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Confusion on amplifier impedance

Hello to all. My question is, is the imput impedance of an amp the same in RCA and Balaced mode. In setting up the M5-HP crossovers ( Balanced) for my Vandersteen 5a speakers  I have to use the input impedance of my amp. Now what I dont understand is the amplifier, a Musical Fidelity M6PRX, is rated at 50K of input impedance but when I decided to check with a voltmeter it was way off the 100Hz target of the crossover. Requirement is play 1KHz tone at 1VAC and measure, then play a 100Hz tone and measure. The resuts should be .707 VAC of the first reading or as close as you can get.
In order to get a proper reading I had to set the M5-HP for 200KHz. My results were then 1KHz at .990 VAC and 100KHz  .722 VAC. I previously had a set of RCA crossovers on a set of Vandersteen 5 with the same amp and had no sonic problems however I never did the check I did now. I just set it at 50K impedance and that was that. Can the difference between RCA and Balanced be so great ? Not sure how this works. Thanks in advance for any help.
cerberus79
6 responses Add your response
erik_squires
16,056 posts
 
Depending on how the amp’s inputs are configured. Some use XLR plugs as convenience only. The (-) pin is grounded, and therefore, it’s really just an unbalanced input, with an XLR plug. In this case, the RCA and XLR inputs will have identical impedance from + to - (which is what is being asked for).

If it’s a true balanced input, it may have 2 x the input impedance from + to -. I suggest using the current flow at 60 Hz to tell. Most meters are only good up to a few hundred Hz, if that much. See how much current 1V of input causes, and if it is cut in half on either input.


Of course, another way to tell may just be to test the impedance from pin 2 and 3 to ground with the amp off. you won't get an accurate Ohms reading, but if pin 2 or 3 are grounded, that's your tell.



Best,
E
oldears
386 posts
 
 Normally for differentially baIanced amps the impedance doubles for a balanced input. The Musical Fidelity manual only gives the 50 kohm spec. Reading the manual leads me to think they tout balanced connections, but I see no remarks to lead me to think the unit is differentially balanced.I was not aware that AC impedance can be reliably measured with a DVM. Therefore you probably should presume the 50 kohm spec for your connections. You might wish to contact Vandersteen to see if they approve using the balanced crossover it the unit is not truly balanced, and also contact Music Fidelity to confirm the M6 series topology.
cerberus79 OP
52 posts
 
Well I opened up the amp and each XLR has an independent plug going to the main board. The connections where the plug connects are labeled, ground, positive, negative. i measured resistance to ground on pin 1 and got 39 ohms. To ground on pins 2 and 3 it jumps to  a fluctuating 176k ohms, equal reading on 2 and three. I read an article on the internet that stated "balanced operation gives an increase in loudness of 6 db". There is a "balanced" switch on the back and the manual states that not having it in the balanced position results in a 6db LOSS of loudness. I am using the amp with the companion M6-PRE wich is definitely fully balanced with inputs and outputs. I think all this means I am safe thinking the amp is balanced. So if the 50k impedance is for RCA ? then I suppose I have to double it for Balanced XLR. I have tried on previous ocassions to email Musical Fidelity with no reply. Now that Project owns them I have no idea whom to contact.
chipl3
2 posts
 
chipl3
2 posts
 
I just re-read, a few days ago, the M5-HP sheet that is part of Vandy 2Wq info. It says to try the settings at, above and below the  input impedance for your amplifier.  You might still call Richard and ask why. I guess, not that is satisfying, that the filter is not so precise across different amps and subwoofers, that one might get better sound - to your ears - off an alternate setting. I have run two separate amps, ARC and Ayre, with these crossovers at the amp-indicated sitting. However, I am not "electric' enough to understand circuit math and so forth. I use the input impedance from the amp manual, and read elsewhere (Vandy's Ask Richard page, maybe) to not double for balanced. (The reason I read it this week is, I am not always good at reading the whole manual, and just picked up somewhere that an alt setting might sound preferable.

A call to Vandy, or perhaps some online research, can also give you correct safety guidance on changing M5-HP dipswitches with main amp on-off, or preamp volume up-down.
celander's avatar
celander
1,503 posts
 
To answer your Q, the input impedance of an amp is fixed regardless of input connections. 
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