quad eight mic pre [pics]

hi all, thought i'd show my first project since joining the group. my thanks to everyone for the tidbits i've gleaned from day to day which made this possible. still some tidying up to do but it's alive so i'm happy!

cheers, -scott

EDIT 1/04/08: pics were low quality to begin with, then links went bad so while fixing broken links i went ahead and updated to new photos. new pics show gain switch instead of gain pot. also input transformer primary selector has been replaced w output fader and cinemag input R has been hard-wired for 150 ohm source and held down w double stick foamcore.

mmmmhhh - nice gain knob!
how did you get the infill of red on the engraving??

SWEET!

I'm curious about the red fill myself.

cheers,
kent

What kind of input transformers are you using? That's a nice looking box. Let us know when you get some better pictures. Nice work.

thanks, glad ya'll like the box. the red infill is a standard option w front panel express, they did the panel. using FPD was the first app i've tried on mac osx virtual pc too. it works great save the encrypted 'order' function which doesn't work, so you stil have to phone them up to complete the payment details.. just as well, they're very nice people and helpful.

the input trafos are cinemag cmqe-3440a. my intention is to have all 3 input impedances selectable but i've only been able to figure out the switching logic to use 37.5 and 150 so far. still thinking on that one, it's tricky. miko, i missed your group buy on the trafos but david at cinemag was very gracious in offering me a couple at the discounted price since there was a larger order in the werks, so i have you to thank there too. they're lovely trafos! cheers.

Excellent, stuff. I was going to quietly announce that I may have an extra pair of the 3440A's. After some listening tests and such, I WANT DETAILS! :grin: Seriously, I have the same CA127A's, JLM supply, and Case. Shoot, I was even going to use the same knobs! This place is creepy sometimes.

Wow those look great-- I have CA127A based pres myself but I had them racked by the seller. Just gearing up to dive in to DIY here.

Can anyone tell me what value pots should be used for the gain knobs?

I think mine are incorrect, all the action is between 12 and 5 o'clock.

BTW I'm loving these things on guitars--

I'd by a 12 positing single pole shorting switch and gear it out to 2db steps
10 k will start you out at 10db of gain so Wil

Wilebee

So a 12 position, single pole shorting switch-- at 10k?

When you say gear it out to to 2db steps-- I'm not sure I follow.

Thanks though--

Liam

Liam if you start at 10 db with a 10k resistor and go up to 40 db of gain with just the amplifier with a 330 ohm your range is 10-40db you should be able to figure out the different values of resistors to switch you between 10 and 40 db remember you will get gain from your input and output transformers. if not usae this as a guideline
Gain Resistor Chart
0 DB = none
Unity = 33k ohm
5 DB = 30k ohm
10 DB = 15k ohm
15 DB = 6.8k ohm
20 DB = 3.6k ohm
25 DB = 2.0k ohm
30 DB = 1.0k ohm
35 DB = 560 ohm
40 DB = 330 ohm
Wil

Wilebee

oops looks like i contrdictied myself use 15k for 10 db. fFigure out your step up gain with the transformers (in/out) the figure out what gain range you need from your amp and select resistors in between that range Wil

Hey thanks--

i'm using the fudged rev log pot getup on mine for now, 100k linear w 47k resistor across the wiper and input. it's usable but i'm gonna change to rotary switches as soon as i can and will build future units w stepped gain.

upon initial listening tests, these input transformers sounds great! thick and meaty w a nice hi fidelity sheen. i usually hate my voice w a 57 but it sounds good here. headroom out the wazoo, not a trace of noise. man i love em! got 6 more to build now, just need some do-re-mi to buy more cinemags and cases. damn these sound good! :grin:

thanks again to everybody here.

and miko, that's damn spooky! great minds think alike? (or at least read the same forum) :green:

:guinness:

Mine are definately usable as is, just a little hard to dial in when your just slightly too hot to the a/d converter.

I wonder if the cinemag would be a match for my electrodyne module?

Very nice! These things sure are (cult) popular, aren't they? Gotta put some pads on mine, like you have...these pres are HOT. Either that or change the circuit some...but I am untrained in such matters. So far, at least. Can't wait to get that Cinemag iron and try it out... Great job!

-- Jesus

got my rotary switches today so took out the gain pots and installed switches built w resistor values posted by wilebee. big improvement w gain control. i got some cheap 12pos 1pole shorting alphas from mouser (part #10ww112), they work fine. the 12pos gives 4 extra positions compared to the chart so i added intermediate values for the top of the gain range. so it goes from 5-25db in 5db steps, then the rest of the way in 2.5 db steps. great way to go for you guys working on these.

also, if somebody figures out a way to wire those cinemag 3440a's to a switch that enables all 3 primaries, please share. mine are wired for 37.5/150 for now. adding 600 gets complicated.



:guinness:

Nice to see both of the QE racks have JLM Power Supplies :grin:. We are about to rack 30 of the QE cards as stereo pairs. Below are two new modules that we use to give our Quad Eight, API etc Amp cards full Mic Pre features.



The Go Between PCB on the left has slow ramp 48v on/off, 20dB Pad & Phase Reverse. The Active DI on the right can be run from a 12v to 48v power rail. Both can be used separately and are designed to cut into the cable coming in from the Mic Input Chassis XLR & The Mic Pre PCB or Input transformer. But when used together only the Go between PCB needs to be cut into the cable and the DI can be connected by one IDC cable.

These will be up available on the JLM web site shortly as PCBs & Full Kits.

Joe

www.jlmaudio.com

How are you guys mounting your input transformers so they don't rattle around the case :grin: ??

Justin

I'm just about to start gathering parts for my CA-127s. I just ordered 6 more trannies after getting two from Miko's group buy. I have 8 cards to rack up.

Does anyone know how many cards can be powered from a single JLM Audio power supply (either the 3 pole or 5 pole version?)

The Go Between and DI PCBs look great, too! I'll probably get some of those when I order the power supply!


Where did you guys get those cool knobs?

Does anyone know how many cards can be powered from a single JLM Audio power supply (either the 3 pole or 5 pole version?)

Click to expand...

Hi Jim I was just answering your email when I saw this post drop in.

The CA127 or CA227 each draw a max of 85mA each at full output power. So the 8 can be run from one AC/DC power supply kit easily as long as the regulators are bolted to a metal case or a good heatsink. OR you can use one new 5 rail power supply and run 4 x CA127 on each set of +/- rails if you want to spread the heat more.

Joe

www.jlmaudio.com