Can i make a balanced preamp only using OPA2134 ? - diyAudio
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Old 1st November 2012, 10:33 PM   #1
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Default Can i make a balanced preamp only using OPA2134 ?

the title of the tread says all...
i have 10 opa2134 so i want to use it
in my PA soundsystem i have this problem:
the output voltage of the electronic 4way crossover is not enough to bring the power amplifier near the "clipping region"(full power),neither if i make all its input and output gains to the max.
so i think that a preamp with a +15dB gain is what i need.
for an unbalanced preamp i was looking at the project88 of sound whesthost, but i need a balanced one,so i need some help.
some question:
i really need a differential opamp like the opa1632 after the two stage of the preamp?
cause i don't need of volume or balance pot between the two stage, i really need of two stage for the preamp?
thanks in advantage for your reply!
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Old 1st November 2012, 11:00 PM   #2
jcx is online now jcx  United States
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if you don't need extreme CMRR then you can roll your own with several op amps and ratio matched resistors

"instrumentation amplifier" is another name, can be done with 2 or 3 op maps:

Building a differential amplifier : OPERATIONAL AMPLIFIERS

Doug Self has a book on preamp design

some info in http://www.douglas-self.com/ampins/b...d/balanced.htm - fig 13 is a 2-op amp hi-Z differential reciever

Last edited by jcx; 1st November 2012 at 11:07 PM.
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Old 1st November 2012, 11:26 PM   #3
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yes,but in the schematics i've seen for balanced preamp,there was (after the preamp stage) opa 1632 that is a fully differential opamp and has 2 different output, (out+/out-)
the instrumentation amplifier has just 1 out...
or i've missed something?

i'm studying something like this schematics,but i would not required the opa1632..

Last edited by PuccioQM; 1st November 2012 at 11:40 PM.
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Old 1st November 2012, 11:43 PM   #4
jcx is online now jcx  United States
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strictly speaking "balanced" in audio interfaces only refers to balanced impedance in the +,- lines source's output, and differential receiver with balanced (or very high) Z inputs

the differential receiver gives common mode rejection - breaks gnd loops, often cures hum

if your amps have balanced inputs then you only need to match the series Z in +,- lines of your pre's output and can use the instrumentation amp output gnd as the negative line out "source"

or you can invert the instrumentation output with another op amp if you think you need bipolar drive of the +,- balanced amp inputs

Last edited by jcx; 1st November 2012 at 11:47 PM.
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Old 1st November 2012, 11:51 PM   #5
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i'm trying to do a preamp similar to this schematics, preferably without using the opa1632 (that i have to buy).

i have balanced output at the crossover and i have balanced input at the power amplifier,i have to rest balanced even in my preamp.
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Old 2nd November 2012, 12:10 AM   #6
jcx is online now jcx  United States
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I can't get excited over "eye candy" symmetry when there is little reason to belive in a audible difference - much less clear "better"

instrumentation amp diff receiver > volume pot (or not) > output buffer/amp (skip this too if you have enough gain in the front end)

put the same value R in series with the pre's buffer/amp output and the local gnd and be done


do look through Doug Self's balanced audio page - some of his diff amp cicuits can be changed to have fixed gain by changing the R ratios

for instance his fig 12 R3,4 can be upped to give gain, fig 4b shows the balanced impedance output

Last edited by jcx; 2nd November 2012 at 12:19 AM.
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Old 2nd November 2012, 09:49 PM   #7
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OK
yesterday i have not seen your edit and i didn't understand your post.

the circuit in fig12 is very interesting
i can up r3 and r4 to 47K to have a 4.7voltage gain (it's right?) letting all other resistance to 10K.
the input resistance of the amplifiers is just 10K for the hot and the cold input.
you think it will be fine?

thank you for helping me!
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Old 2nd November 2012, 10:58 PM   #8
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Puccio, I have a couple hundred OPA1632 and THS4131 if you need any.
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Old 3rd November 2012, 12:23 AM   #9
jcx is online now jcx  United States
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at least use 1% metal film which are pretty cheap nowdays, should keep CMRR over 24 dB without sorting

but even a cheap digital multimeter can sort R to much better than 1% match - if not absolute accuracy

the formula for diff input Z is a little more complicated but it is balanced, and higher than the input R
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Last edited by jcx; 3rd November 2012 at 12:27 AM.
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Old 3rd November 2012, 02:09 PM   #10
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ok, i have several 10K and 47K 1% metal film resistor at home
i can sort it with a digital multimeter
in your design i think i have to add a R from hot input to gnd and one from cold input to gnd,something like 47K or less,to set input resistance,or not?
a CMRR low as 24dB could be a problem?
this pre is connect with the xover and with the amp via 50cm shielded cables so noise on common mode should be not so a real problem,or i 've do't understand the meaning of CMRR?
sorry for my newbie question
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