Why Single-Ended?

bdp2The Tim Paravicini-designed EAR-Yoshino tube circuits are all single ended, the pre-amps and power amps having transformers on the XLR/balanced inputs and outputs.

This brings up an interesting concept that I have been thinking about. In the case of a SE designed preamp, what would be the preferred way to send the signal to the amp; use unbalanced RCA cables, or go thru transformers with XLR’s which will be converting the signal back to single-ended? And depending on the type of transformer, the gain may be increased on the output, or it may be a 1:1 conversion. Seems like an unnecessary step.

But if the preamp was fully differential, then it would be advantagous to output a balanced signal to the amp’s XLR/balanced inputs.
Am I correct?

Converting single ended - balanced - single ended using transformers, makes sense if you have a lot of electrical noise and connection is long. Otherwise transformers won’t add anything positive to sound. They will introduce some low frequency distortions. If your preamp and amp are both differential you still might like single ended connection more. (my power amp has only balanced input - I had no choice).  XLR cable's shield is grounded on both ends shorting that way chassis and possibly creating ground loops.

DB, yes, it appears that the JC2 BP does not provide a tape loop or anything equivalent, so assuming you have more than one source I don’t see a way of connecting the KUBE into the system other than between the preamp and power amp.

Kijanki, thanks. It’s interesting, though, that the Pass amps which are fully balanced and therefore presumably have minimal even order distortion (for example, John Atkinson’s measurements of the XA30.5 state that its "THD is almost pure third harmonic") seem to generally be considered as having a sonic character that is a bit on the warm side. As always, there are countless factors that contribute to the sonic character of a design, in addition to its basic topology.  And therefore, as you said, "I would buy an amp if it sounds good and not because it is fully balanced."

Best regards,
-- Al

Thanks Al,  Third harmonic, that supposed to be euphonic, will be the same in fully balanced or single ended design.  Removal of the even harmonics cannot make amplifier sound "warmer".  It is achieved by reducing higher order odd harmonics produced by the amp.  It is likely related to excellent design and not topology itself.

Also, FULLY balanced amp has practically two amps inside and costs much more.  I could buy much better single ended amp instead. That's why I would never buy FULLY balanced amp ("Fully" is not always "Better").

This statement is false. A fully differential amplifier does not have twice the parts and is not nearly two amps inside! This is a very popular myth.

Our amps tend to be less expensive than SET amps with a fraction of the power, and yet we have more bandwidth with lower distortion (without using feedback). As any transformer designer can tell you, a good SET Output transformer is a bit of a trick! Its very hard to make them in higher power levels- for this reason you can easily build push-pull amps that outperform them for less money.

Of course, we have the added benefit in the cost department of no output transformer...

Balanced input of my amp goes to instrumentation amp (THAT1200) that has CMRR of 90dB@60Hz and 85dB@20kHz. I don’t believe that it can be achieved without cross-feedback in Fully balanced amp. 90dB would be equivalent to overall gain setting (all stages) resistors matching to 0.003%. This cannot be done (be stable) even with multiturn trimpots, etc. Perhaps Fully balanced design offers better rejection at higher frequencies, but I doubt it - frequency response of both halves would have to be identical.

We tend to get about 87-92db CMRR without feedback of any sort. The key is proper Constant Current Source design and I can safely say that most CCS circuits I see in most amps are terrible. Hint: a good CCS design will employ two stages. It simply isn't possible for a CCS with one stage to work right.

I can hear the difference between different XLR ICs - perhaps Ralph was talking about matched 600ohm input/output designs that I’m not familiar with.

If this is the case its probable that your gear does not support the balanced standard. Let me guess- you can run the signal single-ended just by disconnecting pin 3, right? IOW what I am saying is that in your gear, the non-inverted signal occurs between pin 2 and ground (pin1) and the inverted signal is pin 3 and ground. If that is true, then the gear does not support the standard as ground is not ignored, and all of a sudden the cable becomes audible. This is a very common design error and to give you an idea of how common, I saw that mistake being made in a piece of Audio Precision test equipment 20 years ago! I don't know if they ever fixed that...

Third harmonic, that supposed to be euphonic, will be the same in fully balanced or single ended design.  Removal of the even harmonics cannot make amplifier sound "warmer".  It is achieved by reducing higher order odd harmonics produced by the amp.  It is likely related to excellent design and not topology itself.

This harmonic structure thing is another myth. How it works is, if the circuit is fully differential and balanced, the primary distortion product will be the 3rd harmonic, at a diminished level (IOW much less than you would see of the 2nd harmonic in a tube amp). This is true whether the amp is tube or solid state! The implication here is that the topology in fact plays a huge role. In a tube amp the higher orders will be absent given proper design.

When you think about it this makes sense. After all, triodes are quite linear so why should a tube amp make more distortion than a solid state amp? The answer has a lot to do with topology and how much feedback is applied. We don't use much in the way of feedback as we are trying to avoid higher ordered harmonics (and its the feedback that contributes to that in most designs).

One thing you are not taking into account is how distortion compounds from stage to stage. If the gain stage just does not make the distortion, it can't be compounded by the distortion of the next gain stage. We only have one stage of gain in our amps, so higher ordered harmonics really don't play a role. This allows the amps to be very relaxed.