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Old 13th February 2008, 01:37 PM   #1
Dxvideo is offline Dxvideo  Turkey
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Smile Another capacitor question...

Hi all,

I am working on a regulated gainclone project that;

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and the psu is;

Click the image to open in full size.

The amp will work with ~25.5v symmetrical. And will give ~37.5W into 6R which is fairly enough to me. Under that conditions, "overture design guide" says load will sink ~3.5A peak. I am planning to use Panasonic FM series (35v) as reservoir capacitors in both PSU and gainclone circuit. (1500 for PSU and 1000 for onboard)

And my question is;
As I read in Farnell catalogue, Panasonic FM series ripple capacities are;
1,500F/35v --- 3630mA,
1,000F/35v --- 3190mA..
Thats why I select these caps... However I am still not sure that they will enough or not for these positions...

Has anybody worked with this type capacitors before? Do you think they will enough?

Thx in advance...
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Ozgur
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Old 13th February 2008, 02:33 PM   #2
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High ripple current ratings come from low esr. That's because low esr means low heating, which is what destroys caps. Low esr is important for audio, since it means the cap can deliver its energy with a lower voltage drop. Ripple current ratings, however, aren't very important for audio, since you never have a continuous full power signal, and should have very little ripple on the cap. IMO, your caps are fine.
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Old 13th February 2008, 07:35 PM   #3
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Hi Conrad,

is this really true, the speaker return path is through the capacitors. Whatever current flows through the speaker flows through the capacitors.

A simple example is when you disconnect the mains, the capcitors are sourcing the current to keep on playing until the energy is depleted.

The ac current through the speaker must be sourced and sunk from some low impedance source definately not the center tap of the transformer.

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Nico
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Old 13th February 2008, 07:44 PM   #4
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Hi Ozgur,

I hope you are keeping well. Your choice and understanding of ripple current is correct. Connecting two similar caps in // would double the ripple current capabilities.

Kind regards

Nico
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Old 13th February 2008, 08:02 PM   #5
AndrewT is offline AndrewT  Scotland
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Default Re: Another capacitor question...

Quote:
Originally posted by Dxvideo
The amp will work with ~25.5v symmetrical. And will give ~37.5W into 6R which is fairly enough to me. Under that conditions, "overture design guide" says load will sink ~3.5A peak
the prediction of 3.5Apk is into a resistive load. Into a reactive load (real speaker/crossover) the peak transient current can increase to well over double the 6r0 value. I use the formula Ipk=Vpk/R/0.35 and for your 6ohm speakers could approach 10Apk.
Can that sort of current be sourced from the parallel combination of 220uF and lt1083?

When current demand rises, how high does the ripple become on the main smoothing caps? Will the regulators drop out at the bottom of the ripple waveform?
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Old 13th February 2008, 09:17 PM   #6
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Default Re: Re: Another capacitor question...

Quote:
Originally posted by AndrewT

When current demand rises, how high does the ripple become on the main smoothing caps? Will the regulators drop out at the bottom of the ripple waveform?
Hi Andrew,

I would agree fully with your rule of thumb Andrew.


Kind regards

Nico
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Old 14th February 2008, 12:08 AM   #7
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Hi Nico, ripple current ratings are really thermal ratings and have to do with the capacitors ability to dissipate the heat potentially generated by current through the esr. That part of things just isn't significant in audio, as signals have a high crest factor. Esr and capacity certainly are important, for the reasons mentioned above. Thus, I tend to look at esr (actually I never really look at esr since it's highly frequency dependent, but look at dissipation factor instead), rather than ripple current ratings. DF is a much better indicator of cap quality, though not the only factor.
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Old 14th February 2008, 07:24 AM   #8
Dxvideo is offline Dxvideo  Turkey
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Quote:
Originally posted by Nico Ras
Hi Ozgur,

I hope you are keeping well. Your choice and understanding of ripple current is correct. Connecting two similar caps in // would double the ripple current capabilities.

Kind regards

Nico
My dear friend....
How are you? I know I am guilty for "no writing". Please forgive me.. Thats a project for our club (as usual)....
I want to use that capacitors, because of theyre very very small (12,5mm; half of a 10,000!) And hear really good things about FM types..
Thanks for your comment. I will write you soon.
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Old 14th February 2008, 07:28 AM   #9
Dxvideo is offline Dxvideo  Turkey
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Default Re: Re: Another capacitor question...

Quote:
Originally posted by AndrewT
the prediction of 3.5Apk is into a resistive load. Into a reactive load (real speaker/crossover) the peak transient current can increase to well over double the 6r0 value. I use the formula Ipk=Vpk/R/0.35 and for your 6ohm speakers could approach 10Apk.
Can that sort of current be sourced from the parallel combination of 220uF and lt1083?

When current demand rises, how high does the ripple become on the main smoothing caps? Will the regulators drop out at the bottom of the ripple waveform?
Dear Andrew,
I think youre right. But LT1083 can source up to 10A peaks while Vi - Vo < 10v, thats ok. However, do you say should I put a bigger cap on output? So whats onboard (1,000uF) cap is there for?

Thx...
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Old 14th February 2008, 08:34 AM   #10
AndrewT is offline AndrewT  Scotland
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Default Re: Re: Re: Another capacitor question...

Quote:
Originally posted by Dxvideo
.... LT1083 can source up to 10A peaks while Vi - Vo < 10v, thats ok. However, do you say should I put a bigger cap on output? So whats onboard (1,000uF) cap is there for?
Hi Dx,
You have answered the question, the 1083//220uF can source 10Apk.
As you have correctly pointed out the amp is fed by 1083//220uF//1000uF and this will find it even easier to source that same 10Apk.

You will need to measure the ripple at the input to the regulator when mains is at minimum supply voltage and amp is delivering significant power. Then decide if the smoothing in combination with the transformer ensures that the regulators do not drop out in worst case "normal" working conditions.
A scope on the reg input works for this, similarly a scope on the reg output will detect if drop out is occurring.
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