Simple scalable mic preamp

Recording Techniques, People Skills, Gear, Recording Spaces, Computers, and DIY

Moderators: drumsound, tomb

User avatar
Drone
re-cappin' neve
Posts: 678
Joined: Fri Dec 18, 2009 4:01 pm
Location: Uranus

Simple scalable mic preamp

Post by Drone » Fri Feb 26, 2016 7:19 am

I'm realizing with my current recording setup, a console is rather unnecessary. I have an old 24 track Soundcraft 500, and I'm using 8 channels of it, just as mic preamps, I'm doing everything else in the box.

Rather than have the giant console, I'm wondering if I should just get a mic preamp unit, one of those 8 way affairs. I just need something that takes 8 XLR's in and outputs 8 1/4" TS with a gain or level on each channel. No phantom power, phase reversal or anything.

Coming from the less is more school, I want to build something super simple, like a couple of FET's or a single op-amp per channel. Any suggestions, already available kits or whatever?
The previous statement is from a guy who records his own, and other projects for fun. No money is made.

The Scum
moves faders with mind
Posts: 2745
Joined: Thu Jul 03, 2003 11:26 pm
Location: Denver, CO
Contact:

Post by The Scum » Fri Feb 26, 2016 10:43 am

The THAT 1510/1512 are probably the best bang for the buck, low parts count preamp chips around. Add a 1646 if you need a balanced output.

You could always wire a 5534 as a diff amp with gain...or build out the front end with more opamps (instrumentation amp config) or transistors (which is very close to your 500's pres).

Most fully discrete designs use a transformer on the front end - see the Hamptone JFET design. Transformers are great for low parts count, but not so great for low cost.

Maybe I should revive my doorbell transformer preamp...
"What fer?"
"Cat fur, to make kitten britches."

User avatar
Drone
re-cappin' neve
Posts: 678
Joined: Fri Dec 18, 2009 4:01 pm
Location: Uranus

Post by Drone » Fri Feb 26, 2016 11:45 am

I have an old Tapco 6200B, that has transformers on each input. It's too noisy and scratchy to use, but I imagine the xfmrs are usable.

I had been looking at a wchematic with the 5332, which is just a pair of 5334's right?


Image
The previous statement is from a guy who records his own, and other projects for fun. No money is made.

The Scum
moves faders with mind
Posts: 2745
Joined: Thu Jul 03, 2003 11:26 pm
Location: Denver, CO
Contact:

Post by The Scum » Fri Feb 26, 2016 12:40 pm

Careful with your parts numbers here: it's 5534, not 5334. You'll have trouble sourcing 5334's.

But yes, a `2 is a twin `4.

And those circuits are basically the "diff amp" configuration I mentioned.
"What fer?"
"Cat fur, to make kitten britches."

The Scum
moves faders with mind
Posts: 2745
Joined: Thu Jul 03, 2003 11:26 pm
Location: Denver, CO
Contact:

Post by The Scum » Fri Feb 26, 2016 8:53 pm

Also, what's the source of the schematic?

I'm kinda curious about what they say about the "see text" resistors.
"What fer?"
"Cat fur, to make kitten britches."

User avatar
Drone
re-cappin' neve
Posts: 678
Joined: Fri Dec 18, 2009 4:01 pm
Location: Uranus

Post by Drone » Sat Feb 27, 2016 12:09 am

Dunno, google images.

It looks like a differential amp, so they'd be gain setting, or at least R6 in ratio with R3 would be.

Hmm R5 would have to equal R6 to equal the balance on both sides.

Don't know how large you could make them for gain, guess it would depend on the device.
The previous statement is from a guy who records his own, and other projects for fun. No money is made.

User avatar
floid
buyin' a studio
Posts: 983
Joined: Tue Jan 03, 2006 1:39 pm
Location: in exile

Post by floid » Sat Feb 27, 2016 6:40 am

If you decided to use the Tapco transformers, do you necessarily need a diff amp? The advantage I suppose would be the ability to compare with/without to see if response justifies gain.
Village Idiot.

Jim Williams
tinnitus
Posts: 1135
Joined: Sat Jun 03, 2006 8:19 am
Location: beautiful Carlsbad, CA
Contact:

Post by Jim Williams » Sat Feb 27, 2016 8:39 am

The Scum wrote:The THAT 1510/1512 are probably the best bang for the buck, low parts count preamp chips around. Add a 1646 if you need a balanced output.
If you layout for those mic preamp chips you can also use the Audio Upgrades PIP module. That is a hybrid/discrete mic preamp fitted on a 7/8" square pcb that plugs into the SSM2017/2019/INA217 and THAT 1510 layouts.

It features a super low -133 db EIN at 50 ohms source impedance, a 60v us slew rate, 50 ma output current and .0005% THD, .00015% IMD. None of the current mic pre chips can approach those specs.

If you want higher performance I also have the High Speed mic preamp cards for $175. It is by far the world's fastest mic preamp with a 7000V/us slew rate, 110 ma output current (drives 150 ohm loads easily) -133 db EIN and .00012% IMD. Bandwidth is 30 mhz and bandwidth is maintained up to +60 db gain. It is an all current feedback design.

Sonically it sounds much more like my tube designs than anything solid state. The current feedback design features low odd harmonic partials and soft clips unlike any transistor designs. The clipping produces mostly 2nd harmonics and the waveforms have rounded edges, sort of like tubes.
Jim Williams
Audio Upgrades

User avatar
Drone
re-cappin' neve
Posts: 678
Joined: Fri Dec 18, 2009 4:01 pm
Location: Uranus

Post by Drone » Sat Feb 27, 2016 8:45 am

Really what I'm doing is spitballing ideas, and seeing if anyone says, oh there's this super simple circuit you can try, or hey the FLAPDAP1865 is a perfect chip for this

I'm just looking to reclaims some space, I might just get something like one of those Nady boxes, but I figured for the same price, or less, it might be easier to get 8 XLRs, 8 1/.4"s, 8 pots and 8 FET's or op-amps or whatever and build something nice and clean, no extra bells or whistles. Then I can put the console back in it's roadcase, until my setup expands enough to require it

Not set on anything except simplicity, I mean with the transformers I can get say 8 x 12AU7 and go tube, long as I can source a transformer that will supply that kind of heater current.

Just looking for peoples favourite ideas really.

**EDIT Sorry Jim, I was writing this post while you posted yours, this was not a response to that, I will check it out.**
The previous statement is from a guy who records his own, and other projects for fun. No money is made.

User avatar
Nick Sevilla
on a wing and a prayer
Posts: 5555
Joined: Mon Mar 03, 2008 1:34 pm
Location: Lake Arrowhead California USA
Contact:

Post by Nick Sevilla » Sat Feb 27, 2016 8:50 am

I would recommend getting some DIY 500 series preamps.

There is a whole thread here that has most if not all the 500 series manufacturers.

Getting some of those, you alleviate a lot of self build issues, and you get a lot of support from the builders as well as many people here who have built them.

And you can scale it up as you need to.

Cheers
Howling at the neighbors. Hoping they have more mic cables.

User avatar
Drone
re-cappin' neve
Posts: 678
Joined: Fri Dec 18, 2009 4:01 pm
Location: Uranus

Post by Drone » Sat Feb 27, 2016 9:52 am

I think you are missing my budgetary constraint here, sub the cost of the Nady unit.
The previous statement is from a guy who records his own, and other projects for fun. No money is made.

Jim Williams
tinnitus
Posts: 1135
Joined: Sat Jun 03, 2006 8:19 am
Location: beautiful Carlsbad, CA
Contact:

Post by Jim Williams » Sat Feb 27, 2016 10:16 am

The Hairball "Elements" series raw 500 mic preamp is a great choice, only about $125.

I put a High Speed mic pre card into one, works great.
Jim Williams
Audio Upgrades

User avatar
Drone
re-cappin' neve
Posts: 678
Joined: Fri Dec 18, 2009 4:01 pm
Location: Uranus

Post by Drone » Sat Feb 27, 2016 10:24 am

But I'd need a 500 rack, and that's $125 per channel?
The previous statement is from a guy who records his own, and other projects for fun. No money is made.

dfuruta
re-cappin' neve
Posts: 697
Joined: Fri May 08, 2009 11:01 am

Post by dfuruta » Sat Feb 27, 2016 11:49 am

if you like lo-fi, get some transformers from edcor, use a couple common drain jfets or common emitter bjts as the gain elements (or just one if you don't need a lot of gain), stick an emitter follower after it, and impedance balance the output. cheap, easy & fun. i built one of these the other day (with p48, an input pad & transformer balanced output) and it sounds good:

Image

if you want hi-fi the THAT chips are simple to implement and won't cost you more than what, $10 per channel.

User avatar
Drone
re-cappin' neve
Posts: 678
Joined: Fri Dec 18, 2009 4:01 pm
Location: Uranus

Post by Drone » Sat Feb 27, 2016 12:06 pm

I'm not atall sure how-fi I am :mrgreen:

I think I'm more of a DIY 4-tracker soul, with a pawn store budget, which is where I got my 1010-LT which is my sole interface. The Soundcraft was from when I had thoughts of making a real studio, until I realized that was a lot of work and I probably didn't have the social skills :mrgreen:

Nowadays, I keep the cover on the Soundcraft, and just turn it on, unless I'm moving mics around. In which case I'll solo each channel and then ignore it again. I have cats, they shed, a lot.

So the THAT sounds like a possibility, how sane would a tube version be? I'm thinking maybe a 12DW7, using the AX side for the gain stage, and the AU side for the output? Those little Fender reverb clone transformers have 3A of heater, which is more than enough for 8 tubes, not sure if the plate current will be super high? Use the Tapco transformers on the input.

I'm a lot better at scavenging and tweaking other designs from cookbooks and stuff as I am from doing something from scratch. :rolleyes:

What were the Edcor transformers? According to the Service Manual these Tapco ones are 150ohm:600ohm
The previous statement is from a guy who records his own, and other projects for fun. No money is made.

Post Reply

Who is online

Users browsing this forum: No registered users and 52 guests