A new difference

November 30, 2016
 by Paul McGowan

We make friends by focusing on our common experiences, thoughts, opinions, and tastes. If our differences are too great, we typically don’t bond. The opposite is true with a difference amplifier, the heart of most electronics you listen to.

We’ve looked at balanced inputs and cables to discover how balanced audio ignores common noise and amplifies only differences. Now, let’s look at a balanced amplifier itself.

A balanced amplifier is more properly called a difference amplifier. Both terms are accurate. Differential amplifiers are balanced, and balanced amplifiers are differential. But I prefer to think of this type of circuit as a difference amplifier because it more accurately describes how it does it, rather than what it does.

Difference amplifiers are based upon this circuit, called a differential pair (diff pair for short).

It looks complicated but it’s not. Q1 and Q2 are the pair in diff pair. What’s drawn are regular transistors, though they could be FETs or vacuum tubes. What they are doesn’t matter. It’s how they are connected that does.

V+in is where the + of a balanced cable would go, V-in is where the second, out of phase, input from the cable goes (Remember? There are two signal wires in a balanced cable).

Ignore all the rest of the squiggles and notes.

Focus instead on where Q1 and Q2 are connected directly together—the two arrows are pointing at this junction. See it?

For purposes of our understanding, this connection of the two transistors is all we’re interested in.

Whatever signal is placed on either of the two inputs, an identical copy comes out where the respective arrow is (called the emitter).

If the two emitters have the same signal (because the inputs have the same signal), there is no difference between the two, thus, no current flows. Like a teeter totter with two equal weight people, or closer to home, our example of the two wires of a lightbulb placed on the + terminal of a battery. Nothing flows, because there are no differences.

However—and of course you saw this coming—place opposite signals (out of phase: one rising as the other is falling) in the diff pair’s inputs, and current flows.

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11 comments on “A new difference”

  1. Sorry to mention this Paul but there is nothing new about this. It’s been around since just after the flood. If it is so superior, why hasn’t it been universally adopted by everyone else. As you know and should admit, the cost of the extra parts is peanuts compared to the ridiculous prices of the equipment on the retail market. How much does the extra transistor cost, 25 cents? 10 cents?

    1. Of course you’re right, Mark, hence my point in this series. There’s no reason I can imagine that companies don’t add balanced inputs – the only excuse, as in our own Sprout product, is a lack of back panel space for the large connector.

  2. Hello Paul. Your topic focussing on differential voltages triggers the following question concerning a stable reference voltage: Having no clue about the specific behaviour of slow moving electrons and light-speed fast EM-waves pushing the electrons I wonder the amount of voltage these electrons create when arriving at the neural rail after having passed the speakers chassis and lost some energy for accelerating the membranes and having created a lot of heat (dissipation). From my point of view there never will exist a stable “zero-voltage”. The voltage should fluctuate with the current arriving at the rail. Maybe this dilemma has inspired some snake-oil guys to offer ground blocks? Or putting things to the water and pipeline analogy: the negative speaker wire should have a much bigger “volume” (lumen, electron capacity) than the positive wire???

      1. My question simply refers to the voltage on the neutral rail, Paul. I guess this voltage is not constant as assumed in theory. VEE should create a current flowing to the speakers and being finally guided to the neutral rail.

  3. Hello Paul,

    I have a phono pre-amp that uses rca connections exclusively. Is there an advantage to using an xlr adaptor to feed into my balanced pre-amp and then into my balanced amplifier or will it be too late by the time I use an adaptor?
    Please help. I’m excited about the concept but concerned my phono pre-amp will not benefit from this treatment.

    1. Hello Drew,
      I have a little component made by Audio Research called a BL-1, which transforms a single ended signal to balanced. I used it for a while to convert my phono preamp output to a true balanced signal, but it can be used elsewhere.
      The bonus was that it increased the signal by 6 DB, which is useful when you’re using a really low output cartridge.
      All that is debatable, and I’m not sure ARC makes it any more, but every once in a while one comes up for sale on Audiogon or whatever.
      ARC offered a line stage preamp with only balanced inputs and outputs called I believe an LS-3, and came up with the BL-1 to boost sales for folks with only single ended equipment.
      I looked at the innards of the BL-1 and there was a lot more than an op amp in the circuit.
      Funny that after the LS-3, ARC always included the balanced as well as single ended inputs and outputs.
      My dealer at the time told me I was missing 40% of the ARC equipment’s performance if I didn’t use the balanced connection.
      Hype? I don’t know, but it sure sounded a lot better using the balanced connectors.
      LS

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