API 312 Thread!

Shop deals on ebay
shop deals on amazon
Advertise with us
GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
Hey Bob, thanks for that, thats great! I just wanted to make sure i had all the right parts in the right places. I haven't been able to do much fiddlin lately on account of the semester coming to a close thus alot of work. But i'll keep ya posted. Thanks again!
 
I'm working on a pair of 312 type preamps using some melcor cards rewired to a 312 circuit. I'm using Altran inputs, melcor opamps and Fabio 2503 outputs. The Altrans are mounted on JLM Go Between Plus. I need some help with the loading of the input transformer. Is 100k correct (like peter purpose used for his Jensens)? And 9K76 and 22pf instead of the 5.1K 22pf shown in the 312 schematic? What do I need to do on the Go Between plus? Zobel network?
Altran doesn't seem to have too much info on their website
 
The input load resistor depends on the winding ratio or the transformer. I forget the exact equation but as I recall, you take the square of the impedance ratio (eg. 10/1 or 8/1) and multiply that by the desired input impedance. A 100k load resistor gives you something like 1.5k at the primary with a 1:8.

Hopefully I'm not too far off here, someone please correct me if I am.
 
Somewhere around 1.5k is pretty standard I believe. If you're wiring your input for 1:8 100k is a good value for the load.

As far as the zobel goes you'll want to find the network that's appropriate for that transformer. That may take a little more searching or experimentation. If you can find a schematic of the circuit the transformer was used in you can probably get some good information that way.
 
This is a response to another 312 thread I started before realising that this one was the place to post (got a bit of a telling off for starting that one  :( oops). The question I asked is the same one being discussed here regarding the Altran input tranny, so good news all round. Here was my original post:

I'm embarking on a project to build a 2 channel API 312 Clone and have a few questions about adjusting the circuit for different types of transformers. I am going to use an Altran C3402-2 input tranny and possibly a Sowter 9825 as the output. I am going to attempt building the op-amp using mnats JE-990 design. The main circuit I am basing on the attatched schematic.

My question is whether or not I need to adjust the circuit to suit the transformers/op-amp options that I have chosen, or is it just a matter of plonking them into the circuit as it is? Any words of wisdom for me?
Click to expand...

To which joechris responded:

First of all, do a search on transformer loading.  With a 600/10K transformer you migth want to load the secondary of transformer with a 20k resistor, so the mic see a 1200ohm load.
Or make it variable, so you can play with the hi frq response. Check if you transformer need a znobel network to tame hi freq garbage.
Click to expand...

So am I correct in thinking that the load that the mic sees = PrimaryImpedance*(LoadResistance/SecondaryImpedance)? In the case of the Altran that would be 1500 ohms if a 100k resistor is used, which is what the above conversation seems to imply.

My second question is about what load is ideal for the mic to see. Obviously this will depend on on the microphone impedance, and I could use a variable load resistor as suggested by joechris to find a good value, but is there an equation I can use to find what the ideal match between mic and input is? Then maybe I could have some interesting feature on my preamp whereby the load impedance is adjustable and an led indicates when the impedances are matched. Just a thought though.

By the way, I should introduce myself. My name is Dave and I live in England and am a first year Electronics Engineering student.
 
Ideal is all relative to what ends up sounding the best. Vague but true. There's no equation for that. But the very general rule of thumb for efficient voltage transfer is a 1:10 ratio between source and load.

But I believe many or most mics have an output impedance between 50-200 ohms, and some will be more affected by changes in the impedance of the preamp. 1200 seems like a value that's in the area of pretty common.
 
Dave, for the Altran 3402-2 with the primaries in parallel, use the suggested 100K as RL. You should not need a zobel, but if you do, Dave from Altran suggested starting with  27k Ohms and 110 pF.  I tried it but in the end removed it.
 
Thankyou all for your responses, it's been really helpfull. I've decided to go for the Lundahl LL1587 for the input tranny in order to get lower noise as Rodabod suggested in the other thread. I do have a question about the output tranny though.

I noticed some people talking about possibley using the OEP A262A2E as a possible output on another thread somewhere, but I could not find any conclusion to this idea. Has anyone tried it successfully? It would be handy for me to use this model because they are fairly cheap and available form the UK.
 
Hey bob,

Still workin on the same issues. I'm a little curious about some of the R values for the grayhill...

Looking at your 312 bom file, it looks like the resistors are increasing from 255 ohm to 20k. Makes sense. Then i look at R17-R19 drops back down to 63r4 to 160 ohm. See what i'm referring to? How come those values drop back down so much? Seems like that drop the gain value significantly.

Also, what kind of 2520 opamps have you tried in your hybrid boards? Have you (or anyone else) tried a peter lazaro opamp?

Thanks!
 
Ok got a chance to do some more testing today. And... got MORE weird results.

After double checking all my component placement again and cleaning up some solder joints, i tried tying pins 6 and 7 of the input tranny together and taking them directly to star ground point. I decided to do this because i had noticed that touching the can of the tranny significantly reduced all the noise at high and low gain values.

Here's the results: I pretty much get great sound at all gain settings except for the third position from counter clockwise (lowest gain) which still has lots of clicking/humming (oscillation i assume). Another weird thing is that the gain settings below this one (settings one and two) both are very high gain... which seems weird to me.

I have double checked (with a multimeter) that there are no shorts on any of the gain resistors and grayhill switch. I have also checked my R values many times. I cannot figure it out!!!
 
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 (as suggested by owel). Also, this is with the input transformer grounded (both pins 6 and 7 of the input transformer).

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.
 
API 312 Problem (Just moving my post...)
I've done a bunch of searches and haven't found a solution for this...

I built 4 channels of API 312 using 3nity's kits, Cinemag ins/outs and John Hardy 990c's. Being that it was my first DIY build I was sure I would miss something so I tested many many times before dropping the op amps in. Here's the problem: I am measuring 18+/- volts for each preamp card, but on one of the preamps the opamp overheated and died.  :'( I quickly powered down and took the opamp out, tested the that I was getting the right voltage (which I am) and I even went as far as putting another opamp in to see if it would heat up the same way. It did, but I didn't leave it in long enough to die.

I have been over the resistor values multiple times and like I said I am measuring the correct voltage for the + and - of the op amp. Where could the problem be? Am I missing a measurement somewhere?

PS. the other 3 preamps work flawlessly and sound fantastic.  I will post some pics soon  :)
Click to expand...

From Ruairi

Hi,

You should put this in the 312 thread but while I'm typing I may as well say that I suspect that you have a short somewhere on your output.  You are most likely asking the opamp to drive a short and in doing so you are burning it out.  I'm not an expert by any means but this would be my suspicion so check the wiring and soldering after your opamp on the dodgy card. It's good that you have good cards to check against.

Cheers,
Ruairi
Click to expand...


Just looking at the 312 schem there really isn't much happening after the op amp. Could this problem possibly be the cause of a damaged output transformer?
 
compare the non working vs the working pcb's.
make sure you right wired the output transformers...
look the colours...
 
Hi 3nity, yep I definitely made sure that the output of the transformer leads were all going to the right place.

I'm curious. If the gain pot is wired wrong or if I had the wrong value gain pot could this cause a load problem? It's probably not this as the other 3 preamps are wired with the same pots.
 
could someone tell me what the purpose of CDI1 (18pF) is?
I'm trying to find a suitable wima cap replacement for it but the smallest polypropylene they make is 33pF.
From what I understand, it is simply a tone shaping cap. i.e. reduces bass at the DI input. Am I right?

The -3dB point of that RC (RDI1 and CDI1) network seems to be around 4kHz.
Replacing it with a 33pF cap would yield a -3dB point of roughly 2.1kHz.

Is there a reason for the 4kHz figure?

Thanks for the help.
 
I think this is what I'll do in this case.
Ceramic caps are quite popular in guitar/bass amps anyway right?
Maybe there's something nice about those even-order harmonics it produces when it distorts.

anyone else though? before I click "order"  ;D

thanks,
Mike


 

Similar threads

frob
312 collected info, is this right?
Replies
1
Views
1K
soundguy
S
T
Calrec 1061 Filter Simulation - help needed.
Replies
7
Views
6K
TomWaterman
T

Latest posts

Back
Top