Muting relay problem - Page 2

decky · 21st March 2008, 02:06 PM

You are right about the relay arrangement and regarding the noise- there is a shot delay on the power On sequence and no noise. There is a short delay on power OFF and there is that thump noise. So, the relay does NOT react instantly. The output impedance of the gain stage is only 300ohm - and that can be defined as low.

Mooly · 21st March 2008, 06:33 PM

O.K. What I would try is this, first connect a LED + resistor across relay so you can see when it mutes and if it switches cleanly. Without component values, it's a bit hard to say but I think C11 is keeping Q1 turned on to long. Try a smaller cap, perhaps even less than 1mfd depending on the values of R7/8 . You should aim to keep Q1 only just on. If you could tell us values of R7/8 and C11 it would help.You could also try adding a resistor across C11 to speed up discharge.
Karl

Mooly · 21st March 2008, 06:44 PM

Me again

Thinking about this, it's not good practice to define the discharge rate by just dumping the charge into a B-E junction. The transistor will linger in no mans land as the base current reduces slowly. Without major mods look at this idea. Not shown on sketch but add resistor across C11.

john blackburn · 21st March 2008, 06:55 PM

As I understand it, the relay Diode spoken of by Frank Berry isn't the one shown on the schematic.

Scroll down a way to potection diodes on this page...........
http://www.kpsec.freeuk.com/components/relay.htm

John

Original Burnedfingers · 21st March 2008, 10:20 PM

How about putting a relay on the input of the Gainclown that shorts the input to ground when the amp is powered off?

Power it off the AC going to the bridge. Have a delay upon turn on. On turn off it will turn off right away.

decky · 21st March 2008, 11:48 PM

OK thanks guys, I will try with the resistor across C11 which is only 1uF - is 100ohm enough?

Mooly · 22nd March 2008, 08:29 AM

Sounds low to me, what's value of R7/8 ?

decky · 22nd March 2008, 10:15 AM

22k and 2k2 respectively. I hope Rod does not mind this little info disclosure of mine

Mooly · 22nd March 2008, 12:38 PM

Add a resistor across C11, try 56Kohm to 120Kohm. Made some assumptions, AC from transformer 18-0-18 volts. R9 around 4K7. Also add a resistor across the Base Emmiter junction of Q2 the BD140. Use a value similar to R9.

AndrewT · 22nd March 2008, 12:54 PM

have you removed C12 yet?

22nd March 2008, 12:54 PM	#20
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	have you removed C12 yet? __________________ regards Andrew T.

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21st March 2008, 02:06 PM	#11
decky diyAudio Member Join Date: Sep 2006	You are right about the relay arrangement and regarding the noise- there is a shot delay on the power On sequence and no noise. There is a short delay on power OFF and there is that thump noise. So, the relay does NOT react instantly. The output impedance of the gain stage is only 300ohm - and that can be defined as low.

21st March 2008, 06:33 PM	#12
Mooly diyAudio Member Join Date: Sep 2007	O.K. What I would try is this, first connect a LED + resistor across relay so you can see when it mutes and if it switches cleanly. Without component values, it's a bit hard to say but I think C11 is keeping Q1 turned on to long. Try a smaller cap, perhaps even less than 1mfd depending on the values of R7/8 . You should aim to keep Q1 only just on. If you could tell us values of R7/8 and C11 it would help.You could also try adding a resistor across C11 to speed up discharge. Karl

21st March 2008, 06:55 PM	#14
john blackburn diyAudio Member Join Date: Sep 2006 Location: Wirral UK	As I understand it, the relay Diode spoken of by Frank Berry isn't the one shown on the schematic. Scroll down a way to potection diodes on this page........... http://www.kpsec.freeuk.com/components/relay.htm John

21st March 2008, 10:20 PM	#15
Original Burnedfingers diyAudio Member Join Date: Nov 2002	How about putting a relay on the input of the Gainclown that shorts the input to ground when the amp is powered off? Power it off the AC going to the bridge. Have a delay upon turn on. On turn off it will turn off right away.

21st March 2008, 11:48 PM	#16
decky diyAudio Member Join Date: Sep 2006	OK thanks guys, I will try with the resistor across C11 which is only 1uF - is 100ohm enough?

22nd March 2008, 08:29 AM	#17
Mooly diyAudio Member Join Date: Sep 2007	Sounds low to me, what's value of R7/8 ?

22nd March 2008, 10:15 AM	#18
decky diyAudio Member Join Date: Sep 2006	22k and 2k2 respectively. I hope Rod does not mind this little info disclosure of mine

22nd March 2008, 12:38 PM	#19
Mooly diyAudio Member Join Date: Sep 2007	Add a resistor across C11, try 56Kohm to 120Kohm. Made some assumptions, AC from transformer 18-0-18 volts. R9 around 4K7. Also add a resistor across the Base Emmiter junction of Q2 the BD140. Use a value similar to R9.

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