API 312 Thread!

[Mike Keith]

I would say the schematic in flawed, disconnect the 2 caps feeding the 48V and see if it doesnt start working properly.

 

[pmroz]

How much voltage do you have on the input of the regulators?

That's what I meant. the reg might be happiest with around 12v to reg to 5v, since it 's probably not straight DC at C1/R1, so dips/peaks.
Probably at least double that voltage at C1/R1...
12V =Less heat to reg... the R could be 5w to do it's part.

Mike Keith, aren't those just smoothing caps? Why would they make a difference?

 

[Svart]

at c1/r1 the Vreg sees 35v on it's input.. this is about the max it can take. it gets hot since it is turning 30v into heat.. but it's still working fine after a few hours once i stuck a big heatsink on it..

the ideal situation would be to indeed use 24v relays and tap the input of the 24 v reg to r1/c1. those little guys are too expensive to keep replacing so i'll stick with the big heatsink.. but for those of you building these, get the 24v version of the relay and a 7824!


:thumb:

 

[Svart]

just a thought: I have a pair of hot mics that i like to use but the minimum gain of the 312 preamps is still a little high.. when i engage the pad there seems to be something lost! something doesn't sound the same as before.. almost muffled i would say. These mics tend to overload most preamps but most preamps I have are sandstate inputs so they clip but these are pretty damn smooth at such high levels.

i guess it got me thinking about a pad AFTER the preamp so that I could saturate the hell out of the input trafo.. :green:

 

[dasbin]

Yeah, even the real API preamps seem to sound worse when using an input pad.
There was some discussion a while back about implementing an output pad instead.
OSA uses a pot on their output, which seems to work OK for me, but I haven't investigated how good the circuit is and how much it changes the sound really.

 

[Bauman]

Yeah, even the real API preamps seem to sound worse when using an input pad.
There was some discussion a while back about implementing an output pad instead.
OSA uses a pot on their output, which seems to work OK for me, but I haven't investigated how good the circuit is and how much it changes the sound really.

Well, I really need a input pair there cos the sources are always super loud... maybe the resistors value on the pad can be changed for better results?

cheers,
Fabio

 

[Mike Keith]

The Neve modules I racked up a while back I used a 15db pad, depending on the mic used the pad can load the mic.

 

[Bauman]

depending on the mic used the pad can load the mic.

Yea, I was wondering that in the 312 case...

 

[Mike Keith]

I checked a SM57: loaded with a typical 1.4K mic input the 57 dropped about 5 to 6db simply plugging it in, compared with less expensive dynamics they where only .5 to 1.5db, interesting....
Maybe thats why a SM 57 works so well with drums, it has a built in 6db pad...

 

[robomatique]

Hello,

Sorry for bringing up the 5V problem again, but I want to ask about a new solution.

I have tried all the different mods, but it is not working good for me, so I thought of putting in a new 9V transformer and connect it via some rectifier diodes and capacitors to R10.

1. Is this a bad idea? Would it for some reason disturb the "other" power supply?

2. How should I connect the 0V of the new transformer? Should I not connect it to the big power supply's ground?

3. Would it be better to just build a new small completely separate 5V supply (traffo - rectifier - caps - 7805). I guess it doesn't have to be super good regulated supply, just a basic to control the relays.

Please help! I have an important recording this weekend, and tomorrow is the last day I can work on it!

Robert

 

[Jonkan]

the GND on the opamp board is used for 100n caps between +v gnd and -v gnd.
I'm fitting mine under the board. This GND is also used by the original maker to shield the opamp, I have been told about some unetched board connected to GND over the opamp board.
Someone said that a metalic case grounded around the opamp is a good idea...

I think the empty GND hole was for the shielding...

Does this apply to the melcor opamps aswell?

I soldered myself a little "opamp hat" in copper foil that fits really nice. Sorry no pictures yet, but it looks really neat.

If i want to try this, where do i connect the wire from "opamp hat" to ground?

On the Opamp itself, or on the mainboard "audio ground"?

 

[Svart]

I have another issue that isn't much more than annoying.. at the highest and lowest gain settings I have an incredible amount of hiss. the audio does not seem distorted and is otherwise clean but has a loud hiss much like turning an open fader all the way up. I saw that someone mentioned the opamp oscillating but this doesn't quite sound like any oscillating opamp i have ever come across.. but that doesn't mean much..

I'll be trying to up the capacitance of the suggested cap and see if that fixes it anyway but any suggestions?

:thumb:

 

[Bauman]

back in the 312 thread!!

I have spend some time putting the channels I did in the past (6 ch) in the right 1U cases, it was hard work (did it hardwired - no connectors) but fun. Will post some pics.

I have another issue that isn't much more than annoying.. at the highest and lowest gain settings I have an incredible amount of hiss. the audio does not seem distorted and is otherwise clean but has a loud hiss much like turning an open fader all the way up. I saw that someone mentioned the opamp oscillating but this doesn't quite sound like any oscillating opamp i have ever come across.. but that doesn't mean much..

I never had this issue... which opamp are you using ?

About the 5V issue, here's my final toughts about it, after finishing those 3 units:

1) You can stick to the plan and use Tq2-5V and 7805 as regulator. Just get some mica and use the case as heatsink. Keep it isolated from the case!! It wont heat too much and it wont shut down 5V as well. Burn in test is like more than 72 hours with all on and max gain, no probs of overheating.

2) switch to TQ2-24V and 7824 regulator, or anythink near it. I think the amount of heat generated will be lower. (haven't tried that yet...) It draws less current than 5V ones, which is good

3) Use a reparate secondary, like 12V to get the 5V out of 7805. Less heat also.

For 2 and 3 I am not sure you wont need a heatsink, but I believe so.

:guinness:
Fabio

 

[Svart]

Hey Fabio, I am using the Melcor ones.. i built two sets of 2520 and neither work for some reason.. but that is another issue

still having the low/high hiss issue but haven't needed either of these settings yet so I haven't looked at the issue in full yet.

 

[Bauman]

First thing to check with Melcor is the feedback cap. It needs to be around 200pF instead of 120pF as for 2520 (in the mainboard). Also make sure C2 = 10pF instead of 47pF (in the opamp).

:guinness:
Fabio

 

[Svart]

Ah i'm pretty sure I used the 120pf instead of the 200.. I'll get that changed and report back!

:green:

 

[Bauman]

Chris, don't change the mainboard cap man, just add a 100pF one over the Melcor board between OUT and -IN pins, so this way the micpre will be compatible with both opamps

That's the trick I do.

cheers!
Fabio

 

[Svart]

hey good idea!

:guinness: :guinness:

 

[Svart]

Oh and maybe someone can give me a hint on the 2520s..

I originally built them with 2n5088s and 2n5087s because i didn't have any bc550/560. i got something that looked like a signal if I input a LOUD source but was aurally nothing more than noise.

I built up another pair using bc550/560 and I get the exact same problem. I am using exact values for resistors and caps.

I've gone over the schematic and I can't see where i've gone wrong.. however I am fairly sure that it's related to the input section of the opamp as touching leads near the output sections make noise and touching leads of the input section does nothing.

anybody run into this problem?

the melcors worked from the start and sound great!

I really like these preamps for overheads.

 

[Bauman]

Just to check, which values are you using for R3 and R4+R5 ?

Got some pics!

:guinness:
Fabio