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Value of Making an SET Amp with Balanced Output

post #1 of 9
Thread Starter 

Hello All.  I am looking for input from those who understand such things....

 

I have a pair of Audeze LCD3's that have both single-ended and balanced cables. I understand that the single-ended cables take the positive signals (+) from each channel and return the signals to a common ground.  I understand that the balanced cables take the positive (+) from each channel and return each channel to the amps negatives (-) for each channel.  At one point I was thinking of using a pair of 300b monoblocks to power my LCD3's, and Audeze told me that I could simply do this by hooking their 4-pin balanced connector to the R+, R-, L+ and L- terminals of my amplifiers (with proper pin matching).

 

At this point in time, I having a Korneff 45 SET amplifier built for my LCD3's.  The planar magnetic headphones can handle the 2-3 watts put out by the 45 SET amplifier. The amp builder is planning on providing both speaker terminals and a single-ended headphone jack.  I'm wondering if I should ask for both single-ended and balanced jacks, or perhaps just a balanced jack.  Or, if he only provides a single-ended jack for headphones, if I can attach the LCD3's to the speaker terminals as discussed above.

 

I understand that the amp design does not make a true balance signal, using either: a) two amplifier circuits for each channel to process the normal and inverted signal, or b) a balancing transformer to invert the signal and create a normal and inverted output per channel.  Asking for either of those options would go beyond the basic amplifier design.  So, if neither of those options are used, my questions are:

 

1) is there any value to including a 4-pin jack for the LCD3 balanced cable as described above?

2) Is it possible to have a 4-pin XLR jack and a 1/4" single-ended jack on the same amp?  Are there issues with the fact that the single ended jack has a common ground?

 

Thanks in advance for your advice.  Peter

 

PS - wiring diagram for LCD3 balance cable below:

 

 

post #2 of 9
Thread Starter 

PS - I forgot to mention that this is a stereo amp, not two separate monoblocks.

post #3 of 9
Quote:
Originally Posted by Peter_S View Post

 

1) is there any value to including a 4-pin jack for the LCD3 balanced cable as described above?

2) Is it possible to have a 4-pin XLR jack and a 1/4" single-ended jack on the same amp?  Are there issues with the fact that the single ended jack has a common ground?

 

Thanks in advance for your advice.  Peter

 

 

 

 

 

 

1) No. Unless the amp is a true balanced design, having a 4-pin jack would only be a convenience feature.

2) Yes it's possible. As a matter of fact some AudioGD amps have this feature.  No issues with ground, unless of course we're talking about a poorly designed circuit or sloppy build.

post #4 of 9
Quote:
Originally Posted by Peter_S View Post

2) Is it possible to have a 4-pin XLR jack and a 1/4" single-ended jack on the same amp?  Are there issues with the fact that the single ended jack has a common ground?

It's possible to have them, but there are potential issues with the fact that the single-ended jack has a common ground. If the XLR is wired to keep the 2 output grounds distinct, then the common ground at the TRS jack will defeat the intent. Wiring the grounds together can increase the crosstalk, but whether this is audible is arguable, crosstalk being one of the less audible performance flaws. OTOH, if you've gone to the trouble of keeping the grounds apart, then it seems a little perverse to connect them.

w
post #5 of 9

Actually, you could use a resistor network to provide balanced input, that would result in immunity to noise that balanced provides.  One leg will feed through a matching resistor to ground, but it will nevertheless provide effective shielding from common mode noise.  Even though only one of the legs is used further into the SET, this would still be so.  Google it and you can find how that works.  So balanced would have value of noise immunity if done right into the input of the SET. 

post #6 of 9
Thread Starter 

Thanks so far everyone!

 

I posted this same question on Audiocircle and got the following input:

 

1) "Since the amp will have output transformers it should not be a problem to wire a 4 pin output with separate grounds as shown in the drawing. The key being that the output of the transformers is floating and doesn't have one side grounded."

 

2) "Also, when you wire an unbalanced connector in parallel with a balanced connector, both connectors become unbalanced. I think you would need a switch of some sort to separate the two. I would be more concerned with the transformer's output impedance at the headphone jack rather than whether it's balanced or single ended. If the primary goal of the amp is headphone use then a standard 8 ohm speaker tap might not be optimal for a 50 ohm headphone load. It would be cool if the amplifier was made with an eight ohm transformer tap hardwired to the speaker binding posts, and a 16 or 32 ohm tap hardwired to the headphone jack. I don't know if that was a consideration or not, but that seems like the best way to use a tube amp for both situations."

 

All of this seems very useful.  I found the question of the impedance of the output transformer to be quite compelling, and have asked the amp builder to let me know what taps the Xformer offers.

 

Any thoughts on these two suggestions?  Thanks, Peter

post #7 of 9

If the output transformer is floated, it's not going to matter whether or not you connect the can with TRS or XLR.  It won't matter if the TRS is paralleled with the XLR.  The signal coming out to your headphone is balanced, without a need for an inverted signal.  Both positive and negative signal have the same balance to ground, that of open loop or infinite resistance.  The signal coming out in theory can take advantage of common mode noise reduction without a need for differential signalling. TRS or XLR is not going to matter.

 

The key is whether the transformer is floated from ground or not.


Edited by SP Wild - 3/4/14 at 8:51am
post #8 of 9
Thread Starter 

Apparently the output transformers are floated, so we are good to go.  Thanks, Peter

post #9 of 9

With the output impedance, seeing you are using LCD3 and damping factor is not an issue, I personally have found that the higher impedance output is more transparent than the lower output impedances.  Best to incorporate a switch if possible to increase versatility for different cans, or different output jacks for different outputs - this option would bypass a switch.

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