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DIY: The SC-1 Mic Preamp

Here's the place to talk about and post plans for building your own DIY projects, such as vocal booths, patch bays, midi switchboxes, preamps--U Name It! This forum includes pics, plans, and ideas by our top contributor Owel.

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Are you interested in the SC-1 Mic Preamp Kit?

I only want the PCB.
9
11%
I want the PCB and complete parts in kit form.
5
6%
I want the complete mic pre kit plus 18-0-18 power supply kit.
40
49%
I want a pre-assembled mic pre kit.
6
7%
I want pre-assembled mic pre kit + PSU.
21
26%
 
Total votes : 81

DIY: The SC-1 Mic Preamp

Postby owel on Fri Mar 02, 2007 10:39 am

This project evolved from the original discussion here....

NOTICE: THIS IS AN ORIGINAL DESIGN BASED ON DATASHEETS AND APPLICATION NOTES PUBLISHED BY THE CHIP MANUFACTURERS.

A lot of other companies are using the same chips/technology we'll be using in this DIY design. We are not cloning any particular product here. We're just using the same mic preamp chips that Rane, Maudio, Grace, DAV and others used.


The SC-1 Mic Preamp is a THAT 1510/1512/SSM2017/SSM2019/INA217 based Mic preamp with phantom power, DC servo and Balanced line output driver.

Image

Image

Image

Design Goals

The goal is to provide a great performing, nice sounding mic preamp that's reasonable in cost in DIY kit form.

This gives the user the chance to build the preamp themselves and use their imagination and creativity in housing this preamp in it's own custom case.

- Simple, Great Performing, Quiet, High-Quality Mic Preamp chips
- with 48V phantom power
- LED indicator for phantom power
- powered by 18-0-18 Volts supply, with +48VDC for phantom power
- balanced output
- 12-position gain selector switch
- insert points for optional Trim potentiometer*
- insert points for future compressor, EQ, signal processing modules
- hook-up points for use of a potentiometer for gain control instead of the 12-position gain selector switch
- 0 to 66dB gain in 6dB increments (UPDATE: +6dB to +72dB gain)
- input clamping protection
- output surge protection circuit
- output RFI protection

* The Trim pot has now been included in PCB Rev1.2

Block Diagram

The SC-1 Mic Preamp has a straightforward design. The approach is to use the minimum number of components in the signal path, but at the same time provide flexibility to the end-user to add in-line signal processing options.

A DC servo approach was used instead of AC coupling. The Balanced line driver is connected directly at the output of the DC Servo.

In case the user wants to use the optional trim level, "insert/tap points" are provided on the circuit board to insert this potentiometer in the signal path. This will function as the volume control and provide more control over the 6dB per step of the gain selector switch.

These "insert/tap points" also provide a way in the future to add additional in-line signal processors. Some things that come to mind are VCA compressors, FET-based compressors, Passive EQ, Active EQ modules, Low-cut, High-cut filters, exciters, etc...

This gives the user customization capabilities in creating his own unique mic preamp channel strip!

Image

TO DO:

Prototype PCB Boards were sent out for manufacturing.
Parts from Mouser and Digikey ordered.
I'll be building the 1st prototype in 10 days.
Sound Testing and Oscilloscope testing

If everything works out great, we'll have a new mic preamp kit.


[Blog]

While the IC mic preamp chips used cost only a few dollars, shipping charges at $12 to $18 a pop everytime you order from a different online store quickly increases the overall cost for the project.

So if I'll be providing this in kit form at a reasonable price, I will have to order parts in bulk to save on shipping costs, and also to get a price break by buying wholesale.

But that would mean I'd be shelling out hundreds of dollars ordering a single part. Multiply it by the number of parts needed, I'll need to invest a few thousands of dollars to stock-up for a few complete kits. :shock:

I also need to ask the PCB manufacturer for a custom quote for, say 100pcs, of the PCB. The prototype PCB cost is not meant for mass production.

So thus, I started a poll to see if the interest is in PCB only or PCB + parts.


March 2, 2007
I have the preliminary PCB done.

Measuing 4" x 2.5". I kept the height under 3.5" so there's the option of installing it vertically on a 2u case... if you want to cram 8 channels in a rack.

I'm using a 1-deck, 12-position Grayhill sealed rotary switch, PCB mounted. There are two solder pads in case you want to use a reverse log pot instead. The rotary switch will also hold it secure on the rack front panel vertically.

Gain is 6dB, starting at 0db up to 66dB, in 12 steps. 6dB is a big jump so I don't know if I should include a pot as a "Trim" level pot. This can be placed after the mic pre, before the balanced line driver converter.

I still have space on the PCB so I'm investigating the use of PCB mounted switches for phantom power, and also installing the phantom power and power on LED on the PCB.

BUT the price goes up everytime I add a PCB mounted component. The rotary is $16.50 ! A reverse audio log pot will be cheaper. An open-frame selector switch can be bought for $1.00. But you'll have to wire all 13 wires back to the board (or solder the resistors on the selector and you'd only have to bring 2 wires back to the board.) But the open-frame switch will get mucky and dirty over time... resulting in pops and noise.



I'm trying to decide if I should use single-sided so it can be home etched, or double sided for convenience (and easier time routing). And if I'll have the PCB professionally manufactured, the cost is the same whether single or double-sided. So because of the latter, I'm leaning more towards double-sided.

For double sided boards, I can put a ground plane/island on top and bottom. I think this should help in eliminating RF noise.

For single sided boards, the numbe of jumpers needed are a lot. DAV BG-1 used 0-ohm resistors as jumpers so it won't look ugly.
Last edited by owel on Mon Jan 21, 2008 7:23 pm, edited 6 times in total.
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Postby gibong on Fri Mar 02, 2007 11:48 am

An all-in-one double sided PCB is probably the way to go for this project. I do have layouts done for "problem solver" off-board gain switch, phantom, polarity, etc. I would use those in more complicated designs where I wouldn't consider 8 pres in a case.

The layouts I have half-finished (one for an API-ish pre and an "AllTHAT" pre 8) ) are double sided and designed to fit in a Frac-Rac. I still have some layout and routing issues to resolve. I've had to back-burner these for now until I have some more spare time.
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Postby tecnolover on Fri Mar 02, 2007 11:30 pm

owel wrote:I have the preliminary PCB done.

I'm using a 1-deck, 12-position Grayhill sealed rotary switch, PCB mounted. There are two solder pads in case you want to use a reverse log pot instead. The rotary switch will also hold it secure on the rack front panel vertically.

Gain is 6dB, starting at 0db up to 66dB, in 12 steps. 6dB is a big jump so I don't know if I should include a pot as a "Trim" level pot. This can be placed after the mic pre, before the balanced line driver converter.


Yeh, I would include a trim pot for sure and I think it would be cool if this trim pot be a 6 postion. First position being 0db (no gain) and the other five positions would be +1db gain each. That way you can dial in the exact gain in +/- 1db steps and it's postion locked. I think it's the little conveniences on top of a great sounding pre that will make the kit a HIT!. :)

I still have space on the PCB so I'm investigating the use of PCB mounted switches for phantom power, and also installing the phantom power and power on LED on the PCB.


Well if they are going to rack a bunch up it's more convenient to have just ONE phantom power switch that powers them all rather than one for each module. Second, like mentioned above you probably should have a gain trim pot and PCB mounted.
Also any extra space would be welcome for a variable DC offset circuit and a mounted trim pot for this. :) If variable DC offset isn't part of your design that is cool, but I think I would want to add it on somehow later since I plan to add a switchable out tranny later.

BUT the price goes up everytime I add a PCB mounted component. The rotary is $16.50 !


:lol: , how ironic when one switch costs about as much as the entire preamp circuit! But then so will the powersupply and/or chassis on their own also. You gotta luv it.

[/quote]
Happy music making!
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Postby Luke on Sat Mar 03, 2007 12:35 am

yeah those grayhill's are spendy, i just cheap out with the lorin knock offs, don't know if they make any pcb mount ones though.

I usually leave off the phase reverse stuff in my designs becuase it is so easy to do it in software, but I guess it would be better to just capture it correctly and have a physical knob for that.
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Postby owel on Sat Mar 03, 2007 3:11 pm

Okay, update... PCB is now 3.5" x 2.6".

I included a PCB mounted DPDT for phantom power, including LED light for ON position, also on PCB.

I didn't add a PCB mounted POT for the TRIM (yet)... but I added 3 solder pads in case you want to add an external TRIM pot. If not, just solder a jumper.

Option to use a reverse log pot for gain (instead of expensive Grayhill switches).

FUTURE
---------
Maybe on Rev.2 design, I'll incorporate the trim pot on the PCB... maybe even add a 48V soft-start phantom power, and phase switch.
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Postby tecnolover on Sat Mar 03, 2007 10:52 pm

Fantastic. Sounds great Owel. I'm curious who you use for PCB manufacturing?... I checked out PCB express.com and even played with their free design software in the past but didn't like it that much. Eagle, I would think is much better..
Happy music making!
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Postby tecnolover on Sun Mar 04, 2007 5:30 am

Wow, I'm trying out the free trial version of Eagle right now! This software rocks is sweet !
Happy music making!
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Postby owel on Sun Mar 04, 2007 5:32 am

So far, I've tried CustomPCB and PCBFabExpress.

Here's my review based on my own experience:

CustomPCB is lower in cost and minimum prototype order is 2 PCBs. They'll also accept native Eagle .brd files. i.e. you don't need to fool around with Gerber and Excellon formats. It's based in Malaysia. They allow panelizing. So if you'll be panelizing, they may be cheaper i.e. price per board.

Cons: No silkscreen... and it's butt-ugly. No silkscreen stopmasks, no nothing. It's their bare board special - their cheapest price offering. But it's really like ugly, like it's just home-etched. I also don't like I have to send them the .brd files. I sent them my gerber files but they still asked me to email them the .brd files.


PCBFabExpress minimum order is 5 PCBs. Accepts only Gerber and Excellon files. Based in Silicon Valley, CA. The PCB is real nice... pro looking and everything. With silkscreen, stop masks, and everything you expect to see. No panelizing though. If you want panelizing, it has to be a custom quote (i.e. expensive). They do offer bare boards too. But minimum order is 4pcs... factor the setup cost, (and the look), the 5pcs regular order is a much better deal.



Now, if you factor in shipping, and divide the total cost of the order by the number of boards you get (without panelizing).... PCBFabExpress comes out cheaper per price of board. And you get a nicer looking board too!

One time, PCBFabExpress sent me the boards where the bottom copper layer was switched as the top layer, and the silkscreen was mirrored backwards printed, I emailed them and they CALLED BACK and profusely apologized. They asked if I want a refund or want a re-do of the job. Now, that's customer service! I told them to re-do the job and I got the new batch good.

That was my very first order from them and looking back now, it may have been my fault since I didn't check my gerbers with another program. Since that time, I now view the Eagle created gerbers with another program just to "verify" everything is correct. (In case I messed up something in Eagle.)

If want to register at PCBFabExpress, here's my referral ID "fivefish". When you place your first order, I get a little credit for referring you to them. Thanks.
Last edited by owel on Sun Mar 04, 2007 5:34 am, edited 1 time in total.
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Postby owel on Sun Mar 04, 2007 5:34 am

tecnolover wrote:Wow, I'm trying out the free trial version of Eagle right now! This software rocks is sweet !


And it now runs on OSX!!!! Schweeet! Projects I did under Windows, I can open and work in Mac OSX (running under X11).
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Postby tecnolover on Sun Mar 04, 2007 5:43 am

What are you doing up at this hour?! Oh yeh forgot you are an Owl :lol:

PCB Fab express sounds great. Minimum 5 is kinda steep but I don't think you will have trouble selling kits if it tests well. I am in for 2 channels myself for sure. Any estimation of what the total current draw will be for the board yet? I am gonna order my parts for PSU soon and need at least a rough estimate.
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Postby owel on Sun Mar 04, 2007 5:40 pm

Here's a sneak peek at the PCB design.

I sent it out for manufacturing... and in 10 days, I'll receive the PCB.

Ordered parts at Digikey and Mouser too.

Image
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Postby gibong on Sun Mar 04, 2007 11:46 pm

Beautiful!
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Postby jar4ever on Mon Mar 05, 2007 12:22 am

And thus starts the boutique preamp company "Owel Limited".
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Postby tecnolover on Mon Mar 05, 2007 1:46 am

wow! :shock: :crazy:

Looks nice!

Owel, why aren't you working for an audio gear manufacturer??!
Seems like the perfect place for you. Or do you??
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Postby tecnolover on Mon Mar 05, 2007 1:27 pm

As I understand, phantom power actually shares the audio path. However it tests as +48V equally on each pin 2 and 3 so equipment sees it as common mode noise and rejects it. Does this mean any ripple noise in the +48V supply would be recognized by the preamp as CMN also and rejected ?

Reason, I'm asking is that I'm shopping for a good DC-DC converter for the phantom power supply and some have higher ripple than others. Ofcourse the dc-dc converter goes after the linear regulator yet im not sure if that would matter if it generates bad ripple noise on it's own. .
:?
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Postby owel on Mon Mar 05, 2007 2:27 pm

tecnolover wrote:As I understand, phantom power actually shares the audio path. However it tests as +48V equally on each pin 2 and 3 so equipment sees it as common mode noise and rejects it. Does this mean any ripple noise in the +48V supply would be recognized by the preamp as CMN also and rejected ?

Reason, I'm asking is that I'm shopping for a good DC-DC converter for the phantom power supply and some have higher ripple than others. Ofcourse the dc-dc converter goes after the linear regulator yet im not sure if that would matter if it generates bad ripple noise on it's own. .
:?


If you measure across pins 2 and 3 of the XLR, you'll measure 0volts, NOT 48Volts. Because there is no potential difference between pins 2 and 3.

(of course, assuming the 6.8K resistor are matched... if one is skewed, you'd get a few millivolts of reading across 2 & 3. )

The 48V is measured across pins 2 and pin 1 and also across pins 3 and pin 1.

The way balanced lines work, any noise present in pins 2 and 3 (with respect to ground) that are in-phase will be rejected.

From what I know, pick a DC to DC converter with high switching frequency. Some of them have switching frequencies of 25Khz... which may be fine for a non-audio circuits (digital logic, computer psu), but is too close for comfort for use in audio circuits.
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Postby gibong on Mon Mar 05, 2007 3:04 pm

I'm in for a couple of boards to add to my collection of projects I will never get to start let alone finish :wink:

technolver: on Phantom noise...the discussion doesn't get any better than this:
http://www.prodigy-pro.com/forum/viewtopic.php?t=16018
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Postby tecnolover on Mon Mar 05, 2007 3:42 pm

Sorry, i should have worded it differently. I should have said pins 2 and 3 each measure 48V when tested in pairs on pins 2-1 and 3-1. Pin 1 ofcourse being the return path. Anyhow, you are absolutely correct.

Ok, so high switching freq for the converter... that answers my question perfectly. The one I'm looking at is $22 and has a switching freq at 200KHZ so it should be ok...
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Postby tecnolover on Mon Mar 05, 2007 4:08 pm

gibong wrote:technolver: on Phantom noise...the discussion doesn't get any better than this:
http://www.prodigy-pro.com/forum/viewtopic.php?t=16018


Just started reading that thread. Excellent info. You always seem to find the right references. Thanks!


@ Owel

On the kits. I wouldn't mind getting just the PCB and ordering parts myself if you included a BOM with Digikey#'s for the select parts. That would be great and would save you investing a lot$$$ up front buying in bulk. That might be easiest way to go and then you avoid all the problems of inventory. Investing in PCB mass replication would then be your only investment.
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Postby owel on Mon Mar 05, 2007 5:55 pm

I'm starting slow here.... we don't even have a working prototype yet :)

Maybe I'll provide the PCB first, then PCB +parts later on. See how the demand is.
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Postby Nimh on Tue Mar 06, 2007 1:07 pm

I'd be interested in ordering the kit with instructions, depending on how good the preamp sounds. If it's comparable to the BG-1 (like you suggested here), I'd most certainly be interested (I didn't read through the thread, sorry). How much is this thing costing you so far? And how much would you sell the kits for?
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Postby tecnolover on Tue Mar 06, 2007 9:32 pm

Nimh wrote: If it's comparable to the BG-1 (like you suggested here), I'd most certainly be interested...


The nice thing is that all the chips are pin compatible so if you solder in a 8dip socket you can interchange and try the different chips and see what sounds best to your ears. The bg-1 uses ina217 and op275. If you use the ina217 it could sound very close to a bg-1 but theres also the op275. How much of the bg-1 sound comes from that chip is uncertain. I'm not sure if you could swap the op275 in this preamp.

The question :in the SC-1 is the DC servo actually in the audio path? Is it configured as both DC servo and audio driver? if so, then OP275 could maybe be swapped for the OP134 Owel innitially plans to use.. They are both pin compatible. Then it could be astonishinly close to the sound of bg-1 i would imagine. Perhaps Owel could elaborate on that as I'm sure a lot of bg-1 fans including myself would like to know :)
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Postby owel on Tue Mar 06, 2007 10:11 pm

My DC servo is just the servo, functioning as an integrator. I have a separate balanced line driver chip using the THAT chip.

The OP272 is a dual JFET. I'm using the OPA 134 FET opamp which is a single opamp but with offset trim. Both are 8-pin DIP.... but they are NOT pin-compatible.

There is an OPA2134 which is a dual opamp, and that one is pin-compatible with the OP275.

I *think* BG1 uses the OP272 as the balanced line driver???? that requires 2 opamps and I think the OP272 fits the job.

I don't think the BG1 uses DC servo. Maybe just AC coupling??? I don't own a BG1 so I don't know... just guessing.

**** Maybe in the future I can create a version without the DC servo and THAT driver chip and instead just use a dual opamp as the balanced line driver.... we'll see.
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Postby axeman69 on Wed Mar 07, 2007 7:57 am

This is extremely interesting to me since I'm down for a while. Might be a good way to pass the 3 months that I'll be living with the Mother-in-law.

Will a parts list be provided with the PCB?

Thanks,

Mark
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Postby nanashiwanderer on Wed Mar 07, 2007 8:05 am

you've got me interested any idea how much a full package w/PSU will run in total? I wonder also how its going to be stack up against the DAV...
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