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Old 7th March 2008, 08:03 PM   #1
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Default Capacitance in PSU

How do I know if I have too much or too little in PSU. I am using SS rectifier diodes.
I mean how much is enough to makeit sound good?
I have experienced with capacitance and more capacitance means stiffer sound. How do I know if it is not too much stiff? Is there any design rule?

Thank you.
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Old 7th March 2008, 08:50 PM   #2
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My rule of thumb that generally works for balanced lows in tube power amps is 100uF per 10W per channel for final PSU cap. For example if I make a common CLC filter for a stereo 2X20W tube amp, I use 2X220uf in parallel for the last cap.
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Old 7th March 2008, 08:57 PM   #3
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Default Re: Capacitance in PSU

Quote:
Originally posted by kacernator
How do I know if I have too much or too little in PSU. I am using SS rectifier diodes.
I mean how much is enough to makeit sound good?
I have experienced with capacitance and more capacitance means stiffer sound. How do I know if it is not too much stiff? Is there any design rule?
Are we talking guitar amp here, or HiFi?

I don't know what you mean by "stiffer sound". When designing capacitor input filters with SS diodes, I like to keep the initial capacitor low -- not over 47uF -- just as with a hollow state PS. The power xfmrs, such as the Hammond "Classic" series weren't designed to accommodate the Isurge values that silicon can easily handle. Working into a big capacitor, with an Isurge well above an amp, is something that a silicon diode will handle easily, but the xfmr wasn't intended to handle that.

As for subsequent ripple filtering with an LC lowpass filter, what's important is keeping the LF poles well away from the dominant LF pole (usually that of the OPT) to avoid such nasties as "filter bounce" and LF instability under gNFB.

For a HiFi amp, good voltage regulation is desireable. Maybe not so much in a gee-tah amp where you like overdrive distortion, and power supply collapse for increased compression and "sustain". (Which is why you frequently see undersized power xfmrs and smaller filter capacitors in these designs.)
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Old 7th March 2008, 09:16 PM   #4
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Talking about HIFI here. Is the your preference of the bass response and tightness in amp which determines thevalue of filter cap? I am just talking about sound, dont want to talk about tech stuff related do PSU design, which I am aware of.
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Old 7th March 2008, 09:23 PM   #5
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Hi,

Quote:
Originally posted by kacernator
...dont want to talk about tech stuff related do PSU design...
Not to be a downer, but you should really consider having it built for you then, rather than trying to do it yourself.

The power supply is the most important stage and can't just be "slapped together" with rules of thumb if you want a decent HiFi amp.

Cheers!
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Old 7th March 2008, 09:39 PM   #6
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Quote:
Originally posted by kacernator
Talking about HIFI here. Is the your preference of the bass response and tightness in amp which determines the value of filter cap?
I've found that bass response and tightness has more to do with the quality of the OPT, the woofies you're using and how much damping they need to keep 'em under control. Adding gNFB helps by making the sonics less dependent on circuit and component peculiarities.

As for ripple filters, I design 'em to be quiet and stable. Whatever values of L and C make that happen are the ones I use. In push-pull designs, the PS is out of the signal path since cancellation occurs at the primary center tap, making it a virtual ground point. What more can you do? Other considerations are an invitation to veer off into the audiophoolery wilderness of unsubstantiated, overblown claims, old wives' tales and urban legends and other nonsense.
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Old 7th March 2008, 09:44 PM   #7
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Quote:
Originally posted by Geek
Hi,



Not to be a downer, but you should really consider having it built for you then, rather than trying to do it yourself.

The power supply is the most important stage and can't just be "slapped together" with rules of thumb if you want a decent HiFi amp.

Cheers!

I didnt mean it like that. I am know the techie stuff of PSU, but I am asking how do you for example determine the last PSU cap for one channel of Aikido preamp? Generaly speaking if you have flabby bass response you go up by 100uF or so? Or is there a rule or something when have certain capacitance in you PSU and increasing it will not change much in a sound point of view? I mean going from 22uF to 220uF can be clearly heard. Can be also clearly heard 440uF to 1000uF?
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Old 7th March 2008, 10:27 PM   #8
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Quote:
Other considerations are an invitation to veer off into the audiophoolery wilderness of unsubstantiated, overblown claims, old wives' tales and urban legends and other nonsense.
Sadly, this is very true. I haven't seen much of it here but at AA there has been an abundance of dogmatic nonsense. Much of it flies in the face of common sense and the chief culprit is a self-confessed anti-engineer.

Quote:
Generaly speaking if you have flabby bass response you go up by 100uF or so? Or is there a rule or something when have certain capacitance in you PSU and increasing it will not change much in a sound point of view?
Flabby bass is more likely to be due to inadequate damping of the speaker - a combination of the speaker's own mechanical damping and the amp's electrical damping - than a problem with the final smoothing cap.

There is definitely an upper limit to what will help in the final cap - a law of diminishing returns, if you like. Salas's 'rule of thumb' seems reasonable to me. In my 15w + 15w stereo amp, with EL34s triode-strapped in PP and a common power supply, I have 470uF as the final cap. That's probably a bit more than I need and I'm sure that adding any more would be of no benefit.
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Old 6th April 2008, 12:06 PM   #9
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In my Aikido preamp I have 47uF-choke-440Uf foreach channel.
I added 220uF parallel to 47uF for each channel and bass accuracy and clearnes is much better. However the sound is tighter but not as in your face.

So how do you know, concerning the sound, that you have enough filtering capacity or too much or too low?
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Old 6th April 2008, 02:56 PM   #10
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Quote:
Originally posted by kacernator
In my Aikido preamp I have 47uF-choke-440Uf foreach channel.
I added 220uF parallel to 47uF for each channel and bass accuracy and clearnes is much better. However the sound is tighter but not as in your face.

So how do you know, concerning the sound, that you have enough filtering capacity or too much or too low?
For technical evaluation, you have enough capacitance when you have low enough ripple.

For sonics, you have best capacitance when in a neutral sounding system the bass is extended and full and bouncy, without being slow nor hasty. The mids must be relaxed but not thick. In general, you must get neither tight and stressed nor sluggish music.

Use good neutral headphones to be sure, since you will avoid speaker & room deviations. This way you will know that you don't compensate for system anomalies elsewhere.

Its better to start low capacitance and add little by little. Also know that lytics have strong signatures and with another brand you may find that another value proves closer to what you need. My best recommendation is Mundorf M-Lytic.
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