Mitsos API 312 / 990 Build Thread

[benlindell]

@benlindell:
colin @ audiomaintenance
or
jeff @ classicapi
gots the pots if you are doing a 312 style (20-22k rev log pot)

It doesn't look like Colin has them in his webstore, found the 2 gang but no single.

 

[benlindell]

So I'm measuring the dimensions of the board with the switches and I think the closest the inside of the front panel will get comfortably is about 3/8" from the edge of the board, this will give 1/8" of the button outside the case when fully pressed in.

So looking at the data sheet for both the bourns and also the omeg pots it doesn't look like either's thread will make it without being wired off board.

Mouser recently started carrying rev log Alpha pots so I'm gonna order one of those to try out, they are only $1.50

 

[mrcase]

ok. Me again : finished 2 of my api 312s. and both got the same problem. a high pitching noise starting from around 5K.
It is not that loud but I can hear it and it should not be there.
I am using Eds transformers, 2520 DOAs (PPAs, GAR2550) and Mike Fantoms PSU (Whistle Rock Audio).
I do not think this is a shielding/grounding issue since I have tried all different kinds of grounding and I dont have any recognizeable hum.
Could this be PSU related or could a wrong zoble network produce this kind of noise?
Im going to post a audio file later...
thank you for your help!

 

[nielsk]

What are you using in the feedback loop?

 

[mrcase]

I am using a welwyin 24k9 resistor. Should be close enough to jeffs bourns potetniometer(25K), I think!?
ah, and a 100pf silva mica cap.

 

[baadc0de]

hey guys, I'm having problems sourcing the specified Panasonic bipolars from the BOM. I'm in Europe and need 8 channels worth, where could I get them? I usually get my stuff from farnell.

 

[mitsos]

So looking at the data sheet for both the bourns and also the omeg pots it doesn't look like either's thread will make it without being wired off board.

Mouser recently started carrying rev log Alpha pots so I'm gonna order one of those to try out, they are only $1.50

I usually use a second faceplate for most cases.. The one I made for the preamps is set back about 6mm IIRC, basically enough to hide the pot mounting hardware.

Mr Case: Doesn't sound like grounding. The PPA's are pre-built right? Both those and the GAr2520s are giving the same problem?  I'd start from the 312 input and work towards the output. Make sure all the components are soldered properly and are the right value.  The input trafo is wired 1:7 right? (primaries in parallel, should have 2 jumpers, one between pins 1+2 and the other between 3+4).  And you are using the zobel Ed/Jeff suggest?  IIRC, it was around 100-150K for the load and 10K || 220pF for the zobel.

The rest of the PCB is pretty straight forward, check the component values/placement and let us know if you find the issue.

Baadc0de: You can get those Panas from TAW and digikey.  You can also fit similar value Nichicon Muse ES bipolars (the green ones) in those spots (mouser and hndme.com). In case you can't find either, you can use a polarized lytic, with the positive lead toward the opamp.

 

[mrcase]

hey mitsos!

thank you for answering!
I have improved my noise issue a little! here is a short clip with full gain and no input (you have to crank your speakers up)
https://docs.google.com/leaf?id=0BwV6TPmQpKsKNjVjNTMyMzktOTNmZC00Zjg2LThiMjctNWE4MjVjZmMzYmFh&hl=en&authkey=CPCMyP4O
what do you think? is this normal or can I still improve this? I would love to have this great preamp as quiet as possible.
I have the input trafo in parallel (1:7) and use this zobel network:
Zobel 10K + 220pf
Input load resistor 120K

thanks again for your support!

 

[mrcase]

hey guys, I'm having problems sourcing the specified Panasonic bipolars from the BOM. I'm in Europe and need 8 channels worth, where could I get them? I usually get my stuff from farnell.

I got Nichicon Muse caps from the uk. still cheaper than mouser or digikey shipping...
here is the link
http://www.hificollective.co.uk/components/nichicon_es.html

 

[mitsos]

Is the input loaded?  Solder 150R between pins 2 and 3 of a male XLR you have lying around and plug that in, then listen. Otherwise, the noise you posted shouldn't be anything to worry about once in real use, but it's good to run it through RMAA or something similar to be sure.

Nice link on the nichicons, btw! The 330 and 470uf / 16V should fit the PCB just fine!

 

[mrcase]

I only had a 130R (does this make big difference??), anyway, the noise stayed the same if not even less, so I guess I am fine!!  ;D
and yes, those caps fit the board fine! thanks again mitsos for all your work!
now 2 more to be tested and then finally engrave the frontpanel and rack them up properly!

cheers

ps: anybody got RMAA working wth the newest fireface 800 drivers and firmware? No MME anymore...

 

[mrcase]

BTW, did anybody implement a VU meter? where would be a good point to tap the output for the meter?

cheers!

 

[volker]

You can use the auxiliary winding on the output transformer for that purpose.
That should be grey and violet.

 

[mrcase]

great. gonna try that!
thanks volker!

 

[benlindell]

I wired and fired up my first channel.

Lots of noise and the occasional AM radio. I tired jumping the input tx with 2-3 and (1-2, 3-4). Tried 2 different opamps. At the most gain I get a little bit of highpassed signal from my mic.

I grounded the IEC earth to the star point on my PSU and then to output and input pin 1. The only thing not hooked up is +48V supply.

Any ideas?

 

[tmuikku]

I wired and fired up my first channel.

Lots of noise and the occasional AM radio. I tired jumping the input tx with 2-3 and (1-2, 3-4). Tried 2 different opamps. At the most gain I get a little bit of highpassed signal from my mic.

I grounded the IEC earth to the star point on my PSU and then to output and input pin 1. The only thing not hooked up is +48V supply.

Any ideas?

Make sure both wires from output transformer connects to the input that you are feeding with the pre e.g. if you have unbalanced input that you have connected to, make sure you use appropriate cable. I got problems with G9 and it turned out to be that my sound card had unbalanced inputs with TS jacks and my cable was TRS so ring was not connected. Symptoms = very low highpassed output.

 

[mitsos]

Interesting, I personally haven't had this issue on an output before, but I almost always forget the jumpers on the input trafo and have these same symptoms  : but you already mentioned that you have those installed.  Hopefully this should be a simple balanced wiring issue (check your molexes for broken wires... I've had quite a few, especially when testing things on the bench, without a case).  Also, make sure there is some clearance under your input trafo so you don't short a winding and have a burnt trafo to deal with... If you have a way to probe the circuit, you should find the problem pretty quickly.

good luck!  cheers!

 

[seavote]

I wired and fired up my first channel.

Lots of noise and the occasional AM radio. I tired jumping the input tx with 2-3 and (1-2, 3-4). Tried 2 different opamps. At the most gain I get a little bit of highpassed signal from my mic.

I grounded the IEC earth to the star point on my PSU and then to output and input pin 1. The only thing not hooked up is +48V supply.

Any ideas?

i still consider myself  pretty green so maybesome one can confirm my suspicions (or explain why they're not valid)
if you are picking up AM wouldntthis point to a sheilding (lack thereof) problem.
my build is notcomplete so i cant speak from that perspective,but i used sheilded wire to connect the main board to ouput pad/phase board and will use it from mainboard to my stepped attenuators when i complete them.

 

[benlindell]

I wired and fired up my first channel.

Lots of noise and the occasional AM radio. I tired jumping the input tx with 2-3 and (1-2, 3-4). Tried 2 different opamps. At the most gain I get a little bit of highpassed signal from my mic.

I grounded the IEC earth to the star point on my PSU and then to output and input pin 1. The only thing not hooked up is +48V supply.

Any ideas?

i still consider myself  pretty green so maybesome one can confirm my suspicions (or explain why they're not valid)
if you are picking up AM wouldntthis point to a sheilding (lack thereof) problem.
my build is notcomplete so i cant speak from that perspective,but i used sheilded wire to connect the main board to ouput pad/phase board and will use it from mainboard to my stepped attenuators when i complete them.

I haven't had time to probe around but i used the shielded quad cable idea the mitos suggested, but I did use an alpha pot wired off board with 1 inch of unshielded wire, could that cause all of this?

 

[tmuikku]

...

...

I haven't had time to probe around but i used the shielded quad cable idea the mitos suggested, but I did use an alpha pot wired off board with 1 inch of unshielded wire, could that cause all of this?

Do you have a proper case? Is case connected to starground? Must be some shielding/ground issue. I've got 2" unshielded wire for pots and proper case with standard grounding scheme and don't have AM radio