Well, maybe that thread has changed in the intervening time: it's just
(COMPLETELY UNFAIRLY GORGEOUS) build pics now.
Nice job Bruno2000: you can build gear for me anytime -
how about starting today?
I had to order some parts to fix a friend's guitar amp and while I was paying shipping I decided to plunk down for a handful of THAT1510 and 1512 8-pin DIPs. Here is my application:
I am a "Sunday driver" - each week I mix house sound, send out a webcast, record live to 2-track for CD duplication, and (when the music intrigues ya) capture multitrack into Pro Tools for possible later mixing/editing/whatever. This is in the context of a Presbyterian church service; the music is a 30-or-so voice choir, and band with drums, bass, Hammond B3, piano and sax.
I do not have a pickup truck.
What I do have is the desire to get some use out of a nice-sounding, well-built 16 channel line-only mixer I was given, for pre-mixing some things like the drum kit, the Leslie for the B3, and (let's face it) the thrill of the chase. My intention HAS been to throw together a few one-tube mic preamps but that project hasn't gained any traction yet. The line mixer has regulated +/- 18V rails off a raw +/-24V supply and it seems to me I could pop together 8 or 16 mic preamps and insert them after the input jacks without a whole lot of effort, expense, or dreaded metalwork. It was from researching that idea that I stumbled upon this forum and so I joined.
First off: this will be a KISS, quite-good-is-quite-good-enough project, I think. I believe I can get away with injecting the THAT chips servoless into the stage after the line receiver chips (EQ stage - on either the right or the left of the 33 uF capacitor at the upper left), so I'd like to try a one-chip implementation first. I have the THAT data sheet and I'm attaching part of it for reference. I'm also attaching part of the Rane MS1 mic preamp schematic, showing their signal/clip indicator circuit, and the schematic for the relevant section of my line mixer.
- basic that 1510 preamp circuit.jpg (75.88 KiB) Viewed 22861 times
- rane ms1 clip indicator.jpg (76.37 KiB) Viewed 22861 times
- LINEMIXERSchematic.jpg (270.02 KiB) Viewed 22861 times
To me, the simplest way to start looks to me like this:
a) build a 48V phantom supply, which will live on the primary side of a batch of mic input transformers. I have enough transformers on hand for at least 6 channels - I will reserve the transformer-input channels for any phantom-powered mics, and I will thus have two flavors of channel - w or w/o phantom & transformer. If I discover a need for more phantom inputs, I can either buy more transformers or bite the bullet and retrofit the rest of the THAT circuit from the next pic in the data sheet. Those channels will have a minimum gain of 10 or 20 dB depending on which transformer they get, so I suppose I'd be better off using the lower-gain THAT chip on those channels. (That's a question.)
b) build a group of THAT 1510 or 1512 mic input circuits, as shown on the THAT data sheet, with switches for 1. polarity normal/invert 2. mic/line (in other words, with the THAT circuit or without, as originally built) and 3. phantom power on/off. A pot (possibly a rotary switch if I see that I have enough room) for variable input gain. These modules will be powered from the existing rails and will inject the mic signal at the 4K7 resistor (or at the coupling cap next to it if the THAT's DC offset is too much.)
Now for my questions and data-sheet "design" ideas, of course....
The question above about which THAT chip to use. I have no ribbon mics at this time, it's a mix of LDC's, SDC's and dynamics. My Soundcraft mixer never needs the gain knobs cranked up terribly high.
I'm concerned about clipping. My two favorite choices are 1) to build the Rane signal/clipping circuit, calibrated for the THAT's clipping point, and so have a warning light just for the mic pre stage, or 2) to recalibrate the existing clip/signal indicator circuit in the line mixer to be accurate for the mic channels' clipping points. I'm leaning towards 2), but is 1) a good/better option?
Also, I think the mic pre signal can be injected after the 33 uF DC blocking cap if I am reading the EQ schematic correctly: the LM833 op amps in the EQ section are all AC coupled so they would not need the additional cap - but if so, why is it there?
The next task (assuming all goes well) might be to build out a few more buses for flexibility and maybe add another 8 ins (I have most of a second identical mixer for parts), but that's not going to happen right away.
Please: comments, suggestions, recommendations?
Thanks,
-j