RCA to balanced XLR - will this work on a DSP?

[simont]

Hi all,

I have a MiniDSP Flex with RCA connections that I use as a DAC and pre-amp. I have started looking into power amp upgrades (I currently use a 1995 NAD 302) and see that many use balanced XLR inputs, either exclusively or preferentially. I know that RCA to XLR connections are not ideal in general, but after reading up on the subject, I got the following idea.

Is it possible to create an RCA to balanced XLR adapter, by making use of the DSP's four RCA outputs?

It would be configured as follows, duplicated for each audio channel (left/right):
• RCA1 carries the regular signal
• RCA2 carries the phase-inverted signal (this can be configured in the MiniDSP software)
• XLR input expects a balanced signal

Connecting both RCAs to an XLR input would require Y-adapter cable, with the conductors linked as follows:
• connect RCA1-signal to XLR-pin2
• connect RCA2-signal to XLR-pin3
• connect the cable sleeve to XLR-pin1, RCA1-ground and RCA2-ground
For the cleanest signal, it's probably a good idea to ensure that the Y-junction is as close as possible to the RCA end and that the signal wires are twisted in the section from the junction to the XLR terminal.

I have searched the forum and the wider internet for examples, but didn't find any, but I think it should work. It should offer a 'true' differential balanced connection, if I'm not mistaken, and with double the signal strength (4 Vrms instead of 2 Vrms, in my case).

Do you see any issues with such a setup?

Thanks!
Simon

 

[twsecrest]

Unless both jacks and plugs are XLR (or the 1/4" TRS) balanced will not perform it's intended function.
Better to just make an unbalanced connection, like RCA to RCA.

 

[NTK]

Hi all,

I have a MiniDSP Flex with RCA connections that I use as a DAC and pre-amp. I have started looking into power amp upgrades (I currently use a 1995 NAD 302) and see that many use balanced XLR inputs, either exclusively or preferentially. I know that RCA to XLR connections are not ideal in general, but after reading up on the subject, I got the following idea.

Is it possible to create an RCA to balanced XLR adapter, by making use of the DSP's four RCA outputs?

It would be configured as follows, duplicated for each audio channel (left/right):
• RCA1 carries the regular signal
• RCA2 carries the phase-inverted signal (this can be configured in the MiniDSP software)
• XLR input expects a balanced signal

Connecting both RCAs to an XLR input would require Y-adapter cable, with the conductors linked as follows:
• connect RCA1-signal to XLR-pin2
• connect RCA2-signal to XLR-pin3
• connect the cable sleeve to XLR-pin1, RCA1-ground and RCA2-ground
For the cleanest signal, it's probably a good idea to ensure that the Y-junction is as close as possible to the RCA end and that the signal wires are twisted in the section from the junction to the XLR terminal.

I have searched the forum and the wider internet for examples, but didn't find any, but I think it should work. It should offer a 'true' differential balanced connection, if I'm not mistaken, and with double the signal strength (4 Vrms instead of 2 Vrms, in my case).

Do you see any issues with such a setup?

Thanks!
Simon

Interesting idea. Can't think of a reason why it won't work. The RCA grounds should be all internally connected inside the MiniDSP Flex Unbalanced.

 

[simont]

@twsecrest : Yes, I agree that seems to be how it is without active components, but my point is that the DSP (which is already in the loop) can make a difference here.

@NTK : Thanks. Indeed, this does rely on the assumption that the two RCA outputs share a common ground. As you say, I see no reason why that wouldn't be the case, and otherwise shorting them with the cable will take care of that

By the way, what would the output impedance of this setup be? It is 200 ohm on the RCA connection. If the 200 ohm is fully allocated to the signal side, I'd say it becomes 400 ohms...

 

[Jukka]

I've thought about doing this, but all my gear uses XLR, so I never went through the effort... Theoretically you got the got and cold pins right. But XLR shield, pin 1, is supposed to go to ground and consumer RCA sleeve is often ground lifted (floating), which can conduct the noise from the shield into a wrong place in the RCA device.

I'd suggest trying to do pins 2 and 3 as you stated, but lead pin 1 to ground some other way. If it's case is safety grounded, that's what I'd try first, that's what pro gear XLR pin 1's go to.

PS. Balanced interconnect is by definition a 2-signal pin connection with the same impedance, so this is not a guaranteed success, but worth trying nevertheless. If it fails, you can get almost the same result with adapter made with microphone cable. Hypex has useful white papers/app notes for this.

 

[twsecrest]

@twsecrest : Yes, I agree that seems to be how it is without active components, but my point is that the DSP (which is already in the loop) can make a difference here.

@NTK : Thanks. Indeed, this does rely on the assumption that the two RCA outputs share a common ground. As you say, I see no reason why that wouldn't be the case, and otherwise shorting them with the cable will take care of that

By the way, what would the output impedance of this setup be? It is 200 ohm on the RCA connection. If the 200 ohm is fully allocated to the signal side, I'd say it becomes 400 ohms...

Nope, I really doubt the DSP function will make a difference when using an un-balanced connection (RCA) and balanced connection (XLR) together.
If you split the RCA signal, into two separate signals (XLR) your just getting 50% of the volume, half the loudness
My best guess.

 

[NTK]

See this reply from an Analog Devices engineer to a similar question.

Balanced IN/OUT on ADAU1701 - Q&A - Audio - EngineerZone

Hello, I am currently designing AMP-DSP board for an active nearfield-Monitor (2 Way bi-amped). As AMP i will use a TPA3116, which has 2 Balanced inputs. And

ez.analog.com

 

[sam_adams]

Differential signaling is not the only thing that makes a connection a balanced connection.

 

[simont]

I've thought about doing this, but all my gear uses XLR, so I never went through the effort... Theoretically you got the got and cold pins right. But XLR shield, pin 1, is supposed to go to ground and consumer RCA sleeve is often ground lifted (floating), which can conduct the noise from the shield into a wrong place in the RCA device.

I'd suggest trying to do pins 2 and 3 as you stated, but lead pin 1 to ground some other way. If it's case is safety grounded, that's what I'd try first, that's what pro gear XLR pin 1's go to.

PS. Balanced interconnect is by definition a 2-signal pin connection with the same impedance, so this is not a guaranteed success, but worth trying nevertheless. If it fails, you can get almost the same result with adapter made with microphone cable. Hypex has useful white papers/app notes for this.

I must admit I had to look up what ground lifting does, but that makes good sense indeed. The Minidsp Flex itself is not grounded, but it does have a grounding connection at the back of the case, so it definitely wouldn't hurt to attach pin 1 (via the cable sleeve) to that point.

 

[simont]

Nope, I really doubt the DSP function will make a difference when using an un-balanced connection (RCA) and balanced connection (XLR) together.
If you split the RCA signal, into two separate signals (XLR) your just getting 50% of the volume, half the loudness
My best guess.

To clarify, I'm not talking about splitting the RCA into two signals, but adding one RCA signal and its inverse to get a signal that has twice the amplitude, is symmetrical and (hopefully) balanced.

 

[simont]

Differential signaling is not the only thing that makes a connection a balanced connection.

Do you mean you don't think such a setup would be balanced? (I'm not entirely sure what to look for in the search results)

I agree that it's not the same thing. I had a look at the Wikipedia page (I know...) for balanced connections, which defines it as "a circuit consisting of two conductors of the same type, both of which have equal impedances along their lengths and equal impedances to ground and to other circuits." Without knowing the circuit diagram of the Minidsp, I'm guessing that the four RCA outputs have identical properties, so the resulting connection would be balanced. Although strictly speaking unrelated, it would also be differential and symmetric, I think?

 

[NTK]

Do you mean you don't think such a setup would be balanced? (I'm not entirely sure what to look for in the search results)

I agree that it's not the same thing. I had a look at the Wikipedia page (I know...) for balanced connections, which defines it as "a circuit consisting of two conductors of the same type, both of which have equal impedances along their lengths and equal impedances to ground and to other circuits." Without knowing the circuit diagram of the Minidsp, I'm guessing that the four RCA outputs have identical properties, so the resulting connection would be balanced. Although strictly speaking unrelated, it would also be differential and symmetric, I think?

For the intents and purposes we are talking here (audio line level signals from a DAC to an amplifier over a short distance), balanced means differential. What the amplifier amplifies is the signal voltage between the 2 wires. Differential (or "balanced") breaks the connection between the audio signal to the "ground". Thus, if the connecting devices are not at the same "ground" potential (thus causing a current flow in the shield wire between the devices), the audio signals are not affected.

The impedance of the individual signal lines to ground is not important here, as the goal is to break the connection between ground and signal.

 

[twsecrest]

Do you mean you don't think such a setup would be balanced? (I'm not entirely sure what to look for in the search results)

I agree that it's not the same thing. I had a look at the Wikipedia page (I know...) for balanced connections, which defines it as "a circuit consisting of two conductors of the same type, both of which have equal impedances along their lengths and equal impedances to ground and to other circuits." Without knowing the circuit diagram of the Minidsp, I'm guessing that the four RCA outputs have identical properties, so the resulting connection would be balanced. Although strictly speaking unrelated, it would also be differential and symmetric, I think?

Your overthinking, go RCA to RCA unbalanced connection.

 

[simont]

Thanks, all. I guess I know how to approach this if I could use such a converter.

@twsecrest, I agree that the practical benefit of creating such a setup for short cables is to be determined, but I do think there are two cases where it may come in handy:
1) If a power amp only has XLR inputs (if I must choose between RCA and XLR and buy 'for the future'). Of course, for RCA->XLR alone I could also use simpler adapter cables per the Hypex instructions.
2) If it enables me to use a low gain setting on the amplifier. Combining two RCA signals gives a 6dB signal boost, which should be sufficient to get away with the low gain settings of Eigentakt and Nilai500 modules.

 

[antcollinet]

Thanks, all. I guess I know how to approach this if I could use such a converter.

@twsecrest, I agree that the practical benefit of creating such a setup for short cables is to be determined, but I do think there are two cases where it may come in handy:
1) If a power amp only has XLR inputs (if I must choose between RCA and XLR and buy 'for the future'). Of course, for RCA->XLR alone I could also use simpler adapter cables per the Hypex instructions.
2) If it enables me to use a low gain setting on the amplifier. Combining two RCA signals gives a 6dB signal boost, which should be sufficient to get away with the low gain settings of Eigentakt and Nilai500 modules.

If you make the cable correctly (using 3 core cable) you should also get the benefits of noise rejection.

 

[Jukka]

NTK writes mostly nonsense. Balanced means balanced impedance of the two inner conductors. Differential input is the differential voltage of the two conductors in the input of the receiver. The voltage in the conductors can be anything as long as they are within acceptable range from each other (the other signal doesn't need to be inversion of the main signal, it can "a straight line" or even a ground and still function as expected). Impedance doesn't need ground. There is no balanced and "balanced", but there is BS. I learned all that from reading Hypex white papers.

Anyway, using just RCA or RCA-to-XLR adapter cable for short distances and home environment is usually fine. But if you get mains hum (50/60 Hz) or other static noises, XLR may resolve those issues. Personally I accept only XLR in my main gear, so I no longer have to deal with this kind of issues.

 

[NTK]

NTK writes mostly nonsense. Balanced means balanced impedance of the two inner conductors. Differential input is the differential voltage of the two conductors in the input of the receiver. The voltage in the conductors can be anything as long as they are within acceptable range from each other (the other signal doesn't need to be inversion of the main signal, it can "a straight line" or even a ground and still function as expected). Impedance doesn't need ground. There is no balanced and "balanced", but there is BS. I learned all that from reading Hypex white papers.

Anyway, using just RCA or RCA-to-XLR adapter cable for short distances and home environment is usually fine. But if you get mains hum (50/60 Hz) or other static noises, XLR may resolve those issues. Personally I accept only XLR in my main gear, so I no longer have to deal with this kind of issues.

Well, Bruno Putzeys refers to it as "balanced connection" too.

https://www.hypex.nl/img/upload/doc/an_wp/AN_Legacy_pin_1_problems.pdf

Well

 

[Jukka]

Well, Bruno Putzeys refers to it as "balanced connection" too.

https://www.hypex.nl/img/upload/doc/an_wp/AN_Legacy_pin_1_problems.pdf

View attachment 254602
Well

Read the long reply and the other papers on this matter as well, they give more insight. The short reply is for people with no patience.

 

[NTK]

Read the long reply and the other papers on this matter as well, they give more insight. The short reply is for people with no patience.

Does his "long reply" say anything different? Tell me when Putzeys said the voltage between the 2 wires in a balanced interface is the signal, what is the difference between that and a differential connection?

I'll say it is technically incorrect to equate differential to balanced. But that is how the audio industry equates the terms. Putzeys went along with it, and I with him.

 

[Jukka]

Does his "long reply" say anything different? Tell me when Putzeys said the voltage between the 2 wires in a balanced interface is the signal, what is the difference between that and a differential connection?

View attachment 254649

I'll say it is technically incorrect to equate differential to balanced. But that is how the audio industry equates the terms. Putzeys went along with it, and I with him.

That's mostly correct, but only when used in balanced interfaces on both ends. As for terms, differential input is the name for the technique to read a balanced output through XLR-cable. But please note, that ground can be wired to travel through any pin or even 2 pins of XLR-cable, if the source is single-ended (per this topic) and differential input can be used like that. And most likely there is gear with XLR outputs where either pin 2 or 3 is ground, since that is allowed by definition.