portable mixer with 4 THAT 1510 mic preamps

[BertH]

Hi all,

somewhat my first post/thread here, longtime lurker

I am planning a pretty ambitious project (for me, anyhow):

I am a film school student, studying sound engineer. I do alot of student films, where there was no real budget for sound equipment, or the sound equipment provided was not ideal for on the spot mixing. Things like a zoom H4n or tascam equivalent are very common. And as a recorder function, they are good for what they are (except the mic preamps, noisy as hell on those zooms). But for on set modulation, the are not handy at all. For quality reasons, and when I am not alone on a set, I also use my focusrite interface to record straight to my laptop. Again, modulation is hard. Pretty much setting at a safe volume, and letting it stand there, and make adjustments between takes.

therefore I was thinking of this:

a portable field mixer. 4 microphone preamps (switchable between mic & line inputs), led VU or PPM meters, for each input. A headphone monitoring system for listening to the inputs, if possible with various listening modes ("stereo" mix, mono (mix), or listening to one input individually (i think this might be the hardest part, and if possible can be limited to fewer options), and a "return" channel, to listen to the recorder return (this is easy).

the mic preamps should include a -20db pad, phase inverter and a low cut filter per preamp, each channel should be able to receiver phantom power.

Also, a gain trimmer should also be useful. (with professional set mixers, there is a gain pot, and a extra gain/trim control after the gain control to make the actual mix. Giving more gain and reducing volume (like the fader on a mixing desk)

I told myself that I should be able to route the independent channels to a Left or Right "bus", but most of the times I will be able to record 4 channels seperately. But a more simplified version where I would be able to sum channels 2,3, and 4 together to one output (or a combination of those) could be an option too. every output should be balanced/floating...


Extra difficulty: It has to be able to be battery powered! AND be portable, the smaller the better I guess.
for saving battery usage, when I am recording on a set with my laptop, or with a recorder on a steady location (with net power supplied), I would like to build it so that the "mixette", can also receive power from a regulated DC supply (external) (this, I don't see such a difficulty, but I think it's important nonetheless)

I was thinking using 4 9V batteries to create a +-18V supply, and 2 more 9V batteries (in parallell), for the phantom power circuit (seperately). I found a schematic somewhere, creating +48V from a single 9V battery. Anyone have similar ideas here? I could also use 5 9V batteries, but I'd rather not have to carry around 20+ 9V batteries all time in case they have to be changed...
On second thought, I might also think of 2 smallish sealed lead/acid batteries, generating a split 12V-0-12V power supply, and enough current. Also, combining the 2 batteries gives 24V DC, with a voltage doubler, this'll give the 48V for phantom.  (Or is this this not a good idea?)


I am now in the phase of wild ideas and lots and lots of reading here and on the internet in general, books etc, and deciding what route I would like to take.

For now, I think that, due to cost and space constraints, I would like to try a THAT 1510 based preamp. But I am looking for opinions and advice.

Also for the meters, anyone has made a DIY version of a PPM led meter small enough for my situation? would it be possible?

Anyone else made a similar project already?


I have designed and built various turntables, and small electronical devices and tools (in the audio genre), so most case design and building will also be done "in house". I really like the challenge of building everything myself, so cases and knobs are no exception.


I am open for all suggestions and improvements. in the meanwhile, I am starting on mock ups of the circuit design

Thanks for any help, in advance
I could really use any opinions or similar ideas to get me going...
I hope this will be an interesting project.

Thanks,
Keep them spinning,
Bert.

 

[Indecline]

That's a lot of batteries! Look into some sort of rechargeable lithium battery like for laptops.

 

[gemini86]

If you're careful about shielding and filtering, a better setup would be to use a couple off the shelf dc-dc isolated converters to generate your +/- and phantom power. Then you could run the whole thing on maybe one big battery, maybe a car battery.

The 1510/1512 is a good preamp chip. Plenty of layouts and even some pcbs for it.

 

[Rochey]

have a look at the Eden Preamp cards. we (at expat audio) have a special offer on a 4pack.

 

[gemini86]

Your site says you're all sold out...

 

[Rochey]

it's tough when they are popular (kidding! the first batch were low quantity just in case they didn't sell!)

more boards coming in on Jan 21st. Gimme a week to rebuild.

/R

 

[gemini86]

we must rebuild

 

[BertH]

If you're careful about shielding and filtering, a better setup would be to use a couple off the shelf dc-dc isolated converters to generate your +/- and phantom power. Then you could run the whole thing on maybe one big battery, maybe a car battery.

The 1510/1512 is a good preamp chip. Plenty of layouts and even some pcbs for it.

I'm feeling like using a dc-dc converter for the phantom power, if a find one that's small enough, and not too powerful (after all, 4 channels with 48V phantom, shouldn't be using THAT much power...)

Possibly DIY one, a NE555 voltage doubler for that might be a solution too.

For now, I think I'm going with Sealed lead/acid batteries (essentially, a safer car battery). 2 x 1,3Ah would make for a good power supply for endless hours of fun (I hope). 2 of those would weigh in at 1,2Kg total, take 1,5kg total, in a small case for them. I've looked at battery packs too, but the cost is much higher, for a comparable output.

Having learned to deal with Nagra's, i'm used to a little weight though, so there's no problem in that.
Laptop batteries would be a expensive idea, and somewhat harder to implement nicely in a design. If, however, the SLA batteries wouldn't work out, I might take the plunge and get myself a couple of NP1 batteries (and be broke)

Rochey, the Eden preamps certainly look very nice indeed, would I be able to use a rotary knob too for gain setting?

something in my head wants me to try the SSL9K too, ideally one or two 9's in the housing i'm planning would be max though, accompagnied with 2 more 1510's... Or keep that for a seperate project?

and lastly, I'd like to install a trimmer after the mic pre, so I can modulate without touching the preamp (and being able to give that little more, or reduce it). But what would be the best way to build & design this? I'm thinking like a smallish VCA might be an answer, but that might be big for in the project...
Ideas?

Thanks,
Bert.

 

[Rochey]

Bert,

I'd keep it a separate project. If your recording multiple sources on set, you may want to be "mic pre agnostic"...

For the Edens, you can use rotary pots. We use 5K reverse log pots.

However, if you really wanted to, you could use a rotary switch with matched resistors.

Do you need differential (balanced) or single ended (unbalanced) outputs?
That will define your output volume control types.

/R

 

[BertH]

single ended will do, as there would be a trimmer section behind it, balancing after that...

But for now, I think the Edens might be a project of their own, i'm really interested in the digital frontend you supply, as it would open new possibilities, but budget doesn't allow now

(but you've got bookmarked for sure )

btw, anyone tried building a transistor gainstage before the 1510's?
and would it improve performance even more?

Regards,
Bert.

 

[SkunkFunk]

I'd say using 9V batteries are a poor choise. If you plan to use rechargable batteries, differences in the cells will cause some to overload and some other to underload as they are all in series, passing the same current. This will reduce their life. If you don't plan to use rechargable batteries, 9V's are not that nice to carry in your toolbox as they are eaily shorted out (both poles adjacent on one side). One 12V battery with DC-DC converters would be a better choice. That would also maximize the uptime, as you will never end up with +rail charged out but 70% left in the phantom batteries for no good reason. It would also make it easier to make the preamps operational while charging. You never know what might happen in the field...

 

[BertH]

Okay, that got me convinced

Do you have any experience with MeanWell DC-DC convertors?
Looking at these at the moment...

combined with a 12V SLA battery (unless found better)

regards,
Bert.

 

[BertH]

Hi all,

I've been looking around for some circuits for the phantom power.

So far I found several, but I'm looking for suggestions. fairly new to DC-DC converters/switching power supplies (as you've probably noticed from the above posts too)

I found this schematic. Could anybody care to enlighten? Would this work well? switching frequency is 115kHz, but I have no idea about the ripple in this circuit...

Thanks in advance!
Bert.

 

[Rochey]

we must rebuild

Gemini86, boards are back We (I) rebuilt.
http://expataudio.myshopify.com/collections/mic-pres/products/eden-microphone-preamplifier-4-pack

 

[alexc]

There's some interesting 20 segment vu/ppm meter kits on ebay. Worth a try - look like they are differential buffered  and mode selectable.