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
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
Attached Files
) It's where the feedback resistor goes if I'm not mistaken.
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