Gear / Technical Help => Battery Boxes, Preamps, Mixers, ADCs, and Processors => Topic started by: weroflu on November 05, 2016, 11:05:23 AM

Title: mini mixer
Post by: weroflu on November 05, 2016, 11:05:23 AM
Working on an altoids sized 6x2 balanced mic pre mixer.
That 1510 pre's, phase, phantom, mute, 3.5mm balanced input jack, bourns volume pot
dedicated panning per channel, no fancy pan(ts)

Shots of preamp card, sd card adaptor included for scale. Just finished soldering the card, could easily get slaughtered with testing, cold joints and a million other design error.

Title: Re: mini mixer
Post by: Ben Turnbull on November 12, 2016, 10:55:34 AM
I dig tiny stuf. +T
Title: Re: mini mixer
Post by: weroflu on December 12, 2016, 03:15:50 PM
Pic of  5 boards attached. Middle purple is the backplane. From top down on left are summing/master card, bipolar supply, p48 supply. Rightmost is the channel strip/preamp card.

For scale the bipolar supply component area is half the size of a thumbnail. I actually got all 15 or so  parts soldered onto one of these but it was a disaster when testing, smoke and then a sizzle and pop so back to the drawing board. Found some pins bridged on the ic probably caused by me in overheating, esd, or other beginner's mistake.

Going to try for round 2 this week with the bipolar supply and then the channel card.  I found applying solder paste on the small pads was just too difficult without a stencil so did the passives with an iron.
Title: Re: mini mixer
Post by: weroflu on December 21, 2016, 01:17:16 PM
preamp/channel strip is working
bipolar supply working
phantom supply working
summing card soldered but untested

birdseye of cards sitting in chassis, 9V battery for scale
2nd pic left to right: summing card, phantom supply, bipolar supply, channel strip
Title: Re: mini mixer
Post by: Gutbucket on December 21, 2016, 03:39:13 PM
Wow that's tight work!  Good to see your progress on all this.
Title: Re: mini mixer
Post by: Whippoorwill on March 01, 2017, 05:45:56 AM
any updates?
amazing to see...
Title: Re: mini mixer
Post by: weroflu on March 01, 2017, 09:32:54 AM
yep. Switched to a different chipset so gain will be controlled via bluetooth android app. Also panning can now be selected for each channel, but it remains LCR, and still a physical switch.

I had almost finished the pot gain control version but bailed since I really wanted at least two channels with perfectly matched gain.

More pics in a few weeks.
Title: Re: mini mixer
Post by: weroflu on March 19, 2017, 11:05:59 AM
New power supply: 48v, 3.3v, +15v, -15v.
Title: Re: mini mixer
Post by: Gutbucket on March 20, 2017, 01:45:54 PM
Cool to see this project steadily progressing. 

..continued encouragement..
Title: Re: mini mixer
Post by: weroflu on March 20, 2017, 06:18:07 PM
<<Curious about your rationale for some elements of the design?  >>

The main goal was to fit a high quality balanced input, phantom powered 6x2 preamp/mixer in an altoids case. Almost every decision revolved around how to fit things into the case. As it stands now all I have to do is punch 6 holes in a 2$ mint tin and the casework is done.

Originally I was using That1510 but did not think through the pots carefully enough. There was some incorrect information on the net (hard to believe) about converting a linear pot to a reverse log, so the Bourns linear pots were a no go. There was one pot made by BI technologies that would have worked, but MOQ was too high and they are impossible to find. I reworked things a bit to fit alpha pots, but at that point I already had my feet wet in the That1580/5171 and I wanted at least two channels of well matched gain which pots can't do.

The Ltc3625 charge pumps worked well, and 2-3 of them would have been fine for the 1510 version. I had that whole thing about 80-90% done when I decided to switch gears.

<You don't say what the new chip is, but THAT1610 is 6mA/channel so that would be 36mA plus summing, say 40mA total, times 30V is 1.2W.  Six-channel phantom at a typical 3mA >

Schoeps 8mA. That 1510 max current with no signal is 8mA plus wiggle room.

<<is another 0.9W.  Plus conversion losses (that LT module has very low efficiency at the low loads you have, it's designed for loads in the 1A range), you're burning about 4W total?  That's not an insignificant amount of heat for a box that small, and it will make a 9V battery cry.>>

at 225v-ish Ltm8049 is near 80% efficient.

I never intended this to run off a battery, certainly not a single 9V pp3. I was entertaining the possibility of the the first version to run from a usb rechargeable battery pack, but that wasn't a priority.

<<Using a bipolar supply is good insofar as it avoids the need for output capacitors, but sometimes an output capacitor can save the life of a circuit from an external mispatch, even if it's not strictly needed.  My entire life is single supply.  But if you stick with bipolar, I think you will find there are plenty of charge pump ICs that can do that job easily for the current output you need.  They will hit close to 90% efficiency in the process and have almost no Iq, and much lower output ripple than the LT module.>>

At one point I was looking into running all of these ic's from single supply, but it was over my paygrade and I really did not want to create any design headaches. The ltc3265 was the only charge pump I found that did +-15v. I have a few of those working cards now in the ic graveyeard.

That1580 preamp: 10mA
That5171 gain control: 11mA
That1286 panpot: 6mA
That1206 balanced line receiver: 8mA
lme49720 summing amp: 12mA

222mA= too high current draw for the ltc charge pumps unless I had one on each channel - which I did consider and probably could have pulled it off.

One reason I didn't want to do that is to not bus supply voltage since it was coming from a wart and I'm not confident in layout or filtering.

I tried to challenge myself with this power supply just to see if I could get 4 rails in that small size. Plus noise and ripple should be lower with the better regulators - yea better spec but probably not audible.  There is a very wide nominal input voltage on the ltm8049 and the lt8330 (phantom boost switcher), so theoretically this box can run anywhere from 5v-20v or higher. Heat might be a problem at 5v but I'll test it out later if it ever works at 12v.

<<You might design backwards from your output requirement instead.  Chips like THAT1610 have low input noise but much higher output noise, so their noise performance isn't exceptional until you have gain of at least 30dB.  There are plenty of chips that will equal or exceed that performance with less than 2mA draw.  The second thing to think about is your operating level--what are you driving?  Do you need 30Vpp peak output?  Most converter chips are only running 5Vpp max input.  So you could find yourself in a situation where you have to run higher gain than you otherwise would to overcome the output noise of your amp, only to attenuate that output to keep your ADC happy.  That works, but it burns a lot of battery life.>>

It's not a battery powered box and never wanted to be. Yea you're right I don't need 30Vpp, but it was one of those just follow the nominal datasheet values. I don't care that much about wasting some power, it's a fairly low power device as is. The main thing was just to make something really small, very good sounding, and power it relatively easily with a wall wart. No actually I just wanted to use the That1510/1580 chips because they sound really good to me.

<<Instead, think about a topology that is designed around only needing 20-30dB of gain, and since you already have a digital circuit, why not go whole hog and use your own ADC?  Now you can define your operating level and run the whole circuit on 5V.  If you choose carefully you can find a DSP chip that will do everything you could dream of in that space (sum, pan, EQ, effects), controllable via UART or I2C via Bluetooth.>>

There's a big difference between spi gain control on the 5171 and rolling your own adc. The thought did cross my mind, but this project has taken up too much time as it is and I'll leave adc's to smarter people. If I can fit an adc into this box maybe in the future I'll give it a go - especially if it involves 5.6Mhz DSD stereo out. I have no use for eq, effects, digital summing, digital panning, or anything in the digital realm aside from adc. The last thing I want to do is get into dsp programming.

Just wanted to keep things as simple as possible, make a really small and good sounding analog box.  I like the features now, proper 48v phantom supply, switchable panning, phase reverse, exact gain setting, switched gain values on the summing section, etc. Anywho it's a ways off from working. I finished soldering one of the preamp cards today. Had some problems with the smaller qfn20 attached to the ground plane - ground copper wicking too much heat away.

Title: Re: mini mixer
Post by: weroflu on June 14, 2017, 12:04:21 PM


Just got the bluetooth android code working today.

The whole board assembly shown above will fit inside the case but it has to be star trek beamed in, otherwise it's a no go.

Next revision will fit. Lots of testing on this one still to do.

I tested some quick and dirty sounds with an sm57 and the preamps sound really great. But I have no idea yet about everything as a whole, noise, summing, etc.

edit: If anyone knows where I can have a heavier grade metal box and hinged lid the same size as an altoids tin fabricated shoot me a pm.
Title: Re: mini mixer
Post by: illconditioned on June 14, 2017, 01:15:42 PM
Working on an altoids sized 6x2 balanced mic pre mixer.
That 1510 pre's, phase, phantom, mute, 3.5mm balanced input jack, bourns volume pot
dedicated panning per channel, no fancy pan(ts)

Shots of preamp card, sd card adaptor included for scale. Just finished soldering the card, could easily get slaughtered with testing, cold joints and a million other design error.

Great project.  Are you doing ADC as well on in this?
Are you going to sell it here ???
Title: Re: mini mixer
Post by: weroflu on June 14, 2017, 01:49:31 PM
I might take a stab at 2 channel adc off the stereo out once everything else is finished. Selling, I do not have the right temperament for it, doubly so for the maze of regulations that comes along with these types of things. And despite the progress so far I'm not much more than a beginner in electronics. But we'll see what happens if and when there is a working box. Maybe I'll make kits available where people can order the assembled boards from china. I priced assembly recently and it's not too bad especially in quantity.

There are a lot of hoops to jump through to use this thing: special 3.5mm to xlr cables/adapters, fairly delicate connectors - if someone tripped over a mic cable you could rip up the whole box, the case as of now even if I fit it in is not super strong, the switches work but they are not so easy to articulate, it's a set-and-forget box - no real ability to change levels or panning on the fly - well you could but nah it's not really designed for that, the android app only works on ble capable phones and I have no intentions of spending time making the app friendly for different phones, etc etc. Lots of caveats; it's basically designed to record live sessions in a known environment.

All said though,  I will make it available in one way or another to those who must have one.
Title: Re: mini mixer
Post by: weroflu on December 10, 2017, 01:38:47 PM
Prototype for the mixer is complete. Picking up 3d printed case this week as I messed up dimensions by about 1mm and it doesn't fit into a tin. Lots of ideas for a next version of this but not sure if I will follow through. The switches on the mixer (phase, phantom, and pan) are just way too tiny to easily get at them with your fingers. Most likely they will have to be actuated with plastic tweezers or something similar.

Also have a 2 channel p48 stereo preamp in the works, 1 9V battery, 2 regular pots.

Looking into doing a mini  5.6Mhz DSD recorder, either as a card in the mixer format, which would be ultra ultra tiny. Or as a standalone box in a tin, probably that could also fit at least 2 channels of preamp. Could be over my head but I'll take a stab at it.


upside down case:

To answer illconditioned: No plans for pcm adc. There are tons of good pcm adc's around. DSD interests me more. And for it to have utility it looks like it will have to be a DSD recorder. If I can pull it off it, it will be very bare bones with no interface to speak of, maybe just one button that starts/stops recording and advances to a new track.

Title: Re: mini mixer
Post by: voltronic on December 10, 2017, 09:29:27 PM
I'm interested to see what you come up with for the 2-ch P48 preamp.

Why DSD over PCM?  PCM is so much more practical for the vast majority of users.  (Unless you're just doing this for a personal challenge...)
Title: Re: mini mixer
Post by: Gutbucket on December 11, 2017, 12:22:56 PM
Way to keep at it! 

Voltronic, the variant I've been long dreaming of is a straight 6 or 8ch 24/48khz PCM recorder in a candy tin.  Single common 9-pin DSUB input connector to accommodate 8 unbalanced mic channels > low voltage mic power > gain trims > write stereo WAV(s) to SD card.  The On/off switch providing operational control - switch it on and it records, switch it off and it stops.  Small and simple.  I'll do the mixing later.
Title: Re: mini mixer
Post by: Gutbucket on December 11, 2017, 01:15:00 PM
Since I'm day dreaming I'll take it one step further with an alternative to my imaginary single 8 channel recorder..

4 separate super-miniature stereo recorders, each with a coin battery, microSD card, and a highly sample-accurate clock.  Controlled using a simple wireless command to start/stop them together.   Each writes a stereo BWAV with time signature for syncing the resulting files come mix time. 

I'd install each into the back of a rubber boundary-mount disc housing a miniature omni Mid/Side pair and place those anywhere I cared to within the range of wireless control.   Each storing data locally, eliminating all external wires and external connections, and only needing to receive the wireless start/stop signal.

Imagine this with a miniature bi-directional co-located along with the omni, plus a coin-battery and a micro-SD slot on the back-
Title: Re: mini mixer
Post by: weroflu on December 15, 2017, 01:55:53 AM
@voltronic: I have always been very curious about dsd. There is enough misinformation on the net to be thoroughly misleading. For a long time I planned on getting a dsd recorder and testing for myself what was hype and what was what. I picked up an Mr-2 over the summer and did a few rough recordings of acoustic material. Both the straight dsd playback and on-the-fly transcoding to pcm both sounded really good to me. It sounded about half way between digital and analog, and definitely has a distinctly different sound vs. all the pcm adc's I have used.

Note: I was unsold on dsd from listening to sound clips on the net, especially the pcm-d100 which sounded no different than pcm to me.

I am well aware of all of the practical drawbacks to the format, no need to go there. The only thing I can say is rent or get your hands on a korg mr2 or 2000 and try it out for yourself. You might well be surprised.

About halfway done with the stereo preamp. I don't think it will sound any better than what you are already using, but the size is nice and compact.

@gutbucket: I like your idea a lot, it's something that I would want for myself. 6 channels is about enough for my purposes. An 8 channel mint tin preamp for 2 wire mics is very doable, I would almost say easy at this point, but I won't since I can't even make a proper case for my mixer. Add in the adc and it's also doable, even though I have not done an adc yet.

I have my eye on working with the newer AK576x adc's. There is an eight channel one available. The recorder part makes it definitely harder, but I think that there are open source or other available ways to easily record pcm streams - not so with DSD. I might have got lucky in the mixer design in that there is no audible digital noise creeping into the audio. Adding an adc and a linux SoC would probably increase the chances of digital noise problems, but I will still most likely go for it anyway.

(If Jon wants to recommend an appropriate compact SoC for said recording purposes I would be appreciative)

It was a tossup when designing the mixer whether I would have direct outs/discrete channels or make a 2 bus. I like the stereo recorder idea just because it forces you to make decisons, mix on the spot and then the recording is done for better or worse

Title: Re: mini mixer
Post by: weroflu on December 21, 2017, 01:52:39 PM
Stereo pre


Sounds good.

Next version will be tweaked a little and have vertical 10 way switches for gain inside the case, a few changes to the power supply, cut off the part of the board that houses the battery.
Title: Re: mini mixer
Post by: weroflu on January 13, 2018, 03:50:24 PM
Probably the last preamp I wind up making for a long while. Lundahl 1538 input transformers, 10 way stepped gain setting, socketed op-amp will  use an interesting mini discrete opamp (ss3602) or for ic's either lme4562 or opa2134. I think the discrete one is biased for class a so battery power would not be feasible.