I m making a 2 channel amplifier that has 2 wires for each channel for the input that is used for RCA. Now I also want to use XLR along with RCA inputs but I’m not sure on the XLR as it has 3 wires Hot, Cold and Ground, how would the wire to a amplifiers board that has only 2 inputs for 2 wires not 3?
There is a long-standing debate over how XLR's should be used for unbalanced in and out. Everyone agrees that pin 1 is ground but which pin 2 or 3 should be hot is not always agreed upon. Most people say pin 2. Then there is the issue of what should be done with the other pin. Should it be grounded or open, or loaded with a resistor.
A balanced circuit mostly solves the debate except there are common mode rejection issues, and potential output short to ground issues. Years ago, when I was in the audio business, I often had to use an Ohmmeter to determine how a given piece of equipment was wired. On a couple occasions, I had to add jumpers from pin 1 to pin~3 on a mixer output. Sometimes we made special cables that swapped 2 and 3, rather than modify equipment.
A balanced circuit mostly solves the debate except there are common mode rejection issues, and potential output short to ground issues. Years ago, when I was in the audio business, I often had to use an Ohmmeter to determine how a given piece of equipment was wired. On a couple occasions, I had to add jumpers from pin 1 to pin~3 on a mixer output. Sometimes we made special cables that swapped 2 and 3, rather than modify equipment.
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Hmmm is there a circuit board that has rca and xlr input with a switch that switches between them both and connects to the input of amplifier board
Keep in mind that an XLR connection implies professional signal levels. That would be +4db today but some older broadcast gear was +8. RCA jacks imply -10 to 0db. I say "imply" as the physical jack doesn't care what the signal level is but connecting a -10 to 0db into a pro audio device expecting +4db may be a problem. Likewise feeding a +4db signal into a consumer RCA jack may cause an overload.
As for the pin3 problem I would leave pin3 unconnected, pin1 to the RCA shield, and pin2 to RCA center or hot. Note that the RCA shield may not be chassis ground but grounded elsewhere. Be sure to maintain the RCA ground isolation if present.
Now leaving pin3 open poses a problem if the source is transformer driven. In that case however the pin3 to pin1 jumper should be done on the XLR cable end connector. As for just tying pin3 to pin1 on your new chassis jack, this will short out the minus phase on an electronically driven balanced source. that can cause issues with some balanced gear. A switch on your amp to connect pin3 to pin1 would be a great compromise here.
If it were mine, I would install an SSM2141 chip and put a DPST switch between the SSM2141 output and the RCA jack. Install a simple voltage divider attenuation as needed if the SSM2141 output is too high. You should be able to get the +/-15 volts from the amp. Alternately, you can build a balanced receiver circuit with any audio OPAMP but the SSM series is much simpler. Burr Brown also make a competing device.
Yes you could use a transformer too. But a quality full range, low distortion, audio input transformer will cost you at least $100 per channel!
As for the pin3 problem I would leave pin3 unconnected, pin1 to the RCA shield, and pin2 to RCA center or hot. Note that the RCA shield may not be chassis ground but grounded elsewhere. Be sure to maintain the RCA ground isolation if present.
Now leaving pin3 open poses a problem if the source is transformer driven. In that case however the pin3 to pin1 jumper should be done on the XLR cable end connector. As for just tying pin3 to pin1 on your new chassis jack, this will short out the minus phase on an electronically driven balanced source. that can cause issues with some balanced gear. A switch on your amp to connect pin3 to pin1 would be a great compromise here.
If it were mine, I would install an SSM2141 chip and put a DPST switch between the SSM2141 output and the RCA jack. Install a simple voltage divider attenuation as needed if the SSM2141 output is too high. You should be able to get the +/-15 volts from the amp. Alternately, you can build a balanced receiver circuit with any audio OPAMP but the SSM series is much simpler. Burr Brown also make a competing device.
Yes you could use a transformer too. But a quality full range, low distortion, audio input transformer will cost you at least $100 per channel!
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You wouldn't. You need a balanced input stage first. In light of the generally higher levels, I would recommend one with a -6 dB gain, e.g. using 20k + 10k or 10k + 5k1/4k7 resistors, respectively. (Ref. to this circuit, the first value is R1/R2, the second is R3/R4. The opamp preferably should have good input stage symmetry, so maybe not a 5532, but most audio opamps suited for mid-highish impedances should get the job done. A trusty NJM4580 should be perfectly fine.)
PS - The next fun part is going to be hooking up XLR pin 1, though "chassis" is rarely a bad idea. Accommodating both balanced and unbalanced inputs well at the same time is fairly tricky business... the former generally are going for "IEC Class I and be done with it", which is not generally what you want for the latter (ground loops). Section 4.5 from this Hypex whitepaper provides a possible solution.
What kind of power supply topology are you looking at (switching, linear, and if the latter, what sort of VA and is there a shield winding)? That closely ties into the whole topic.
What kind of power supply topology are you looking at (switching, linear, and if the latter, what sort of VA and is there a shield winding)? That closely ties into the whole topic.
Pin 1 is shield, extension of protective chassis and contains error current. If one wants error currents to the audio circuitry then please do connect it to the audio circuit 🙂 Usually this is not wanted.
RCA sleeve (-) should go to pin 3, pin 2 is "hot" (+). Although sometimes the rules are broken by manufacturers and problems arise and one has to figure out what is wrong and then build suitable cable.
If in doubt, refer to Rane notes about interconnects
https://www.ranecommercial.com/kb_article.php?article=2107
If more info is required then refer to book by Henry W Ott - Electromagnetic Compatibility Engineering, which I recommend to anyone fiddling with audio equipment as it will remove any doubt about grounding stuff.
RCA sleeve (-) should go to pin 3, pin 2 is "hot" (+). Although sometimes the rules are broken by manufacturers and problems arise and one has to figure out what is wrong and then build suitable cable.
If in doubt, refer to Rane notes about interconnects
https://www.ranecommercial.com/kb_article.php?article=2107
If more info is required then refer to book by Henry W Ott - Electromagnetic Compatibility Engineering, which I recommend to anyone fiddling with audio equipment as it will remove any doubt about grounding stuff.
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Agreed for a proper balanced implementation. But if kludging a balanced connector across an RCA connector, you would want to follow what ever ground technique was used for the RCA connector which may be carrying the ground to the PC board instead of directly to the chassis at the connector.
Using pin 2 & 3 is also a good approach and here I would actually float pin 1. But there is another potential problem with this implementation. If the balanced driver is electronic and not a transformer, then you would be shorting one phase to ground as the RCA shield is essentially chassis ground one way or another. Some driver circuits do not like that. But then if using pin 1 & 2 with pin3 floating. a transformer driver is now not going to work unless you do bond pin1 to pin3.
All this is a bad idea. Just install a proper balanced circuit if true balanced operation is needed. Like we discussed above there are good single chip solutions as well as fairly simple circuits using a basic OPAMP.
The XLR signal pins are 2 and 3, you would connect pin 2 to the RCA hot and pin 3 to the RCA shell.
The recommended practise (I think it is AES17) is to connect the XLR pin 1 to chassis ground only, NOT to signal ground.
That's all there is to it, everything else is speculation and trial by error.
In some rare cases, depending on the actual wiring on the receiver board, this connection causes some hum, in that case pin 3 should be connected to pin 1 in the XLR.
As @tmuikku said above. All this stuff is easily found with one or two Google searches btw, look for 'the pin 1 problem'. There is no 'pin 2 problem'or 'pin 3 problem'.
Jan
The recommended practise (I think it is AES17) is to connect the XLR pin 1 to chassis ground only, NOT to signal ground.
That's all there is to it, everything else is speculation and trial by error.
In some rare cases, depending on the actual wiring on the receiver board, this connection causes some hum, in that case pin 3 should be connected to pin 1 in the XLR.
As @tmuikku said above. All this stuff is easily found with one or two Google searches btw, look for 'the pin 1 problem'. There is no 'pin 2 problem'or 'pin 3 problem'.
Jan
Thank you for the information much appreciated.
I must ask what is the need for a board then like the picture I had posted.
That has RCA and XLR on it and should there be a switch that separates the signals between the RCA and XLR?
I must ask what is the need for a board then like the picture I had posted.
That has RCA and XLR on it and should there be a switch that separates the signals between the RCA and XLR?
Well there's 'different' needs. Some like the option of two input connectors.
Or have the two connected and just flip the switch instead of plugging/unplugging connectors.
I always use an external RCA-to-XLR adapter on the bal inputs.
Some have the need to generate income.
Jan
Or have the two connected and just flip the switch instead of plugging/unplugging connectors.
I always use an external RCA-to-XLR adapter on the bal inputs.
Some have the need to generate income.
Jan
^ Connecting pin 1 to pin 3 at the XLR like that might connect Haffler chassis to its own input and introduce noise to the audio undoing shielding the chassis is for. I'm guessing without knowing how its all connected inside the Haffler, whether pin 1 there is connected and where. Perhaps its not connected at all because of this kind of cables.
It would be better to use shielded twisted pair cable instead, connect RCA to the twisted conductors and to pins 2 and 3 on the XLR end. The cable shield should connect to pin 1 on XLR end only. Now the cable shield would work as extension of the Haffler chassis protecting the unbalanced connection (cable) from stray electro magnetic fields. More importantly noise current of Haffler chassis (pin 1) would not be connected to its own input (pin 3). The DAC ground is now still connected to Haffler audio circuitry via pin 3 but no can do so its as good as it gets without transformers / balanced output.
This is connection # 17 from Rane docs https://www.ranecommercial.com/kb_article.php?article=2107
Anyway, if there is no (audible) problem then its fine 🙂
It would be better to use shielded twisted pair cable instead, connect RCA to the twisted conductors and to pins 2 and 3 on the XLR end. The cable shield should connect to pin 1 on XLR end only. Now the cable shield would work as extension of the Haffler chassis protecting the unbalanced connection (cable) from stray electro magnetic fields. More importantly noise current of Haffler chassis (pin 1) would not be connected to its own input (pin 3). The DAC ground is now still connected to Haffler audio circuitry via pin 3 but no can do so its as good as it gets without transformers / balanced output.
This is connection # 17 from Rane docs https://www.ranecommercial.com/kb_article.php?article=2107
Anyway, if there is no (audible) problem then its fine 🙂
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I don't agree. Pin 1 must be connected to the chassis, which is usually not audio ground (assuming that by "ground" you were referring to audio GND).
Pin 1 does not carry the audio signal!
Pins 2 and 3 carry the audio signal. The unbalanced input (RCA) therefore needs to be connected to those two pins. I have no opinion about the polarity of the RCA connection to the XLR pins 2 and 3, but I'd recommend you follow the recommendations in the RANE techical note.
The Hafler p3000 has a small switch in the back to connect (or not) XLR pin 1 to chassis. In my case it is open and there is no noise (RCA to XLR).
I’m building amplifier and like to have RCA and XLR inputs on the back of the amplifier with a switch also I like to have the XLR wired properly with hot, cold and shield
If you want to be picky, a 3 pin XLR has 4 connections, and the shell is the chassis connection. Often the shell terminal is jumped to pin 1. Connecting a shield to something other than the chassis is an invitation to EMC problems since RF currents have to go through analog ground to get to the environment. But practical considerations are paramount, and you have to work with whatever you are connecting to, which means compatible with the mixer/crossover/pre-amp. This is how Yamaha wired the P2200 power amp. Notice that the only switch is for selecting pin 2 vs 3 as hot. There is no need to select the input. You just don't use the other jacks. You may also notice that there are in and out jacks for paralleling amps and the XLR pin-1 connection to chassis is not shown in the UL model..
