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Old 23rd January 2013, 06:47 AM   #1
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Default OpAmp power amp, would this work?

After reading about the Nelson Pass, Beast with a 1000 JFETs, and also seeing this multi paralleled Op-Amp power amp:

SoundStageXtreme.com | UltraAudio.com | Blue Circle Audio NSL Stereo Amplifier

it got me thinking.

The power supply is +/- 18V. Using the OPA2134PA can give +/- 35mA at the output, so we could safely use them at 30mA, so a 560ohm resistor is used on each op-amp output, giving you 18 / 560 = 32mA. If you used 50 dual op amps per channel, you could get a max of 3.5A (100 op-amps in parallel), that would be 3.5 x 4 (P = I R) = 49W into 4ohm load.

Would this circuit work:

Click the image to open in full size.

Here is a schematic I drew that has 4 op-amps in parallel. Each has a gain of 17.6 and an output resistor of 560ohms to limit the current. R1 and R2 are used at the front end to drop the voltage as the OPA2134 has in input voltage limit of +/- 0.7v. With a pk-pk input of 4v and the volume pot turned fully up, there is only +/- 0.66v at the input to the buffer.

Will this work? And if so, could I simply parallel a load more of the devices?
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Last edited by portreathbeach; 23rd January 2013 at 06:50 AM.
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Old 23rd January 2013, 07:10 AM   #2
Mooly is offline Mooly  United Kingdom
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Check out Elektor (October 2010) for Doug Selfs NE5532 power amp using multiple devices. It used around 40 (in total) NE5532's.

Those 560 ohms can come right down in value the more opamps you use (by a factor of several hundred )
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Old 23rd January 2013, 03:07 PM   #3
CJ900RR is offline CJ900RR  Sweden
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Inspiration: link to copyright material removed.
That is the one in Elector.

Last edited by wintermute; 24th January 2013 at 09:12 AM. Reason: remove link to copyrighted article.
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Old 23rd January 2013, 03:42 PM   #4
johnr66 is offline johnr66  United States
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Bridging aside, you don't get anywhere near 49 watts of non clipped sine wave with +/- 18v supplies and 4 ohm load. To simplify, lets say the amp can swing completely to the rails, therefore:
(18*(1/sqrt 2))^2/4 = 40.5 watts

With any practical amplifier, you get around 28 watts, maybe as much 35 with a design that optimizes the output swing.
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Old 23rd January 2013, 06:06 PM   #5
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OK. My maths was wrong, but 28watts is still a worthy amount of power

Just downloaded the Doug Selfs Op-Amplifier and must say it is an interesting read. The Op-Amps used are NE5532 which have an input voltage of +/- the supply voltage, instead of the +/- 0.7 volts that the OPA2134 has. From reading the article, the gain is done at the front end of the amp and all the parallel OpAmps are unity gain, obviously this dramatically cuts down on parts.

I notice from his schematics that only 1ohm resistors are used on the outputs of each OpAmps. I can see how this would work, but you are relying on the current limiting feature of the OPAmps, will this not damage them if you drive them to the limit? If you built this actual amp and had the audio playing and then removed one of the sections of OpAmps, you could suddenly run into overcurrent in the remaining OpAmps and they would start dropping out, what would happen? Would the audio slowly dwindle away, or would you get spikes and horrible noise from the speaker as each one dropped out?

Also, what is IC6 doing in the schematic?

Is the NE5532P as good as the NE5532?
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Old 23rd January 2013, 06:17 PM   #6
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by portreathbeach View Post
The Op-Amps used are NE5532 which have an input voltage of +/- the supply voltage, instead of the +/- 0.7 volts that the OPA2134 has.
Eh

Where've you got that from.
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Old 23rd January 2013, 06:45 PM   #7
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On the datasheet for the NE5532 it says that the absolute max ratings are +/-22 V for the supply and 'Input voltage, either input(2) (3)' maximum is VCC

Oh, my mistake, I just re-read the OPA2134 datasheet and that has a max input of +/-13.

I'm still thinking of having a go at building something like this and the NE5532s are way cheaper than the OPA2134, but are they as good?
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Old 23rd January 2013, 06:46 PM   #8
Mooly is offline Mooly  United Kingdom
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IC6 looks like some "form" of DC servo. It's used to set the operating point of IC1. Doug must have had his reasons for specifying the OP177 (just ultra precise DC operating conditions I guess... for that its one of the best around) but it seems overkill.
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Old 23rd January 2013, 06:52 PM   #9
Mooly is offline Mooly  United Kingdom
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Yes, the NE5532 has a wider supply range than the OPA2134 if you wanted to run the IC's to the limit.

The 5532 is fine as a "gain block" as here. A wild card could be the OPA2604 (-/+24 volt supply) and the TLE2072 which is slightly less at -/+19 volts but has the benefit of a much higher output current (60 or 80ma from memory)
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Old 23rd January 2013, 07:35 PM   #10
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The OPA2604 is more expensive and the TLE2072 is about the same price. Is there any audio quality difference? The extra voltage and output current is obviously a bonus?


I've been looking at prototype boards like this:

http://www.ebay.co.uk/itm/Copper-Pro...item27c81aee5d

I thought I could build 4 or 5 parallel Op-Amps onto them and then stack several boards using header pins to connect them. That way you could add and remove boards at will.

The big question I am having now is the quality of sound that could be expected from an amp built entirely from OpAmps. I mean the build would be fun and the result would look pretty cool, but would it sound any good compared to my LM4780 chip-amp? My chip-amp sounds very nice, but I love experimenting and building different things.
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Last edited by portreathbeach; 23rd January 2013 at 07:40 PM.
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