston's service dept. confirmed that it's not expected. The amp is under warranty (B. offers 20 yrs). The switch is not a direct mechanical switch, but a "soft touch" one that operates a relay. Presumably, there is a cap or some other circuit across the relay.

I'm getting ready to send it back, but wondering what to do about music while it's gone!

Thanks again for all your comments

[KCLam]

I am trying a number of combination sequence for switching on or off components.
I noticed that turning off the power amp first without muting the preamp but with the volume set to 0 does reduce the pop.
Another option that I'm looking into is to connect the bass drivers to the power amp that pops instead of connecting it to the tweeters.

[Mike-48]

Well, don't forget that Bryston's warranty is 20 yr from the manufacture date, so they will repair it at n/c if you're willing to ship the unit.

[KCLam]

As the system had to be rewired for triamping, all cables were reconnected again. As a result the pop has gone away. I suspect it is due to loose connection or broken banana plugs.