API 312 Thread!

[BR]

Hey guys,

I have a pair of Fabio's racked up and it works pretty good for a few minutes and then I start hearing pops and noises. I don't have a clue where to start. I don't think its the DOA. I've swapped it and it problem is still there.
any hints are welcome.

take care
Gil

 

[shabtek]

pops and clicks--maybe cold solder joint, intermittent switch or relay contacts? happens at all gain settings?all input settings? doa sockets?

 

[CJ]

maybe a backwards output cap
maybe heat is making the transistors get noisy.
try some freeze spray to find out.
try bridging the pwr supply rails with with a .1 cap
then a 10uf cap
10 second test

see if its squelched there.

then the input circuitry, jacks connectors, transformer leads

then....the road goes on forever..and the party never ends.

you just get quicker on your next brain stumbler.

 

[flintan]

[quote author="Sender"][quote author="Phazinhead"]Hi everybody


Does somebody know the mod to do so as to discharge more rapidly the 48V phantom current ? Flitan already asked it, some post before, but anybody seems to answer..

thank for your help ! :grin:[/quote]

An 5.1k, or even a 4.7k resistor in series with an LED to ground will drain the rail faster, and provide a handy little "Phantom On" function.[/quote]

No, that doesn't work.. I tried it, and Mick has also tried it. For some mysterious reason there is a small voltage present on the PP when turned off. When you turn off the mains it's still there, but when you turn on the PP switch (with mains off) it goes away.

 

[warpboy]

hello,
Phazinhead,
I use this solution with my Neve mic preamps.
if this can help you :wink:

[/img]

 

[flintan]

warpboy, i think thats the common way to do it. It's exaktly the same as in Fabios' boards except from that 100R is 200R and the cap is 220µF on the Api.

The switch is before the cap and therefore the cap will hold some voltage when you turn the PP off. So either way you let the microphone draw it before you disconnect it or you will need a bleeder resistor. I think Mick had an idea some page earlier in this thread on how to tweak the PP switch with a bleeder. I will try that next time i find time to..

 

[mick]

Hi Warpboy, Flintan is right, I did a quick drawing of how I wired up my switch here

http://www.twin-x.com/groupdiy/albums/userpics/phantom_drain_2.bmp

For the relays I went with 24v so the 24v in is for those, change for whatever voltage you went with.

 

[warpboy]

Hi,
So either way you let the microphone draw it before you disconnect it or you will need a bleeder resistor

Flintan, I agree completely with you.
The first idea of this schematic is to protect the input stage of mic preamp board
:wink:

 

[flintan]

Mick, does your solution work good? It connects the +/- signal to ground via the PP resistors when not using PP. Have you tried it with dynamic or ribbon mics?

 

[bobschwenkler]

Hi all.

I'm at the final stages of putting together an eight channel 312 type preamp using Fabio's boards. I'm having a very interesting and perplexing issue (which is probably something really easy, but I can't figure it out).

It is, simply, when the phantom power switch is turned to off on any given channel, a DC voltage begins to build up on both mic inputs. The voltage is of the same polarity and is equal in value on both pins 2 and 3 in respect to both chassis and audio planes. This happens regardless if the unit's AC cord is plugged into the wall or not, and whether the power supply is on and running or not. It does not get past the input transformer.

Essentially, whenever a PP switch is set to the off position, a voltage begins to build up, a mV per second or so (roughly, to give an idea). The max voltage varies from channel to channel slightly, but not much (<10mV right now). After about 10 or 15 minutes of just leaving the unit unplugged and the switches set to off, there is about 370mV on all channels. For as long as I've let it sit today, it has gotten up over .5V.

If I were to switched the PP switch right now, the voltage would drain on that channel, and if I switched it back again, the voltage would begin to build up again, starting at 0.

Any clues?

 

[flintan]

bobschwenkler, i have the same problem. Just to confuse you even more..

I tried to drain the PP circuit with the PP LED indicator. So the LED would always be connected at the PP and drain it when PP switch is off. That gave me about 0,5-3V ALWAYS present at the mic input when PP turned off.

I believe there's some construction mistake in the PP section but i just can't figure it out..

 

[Walrus]

bobschwenkler, the voltage you are seeing is coming from the capacitor CPP1 (on Fabios diagram). When the phantom switch is turned off the capacitor is seeing an open circuit and, due to physics, (don't ask as I don't know fisics! :? ) it starts to self recharge. It's a function of electrolytics only I think.

flintan, it's the same in your case, except this time the LED is discharging most of the voltage until the minimum conduction voltage of the LED is reached. At which point the voltage will stay there until the capacitor charge decays to zero. This time will depend on the leakage resistance of the capacitor and more fisics. :grin:

 

[bobschwenkler]

Ok, it is a little bit disturbing to see... a concern of mine is if I hook up the output of a line level device (like my Apogee converters) and run through a passive impedance/level matching resistor network, and into the preamps as an effect, if the DC voltage there has a possibility of damaging the outputs of the converters. Anyone know about that?

Also, I'm trying to figure out the 75101 transformers I have in the cards, and the Cinemag documentation is not very clear to me. Pin one is marked with a red dot. Does it go 1,2,3,4, etc clockwise from there? I'm not getting any sound through yet, and I'm thinking I might need to tie some of the primaries together, but I don't know which pins are which for sure.

 

[bobschwenkler]

Also, is the PP cap necessary? It is a filtering cap right? I wonder if a different value or type of cap would perform the same function but not charge like that.

It seems like the charging is maybe a function of the transformer and the capacitor working together? The transformer inducting EMF from the air or something?

 

[bobschwenkler]

After sitting overnight, the charge is up very close to 1V on the mic inputs. That worries me, but I've not read anyone having problems due to this. Has anyone? Is there any technical reason to be worried about it?

 

[flintan]

[quote author="bobschwenkler"]After sitting overnight, the charge is up very close to 1V on the mic inputs. That worries me, but I've not read anyone having problems due to this. Has anyone? Is there any technical reason to be worried about it?[/quote]

If you plan to connect a ribbon mic it will probably blow. There should be 0V when PP turned off. :evil:

 

[bobschwenkler]

It should not blow if the voltage is equal on pins 2 and 3... I've run ribbons with PP turned on accidentally with no problems.

But regardless of course, I'd like to fix the issue. How many other people are having the problem? All of them? Or just some?

Maybe a rewiring so that the switch is between the cap and the mic inputs?

 

[Phazinhead]

Hi everybody


I've finished 6 API312 channels and I haven't this problem .. maybye i'm very lucky lol. about the 48V mod, I tried the mod with a 10k resistor between switch and led so as to discharge CCP1 through the LED and it works fine. It would works better with a 5k resistor I think, making the led glowing higher and discharging the cap faster. actually the 48V is gone after 10 / 15 seconds, which is OK for me.

But I have another problem.
The first time I ran the first channel of the API, I got a short circuit between 48V and ground wire, which made the negative Led (from the PSU) glowing less and less and then I saw smokes coming out from the DOA !
I changed the DOA, changed all electrolyics caps on the 312 board, changed the +48V regulator, C1 and C2 of the PSU and I checked the negative current regulator, all boards and soldering.

Now the problem is when I run the preamp, the R2 resistor ( the 10ohm, 5W) comes very very warm (I can't touch it with my fingers , 10 seconds after running !) , and when I stop the current, the negative LED ( from PSU), stop glowing instantaneously, while the positive LED stop glowing after 20/25 seconds.

I have tested with another board and the problem disapear, which means that the problem is certainly on the 312board which had the burned DOA

Does somebody have any idea of what to change on the board? ( I already changed eletrolytics caps and DOA)

thank a lot for your help, 'cos I become crazy with this preamp ! lol

 

[flintan]

I guess the only involved components except for the DOA are C1 + C2, CD1 + CD2 and the protection diodes D1 + D2. I would try changeing the 100n caps by the DOA.

But i really don't know what could've happen with 48V to ground.. :?

 

[Phazinhead]

Hi Flitan !


thank for the help ! .. yes I changed also C1 and C2 ... but I didn't checked the diodes (which look "good") yet. CD1 and CD2 are 100nF 500V non polarized.. so, i didn't thought it was necessary to change them, but I'll do it.

Another question : I've made 6 channels and I have lots of "buzz" on all of them. This is not a big "hum" , it seems to be only the high harmonics of PSU trafo (50Hz).. it sounds like a flying bumblebee lol
I have made a star ground of each connector and connected input XLR chassis to earth. But I don't have any magnetic protection of the PSU trafo and audio trafo. Do you think I should use a copper shield, as for example around each output trafo ?