API 312 Thread!

[tmuikku]

In the thread Teemu linked above, I posted the Mill-max socket I designed the boards around:  0344-2-19-01-34-27-10-0

Niels told me that these (0327-0-15-01-34-27-10-0) will also work, and they are cheaper.

If you enlarge the socket holes, be aware that you will lose the internal plating that connects the top and bottom solder pads of each hole. That is not normally a problem, but you may have to solder some of the sockets on top in some cases to ensure connectivity (for example the ground socket). I had to do that on some of the prototype boards since the fabhouse had undersized the socket holes.  Not a huge problem, but something you should be aware of.

Good luck!

Ah yes! I totally forgot that too, solder on both sides

 

[scottlangendyk]

Hey guys had a few questions about the API 312.

I'm building one based off of the original schematic, but I'm having a problem finding all the caps. More specifically the two 12.5uF 25V ones. What can I substitute these with?

Also what's everyone liking these days as far as transformers and opamps. From what I've read a lot of people like the Cinemag transformers, but what are the thoughts on the transformers from classicapi.com how do they compare?

 

[jsteiger]

Hey guys had a few questions about the API 312.

I'm building one based off of the original schematic, but I'm having a problem finding all the caps. More specifically the two 12.5uF 25V ones. What can I substitute these with?...

47uF/25V or 100uF/25V. Anything bigger will be fine to decouple the bipolar V rails.

I won't comment on the tranny question as my opinion may be biased.  ;D  I'm sure you will get enough opinions from others.

 

[Rybow]

I had a transformer question as well. I am flip flopping between the CMMI 8PCA and the CM 75101APC. Anyone have experience with the CM 75101APC? I emailed David about this as well. He told me before that the CMMI 8PCA has more high end bandwidth than the 75101. I am not too concerned about that quite frankly since I plan on mostly recording amps with this pre.

 

[scottlangendyk]

So I finally got my transformers yesterday, and went ahead and started wiring up a channel of my API 312 clone. I etched the board myself as basically an exact clone of the original boards, right down to the edge connector. So I got everything wired up, powered it on, but I'm getting nothing. I'm using the gar2520 op amp, but I'm unsure of how I can verify whether it's working or not.

 

[mitsos]

You get nothing? Silence?  Could be a connection problem (check your in/out XLR, in/out trafo, missing jumper somewhere, etc).

If that's all OK, check the DC voltages on the opamp power pins.

That's pretty much all there is to the circuit, especially if it's the board is a clone of the originals.

If you're using any other add-on boards (JLM pad/pol/phantom board, etc) disconnect it and test the 312 alone first.

good luck!

 

[scottlangendyk]

So I realized, I had soldered everything to the wrong side of the edge connector, haha so I fixed that up. Plugged it in, saw some smoke coming off the board, powered off, couldn't identify what was fried. Now when I plug it in, I get output that is a very low oscillating frequency, that slowly increases. Any ideas?

UPDATE:

Looks like I fried some components in the first op-amp I'll have to investigate that a little bit further, but I swapped it for another one, and nothing blows up and everything is silent. Now I'm still not getting a signal from the mic. I think everything is more or less working, because I was able to short out the mic input and create that buzzing sound. While I heard the sound, I turned the pot and the gain was adjusting as expected, so that's a good sign. Now my question is why I can't get the mic signal into the pre. I'm using a Cinemag CMMI-8-PCA input transformer.

 

[mitsos]

Tthat sucks.. sorry to hear.. 
You can sometimes test the transistors with a a DMM by doing diode tests, as well as check resistors to see if they've gone open circuit.  It takes time and you may have to remove some components to do it, but I've gotten lucky a couple times and found the problem in the first few minutes.

The other bad part is your input trafo could have been damaged. If you exactly reversed the connections (flipped the connector over, for example) according to the pinout on Dan alexander's site, you sent your -V rail into the - input, which is not good. Check the windings for DCR/continuity.

hope that helps.

 

[rrs]

From what I've read a lot of people like the Cinemag transformers, but what are the thoughts on the transformers from classicapi.com how do they compare?

I built my first 4 API (Bob's boards) with Cinemag CM-75101APC and CMOQ-2S .My next 4 with Ed Anderson's.
After comparing I very quickly sold my Cinemags and bought another 4 Ed Anderson in and out trannies.

My feelings were the Cinemag's had a deeper low end and brighter/ sparkly top end and a little soft but clear sounding.
The Ed Andersons I felt had a harder hitting mid range and tighter sounding. More natural.

 

[scottlangendyk]

Tthat sucks.. sorry to hear..  
You can sometimes test the transistors with a a DMM by doing diode tests, as well as check resistors to see if they've gone open circuit.  It takes time and you may have to remove some components to do it, but I've gotten lucky a couple times and found the problem in the first few minutes.

The other bad part is your input trafo could have been damaged. If you exactly reversed the connections (flipped the connector over, for example) according to the pinout on Dan alexander's site, you sent your -V rail into the - input, which is not good. Check the windings for DCR/continuity.

hope that helps.

I'm certain I didn't flip the connector, was very careful about that. DCR across Pins 1 and 4 is 37ohms, and across Pins 5 and 8 is 3KOhms. This is the same on the other currently unused input transformer.

I'm thinking I may just have the wiring wrong. I have the following wired up.

Pin 1: +18V
Pin 2: -18V
Pin 3: 0V
Pin 4: Out+ going to XLR pin 2
Pin 5:
Pin 6: Out- going to XLR pin 3
Pin 7:
Pin 8:
Pin 9 and 10: Gain pot. Was a little confused about this, but I have it wired up where Pin 2 and 3 are joined together.
When I first smoked the Op-Amp, i had the gain pot removed from the circuit completely.
Pin 11: Input+ to XLR Pin 2
Pin 12:
Pin 13:
Pin 14: Input- to XLR Pin 3
Pin 15:

Have the XLR pin 1's going to 0V's as well.

I originally had the input wired up between 11 and 13, but I was pretty sure that wasn't correct.

 

[mitsos]

So I realized, I had soldered everything to the wrong side of the edge connector, haha so I fixed that up. Plugged it in, saw some smoke coming off the board, powered off, couldn't identify what was fried. Now when I plug it in, I get output that is a very low oscillating frequency, that slowly increases. Any ideas?

I wasn't sure what you meant by the part in bold, I thought you meant you reversed the connections. Your trafo sounds like it's fine, which is a good thing. 

Can you post an image of the PCB you used? A decent resolution, in focus picture of yours would be good, front/back, that way people can see what's going on.  btw, are you using a dynamic mic (just to rule out phantom probs)?

 

[scottlangendyk]

So I got everything working, ends up Pin 8 of the input transformer wasn't soldered to the ground plane, did that and everything worked. As far as the origin op-amp goes, I'll have to troubleshoot it and see if I can figure out what components were blown. I noticed on the new op-amp, that there were a few wire traces that were dead (maybe from overheating with the iron) so I was able to correct those with either a small of wire or solder bridge. Everything is operational now, except the phantom power from my JLM Go Between, so I'll have to look into that. I'll also have to do some playing around with the ground and shielding to reduce some of the noise at maximum gain, but even then the signal/noise ratio is low, and the signal is very clean and punchy .

EDIT:
Everything is working, output is quite hot as most report with API preamps. Anyone got any suggestions for putting an adjustable pad on the output?

 

[mitsos]

glad to hear you got it going.  you can use a U-pad with, say 3x 369 or 383R resistors, should give you about 10dB I think. You can use any online calculator to get a different drop if you wish.  You can stick this in the preamp with a switch on the front panel, or stick it in one of those XLR M-F barrel adapters and plug it in the back when you want.

here's a good site about pads:
http://www.uneeda-audio.com/pads/

 

[gar381]

Hey Scott

Everything is working, output is quite hot as most report with API preamps. Anyone got any suggestions for putting an adjustable pad on the output?

Try this methode.  Works great!!

http://classicapi.com/catalog/product_info.php?cPath=48&products_id=179

GARY

 

[scottlangendyk]

Try this methode.  Works great!!

http://classicapi.com/catalog/product_info.php?cPath=48&products_id=179

GARY

I looked at that, but seems a little pricey at 40.00 a channel (would rather buy a Shure inline attenuator). Don't I more or less just need switchable resistors?

 

[mitsos]

oops, I didn't see you wanted adjustment...  I think there are some inline ones (maybe the Shure you mention?) that give you a few different pad options.  The T-pad works like a fader, full-on, full-off and everything in between. You can probably do the same with a rotary switch, there is a diagram somewhere I think by a past member named New York Dave.  Anyway, other than that, you can always breadboard a PCB with a couple choices and use a relay or toggle switch to switch between them. You've got choices, just depends on the amount of work/flexibility you want to have.

 

[scottlangendyk]

I've been looking into some pad options, and keep seeing 600ohm impedance popping up. What's the significance of that?

 

[3nity]

I cant remember where or who but someone did used a 1K pot at the very XLR out and give as an attenuator.
Thanks.

 

[sethmopod]

I am nearing completion of a 2-channel 312 build using Bob Schwenkler's pcbs.  As I've encountered various problems, I've been corresponding privately with Bob and checking this message board frequently.  Recently I realized that instead of corresponding privately, I should have been posting here all along so others can benefit from my experience the way I have from reading here.

In that spirit, I have gotten permission from Bob to share his answers to my questions.  I am going to make several posts as if our exchange happened over the forum.

 

[sethmopod]

My question:

Do you know offhand how much current one channel will draw with phantom?  I'm trying to figure out my fuse requirement and I haven't been able to find much that's useful in the forums.  I'm using the gar2520 op amps.

Seth