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Old 20th March 2017, 01:29 AM   #1
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Default Balanced PGA2311 volume control

Hi guys.

I'm going to build an volume control on PGA2311 which will be used with XLR (balanced) input.
Signal will came from typical DAC (non pro stuff).

And here is a question, may I use one PGA2311 per one channel?
PGA2311 is a single-ended chip and I'm not sure is that will work correctly in terms of signal integrity etc.
So let me explain

XLR Input -> LPF -> PGA2311 (hot and cold pins connected to left and right channel input pins).

Or should I use INA13x (or any other opamp) chip to convert balanced singnal to SE -> PGA2311 -> DVR135 to convert it back to balanced again?

First idea is much more simpler and economic but I'm unsure is that will work as it should.
Could you help me with that?

Thank you for your time and effort to help me out
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Old 20th March 2017, 01:38 AM   #2
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To keep the benefit of using balanced line, you need to be very careful about gain mismatches between the hot and cold signals.

I can't recall the gain specification for the pga off the top of my head, but would be surprised if it were nearly as good as using a balanced line receiver/ transmitter around the pga.

I presume your goal is noise rejection?

In writing the above I starated asking myself questions like : why a volume control on the middle of a balanced line run?

And: is this person going to get onto hot water with grounding issues? Think about where control for the plan comes from.
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Old 20th March 2017, 01:52 AM   #3
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I just want to build a volume control for balanced headphone amplifier.
There is really hard to get good 4ch pot (on the other side they are expensive).

OFC I want to build that volume control properly in any way.
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Old 20th March 2017, 02:43 AM   #4
plasnu is offline plasnu  United States
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Quote:
Originally Posted by mikolaj612 View Post
Hi guys.

I'm going to build an volume control on PGA2311 which will be used with XLR (balanced) input.
Signal will came from typical DAC (non pro stuff).

And here is a question, may I use one PGA2311 per one channel?
PGA2311 is a single-ended chip and I'm not sure is that will work correctly in terms of signal integrity etc.
So let me explain

XLR Input -> LPF -> PGA2311 (hot and cold pins connected to left and right channel input pins).

Or should I use INA13x (or any other opamp) chip to convert balanced singnal to SE -> PGA2311 -> DVR135 to convert it back to balanced again?

First idea is much more simpler and economic but I'm unsure is that will work as it should.
Could you help me with that?

Thank you for your time and effort to help me out
I have done exactly the same thing before. It works as it should be, but the sound is not as good as I expected. Probably due to cheap PSU and bad implementation. I used Ebay China contoller. Also THAT chip is better.
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Old 20th March 2017, 02:49 AM   #5
plasnu is offline plasnu  United States
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And I also tried Opamp in front of 2311, and the result is slightly better. I don't remember what DAC I was using at that time. I ended up using digital volume on DAC which sounds better.
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Old 21st March 2017, 03:50 AM   #6
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+1 for converting to unbalanced and back. Reading Bruno Putzeys' "G-Word" whitepaper is advised.

Make sure your balanced receiver can do less than unity gain, since INAs often don't - common balanced line driver circuitry has 6 dB of gain, so you're generally at least that much hotter than unbalanced, and pro levels could be up to +22 dBu (almost 10 Vrms), while the PGA's input is somewhat limited by its +/-5 V supplies, so you can't run more than 2.5 Vrms into that. Thus depending on where your balanced signal is coming from, the balanced receiver would have to have a gain of -6 dB to -12 dB (maybe -14 dB).

Likewise, it has to be made sure that PGA output level is well-mapped to headphone amp output level. Given that headphones wired up balanced tend to be fullsize models that are not super-sensitive, you'd probably be just fine with enough gain to achieve full output (2.5 V of noise + 12 dB is 10 V or 2 dB SPL into something 102 dB / 1 Vrms). That said, if you do need a variable gain control to accomodate more sensitive cans, after the PGA would be a good spot.

Oh, and XLR pin 1 goes straight to chassis.
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Old 21st March 2017, 08:19 AM   #7
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I'll put in a vote for using a balanced signal path. An advantage of a balanced signal path is that, to a first order, a balanced signal path helps to keep signal currents out of ground, and given a few other techniques, allows signal induced power supply currents to be kept out of the power supples and ground.

The cost is that usually 2x the circuitry is required to provide a balanced signal path, but the benefit is that because signal currents are able to be kept out of ground and the power supplies, PCB layout is greatly simplified.

The typical idea now is that 'balanced' is useful only between separate boxes, and primarily serves to eliminate noise from interface cabling. While that's true, one can consider separate functional blocks within a larger PCB to be 'separate boxes', and so the ground and power supply isolation that one gets from a balanced signal path also helps these circuit subsystems and functional blocks to work properly despite the fact that they share a PCB ground and power supply system. So, while 'balanced' is great between pieces of gear, it's also great within a piece of gear for the same reasons.

In a balanced signaling system, every time a signal is sent to ground through a resistor or other component, there will be an equal and opposite signal also sent to ground. If one arranges the layout so that these two signals to ground can cancel each other within a small piece of PCB foil, then these currents essentially disappear, instead of traveling back to their source, possibly becoming coupled to other circuit nodes due to IR losses in the PCB foil as these currents travel back to their source.

While it's more complicated, there are similar cancellation techniques that can be used to keep transient signal induced currents out of the power supply, and also keep half wave rectified power supply currents out of the ground system, where they can inject significant distortion into a circuit through IR coupling through the PCB ground foil. Those power supply bypass caps can couple half wave rectified signal currents into the ground foil, and as these currents travel back from their source/load, voltages can be generated in the finite impedance PCB foil, and possibly couple them into the signal path.

While the degree of isolation depends upon the degree of signal balance, even basic balanced circuitry that has only modest balancing, for example to 1%, can offer 40dB of reduction of the magnitude of signal currents traveling in ground or the power supplies. With a more closely balanced signal, one can get greater isolation. So, even though the PGA2311 is not precisely balanced, the two channels should match each other to a pretty high degree, and offer some significant advantages.

To summarize, I know that the common parlance is that "balanced is to eliminate cable problems and interference". However, if you dig a little bit deeper, one can take advantage of a symmetric signaling system in order to simplify and minimize the currents traveling within ground as well as the power supplies. I've built some circuits recently that use these techniques, and they perform extremely well, despite their relatively simple and compact PCB layouts.
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Old 21st March 2017, 08:45 AM   #8
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Quote:
Originally Posted by googlyone View Post
To keep the benefit of using balanced line, you need to be very careful about gain mismatches between the hot and cold signals.
This is a non-issue; balanced signals are defined by the level BETWEEN the two lines and NOT to ground, that's the whole idea.

A PGA is perfect for this application because you can use one half for each phase and you do not have to convert to single ended and back. At the instant you convert to SE you again introduce ground as a reference with all the issues that go with that, and destroys the advantages of balanced connection.

Jan
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Old 21st March 2017, 09:13 AM   #9
AndrewT is offline AndrewT  Scotland
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But one does need to maintain a very close matching of the impedances.
It is after all a balanced impedance connection that one is trying to achieve.
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Old 21st March 2017, 09:26 AM   #10
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Originally Posted by AndrewT View Post
But one does need to maintain a very close matching of the impedances.
It is after all a balanced impedance connection that one is trying to achieve.
Indeed, and therefor it is best to use one chip for each channel. The input impedances are not well defined but the two channels on a chip should be reasonably well matched.

Jan
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