Voyager Owners - Slew Rate Mod

[GRollins]

Slew rate is a specified characteristic associated with Operational Amplifiers (op-amps) used throughout the audio chain. It defines the output transition time in response to an input signal and is measured in Volts per second. In general you want the slew rate to be as fast as the maximum frequency running through your critical audio signal path. Now we tend to think of signal frequencies in the kilo-Hertz range but oscillator square and sawtooth waveforms have edges with transients in the Mega-Hertz arena - maybe upwards of 100 MHz IIRC.

 

Anyway if that slew rate is too slow the transients get lost and/or sub-harmonics develop altering the original waveform. This is not desirable as you have no control over this limiting property. The slew rate modification replaces several of the op-amps with faster slew rate chips and loses a few capacitors no longer needed or desired. This all happens before the filter. It's easy to think the filter is being modified based on the results, but it's not. There are other design considerations with faster slew rates. I used to deal with these headaches in a previous life but none of these manifest themselves as "losing bass' or anything else for that matter. There are no other degraded audio artifacts - at least that I'm aware of.

 

Slew rate, eh?

 

Think of a movie wherein our heroes are being pursued by alien bad guys across the surface of a rugged planet. They're hugging the surface, following a canyon. Suddenly, the canyon dead-ends. There's the inevitable shot of the pilot desperately trying to get the ship to pull up fast enough to clear the peak of the mountain. It's not a matter of altitude--we'll assume for the sake of argument that a spaceship has infinite altitude capability--it's that you've got a limited amount of time to react and you need to gain a thousand meters altitude right now, or get splattered across the unforgiving rock of the mountainside.

 

Without getting too far out into high end stereo land, one of the misconceptions that tends to arise is that since human hearing is commonly quoted as being from 20Hz to 20kHz, that that's all the frequency response you need to accurately reproduce an audio signal. It's actually a little more complicated than that. While it's true that human hearing tends to top out around 20kHz, far greater bandwidth is needed in actual use, for (at least) two reasons:

1) Limitations in bandwidth tend to come with associated phase shifts. Those phase shifts have audible consequences far below the nominal -3dB rolloff point of the circuit.

2) Humans don't listen exclusively to simple sine waves. We listen to waveforms. A square wave or sawtooth--any waveform with a vertical leading or trailing edge--has a theoretically infinite sequence of harmonics. On paper, you could make a case for needing infinite frequency response. In practice, it gets expensive to go much beyond, say, 1MHz in consumer audio equipment...and at that point you're inviting problems with radio frequency nasties from car ignition systems, radio stations, what have you. Like so many other things, it's a trade off.

 

Opamps (and the pictures I've seen of the innards of Voyagers indicate a small army of the boogers) tend to be regarded with suspicion by audio folks for a number of reasons. One being that the capacitors they use internally aren't that great. Another being that opamps are almost universally designed for high gain, at which point band-aids have to be put into place to keep the circuit stable. Those band-aids tend to reduce the slew rate, which affects the "verticality" of the leading/trailing edges of square waves and such--and how sharp the corners are at the top and bottom. This stuff is audible, even though it happens at frequencies far beyond the nominal range of human hearing. Use brass instruments to demonstrate this in the audio listening room--lotsa harmonics, going all the way up to the sky.

 

So you don't want to use opamps in designing your synth? Always an option. But it would take more time and effort (hence more money) and the circuit board would be much larger, physically, and heavier. Expensive, large, and heavy not being popular characteristics with musicians, you're not going to see a lot of synths designed exclusively with discrete parts.

 

Add to that the unfortunate fact that parts manufacturers are phasing out a lot of the cool discrete parts (I'm scowling at you, Toshiba--I want my JFETs back, dammit!) and you've got a recipe for difficulties.

 

I, for what it's worth, delight in such nonsense and would love to fiddle with a discrete circuit synth, but I scarcely have time to get the stuff done that I need to do now, much less a time-consuming project like a high end Voyager-ish circuit. So...I'll probably end up getting the opamp version that the good folks at Moog put out. Although I don't necessarily expect that I would twist the knobs to fully open position, I expect that a careful listen in person would reveal that the circuit sounds different at more modest settings.

 

From the above yik-yak you might conclude that the lower frequency response is unchanged by slew rate modifications. You'd be right. The psychoacoustics of low frequency perception is a fascinating subject in and of itself. For the moment, let's just note that a lot of people regard boom-boom car stereos as having a lot of low end. Ain't so. They're loud, of course, but the frequency response generally has a peak around 80Hz (from the music, if not from the system itself), which is where humans tend to perceive the "punch to the chest" sensation. For reference, low E on a bass is roughly an octave lower, circa 41.2Hz, but most bass amps and PA systems are already in free fall at that point (-10dB or worse) and most people wouldn't recognize a 40Hz note if it bit 'em in the butt...much less 20Hz. But that's another story for another day.

 

Grey

[dsetto]

I believe there is something lost along with what's gained with the slew rate mod. Bass response under certain conditions, perhaps?

 

What are you basing this on? I'm curious whether you heard something in my samples or if you read/heard this elsewhere. And as always, I'm not looking to sell anyone on anything.

Come to think of it, I did not notice it in your samples. And I listened to them a few times, to discern the differences in the high end. I read that prior to having the mod done. When I first played it after the slew-rate mod, I felt that the low end under certain circumstances felt less. I knew my perception could very well have been influenced by what I'd read. I never thought about it again. I liked the synthesizer very much before & after the mod.

 

I recall reading the following reasonable notion: Possibly the presence of more high end influences how we hear the sound.

My thoughts: In my particular case, that may have affected how loud I had the overall volume. And, if there's lots of high end present, I'm going to set my master volume softer than if it wasn't there. Hence, the bass - in that scenario - is heard at a lower relative volume.

 

That observation was done with the raw synth & no processing (such as compression or eq.)

-

As of now, I don't know what the slew rate mod actually does. (I have to slowly digest this last page, here.) ... If it results in greater high end, without actually detracting from the low end, then I would imagine awareness of that - or pursuit of bass by all means necessary (compression, eq, ....) - addresses it well. That's probably why I've never thought of it again. (I just got to your explanation.)

[dsetto]

It's possible you only need to send the Analog circuit board for the mod.

 

did you get this information from Moog Music? I had 2 repairs done by Dave Smith Instruments this way (sending only the circuit board that needed repair, not the entire keyboard), but I didn't know Moog Music offered the same service. :idk

That's how I did mine. I was already sending it to address a faulty chip.

[dsetto]

... Is there any external sign that Moog has done the mod? Meaning, do they affix a sticker, etc? ...

 

On mine, it's easily discernible when looking at the analog board. It's an obvious addition.

[Doerfler]

It's possible you only need to send the Analog circuit board for the mod.

 

did you get this information from Moog Music? I had 2 repairs done by Dave Smith Instruments this way (sending only the circuit board that needed repair, not the entire keyboard), but I didn't know Moog Music offered the same service. :idk

That's how I did mine. I was already sending it to address a faulty chip.

 

that's great news.

[Markyboard]

... Is there any external sign that Moog has done the mod? Meaning, do they affix a sticker, etc? ...

 

On mine, it's easily discernible when looking at the analog board. It's an obvious addition.

 

 

Umm (and I'm really not trying to bust your chops here) the slew rate mod only involves changing out a few ICs and removing a few caps. Depending on the soldering skills of the tech it may or may not be that obvious.

 

Depending on the age of your Voyager Moog may have performed some additional ECNs (Engineering Change Notice) unrelated to the slew rate mod. There are some alt wires on the back side of the board to address unlocking the sync between oscillators and a couple others from early on.

 

But no external signs, other than the sound.

 

[GRollins]

...And I doubt that a seller is likely to be all that happy with a stranger wanting to break down his Voyager to look for mods...and even less likely over eBay.

 

A Voyager turning up locally? Not in my wildest dreams. I bought the only Moog synthesizer that's dared to show its head hereabouts in at least the last six years and it was a humble Little Phatty (original version). Voyagers here? Don't make me laugh.

 

Grey

[dsetto]

No chops bust. Reconciling truths is what matters to me.

 

I saw my Voyager Analog board a couple of times, as I had 2 issues with it. (It's a work of beauty.) When I installed it after the slew rate, I noticed a "large" somewhat messily soldered-on "thing" that I hadn't noticed before. I *assumed* that was the slew-rate mod. Everything except for this seemingly new and seemingly jutting-out thing was very tidy. As I'm calling it a "thing" you can see I have no authority to be a reliable source.

 

Grey, I would think Mark's demo would be the best clean-hands guide to ascertaining if the mod's been performed. Opening up to see the board simply requires about 7 or so screws. Very quick & easy, and an accommodating synth-lover seller may oblige.

 

Markyboard, I'm eager to read your slew rate explanation to see if I can make some sense of it.

[dsetto]

Grey,

Based on my conversation with a person from Moog support ...

The slew rate mod is optional. It is not done as a default. It is not part of the original design by Bob Moog. Some purists may prefer Bob's intention for the Voyager, & thusly not do it.

[GRollins]

Grey, I would think Mark's demo would be the best clean-hands guide to ascertaining if the mod's been performed. Opening up to see the board simply requires about 7 or so screws. Very quick & easy, and an accommodating synth-lover seller may oblige.

 

That still leaves begging the question of what to look for.

 

I suppose I could look for pictures on the web, but I'm generally leery of soi disant "experts" pointing at this and that and saying that this is an unmodified board and the other has the slew rate mod. For all I know, they could be misidentifying a repair (there's a Voyager on eBay at this moment that is said to have had an oscillator repaired), rather than a mod.

 

Not having seen a schematic of a Voyager--either stock or modified--I would be unable to tell by looking at part numbers as to what's what. If the mod was done cleanly enough, it might look stock even though it's been changed.

 

Grey