LR Baggs Para DI Acoustic Preamp

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LR Baggs Para DI Acoustic Preamp

Postby lukatosh » 22 Oct 2017, 23:58

i've started tracing this yesterday... So far this is what i have

Here is the power section... You can look at IC1 that pin 4 is not conected directly to ground. Somebody can explain this circuit block? (The unit may be phantom powered).
LR2.PNG


Here, C9 is a NON polarized capacitor (50v 22uf NP). Is there a way that you can replace this using two electrolitic capacitors?
lr3.PNG


Finally, the input signal flow start like this...
lr1.PNG


Thanks!
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deltafred (23 Oct 2017, 05:33), modman (24 Oct 2017, 15:02)
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Re: LR Baggs Para DI Acoustic Preamp

Postby PMowdes » 23 Oct 2017, 12:42

Good work

Why would you want to replace the non-polar cap??? They are readily available.
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Re: LR Baggs Para DI Acoustic Preamp

Postby lukatosh » 23 Oct 2017, 20:26

Not much in my country :(
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Re: LR Baggs Para DI Acoustic Preamp

Postby PMowdes » 23 Oct 2017, 21:35

lukatosh wrote:Not much in my country :(


Where are you??
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Re: LR Baggs Para DI Acoustic Preamp

Postby lukatosh » 25 Oct 2017, 01:48

Apparently... the signal goes like this:

Input-> Gain Stage- >Buffer -> Eq -> Phase invert and line balancing (DI) ->Out

The Eq is based on this:

http://www.geofex.com/article_folders/eqs/paramet.htm

Tree fixed frecuencies usin transistors (treble, presence and bass) and two variabe frequencies (Notch and Mids)

A couple of questions...

1) About the leds... Since non of them have a current limiter resistor, is possible that they have a built in resistor? How can i check this?
https://www.mouser.com/new/kingbright/k ... stor-LEDs/
led.PNG


2) Is this correct? i want to replace the NP capacitor. Are they related to the phantom power? (please omit the "+" sign in the lower circuit, it's a non polarized capacitor )

xlr.PNG
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modman (01 Nov 2017, 18:23)
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Re: LR Baggs Para DI Acoustic Preamp

Postby phatt » 28 Oct 2017, 12:49

Hard to answer but the Led (the snippet you posted) it has a Zener which sets the voltage so there is no need for a drop resistor. Very few circuits would use Leds with the built in resistor.
As for the NP Capacitor Q, In most cases you can just use a Polar Cap with positive facing the active device. (The output of the opamp I'm guessing) it will make no difference to the AC signal.

The pin 4 drawing makes little sense and without a more detailed drawing of the circuit with values on the components (Hint) it's had to give a meaningful answer. You may have misread the tracks which is easy to do.
I often have to re draw a circuit a few times before it makes sense.
Can I suggest you post a picture of the board that might help us to understand the circuit.
Phil.
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Re: LR Baggs Para DI Acoustic Preamp

Postby lukatosh » 07 Nov 2017, 19:07

Well...here we go. I have not had much time... To much to do at work.

1.- The unit uses 3 LMC6032.

IC2a Eq
IC2b Line balancing

IC3a: Notch control freq.
IC3b: Line balancing.

IC1a used to control bass freq.
IC1b used on this arrangement:
cap val.PNG


¿it' possible to replace the LMC6032? (Not available in my country). From what i've readed, mostly it was selected because of its low power comsumption. The unit may be powered by batteries or phantom. I want to add a 9v dc supply so, current comsumption will not be a "mayor problem".

2.- What about the leds and zeners?... Even if the zener sets the voltage, will even be needed a current limit resistor?
out section.PNG
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Re: LR Baggs Para DI Acoustic Preamp

Postby phatt » 15 Nov 2017, 13:19

Again without a full schematic it's hard to know what's going on but as it's a basic audio circuit I'd say any of the common opamps will produce the same outcome,, give or take an electron. :blackeye
Regards the Led's,,, remember that the led only draws the current it needs if the voltage is too high then the led will overheat and burn out, the zener holds the voltage at the correct level so no need to use a resistor as well. :thumbsup
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Re: LR Baggs Para DI Acoustic Preamp

Postby drewl » 15 Nov 2017, 17:23

I've replaced the 6032 and other low power opamps with other more common ones I have laying around like 4560 or 4580 and nobody has complained about short battery life.
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