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Old 2nd May 2003, 04:05 PM   #21
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how can i measure the current my gainclone needs? if i connect my multimeter in series with one of the power rails, i get zero reading and severe distortion on the output.
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Old 2nd May 2003, 04:21 PM   #22
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Talking Truck

Quote:
Originally posted by Matttcattt
its going to be a car amp max voltage is about +/- 14v, i cant aford more batteries.

it doesnt need that big a power supply does it?

my 2 channel gainclone with a +/-12v supply takes about 2A max, which is about 12VA needed? 12x8=96VA. or am i insane?

at 60watts per chip at 24v is 2.5A max, 2.5x16=40A.

re-thinking needed possibly

maybe i could aford +/-24v supplys

edit: i have 66,000uf on each rail


12Vx2A = 24 VA.
Anyway, you need big batteries, and worst than that, how are you going to charge that?
The alternator on your car may not handle with that.
Maby you could go with a switching PSU instead of batteries.
You're confusing me!
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Old 2nd May 2003, 05:20 PM   #23
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its not 12v, its +/-12v, 24v total
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Old 2nd May 2003, 08:08 PM   #24
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Quote:
Originally posted by Matttcattt
how can i measure the current my gainclone needs? if i connect my multimeter in series with one of the power rails, i get zero reading and severe distortion on the output.
I also tried to measure the current exactly the same way as you tried, and had hudreds of mA reading on multimeter. Of course I had SEVERE distortion on the output. There should be some other ways to measure it. Any comment, please?
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Old 2nd May 2003, 08:30 PM   #25
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are the capacitors circled in red electrolytic? can i use electrolytic capacitors? can i use 1000uf ones? i have loads. i am using 4 on one circuit board, can i use one 1000uf capacitor per rail for all of them?

where do the input capacitors go? after the potentiometer? where the blue circles are?
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Old 5th May 2003, 08:20 AM   #26
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Red face waste of parts and time

Matttcattt,
Something has hit me in my had, I was too confused.
Have you read the datasheet of the LM3875?
With +/-12 volts you get something around 10 watts.
If you bridge two of them, you get 20 watts.
Paralleling you double the current.
So... with 4 chips in bridge-parallel configuration per channel you'll get 20 watts!
It's a big waste of parts, time and money.
These power op-amps are not made to work at these low voltages.
You should investigate the TDA1562.
You'll get around 3 times more power with one chip per channel than with four LM3875s in bridge-parallel at +/-12v.
And it's made to work on single voltage supply.
For these voltages I don't know a chip with more power than this.
It was made for car-audio.
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Old 5th May 2003, 09:49 AM   #27
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Carlos,
Your power calculation isn't quite right: when bridging an amp the power theoretically increases by x4 since power = V^2/R

Single ended, assuming an 8R load and +/- 9v output swing....
9v^2 / 8 = 10 watts

As above but bridged to give a +/- 18v output swing....
18^2 / 8 = 40 watts.

When bridging an amp, the output current doubles compared with the single configuration. In the case of the bridged amp above, the peak current is 18v / 8R = 2.25A, which is well within the capabilities of a single pair of LM3886 devices so paralleling isn't necessary.

Bridging is an efficient way of getting high power from low supply voltages.

Regards,
David.
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Old 5th May 2003, 10:11 AM   #28
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David,

You're absolutely right, now I wake up...
But I still think it's much cheaper to use 4 TDA1562s (only one per channel).
Or else it's going to be a huge amp, and the man will loose all the baggage space of his car with the amp and the extra batteries.
And to charge those batteries...
I think he wants a Krell in his car.
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Old 5th May 2003, 10:11 AM   #29
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Default Re: waste of parts and time

Quote:
Originally posted by carlosfm
Matttcattt,
Something has hit me in my had, I was too confused.
Have you read the datasheet of the LM3875?
With +/-12 volts you get something around 10 watts.
If you bridge two of them, you get 20 watts.
Paralleling you double the current.
So... with 4 chips in bridge-parallel configuration per channel you'll get 20 watts!
It's a big waste of parts, time and money.
parts: chips, free, others about 1 per amp (about 16)
time: i have loads of time
money: as above
Quote:

These power op-amps are not made to work at these low voltages.
You should investigate the TDA1562.
You'll get around 3 times more power with one chip per channel than with four LM3875s in bridge-parallel at +/-12v.
And it's made to work on single voltage supply.
For these voltages I don't know a chip with more power than this.
It was made for car-audio.
im using the OPA541, not the LM3875. it can run with +/-4v.
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Old 5th May 2003, 10:16 AM   #30
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for the third time :
are the capacitors circled in red electrolytic? can i use electrolytic capacitors? can i use 1000uf ones? i have loads. i am using 4 on one circuit board, can i use one 1000uf capacitor per rail for all of them?

where do the input capacitors go? after the potentiometer? where the blue circles are?
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