Hi, I make diy audio stuff occasionally and when I make them I want them a bit of higher quality. I'm not that informed on everything, so please excuse me if I miss out anything or if I'm not too clear.
I want to make an onboard bass guitar preamp, for now I'm considering 2 bands only, low and high of course, probably will be boost only, I want it simple, effective and with lots of headroom and transparency.
My intention is to build it with opamp AD8620, which should be ideal for battery powered preamp considering its performance. I'm not too fussy about lowest power consumption as I want performance as well so hitting the sweet spot works for me.
What I'm concerned about mainly is the type of capacitors to use. I need this to be as small as possible but also as transparent as possible. Having hard time narrowing that down, good audio capacitors and physically small enough. The pico values I find the most difficult to figure what to use as I guess I need to avoid ceramics if I want performance...? The nanos could be MKP or MKS I guess.
Does probably anyone have any idea or a suggestion? I welcome suggestions on anything related to the preamp as well.
Thanks!
I want to make an onboard bass guitar preamp, for now I'm considering 2 bands only, low and high of course, probably will be boost only, I want it simple, effective and with lots of headroom and transparency.
My intention is to build it with opamp AD8620, which should be ideal for battery powered preamp considering its performance. I'm not too fussy about lowest power consumption as I want performance as well so hitting the sweet spot works for me.
What I'm concerned about mainly is the type of capacitors to use. I need this to be as small as possible but also as transparent as possible. Having hard time narrowing that down, good audio capacitors and physically small enough. The pico values I find the most difficult to figure what to use as I guess I need to avoid ceramics if I want performance...? The nanos could be MKP or MKS I guess.
Does probably anyone have any idea or a suggestion? I welcome suggestions on anything related to the preamp as well.
Thanks!
From a technical point of view, there is nothing wrong with class 1 ceramic capacitors: they are linear, stable, accurate and have very low losses. Class 2 ceramic capacitors, on the other hand, are terrible for just about any application except supply decoupling: they are grossly non-linear, inaccurate, only have a Q of about 30 to 50, vary with temperature and very slowly lose capacitance.
There used to be many class 1 dielectrics with different temperature coefficients around, but nowadays almost all class 1 capacitors have the most temperature-stable dielectric with code name NP0 or C0G (which are two names for the same stuff). Ceramic capacitors with low values are almost always class 1, so I think you can just use ceramic capacitors for the low values.
There used to be many class 1 dielectrics with different temperature coefficients around, but nowadays almost all class 1 capacitors have the most temperature-stable dielectric with code name NP0 or C0G (which are two names for the same stuff). Ceramic capacitors with low values are almost always class 1, so I think you can just use ceramic capacitors for the low values.
By 'onboard', I'm guessing you mean 'built into' the bass guitar?
Capacitor-wise, for smaller values - say under 680 pF - silver micas are pretty great. But you might have to track down an overstock or salvage vendor -- they're getting a lot harder to find.
How did you choose the AD8620? That's a pretty fierce part, especially for bass guitar frequencies. Also, there's no spec anywhere on its 24 page PDF concerning single-supply operation, or less than +/-5V operation. How are you planning to power it?
The high impedance inputs are nice for pickup loading, but not necessary. The very low input offset serves no advantage at all. And they're rather expensive. DigiKey lists Analog Devices p/n's as 'Active', but zero stock in each of the 7 listed. They do have inventory in Rochester Electronics parts, only a few hundred, though, and they're all listed as 'Obsolete'.
Fun project. Takes me back to 1977 and a best buddy's Gibson Ripper.
Cheers
Capacitor-wise, for smaller values - say under 680 pF - silver micas are pretty great. But you might have to track down an overstock or salvage vendor -- they're getting a lot harder to find.
How did you choose the AD8620? That's a pretty fierce part, especially for bass guitar frequencies. Also, there's no spec anywhere on its 24 page PDF concerning single-supply operation, or less than +/-5V operation. How are you planning to power it?
The high impedance inputs are nice for pickup loading, but not necessary. The very low input offset serves no advantage at all. And they're rather expensive. DigiKey lists Analog Devices p/n's as 'Active', but zero stock in each of the 7 listed. They do have inventory in Rochester Electronics parts, only a few hundred, though, and they're all listed as 'Obsolete'.
Fun project. Takes me back to 1977 and a best buddy's Gibson Ripper.
Cheers
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For a bass guitar preamp, I think the OPA145 is pretty hard to beat---still VERY miserly on power draw (~1/4mA each amp) and exceptional THD and slew rate specs, whilst still offering very good noise performance. For capacitors, polypropylene is the best. And, of course, use only metal film resistors.
I didn't know that about ceramics, now it's easier to know what to look for. Thanks for the tip, that saves a lot of time researching, I'll keep an eye on the codes since I don't want to use the unstable ones for high quality audio.
Yes, like encapsulated in a small plastic case or something and installed inside the eletronics cavity at the back with the rest of the electronics. Those types they call them "onboard" for musical instruments.
The picos go all the way down to 22p for this specific schematic that i found. I've seen silver micas, but only 250V and up. Isn't the high V having any effect on the signal? I know that kind of it also doesn't really make sense what i'm saying but i've heard if the rated voltage on the cap is way higher than the line it's placed into kind of doesn't get the full potential of the cap..?
For the AD8620 i've read several reviews and audio comparisons among this league of opamps, and performance and voltage wise turns to be the "best" or the sweet spot option. So this is an audio comparison instead of the data sheet where you can't really tell how it's gonna sound other than in what category is and some technical info. For the power supply it's gonna be +-9V (two 9V batts), in audio test performance it was mentioned that it is stable enough at as low as +-3.5V or +-3.7V.
They compared the OPA627, OPA2132, OPA2134, OPA2107 and the AD8620, which turned to be the best option if you go with portable power supply. Anyway, i ordered all of these to try and see which i'll like the best (except the OPA627 which is not really battery friendly so I ommited that one). Found a chinese seller with really good prices on all so i got them all for about $25-30. The seller was long time member with pretty high feedback so i assume his stock is genuine as he says, we'll see, i just purchased them. But if you might have any opamp suggestion i'm open, at the end is a taste preference in the high end, that's why i got several different to hear.
Why obsolete though?
Yup i got metal film resistors, thanks for the cap tip, i'll go with something of what you guys suggested, probably what i can find and that is small enough, i need portability.
I've checked the OPA145, it may have ridiculously low power consumption, but Texas Instruments doesn't list it in High-Fidelity opamps. How do you know it's that good for a bass guitar preamp? By the information out there it's pretty good for very low power systems but no one mentions audio performance...?
What exactly do you mean by "fierce part" for bass guitar frequencies?
Yeah, definitely fun project. It's even more fun when you want to experiment to get the sound you're after.
How would it know it is "single supply"?
Figure 9 shows it is probably fully-biased at 9V (+/-4.5V) but may get funky at 7V (+/-3.5V). Combined with 2.8mA per opamp (you may need two), it seems a power-hog for this use.
The price makes me gasp.
But it comes down to what makes the user feel good. If a bassist thinks 25MHz enhances his bass sound, then he should have 25MHz.
By looking at the datasheet:
https://www.ti.com/lit/ds/symlink/opa145.pdf?ts=1592282563109&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FOPA145
Particularly looking at the slew rate (20v/µSec), noise (7nV√Hz voltage,0.8fA√Hz current), THD (0.0001%), and the fact it will operate on a wide range of voltages (2.25-18v) and still swing within 0.2v of that rail. It's a JFET with high input-Z, which bass guitars like. T.I. touts it as a good "choice for amplifying small signals from high impedance sensors [such as] battery-powered instruments."
It pretty much checks all the boxes for a guitar preamp, IMO. If there's a better IC for this application, I am not aware of it. Anybody else?
However, I have NOT had the opportunity to try it myself.
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It wouldn't, of course. I just worry about an inexperienced designer sorting out a supply ramping issue if the opamp misbehaves at common-mode extremes.
All I meant by 'fierce' was that it has a lot better high frequency performance than you need. (And that bandwidth could cause stability issues.) But PRR is right about that, too -- if you want 25MHz, then you should have it.
Thanks for keeping me honest.
Cheers
Somebody (cbdb)
just answered this in another thread---an OPA1692:
https://www.ti.com/lit/ds/symlink/op...=1592156716662
This might be even better!
These specs are SOTA, but certainly overkill for a bass guitar preamp. Be assured you will hear no difference compared to good old TL072 except some slight increase of noise. Back on earth again I would suggest OPA2196 and the small Wima MKS capacitors.
The TL072 is OLD, but there's very little GOOD about it---it's NOISY and won't drive anything below ~10KΩ. Besides, that OPA1692 is only $1.69.
Yeah is it expensive but you buy it once and i found it for a good price from China so we'll see how it's going to perform.
I'm mainly looking at those opamps from the reviews on how they sound, and a little only cause of the datasheet. So i don't care that much about the bandwidth as i do for how it sounds. In the audio reviews they define them for example "low end or bass reproduction is tight and well defined with open midrange" or "low end is extended but flat with somewhat muddy midrange" etc. That's why I look at these specific preamps, based on on tested sound rather than what's on the datahseet. The few things I guess I would need from the numbers is current consumption, THD, low noise etc, the rest is definition of the sound output i'm after. At the end there may not be even that much difference, I don't know yet.
I can think one thing but if there is no real life or audio difference there's no point pushing high or expensive for no reason, that's why i'm testing several opamps.
You're right about the specs, they are great. I'm willing to try it cause the battery life would be perfection with it. If i can't hear any difference from the others or if i just like it then it's definitely worth using it.
The thing is the factory one that i like and want to base my preamp is Aguilar OBP-1, and it's expensive as hell, that's why i want to make one similar to that but i don't know what opamp it uses.
Maybe it has, i'm aiming for center low freq at 40Hz and high at about 4-6Kz for the two bands. I'm looking at those opamps based on reviews on how they sound instead of all the numbers. They explain the sound output like defined bass, open midrange etc... i'll end up with the one i like its sound output the most at the end and of course to be somewhat suitable for batt power
By the numbers and that that it fits their high fidelity category is a very worthy contender for portability and sound... Now even more opamps to try. Haha, i would do this mainly for fun with this much experimenting rather than the fact there will be any significant audio difference among the high end opamps.
Thanks for finding that one!
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