New user and Bryston 7BST question

Alfalfa · « **on:** 26 Feb 2005, 04:30 pm »

Hi all,

Since I am in the waiting process for my newly ordered IB2's to arrive at my door next week, I wanted to introduce myself and ask something that has worried me about my 7BST's.

With my former speakers (KEF Reference 4) and my current mini-stand ins (Celestion DL8II) I hear a terrible noise though the tweeters when I turn off the 7B ST's. With the KEF's this sound was best described as 'screaching'. It was at rather high levels until it died away after a few seconds. Very uncomfortable with expensive speakers I can tell you..! Now with the Celestions the problem presists, only it takes some seconds before the noises start and later dies away (bleeding of the caps??). I've mailed the Dutch importer about my worries but got no reply.

The rest of the system comprises the Musical Fidelity Nuvista 3D CD and the Bryston BP20. I live in THe Netherlands and heard the Bryston/PMC combinations play very well at several shows. Since the Brystons lived in my system for a few years and I was never happy with the new KEF's (loved and owned the old 107 and 104/2's though), I sold them and ordered a pair of IB2's because of my memories and the synergy between Bryston/PMC. I took out the extra cash and took the IB2's as I did not wanted to think "I wish I got the IB2's everytime I listen".

Does anyone recognize the screaming noises after turning off the Bryston power amps?

Greetings & happy listening!

Alfalfa

jethro · « **Reply #1 on:** 26 Feb 2005, 05:46 pm »

Welcome to the circle alfalfa. It's good to see more and more PMC owners participating in the circle.

I have IB1's powered by 4B-ST's using the SP-1.7. I get a single pop from my speakers (IB1's, sub, and B&W surrounds) when I power down. Basically the 4B-ST's LEDs get dimmer and then before they go dark, I get a single low frequency pop that is not very loud. I believe the low pop is known behaviour and not damaging, however, I haven't heard people talking about long screeching noises. I always turn down the volume before powering on and off, and before switching inputs and music modes just in case. Hopefully somebody else with more knowledge can comment on the screeching.

saea501 · « **Reply #2 on:** 26 Feb 2005, 06:22 pm »

I have 2 3B SSTs......I get a very faint snap through the speakers when I power down, it is barely audible. Both power amps are triggered by a Krell processor. What you are describing does not sound at all normal for any kind of electronics when being turned off. Yours sounds like it is potentially damaging to the tweeters. Probably should have the amp and pre amp looked at.

John Casler · « **Reply #3 on:** 26 Feb 2005, 06:52 pm »

The "screeching" is simply the caps discharging.

It is likely not harmful. If you find it disconcerting, play music as you shut off the amp and the music will slowly die away and little if any screeching will result.

Some manufactures add "buffering" circuits to eliminate this, but they also add coloratios to the sonics.

Always "power up" from the source in this order:

1) CDP
2) Preamp
3) Power amp

And "power down" in the opposite direction in this order:

1) Power amp
2) Preamp
3) CDP

BeeBop · « **Reply #4 on:** 26 Feb 2005, 07:03 pm »

I Have a 3BSST and a BP25 pre-amp. I get no sound at all when I power down. My pre is connected to the amp with a 12v trigger. Maybe the problem is your pre.

KJ · « **Reply #5 on:** 26 Feb 2005, 07:31 pm »

Is your CD player still powered up when you turn off the amp? If so, does the screaching noise still occur if you turn it off prior to shutting off the amps?

-KJ

BeeBop · « **Reply #6 on:** 26 Feb 2005, 08:45 pm »

Yep. You got it. Not a CD player but a DAC - or more accurately, a dAck! One rule I have always followed is to shut off the pre and amp before the source, and to turn on the source before I turn on the pre and amp. Training from days of yore. Anyways, the screeching is from shutting down the source, not the amp.

Alfalfa · « **Reply #7 on:** 26 Feb 2005, 08:45 pm »

First: thanks for the replies!

To answer or comment your questions:

1. when I play music while powering down, the same thing happens (maybe anly slightly less)
2. the preamp does not influence the screaching (other preamps tried, same happened)
3. cd-player on or off does not make a difference
4. same powering up and down sequence has always been my ritual

My only concern is that there is something wrong with the amplifying circuits (and therefore also inlfuencing the music qualitty). If it "only" concers the output mute circuit I can 'fix' it by leaving the power on al the time. Of course I also don't want this to ruin the (costly!!) midranges of the upcoming IB2's..

Alfalfa · « **Reply #8 on:** 27 Feb 2005, 07:55 am »

I have to correct myself a bit. When I powered donw yesterday while the music was still playing through the small Celestions, the amps made no screaching, pulsating noises. THe music died away and there were some minor pops audible. Strangely the big KEF's showed no difference in this. THere was always this schreaching... Maybe differenct crossover filters an/or drivers (the KEF uses the Uni-Q unit) affect this as well. Anyway, I hope the new PMC's behave in the same way as the Celestions. Can't wait to hear them play (actually, this is one of the reasons I am hanging around here

afblaster · « **Reply #9 on:** 27 Feb 2005, 11:10 pm »

One channel of my 3-channel 5B-ST screeches after power down, despite having been modified by PMC with parts supplied by Bryston.
When the amp was first returned to me the screeching was very much reduced, but now it is starting to increase to the previous, before modification, level. This occurs with the pre-amp powered on or off.
I want to sell the 5B soon and replace it with a 6B-SST, so I trust Bryston/PMC will be able to effect a proper, permanent repair - after all, two of the channels power off silently.
Plus - this sort of problem shouldn't occur with this calibre of equipment, imo.
Any comments James?

Alfalfa · « **Reply #10 on:** 1 Mar 2005, 07:15 pm »

Hmmm, seems like a Brystons' achilles heel?

A friend on mine used to own 7B ST's as well. He said the amps screamed at him as well when he powered down. What can it be?

afblaster · « **Reply #11 on:** 2 Mar 2005, 07:12 pm »

Before I sent it to PMC for repair, I had a look inside my 5B-ST, and I remember that there was a relay on each of the driverboards which feed their associated output stage.
My guess is that these relays are meant to disconnect, or maybe crowbar the signal to the output stages in the event of a fault situation, or if the supply rails fail, as in a power-off situation.
Since two channels on my amp are powering down quietly, I suspect there is a problem with the relay on the driverboard feeding the screeching channel.
Anyone at Bryston care to comment?
Anyone?

Mike Pickett · « **Reply #12 on:** 2 Mar 2005, 09:34 pm »

Hi all;

Turn off noises have been fairly common on most pre SST amps, and they can vary from the small pops and ticks to some screeches as mentioned. None of these noises should be damaging to speakers, although they can be annoying. The relays in the ST amps are designed to mute the outputs as soon as the voltage drops to a certain level. These work fairly well, but they are dependent on the voltage dropping before the noises are produced, and larger amps like the 7B's have lots of capacitance to drain. Playing music while the amp is shutting down will discharge the power supply more quickly, but the downside to this is the fact that the waveform will begin to clip as the voltage drops. This most certainly can and eventually will damage speaker components, so I don't think it's a viable alternative to the regular turn off noises.

On the 5B, because it was based on the 8B four channel amp, one channel actually has double the capacitance available to the low voltage section, since it's twin channel isn't present. This means that one channel will take twice as long to discharge as the other two. Afblaster; I'm assuming this is the case with your amp, but I'd have to spend some time with it to know for sure.

With the SST series, the muting at turn on and turn off is controlled by logic, so as soon as you hit the power switch the channels are muted and the LED's are switched off immediately. This makes the amps more esthetically pleasing to the eye and ear, and luckily, doesn't degrade sound quality.

I guess my only advice is to rest assured that you aren't going to damage your speakers by turning off your amps.

Mike

afblaster · « **Reply #13 on:** 3 Mar 2005, 12:32 pm »

Hi Mike

Thanks for your speedy reply.

I'm sorry - I have to disagree with your reasoning. My 5B-ST (bought in 2000) has three toroidal transformers and three separate supplies, one for each channel (isn't this the case for all Bryston power amps - a power supply per channel?).

If I'm following your logic correctly, I should be hearing the noise from channel 3, which you say should be the 'solo' channel with double the reservoir capacitance available? However - it's channel 2 that is screeching.

I'm still of the opinion that the problem is caused by the relay on channel 2 not operating quickly enough, for whatever reason. Perhaps channel 2's reservoir capacitors require some bleeder resistors to discharge them more quickly?

Richard

Alfalfa · « **Reply #14 on:** 3 Mar 2005, 09:13 pm »

Thanks Mike, it seems that my answers/concerns have been well adressed! I will not turn off my 7BST's anymore as soon as my IB2's have arrived. Terrible thought to damage a new and very costly component!! Can you tell what components will or can get damaged, and what kind of damage: so far as that the speakers(units) breakdown, or could it degrade the quality?

Many thanks again for your answers!