Poor Man's API 312, Anyone Interested?

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rp3703

Well-known member
Joined
Mar 23, 2014
Messages
53
After four or five months of figuring this thing out, I finally got it working, well, one channel that is. Yes, I know it’s ugly as hell but I was only trying to prove that it could be done. Now this is where you ask why? So here is my reasoning, besides having fun and learning a lot, my main goal was to save money. The cheapest set of 8 API mic pre's on the market today would be either the CAPI VP312 or the SCA A12. One requires a 500 series chassis and the other requires a chassis similar looking to this unit. A set of 8 from either manufacturer with transformers, opamps, chassis and powers supply would run you around $2400. This unit costs roughly $1200 if you reuse the existing PCB’s like I did and requires pretty much the same amount of work to put together.
Now I’m not going to lie, removing all the parts off of one of the existing PCB’s is a pain in the ass and took about an hour. Plus wiring up a 312 circuit using existing traces can be confusing as hell. That is why my next goal is to design new PCB’s for this unit. I already have most of the design complete for the channel PCB but I have never had a PCB manufactured before and would greatly appreciate it if there were others interested in building this unit to split the cost of manufacture with. Now I’m going to carry through with this either way but once I send it off to the plant, that is it. I’m not looking to get into the preamp business. So let me know soon. Here is my cost break down:

AMS-8000- $30-50 (I paid $30)(does not need to be working)

Power supply:
Power transformer- $30
Parts-$20(Estimated high but should actually cost less)

Per Channel:
Input transformer, Output transformer & DOA-$115(purchased from CAPI)
Parts-$20(Estimated high but should actually cost less)

Total: $1160-$1180(Does not include cost of new PCB’s which are unknown at this point)
 

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I realize that this is not the first DIY 312 ever made but in my opinion, the reason most of them have come and gone is because all they offered was the preamp PCB. Having to figure out how to power it and box it up on your own can be pretty overwhelming and in the end, probably not costing too much less than one of the 500 series DIY kits on the market today. I’m guessing that’s why the ones that are still around have either switched to strictly 500 series or they sell the box and PSU as well. Regardless of how much cheaper this unit is compared to other DIY 312’s, $1200 is still what I would consider to be a lot of money and I would want some guarantees that this thing actually works as it should. While I have turned it on plugged it up and verified that it’s not buzzing or humming and passing signal, I still have a few other tests to perform before I can claim that this thing works as it should. If anyone can think of any other tests I should perform, please feel free to share them with me.
 
You can etch single/double sided PCBs at home.
All you need is a laser printer, some transfer paper (ebay), acid and blank PCBs. I made dozens of PCBs for my modular synthesizer project (work in progress) and it was about 2 euro per module.

QEv_pmmt_cs.jpg


You do need a good precision bench drill for 0.8mm holes. (I use Proxxon TBH, which is great for both PCBs and metalwork).

If I would want to build multi channel API312 (and I might some day), I would buy audio transformers and unassembled opamps from CAPI, make my own PCBs, caps/resistors from big distributor, put it in 2U enclosure (you can buy an empty enclosure for 35 euro, big enough for 4 channels) and make my own PSU, inside the unit.

I think this would be the most cost effective solution.
 
This project does exactly what you stated except that it reuses an existing 2u enclosure. Drilling holes and mounting PCB's in an blank enclosure is not an easy task and requires some expensive tools if you want it to look half way decent. Reusing the AMS-8000 would only require drilling the holes for the switches on the front panel.

Sure, you could make your own PCB's at home but the cost savings for buying all the supplies as well as the coated boards is not significantly different than just having them manufactured in China and a lot less clean.
 
It’s been a while but after much back and forth on various forums trying to get answers to questions plus multiple design revisions, I finally have PCB’s in hand. I decided to take my design a step further by incorporating the DOA into the layout of the PCB instead of having it socketed. This only saves maybe $50 in costs but makes assembly much easier since the parts no longer need to be crammed onto a 1-1/4” square. So far all the important parts fit. Now I need to put the PSU and one of the channel cards together and try it out. Here is the cost run down so far:

Sure AMS-8000- $30 used off eBay
Power Transformer- $30
PCB’s- $155
Input/Output Transformers & HFE Matched Transistors-$690
Metal plate-$10
Parts & Tools-$200

Total for 8 channels of API-312 preamps= $1115
 

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As with pretty much every electronic project I’ve built, they never seem work on the first try. This project would not disappoint either. I put everything together and started by checking the output of the PSU. All voltages were right on. Then I  hooked up the channel card and turned it on. The first thing I noticed was an electrical burning smell and immediately turned it off. Feeling around, I noticed that BD140 was extremely hot yet BD139 was not. Checking the data sheet, the BD140 can get as hot as 300 degrees Fahrenheit so I decided to try it out again and see if signal was passing. Using a tone generator and earbuds, I got a barely audible tone and after a few seconds a squealing that was increasing in volume. I shut if off. I went over all my notes and drawings and noticed that I had C5 hooked up to the collector of Q2 instead of the base like it should be. Oops. How did I get this far without noticing that? Anyhow, I cut the trace and ran a jumper wire to connect it correctly and fired it up again. This time I got a different squealing sound out of it so I turned it off again and went back to my notes. After a day of head scratching, I realized that I had mis-labelled the position of R14 and R16. After swapping them out, I fired it up. SUCCESS! Everything works and BD140 no longer gets hot! Now its time to order the rest of the transformers and finish putting this thing together. I can’t wait to try it out.
 

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did you design your own pcb's?  I'm thinking about doing this, but buying  the gar 2520 kits http://capi-gear.com/catalog/product_info.php?products_id=136

and then wiring the rest of it point to point - maybe use the original cards as a base and screw in a turret board, or try my hand at etching and attach that to the original pcb with standoffs?

I found a cheaper source from transformers from a group diy member in russia.  Also, did you DIY your power supply PCB, or use one of the available kits?
 
What you’re describing is the reason I did mine the way I did. Repurposing the Shure AMS-8000 and reusing the chassis, inputs and outputs as well as some of the wiring connectors saved me a lot of time and a good bit of money. Yes, I did have to design all new PCB’s for the preamps and the PSU that had to fit into the existing unit but once that was done, there was very little work required for the chassis itself. It’s a lot of work drilling holes into a metal enclosure and if you want it to look decent, you need to use punches that are really expensive. I only ended up having to drill the holes for the toggle switches on the front. All in all, I spent around $1200 in actual money to get 8 API 312 preamps. If I were to have paid myself $5 an hour for the time it took me to design the PCB’s I would have probably spent another $20K in wages. I have no regrets though. I learned how to design a PCB and a PSU from scratch and I have an awesome set of preamps. I hope your endeavor is as rewarding. Good luck.
 
Awesome, do you have pcb's for sale or would you share the files to etch my own?  I'm a total noob with some experience in diy pedals :D
 
Like I wrote earlier, there is a mistake on the channel PCB in the layout of the DOA. There needs to be a capacitor added to the phantom power switch section to eliminate popping when phantom is engaged. I also need to reverse the labelling on one of the charge capacitors in the PSU that is labelled backwards, which if installed as directed will cause it to explode. All these changes would take me hours to complete. I know the title of this post suggests I was looking to sell these but I have since changed my mind. This is far from a project for a novice. While yes this project cost me very little compared to other DIY kits, many of the 500 series DIY kits offered today will be much easier to put together for someone who is new to this sort of stuff plus they will be able to offer you help which I don’t have time to do. Sorry for the misinformation.
 
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