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Old 12th April 2009, 10:29 PM   #1
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Default Adding capacitors to power supply

I just completed one of Brian's LM3886 amps. It turned out wonderful! I was looking an Audiogon at a LM3875 that has 35000uF per channel on the power supply. I was wondering if it was possible to add 2 more 10000uF to each side of mine. At this point mine is totally stock. If this can be done could you just wire them in parallel with the existing caps (from the underside of the board) with snubs accross each new Cap? Or would I have to make additional modifications? to add the additional storage?
It probably doesn't need a thing else done to it....let me rephrase...it doesn't need a thing else....but I like to tinker!

Thanks for your input,
Jeff Miller
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Old 13th April 2009, 01:18 AM   #2
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Default adding caps in ps for LM3886

Good day sir,

I used to build chip amps for simplicity, until i got serious with SS amps, I still have one I used occationally using TOSHIBA TA8416H, 12w/ch, 30v single ended ps. I modified it recently implementing capacitance multiplier power supply I use in SS amps, I was amazed with the results, sounds smoother and cleaner, a big diffrence from conventional ps, gone is the ripple i hear when i stick my ears close to the speakers, hissing at full volume w/o signal is very negligible. At normal listening levels, IMHO i'ts Hi-Fi in a chip amp.! I'ts worth a try.

Regards,

Efren
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Old 13th April 2009, 10:19 AM   #3
AndrewT is online now AndrewT  Scotland
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Default Re: Adding capacitors to power supply

Quote:
Originally posted by jmillerdoc
I just completed one of Brian's LM3886 amps. It turned out wonderful! I was looking an Audiogon at a LM3875 that has 35000uF per channel on the power supply. I was wondering if it was possible to add 2 more 10000uF to each side of mine.
what speaker impedance are you using?
What smoothing capacitance are you using? giving RC=?
What NFB DC blocking cap are you using? giving RC=?
What DC blocking input cap are you using? giving RC=?

I have found that these all interact with each other. Simply changing one may change almost nothing or could bring the amplifying system into balance resulting in an audible change.
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Old 13th April 2009, 04:32 PM   #4
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Default HUH???

you wrote:
what speaker impedance are you using?
What smoothing capacitance are you using? giving RC=?
What NFB DC blocking cap are you using? giving RC=?
What DC blocking input cap are you using? giving RC=?

I have built a standard CHIPAMP.COM unmodified LM3886. I am sorry, I'm not that savy with electronics yet. I just started "The Art of Electronics" by Horowitz/Hill. It is 1100pgs long I I am still learning chp2 transistors. I do understand a little about RC filters and understand that R*C is important with filtering in the RC filter design and that with the increase in capacitance you begin to have some problems with the inductance of the circut and larger caps in the RC causes problems with this (I think) but I thought the snubber took care of that. I guess I should read the RC stuff in detail huh?

I , otherwise, can't answer above because I really don't know the last 3 are. As for impedence of the speakers they are custom built and are effectively around 7 (8ohm basically).

Can you explain breifly what NFB, blocking input, and smoothing cap. means? in terms of the RC=.

Thanks.
Jeff
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Old 13th April 2009, 05:48 PM   #5
AndrewT is online now AndrewT  Scotland
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The smoothing capacitance is the capacitance that is fitted after the bridge rectifier. Usually you have a dual polarity supply so you have two sets of smoothing, split into a +ve half and a -ve half. What value is fitted. The R part of the PSU RC is the speaker impedance, confirmed @ 8ohm.

The Negative FeedBack loop (NFB) determines the gain of the Power Amp.
It usually consists of two resistors from output to signal ground of the chipamp. From the junction of these two series connected resistors, a trace connects to the -IN input pin of the chipamp.

The ground connected resistor can have a capacitor in series. This cap blocks DC from passing through the lower leg of the NFB loop.
This achieves three parameters of the operation of the amp.
1.) it helps set the impedance seen by the -IN input pin.
2.) it sets the DC gain of the amplifier to 1 (+0dB).
3.) it filters out extreme low frequency non audio signals, it behaves as a high pass filter.
The value of the cap and the value of the lower leg resistor give you the RC of this filter.

The input usually has two filters that determine the passband of the amplifier.
A low pass filter attenuates the RF interference at the input of the amplifier and helps prevent the amplifier misbehaving.
A high pass filter attenuates the extreme low frequency signals at the input. The DC blocking capacitor is half of this high pass filter. Again the R and C values determine the RC of the filter.

Tell us what these 8component values are and we can then help determine if they are in correct proportion to allow the amplifier to operate properly and amplify the audio signal presented to it.
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