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Output Cap Loading

post #1 of 5
Thread Starter 

This is my first post. I Have been sand bagging for a while and have a question. In all the amp sch's using cap isolation at the output, a relay is used to delay turn on during which time the cap charges to equal potential as ground on the speaker side of cap. However, most sch's leave only a high value resistance to ground when contacts are open. (i understand the res's need for being there), but this tends to leave a bit of dc remaining causing a click when phones connect. I put a value of resistance equal to the phones (connected at the "NO"contacts of relay) so cap is charged fully and NO click is heard when relay energizes and resistor impedance is replaced by phone's impedance. Makes sense to me but i never see it done. Extreme differences like 32 and 600 ohm drivers, could utilize something like 220 ohm for loading before relay switching on. Am i missing something?

post #2 of 5

Correct me if I am wrong but without looking at the circuit the resistor acts like a surge suppressor in case of a fault in the circuit/capacitor . If it is now using the ohmic resistance of the head-phones what happens when a large fault current goes through them and not the resistance?

post #3 of 5
Thread Starter 

You misunderstand what i was saying. When an amp has a DC at the output due to that point being at 1/2 power supply voltage, a cap is used to block the dc from destroying the phones. This is common practice and using a transformer would be the other method. But, when first turning on the power supply,, a dc potential will be present at the output until the cap is charged. To prevent this dc from causing anything from a "thump" to 'flying cones", a relay is used to delay the spkr/phns from being connected until the cap is charged. While the cap is charging the spkr/phns are NOT connected to amp. Usually 15 to 30 seconds. Also usually the relay used is a DPDT type and the NO contacts are not connected. Therefore during the 15 to 30 sec wait the cap is taking on a charge so when rly is energized and spkr/phns are connected no thump is heard.   But many times i read people extending the 15 to 30 sec wait period because cap has not fully charged to 1/2 supply potential. I find that the cap will not charge to a potental that results in zero volts when phones are connected because the cap spkr/phns side is not at the resistance of the phones but instead is loaded with the high value res.

So:

1- Place one resistor per channel, (the value of the load) from N.O. contacts of relay to GND so when rly is not yet energized, the cap will be charged to it's proper potential for the spkr/phns when rly IS energized and the loads are connected up. It is like having a second set of spkr/phns hooked up during this turn on period. BUT a set of spkr/phns that can take the dc surge and not be affected.

post #4 of 5
Quote:
Originally Posted by Sunshine55 View Post

So:

1- Place one resistor per channel, (the value of the load) from N.O. contacts of relay to GND so when rly is not yet energized, the cap will be charged to it's proper potential for the spkr/phns when rly IS energized and the loads are connected up. It is like having a second set of spkr/phns hooked up during this turn on period. BUT a set of spkr/phns that can take the dc surge and not be affected.

Everything is a compromise ... and the ratings of the resistors tied to the output capacitors are just that - a compromise between the ideal of the headphone load and what may come after.wink.gif  The thing is, what may come after is totally out of control of the relay.

 

What you've done is to optimize the turn-on scenario, by allowing a greater current (faster charging of the caps) as a result of the current through the resistor being equal to the headphones.  However, the relatively small ratings of the resistors (32 to 300 ohm?) will allow much more current to flow when the voltage is absent.  The normal resistance to ground from an output cap is 1K or more.)  IOW, turn-off will release the cap's charge very, very quickly through the circuit and to the headphones.  Unfortunately, a delay-relay does nothing for the turn-off thump.

 

You may have degraded the advantage of having the relay in the first place.

 

P.S. Back to packing orders - was just taking a break. wink.gif


Edited by tomb - 3/14/13 at 7:26pm
post #5 of 5
Thread Starter 

Yup I just had turn on thump and increased delay time to 30 seconds and still a bit so i put 32-ohm on unused contacts so caps seen same res as phones and now no thump after 10 sec. Just don't see it done and was wondering why thanks Tom. I'll post pics of build when i can. (and others)

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