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Old 3rd February 2009, 05:32 PM   #1141
rob3262 is offline rob3262  United States
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Quote:
Originally posted by AndrewT

For 8ohm speakers C must be >=+-20mF/ch.
For 4ohm speakers C must be >=+-40mF/ch.

For very high power amplifiers, a second rule becomes applicable.
>=2mF/Apk of output current, to reduce cap ripple to an (arguably) acceptable level.
Hi Andrew,
Please confirm, is your term mF equal to millifarad, or microfarad?

I read it as micro, but the value seems to be much to small.

Thanks,
Rob
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Old 3rd February 2009, 05:34 PM   #1142
AndrewT is offline AndrewT  Scotland
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m=milli
u=micro

note that RC>160mS (milli Seconds)

If R=8r0
then R * C > 0.16 S
C > 0.16 / 8 > 0.02 F
0.02F = 20mF = 20,000uF
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Old 3rd February 2009, 06:12 PM   #1143
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Hi,

For those in doubt about SI units

m = milli (10^-3)
= micro (10^-6)
s = second (note that s is lower case)

S = Siemens (Ohm^-1) (Conductance S is upper case)
S is actually a derived unit [Amp/Volt]

ms = milli second
mS = milli Siemens!

Find more here: http://en.wikipedia.org/wiki/SI_units

\\\Jens
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Old 4th February 2009, 04:40 PM   #1144
rob3262 is offline rob3262  United States
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Quote:
Originally posted by AndrewT
that 1.5V across the resistor will affect the accuracy of the testing.

if you put a 19.9V Zener in it will leave 21.5-19.9=1.6V across the 1k0, passing 1.6mA through the Zener, ~32mW (<10% 0f 400mW rating).
now put in a 20.1V Zener. It will pass just 1.4mA. this is even less than the 32mW of the previous and will result in a voltage measurement that is not comparable to the first test.

You need a more consistent current source. Either a higher regulated voltage or a CCS.
I recommend that Zeners dissipate >=10% of rated power for consistent voltage regulation.

But the final test is in the powered amp, that is the one that matters.
Not a suprise, Andrew was correct on this. My first front-end test revealed a voltage mis-match (@ 2/10v). I will have to rematch all my zeners with a proper test
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Old 6th February 2009, 11:17 AM   #1145
clm811 is offline clm811  United States
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Default Which transformer

I have two pairs of transformers from which to choose:

Type#1 (from 120W stereo receivers) has a single set of center-tapped secondaries(45-0-45vac), weighs 13.5 lbs and I don' t know the VA rating;

Type#2 (from a 100w dual mono amps) has TWO sets of center-tapped secondaries(40-0-40 vac @ 1.6Aea), and weighs a tad over 9 lbs .

I plan to use the BrianGT power supply board, with between 22,000 and 33,000uf per rail.


For use here in assembling stereo amps, which would be more advantageous?

How about for assembling a pair of mono amps?

Here's a photo of each:
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Old 6th February 2009, 11:56 AM   #1146
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Both stated VA ratings seem low for a stereo amp. However, depending on how much of the weight is potting, they may be suitable.

Avel's un-potted 650 VA transformers weigh in at 10.8 pounds. The larger one could be capable of similar output. The problem is the voltage rating demands a higher VA rating if you want to extract the maximum power from your amps.

In normal home use (is that becoming my trademark?) you can use either, especially if your speakers are near 8 ohms nominal. No, you won't get as much power as the amp is capable of into low impedance loads. However, toroidal transformers can handle significant intermittent overloads. I'd lean towards the smaller one for its separate windings and the fact that you can use 63V caps.

If you want to get higher power into lower impedance loads go for at least 600 VA. You know where I'd look for inexpensive transformers...
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Old 6th February 2009, 12:03 PM   #1147
AndrewT is offline AndrewT  Scotland
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I got 170W into 8r0 from a 0-40, 0-40Vac transformer using Jens' 3pair board. It was over 300W into 4r0 but SOAR is well used at this combination of voltage and low impedance.

If you plan to never use 4ohm, nor 6ohm, nor 4 to 8ohm speakers then the higher voltage transformer can be used for 8ohm speakers and above.
But, the 45Vac will require >=75Vdc capacitors.
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Old 6th February 2009, 12:37 PM   #1148
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Chas, With the interest you are applying to this project I think it might be advantageous to spring for a toroid from Bob's favorite hobby store, Apex Jr., and then be assured you will achieve the results you are after.
Unless you know the history behind your present trannys it might not turn out pleasant. Decide what results you wish to achieve and place the appropriate item in to get there. You could also include some power supply caps in with the purchase and save on overall shipping charges. Apex has some good cap deals. Antek is another good choice for quality trannies. There service is great. I think the transformer is one of the most important choices.
Tad
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Old 6th February 2009, 11:05 PM   #1149
rob3262 is offline rob3262  United States
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I'm very satisfied with the Antek transformers I picked up, they are a good value. Thanks to Tad for their mention.

Also, got some nice big electolytics form Steve @ Apexjr. Send him an email with what you're looking for, he'll keep an eye on hooking you up.
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Old 7th February 2009, 01:47 AM   #1150
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Quote:
got some nice big electolytics from Steve @ Apexjr. Send him an email with what you're looking for, he'll keep an eye on hooking you up.
A while back I looked at Apex Jr. and he was offering Hitachi 22,000/100v caps but they're no longer listed.

Anyone know where I can get some 80v-100v caps >15,000uf?

Joe
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