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May you help me converting XLR to RCA ?

ohtoulouz

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Hi guys,
After reading docs about XLR to RCA conversion here and there, I still have questions about pluging a balanced DAC to an unbalanced AMP in a specific case: I have a HRT musicstreamer II+ (unbalanced) and a HRT musicstreamer PRO (balanced). I would like to use the PRO as unbalanced, like the II+, with a XLR to RCA adapter. II+ is said to deliver 2,25 V RMS. PRO is said to deliver 4,5 V RMS.
My question is: Is it right to say that signal at the AMP input would be 4,5V RMS with XLR pin 3 shorted to pin 1 and 2,25V RMS otherwise; and thus, that PRO should behave like II+ if pins 1 and 3 are not shorted. If I am right, the other reason for not shorting pins 1 and 3 is that 4,5V RMS seems (at least for me) high for an unbalanced input.
But I might be completely wrong due to my poor knowledge in electronics... You tell me.
Thanks for your help (and sorry for my frenglish...)
Pierre.
 
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ohtoulouz

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Replying to myself, as it may help someone else.
Perfect matching between PRO and II+ with the following adaptation:
XLR side:
cable 1 connected to pin 1
shield connected to pin 1
cable 2 connected to pin 2
nothing connected to pin 3
RCA side:
cable 1 connected to ground
cable 2 connected to center pin
shield not connected
Pierre.
 

Digby

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I think you can buy XLR to RCA cables, might save some time and energy?
 

LTig

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Replying to myself, as it may help someone else.
Perfect matching between PRO and II+ with the following adaptation:
XLR side:
cable 1 connected to pin 1
shield connected to pin 1
cable 2 connected to pin 2
nothing connected to pin 3
RCA side:
cable 1 connected to ground
cable 2 connected to center pin
shield not connected
Pierre.
This the correct solution for your devices but it might be different for others. In case the balanced source uses a transformer at its output you must short pins 1 and 3. The same could be true for a floating output circuit.
 
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ohtoulouz

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Yes, probably. I have read that "shorting pins 1 and 3 or not shorting pins 1 and 3" greatly depends on the output architecture. But something remains unclear to me: when shorting pins 1 and 3 is recommended (output transformer or cross-coupled op amp), what is the drawback of not doing so? Is it simply a lower output voltage? Or is it much more than that?
 

Zek

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Is it simply a lower output voltage? Or is it much more than that?
Some manufacturers do not recommend that balanced outputs be connected to unbalanced amplifier inputs, as this may result in damage of the source output stage (e.g. balanced DAC output).
 

NTK

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Yes, probably. I have read that "shorting pins 1 and 3 or not shorting pins 1 and 3" greatly depends on the output architecture. But something remains unclear to me: when shorting pins 1 and 3 is recommended (output transformer or cross-coupled op amp), what is the drawback of not doing so? Is it simply a lower output voltage? Or is it much more than that?
The signal from a balanced (differential) output is the difference between pin 2 & pin 3. When connecting to a single ended input, if you connect pin 3 to chassis/signal ground (i.e. pin 1), the receiving end will receive the proper voltage difference between pins 2 and 3.

If pin 3 is left unconnected, there is no guarantee that the receiving end will be sensing the true difference between pins 2 & 3, as it only senses the difference between pin 2 and ground. You may run into problems such as DC or other offsets.
 

LTig

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Yes, probably. I have read that "shorting pins 1 and 3 or not shorting pins 1 and 3" greatly depends on the output architecture. But something remains unclear to me: when shorting pins 1 and 3 is recommended (output transformer or cross-coupled op amp), what is the drawback of not doing so? Is it simply a lower output voltage? Or is it much more than that?
Not shorting pins 1 and 3 with an output transformer will lead to no sound at all. With a cross coupled amp I'm not quite sure whether it really matters at all, need to see the schematics. At the end the best is to just do what the manual recommends.
 
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ohtoulouz

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OK, Thank you.
I have just found a review of the PRO saying: "The maximum level at 1kHz from the balanced outputs was to specification, at 4.5V RMS, and was exactly half that from the single phase used for the TiniQ/RCA adapter supplied with the review sample". I think this confirms that pins 1 and 3 should not be shorted. Strangely, the review is also saying: "With both the TiniQ/XLR and TiniQ/RCA adapters and the balanced cable supplied by HRT for the review, the Music Streamer Pro's output was in inverted polarity". I suppose that pins 2 and 3 are inverted. Is it a problem? I do not think so, as far as left and right channels are wired the same way. Wihout knowing if the problemw come from the cables or from the DAC internal wiring, I cannot solved the problem.
Well, XLR world seems quite complicated...
 

LTig

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Strangely, the review is also saying: "With both the TiniQ/XLR and TiniQ/RCA adapters and the balanced cable supplied by HRT for the review, the Music Streamer Pro's output was in inverted polarity". I suppose that pins 2 and 3 are inverted. Is it a problem?
No, and you can still rewire the XLR cables if it bothers your mind.
 
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ohtoulouz

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I think I will end the story with this last post. I got additional information that are partly useful, but it is a little bit time-consuming to go deeper and deeper on this topic.

I have found a photo of the Musicstreamer PRO PCB on the web: it is using 2 TI4131 FDA, one per channel, that are used to transform the SE signal from the DAC chip into a balanced signal. According to TI4131 datasheet, this is done by sending the SE signal to one input pin and grounding the other one (pins 1 and 8). Vout+ is then got from pin 4 and Vout- from pin 5 of TI4131. This is what seems to be done on the PCB:

HRTPro.png


Looking at the following TI4131 diagram, I am not clever enough in electronics to determine if grounding one of the output pins is necessary or not, or even detrimental, for balanced to unbalanced conversion. If someone clever enough has some answer about that, he is more than welcome!
TI4131_3.png

As far as polarity inversion mentionned by the reviewer is concerned, I think that I have understood what is going on:
Tiny XLR hot pin should be the center pin (2) and cold one should be the upper pin (3) on this PCB diagram. So, pin 2 is probably connected to the lower output lane of the TI4131 and pin 3 to the upper one. Looking at TI4131 pins, this means that SE signal from the DAC chip is connected to Vin- (TI4131 pin 1), XLR pin 2 is connected to Vout+ (TI4131 pin 4) and XLR pin 3 is connected to Vout- (TI4131 pin 5):
TI4131_2.png

TI doc has also interesting simplified diagrams and equations:

TI4131_4.png

TI4131_5.png


So because the DAC chip output is connected to Vin- (beta2=0) and Vin+ is grounded (beta1=1), Vout+ = -Vin- + .... Thus, pin 2 of the XLR output is inverted compared to the input signal: polarity inversion is due to the internal wiring of the Musicstreamer.

Pierre.
 

LTig

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Looking at the functional diagram you must not shorten the unused output. Just leave it open.
 

Extreme_Boky

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That DAC uses a current-out, differential output DAC. The current-to-voltage conversion is done exactly the same as on page 18, figure 23 of the TI PCM1794 data sheet; HRT used 5534 x 4.

Instead of summing the differential signals (after this first I/V stage) into RCAs single-ended outs, HRT decided to keep everything differential all the way to the XLRs by implementing low output impedance fully balanced ins and outs driver ICs.

You could add the differential-to-single needed stage inside that DAC... if you want... as per page 20, figure 24 (one LT1028 per channel)... and then you'll have a true differential as well as RCA outputs at your disposal.

I'd use only one half of XLRs for RCAs outs... (if you do not want to do the above...) the other half should be loaded with let's say.. 100ohm resistors to ground. Which side (hot or cold) to use for RCA?? Try both and see which one sounds better in your system.

1682944082655.png
 
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