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Old 7th December 2011, 11:52 AM   #1021
qusp is offline qusp  Australia
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i knew there was something going on i didnt see properly, initially i thought the gain was 0.1, then it started to look weird and i gave up to go have a coffee and shower. note to self, never use words like definitely when your spidey senses are tingling. i didnt take into account the initial very basic 1 in the 1.1 on my initial glance, then 'over thunk' it forgetting the buffer wrapped in the loop and thus the non inverting FB runs the other way towards ground from the output of the lme49990 through the buffer and back through the 1k0, departs to the inverting input with the 10k0 to ground, while the non-inverting input also flows straight through 1k0, splits off to the input while 10k0 goes to ground; so the symmetry works out with the 0.1 + 1

doh, now that cuts one down to size

Last edited by qusp; 7th December 2011 at 12:12 PM. Reason: added inverted commas
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Old 7th December 2011, 12:27 PM   #1022
opc is offline opc  Canada
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qusp:

It's an easy mistake to make at a glance, so I wouldn't worry about it if I were you!

In fact, I actually built the first version as it was in the original schematic thinking everything was good, then when I measured it I got a gain of 1.81 and thought "WTF is going on here" before I realized that I had forgotten the +1 for the non-inverting setup.

I have tested the amp at a gain of 2 (and 1.8) and both work just as well, but I have yet to test at 3x or 4x. I'll get around to it eventually, but for now I would say that it's safe to build with gains up to 4x at least, and much beyond that there may be a need to increase Rf value along with the Rg value.

Cheers,
Owen
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Old 7th December 2011, 12:59 PM   #1023
sek is offline sek  Germany
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Regarding differential current drivers as an output stage:

Quote:
Originally Posted by qusp View Post
doesnt exist for audio, perhaps there are some RF or ADSL line drivers that provide a high current differential output
ADSL line drivers can make for good audio devices, just consider the TPA6120A2 ("specified for audio" version of an ADSL counterpart).

But look how they do it. With dual single ended buffers in the feedback loop of a differential input stage. There are other approaches with all of the stages on the same die, too. But the principle stays the same.

So, no, even in that field it's done exactly the same way.
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Old 7th December 2011, 02:03 PM   #1024
Wolfsin is offline Wolfsin  United States
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You're still my favorite curmudgeon, qusp
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Old 7th December 2011, 02:12 PM   #1025
qusp is offline qusp  Australia
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Quote:
Originally Posted by sek View Post
Regarding differential current drivers as an output stage:



ADSL line drivers can make for good audio devices, just consider the TPA6120A2 ("specified for audio" version of an ADSL counterpart).

But look how they do it. With dual single ended buffers in the feedback loop of a differential input stage. There are other approaches with all of the stages on the same die, too. But the principle stays the same.

So, no, even in that field it's done exactly the same way.
sek, i'm really not quite sure what you are contesting. yet here you are again picking a line out of my post,m ignoring the rest and taking it out of context so you can more adequately disagree with it.
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Old 7th December 2011, 07:03 PM   #1026
sek is offline sek  Germany
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That was in support of your point (of ADSL drivers not being better, as AFAIK they incorporate the same idea). Go figure.
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Old 7th December 2011, 11:26 PM   #1027
L-Train is offline L-Train  Canada
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Thanks for the great explanation Owen, I'm really looking forward to building this thing
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Old 8th December 2011, 12:39 AM   #1028
Nevod is offline Nevod  Russian Federation
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Quote:
Originally Posted by qusp View Post
several reasons not to do that.

for starters, an instrumentation amplifier usually has (perhaps by definition has to have) a differential input and a single ended output, how would this be useful for a balanced output amp? you would need to have 2 of them and supply it with a dual differential dac output to get a single balanced output.
There is a scheme of an instrumentation amplifier with differential input and differential output, consisting of 2 SE-out opamps and 1 diff-out opamp.
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3 x opamps for starters doesnt make sense for balanced output unless performing a balanced-SE conversion, or if one of them was a differential I/O chip like the opa1632, noise performance with 2 series active elements will compound and be worse than the single diff chip. thermal and therefor common mode coupling/matching will be worse and thus noise performance will suffer again. this is handling 2 phases of the same channel, not 2 different channels. there is no benefit at all to using separate chips as could be argued (in a rather academic fashion) when talking about a left and right channel.
Then, well, why the BAL-SE version uses 3 op-amps, when 1 op-amp already can convert balanced input to a single-end output? Or it can't?
I understand about noise, but I don't think that at this level it's critical. I.e. -130dB or -160dB is anyways so low a noise it doesn't matter. While other moments may be significant. I've thought that 3 op-amp instrumentation amplifier circuits are used due to their high performance as in low distortion etc, and that was the reason to use it in The Wire?
Sorry for lame questions, I'm no expert in electronics.
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Old 8th December 2011, 01:39 AM   #1029
qusp is offline qusp  Australia
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Quote:
Originally Posted by sek View Post
That was in support of your point (of ADSL drivers not being better, as AFAIK they incorporate the same idea). Go figure.
hmm, well if i took you the wrong way (and it appears i did now reading back over it) my apologies, its what i'm used to you doing. i was confused because it seemed like you were countering me, without countering me.

but perhaps i was too jumpy there, i had been dealing with a bit of a situation we have going on here in Oz with a GB where the organiser appears to be pulling some kind of swifty, so was a bit wound up.
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Old 9th December 2011, 01:28 PM   #1030
Wolfsin is offline Wolfsin  United States
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Default A tiny 'parcel' arrived yesterday . . .

Wow for the Bal-Bal! Owen, take a bow. Encore, encore . . . :-)
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