API 312 Thread!

[bobschwenkler]

The N version is non-shorting. You don't want that as when you switch gain levels more of the negative feedback may be routed to ground and the gain of the opamp may jump momentarily. You need a shorting switch.

 

[RedNoise]

Thanks bobschwenkler...
Will try to find a S one!!
Cheers!

 

[spica]

Hey guys,

I just finished my second set of 4 312's and things are just gravy (many thanks to Fabio, Greg, and everyone else on this thread)... except for some slight ground hum on the first channel that I just can't track down.

What is really bothering me is that it is only on the first channel... each channel is grounded the same so this is a bit odd. The hum seems to be introduced before the opamp as it increases in volume with increased gain. It is pretty quiet but definitely something I want to fix before I call this project done. Any ideas?
:?

Thanks.

 

[Walrus]

Are you checking the hum with nothing connected to the input?
When I built my 4 way 312's (using 325 boards), I too had hum on one channel. But loading it with a 150 Ohm resistor across pins 2/3 and then measuring with a Neutrik ML1 Minilyser showed that all channels were now equally quiet and hum free.

 

[spica]

I am testing with no load and with a microphone attached to the input, but with no difference.

Thanks.

 

[3nity]

Wich boards are you using?

 

[spica]

Fabio's.

 

[3nity]

are you using the DI boards?
if so take care of the wiring...long wires induce hum...

 

[bobschwenkler]

If you haven't done any of the following yet, this is my basic troubleshooting process for stuff I build.

The first thing I do when I have any issues is check for solder shorts. Often times I'll check resistances between certain points in the circuit and compare those to known working examples. If you have an incorrect value resistor in there this can help find that (I recently ran into my first example of a resistor reading not matching the color coding, what was supposed to be a 10r measured about 110). Check to make sure you've got the correct components in the correct spots.

You might also check for shorted traces on the PCB.

I've not had a problem I didn't fix by checking all of the above mentioned things.

 

[spica]

Thanks, Bob. I will try that.

 

[mitsos]

Hi everyone, a couple of 312 questions...

First an odd problem... I had 2 channels (home-etched HandcraftedTone boards) working, with various component differences to check how the circuit was affected by each. Anyway, they were both working fine but today, I swapped some caps on Ch 1 and then hooked it back up to test against the other (untouched and supposedly working) Ch 2. 

Turned on without Opamps, checked voltages, turned off, and inserted 2 melcors.

I set up the signal gen in PT LE and sent a 1K tone through a DI to Ch 1. It worked just as it did before. When I switched to Ch 2 (swapped cables from one input to the other is all I did) the signal going back into PT was half as strong, at full gain. I hadn't touched the gain loop, but checked anyway and everything is as it should be.  I tried with different opamps but same problem... Could it be a short, or bad connection somewhere?

I'm using a 100K rev log, 100K Rfb, 1K Rshunt and 47uF cap.  The only thing strange is that the Ch 2 pot only goes to 85K maximum but this should really only affect my minimum gain, right?


Second question: Unrelated to this, I was checking Eisen Audio's schematic and BOM for the 312 and it's got a load on the secondary of the output trafo, R17= 1K2. Is there a reason for this?

Thanks!

 

[bobschwenkler]

I could be wrong, but isn't the input transformer on a 312 1:8?

I think the Jensen model is 1:8 and Cinemag has a couple API options one of which I think is called the CMMI-8 or something. I believe it's a 1:8 as well.

 

[Dsaudio]

Hi from Argentina, one pic of my DIY 312 , diy 2520 , diy output transformer and Jensen input.

very quiet( no hiss or hum) 18 Vrms of output.

 

[mitsos]

That's very cool... All in one board.. What did you use to do that trafo? Where did you get the parts? I ask because I'm in Brazil and have been looking for output trafos, but no one wants to make one here! Thanks!

 

[khstudio]

Hi from Argentina, one pic of my DIY 312 , diy 2520 , diy output transformer and Jensen input.

very quiet( no hiss or hum) 18 Vrms of output.

That's cool! 

I was going to try the same thing... building the 2520 circuit right on the pre card.

Any chance you could post & share the files - for etching & component placement???

Also, what version & schematic did you use for the 2520 circuit?

Kevin

 

[Dsaudio]

Sorry for my english
For the output  i used grain oriented silicon ST audio output transformer ( 10 hz to 200khz) with 1:2 -1:4 turn ratio, mine are one primary and 2 secondary
supports +26 dBu @ 50hz without distortion.

you have linx audio transformers in BRAZIL.

http://www.audiolink.com.br/audiolink/transformadores.htm

You don't need a special transformer for api 312 output, try with any output trans , with a regular 5k 600 ohm trans inverted works fine with lots of headroom.

Some people uses power transformers instead audio transformers with good results.

Greetings from Argentina

 

[Dsaudio]

Hi from Argentina, one pic of my DIY 312 , diy 2520 , diy output transformer and Jensen input.

very quiet( no hiss or hum) 18 Vrms of output.

That's cool! 

I was going to try the same thing... building the 2520 circuit right on the pre card.

Any chance you could post & share the files - for etching & component placement???

Also, what version & schematic did you use for the 2520 circuit?

Kevin

The 2520 schematic, works fine!

http://www.thediypill.phx.com.br/forumfiles/sch2520BC_r1.gif

I`ve been doing comparisons between my 312 and Original 512c in one studio in Buenos Aires, sounds very very very close to the original, so the 2520 clone works fine ,i suppose.

 

[mitsos]

Sorry for my english
For the output  i used grain oriented silicon ST audio output transformer ( 10 hz to 200khz) with 1:2 -1:4 turn ratio, mine are one primary and 2 secondary
supports +26 dBu @ 50hz without distortion.

you have linx audio transformers in BRAZIL.

http://www.audiolink.com.br/audiolink/transformadores.htm

You don't need a special transformer for api 312 output, try with any output trans , with a regular 5k 600 ohm trans inverted works fine with lots of headroom.

Some people uses power transformers instead audio transformers with good results.

Greetings from Argentina

Thanks for the info.. I had already tried lynx, they don't make API outputs.

 

[budhastudio]

Hola!!!
Donde has encontrado los transformadores jensen en argentina???
Solamente pude encontrar los oep.
No me decidi a importarlos por la ley de electricidad que rigue en el pais, pero tu has podido???
Bueno, saludos y felicitaciones por el trabajo

 

[Dsaudio]

Hola, los compro directamente a jensen, conviene comprar 12 o mas pero terminan costando el doble por los impuestos, se consiguen los oep en argentina?, me interesan los OEP porque son lindos buenos y baratos!
Te digo sinceramente que los jensen no te dan una diferencia de noche y dia en el sonido, suenan te diria un 10% mejor que un oep, tambien uso neutrik y suenan de primera como los oep.
Un abrazo.