SWR LA-15 'noise suppression' at power up.

[Mars Amp Repair]

Hello,
I have this amp for repair that hums & pops badly at power up. It is the typical speaker excursion while the + & - 16V supplies in the preamp stage stablize.

Anyway, I see where SWR used to have a little add-on board to suppress the output of the preamp for a few seconds at power up.

This board is no longer available since Fender took over SWR.

Has anyone seen these boards & possibly drawn up a schemo of it? From the pic it looks like it might have had an IC on it, but no real way to tell.

Has anyone actually have a circuit they us for this type of issue. Any ideas greatly appreciated.

BTW:
I had an Ampeg BA210sp that would make a really nasty cracking sound badly upon shutdown. The noise was coming from the Digtal pcb & I could not find anyway to keep that from occuring. Replacing the digit pwb did not change anything.

This Ampeg has a circuit built in to mute the output stage upon power up, but nothing to mute it upon power down.

I stole a portion of the mute ckt from a Marshall & adapted it to work with the Ampeg.

Unfortunately, I cannot steal the ckt that muted the Ampeg upon start up for this SWR amp as it works with the diff amp in the power stage.

Any ideas?

Thanx, glen

[Enzo]

I'll look after dinner, but power up noise and power down noise are separate issues.

[Enzo]

Well there is a bulletin, I attach it below. But it contains nothing about the item itself. The photo shows a small board with a chip on it.

I also include the power amp schematic from the SWR version. Upper left portion shows what looks to me to be a power on mute. Current for the input diffy pair has to come through Q10. Q9 will turn it on only after C16 charges up. C16 is 2.2uf but could easily be made larger for more delay. This is not the circuit of the little board, but it is a direction to consider.

ANy help?

[Steve Conner]

IMO, you can't beat a speaker relay for this kind of thing. I've seen a few standalone boards that use a relay to do power-on/off muting and DC protection, maybe you could squeeze one in there?

The manufacturers don't like relays because they cost money, but it's not so significant when you compare it to the labour costs of a skilled tech. And if it saves a couple of speakers from roasting by DC one day, there's some more money saved.

[Mars Amp Repair]

Yes Steve, I remember all those old stereo amps always had relays. Only issue would be the contacts getting oxidized.

Enzo, that mute circuit actually only mutes the power stage at power down. If you look at it from the perspective of the AC input:

1. the AC input to the mute ckt will disappear immediately at power down, but the -59 remains for quite a while until the main filter caps discharge.

2. That neg charge holds the voltage at the base of Q9 to a neg 0.6V which turns off Q9 hence keeping Q10 from conducting.

3. At power up, the ac is there immediately (minus a very slight delay while C16 charges I believe C16 is really just there for smoothing), so Q10 is on almost immediately.

4. Essentially as I see it the whole purpose of the AC input to this ckt is to enable the delay ckt at power down and to mute the mute ckt (if you will ;-] ) during normal operaton.

5. if I mess with the value of C16, the mute ckt won't do it's job properly at power down.

HMM, that whole dissertation seems to have given me an idea...I'll get back to you...thanx, glen

[Enzo]

Oh well, it did something, anyway.

[Enzo]

What then in your estimation would be the result of increasing the cap C16, and decreasing or removing it?

[Janou]

I have a board with the soft-start circuit. No the small board, but i think you can find the schematic because it's not very hard to understand.

I put pictures with good resolution in a .rar file

[MarkusBass]

Of course this is not a soft-start but rather speaker delay. It looks very similar to the circuit published in Elektor: http://www.audiostereo.pl/forum_wpis...emat=27933&p=1 (schematic in the middle of the page). Janou, can you read the components values (the link with the .rar file does not work)?

Mark

[Janou]

Oups, i just correct the link

For values
Relay : Omron G5V-2 24vdc
C20 : 47uF/63v
C19 : 100uF/63v
R35 : 2.2k
R36 : 33k
Q11 : MPSA06

If you need more informations, no problem

[Mars Amp Repair]

Thanx eveyone,
I've been a bit busy. My wife just had the surgery for a cochlear implant. It's been a very interesting road. They'll activate the implant in a few weeks & begin 'mapping' the processor for what bascially amounts to her 'brain' response the the 16 electrodes now inside her cochlea.

Modern technology...facinating

I appreciate the info on the 'muting' ckt from all. I'll look into that & get back. Thanx, glen

[Amp Kat]

I thought about being an Audioologist and still am but that there is some serious stuff. Hope all goes well and wish her our best of luck. I've worked on several of those Bass amps and the muting circuit sounds like the right thing the last one I did had a loose filter cap connection I couldn't see with the naked eye. At least touch up those two caps and glue back down if they came loose.

[Mars Amp Repair]

thanx all & AmpKat for the good wishes. I'll look into all that. glen