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Old 1st January 2016, 08:34 AM   #1
Mayday is offline Mayday  Sweden
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Default Yet another headamp..a few questions

Well, I know I've got a ton of headamp projects going...but here's another one.
I got a, cheap, TPA6112 board to use with batteries(3xAAA) to use as a work station/aka solder station headamp.

It's also, according to ebay listing, able to do balanced to SE conversion.

Now to the question...the first one atleast.
How do I hook up a SE source (cellphone) to the balanced inputs of this tiny board?
How do I hook up "SHDN"?
Attached Images
File Type: jpg TPA6112_mini_headamp_1.jpg (164.0 KB, 180 views)
File Type: jpg TPA6112_typical_application.jpg (130.7 KB, 177 views)
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Old 8th January 2016, 01:43 AM   #2
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A phone should be a near-0-ohm source, so you should easily be able to connect signal --> +in, shared output ground --> -in, for both channels. (Ideally you want impedances to match exactly but I don't think you'll ever see more than 47 ohms on a phone, which isn't much of a mismatch vs. 20k.) Maybe another connection output ground --> power GND, but separate (e.g. shield of balanced cable), so that ground levels don't float off.

I don't think anything of this is super critical in your application. It's just that balanced can be super handy at times (e.g. for keeping pulses on the power rails from getting into input circuitry and worsening distortion, or keeping ground loops at bay), and it's fairly easy to attach unbalanced sources anyway. (See Bruno Putzeys' "G-Word" article.) With a purely unbalanced input you're possibly screwed.
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Old 8th January 2016, 08:13 AM   #3
Mayday is offline Mayday  Sweden
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Hi there.
And thanks

Something like this then? (see attached pic)

Is CTRL GND?
Attached Images
File Type: jpg SE_to_balanced_wiring_TPA6112_1.jpg (395.0 KB, 112 views)
File Type: jpg TPA6112_mini_headamp_1.jpg (164.0 KB, 98 views)
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Old 8th January 2016, 01:09 PM   #4
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I imagine CTRL connects to SHDN via 10k, but you'd have to verify that with a multimeter. Ask the datasheet re: how that's supposed to be hooked up or whether you can just leave it unconnected.

Re: signal connections, yep, that's what I meant. Should your source happen to have substantially nonzero output impedance, you would replicate that in the (-) signal connection directly at the SE output. In fact, a lot of semi-pro gear uses "balanced" outputs like that (the various Behringer xxx2496 devices come to mind). These can be readily adapted to SE without anything bad happening (important when using TRS jacks, as someone might be plugging in an instrument cable, which is "mono" TS only), while some other types of balanced outputs may end up pretty unhappy if you short out one side to ground. You'd need a "floating balanced" output otherwise.
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Old 8th January 2016, 03:05 PM   #5
Mayday is offline Mayday  Sweden
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Quote:
Originally Posted by sgrossklass View Post
I imagine CTRL connects to SHDN via 10k, but you'd have to verify that with a multimeter. Ask the datasheet re: how that's supposed to be hooked up or whether you can just leave it unconnected.

Re: signal connections, yep, that's what I meant. Should your source happen to have substantially nonzero output impedance, you would replicate that in the (-) signal connection directly at the SE output. In fact, a lot of semi-pro gear uses "balanced" outputs like that (the various Behringer xxx2496 devices come to mind). These can be readily adapted to SE without anything bad happening (important when using TRS jacks, as someone might be plugging in an instrument cable, which is "mono" TS only), while some other types of balanced outputs may end up pretty unhappy if you short out one side to ground. You'd need a "floating balanced" output otherwise.
Thanks for your help

Don't know if I'll go as far as to build some extra module for this as that would probably be more in parts value than the headamp module is worth.

The listing on ebay suggested this circuit could also be used as a balanced to SE converter (something like the DRV134's I'm guessing?).
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Old 8th January 2016, 07:45 PM   #6
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Quote:
Originally Posted by Mayday View Post
The listing on ebay suggested this circuit could also be used as a balanced to SE converter (something like the DRV134's I'm guessing?).
DRV134 is a balanced line driver, not a receiver. You will find that the TPA6112 circuit as shown is pretty much a bog standard bal/SE job like everyone and their dog uses in (home) studio gear (see aforementioned Putzeys article, Fig. 9/10), only in a single-supply version (observe R/RC voltage divider for virtual ground generation).

While the chip has a good bit more oomph to offer than your average opamp, the circuit's utility as an actual balanced line receiver would be limited by low supply voltage. Unless you were to swap out resistors to reduce gain considerably below unity, that is, but then you'd require extra compensation. A typical pro-level line receiver at unity gain would require no less than about a +/-15 V supply, since actual peak levels for a nominal +4 dBu can be up to +22 dBu or so, which is >9 Vrms.

BTW, there still is some tweaking potential left in this circuit:
It looks like there only is a single 100n bypass capacitor on the supply, C7. That won't cut it in battery operation - another ~220 F in parallel at the power input wouldn't be a bad idea.
The input coupling caps C1-C4 seem to be small ceramic jobs, probably some rather crummy dielectric at that kind of size. If you have some film caps floating about (ordinary polyester ones will do), you may want to use those externally and short out the original caps. Not sure what kind of quality the output coupling caps are, looks like pretty ordinary stuff.
You should also be able to tweak low-frequency PSRR and distortion performance a bit by adding some capacitance in parallel to the cap already on the bypass (+Vdd/2) pin (C8), maybe around a 10 F lowish-ESR electrolytic of good quality. Should reduce turn-on thump as well.

It looks like the exposed thermal pad may be soldered to the ground trace underneath. Better than nothing, but hardly what I'd call perfect heatsinking.
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Old 10th January 2016, 03:25 PM   #7
Mayday is offline Mayday  Sweden
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Thanks for your reply
I'm sorry for my late answer. This project is amongst the lowest in priority of the ones I work on atm.

I'll get back to it as other stuff gets done.
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