Line Amp Schematic Suggestions

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sr1200

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Dec 6, 2010
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I'm in need of a stereo variable line level amplifier that can push out about 16dB of gain.
Most of what i've found takes a balanced line in and gives me an unbalanced out.  Ideally, i would like a balanced output as well, but that would require another set of chips (if thats the way its gotta be, so be it, but im looking for as simple as possible).
I have a passive audio circuit im trying to drive harder to saturate.  If anyone has any suggestions for something simple, I'd love some suggestions! :) thanks
 
sr1200 said:
I'm in need of a stereo variable line level amplifier that can push out about 16dB of gain.
Most of what i've found takes a balanced line in and gives me an unbalanced out.  Ideally, i would like a balanced output as well, but that would require another set of chips (if thats the way its gotta be, so be it, but im looking for as simple as possible).
I have a passive audio circuit im trying to drive harder to saturate.  If anyone has any suggestions for something simple, I'd love some suggestions! :) thanks

Just take a design that takes balanced in and unbalanced out. Then modify to give a simple impedance balanced output.
Typically minimum extra component count of one resistor. Maybe two resistors and a cap if being a bit picky (although matching tolerance on caps may negate the advantage of this). Add a trimmer if you want to tweak CMRR at that end. But for general applications single resistor should do it on a standard op amp output.
 
Agreed. Impedance balanced is more than good enough for 90% of scenarios. So a THAT120X circuit into +16dB amp with an extra resistor to make it impedance balanced out is probably about what you want. But the resistor values depend on the load and the output level. What is the load impedance and desired output level?
 
Signal is coming from the interface which is 10-20k or something crazy high.  Output level is arbitrary since im just looking to saturate the input of the passive box.  Input impedance according to specs is something nuts like 140k or something. I was planning on having a 6db buffer on the out of the passive box to Make up the loss as well.
 
squarewave said:
Agreed. Impedance balanced is more than good enough for 90% of scenarios. So a THAT120X circuit into +16dB amp with an extra resistor to make it impedance balanced out is probably about what you want. But the resistor values depend on the load and the output level. What is the load impedance and desired output level?

Thanks. But I don't see that the output level is really relevant ? The resistor value just needs to be the same as the 'series output' resistor from the (assumed) op amp output. Typically 68R but in any case between 47R and 100R or so. That will generally avoid any cable loading problems due to capacitance on a reasonable cable length, Precision resistors preferred  but not essential assuming standard tolerance of 1% tolerance and Metal Film or SMT COG (MELF) or Thin Film  type. And, say, minimum 10K input impedance at the relevant input.
 
Newmarket said:
Thanks. But I don't see that the output level is really relevant ?
If the load is low and the impedance balancing resistors are large, then the output amp could clip. For example, if the load was 600R and the output level required to saturate it was say +20dBu, that's 11V peak into 600R which is 18.3mA. So 18.3mA through a 100R impedance balancing resistor is 1.8V drop on top of the 11Vp = 12.8Vp on the output of the line amp. That's about where a typical amp would clip depending on the amp and it's supply.

But if the load is 140K, it's not an issue.
 
squarewave said:
If the load is low and the impedance balancing resistors are large, then the output amp could clip. For example, if the load was 600R and the output level required to saturate it was say +20dBu, that's 11V peak into 600R which is 18.3mA. So 18.3mA through a 100R impedance balancing resistor is 1.8V drop on top of the 11Vp = 12.8Vp on the output of the line amp. That's about where a typical amp would clip depending on the amp and it's supply.

But if the load is 140K, it's not an issue.

Yeah. I trust the maths (note its maths not math  :)). I wasn't thinking about 600 Ohm inputs - I don't have anything so 'esoteric' !
 
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