[Rocinante]

So I've decided id like to add an output trim to the ml12's I am building. I'd also like to possibly add a stepped gain switch. I've researched a good bit reading various 312 threads but I'm not sure how I can implement this on the ml12? Could I just use one of the gain switch PCB's and grayhills that CAPI sells replacing the 25k pot?  Where would I tap for the output trim?  A point in the right direction would be great. I'm not positive about the gain switch but I'd like to explore my options and also it'll help strengthen my understanding.

[buildafriend]

Sweet build!
Ahh good to know about that star ground. Sharp, Mike.
I decided to daisy chain a couple more pres off of my psu.
What kind of 5 pin cable are you guys using?
I've got a DMX cable. Is it cool to run the ground through the (unshielded) shield strand?

Hello,

I am using a Mogami tube microphone cable from Redco.com

The connectors are these:
http://www.pmdx.com/6Pin-PlugAndJack

I used the two slightly heavier guage heater trace wires as my +/- 16V leads.

Best,

[mikefatom]

So I've decided id like to add an output trim to the ml12's I am building. I'd also like to possibly add a stepped gain switch. I've researched a good bit reading various 312 threads but I'm not sure how I can implement this on the ml12? Could I just use one of the gain switch PCB's and grayhills that CAPI sells replacing the 25k pot?  Where would I tap for the output trim?  A point in the right direction would be great. I'm not positive about the gain switch but I'd like to explore my options and also it'll help strengthen my understanding.

Hi Rocinante,

For the output trim, I suggest that you get the one from Classic API or Hairball Audio:

Bourns 51 T-Pad & PCB Bundle from Classic API
Bourns 600ohm T-Pad Attenuator from Hairball Audio

This attenuator would be wired between the output transformer and the male XLR jack of the unit.  So you have 4 connections to the attenuator PCB: +/-IN and +/-OUT.  See the images in the links above.

As for the gain switch, your thinking is spot on!  I suggest you get this bundle from Classic API:

Stepped Gain Switch from Classic API

There are only two connection between that switch PCB and the main ML12 PCB.  There are actually two extra pads next to the gain pot footprint on the ML12 PCB for hooking up an off board gain pot/switch.  So the two wires from the gain switch PCB will go right to those pads.  Nothing needs to be modified on the ML12.  The gain switch bundle from CAPI is perfect for what you want to do.

I hope that helps.  Please don't hesitate if you have more questions.

Cheers,

Mike

[Rocinante]

Thanks Mike. After reading the other 312 threads I came to that conclusion, but I still feel pretty green and part of me just wouldnt believe it was that easy. 
Awesome project btw. PSU is done just gotta get up.some loot to go shopping at CAPI.
Cheers

[mikefatom]

Thanks Mike. After reading the other 312 threads I came to that conclusion, but I still feel pretty green and part of me just wouldnt believe it was that easy. 
Awesome project btw. PSU is done just gotta get up.some loot to go shopping at CAPI.
Cheers

It IS that easy!

Enjoy.

Mike

[buildafriend]

I got a few of my 6 channels up and running. I'm very happy with the sound. 

Best,

[John Isaac]

Hi Mike! (or anyone else gracious enough to help me) I'm putting together a 4-channel unit (Whistle Rock 312, psu, and opamps) and had a couple quick questions for you.

1). Earlier you mentioned a 60va transformer would be sufficient power for a 6-channel unit. For a 4-channel unit, could I get by with a 2x18v 50va? Something like this: http://www.digikey.com/product-detail/en/VPT36-1390/237-1334-ND/2090072

2). I'm mounting the pre's in a 1u chassis using your recommended front panel design. Would 3/8 inch tall standoffs be sufficient to elevate the pcbs properly to line up the pot's and switches correctly with the front panel holes?

Thank you!

[MountCyanide]

That 50VA will definitely be enough. That's 1.39A for 4 pres. 347mA per. The 60VA Mike recommended would give 278mA per.
If I were in your shoes though I would grab a 100VA and build an external psu box.
These sound so good you'll probably want 4 more to track the whole drum kit. 
I used nuts and eyeballs as standoffs for mine so I won't answer #2.

[mikefatom]

Hi John,

Sorry for the late reply. I am away from home and internet access is a bit dicey

A 50VA @ 2x18vac transformer is just fine for 4 preamps.
As for the front panel, it is impossible to answer that question.  The design that I provided is only a guide.  The horizontal distances are good but everything else is not necessarily so.  Verify all dimensions before drilling your case.  The standoff height of the PCB will depend on the dimensions of your enclosure.  Arm yourself with a good pair of calipers and measure all relevant dimensions.  Better yet, get the mechanical drawings from the case manufacturer.

Hope that helps,

Cheers!

Mike

[buildafriend]

If I were in your shoes though I would grab a 100VA and build an external psu box.

You mean like this?

All done! Thanks for the help guys. My only mistake on this build was that I made it for a friend and not myself.

[John Isaac]

If I were in your shoes though I would grab a 100VA and build an external psu box.
These sound so good you'll probably want 4 more to track the whole drum kit. 

BAH! Cyrus, i have far too big a fear of commitment for that. Besides who needs more than four mics on a kit?!?   

A 50VA @ 2x18vac transformer is just fine for 4 preamps.
As for the front panel, it is impossible to answer that question.  The design that I provided is only a guide.  The horizontal distances are good but everything else is not necessarily so.  Verify all dimensions before drilling your case.  The standoff height of the PCB will depend on the dimensions of your enclosure.  Arm yourself with a good pair of calipers and measure all relevant dimensions.  Better yet, get the mechanical drawings from the case manufacturer.

A well-timed and appropriate response! This is my first build with transformers, and I was a little iffy (those sticky nylons standoffs aren't gonna cut it). I think I've figured it out though.

Thanks all!

[Dyonight]

Ohhh nice thread!!

I see a bunch of transformer rating but I can't figure how much current each preamp is drawing on its own.

Let's say they are loaded with Whistle Rock's own ML2520.

I ask cause I got a [email protected] torroidal and it will deliver 4.44A to 3xML12psu driving 21x ML12 which give 211mA each. Am I in trouble?

I know I probably won't ever drive all 21 pres at max setting (that would be a strange situation...) but I'd like to know if I'm on the safe side...

[mikefatom]

Hi Dyonight,

The 160VA transformer is more than fine.  The absolute worst case current draw per module is about 120mA the +16V/+12V (combined) rails.  That's is if you use the ML2520.  The quiescent current draw of the ML2520 is < 30ma.

I hope that clears that up.

Cheers,

Mike

[mikefatom]

Is anybody aware of a simple (read unexpensive...) +/- 5 leds VU meter circuit that could be fed by the 3rd winding of the output transformer?

I found this one http://groupdiy.com/index.php?action=dlattach;topic=43474.0;attach=6684 but I not qualified enough to shrink it to 5 leds...

I believe Rochey @ Expat Audio was working on a small VU meter.  It may be worth shooting him a PM to see what is going on with that.
The meter you posted will work fine as well.  If you only want 5 LEDs, only wire the ones that you want.

Cheers,

Mike

[Dyonight]

He another question! (I know I never quit.....)

Am I wrong or the relay RL2 is used exclusively for the DI option?

Let say I won't ever install a DI on the preamp (I use external ones through mic pres), can I remove the relay and use it elsewhere?

[mikefatom]

Dyonight,

The Grayhill should fit fine. Others have done it.  You have to put it on a 90 degree angle and the clearance from C6 won't be an issue.

As for RL2, you are correct.  It is strictly for the DI signal switching.  If you decide to do without the relay, you will have to jumper the contacts.  If you need help with that, let me know.

Cheers!

Mike

[mikefatom]

Hi Dyonight,

RE: High Pass Filter:  This feature was implemented in the previous version by switching in or out a RC network before the op-amp (I will send you the schematic via email).  This never worked out great to be honest.  One issue was that the op-amp would develop a DC offset when the HPF is engaged.  This means that the DC blocking caps became mandatory.  One way to reduce that problem is to chose a low-enough value for R11 to keep the effect of the op-amp's input bias current tolerable (something around 15K).   To completely eliminate that problem, you would have to use an op-amp with very low input bias current, which are typically using JFETs at their input.

Re: The VU meter:  I would add a differential amplifier stage in parallel with the main output to feed the VU meter circuit you posted.  The reason is that the VU circuit expects an unbalanced signal while the preamp's output is balanced.  Alternatively, you could tap the VU signal from right after the DC blocking caps and before the output transformer primary.

Also, I'm not a big fan of this circuit because it draws way more current than needed.  Every time, a LED lights up, you will be drawing an additional 10mA (or whatever the LED brightness resistor dictates) from the V+ rail.  Take a look at the schematic on page 25 (figure 38)  of the Texas Instruments datasheet for the LM3916 for a better circuit.  Although it says that V+ should be between 22 and 25V, I think it will work fine on the +12V rail.

Prototype it first and verify that it works the way you want before investing time in a layout.

As for getting your PCBs made, I suggest getting them from http://dirtypcbs.com/.  I have not used them myself (yet) but I’ve only read good things about them.  I don’t suggest any of the places I usually get my PCBs from because the price will be prohibitive for a small one-off run.

I highly recommend making your own layout, even if only for the experience!

Cheers,

Mike

[mikefatom]

Hi Dyonight,

What you want to do here is use a t-pad simply for the sake of simplicity.
Check out this link for a nice calculator:  http://www.nu9n.com/tpad-calculator.html

The t-pad would connect between the + and - of the output.  There is no connection to ground because you ideally want to keep the output floating.

Also, I would suggest to go for something a bit more drastic than 6dB attenuation.  I think that 12dB would be a good point to start.  Either way, try it out and tweak as required.

Mike

[mikefatom]

I've always thought that the Hi wire carry the positive signal and the Low wire the negative one (same as Hi but flipped 180°) to provide a perfectly balanced image ready to be inverted in the next device to remove noise.

You're describing a differential signal here and not necessarily a balanced signal.
See here for more details:  http://www.hairballaudio.com/blog/balanced-differential/

So if I use the T-pad, it will affect only the positive side of the signal and leave the flipped one at full volume and will unbalance the signal? is the low wire used as a "sink" like the ground is used in circuit I understand? (or think I do...)

In the case of the transformer here (floating output), the amplitude of the output signal is really just the voltage difference between the positive and negative.  One is referred to the other.  The t-pad is just a voltage divider between these two signals which, as a result, attenuates the difference between them.

Hope that helps,

Cheers!

Mike

[Dyonight]

I just realized that I flooded this thread with all my questions and since Mike kindly quoted all my posts I erased my original questions  to shorten the thread.

I just got 21 stepped attenuator kits yesterday from CAPI and finished soldering everything this afternoon.

Also, I ordered 14x CM75101 and 14 CMOQ-2L from Cinemag and should recieve them this month.

I screwed the extra options (severely lacking money and need preamps!) so I'll do 3 rows of seven preamp cards  with 48v, PAD and input Tx ratio. I'll flip the phase in the box when needed or built some xlr adaptor.

Here are the combinations I went for:

1st row (312) = EA2262 -> ML2520 -> EA2503

2nd row (Pacifi-kind-of... well,  cleaner)= CM75101 -> ML2520 -> CMOQ-2L

3rd row (Cleanest-Hardy-ish...)= CM75101 -> ML918 -> CMOQ-2L

I think those combinations will highlight Mike's GREAT work with the opamps and ML12 pcb.

Anyway I have a bunch of pictures, will take more and will condense everything in one mega post with all the built process.... or wil make a new thread maybe if it's too long

Next will be sending the front and rear panels to front panel express.