balancing a mic pre output

[electrichild]

Hello,
I would not change the output stage of this circuit
http://gyraf.dk/schematics/Amek_2500_micamp.gif
from balanced to unbalanced.
Removing the NE5534 can the pins 1 and 7 of the TL072 go directly to output + and - or must I add some components?
Thanx

 

[Bo Hansen]

Electrochild,

The last OP-amp "U1" is a important part of the whole balancing topology to get good "common mode" on the balanced input, so do not change the original internal design.

If you want a electronical balanced output, you can easy add a unity gain inverted NE-5534 after the original mike pre output, and you get in phase from the original output and out of phase from the inverted output.

This inverted amp is easy to build, only connect + input to ground, and a 10 k resistor between - input and output, and a 10 k resistor to - input, and the other end of this resistor connected to the original mike pre output. (before C7)

Don´t forget the output 100 uF capacitor and the 47 ohm resistor, and also the 10 pF capacitor between pin 5 and 8.
(and of course +16v to pin 7 and -16v to pin 4)

The mike pre amp get +6 dB more gain with this balanced output. (between the in phase and out of phase outputs)

Note,
Do not "side grounding" the out of phase inverted output in unbalanced application, in this case, use only in phase output.

A other idea, is to use a balanced driver as SSM-2142 instead for the inverted add on amp, but I think it is same work, count of components and cost.

(but the most easy are to use a good 1:1 transformer on the "U1" output)

--Bo

 

[JohnRoberts]

Hello,
I would not change the output stage of this circuit
http://gyraf.dk/schematics/Amek_2500_micamp.gif
from balanced to unbalanced.
Removing the NE5534 can the pins 1 and 7 of the TL072 go directly to output + and - or must I add some components?
Thanx

The tl072s are not very robust for driving low impedance output lines, but yes indeed taking those signals before the differential would give you a balanced output. The circuit would have no CMR and simply pass CM signals through to the output so the following stage would need to have a good differential input.

Depending on what you are trying to gain from a different output configuration you may be better of staying with what you have. If you add a proper balanced output driver you should probably perform a differential to SE before the driver to scrape of CM from the raw input. There are plenty of canned circuits or chip sets for line drivers.

JR

 

[electrichild]

This inverted amp is easy to build, only connect + input to ground, and a 10 k resistor between - input and output, and a 10 k resistor to - input, and the other end of this resistor connected to the original mike pre output. (before C7)

Don´t forget the output 100 uF capacitor and the 47 ohm resistor, and also the 10 pF capacitor between pin 5 and 8.
(and of course +16v to pin 7 and -16v to pin 4)

The mike pre amp get +6 dB more gain with this balanced output. (between the in phase and out of phase outputs)

Note,
Do not "side grounding" the out of phase inverted output in unbalanced application, in this case, use only in phase output.

Bo and JR,
thanks for your answers. I make a redrawing of the schem with another IC as you Bo suggested.
I'm not sure I understood well the thing so let me know about it...is the "side grounding" of unbalanced design R17? Must I remove it?
http://www.nipperstudio.net/electrikaLABMP1bal.bmp
Thanks for your time and patience

 

[peterc]

You could also use this standard cct:

http://1176neve.tripod.com/id6.html

Peter

 

[Harpo]

I make a redrawing of the schem with another IC as you Bo suggested.
I'm not sure I understood well the thing so let me know about it...is the "side grounding" of unbalanced design R17? Must I remove it?
http://www.nipperstudio.net/electrikaLABMP1bal.bmp

When you connect to an unballanced following stage, only connect to XLR-Pin2 and shield XLR-Pin1. Leave XLR-Pin3 open (don't short it out to 0V).

While at it, you might fix some 1st. sight bugs in your schematic:
A 10K resistor to 0V after C7b is missing (just like C7/R17).
Both current limiting 47R resistors (R14 and R14b) after C7 and C7b are missing.
C7 and C7b with 3V rating might blow from accidently connected phantom voltage (some AD/DA converters come with mic-amp frontends these days, so this actually can happen), so better use at least 35V rated caps.
Compensation cap C11 connects between U2 pin5/8.
U1 is drawn a little funny (this is a dual op-amp, symbol for usual shown as 2 separate triangles),
non-inverting inputs pin 3+5 connect to junction R25/C9,
inverting input pin 2 connect to junction R23/C8 instead.
C10 with 63V rating will be giant, compared to a sufficient 6.3V rated cap.
Collector and emiter connections Q4,5,6,8,9,10 obviously missing.
C2 and C4 seem polarity reversed (2 bipolar 4.7uF/63V caps for C1+C2 and C3+C4 might fit better, 2 huge 4.7uF poly caps might sound better).
just my 2ct

 

[Bo Hansen]

Electrochild,

Yes, you have understand my explenation, so you have connect the inverted "U3" amp right.
The R17 resistor shall go to 0v ground, and put also a "R17" on the inverted output.
The original R4 (47 ohm) is also nice to have on both outputs.

But on your redrawing, it have now happend some strange things with the original drawing, you have done some error around U1 and U2.

--Bo

 

[electrichild]

While at it, you might fix some 1st. sight bugs in your schematic:
A 10K resistor to 0V after C7b is missing (just like C7/R17).
Both current limiting 47R resistors (R14 and R14b) after C7 and C7b are missing.

Fixed!

C7 and C7b with 3V rating might blow from accidently connected phantom voltage (some AD/DA converters come with mic-amp frontends these days, so this actually can happen), so better use at least 35V rated caps.

I used 470uF 6.3V because is difficult to find original one 100/3. I' ll use this pre with converters without mic-amps.

Compensation cap C11 connects between U2 pin5/8.
U1 is drawn a little funny (this is a dual op-amp, symbol for usual shown as 2 separate triangles),
non-inverting inputs pin 3+5 connect to junction R25/C9,
inverting input pin 2 connect to junction R23/C8 instead.
C10 with 63V rating will be giant, compared to a sufficient 6.3V rated cap.
Collector and emiter connections Q4,5,6,8,9,10 obviously missing.

All fixed. I'll fix another appointment with my oculist...

C2 and C4 seem polarity reversed (2 bipolar 4.7uF/63V caps for C1+C2 and C3+C4 might fit better, 2 huge 4.7uF poly caps might sound better).

I think C2 and C4 are right. Are they 10uF 80V???

Thanks a lot

 

[Harpo]

C7 and C7b with 3V rating might blow from accidently connected phantom voltage (some AD/DA converters come with mic-amp frontends these days, so this actually can happen), so better use at least 35V rated caps.

I used 470uF 6.3V because is difficult to find original one 100/3. I' ll use this pre with converters without mic-amps.

The schematic shows a preamp stage that is part of a console. This internally only connects to known loading and voltage values like the this stage following eq.
If you want to make this a stand alone unit, you might take this into account.
For a heavy 600 ohm load a 100uF cap setting this HPF @ 2.6Hz. More common > 2k load will drop this HPF to less than 0.8Hz. Maybe missing something, but I see little use in increasing this cap to 470uF. Upping the voltage rating would protect it instead.

C2 and C4 seem polarity reversed (2 bipolar 4.7uF/63V caps for C1+C2 and C3+C4 might fit better, 2 huge 4.7uF poly caps might sound better).

I think C2 and C4 are right. Are they 10uF 80V???

Might be a drawing error or intentional reversed.
Looking at these 2x 10uF caps in series as a single 5uF cap and expecting +48V phantom voltage at the left/input side, this has to be the +end. Now switching phantom off, this input side drops to 0V, but the other side of the cap is biased  at ~+0.3V, so now more positive than input side, hence my suggested flipping C2 and C4, or substituting the series caps C1+C2 and C3+C4 by single 4.7uF/63V bipolar or poly caps.