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In Reply to: RE: Questions about balanced and unbalanced interconnects posted by John Elison on January 18, 2015 at 18:49:26
Thanks for this - I was not previously aware that internally balanced amplifiers loose power if fed with an RCA-to-XLR adapter and I would be interested to know why this is.
I have (I think!) a fully balanced power amp (ARC VT130SE) fed by balanced XLR-->XLR inputs from a passive pre-amp. However, the CD player has only RCA outputs and these are connected to the pre-amp by a custom XLR connector on the interconnect. It sounds pretty good and powers the Martin Logan electrostatics well.
My question is: does the CD --> pre-amp RCA-->XLR connection have any adverse effect?
Many thanks,
garf
Follow Ups:
A fully balanced amplifier would be equivalent to a stereo amplifier that is bridged to mono. You connect a speaker to the two hot (red) output terminals so that each speaker wire contains an audio signal. An unbalanced amplifier has a signal on the hot (red) terminal only while the other is ground. Consequently, if you are providing a signal to only one speaker terminal of each channel on a fully balanced amplifier, the maximum power it can produce is only one-forth of its rated power.
If your ARC VT130SE is fully balanced, it will have an in-phase signal on the positive speaker terminal and a 180-degree out-of-phase signal on the negative speaker terminal. If it has only one audio signal on the positive speaker terminal with ground on the negative terminal, it is not what I consider to be fully balanced.
For example, I own a Parasound Halo A21 amplifier. It has both balanced and unbalanced inputs, but its speaker terminals have the audio signal on the positive (red) terminal only, so it's not what I consider to be fully balanced from input to output.
> My question is: does the CD --> pre-amp RCA--> XLR connection have any adverse effect?
That would depend on whether or not your preamp is fully balanced. A fully balanced preamp never combines the two signal legs of a balanced input. In other words, a fully balanced preamp has four discrete signal paths from input to output -- two for each audio channel. If your preamp is fully balanced and your amplifier is also fully balanced, then you will be able to extract only one-fourth of your amplifier's rated power when playing your CD player. If, on the other hand, your preamplifier combines its balanced input signals into a single path for each channel and then splits them into balanced signals again at its outputs, you are in good shape.
Best regards,
John Elison
Thanks John - and all the others below - for taking time to answer my questions.
garf
"I was not previously aware that internally balanced amplifiers loose power if fed with an RCA-to-XLR adapter and I would be interested to know why this is."
They don't necessarily lose power, they lose voltage gain. That's just due to seeing only half of the applied voltage. Probably the gain structure is such that that's still plenty to drive them to clipping so all you have to do to 'fix' it is to turn up the volume.
Rick
Thanks for this - I was not previously aware that internally balanced amplifiers loose power if fed with an RCA-to-XLR adapter and I would be interested to know why this is.
They don't necessarily lose power, they lose voltage gain. That's just due to seeing only half of the applied voltage.
Actually both statements are false unless the unit has poor Common Mode Rejection Ratio. If the ratio is high, the gain is the same using either input- RCA or balanced. Now if you apply half the input voltage, the amp will have 6 db less voltage output. That's not a gain issue- its simply not putting in the voltage in the first place.
My idea of a fully balanced stereo amplifier is one containing four discrete and separate amplifiers. In other words, each speaker would be powered by two separate amplifiers -- one being fed from Pin 2 of the XLR and the other from Pin 3. The output terminals to each speaker would both be hot with audio signals of opposite phase. In this configuration, if an RCA-to-XLR adapter is used, the amplifier being fed from Pin 3 will have no signal because Pin 3 is grounded in the RCA-to-XLR adapter. Consequently, each speaker can receive only one-fourth of the amplifier's rated power and voltage gain will be cut in half.
Best regards,
John Elison
Such an amplifier would have a terrible Common mode rejection ratio, which is actually pretty important if you want to run a balanced connection.
This is why transformers are often used as you can get really good CMRR numbers.
I'm not sure if I have ever seen such an amplifier or what it would be used for. I had a balanced Kepco lab amp years ago but even that employed a differential input.
But I see this idea a lot. It should be put to bed as such an amplifier is really just plain impractical.
Well, I didn't know that. I was basing this idea on several other posts I've read where people talked about a fully balanced power amplifier. So, you say it is very uncommon to have a fully balanced power amplifier? I do know that when stereo amplifiers are bridged to produce a mono amplifier of greater power, they basically operate like a fully balanced amplifier with a phase inverted signal on one speaker lead and the in-phase signal on the other. I just thought that perhaps there were fully balanced amplifiers designed that way, too.
I do know that some preamplifiers are fully balanced from input to output because I own one. My Pass Labs X1 preamp is fully balanced with four discrete signal paths from input to output -- two for each stereo channel. Most of my components are balanced but one of my two turntables uses an unbalanced phono stage, which is connected to one of my preamp's unbalanced inputs. The problem with the unbalanced inputs is that they send a signal only to Pin 2 on the output XLR. My X1 also has a balanced tape loop, which receives a signal only on Pin 2 from the unbalanced inputs. I have a TASCAM DA-3000 DSD recorder connected into the balanced tape loop and I recently discovered that when I make a recording from my unbalance phono stage, the loud passages are severely clipped yet the meters do not indicate a problem. In fact, I was unable to even get the meters to go into the red. Apparently, when I turned up the input level control on my TASCAM DA-3000, it actually clipped the input amplifier on Pin 2 because there was no signal on Pin 3 of the XLR. I now know that I can record LPs only from my other turntable with its balanced phono stage.
I suppose I could get around this problem by connecting my other phono stage directly to the TASCAM DA-3000's unbalanced inputs, but it is really not a concern since my other turntable sounds better for making recordings anyway.
Best regards,
John Elison
You may not know this but we made the first balanced line preamps offered to high end audio (1989- the MP-1; complete with balanced differential phono section). We also made some of the first balanced amps (1985).
However, they are also fully differential, which IMO is the way to do it- less parts and better performance (lower noise, less distortion). I'm not saying the balanced amps are uncommon, just that balanced amps that are also not differential (two discrete signal paths) are.
Bridged amps are an exception. I have dealt with a few of them that have a balanced input that only works if the amp is in bridged mode. Their performance is really not good!
Thanks, Ralph! I appreciate all the great information.
When you say there are plenty of fully balanced power amps, I interpret this to mean that the balanced input signal is converted to an unbalanced signal through a differential input stage before being sent to the power amplification stage. Am I correct? In other words, would you consider my Parasound Halo A21 to be a fully balanced power amplifier?
Thanks again,
John Elison
When you say there are plenty of fully balanced power amps, I interpret this to mean that the balanced input signal is converted to an unbalanced signal through a differential input stage before being sent to the power amplification stage. Am I correct? In other words, would you consider my Parasound Halo A21 to be a fully balanced power amplifier?
In our amps and preamps, the balanced signal never goes single-ended at any point in the circuit and everything is handled by differential circuits.
The Parasound I would expect has a differential input and then goes to push-pull in the driver and output section. So yes, its fully balanced even if not fully differential.
Usually a differential amplifier is used to convert from single-ended to balanced, but it can go the other way as well. In the case of the Parasound, I don't think so as it has a balanced input in which both phases are processed by the amp.
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