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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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Meet ‘The Raminator’

'The Raminator' finished

Recently I’ve been missing a certain guitar sound that I had at my disposal as a kid. It was a ridiculously scooped, buzzy, fuzztone that was nothing particularly useful by itself, but could be outrageously good in the right context. I mainly used it with volume swells to simulate low orchestral strings within thicker arrangements. Executed and mixed properly, it was indistinguishable from a quartet of cellos.

This effect was one of many I could dial up with my treasured old Boss ME-10 multi-effects – a heavy-duty floor unit made in the 90s. I evidently sold mine at just the right time given that most seem to have given up the ghost by now. Something about cheap electrolytic caps being slowly cooked in a particularly hot location, but that’s a whole other topic.

In the past few months I’ve whipped up a few ultra-wide, passive, notch filters hoping they could be combined with some of my existing distortion sounds to achieve the same effect, but to no avail. The sound I was seeking was indeed more integral to the signal than that.

By now I’m sure you’ve guessed that the only course of action left to a certifiable lunatic like myself was to design and build a new effect pedal based on the topologies most likely to deliver the desired sound. The result is… The Raminator

Raminator freshly drilled enclosureRaminator guts instalation'The Raminator' finished

Astute readers may speculate whether the name is derived from a certain EHX Big Muff variant, and they would be on the right track. However, I’ve made too many changes to comfortably call it a clone. This poor thing is more machine now than sheep. Twisted and evil.

It still features four gain stages with the two clipping stages in the middle, but with some pretty different components and values, particularly in the filtered negative feedback loops. Those probably account for my most significant alterations – to effectively widen the Q of the mid cut, scooping out much more of the fundamental signal on that foldback. And just for a little versatility, I added a switch that can effectively “undo” my gutting of the signal and produce something a bit flatter than an actual Ram’s Head would be normally. I figured between those two modes, the amp’s tone stack, EQ on my mic’s signal paths, etc., I’d end up with a fair amount of tonal range beyond the one trick for which this ruminant was built.

Those and some other minor but deliberate tweaks (including pot values & tapers, transistor models, diodes, etc.) seem to have achieved something just slightly unlike anything. It sure was a hell of a lot of circuitry to cram onto that little soup cracker, but does it work?

Well the proof would be in how well it fakes a chamber full of moaning f-holes. Personally I think it’s bleating brilliant, but you tell me. I’ve posted some test audio below.

Until next time, I’ll be ba’a’a’a’a’ck.


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What’s all this then?

GEV Model 638H Record Player

Why the blog?

Whenever you reboot your website as I recently have, you naturally need to refresh all the carefully written copy it contains. Then if you’re anything like me, you quickly discover how easy it is to completely lose the plot along countless long, deep tangents. Perhaps you disappear down a few of these rabbit holes for days or even weeks at a time. But then like any creative process, you make choices. You cut the fat, resume course, and strongly communicate what’s central.

Nothing particularly groundbreaking there, except that this time I felt a much stronger sense of loss after the final cuts were made. Granted, most of my unused ramblings drifted far from the succinct points that a website needs to make, but that doesn’t necessarily make them irrelevant either. On the contrary, in terms of representing my valuable idiosyncrasies, and in turn, highlighting the competitive advantage of my services, some of the outtakes performed better.

Now believe me, I of all people get it. This is a keyword-searching, elevator-pitch world that we live in. You have to produce short attention span theatre nowadays. But goddammit, sometimes it’s the sum of countless peripheral factors that gives all the color to the middle. Sometimes you have to take a side trip to best view what’s along the main route.

GEV Model 638H Record PlayerGEV Model 638H Record PlayerLabel of Queen's 'News of the World'

For instance, let me take you back for a bit. One of my earliest memories of being alive is as lucid to me as if it were yesterday. I was sitting on the floor of my tiny childhood bedroom, and I was being hypnotized by the pale blue & violet label of Queen’s ‘News of the World’ as it spun above the orange plinth of our portable GE record player. I was probably 2 or 3 years old (no joke, see figure 1). The record was my older brother’s, the player might have been my parents’, but the transcendent experience I had with those sounds and the resulting drive for profound connection, those were completely my own.

A little later I remember my brother demonstrating how a phonograph worked with a rolled-up paper cone and a thumbtack. He joined the two into a makeshift gramophone that could amplify the music from a record mechanically. No electricity required. Well, I was positively transfixed. It was so ghostly and enchanting to distinctly hear those familiar sounds (albeit faintly) with nothing at work beyond what I could observe right before my eyes. No electronic components hidden away inside a mysterious enclosure, just simple things. Materials that us kids had handy. Well it was a rush of bewilderment that has never left me. I remember wanting to somehow squeeze down into the groove of that LP to explore it like a climber in a slot canyon.


Figure 1: Brief excerpt from a cassette that my late mother gave me, labeled “July, 1978”. I would have been two and a half years old. She was coaxing me into singing for the tape, accompanying myself on ukelele. Although I was not particularly cooperating, I did provide a few recognizable bits. Incidentally, this is probably my earliest recorded musical performance. (At least I hope so.)

Today, although the experiments and the equipment are much more elaborate, the same compulsions persist, and they stem from the same fascinated state. Music by itself is more than enough to stand me transfixed and stunned drooling, but the study of its nature has always held rewards beyond this already priceless gift. It’s the unshakable sense that such investigation provides a commensurate understanding of our very universe.

And it’s in that spirit that this new blog is dedicated. While the rest of my website dutifully holds down the SEO-friendly core of my message, this section can be a disorderly playground for any ‘meta’ rumination I wish. From philosophical rants to technical adventures, it will be an open journal for any share-worthy notes too long-form for the vaunted social media, but still chewable enough to encourage my own spontaneity. My hope is that it will convey more than merely what I do and much more of the reasons why.

So here’s to those scenic backroads. Yell if you wanna’ pull over.


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