Is the DC supply not flat in GainClone? - diyAudio
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Old 21st January 2004, 05:34 PM   #1
lgreen is offline lgreen  United States
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Default Is the DC supply not flat in GainClone?

Having just looked at my DC supply in my LM3886T gainclone via a scope, I noticed it is not flat. In fact, its very wavy, not even close to a flat line. The supply is + and - 35 V, and probably drops to 31 V or so every cycle, even when not under a tough load. When the load is tough it drops even more. And I'm using 8 discrete MUR diodes and low ESR panasonic caps. 1000 uf per rail, separate rails for each channel, so 4 caps total for the stereo amp. Transformer is 500 VA used for both channels.


I have read many discusions here about desired capacitance levels for GC, in terms of sound, but have not seen much in terms of results and measurements. For example, I thought 1000 uf was considered the ideal by many (not all of course, some like more, some like it distributed, some like less....), but the measurement I took was a surprise, I thought it should be flat under a low load. Anyone have pics of the DC waveform that they get? (I can't post any for a few days...but can if you want me to after that).

Just so we are clear, those of you who use 1000 uf or less per rail and who actually have measured your DC supply don't actually get a flat DC supply, do you? Is this something to be worried about?
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Old 21st January 2004, 05:47 PM   #2
macboy is offline macboy  Canada
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I've said it before and I'll say it again:
1000 uF is not enough capacitance for a power amp

Somebody saw in the datasheet that each rail needs to have a 1000 uF cap on it, close to the amp. They thought that this capactiance is all that is necessary, but in fact it is intended to be over and above the power supply capacitance. That can be determined using solid engineering theory and methods. The people suggesting that 1000 uF total per rail obviously do not understand those basic electrical theories. The fact is, that for most of each AC half-cycle, the only thing powering the amp is the energy stored capacitors, because the AC voltage has fallen below what is necessary to power the amp. With too little energy storage, bad things happen.

I would suggest 5000 to 10000 uF per rail per 100W of output power.
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Old 21st January 2004, 10:05 PM   #3
Previously: Kuei Yang Wang
 
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Konnichiwa,

Quote:
Originally posted by macboy
I've said it before and I'll say it again:
1000 uF is not enough capacitance for a power amp
Maybe, but in case of Gaincard/clone it is, to listen to music, no matter what the 'scope says. So what?

Quote:
Originally posted by macboy
I would suggest 5000 to 10000 uF per rail per 100W of output power.
Making it 1250uF per rail for a 25W Gainclone (original is rated at 25W), NPV is 1,000uF. So are you actually recommending using a 1,000uF Capaictor per rail? You are contradicting your first statement, so it seems...

Sayonara
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Old 21st January 2004, 10:22 PM   #4
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Hmm, that seems like a good point. If a 25W gainclone uses a 1000uf per rail, shouldn't a 50W clone use 2000uf?
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Old 21st January 2004, 11:13 PM   #5
azira is offline azira  United States
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1000uF is pretty low for supply voltage ripple reduction. W/o going into how it sounds and etc. You are getting about 5.7VRMS of ripple from your supply. This is about what you would expect from only 1000uF of filtering. By comparison, my +24V power supply with 6300uF of filtering has around 2.1VRMS of ripple. If you want to flatten out your power source, you will need more capacitance. Look up bridge rectifier equations in your analog electronics textbooks for the ripple calculations.

I think you'll be better off with atleast 4700uF and probably 10000uF of filtering.
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Old 21st January 2004, 11:25 PM   #6
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I'm puzzled by this: some people run gainclones from batteries.

Do battery powered gainclones sound inferior to those with >5V psu ripple?
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Old 21st January 2004, 11:32 PM   #7
chrish is offline chrish  Australia
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I have also found this point interesting. I seem to recall that a very well respected poster to this thread has said that he has not even tried more than 1000 uF per rail, but later stated that a battery supply sounds better! My somewhat limited electronics knowledge suggests that a well filtered power supply will approximate a pure DC (battery) source better than 1000 uF per rail, but what would I know
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Old 22nd January 2004, 12:15 AM   #8
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lm1875 - 20watts - reccomended near-chip capacitor per rail - 100uF bypassed with 100nF ceramic.

lm3886 - 60 watts - reccomended near-chip capacitor per rail - 470uF bypassed with 100nF ceramic

lm4780 - 120 watt - reccomended near chip capacitors 1000uF bypassed with 10uF bypassed with 100nF ceramic.

these were intended to be close to the IC. ceramic was intended for the better RF performance.

the chips were made with a high PSRR in mind, so supply ripple won't affect musical use by as much.
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Old 22nd January 2004, 02:45 AM   #9
Alex S is offline Alex S  Singapore
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there was a perceived need to balance the advantages of a stiff, low-ripple supply against the ill-effects of huge current spikes caused by large power supply capacitances.

since the LM3875 has very high built-in PSRR (power supply ripple rejection), it was felt this should be exploited to sacrifice the former to favor (minimise) the latter.

to have your cake and eat it, i.e. to have both low-ripple supply and low suppy line spikes, there are a few ways, namely
a. battery
b. choke filtering (this is suitable for the large, constant current draw of class A, so less applicable here)
c. active regulation or capacitance multiplier

note all the above three methods then introduce the problem of high power supply output impedance (which can be mitigated somewhat by having large/low-ESR capacitances after them).
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Old 22nd January 2004, 10:18 AM   #10
NealG is offline NealG  United Kingdom
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Quote:
I've said it before and I'll say it again:
1000 uF is not enough capacitance for a power amp
Don't flame me this is just MHO and observation.

I am leaning more and more to this way of thinking with GC. Yes the 'scope shows lower ripple with 10,000uF / rail but to my ears it also sounds better. With lower capacitance (1000uF / rail) I hear a gritty sound presentation, over bright but with clear mids. Bass is thinner and overall the sound is less cohesive, I find it fatiguing to listen to for any length of time.

With greater capacitance the bass fills out and I detect no loss of detail or clarity, the grittiness seems absent and the overall presentation is smoother, dare I say it, more valve like, more natural. I have no problem listening for several hours.
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