Explain to me like I'm 5: The difference between fully balanced and bridged mono


2014-07-16 1:39 am
I have been intrigued by the Monarchy Audio SM-70 Pro amplifier. It is a pure class A "true balanced design".
For those not familiar with this little amplifier, it is a stereo amplifier that can be run as a mono block in two different ways: 1. with a balanced input (XLR) or 2. single ended (RCA). Infact Monarchy says it was designed to be a balanced mono block, stereo use seems to be the second use case.

Here is what I'm trying to wrap my head around. According to a review of the amplifier: "
The SM-70 Pro is designed as a 25Wpc stereo amplifier that can also be bridged to perform as an 80W monoblock. What the amp is really designed for, however, is to be used as a fully balanced mono amplifier -- when its XLR input is used, it routes the positive and negative legs of the signal to the left and right channels of the amplifier. The loudspeaker is then connected to the positive binding posts for each channel (right being positive and left being negative). The SM-70 also has a switch that will bridge the amplifier when a single-ended input is used; the amp then routes the output of the left channel's gain-stage op-amp (a Burr- Brown OPA2604) through a resistor into the inverted input of the op-amp's right channel. Thus the right channel's voltage gain is controlled by the left's op-amp.

What is the benefit in sound quality/design that is likely to experienced from a balanced XLR source vs. unbalanced? Would it be even worth it to run unbalanced? What other amplifier designs or manufacturers use a circuit architecture that is "fully balanced" in this way?

Details on the amplifier can be found here: Monarchy Audio

Thanks for your input.


2018-08-31 12:32 pm
A balanced cable has better common-mode noise rejection and typically runs at a higher level compared to unbalanced connections.

Unless you're using a long run of cable it should make little difference provided the inverting stage for the unbalanced bridge mode is well designed.
There is actually nothing in the specs to indicate that this is a fully Class A design... power just keeps on increasing as you go down in load impedance. It would have to dissipate something like 350 W to meet its specs, and that seems unlikely given the amount of heatsinking on there. It would weigh a whole lot more than 24 lbs, too. Looks like a rather normal AB design with MOSFET outputs to me.

The specs are full of red flags btw... not being able to spell "impedance"... minimum load impedance being the same in unbalanced and balanced mode (yeah right)... S/N "better than 100 dB" (pretty meh by modern standards) in balanced mode but only "better than 90 dB" in stereo mode (I betcha they had a ground loop when measuring this, not a surprise since the unit appears to be an IEC Class I device)... This is a "someone's backyard shop" type product.

I would argue that operating two single-ended amps out of phase actually is not fully balanced, even though it carries much of the same benefits (some common-mode rejection, symmetrical power supply loading). It's comparable to using an unbalanced stereo line input as a balanced mono input.

Its problem area consists of common-mode gain being as high as differential mode gain. Let's assume the input to be DC-coupled for simplicity's sake. Then a common-mode offset of 1 V (peak of 0.7 V nominal input sensitivity) would be sufficient to bring both outputs to maximum positive voltage, and every output signal would end up being clipped.

By contrast, a balanced line receiver would eliminate most of the common-mode component and keep it from becoming a problem... Some manufacturers have actually used the entire power amp as a balanced receiver (not necessarily my preferred approach due to noise considerations) - then common-mode tolerance is being defined by power amp common-mode input limits, which could be in the tens of volts.

There could be another problem when using the XLR... no input level control like a pro amplifier would. Balanced outputs not uncommonly have both higher output level and more noise.

IMHO this amp could have been made substantially better with fairly little effort, a handful more parts and a proper understanding of balanced inputs (reading the various articles on Rod Elliott's site and some of what Bruno Putzeys has written would help).