Hi, I want to add balanced line out to an amplifier.
The circuit I consider: Balanced Line Driver & Receiver
Some questions about it:
A. Why there are no coupling capacitors at the output?
B. What would happen if someone switches the phantom power on the mixing console (+48 volts on XLR pins 2 and 3) ?
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I also want to add ground lift to reduce possible hum.
Rod Elliot states:
"Although this transmitter and receiver pair will probably allow the use of unshielded interconnects, I don't recommend this. Use a good quality shielded twin microphone cable. The earthing of the shield should normally be done at the receiver end, but in some cases you might find that the noise rejection is better if the transmitter end is earthed. Experimentation will be needed, and often both ends will be earthed. One trick that often works well is to connect the 'floating' end of the shield to earth with a 100nF multilayer ceramic capacitor."
So, Plan to add a switch on pin 1.
1. Position one: dissconects the pin 1 from my circuit (transmitter)
2. Position two: connects pin 1 through a capacitor of 100nf.
The idea for the capacitor is twofold:
- Because Rod Elliot says it might have good results 🙂
- I don't know what would happen if someone turns the phantom power at the mixing console (the question B from the above).
Where do I connect the XLR pin 1 ??
2.1. Connect to my PSU ground.
2.2. Connect to my mains earth. I don't think there would be much of a difference, because the mains earth (chassis) would be connected to the PSU ground through this circuit:
^^ The above is to protect me from potentials/leakage on the PSU ground. The amplifier is a guitar amplifier and the PSU ground is connected directly to the guitar strings.
The circuit I consider: Balanced Line Driver & Receiver
An externally hosted image should be here but it was not working when we last tested it.
Some questions about it:
A. Why there are no coupling capacitors at the output?
B. What would happen if someone switches the phantom power on the mixing console (+48 volts on XLR pins 2 and 3) ?
---
I also want to add ground lift to reduce possible hum.
Rod Elliot states:
"Although this transmitter and receiver pair will probably allow the use of unshielded interconnects, I don't recommend this. Use a good quality shielded twin microphone cable. The earthing of the shield should normally be done at the receiver end, but in some cases you might find that the noise rejection is better if the transmitter end is earthed. Experimentation will be needed, and often both ends will be earthed. One trick that often works well is to connect the 'floating' end of the shield to earth with a 100nF multilayer ceramic capacitor."
So, Plan to add a switch on pin 1.
1. Position one: dissconects the pin 1 from my circuit (transmitter)
2. Position two: connects pin 1 through a capacitor of 100nf.
The idea for the capacitor is twofold:
- Because Rod Elliot says it might have good results 🙂
- I don't know what would happen if someone turns the phantom power at the mixing console (the question B from the above).
Where do I connect the XLR pin 1 ??
2.1. Connect to my PSU ground.
2.2. Connect to my mains earth. I don't think there would be much of a difference, because the mains earth (chassis) would be connected to the PSU ground through this circuit:
An externally hosted image should be here but it was not working when we last tested it.
^^ The above is to protect me from potentials/leakage on the PSU ground. The amplifier is a guitar amplifier and the PSU ground is connected directly to the guitar strings.
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Just but the analog device "ssm2142"
Or the "DRV134" if you prefer burrbrown sound.
The That 1606 would work too but I do not know if they sound better than the AD...
Or the "DRV134" if you prefer burrbrown sound.
The That 1606 would work too but I do not know if they sound better than the AD...
I hoped it can work with any opamp.
p.s. Have to recalc the power requirements, driving 600ohm load at max line level (~3.4v peak to peak)
Just wondered if I can burn the circuit with the phantom power from a mixing console.
Is there any specific reason that there are no decoupling caps at outputs?
What if I connect the ground (pin 1) through decoupling cap? (also disconnecting it - "ground lift")
p.s. Have to recalc the power requirements, driving 600ohm load at max line level (~3.4v peak to peak)
Just wondered if I can burn the circuit with the phantom power from a mixing console.
Is there any specific reason that there are no decoupling caps at outputs?
What if I connect the ground (pin 1) through decoupling cap? (also disconnecting it - "ground lift")
Phantom power is typically on microphone inputs. So it's unlikely your circuit would be connected to a microphone input.
But, hey just put a 22uf non-polarized electrolytic on each output phase. And ingore the non-engineering purists. It's not going to make any audible difference. Especially using a TLO72, which is fine BTW.
As for XLR pin 1, I always connect it to the chassis right at the connector. They have a lug just for this purpose. This is the proper way and it has never failed me. The shield should go to the chassis and not the circuit ground as is often improperly done. Never mind that the circuit ground ultimately goes to the chassis anyway. It's the creation of a current loop you don't want.
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Ok, so, just have to check if TL072 can drive the load... 😀
p.s. If I take the signal right before the power amp, It would probably be less than 3.4v p-p
I haven't check if the power amp is going to like that much of an input 😀
As for the mic input - the mixing console in our rehearsal room has XLR balansed inputs which are exactly the MIC inputs. It also has on/off button for the phantom power.
I am going to test it with this console and I hope to make the line out work OK with ANY possible console we may possibly plug it in.
(f.ex. if we take the amp to some gig with some PA)
That's why I think abt the "ground lift" as well.
p.s. If I take the signal right before the power amp, It would probably be less than 3.4v p-p
I haven't check if the power amp is going to like that much of an input 😀
As for the mic input - the mixing console in our rehearsal room has XLR balansed inputs which are exactly the MIC inputs. It also has on/off button for the phantom power.
I am going to test it with this console and I hope to make the line out work OK with ANY possible console we may possibly plug it in.
(f.ex. if we take the amp to some gig with some PA)
That's why I think abt the "ground lift" as well.
btw, Rod Eliot mentions to use higher tolerance resistors at the outputs.
(%5 or bellow)
So that the output is 'ballanced'.
Shall I look for 'mathced' output capacitors as well 😀?
Mby this is the reason there are no caps there...
(%5 or bellow)
So that the output is 'ballanced'.
Shall I look for 'mathced' output capacitors as well 😀?
Mby this is the reason there are no caps there...
Pin1 does NOT go to audio.
Pin1 only connects to Chassis/Enclosure.
Connect the screen/shield to Chassis/Enclosure via Pin1 at both ends.
There are rare ocassions where the Receive end may require a capacitor coupling to Chassis/Enclosure. Keep a "special XLR cable" with this arrangement built into the XLR plug and label that plug so you don't forget what is inside. This is described in a Jensen App Note.
No. See above
No. See above.
All your metal cased PA gear that gets connected directly to mains power or connected via other gear to mains power should have Protective Earth wired directly to Chassis/Enclosure.
Final note:
Balanced Impedance is all about IMPEDANCES. See the two 100r (R4 & R5). They are what make the two signal lines Balanced Impedance. They need to be matched to <<1%. If your can get better than <0.1% then go for it.
Balanced Impedance connections have nothing to do with voltages active. or passive. on the two signal lines.
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The balance required is much better than 5%.
For poor rejection of interference use 1% matching of impedances
For better rejection use 0.1%
For best rejection use 0.01%
The capacitors only affect the upper frequency range and as such I think you can get away with slightly wider tolerances on capacitor matching, especially since their impedance at HF is a tiny poprtion of the typical output padding resistor. I often suggest one aims for 1% for the capacitors, even the electrolytics. Panasonic Bi-Polar are recommended by Jensen in AN003.
These will also solve your phantom power blowing up the transmitt end.
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