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Old 9th March 2006, 01:18 PM   #1
SRMcGee is offline SRMcGee  United States
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Default Two Chassis Preamp -- Capacitor Question

Folks:

Please go easy on me -- I'm pretty new at this and have what may turn out to be a fairly dumb question. I am building an Aikido preamp in 2 enclosures using Bas Horneman's PCBs. The power supply chassis includes the transformers and the power supply PCB and will be connected to the preamp chassis using an umbilical cable.

Here's the question: I understand that some kind of RC network may be useful in smoothing out the two B+ lines when they enter the preamp chassis. I have some 10 ohm 5 watt resistors and little 47uf 400V electrolytic caps on hand. If I run each B+ line through one of the 10 ohm resistors followed by one of the 47 uf caps (pos side to B+, neg side to ground), will that do the trick?

Or have I got this all wrong?

Thanks for the help!

Regards,
Scott
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Old 9th March 2006, 02:58 PM   #2
sreten is offline sreten  United Kingdom
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Hi,

if your talking about modifying an existing
design you've probably got something wrong.

(C)RC smoothing is common, but note the voltage drop depends
on R and the load current, 47uf sounds too low (or R is not high
enough) to be effective.

However it is unlikely that you've thought of something the designer
of the preamp hasn't already considered and included or rejected.

Can't really say without more circuit details.

/sreten.
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Old 10th March 2006, 08:45 AM   #3
AndrewT is offline AndrewT  Scotland
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Hi,
what is the voltage and current you are playing with? Talking about B+ sounds like valve (tube) type voltages.

The 10r and 47uF gives a low pass filter of about 3400Hz.

This will not filter out mains hum or it's buzz type harmonics
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Old 10th March 2006, 06:55 PM   #4
AndrewT is offline AndrewT  Scotland
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Default oops!

Hi,
the roll off of a low pass 10r + 47uF is 340Hz.

The substance of my reply stands although a small amount of high harmonic buzzing will be removed at this lower cut off.

You need at least ten capacitors or 100r to give 34Hz as a starting point.

Can you afford the volts drop using 100r?
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Old 10th March 2006, 10:43 PM   #5
lndm is offline lndm  Australia
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Why do you need it?

If your ps has filtered the DC suitably for driving your amps, the only reasons you may want to do it again after the umbilical are to filter RF picked up by the umbilical or to compensate for the impedance of the umbilical.

Problem 1 can be dealt with by keeping the umbilical short or by shielding it. Problem 2 is less of an issue for valve equipment than high current low impedance solid state equipment, but if you wish, use heavy cable.

Be wary of adding an extra stage to your ps unless the last capacitor is large enough to supply current to the circuit down to a low frequency.
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