Auto soft-start for class-T amp not working

Nuuk · 16th June 2006, 10:02 AM

I have built the circuit below to auto soft-start a Charlize amp

The idea is that there is 330 ohms in series with the 12 volt power supply until the relay shorts the resistor, allowing full voltage to the amp. I have used a single pole relay rated at 8 amps.

When tested on its own it works OK but when the amp is connected, the relay repeatedly trips every few seconds. Can somebody suggest a way to fix this problem or even tell me what is happening?

richie00boy · 16th June 2006, 10:47 AM

Try 10nF on pin 5 to ground.

Nuuk · 16th June 2006, 12:33 PM

Thanks Ritchie - it appears to work OK!

I had no idea such genius lurked just across the county border!

richie00boy · 16th June 2006, 01:31 PM

Had you considered using a simple transistor off an RC charging network?

Nuuk · 16th June 2006, 03:27 PM

Quote:

Had you considered using a simple transistor off an RC charging network?

In a word - NO! Any more info?

richie00boy · 16th June 2006, 03:59 PM

TBH the 555 has a more elegant edge, but this could be worth a try as a comparison.

Ignore the transistor type - use something like a BD139.

You could add R1 and D1 to your circuit to give a faster reset.

Bearman · 16th June 2006, 09:36 PM

That looks interesting, All I have is a TIP3055 which I hope works, but what values would you use for a 3-4 second delay?

Thanks for the circuit.

cpemma · 16th June 2006, 11:34 PM

The low-gain 3055 will I think need too much base current so R2 would need to be much smaller, hence C7 much larger for the same delay. A 2N2222A will drive any relay with ease, the 2N3904 shown most small relays.

An improvement for better reproducibility and longer delays without big caps is a zener in the base lead; the transistor won't turn on until the cap has charged to (Z+0.7)V rather than only 0.7V. Pick one about 60% supply to ease calculations.

Nuuk · 17th June 2006, 06:41 AM

Thanks Ritchie, I'll give it a try when I can find time. My hi-fi projects list is growing longer every week!

richie00boy · 17th June 2006, 09:50 AM

cpemma's comments are all good. He beat me to it

I've also thought about using a little power Darlington from the TIP range as this would raise the threshold a bit as well as requiring less base current. You could even try a MOSFET.

16th June 2006, 10:02 AM	#1
Nuuk diyAudio Member Join Date: Feb 2003 Location: Somerset, SW England	Auto soft-start for class-T amp not working I have built the circuit below to auto soft-start a Charlize amp The idea is that there is 330 ohms in series with the 12 volt power supply until the relay shorts the resistor, allowing full voltage to the amp. I have used a single pole relay rated at 8 amps. When tested on its own it works OK but when the amp is connected, the relay repeatedly trips every few seconds. Can somebody suggest a way to fix this problem or even tell me what is happening? __________________ The truth need not be veiled, for it veils itself from the eyes of the ignorant.

16th June 2006, 10:47 AM	#2
richie00boy Did it Himself diyAudio Member Join Date: Nov 2003 Location: Gloucestershire, England, UK	Try 10nF on pin 5 to ground. __________________ www.readresearch.co.uk my website for UK diy audio people - designs, PCBs, kits and more

16th June 2006, 12:33 PM	#3
Nuuk diyAudio Member Join Date: Feb 2003 Location: Somerset, SW England	Thanks Ritchie - it appears to work OK! I had no idea such genius lurked just across the county border! __________________ The truth need not be veiled, for it veils itself from the eyes of the ignorant.

16th June 2006, 01:31 PM	#4
richie00boy Did it Himself diyAudio Member Join Date: Nov 2003 Location: Gloucestershire, England, UK	Had you considered using a simple transistor off an RC charging network? __________________ www.readresearch.co.uk my website for UK diy audio people - designs, PCBs, kits and more

16th June 2006, 03:59 PM	#6
richie00boy Did it Himself diyAudio Member Join Date: Nov 2003 Location: Gloucestershire, England, UK	TBH the 555 has a more elegant edge, but this could be worth a try as a comparison. Ignore the transistor type - use something like a BD139. You could add R1 and D1 to your circuit to give a faster reset. __________________ www.readresearch.co.uk my website for UK diy audio people - designs, PCBs, kits and more


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16th June 2006, 09:36 PM	#7
Bearman diyAudio Member Join Date: Apr 2005 Location: Utah	That looks interesting, All I have is a TIP3055 which I hope works, but what values would you use for a 3-4 second delay? Thanks for the circuit.

16th June 2006, 11:34 PM	#8
cpemma diyAudio Member Join Date: Jul 2003 Location: S Yorkshire OK	The low-gain 3055 will I think need too much base current so R2 would need to be much smaller, hence C7 much larger for the same delay. A 2N2222A will drive any relay with ease, the 2N3904 shown most small relays. An improvement for better reproducibility and longer delays without big caps is a zener in the base lead; the transistor won't turn on until the cap has charged to (Z+0.7)V rather than only 0.7V. Pick one about 60% supply to ease calculations.

17th June 2006, 06:41 AM	#9
Nuuk diyAudio Member Join Date: Feb 2003 Location: Somerset, SW England	Thanks Ritchie, I'll give it a try when I can find time. My hi-fi projects list is growing longer every week! __________________ The truth need not be veiled, for it veils itself from the eyes of the ignorant.

17th June 2006, 09:50 AM	#10
richie00boy Did it Himself diyAudio Member Join Date: Nov 2003 Location: Gloucestershire, England, UK	cpemma's comments are all good. He beat me to it I've also thought about using a little power Darlington from the TIP range as this would raise the threshold a bit as well as requiring less base current. You could even try a MOSFET. __________________ www.readresearch.co.uk my website for UK diy audio people - designs, PCBs, kits and more

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