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Old 28th December 2001, 06:00 PM   #11
PH104 is offline PH104  United States
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Hey, why would anybody want to talk to you -- you only started this topic!

Thanks for the tip on the 4541 -- it looks good. I'll have some applications for it but I'm not sure I agree with all you've said.

The 555 takes no more parts to implement as a delay than the 4541 plus it can source/sink up to 200 mA which often eliminates the need for a pass transistor, although I usually use one based on the belt and suspenders philosophy. It's also cheap, and isn't cost-effectiveness also a good engineering principle? (Besides, I have a bunch of them I inherited from somewhere that I want to use up). This is mostly tongue-in-cheek --- we're talking about pretty small differences here, aren't we, especially for those building onesies or twosies of something. But I believe that Thomas Edison was the first to comment on the benefits of laziness.

I guess the one thing that would bother me about the 4541 for my present application is the built-in oscillator. I run an all-analog system most of the time and keep any clocks or digital oscillators out of my equipment, even if they're not directly in the audio circuit. Maybe just superstition.

Anyway, time to stop yackin' and start soldering and finally finish those delay boards like I promised myself. Thanks again for pointing out 4541 (I wish I had known about them earlier) and for starting this post. It's been interesting especially since I've been thinking about this kind of stuff recently. Good luck with the project!

Phil
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Old 28th December 2001, 08:37 PM   #12
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Thinking...?
Whuzzat?
I've never bothered to come up with an original delay-relay circuit because so many people have solved the problem so nicely in the past. Saves brain cells (& time) for other things.

If I have seen further, it is by standing upon the shoulders of Giants...
Sir Isaac Newton

Grey
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Old 29th December 2001, 01:31 AM   #13
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I've built a small board for my amp, containing two led vu-meter chips and two of Rod Elliots' "Loudspeaker protection and muting" designs... this is a very simple circuit, and as far as i've tested it, it works flawlesly. It'll drop the relays if any DC is present at the input and you can also set up a turn-on delay and inmediate turn-off muting to kill the transients. I just added some minor mods (one circuit per channel, ability to mute from a switch and a led indicator to match the nifty vumeters ). Highly reccomended.
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Old 29th December 2001, 02:28 AM   #14
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Lisandro_P about that protection circuit using relays...

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Old 29th December 2001, 05:04 AM   #15
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A simpler version of delay timer uses just a cap and resistor wereby the charging cap removes the mute after reaching the trigger voltage. 2 parts, real simple, and it works. Follows Pete's KISS principles

Cheers,

Pete Fleming
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Old 29th December 2001, 06:47 AM   #16
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Quote:
Originally posted by AudioFreak
Lisandro_P about that protection circuit using relays...

Read What do you thnk of this amp?
You mean the relay relaiabilty issue i presume... well, i'm using a pair of SPST Goodsky relays rated at 10A 30VDC (about $4 each). I once opened one and the contact area is a solid disc of some material i can't identify (seems like a gold alloy), so i presume they can take small DC arcs without problems. My amp is 2x20w, so i have quite a safety margin. I haven't had a chance to measure the switching time, but the switching itself is INAUDIBLE; guess that's good enough.

I strongly reccomend using relays for output muting. Good relays are not that expensive or uncommon, and from previous experiences, they're can be very rugged and have a long life. The Elliot project for muting and DC protection can be built for about $20, what do you have to loose?
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Old 29th December 2001, 08:32 AM   #17
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Lisandro....

What do you have to lose? how bout the amp and speakers.

The arc is potentially huge and can/will easily weld the contacts which can then lead to a totally dead amp and fried speakers.

The relays are great for turn-on/off muting but this is where their use should finish ... a crowbar circuit should be used for DC protection as a relay will easily be arc welded when used in the protection circuit.
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Old 29th December 2001, 02:26 PM   #18
paulb is offline paulb  Canada
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The DC-rated relays I used once have a permanent magnet built in next to the contacts. On disconnect, the magnet pulls the arc over, which lengthens it so that it quenches sooner. Cool or what? Quite expensive, as I recall.
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Old 29th December 2001, 03:13 PM   #19
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nifty yeah but that magnet will not do very nice things to the signal traveling thru the relay.
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Old 29th December 2001, 05:00 PM   #20
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A constant magnetic field (such as the Earth's--if you're going to worry about 'DC' magnetic fields, don't forget that one) will not have an effect on the current in a wire. Only a varying magnetic field will create a current in a wire.
Relays do sometimes have problems, but rarely. At the moment, I can only remember seeing one welded relay and it was smaller than I would have liked to have seen. Crowbar circuits aren't perfect, either. For that matter some people seem congenitally unable to drive a car safely. Does that mean no one should drive a car?

Grey
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