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Old 18th May 2005, 05:37 PM   #1
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Default choke input power supply filter

I have just acquired some of BrianGT's excellent aleph 30 boards and am building a power supply.

The Aleph 30 requires 25 volt rails, but with the transformers that I have on hand, I get 40 volt rails with one Aleph channel attached to the CLC filtered supply.

When I put the 2.2mh air cored inductor ahead of the capacitors in the choke input configuration, the rail voltages fell to 33 volts. Still high but better than 40 volts.

Is the choke input configuration appropriate for the Aleph 30. I have read that the choke input needs current draw to work properly...is the draw from an Aleph 30 constant enough to make the choke input work well??

Would there be anything wrong in adding another 2.2mh inductor between banks of capacitors(LCLC)...this would knock off another volt or so. But I thought I read somewhere that I might be hurting the voltage regulation properties of the power supply.
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Old 18th May 2005, 05:43 PM   #2
Magura is offline Magura  Denmark
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CRCLC ??

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Old 18th May 2005, 07:59 PM   #3
eLarson is offline eLarson  United States
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As long as the minimum current draw doesn't fall below a certain threshold, the filter should continue to act as a choke-input type. (You'll get about 80% of the secondary's voltage, rather than ~1.4 times that amount as with a capacitor input.)

Search around the archives for choke-input power supply if you are curious about the particulars.

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Old 5th June 2005, 05:36 PM   #4
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audionut,

You're using the input choke filter in a wrong way. A choke filter (CLC) is used to reduce power supply noise and hum, not to reduce the voltage itself.
Never place an extra inductor between the rectifier and the capacitors, like you did in the 'LCLC' filter. This will put a lot of stress on the rectifier diodes because of high di/dt at turn-off each period. The fact that you experienced a lower rail voltage in this configuration is because the charge current of the capacitors is affected by the inductor in a negative way.
In my opinion the best solution is to look for a better matching transformer with lower secondary voltage windings.


Leroy
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Old 5th June 2005, 08:14 PM   #5
MRupp is offline MRupp  Germany
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The position of the inductor in audionut's input choke (LC) filter is actually correct, except that it's inductance is too low (lower than the critical impedance). As a matter of fact it would still reduce the current peaks in the diodes, not the other way around. A short overview of choke input filters, including formulas for calculating the critical impedance, can be found here:

http://www.qsl.net/i0jx/supply.html

In addition, PSU designer from Duncanamps can simulate input choke supplies. Some more theoretical background on LC filters can be found here:

http://www.tpub.com/content/neets/14.../14179_193.htm
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Old 6th June 2005, 06:25 PM   #6
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Hi,

the first link doesn´t seem to work. Any ideas?


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Old 6th June 2005, 07:17 PM   #7
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Default Re: choke input power supply filter

Quote:
Originally posted by audionut
Is the choke input configuration appropriate for the Aleph 30. I have read that the choke input needs current draw to work properly...is the draw from an Aleph 30 constant enough to make the choke input work well??

Would there be anything wrong in adding another 2.2mh inductor between banks of capacitors(LCLC)...this would knock off another volt or so. But I thought I read somewhere that I might be hurting the voltage regulation properties of the power supply.
The amp has to be drawing the ordinary amount of current to
make this work, and assuming that the channels are biased
up, adding more inductance is OK. The average draw of an
Aleph is constant, so I don't expect that you would have a
problem. Putting inductance in CLC will also reduce the noise
by an order of magnitude or more, and is very desirable.
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Old 7th June 2005, 06:20 AM   #8
Stefano is offline Stefano  Italy
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Hi guys! I would ask you a question, it's real that, since in a class A amp current drow is costnt, the input supply choke may be iron core or similar without risk of saturation?
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Old 7th June 2005, 06:43 AM   #9
rchua77 is offline rchua77  Singapore
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Quote:
Originally posted by Stefano
Hi guys! I would ask you a question, it's real that, since in a class A amp current drow is costnt, the input supply choke may be iron core or similar without risk of saturation?

You can use iron core chokes but be careful of their saturation values. Once saturated, the choke doesnt have any more inductive behaviour and starts behaving like a resistor.

I hope I got it right and answered the question.
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Old 7th June 2005, 07:20 AM   #10
AndrewT is online now AndrewT  Scotland
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Hi,
there is only one ClassA topology that has constant current draw.

All the other ClassA amplifiers whether single ended or push pull have modulated current on the supply rails.

You can used bridged amps off common rails to achieve constant draw while in ClassA mode.
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