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Left, Right… What’s the difference?

A–B Switch Installed

Well let’s listen to A minus B and find out.

It’s often the simplest things that can get you excited all over again. Here’s a short anecdote about trusting a whim even when it isn’t immediately clear how it factors into the path that you think you’re on.

A couple months ago I found a small treasure while antiquing with my wife. I’m referring to the 1950s Grommes 24PG HiFi amplifier which I’ve already written about in a previous post. As I type this now, I’m still waiting for my replacement transformers to be custom wound. COVID-19 has understandably slowed down production of many such things, and luckily I’m not in any rush. But the symptoms I observed before diagnosing this amp have had my mental gears turning ever since.

Grommes 24PG
Inside the chassis of the now-much-cleaner Grommes 24PG
To briefly recap, one of the the Grommes’ two output transformers (it’s a stereo amp) has a break somewhere between its center tap and one end of the primary coil. As a result, this push-pull amplifier was entirely missing one half of the waveform on one of its two channels. After my first listen, I told a close friend and fellow sound engineer, “It’s exactly like I’m listening to A minus B. The center program is missing and I’m only hearing what’s unique to each channel.” Some further troubleshooting revealed that it sounded that way because that was exactly what was happening. Measuring 0v at the plate of one power tube was the clincher. What I had purchased (for the time being) was essentially a bulky differential amplifier.

Without getting too deep, I’ll back up and define a few things. A differential amplifier, in broad terms, is an amp that suppresses all voltage that is common to its two inputs and then amplifies only the difference that remains. In the case of a stereo audio signal, this would mean removing anything panned toward the center of the mix and leaving only the portions that are far-flung toward the left or right sides. Suffice it to say that because of my amp’s compromised power stage, (through a phenomenon called phase cancellation), the left and right channels were being subtracted from each other instead of summed. Hence, A (left) minus B (right) as opposed to the usual A plus B we’re all used to hearing.

Since I recognized its sound and noted it in conversation, this clearly wasn’t the first time I’d ever heard that particular effect. Far from it. But it certainly was the first time I had thought about it in a long while. When I was a young house engineer in the ’90s, I was introduced to the technique of monitoring in A–B during mixdown for all kinds of useful purposes. For instance, you could zero in on phasing problems you might be noticing in the periphery, check if separate reverbs were playing nice together or combining poorly, verify desired separation between particular tracks, among other handy evaluations. I even read that the legendary mastering engineer Bob Ludwig was a frequent proponent of this listening technique and the insights it provided. Now, having had an accidental taste of it all these years later, I found myself craving this tool again.

And all because of an ancient, broken stereo that I, for unclear reasons, wanted so badly to buy and refurbish.

Stereo Reconsidered
A shamelessly-stolen, textbook illustration showing a typical stereo field in terms of “A” and “B”.
The steps to creating this subtractive image could not be simpler. It’s as easy as reversing the phase of one of your stereo channels and then summing them in mono. Either of the two channels will work, but general convention is to flip the right “B” channel (following the “A–B” nomenclature). My first thought was to build one of my usual party trick enclosures which I could patch between any source & load that I wished. However, what I really wanted was to hear A–B while mixing, and an external box was not going to accomplish that. I needed something that could flip the phase of B at a point after all my busses were funneled down to 2 signals, but before the console’s mono summing amplifier, and that meant I had to add something internally. (Yay!)

I’ll note that some mixing consoles already provide switching for A+B/A-B listening, but my M-520 is not one of those. As luck would have it though, this desk did come into my possession with a small modification that has never been of any use to me. The upshot was that it already had an extra hole drilled through the monitor section for a toggle switch of exactly the size I would need for my new function.

I’ll spare you the details of carving out the previous owner’s mod to the talkback mic switching. Let’s just say I deleted all that first. From there it was mostly a matter of finding a smart location for two 1:1 ratio “unity” isolation transformers. These small transformers, made specifically for audio, serve a couple of purposes. I originally bought them to eliminate some low-level noise I was sometimes hearing from my outboard effect returns during mixdown. As the name implies, isolation transformers are made for this very thing, and they would still serve that purpose beautifully. But now, secondly, the one for the right channel would also facilitate the phase inversion for my new function. A small DPDT toggle placed in the afore-mentioned hole was wired to optionally cross the output leads from that secondary coil, and voila! A–B available at the flick of a noiseless switch.

Transformers and SwitchM-520 Monitor Section SurgeryA–B Switch Installed

So again, beyond my general fetish for all things audio, I had no real idea why I had such a strong impulse to take on that old Grommes stereo at a time when I had so much work to do with endless ground to cover on other projects and on my professional development. Nevertheless, I gave in to the idea that, as long as I was following my fascination, the benefit would reveal itself eventually. Happily, I was right. I can’t believe it’s been so many years since I’ve had this trick in my bag.

To anyone taking the deep dive into a lifetime of continually improving your own mixes, I can’t recommend this simple technique enough. Especially if you’re at a stage where you’re concentrating on improved imaging. The ability to expose everything beyond the music’s natural focal point and foundation is wonderfully revealing and addictive. You’ll begin to think of it as the perfect tool in all kinds of instances, perhaps becoming as ingrained in you as the humble “solo” or “mute” button.

But some things are best when demonstrated. Some fun A–B examples of extremely well-known recordings are provided below. (I’m claiming “fair use” of these bastardized excerpts for educational purposes, so we’ll se how that goes.)


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Getting A Little Grommes Under My Nails

I must be crazy. To put it simply, I’ve got a lot on my desk at the moment, and here I am digging into a fresh can of worms. But…

Worth it! Totally worth it.

I found this old Grommes stereo amp at an antique shop about a month ago. No idea why I was so drawn to it, but very glad I was. I initially resisted admirably, but after snapping some pix on my phone I returned home and located a schematic for it online. That did it. I returned the next day and took it home.

Grommes 24PG

Grommes 24PG

Grommes 24PG

It’s a model 24PG from the mid to late 1950s. Rated 12 watts per channel (hence the “24”), it’s powered by push-pull 6BQ5s (aka EL84s) with 7025s (similar to 12AX7s) at all earlier gain stages. It’s also tube-rectified with an EZ81.

For the most part, this thing seems like a genuine time capsule. Appears to be untouched inside since it left the factory. I began my refurb with a much-needed cleaning (see photos below – was positively filthy), and testing values of all key components. After that I felt confident enough to place it on some concrete outside and power it on. Happily, it warmed up normally, so I didn’t need the fire extinguisher I was holding. (Never know with power transformers that old.)

Grommes 24PG

Grommes 24PG

Grommes 24PG

Feeling even more encouraged, I then decided to give it a full audio test. A pair of Klipsch 2-ways were attached to the 8 ohm terminals, and I delivered some audio to its auxiliary inputs – to effectively troubleshoot the shortest possible portion of the signal path (skipping the earlier phono and tapehead gain stages).

The good news was that the overall sound quality is glorious! The bad news was that, in stereo, the mix became somewhat ghostly with the balance set at center. I suspected a handful of different causes, and after some further testing, discovered “infinite resistance” between one side of the left output transformer’s primary coil and its center tap. I was hopeful the short was only in the lead, but I’ve now confirmed that it’s an internal failure. Bummer.

So, off to Edcor‘s website I went to find a replacement. Since the right channel’s OT is intact I was able to measure & calculate the primary resistance and find the most suitable replacement. Order placed… so now we wait.

Nevertheless, I’m extremely happy to have had an inspiring taste of this little treasure. It’s definitely a sound that I don’t yet have in any of my listening rooms, and I’m looking forward to nursing it back to 100% health. Will post again once progress resumes


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