Parellel Opamps - Why it works and is it safe ?  - Page 1
Forum section: electronics / projects

author: VirtualHiFi
author_rank: Newbie
timestamp: June 21, 2024, 03:47:46 am
content: Hi Guys,I'm an down to earth audiophile who likes gear for the sound it offers and not the looks or high prices and also I run my YouTube channel where I test the gear and record high quality audio samples comparison. When I was recorded Opamps comparison -,I stumbled on something very interesting - Stacking Opamps.http://WhatsApp Image 2024-06-21 at 05.39.34.jpeg(243.41 kB. 1200x1600 - viewed 134 times.)This is very interesting thing. After soldering opamps stacked on top of each other the sound is amazining and 2 stacked NE5532 IMO outclassed all other opamps in the test.Since it's a very cost effective option, I would like to make the video on that topic, but I'm not an electronics engineer so I'm not sure how and why it works, what happens by doing that and is it safe to do so (the stacked opamps get really hot) ?Thank you very much foy your insight

author: Someone
author_rank: Super Contributor
timestamp: June 21, 2024, 03:55:10 am
content: People who manufacture opamps have talked about this many times:https://e2e.ti.com/blogs_/archives/b/thesignal/posts/paralleling-op-amps-is-it-possible

author: moffy
author_rank: Super Contributor
timestamp: June 21, 2024, 06:03:54 am
content: If the input offsets of the opamps are not identical, and there is no reason to believe they will be, the outputs of the paralleled opamps will fight one another, with one sourcing and the other sinking current even without a load i.e. they run hot and could even damage one another. You have to allow each opamp to find its own equilibrium with feedback.

author: magic
author_rank: Super Contributor
timestamp: June 21, 2024, 09:42:10 am
content: NE5532 has 100dB voltage gain at DC, so with shorted IN+ and IN- pins a mere 0.013mV difference in input offset voltage is enough to produce 1.3V output difference. As the outputs are shorted too, this drops 650mV across one output resistor of each opamp and overcurrent protection is activated in one of them (which one limits first depends on part-to-part variation and whether the sourcing or sinking current limit is lower, as they are unlikely to be exactly equal).Let's start with the obvious: the chips run hot and they may overheat and die over the long term if supply voltage is too high. Power is voltage times current, do the math. You probably shouldn't exceed 0.5~1W because a pair of hot chips stuck together may dissipate not a lot more heat than one alone.A more interesting question is how and why the amplifier ends up working anyway. Under no load, the "weaker" side which runs into overcurrent limit first is simply a constant current source, while the "stronger" side gets biased in its linear region and drives the output in class A. So far so good.But assume that one chip sources constant 38mA and the other can sink up to 40mA. When the load demands more than 2mA to be sunk, the sinking chip runs into current limiting too and loses control of the output. Then the differential input voltage needs to move the difference between offset voltages of the two opamps before the sourcing one desaturates and starts to reduce its output current below 38mA, allowing more than 2mA to be sunk from the load. Now the sourcing chip drives the load in class A and the sinking chip provides active load. But there is a nonlinear crossover glitch at every point where load current crosses 2mA. This step in differential intput voltage appears on the output amplified by the noise gain of the circuit, which is typically about equal to the closed loop signal gain.In reality, I made up the 2mA difference - it could be anything from zero to tens of mA as the guaranteed current limits are quite wide (see the datasheet). The difference of offset voltages and the magnitude of the crossover glitch may also be anything from zero to a few mV. Definitely not a reliable and reproducible trick and if you are actually hearing anything that isn't there otherwise, it is probably crossover distortion, although of a different sort than you will get from LM358.I would test this with RMAA or something like that out of curiosity, but I wouldn't expect (positive) surprises. And the outcome may be highly random between samples and even between the left and right channel of the same stacked pair.If driving opamps in class A is your thing, there are better ways.

author: rkabz
author_rank: Contributor
timestamp: June 21, 2024, 11:46:12 am
content: Check this out...https://www.diyaudio.com/archive/blogs/alexcp/1016-ne5532-power-amplifier.html,note that each amplifier has a resistor in series with its output to help with offset voltages and current sharing. The other consideration is that the input bias current adds up and the input impedance goes down.

author: Coordonnée_chromatique
author_rank: Frequent Contributor
timestamp: June 21, 2024, 01:36:54 pm
content: https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwirgc2ksIqCAxUFMDQIHcCwBUgQFnoECBcQAQ&url=https%3A%2F%2Fwww.physicsforums.com%2Fattachments%2Fdouglas_self_elektor_amp5532_compressed-pdf.254881%2F&usg=AOvVaw0nC43peZf7Po8gK7OQGR3O&opi=89978449Have you seen the styroflex capacitors

author: ConKbot
author_rank: Super Contributor
timestamp: June 21, 2024, 04:17:20 pm
content: It all depends on what you do with the opamp. With 5532's they have a relatively weak output, so if you load them heavily (direct driving headphones as an example) The sound will be affected dramatically. Paralleling them can lessen the load per amp, or as alluded to above, adjust bias operating points, or even drive towards clipping, so it's perfectly reasonable to me to expect the sound to change as the flavor of abuse the output experiences changes.If you have something like an O2 amp where the opamps are specified correctly with lots of drive and plenty of margin, if you started paralleling unity gain output buffers I'd call hogwash on hearing a difference. Unless you didn't find some corner case (goofy sub 8 ohm multi driver IEMs as an example)

author: Kleinstein
author_rank: Super Contributor
timestamp: June 21, 2024, 06:43:39 pm
content: A direct parallel connection is usually not working (mainly when used as comparator / saturated). It need a little extra ciruit modification so that 2 or more OP-amps can be combined without fighting each other.

author: Zero999
author_rank: Super Contributor
timestamp: June 21, 2024, 08:03:12 pm
content: Quote from: VirtualHiFi on June 21, 2024, 03:47:46 amPlease post a time stamp. Few people are going to sit through and hour+ long video.Whether it's a good idea or not, to just connect the outputs in parallel, depends on the op-amp. The general advice is to use output resistors, with one op-amp acting as the master and the other the slave.