caps for quad 405 amp

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goodguys

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Joined 2011
Hi. I want to add a few caps to my quad 405 amp, near the output and driver transistors, so the transistors have a local power supply to tap into rather than waiting for the mains power supply to catch up.

What would be a good location for these caps

I can see c15 and c16 decoupling caps near the two fuses, and am thinking about a 300uf across both of the caps, that is close to one of the driver and output transistors, but what about the others on the opposite side of the board.

Many thanks.
 
JonSnell Electronic

JonSnell Electronic

Member
Joined 2012
jonsnell.co.uk
goodguys said:
Hi. I want to add a few caps to my quad 405 amp, near the output and driver transistors, so the transistors have a local power supply to tap into rather than waiting for the mains power supply to catch up.

What would be a good location for these caps

I can see c15 and c16 decoupling caps near the two fuses, and am thinking about a 300uf across both of the caps, that is close to one of the driver and output transistors, but what about the others on the opposite side of the board.

Many thanks.
Click to expand...
Do you think if it was required in the original design as an improvement, Quad didn't notice the need and missed them out?
 
G

goodguys

Member
Joined 2011
Well Quad did make a few mistakes with their amps, and this mod is endorsed by bernd ludwig.

I have a clone quad amp made by net audio and they use caps all over the place, i have fiddled with these trying different values and it does make a difference, wouldn't mind trying the mod on a stock 405.

Do you know what the voltage is across the op amp, could i put a cap across the power lines, say 500uf

Thanks.
 
D

DF96

R.I.P.
Joined 2007
myweb.tiscali.co.uk/g8hqp/
If you want to improve something you need to understand the circuit better than the original designers.
goodguys said:
What would be a good location for these caps
Click to expand...

Hi. I want to add a few caps to my quad 405 amp, near the output and driver transistors, so the transistors have a local power supply to tap into rather than waiting for the mains power supply to catch up.
Click to expand...
Perhaps you could show us your calculations on how long the transistors might have to wait, and how your extra caps would improve this?
 
Mooly

Mooly

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DF96 said:
Perhaps you could show us your calculations on how long the transistors might have to wait, and how your extra caps would improve this?
Click to expand...

:)

goodguys said:
Do you know what the voltage is across the op amp, could i put a cap across the power lines, say 500uf Thanks.
Click to expand...

The voltage will always be equal or less than the maximum recommended supply for the opamp in question.

The 405 is one amplifier I would be very wary of adding a significant time constant to, at least in the area around the opamp. You would need to observe the behaviour of the amp at power on and power off to ensure you have not altered the settling time i.e. you could end up with an amp that produces a large thump at power on/off (or worse if it latches in some non valid state).
 
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