another API inspired pre

[oran.outan]

Hi all

I arrange a 500 pcb based on API 312 using alternative transformers like OEP... it come to work but I get a tiny huumm... they're usable with loud sources but while I push a bit on the gain I can hear that Huumm. ground loop issue !?!? so I'd like to improve that.
I revised the circuit looking at others schematic, read and try to learn, understand more but as I'm just a composer with little skills on electronic and engenering I'd be happy to have some advice/feedback as I don't know exactly where to look or what I could have make wrong.
So, if you find any mistake please tell me !!! or things to improve ?

On the circuit, I link all ground together. I mean audio ground and power ground. could that be wrong ?
Also, I test with an OPA134 or quadeight AM10, the Humm still there...



for the gain I choose to use a 10k Rev log pot as they're easier to find and common.
RF: 10k (feedback ) and parallel 120pf : CF
R11 : 100R
C8 : 330 uf
that should give + 6 db at minimum Gain / + 40db at maximum gain but I'm a bit confused about R11 and C8.

Input is OEP A262A3E:
RZ= 47K
CZ= 220pf
RL= 33K

output is OEP A262A6E

On the circuit, you will see I added footprint for alternate transformers but I only try the OEP... it be a second step...

thanks a lot for your feedback...
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[Ricardus]

I would definitely be interested in building some of these in the future.

 

[Ricardus]

If you get this working properly I could do a panel for it and make the FPD file available.

 

[Flatpicker]

Is the OEP input transformer shielded? If not, and your 500 rack has a linear power supply, the OEP can potentially pick up electromagnetic interference from the rack's power transformer (assuming that the "hum" is 50-60 Hz). Try it in a slot furthest from the power supply. If that's not the issue, maybe look at the +/- 16V and 48V on the board using a scope and check for ripple. You don't say what kind of mic you are using, but try a dynamic mic - that will eliminate the +48V.

Lastly, but maybe most important, never connect the grounds in a serial succession as you have here. ALWAYS use 'star' grounding. The best way to implement that on a PCB is to make one of the layers a ground plane that covers the entire board and connect all of the ground connections to that instead of running traces. This is done by adding a filled layer connected to the GND net. You can still run traces on that same layer - the ground plane will just fill in around them.

 

[oran.outan]

The OEP were not shielded/on a can. I used a Bento2 which is a Switch Smps without transformer and with the link audio design stereo modul tester with an external linear power supply. I don't think it's front electromagnetic field.

I have seen 'ground plane' design on some pcb/gear I have but most of the circuit I have are not that way. I imagine it should work on traces.

About the 'star ground' I understand I should take all Ground point/pin and make them meet in one point more than link them as a line.
Also, on some PCB audio Ground are not connected on the PCB (which are connected between the audio connectors and chassis/enclosure).
My others 500 gear seems to have audio Ground (pad 5) and power ground (pad 13) connected but does in internal the chassis already have that connection ? or it's the circuit who made that 'link'.

thanks
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[Ricardus]

OEP makes a shield that you can drop onto those transformers I believe. I used them when I built my Bo Hansen DIs.

But did I read right and the maximum gain from this is 40 dB? That seems really low for an API style preamp.

 

[oran.outan]

OEP makes a shield that you can drop onto those transformers I believe. I used them when I built my Bo Hansen DIs.

But did I read right and the maximum gain from this is 40 dB? That seems really low for an API style preamp.

I know about the shield and already used them on a previous build... but I don’t think it’s the cause...

The 40db gain will be from the pot feedback with the DOA, the final gain will depend on transformer and ratio...
It’s what I understand!

 

[moamps]

So, if you find any mistake please tell me !!! or things to improve ?

Why the pad resistors are active at the input all the time?
What is the input impedance of the preamplifier when the pad is on and when it is turned off?

 

[Ricardus]

Why the pad resistors are active at the input all the time?

They aren't. Only when you engage the Pad switch/relay.

 

[moamps]

They aren't. Only when you engage the Pad switch/relay.

Microphone is all the time loaded with pad resistors. This solution is far more commonly used:

 

[Ricardus]

Microphone is all the time loaded with pad resistors. This solution is far more commonly used:

View attachment 93979

Far more commonly? Citation needed.

The ML12 circuit wired the pad exactly like he did. Works fine. Isn't electricity following the path of least resistance here?

 

[moamps]

The ML12 circuit wired the pad exactly like he did.

Sorry, but isn't.


R

 

[Ricardus]

Sorry, but isn't.

View attachment 93982
R

Ummm, Yes it is. That is unless we're talking about different things here. Are you talking about the input pad?

With the switch/relay in the position shown in both schematics the audio signal flows through the switch and around the pad resistors. When switched the other way, it engages a 3 resistor pad very much like the pic on this page on the upper right.

http://www.uneeda-audio.com/pads/

 

[Walrus]

I’m with moamps here, it isn’t.
Relay *shorts out* R7 and R8 on the ML12.
But on the cct at the start of this thread, the relay doesn’t short the pad resistors.

 

[Ricardus]

Ummm, Yes it is.

I’m with moamps here, it isn’t.
Relay *shorts out* R7 and R8 on the ML12.
But on the cct at the start of this thread, the relay doesn’t short the pad resistors.

Ohh, I think I see it now. His shunt resistor (R9) is always in circuit. On the ML12, the shunt resistor (R6) is switched out when the pad is off.

He just needs to rewire it like the ML12.

 

[oran.outan]

Hi

I still continue working on that project.
I make a build, No more hummm and working quietly but I run a test on REW and get a 'bad' frequency response, with a big loss in the Low, acting like a low cut filter. I suspect that is from the zobel network after the input Tx !? but I'm a bit lost understanding what is wrong...



My circuit is based on the CAPI VP 312 schematic, except using OEP A262A3E on input which is 1:6.45 (EA2622 (1:7) which is less than 1db different). If I make it right the 150k load resistor should give me roughly 1500 input impedance.

Theorically, I read that if the inductive reactance of the TX equals the effective source impedance, low-frequency response will fall.
I supposed I have to change CZ/RZ for another values ! but I missed the calculation process.
I make few test with others values that I find on thread around (800pf / 22k) & (1000pf / 20k) but I still get that loss in the low...
Any tips is welcome !!!



I also have trouble understand what the purpose of the 2x (C5/C6) 1000pf capacitors ! I just remove them for now...


thanks
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