Building the Jensen AS016 Microphone Preamp Front End

[straypacket]

I recently acquired a pair of mostly-populated "Keith 9K" preamp boards.  In building a parts list for completion, I started looking for something to use for the standard mic-pre functions that are missing from the DIY 9K boards (pad, polarity, phantom power).  I could have used the excellent candidates that are already out there, but I decided to see if I could roll my own.  I decided on the Jensen AS016 schematic.  This can be found here: http://www.jensen-transformers.com/as/as016.pdf.

I have created a schematic and PCB layout using Eagle, and created the Gerber files using the SparkFun CAM script with Eagle.

Since this is the first time I've done this, I have no idea if what I have created will work or not - but it sure looks cool on paper   Since it's a passive design, and is relatively simple I am hoping for the best!

I am about to send it off to http://batchpcb.com to have a prototype made, and see if it will work.  If it works and there is interest, I can see about getting more boards made.  BTW, does anyone know if there would be legal problems with me doing this?

The Eagle board file is attached to this post, for anyone who is interested in taking a look.  If you do take a look, and have any pointers for a noob, please feel free to chime in
I will try to attach the schematic and gerbers.

Thanks to all here, for the inspiration to try this.

EDIT:  Version 1.7 of the board file has been attached:  the part for the DPDT switches has been changed to Mouser part #108-2MD1T2B2M6QE-EVX and the board has been organized to be vertical-mount-friendly.

 

[straypacket]

Here is the Eagle schematic, as an attachment.

EDIT:  Version 1.7 of the schematic file has been attached:  the part for the DPDT switches has been changed to Mouser part #108-2MD1T2B2M6QE-EVX and the board has been organized to be vertical-mount-friendly.

 

[straypacket]

And here is an image file of the board layout.

EDIT:  Image of Version 1.7 has been attached:  the part for the DPDT switches has been changed to Mouser part #108-2MD1T2B2M6QE-EVX and the board has been organized to be vertical-mount-friendly.

 

[Rochey]

this front end is pretty well known. So your schematic I have no major concerns with.
(In fact, the digitally controlled mic pre frontend over at expataudio uses the same schem!

Good luck! (and don't forget to pair it with a pair of edens!  ;D )

 

[straypacket]

this front end is pretty well known. So your schematic I have no major concerns with.
(In fact, the digitally controlled mic pre frontend over at expataudio uses the same schem!

Good luck! (and don't forget to pair it with a pair of edens!  ;D )

Thanks Rochey!  I've been so tempted by Expat's front end - the buttons just look so sexy.  I think I'll have to get this analog technology thing sorted before I go digital though

 

[Harpo]

a little critic ...
when revisiting your layout, you might place the phantom LED centered above the switch. The LED is missing a current limiting resistor anyway.
Your label GND will be Chassis (XLR-pin1/shield and phantom current return) and the LED might have a seperate connection for your 18V current return. These +48V and +18V current return paths are not necessarily the same. For the LED maybe use a different or edit the silkscreen symbol for a better visual of this polarized part as well. Just my 2ct and good luck.

edit: What type are your selected toggle switches ? With 220uF/63V cap (hight 16mm for usual) fitted from component side, this might exceed the hight of the switch body below frontpanel.

 

[straypacket]

Hi Harpo,

Thanks for spotting the missing CLR and ground.  I have made all the modifications suggested except the 18v ground.  Working on that now.

Also, good call on the cap height.  When I was investigating that, I saw that the switch in my BOM apparently doesn't match the Eagle component I used - so I'm glad you encouraged me to check -  as I was just about to order the parts from Mouser

I will check back in when I get that sorted.

 

[straypacket]

I added the current limiting resistor for the LED.  I also added the second ground for the +18v supply line.  I made this the ground used for the ground plane on the board.

For the switch part on the board/schematic, I used the Marquardt 9070-2W from Eagle's Switch library.  The footprint and pinout looked like a match for Mouser part #108-2MD1T2B2M6QE-EVX, so I am using that in my BOM.  Hopefully they are compatible.

I've adjusted the layout to be vertical-friendly, and the toggle-directions to be consistent with switch-right="function off", switch-left="function on".

 

[straypacket]

2 PCBs arrived last Thursday...

 

[straypacket]

... and assembled.

The switch legs needed quite a bit of filing to get into the holes in the PCB.  Guess I'll have to modify the Eagle package...

 

[straypacket]

I finally got these installed on the 9K Preamp clones.  I have two questions about the installation though:

(1) When I have the 48v indicator LED in the circuit and the 48v is one (therefore the LED is on), it buzzes enough to make the preamp unsuitable for recording.  If I simply remove the 18V wire that feeds the LED, then everything works fine.  I'm just wondering if the my layout of the circuit is somehow causing this?  I think I've seen other preamps that have the phantom LED hanging off the switch legs...


(2) Most of the switches make annoying pops when switched.  Is there standard way to fix this (or somewhere I can read up on why this happens and how to fix it)?

 

[ksanura]

Hi,

Sorry for ressurecting old thread, but did you manage to solve these two issues?

I'm currently looking for Mic Pre front end for my dual SSL9k, that would be available to do as self-etch.

 

[straypacket]

Hi ksanura,

I didn't find any solutions (granted, I haven't looked too hard).  Regarding the buzzing, someone had mentioned that I should look into a "constant current source", but I'm not enough of an engineer to do much without specific circuit recommendations.

At the present time, I have just disconnected the LED from the circuit.

I would be interested to know if you come up with a different front end, or any solutions to these issues.

Thanks!
Mark

 

[ksanura]

Ok, thanks Mark, will try to come up with something, or will try your layout and see how it goes..

Was thinking of JLM go-between-kit initially, but 2x25$ for kits + 25$ for shipping to my country seems a little overhead for this..

 

[straypacket]

I think I have a couple PCBs that I can send you. 

I probably also have a few of the toggles too.

Let me know if you are interested.

 

[straypacket]

Hi All,

I've had some questions recently via PM about the AS016 PCBs. 

First,  the board is available on oshpark.com:

  http://oshpark.com/profiles/straypacket

You should be able to see and order the pcb from that link (let me know if you cannot).  It's $23 for three boards, so not cheap but not terrible either.


Second,  regarding 0v/ground:

I noticed you don't have an ov (shield, ground etc..) for the input and output.  Not needed?

The Jensen schematic ties the ground pin from the input jack to chassis earth, and specifies a ground for the phantom portion of the circuit.  The output ground is not specified, as  the circuit is meant to interface with the electronics of your preamp, which will supply their own grounding. 

This PCB provides the phantom ground pad, and the ground pad for the phantom LED indicator.  The ground plane for the PCB is tied to the LED ground pad, and the ground for phantom is separate (although I have them tied together in my build since they are coming from the same PSU).

 

[Rocinante]

Thanks.  I'm gonna meditate on this for a few hours.  Digging through the cobwebs in my head I seem to recall that the popping that occurs in switches is from capacitors needing an independent dc discharge path???  Part of the reason using shorting and/or non-shorting I think???  I believe the remedy was 4m7 resistors.... 

Ahh here it is; Ian (Ruff Records) schooling me in regards to using non-shorting Lorlins and Grayhills in the Helios circuit :

    "Non-shorting Lorlins for the variable resistors should be OK. There is not possibility of switching capacitor charged with dc voltages at that point in the circuit so shorting is not required. If you are really concerned you can put a 4M7 resistors from the wiper to each end of the Lorlin to ensure there is always a dc connection path. For the Grayhills, non-shorting are OK for the Hi amd Mid controls. The Lo control really should use a shorting type for the low cut part of the circuit as the capacitors in the part of the circyuit have no independent dc path to discharge them. Again, if you are really concerned about this you can wire 4M7 resistors between all successive pairs of contacts on the lo cut section."

Possible solution?

 

 

[Rocinante]

And then there's this thread where Rochy from ex-Pat explains how they were planning to fix the pop in their mic pre front end brought on by non discharged caps.

http://groupdiy.com/index.php?topic=22683.40

It appears the circuit needs a bleeder resistor(s).

I think its a done deal and am going to get a batch made.  The LED problem I have no clue about but I am sure there is a work around if i begin to care. 

 

[straypacket]

Thank you for the link to Rochey's info on the bleeder resistors.

For the LED buzzing, you can try to take the V+ feed straight from the regulator, I know that solved an issue for my GSSL.

I can't test that theory at the moment with my 9K preamps, as the PSU lives in another enclosure, and the preamp enclosure just gets  +V, -V, +48V and ground.

 

[straypacket]

Also,  if you are planning on using the on-board toggle switches from Mountain, I am working on updating the PCB layout to accomodate those switch legs.  I just need to remember how to get Eagle to spit out Gerber files and upload them to OSHPark.