mini mixer

[Gutbucket]

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-

[weroflu]

@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

[weroflu]

Stereo pre

https://1580mixer.shutterfly.com/pictures/69#69

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.

[weroflu]

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.

[weroflu]

https://1580mixer.shutterfly.com/pictures/79

https://1580mixer.shutterfly.com/pictures/80


testing with an ic then popping in discrete opamp once everything is ok

[Gutbucket]

Clean and tight.  Very nice work!

[spyder9]

Any pricing?  I'm interested.  Thank you for including the Lundahls.      

[weroflu]

No pricing. I am not sure how to make these available. There was almost no interest aside from you. Dealing with any sort of regulatory agencies would be a nightmare for me.

Specifically, there are 3 designs now: mini mixer (wall wart powered), battery powered that1510 preamp, and the discrete opamp transformer preamp above which is wall-wart powered. The transformer preamp needs a new layout: if you look closely the right rotary switch is almost impossible to turn since there is no space around it.

I  imagine of the 3, the battery powered stereo preamp is more geared toward what most people here do.

The transformer preamp is very costly in parts alone. Assembly is not that difficult, but unless you have smd soldering skills it will need to be assembled at a factory.

In quantity of, say more than 10 or so, the battery preamp is feasible and cost effective. But this is more of a diy thing for me and I'm not going to put in any time into selling  or making that side of things happen. I will stand by what I said earlier though, if someone must have one then I'll help make that happen.

I can easily make boards available to anyone.

[illconditioned]

Very nice projects.

Thanks for sharing ...

[mnm207]

I'd be interested in a board or two for the battery powered version if you make them available.

[voltronic]

Wereoflu, have you taken any measurements of any of these units?

[weroflu]

No I have not. For a while I was into the idea and made a t-pad drv135 adapter board to interface with a soundcard to test the mixer mic preamps  for noise and ripple but I lost interest once everything started to work. Everything sounds dead quiet, but that doesn't mean there aren't some high frequency anomalies or other problems that I can't hear. Maybe I'll get motivated at some future point to do that.

[voltronic]

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.

I didn't really see this post until now.  Wow, this sounds like an outstanding idea, sort of a DPA d:vice on steroids and not requiring an external control / recording device.

As long as the preamps have low noise such that you can set the initial gain low enough not not require any limiting, this could definitely work.  Maybe have 3 fixed gain levels, such as what Jon had on the Littlebox / Tinybox - no need to worry about channel linking since all 2/4/6/8 channels would be fixed at those switched levels.  That would also make it useful for Soundfield or similar applications.

Such a device wouldn't even need a display if it's a one or two switch operation.  Maybe just a few cheap LEDs to indicate power, signal presence, or error.  Card formatting and similar operations would be handled on the user's PC as it should be.

The only thing I'd possibly change to what you propose would be to have it write a poly WAV rather than separate stereo WAVs, such as what you see in the new SD MixPre line.  Writing one file at a time is likely to be more reliable than writing 3 or 4 simultaneously.  You could make it such that all 8 channels are recording no matter what, or arm/disarm using internal DIP switches.

The closest thing to this that actually exists might be the Sonosax miniR82, but the device you're proposing could be much simpler (and much cheaper).

Someone needs to build this thing!

[Gutbucket]

Glad you are in agreement!

The challenging part seems the portion post-ADC, handling writing a poly-WAV file to an SD card.

[weroflu]

On to version 2 of this mixer. Mostly this is the same idea, but there were a number of things I wanted to correct in the prototype.


The bluetooth thing sounded like a good idea but in reality having another device (phone) to control preamp functions was a drag.
Everything is now controlled on board via mini joystick and mini display. Channel count goes down to 5 now instead of a 6x2.

Just worked out metering, so there will be a nice 10 bar peak meter on each channel.

All switching is digital, no more analog switches for polarity, phantom, mute.

5v rails instead of 15v. I changed the power supply around and will use different regulators but in theory it should be fine.

The main thing I'm happy about is finally making the channel cards truly modular. There are miniscule edge connectors for the preamp cards now. Working with mini pitch  header pins was a disaster last time. Every time I worked on a card the pins would desolder and misalign.  If these edge connectors work out then I can see making different types of preamp cards in the future. Also I never really got the last one to fit in a mint tin properly, so I shaved off some dimensions here and there so hopefully things will fit properly.

Direct outs would be nice, but I could not find any more room for the output connectors.

Design is finished, I'll start to leisurely build the first one soon.