API 312 Thread!

[Guest]

anyone?

 

[laitue5]

* is it possible to have an output level switch in the preamp? im wondering if overdriving it a bit can bring a nice effect, and controling the amount of overdrive would be better controled with a output level.

Great idea, I have no idea if it is possible but I?m interested. I?ll dig in this direction as soon as I?ll get a working opamp.

 

[Bauman]

Hello,

here are some pix of my current state:
www.o-zen.com/saffi/api

looking good!

* for the relays i used Axicom FP2 D3012 (24v) http://relays.tycoelectronics.com/datasheets/fp2.pdf
are they worthy or am i better off using switches instead?

give it a try, they look ok.

* on the melcor and 2520BC, instead of the BD139 i used BD441
http://www.ortodoxism.ro/datasheets2/0/01glh2g3agku3e7l7q75q2rt41py.pdf
is that ok?

looks like they have lower beta... not sure...
you can use BD135, 137 or 139 for NPN and BD136,138 and 140 for PNP.
Can't you find it?

* will the servo output level be the same as the line out? im thinking of making a switch between them (SW_IRON ), has anyone tested the servo out, and can say how it sounds?

I have tested, it sounds good. Also, using good caps, the unbalanced out sounds good too. Some different stuff going on in low end, not time to compare them a lot til now. Both are ok.

* I'm planning to make a small hatch for each opamp that you can open and change the opamp from melcor to 2520bc at will. i understand there is an issue with the c5 cap. right now i put 220pf, which i read is better for the melcor. will it work ok on the 2520bc aswell? or shoudl i just add another cap on the melcor to balance it (C5=120 + another 100pf on the melcor)

I did this! On the micpre I have 120pF and on melcor omap I have a 100pF under the board to make it compatible...

*what are the leds in the PSU for? are the mainly acting as diodes, or do they have some calibrating usage? im wondering if i should mount any of them on the chasis...

I like leds in PSU cos you can see if everything is ok with the voltages, but they are completely optional...

* is it possible to have an output level switch in the preamp? im wondering if overdriving it a bit can bring a nice effect, and controling the amount of overdrive would be better controled with a output level.

someone suggested a pad after the output transformer. a U pad, 2k2 - 10k pot - 2k2. never tried.

* was the melcor's oscillation fixed in the end of the proccess of trying to fix it? there is so much technical talk i have lost track of what parts are the problematic ones.

I never had any issues doing them the "original" way. Some guys did experiments (see Melcor Deconstructed thread) and reported some improvements. I guess I never see low gain probs cos I run mine all the way up all the time

hope this helps!

Fabio

 

[Greg]

Fabio: I also have problems at low gains with the Melcor 1731. I usually have my gain very high so it's not a huge problem. Sorry to make you repeat yourself, but I think might have missed the talk about changing the feedback cap with the Melcor. So you added a 100pF cap in parallel with the existing 120pF... ie 220pF now? Did this help out with the stability issue?

 

[Neeno]

I had the same problem, oscillation at higest and/or lowest gain setting...
220pF cap on the feedback solved the problem (thanks Mr. Bauman) :thumb:

 

[Greg]

Thanks !!! :guinness: :sam: :thumb:

 

[Guest]

Thanks fabio for an excelent answer!

im still wondering if the servo output and the line output would give me the same level so i can switch between them.

also would i need a shorting switch or a non shorting one?

 

[Svart]

Muze, Shorting for switching during tracking. you can use NON shorting if you don't plan to switch during tracking. the non shorting will cause an audible pop usually.

As for the servo and servo out.. does the servo affect the normal output of the board if it is installed? I assume it does..

 

[Greg]

[quote author="red_muze"]im still wondering if the servo output and the line output would give me the same level so i can switch between them.[/quote]
The "servo" output and the "line" output should both be at a line level. Only difference should be the "servo" is electronically balanced, and the "line" output is transformer balanced. But both should not be hooked up at the same time, so I'd suggest a non-shorting (break-before-make) type switch. But a pop might occur. If shorting were used both outputs would be loading the opamp for a short period of time... this doesn't seem like a good idea to me. Fabio or someone correct me if I'm wrong.

 

[robomatique]

quick question, should I ground the PSU (fabios API psu)? Fpr the moment I am connecting the XLR inputs and the IEC to a starground scheme, should I connect the PSU there as well?

Robert

 

[bubba_b]

Yes. There's a solder point right between two big electrolytics (C1 & C2). Hook that up to your star point on the chassis.

 

[robomatique]

THanks for answer!!!

Another small question. I am using sowter input transformers, and unfortunately I have to mount them off board, with the wires connected about three centimeters to the board.
Do you think it would help if I shielded them with some metal foil connected to ground?
It just looks so bad with such thin wires running my microphone signal in the chassi...

THanks again

Robert

 

[Jonkan]

Robo: I just twisted the wires in pairs according to their connections. Works great!!

Dunno if it makes any difference to just having them untwisted. I suppose i cant hurt though.

Also, be sure to ground the mumetal can of the input transformer, or you'll get loads of hum and interference.
/Jonas

 

[robomatique]

OK, another question...

The phantom power is not discharging very fast from the circuit, I guess it is CPP1 that makes the slow discharge. I would however want to be able to connect a ribbon without having to worry about phantom power still being in the circuit.

I know this has been up before, but I thought of connecting the phantom power led from the 48V around RPP3 instead of the switch source to be able to see the phantom discharge thru the led.

1. Would this be a good idea? Will it effect the phantom supply in any way?
2. What resistor value should I have in serie for a 48V source?
3. Any special ground point I should use? 48V ground?

Thanks again

Robs

 

[bubba_b]

I wouldn't suggest running your phantom power through the LED. In order to not blow the LED, you'd need to step down the voltage so much it wouldn't be enough to power your mic anymore. If you're thinking of paralleling it with the 48V supply, that will probably still affect your circuit in some way, but I'm not sure how. It doesn't sound like a good idea to me. Anyone expand on that?

If you turn off phantom with the mic still hooked up (which you should do anyway), it drains much quicker through the mic itself and you can hear it (when the mic dies out). Also, I think I heard somewhere that disconnecting a condensor with the 48V still on, leaves a lot of charge in the mic itself and causes the capsule to attract dust and particles that gunk it up.

Simple, non-modding solution to your problem, I think. :grin:

 

[Greg]

I noticed the same problem when I built the PSU. This topic was discussed earlier either in this thread or another... I can't remember it was a while ago. I decided not to any mods and do what bubba_b suggested, just simply turn off the phantom with the mic still hooked up. Everything discharges and then I plug in my ribbon.

But I could see why you'd want to change this...

 

[robomatique]

Thanks guys! I know what you mean, but I still would like to know that I can't blow any of my equipment with it just because I am forgot.

I was thinking if I could use a bleeder resistor that gets connected when I turn off the phantom power and drains the phantom power. What value should you use for a bleeder?

Thanks again!!!

Robert

 

[Mike Keith]

If you use a LED for the phantom use a 15K in series, you could also place a 33K across the 48V will also dissipate the voltage a little quicker. That's what I use on my JM 130 mic pre and works just fine..

 

[Flatpicker]

I use 200 ohm.

 

[Mike Keith]

I was referring to directl across the 48 volts, 200 ohms at 48 volts would pop the LED in a few seconds...