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Old 17th December 2010, 10:10 AM   #1
Kudlaty is offline Kudlaty  Poland
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Default Fully Balanced Input/Output

Dear Friends,

I currently designing preamp/headphone amp. I have some problem with design fully balanced input. It should accept balanced and unbalanced signal comming from input selector. This unit will be driving relay volume control.

Relay volume control input is about 2,5kOhm, but it would be great to lower is as this will lower output impedance and overall noise floor.

I would like to use AD8599 in this unit, but if you look into data sheet there is showed that THD vs freq for 2kOhm

load is much better than for 600Ohm. So I want to have load of AD8599 no lower than 2kOhm, but also I want to have S/N ratio better than 120dB. This is main problem. Gain for this input should be 10dB (best swichable to 16dB for unbalanced inputs).

I was thinking about using voltage followers (maybe on AD8397) in nested feedback on main outputs.

Can you help me with this design ??

U1, U2, U3, U5 it will be AD8599, and U4, U6 it could be AD8397.

For now I have two solutions:
1 - not symetrical, and U4 load is different than U6 load, noise is different on + and - output

2 - symetrical - it could be good, but S/N would be lower by about 3dB. I prefer this version.

Do you have some clever idea how to make this better

Best Regards
Kudlaty
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File Type: jpg solution v2.JPG (29.4 KB, 499 views)
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Old 18th December 2010, 01:09 AM   #2
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I would have a balanced input stage and a un-balanced input stage. Then an un-balanced selector and volume control. Finally a balanced output. When the signal is inside the box, un-balanced is fine.
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Old 18th December 2010, 04:21 AM   #3
jcx is offline jcx  United States
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it helps to know source impedance, V as well as supply V and headphone Z, sensitivity to know what to put in between

the AD8397 isn't totally happy in unity gain and its input doesn't like to swing full Vsupply so it should be used with gain of +2 or more - this can still be done inside a feedback loop
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Old 18th December 2010, 05:45 AM   #4
Kudlaty is offline Kudlaty  Poland
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Speedskater, I want to do this all balanced inside. I know it could be done unbalanced. The input stage for unbalanced output in on my schematics just look only on upper part. You can have one input stage for balanced and unbalanced and switch gain by relay (or even not ).Unbalanced is not the fun.


jcx,
Quote:
Originally Posted by jcx View Post
it helps to know source impedance, V as well as supply V and headphone Z, sensitivity to know what to put in between
I didn't write this well. This is one part of the project. Project consist of:
1)Input selector
2)*** Op amp input balanced/unbalanced - balanced output *** - this is it
3) Relay volume control - partially design
4) Line driver
5) Headphone amp - already designed


Quote:
the AD8397 isn't totally happy in unity gain and its input doesn't like to swing full Vsupply so it should be used with gain of +2 or more - this can still be done inside a feedback loop
It don't need to be AD8397. It could be better to use TPA6120 with gain a little like Walt Jung style.

I just asking you with help with topology, then I can continue.
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Old 18th December 2010, 06:24 AM   #5
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Why not use a fully balanced OP-amp like AD8132, AD8137, AD8138, LME49724 or OPA1632 ???
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Last edited by Robert GS; 18th December 2010 at 06:48 AM.
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Old 18th December 2010, 06:30 AM   #6
erin is offline erin  Australia
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How about an SSM2142 and SSM2141.
SSM2142 | Balance Line Driver | Audio Signal Processors | Audio/Video Products | Analog Devices

SSM2141 | High Common-Mode Rejection Differential Line Receiver | Audio Signal Processors | Audio/Video Products | Analog Devices
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Old 18th December 2010, 09:43 AM   #7
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Even better:
THAT Corporation 1606/1646 OutSmarts Balanced Line Driver ICs

I recently ordered parts to build a couple of those (circuit in datasheet).
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Old 18th December 2010, 08:10 PM   #8
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OK, so your heart is set on a fully balanced circuit:

Un-balanced to balanced buffer
Input selector (balanced)
Balanced buffer
Volume control (balanced)
Balanced output stage
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Old 18th December 2010, 08:32 PM   #9
Kudlaty is offline Kudlaty  Poland
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Thank you for your help.

I am a little bit afraid of fully balanced op amps, not from technical reasons, but from sonics.

You know AD8599 have very good opinion, about fully differential op amps there is less information.

Which one would you choose:

AD8132 - cheap, easy to buy, very fast, seams low THD
AD8137 - my supplier don't have it, cheap, very fast
AD8138 - cheap, low noise, fast
LME49724 - my supplier don't have it, impressive datasheet
OPA1632 - cheap, low noise, fast
SSM2141 - cost like others, but specification is far worse, higher noise, big resistors in feedback
THAT1646 - it is amazing for me, but my supplier have it, cost is ok, specification is ok,



For now I would give a try to OPA1632. Which one you would choose.

PS. Speedskater, almost as I want to do this, but selector will be before op amp input (not buffer because it has gain), this input can be make to accept balanced and unbalanced signals.
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Old 18th December 2010, 11:33 PM   #10
erin is offline erin  Australia
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Quote:
Originally Posted by Kudlaty View Post
I am a little bit afraid of fully balanced op amps, not from technical reasons, but from sonics.
I only recommended the SSM2141/2 based on sonics. When I have heard them inserted into the signal path my immediate surprise was that they were very neutral sounding and didn't hear a difference between them being in circuit or bypassed. just my 2 cents
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