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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jcx

Member
2003-02-17 7:38 pm
..
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
 
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,
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


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.
 
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.
 
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 :)
 
If you convert the un-balanced input to balanced before the selector,you can:
a] Adjust the input gains to the same level.
b] Build a more symmetrical selector and level control.

It's not easy to build a balanced circuit that is symmetrical at high frequencies. Heck, lots of 2 conductor shielded cable is not symmetrical.
 
Speedskater, I think that one additional metal-metal contact (relay) before input is not problem. I can adjust input level to be the same by using one relay and 2 resistors in feedback circuit. I have seen pre amps with op amps for each and every input. Days before I thot that this is good ideal, but now... Think what happens if the are with out any cable on input in noisy environment. All that noise will be on power lines. Cost is much higher. It is easer and cheaper make perfect input - best op amps you can afford (you need only fie) and listen...