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Old 11-17-2021, 12:49 PM   #1
Dannyx
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Talking Op-amp preamp discussion

Good day folks. In another one of my "sonic projects", I got myself a couple of these little pre-made pre-amp boards off Ali (photo 1), mostly to experiment with them and learn something in the process by hopefully combining my modest electronics knowledge and my passion for audio, although I don't claim to be an audiophile who likes to spend thousands on pro audio gear....which itself is often regarded as just snake oil and marketing BS from what I've read. I'll try to keep the wall of text to a minimum so:

The op-amp they came with is an AD828, which the datasheet calls a "video" op-amp, which is pretty ironic and is much the polar opposite of "audio". It doesn't matter, because I can swap this out with another one, which leads me to point #1: would changing the op-amp to one that costs more than these 3 boards together, like an OPA1612 really have an noticeable effect ? I'm talking something you can feel with your ears and not something you can pick up on a piece of laboratory equipment which I don't have anyway...I looked at what others have to say about "upgrading" op-amps in general and it ranges from "must-do" to "useless and mostly psychological" as long as the op-amp is not 30 years old...

Point #2: frequency response and how to calculate it. It was really easy for me to draw a schematic and even simulate the circuit here, since it's only a handful of components. Not knowing what this configuration is called, I just browsed around until I found something that looked similar here. It obviously has to be some sort of amplifier configuration and there's clearly a feedback resistor there, but it wasn't clear what that voltage divider+cap is doing on the + input. The chap had the same question and that is what values the cap and resistor on the Inverting input should have. Someone replied, mentioning the high-pass filter, but I can't possibly figure out how he got those values in the reply: Fc = 0.16/Rc =1.6kHz. Hence C = 0.16/(8Hz * 1k) = 22uF.

In my case it's 10u for the cap and 2.2k for the resistor (I THINK that's what 10 stands for on the cans there). I tried figuring out how to calculate this and I DID find something about high-pass filters here. The formula appears to be fC=1/(2pi RC). There's an example there which demonstrates how to calculate for 82pF and 240kOhms, and indeed I was able to get to that example result myself, but I'm not sure how and IF this applies to my circuit as well.

Point #3: gain. Does a 2.2k resistor and a 20k feedback one result in a gain of 10 there ? I found the formula to be Av = Rf/R1 (thank you Dave from EEV ) The simulator seems to confirm this: I get 10mv output for a 1mV input...unless the input frequency is very low, like 10hZ, at which point the output starts to drop too, to like 7mV instead of 10mV, unless I increase the value of the cap too. I'm not interested in filtering anything - just passing the amplified signal to the downstream "power" amp. I'm aware it cannot drive a low-impedance load like a speaker or headphones directly - you know me - not that stupid Decreasing the value of R1 obviously increases the gain, on paper at least, because I'm not sure it can be done in practice, otherwise the boards would've come like that from the factory...

While audio quality is strictly subjective and not quantifiable, aspects such as frequencies and gain at least have some calculations and formulae behind them which we can all agree upon, regardless of what our ears say
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File Type: png OpAmp.png (30.3 KB, 16 views)
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Last edited by Dannyx; 11-17-2021 at 12:58 PM..
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Old 11-17-2021, 03:32 PM   #2
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Default Re: Op-amp preamp discussion

This one has resistors you can change to increase the gain. https://www.aliexpress.com/item/1005001703051433.html

Whether you'd be able to tell a difference depends on the source audio and what it's driving and how good your ears/brain are.
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Old 11-17-2021, 11:23 PM   #3
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Talking Re: Op-amp preamp discussion

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Originally Posted by diif View Post
This one has resistors you can change to increase the gain. [url]https://www.aliexpress.com/item/1005001703051433.html[/url
Well, yeah - that's the one I got - see the attached pic
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Old 11-18-2021, 01:21 AM   #4
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Well, yeah - that's the one I got - see the attached pic
I was showing it to you not as a better purchase but to confirm what I think you were saying about increasing gain.
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Old 11-17-2021, 05:03 PM   #5
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Default Re: Op-amp preamp discussion

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While audio quality is strictly subjective and not quantifiable, aspects such as frequencies and gain at least have some calculations and formulae behind them which we can all agree upon, regardless of what our ears say
Tell that to the tube-o-philes. They love distortion.
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Old 11-18-2021, 08:07 AM   #6
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Default Re: Op-amp preamp discussion

i would replace the caps with tantalum - lower esr and probably more linear response.
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Old 11-18-2021, 09:36 AM   #7
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i would replace the caps with tantalum - lower esr and probably more linear response.
True. I too was thinking about that - better caps. I guess I'd keep the existing values. Should I bother replacing the power caps as well, or just the signal ones ?

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I was showing it to you not as a better purchase but to confirm what I think you were saying about increasing gain.
I think you're looking at those holes where a thru-hole resistor can go. I just noticed those ) It's where the feedback resistor goes if I'm not mistaken.
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Old 11-18-2021, 10:27 AM   #8
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Default Re: Op-amp preamp discussion

i would normally not change power caps to opamps - it's not like they are subjected to high frequencies.

that said, i dont trust smd electrolytics.
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Old 11-18-2021, 11:38 AM   #9
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Talking Re: Op-amp preamp discussion

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that said, i dont trust smd electrolytics.
Especially from obviously questionable sources
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Old 11-18-2021, 08:50 PM   #10
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Default Re: Op-amp preamp discussion

Just to pedantic, you can increase the gain of an op amp circuit, but can't increase gain of an operational amplifier, you can only reduce it...

And the two boards/pictures are eversoslightly different... dang those bits of rice.

TBH it looks like the OPA1612 was specially designed for audio and would be guaranteed to be better for audio usage. The AD828 is a faster op amp and hence better for video and other high speed stuff.

Someday it might be worth it for me to "hear" a LM741, LM358, TL082, LF353, LM6142, etc., etc. in my junkpile, and if I ever get my hands on a OPA1612 and AD828 too, but I don't think I really can tell the difference at a fixed, reasonable gain. I'm no audiophile. Heard a lot of hearsay about LM358's XO distortion but I'll have to be the judge of that.
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Old 11-21-2021, 12:28 PM   #11
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Talking Re: Op-amp preamp discussion

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Originally Posted by eccerr0r View Post
Just to pedantic, you can increase the gain of an op amp circuit, but can't increase gain of an operational amplifier, you can only reduce it...
Ok, I was curious about the gain of the op-amp CIRCUIT then

Also, yes the board in the link has polarized caps, while mine seem to be non-polarized. Also, the resistors are arranged slightly differently, but overall I'd say it's the same circuit...
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Old 11-21-2021, 05:20 PM   #12
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Default Re: Op-amp preamp discussion

Are they really bipolar capacitors or are they just missing paint ?
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Old 11-21-2021, 07:19 PM   #13
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Default Re: Op-amp preamp discussion

i would say they are far too large to be polarised caps
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Old 11-21-2021, 11:20 PM   #14
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Default Re: Op-amp preamp discussion

If I were to replace them with polarized ones, where would the positive go ? Looking at the other board which appears to use polarized caps, I can see the inputs go to the negative side, while the outputs come OUT of the negative side....
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Old 11-22-2021, 10:39 AM   #15
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Default Re: Op-amp preamp discussion

Orient them the same way as the other board which does come with polarized capacitors
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Old 11-22-2021, 10:42 AM   #16
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Default Re: Op-amp preamp discussion

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Orient them the same way as the other board which does come with polarized capacitors
That aside, I'm mostly interested in the "electronic" reason why signals go "in"/come "out" of the negative/positive leg of caps in audio circuits

It's true that most schematics I've seen use non-polarized symbols for the caps....
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Old 11-28-2021, 04:50 PM   #17
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Default Re: Op-amp preamp discussion

Generally a bipolar cap should be used when unsure. A polarized cap can be used if the circuit charges it the right way.
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Old 11-29-2021, 02:37 AM   #18
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Default Re: Op-amp preamp discussion

I ordered some low ESR caps, though they ARE still electrolytic, not knowing what I should go with...mostly to replace the original ones with something *supposedly* better quality. I went over plenty of sources and discussions trying to work out what the best choice would be and came to the conclusion that the minimalist nature of this circuit probably outweighs the quality of the caps (unless they're complete trash), even if they cost 2-3 times as much ! Poly film caps seem to be the most popular for this application, but they are too expensive and too large to be worth the effort, especially since, as previously mentioned, there are probably other things which outweigh the caps...
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Old 12-03-2021, 10:04 AM   #19
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Talking Re: Op-amp preamp discussion

I finally got around to actually running these things to see how they sound out of the box, with no alterations. Again, only having my ears as my primary test instrument at my disposal, I can say they exceeded my expectations: I was worried I'd run into all sorts of issues like buzzing/humming, all of that BS, but everything went really well. The amp is also really quiet when idling, even with the Aliexpress pre-amps hooked on its input. Often times, amps like to hiss when nothing is playing through them ! I managed to achieve decent loudness, with no noticeable distortion. I was afraid I may damage the speakers I used, so I didn't go all the way to the max on both the amp and the PRE-amp, which would be just silly.

Since this is a Sony car amp I scored for cheap, it's got a gain control which ranges from 6v to 0.3v. This corresponds to the sort of output your headunit is expected to put out, so if it's got a higher output, you turn it towards the higher value and the gain actually DECREASES to compensate and vice-versa. I set it to right about when that hissing was just starting to come through at idle, which in in this case is a little over the half-way point, which is 2v.

Not sure whether I should even bother to replace the AD828 amps or even the caps for that matter
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Old 12-03-2021, 11:41 AM   #20
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Default Re: Op-amp preamp discussion

I wouldn't have thought it would have been necessary for most people... on the other hand I'm curious about people who could tell the difference between op amps and whether or not they can tell with a double blind test.
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