Analog pre and post axoloti addons

Blindsmyth 2017-10-05 23:29:24 UTC #1

As we all know Axoloti has it's flaws considering signal level. This has been quite intensly discussed here:

Although I agree that clipping axolotis input and low output volume can be a problem I don't want to conclude that axoloti is unusable and would like to discuss practical solutions, like adding analog circuitry to the ins and outs.

I've noticed that there is a lot of these cheap preamp boards on ebay. It seems like this could be a convient way of raising the low output volume of axoloti. Does anyone here have experience with these?

Further more I would love to have compression and eq/filtering at both input and output in a very small form factor. I'd prefer not to build this on breadboard but rather have a ready board like above or a soldering kit. Any suggestions welcome!

strum 2017-10-06 13:42:09 UTC #2

My long term solution is to bypass the whole existing analog section and add on a new adc/dac and balanced analog audio side then get it to talk to axo with spi or some other protocol.

In the meantime i am running the Axo output into a limiter to boost it up a bit and just not using the input for fear of burning it out.

Blindsmyth 2017-10-06 17:06:07 UTC #3

That sounds not easy to achieve but like a super good plan!

Wich Limiter do you use for that? I'm just thinking if I want a compressor/limiter at the master stage anyway I could just skip using a preamp. But I would really something in a small form factor...

strum 2017-10-07 01:26:21 UTC #4

I'm using an alesis micro limiter, it's a 1/3 of a rack unit size.

I picked it up cheap at a music shop clearance sale

Ersatzplanet 2017-10-07 04:08:22 UTC #5

Sparkfun has a few boards that are under $10 that are amp boards.

rbrt 2017-10-07 05:39:25 UTC #6

Hey-ho,
...same here,
I would like to use the axo as an all-purpose tool in a club/studio environment where you need 'pro-audio' input and output levels.
What I'm going to try is to simply scale down the input with a potentiometer,and add some cheap pre-amp at the output.
I already got myself a cheapo kit at 'conrad electronics.I can post the results once I got it up and running

Blindsmyth 2017-10-09 22:30:50 UTC #7

ok thx. Since I'm building something ultra portable and light this is already too big for me.

But these are power amps right?

That could work. Right now I'm using this cheap conrad mini mixer before the axolotis to hook up different inputs and turn them down with the pots.

Cool, keep us updated!
I for myself have figured out that I want something compressing/limiting/saturation at the masterstage wich should be at the same time be able to raise the output volume.

At the moment I'm looking a bit in the guitar diy world like here for example:
https://www.musikding.de/

Only thing that might be a problem with guitar stuff is impedance...

Ersatzplanet 2017-10-10 17:00:21 UTC #8

I was thinking ,more along the lines of this - https://www.sparkfun.com/products/9816

"This is a simple 5V Op Amp breakout board, set up as a 2-stage amplifier with a gain of 100 (gain of 10 for each stage). The on-board trim pot sets the signal level between the stages, not the feedback path. The bandwidth is set to 15.9kHz by a pair of feedback capacitors, or over 100kHz with the caps removed. The LMV358 opamp can source up to 160mA and works well as a low impedance driver/buffer."

Blindsmyth 2017-10-11 13:02:03 UTC #9

Ah doesn't sound too bad and looking at the demo might be able to do a bit of distortion as well...

johannes 2017-10-11 22:19:59 UTC #10

The reason that Axoloti Core can only output around 2V peak-to-peak is that the +5V voltage from usb (or regulated to 5V from the DC input socket) is considered noisy, and is regulated down to a clean 3.3V for the analog circuitry.

An opamp like the NE5532 can be used to amplify the analog output, but this particular opamp requires a +5V/-5V symmetrical power supply minimum, and recommends a 2V margin between supply and output voltage, so the output swing with a +5V/-5V supply would be 6V peak-to-peak. That would work, but would need to get the +5V/-5V supply from somewhere. Alternatively an asymmetrical +10V supply (or even up to 30V) could work

The AD828 Stereo Preamp Power Amplifier Board Preamplifier Module D has a bit of a confusing name, first I thought it would be a class-D power amp, but it is really a preamp. The AD828 opamp is also a bit of a strange choice as it is designed for video signals. The output signal when using a +5V supply is 3.5V peak-peak, so the optimum maximum gain setting would be around x1.75.

The LMV358 opamp on https://www.sparkfun.com/products/9816 is a "rail-to-rail output" opamp, that means you can get close to 5V peak-peak output swing when powered from +5V. The two stages of x10 gain combined to x100 gain will be too much, it would be better if it was designed for two channels of x2 gain.

Blindsmyth 2017-10-12 14:36:26 UTC #11

Thanks for bringing some clarity into this!

Ok but does that mean If you would feed it clean 5V you could get higher output level/voltage?

Ok, this also means higher supply voltage higher gain/output voltage?

I think you could change the gain with the trimpot right? Sounds like the most convenient option so far.

Although I'm still intrigued by some analog compressor/drive combination that at the same time raises the output, so if anybody has suggestions, small form factor and either ready made board or kit with ready pcb preferred.

Blindsmyth 2017-10-19 22:15:47 UTC #12

Hey there just to keep the discussion alive:

Have any of you guys experience in noises when hooking up different elements pre and post axoloti to the same power supply?

I've had this setup
guitar preamps ---> littler mixer --> axo
usb midi controller ---> also axo
In this configuration I had a lot of noise from the controllers LED but also other noises/interferences.

I kind of solved it with this thing: https://www.amazon.com/Andoer-Electric-Isolated-Stabilized-AC100-240V/dp/B015X9E4VI?th=1
But it stands in my way for a ultra compact design. Does any of you know what these kind of things actually do and how re DIY it?

I also found the sparkfun thing as a cheapo/china variant:

I'm going to let you know how it works.

Furthermore, does anybody have a clue what this actually does?

I'm still looking for a miniature mixing board. simply actively summing 2 or more channels. as it also says "mixer" in the description could the be done?

Blindsmyth 2018-03-22 14:53:01 UTC #13

So little update here, I tried the "video" preamp, AD828, it kinda sucks and does the oposite of what it should, it makes the signal less loud.

With the LMV358 sparkfun clone, I first couldn't get it to work with 3 different boards, untill the 4. did it, wich means the first 3 were propably broke. So if you order that one, get a bunch, test them and ask for a refund for all of them that are broke.

I also found this ready piezo preamp, again based on the video preamp chip and ordered one:

My experience with piezos is that you mainly need the high input impedance buffering. Unfortunately there is no info about input and ouput impedances with this little chip, but for 3bucks it's shurely worth trying.

I also got one of these lying around

Will let you know how it works/sounds.

patatos 2018-03-22 23:12:51 UTC #14

I have no problem to match axoloti output with my other equipment into my old mackie vlz, why do you need a preamplifier? Don't you have a mixer with trim?

Blindsmyth 2018-03-23 15:23:08 UTC #15

I use axoloti often in a portable battery powered setup, so bringing an extra mixer is not an option.
Furthermore I work with live input and looping, so I have a big diynamic range and need to leave a lot of headroom internally to not overload axoloti because of too many loop layers, so my output is fairly low.

I've very often made the experience of not having enough gain from my system, especially when plugging into other portable sound systems.

patatos 2018-03-23 20:24:00 UTC #16

Ok! So maybe that stereo preamp can be really a solution.. keep us informed I'm interested as well.