DIY API 312

[guitarguy12387]

Well i'm back again with another project i had been working on. Posting it for anyone interested. This one is a clone of the API 312 preamp. Again, i haven't had much time for research so i don't know a whole lot about it. At the heart of this preamp is the discrete opamp. It is the classic 2520 opamp thats used in alot of API stuff (although i think other DOAs are compatible as well). An original 2520 opamp from API can be hard to come by and quite expensive so there are a number of replicas out there. It also uses Cinemag input/output transformers. It seems to be a pretty simple design... its basically the lone opamp which provides the gain and the transformers.

Here's where i'm at at this point: I have most of the components... opamps and transformers just came in today. Just need a few parts (PCB mount xlrs, grayhill selector switch which i am having trouble finding, power supply) and then i'll try it out!

Here's pics thus far:


DOA: This thing's quite small. Glad i didn't build one... that'd be quite the headache.


Cinemag I/O Transformers: These things are always smaller than i expect haha.



Pics of the mostly stuffed boards.

Enjoy!

[owel]

Schweeet! Looks good!

[davidaltemeier]

yes, very nice.

who'd you get your op-amp from? i just got the seventh circle audio one's for my API clones.
(which also uses cinemag transfo's)

[guitarguy12387]

Thanks guys.

who'd you get your op-amp from? i just got the seventh circle audio one's for my API clones.

Ahh yeah i thought about those. They were a bit more pricey then i was looking for at the moment though. How do they sound? I'd like to get some SCA kits sometime too... One day haha.

I got mine from a guy named Peter from the prodigy pro forums. More info on it here:
http://www.lazpro.com/laz25.htm

I'm anxious to hear how it all sounds!

[owel]

What's the latest on this build?

[guitarguy12387]

i've got everything installed. I'm just waiting on the gain selector switch. It's a non-stocked grayhill part that's still got a week or two lead time if i recall.

Once that gets here, i'll test it all out with a bench supply and make sure she works!

[guitarguy12387]

Update:

Finally got the PSU in yesterday and whipped it together. Ordered from JLM audio all the way over in australia (sorry owel... woulda went with one of yours but i needed the 4th rail for the relays). I get it mostly pieced together and realize that they forgot the trimmer resistors! Thought i was going to finally be able to test this thing out this weekend but STILL not yet. Im waitin on the part no for the R's from JLM and will just order some with my next mouser/digikey shipment haha. Save time on shipping cuz i'm THAT impatient haha.

Anywway, here's pics of the completed boards (finally got some grayhills) and psu section:


PSU


Associated power tranny from JLM as well


Finished preamp card

If this thing sounds as good as it's reputation suggests... i'm considering making this a 4 or 8 channel. I have some extra grayhill switches now and the overall cost wasn't too bad. Plus there is minimal wiring.... just tie each card to the PSU in parallel and thats IT. So it will make for a relatively easy implementation.

[guitarguy12387]

Update:

Got a shipment from mouser today with the necessary trimmers for the PSU (couldn't wait for aNOTHER package all the way from Australia haha). Fired up the PSU first and took voltages. All well and dandy. Volts check out. Hook it up and plug it in. Power it up........ much to my dismay, i'm getting a pretty nasty hum/hiss/oscillation at different frequencies depending on the gain settings.

Sigh! It's a pretty simple circuit with ZERO wiring so i figure it shouldn't be TOO bad to debug. I was a little worried at first that i got a bad DOA. But i tested out both cards and they seem to have the same problems. This also probably eliminates a bad joint/short. I'm thinking either i have extra parts or am missing parts. Or maybe i've read of people having to mess with different capacitor values around the opamp to stabalize.

I'll have to do some more research and figure out some things to try!

Wish me luck!

[owel]

i'm getting a pretty nasty hum/hiss/oscillation at different frequencies depending on the gain settings.

Hmmmm....

Did I ever told you I'm working on my own API-like pre?


From what I've learned in testing of my pre, yes, it is a simple non-inverting circuit. But care has to be taken with the layout and bypass capacitor values. And just because something is a discrete opamp, doesn't mean it's better than a PDIP8 chip!!!!

Depending on the opamp/DOA you put in, some are well behave than others. All of them works in the circuit, but if you do close scrutinizing with a signal gen and oscilloscope, you'd find that not all are created equal.

For a given bypass capacitor value, some opamps will be well behaved even at max gain, 72dB.... even at high frequencies, and even if you shoot it with a square wave.

Some opamps will start to oscillate at too low gains or anything above 40dB. Or some opamps start to oscillate if the freq. is too high. or the gain starts to get too high, or some combination of the two.

** note: Listening tests does not reveal these oscillation I'm talking above. You have to see it and magnify it closely with an oscilloscope. If you're already hearing your oscillation, without even using an oscilloscope, that's pretty substantial.

You can increase the value of your bypass caps to stabilize it more, but too high a value and it's going to act like a Low pass filter and your preamp's upper frequency limit will be reduced.

I say start playing with your bypass cap values, and see if you can find the best compromise.

Also, you may need different values of bypass caps for different discrete opamps to stabilize them.


The (2) prototype preamps I've built (and both worked very well) can use PDIP8 or DOA, and uses a high-speed video, high-current line driver (2000V/us). Lighted push buttons, relay controls, pad, polarity, soft start phantom, gain and trim knobs, VU meter, input and output transformers. ... ETA by end of February. (I'm already having some problems sourcing 2 out of the 3 chips I'm using. They're hard to come by and expensive! But they sound awesome in this pre!!! )

[guitarguy12387]

Wow! That looks awesome owel. Now that you mention it, i think i remember you looking for a DOA over at prodigy and i got quite curious haha!

Well the particular boards i got support about a billion different options... monolithic amp, doa, different i/o transofmers, servo, coupling caps, gain selector switch, gain pot etc. So i'm probably gonna go through again and check yet again that i have all the right parts in place or NOT in place for the right version of the pre that i'm building. I'd say it's probably not unlikely that i have a part in there that's not supposed to be lol.

Thank you very much for the input. I will definitely try messing with the bypass caps as well. Hopefully i get it figured out soon! I'm anxious to her 'em.

Thanks again!

[guitarguy12387]

Apparently i needed to tie the primaries of the input trafo together. Who knew haha. I woulda thought that would have been taken care of on the pcb layout.

Now it seems to be working great except at the lowest and highest few gain settings. At which point the hiss/buzz comes back. Hmmmm NOW might be time to start messing with the bypass caps!

[guitarguy12387]

Update:

Most of the noise @ high frequencies seem to be a grounding issue. I haven't been able to properly ground the thing very well because the xlr's are pcb mount and are grounded via the mounting screws. So i figured i'd order the par-metal cases and get it all setup properly and see if i can't isolate the problem. Recieved the cases last night and guess what. They forgot to include the backpanel for this case

So i gotta wait longer. We'll see what happens.

[guitarguy12387]

Hey owel (or anyone else),

If you have a free min, would ya mind helping me out? I've exhausted all of the other solutions to my oscillation problems... so i'm about to try bumping up the bypass caps.

My question is which ones should i change?

Looking at this schem: http://www.bicyclerecordings.com/312/fi ... ematic.png

What do ya think? I'm thinking C2, C4, and C5... or are those not the caps you're talking about...

FYI, i'm not using the servo amp (U2) so none of those caps are installed. Thanks for the help!

[owel]

I saw this thread on Prodigy Pro... was there any suggestion that helped?

Can you make a recording of the oscillations?

Some tips:

1. If C2 is left out, there may be oscillations at low gains. There may also be oscillations at high gains. It depends on the opamp used for U1.

2. You can try bumping up the value of C2.

Maybe add another 0.1uf for each leg of the PSU voltage rails.. i.e. parallel to C13 and C14.

Ground the input transformer T1.

Do you have a scope? Maybe you can trace the signal to see at what point in the circuit it oscillates.

Were there others having the same problem with the preamp, or just you?

Maybe you have a bad U1?

Did you finish the X12 preamp? What do you think?

[guitarguy12387]

Hey thanks for the help owel.

Can you make a recording of the oscillations?

Will do.

2. You can try bumping up the value of C2.

Just tried bumping it from a 120pf to .1uf (thats the next closest higher value cap i had...). Which significantly lowered the HF response... but there was still some hum (no clicks/pops... i assume those are just being filtered out, leaving the LF crap).

Ground the input transformer T1.

I've been taking pins 6 and 7 (ground and chassis, from what i can tell from the cinemag datasheets) directly to earth on the IEC connector (tried taking each pin on its own, but have had the best results taking both).

Do you have a scope? Maybe you can trace the signal to see at what point in the circuit it oscillates.

I do not have a decent scope up at school with me. I have been trying to dload some Mac freeware scopes, but haven't found any good free ones yet. But i'll keep trying and post that if i can.

Maybe you have a bad U1?

I'm starting to think i might wanna try a different model opamp. I have tried three different opamps (all from the same guy), but the result is the same.

Maybe add another 0.1uf for each leg of the PSU voltage rails.. i.e. parallel to C13 and C14.

I'll try that next.

Did you finish the X12 preamp? What do you think?

YES! I haven't gotten much chance to play with it, but i think i will try and make some recordings with it this weekend. I DO still need to get a PSU and case... i've just bench tested it with a bench power supply. But i think i'm gonna get another channel and the case i keep hearin about once funds allow. I would like to do some comparisons between the SC-1 i have and maybe another preamp comparison. I'll post that up here once i get 'em done! How have they been selling so far?

Thanks again for all your help.

[owel]

Just tried bumping it from a 120pf to .1uf (thats the next closest higher value cap i had...). Which significantly lowered the HF response... but there was still some hum (no clicks/pops... i assume those are just being filtered out, leaving the LF crap).

No no no.... that's too much. Go from 100, 120, 160, 180, 220, 300, 330pf... would help if you have a bunch of different value capacitors in your stash.

Hum... could be related to grounding. Maybe PSU problem? (Is it my PSU? ) Maybe cold solder joint somewhere?

I've been taking pins 6 and 7 (ground and chassis, from what i can tell from the cinemag datasheets) directly to earth on the IEC connector (tried taking each pin on its own, but have had the best results taking both).

If you notice on my X12, pin 6,7,8 are all tied to GND on the PCB, which is connected to GND of the PSU, which eventually connects to the case GND via pin1 of the XLRcase.

I have been trying to dload some Mac freeware scopes,

Have no experience with them... don't know if they'll be helpful. Sometimes you want to see if there's oscillations in the higher frequencies (i.e. beyond 20Khz) which a Mac/software scope wouldn't show because it will probably get filtered in the audio interface.

Maybe add another 0.1uf for each leg of the PSU voltage rails.. i.e. parallel to C13 and C14.
I'll try that next.

Please do try... on my original SC-1, the difference between noise/garbage and quiet operation is just one 0.1uf on the supply rail! Prototype #1 was lacking a couple of 0.1uf, and only adding another 0.1uf fixed the noise problem. Even manually holding the 0.1cap by hand, I can hear a dramatic difference.

In the final version, I used a total of (10) pcs of 0.1uf on that little bitty SC-1 board!

[guitarguy12387]

I recorded the pre at different gain settings so you can hear whats goin on. You can find it here:

http://www.soundclick.com/guitarguy12387

This is with a few .1uf caps in parallel with C13, C14. Also, this is with the input transformer grounded.

Judging by the way the gain is going... it really seems like i have the gain resistors messed up... but i have checked all the gain resistors SEVERAL times and checked (with a DMM) for shorts in these R's and in the grayhill and didn't find anything out of the ordinary.

[owel]

It *sounds* like the resistor placements are out of order. The gain stepping is out of order.

I wonder if that gets fixed, the oscillation on the 3rd step might disappear (or move to the 1st setting).

Maybe the 3rd step is actually (or should be) the lowest gain setting... (#1)...

Oscillation for opamps usually happen at the lowest gain setting.. if it's not unity stable.

How about the "stop pins" for the Grayhill switches... were they put in the right location?

[guitarguy12387]

Ahaaaa! As suspected... the resistors were in the **wrong** place. I use the term loosely. Apparently... the way the BOM i'm using was set up had different resistor placement depending on how it's going to be mounted... i just assumed that fully CCW was the lowest gain setting and fully CW was highest... not so.

Anyway... that explains the weird gain. The actual oscillation-type problems were caused by the output transformer. For whatever reason, when the transformer is mounted directly on the board, it gives these oscillations. It was suggested that i pull the output tranny off of the board and try it... and BINGO! Great sounding preamps!

Woooooo! Finally... i can wrap this project up hehe. I'll keep ya all posted.

And thanks again, owel, for all your help. I owe ya.

[owel]

Congrats on getting it to work!

>It *sounds* like the resistor placements are out of order. The gain stepping is out of order.
>>the resistors were in the **wrong** place

Ha! I knew it... glad you got that sorted out.

For whatever reason, when the transformer is mounted directly on the board, it gives these oscillations.

Ahemmm... my X-12 doesn't do that.
I've built 6+ prototypes and even if the trafo is butted against the PCB, no oscillation. I prefer it butted on the board, the ground plane helps.

[guitarguy12387]

Hehe yup! Thanks for the help!

Ahemmm... my X-12 doesn't do that.
I've built 6+ prototypes and even if the trafo is butted against the PCB, no oscillation. I prefer it butted on the board, the ground plane helps.

Haha yeah, my X-12 went together with ZERO problems. I was talking a bit to the guy who designed these boards and i guess there's an issue with this revision of the boards that the input signal runs under the laminates for the output transformer or something... i dont really know, i know nothing about PCB layout/design lol.

Any word on the rumored 2ch. X-12 case?

[guitarguy12387]

Hey! Been a while since i posted here. This one's coming along very slowly still! So i decided to make this thing a work in progress 8ch (cuz i can't afford to put it all together at once haha). So i racked up the PSU in a separate case and made a cable for that and its finally starting to come together. Here's some pics!