Balanced Outputs

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Line Driver Distortion Comparison Using DIM

Postby mediatechnology » Fri Dec 02, 2011 3:32 pm

I ran some tests checking the relative distortion levels of the THAT1646, DRV134 and SSM2142 both with and without 22R/leg build-out resistors.

I decided to give the DIM method described by Leinonen, Otala and Curl a try.
See: http://www.ka-electronics.com/images/pd ... rement.pdf

Based on the high performance of all the line drivers I modified the test to increase the ratio from 4:1 (3.18kHz, 15 kHz) to 1:1.
There were DIM-30 and DIM-100 (30 and 100 kHz) measurements which limited the risetime. I did not apply rise-time limiting.
The line drivers are driving the 1000 feet of Brand-Rex cable, received by a 1246 and an FFT performed.
The differential drive was 24 V p-p.
The peak slew rate appears to be 6V/us.
Tests were run with the native 50 Ohm Zoutdiff and with 22R/leg added to provide an ~94 Ohm Zoutdiff.

The displayed FFTs show only two distortion products at ~2.28 kHz (f2-4f1) and ~4.08 kHz (f2-6f1).
I did this to simplify interpretation.

SSM2142 No Added Build-Out
Image
SSM2142_DIM-30M_0R

SSM2142 Zoutdiff ~ 94 Ohms
Image
SSM2142_DIM-30M_22R


DRV134 No Added Build-Out
Image
DRV134_DIM-30M_0R

DRV134 Zoutdiff ~ 94 Ohms
Image
DRV134_DIM-30M_22R


THAT1646 No Added Build-Out
Image
THAT1646_DIM-30M_0R

THAT1646 Zoutdiff ~ 94 Ohms
Image
THAT1646_DIM-30M_22R

The added build-out resistors seem to reduce the IM sidebands around 3.18 kHz in each case.

Note that the 1646 appears to out-perform the DRV134 by 6-8 dB when build-out resistors are not used.

DRV134 distortion tends to increase after about 30-60 seconds after the die temperature rises.
I can heatsink the DRV134 with my fingertip and lower the distortion.
I did not see thermal drift in the THAT1646.
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Re: Balanced Outputs

Postby mediatechnology » Mon Dec 12, 2011 10:48 am

ricardo had asked earlier in the thread about common mode chokes.

See: http://www.eeweb.com/blog/andrew_carter ... inductance

The common mode connection is shown in the lower two examples in figure 2:

http://www.eeweb.com/images/sized/image ... 00x318.png
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Re: Balanced Outputs

Postby mediatechnology » Wed Dec 14, 2011 5:51 pm

I ran some measurements using two THAT1646 operating in parallel under identical test conditions.
Outsmarts was enabled.

The first is 24V p-p showing the reduced sidebands from having twice the available drive current.
The differential output impedance is ~97 Ohms.

Image

The following image is 35 V p-p with a ~97 Ohm Zdifferential:

Image

Under identical test conditions two 1646 have approximately 15 dB better performance than a single THAT1646 or DRV134 and about 25 dB lower distortion (these two sidebands) than an SSM2142.

Comparing the above 24V p-p examples using two THAT1646 to a single THAT1646, DRV134 or SSM2142 in "stock" circuits without added build-out resistors the improvements are 25, 31 and 33 dB respectively.
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Re: Balanced Outputs

Postby mediatechnology » Wed Jan 11, 2012 8:27 am

I wanted to post one more set of FFTs.

This one is the "Dual Class-A Output" we've shown previously as a headphone amplifier.
The cable load and line receivers are the same test circuits as before.
The build-outs are 47R/leg ~94 Ohms differential.
Drive for the dual Class-A is from the cross-coupled "double-balanced" 1286.


The Dual Class-A output at 24V p-p.
Compared to the SSM2142 with no build-out the DIM products are about 35 dB lower.

Image
Class-A 24V p-p 1000 feet

This is at 55V p-p differential.
None of the other amps could handle this test without serious distortion.
The performance shown below is similar to a THAT1646 (with no build-out added) but at a level that is ~7dB hotter. (55V p-p vs. 24V p-p.)

Image
Class-A 55V p-p 1000 feet

THD Driving the line differentially at +24 dBu is nearly equal to the generator residual.
Image
THD Class-A +24dBu 1000 feet

It's a current slewing thing...
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Re: Balanced Outputs

Postby Noise50Hz » Sun Jan 13, 2013 5:34 pm

Hi people here.
Sorry to wake up this thread asleep for one year.
I'm stuck in an issue with balancing a preamp. I read this 20 pages but my mean English and electronics skills didn't help me to resolve it.
If this is a forum to get help, can I post here below, or should I open a new thread?
Thank you
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Re: Balanced Outputs

Postby mediatechnology » Sun Jan 13, 2013 5:36 pm

Go ahead and ask in this thread and we will see where it heads.

Thanks for joining us and posting!
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Re: Balanced Outputs

Postby Noise50Hz » Mon Jan 14, 2013 6:33 pm

Thank you Wayne (I think you are Wayne isn't it?)
First, sorry I forgot it in my first post, have a happy new year, you and all people here.
Here is the story: In my 20 years in the 80' I built a preamp and amp according to Elektor's magazine, namely the "Crescendo" and "The Preamp". Later I moved to more little house where I did not had room enough to host my home made boxes. Then I recovered room and last year I decided to plug again the boxes to the mains. In the lap time, a huge change happened; do you guess?... Internet arose all around :). Do eldest remember and can youngest imagine how making one's first steps in electronics was more difficult without it?
OK, enough with the long story, the more recent one now: when I plugged the boxes I got huge hum that a gentle French guy in the diy forum helped me to remove after I dug around for ground loops without any success. Once done, I found my passive home-made LS filters were say... rubbish.
Unfortunately, a lower hum remained, I could not say it was LS filters' fault, so I decided to tweak the amp and preamp, with the help of affordable hardware we can find nowadays, active LS filters, so the way went: go active + go balanced. After digging the net around, I first discovered there were dedicated OAP for this, SSM and DRV things (OAP and few build-out components got my preference because of room in the boxes and also because in the mid 2000's I lost my job in the electronics facility of my 20's, which was a big help to do my own PCB and find/buy components, and also because my audio system and skills do not pretend to reach far-end quality), and in the end I found the THAT's chips would be the definitive ones. 120x and 1646 would be the ones, 1646 instead of 1606 because I do not dare SMD.
So I made my own PCBs design and built a bunch of them, and tested successfully a pair of balanced outputs in a recent preamp I offered to my daughter this very last Christmas (she already has pair of bal/unbal amplified shelf-speakers).
For my own preamp I have an issue with the add-on line drivers. I designed the pcb driver to be able to choose to install or not install an additional OAP (OPA227) depending on the genuine outputs impedance, to reach the THAT1646 source requirements. For my daughter's preamp, because of the 1kΩ outputs, I used the OPA227. In mine I cut PCB to remove the unwanted OPA227 as the outputs are direct from the original OP27 through a 15µF film capacitor.
When I bring my hands near to the metal box I get a growing hum (up to huge) on the bal outputs although the case is earthed and grounded. I draw a picture of the output, that shows how the supplies and zero are wired.
When the add-on input signal is picked directly from OP27 output instead of directly on RCA after the 15µF, the hands effect is no more there. But if I plug a phone amp (Sennheiser 170 transmitter) in unbal outs, I get hum in bal outs. A very strange thing is that even if I just touch one preamp's out RCA ring with one Sennheiser's input RCA ring (RCA tips untouched at preamp and phoen amp) I get similar hum as when RCA are plugged the normal way.
Sorry for this long description.
Please, I would really appreciate any suggestion. thank you in advance.
Fabrice
Attachments
bal with OPA227.PNG
My driver pcb
bal with OPA227.PNG (25.82 KiB) Viewed 2852 times
tweak.png
wiring my add-on
tweak.png (366.56 KiB) Viewed 2852 times
Last edited by Noise50Hz on Wed Jan 16, 2013 8:12 am, edited 1 time in total.
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Re: Balanced Outputs

Postby mediatechnology » Tue Jan 15, 2013 6:44 am

Fabrice - Thank you for joining us and Happy New Year to you as well.

I haven't looked at it too closely but I see a few things.

First, in the layout it appears that the 1646 input, pin 4, is tied to the OP27 inverting input summing node and not the output.

In the schematic and layout it also appears that you are attempting to "telescope" the THAT1646 pin 3 to some other ground point location.
I would not do that: Inductance at pin 3 - and it takes very little - will make it oscillate. Ground pin 3 as close to 0V as you can.

The OP27 has no series isolation build-out resistor, 47R - 100R, to isolate it's output from capacitive cable loading.

I'll keep looking but these are the things I saw off-the-top.
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Re: Balanced Outputs

Postby Noise50Hz » Wed Jan 16, 2013 8:34 am

Thank you very much for feedback Wayne:
Resorted,

About OP27, you drove me in reading the datasheet, in which if found this "The OP27 provides stable operation with load capacitances of
up to 2000 pF and ± 10 V swings; larger capacitances should be decoupled with a 50Ω resistor inside the feedback loop. The OP27 is unity-gain stable."
Good info to dig. Maybe this was the culprit when I first used the yet unbalanced preamp with longer (8m) cables. I also have to check the input characteristics of the phone amp I use for the tests.

In my layout design, you found an error: well seen: only when used with gain resistors for the OPA227, the THAT1646 input is bound to the inverting input of the OPA instead of output: shame on me :oops: Fortunately (for me) I don't use the OPA (used a saw against the thin vertical yellow line), and for my daughter I used unity gain so everything was OK and I couldn't see the mistake. I'll publish the correct layout quickly.

Then the 0V/ground: this is the great headache when it comes to go to real world. Pin3 is a well located thing, when 0V is a widely spread one in the box, and so the frightening inductance hides itself between them, if I understand well. Reading things about that (e-g this very thread pages beginning at ~ #5 / #6) I learnt I'd try to remove this evil, but I remained stuck to the way a resolved a previous issue, namely, building a star for 0V. So I though I'd better pick the 0V for the 1646 pin3 directly at genuine RCA rather than between the 1646 supply decoupling caps, because I was afraid the current sunk by the 1646 driving ~long~ lines would pollute Pin3 reference. Is this my (other) mistake Wayne? I see 2 other ways to link Pin3 to 0V: first one, to the 1646 supply 1000µF/100nF caps node in my layout, the wire to 0V bus being already here, or second one, through an additional dedicated wire (~10 inches) for Pin3 to 0V bus where I pick the supply?

Thanks again (surely they won't be the last ;) ) for your fine advises Wayne.
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Re: Balanced Outputs

Postby mediatechnology » Wed Jan 16, 2013 10:59 am

Glad to help.

Well, you really don't want the 1646 input connecting to an RCA jack at all.

The 1646 pin 4 differential input requires a near-zero source impedance driving it or output balance and noise suffer.
Once you buffer the 1646 input with an op-amp there is no reason to telescope pin 3 to the RCA.
One thing you can do to reduce the risk of oscillation is to try a 100 nF directly from pin 3 (gnd ref) to Vee pin 5.
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