Roctron Velocity rant ...

[Kaz Kylheku]

Recently I became curious about what exactly the "reactance" control of a Rocktron Velocity 300 does.  (I don't have one of these units, thank goodness).  According to the marketing spin, it has something to do with speaker interaction. Quote from website:

The Velocity 300 has a unique "Reactance" circuit that actually replicates the output impedance of tube amplifiers—so you can get the same great sound that a tube amplifier delivers in a reliable solid state design. And, because it is a variable control, you can customize your Velocity 300 to sound like any of your favorite tube amps. Best of all, this feature is available in the mono bridged mode too!

If you Google for what users say about the amp, you can see that people are falling for this bullshit.

The old 1990's schematic shows this to be purely a tone control which mixes between two paths through different op-amp filters. There is no possible way it has any effect on output impedance. I'm guessing that it produces various amounts of a "frown curve" EQ.

In the newer amp, there are two tone controls. The schematic reveals these to be an obvious variation on the Baxandall tone control topology, again, purely in the preamp. Bass must be the reactance, and treble is called this:

In addition, the Velocity 300 has "Definition" controls to give you that little bit of edge you need to bring your playing out in the mix.

Good grief!

I suppose that if you hear tube sound when you tweak a pre-amp Baxandall bass knob, you deserve Rocktron equipment and the lies that sell it.  xP

[Kaz Kylheku]

In addition, the following marketing bull is found on various websites, without any attribution as to where it came from. I cannot find it on the Rocktron web site itself:

The Rocktron Velocity 300 Rack Power Amp was designed in a joint effort between Rocktron and integrated circuitry design engineer Derek Bowers. The Velocity 300 is the first commercially available amplifier to be based on current-feedback technology. This technology provides extremely high-speed, wide-bandwidth amplification. The bandwidth of the power amp section is considerably higher than typical amplifiers based on voltage feedback designs therefore providing much more punch and definition than previous designs have allowed for.

What? First commercially available amplifier with current-feedback? Just yesterday I was looking at some Roland JC-120 schematics from 1979 with CF. (teemo piqued my interest.)  Straight-faced lie.

Okay, never mind the claim that it is first. I'm pretty sure the thing does not have current feedback!!! There are no obvious signs of any current feedback in the old 1990's schematic or the newer one. Unless the schematics are a decoy.

Bandwidth? Lots of audio voltage amps have bandwidth way beyond 20 Khz.

Another of the Rocktron Velocity 300 power amp's innovative features lies in the reactance simulation circuitry (controlled by the front panel "Reactance" control. At the minimum setting of the reactance control, the Velocity 300 will provide the added punch and improved transient response of the high-speed current-feedback design. As this control is turned up, the Velocity 300 will begin to simulate the interaction that takes place between the tube amplifier and the guitar speaker cabinet.

If so, the schematics are phony.

[J M Fahey]

Well, don't be so hard on them.
Marketing is always B.S., in varying degrees, so I usually let it slip away and just concentrate on the product itself.
The amp *does* sound good, at least as good as any other good SS power amp.
Its having a pair of little tricks added, such as current feedback or some pre-EQ doesn't hurt either.
Marketing hype, as I said before, is better left alone.
I have seen its schematic very long ago, don't remember details.
Would you be so kind as to publish (or link) it?
Thanks.

[teemuk]

There was at least one "Velocity" amp that truly used sampled speaker current  as feedback (if I remember right it was the old 150 or the 120) but most of them (and I guess that includes all the new ones) indeed merely simulate the effect of high output impedance with some filters. The newest 300 doesn't even bother that, it simply labels your run-of-the-mill Baxandall bass and treble controls differently.

I once had a discussion with some person about these new Velocity 300 amps and pointed out the same thing. Before that, the guy had practically described how wonderfully the amp emulates the tube sound, including the sag effect. After setting facts straight about those controls being just cunningly named treble and bass controls he just got downright mad at me because things he was hearing were real to him.

Lessons learned: For some, slight bass and treble boost can be everything about tube power amp sound, and I mean everything. Some people hear that very same thing only as a slight difference, one with almost no importance at all. Then... When objective (eg. purely technical) discussion extends to subjective issues (eg. tone) *s!!t* can hit the fan quickly.  

[teemuk]

...Additionally, the term "current feedback" literally means just that. It doesn't neccessarily relate to the common trick found from musical instrument amplifiers where speaker current is sampled to provide negative current feedback. Basically, it just means using current for feedback, and quite a many opamps do this too.

Here's a nice explanation of the current feedback concept, and you can likely notice the similarity to Rocktron Velocity design:
http://www.andiha.no/articles/audio/dcamp.htm#4.Current

Here's more...
http://www.elenota.pl/pdf/National_Semiconductor/an-597.pdf
http://www.intersil.com/data/an/an9420.pdf

[J M Fahey]

indeed merely simulate the effect of high output impedance with some filters.

Just what Dr Phatt recommends.   :tu:
I have even seen a variation on this in reverb tank drivers.
Since they are highly inductive, one preferred method is to drive them with constant current, by putting them in the feedback loop of an Op Amp, which follows foltage across a small (often 47 ohms) resistor to ground.
But others, such as Music Man, achieve the same effect in a simpler way, just by using a very small capacitor at the input of said Op Amp, and so getting a raising-with-frequency response, the same as the other way.

I once had a discussion with some person about  ....  and pointed out the same thing. Before that, the guy had practically described how wonderfully  ....  emulates the tube sound, including the .... effect. After setting facts straight about ..... he just got downright mad at me because things he was hearing were real to him.

Fill the blanks with *a lot* of things and you'll soon be hated in some particular places.
Oh well.
At least, stoning and burning at the stake are forbidden now.
I mean, they *are* forbidden, aren't they?
Maybe one of these days they'll  :trouble me and I'll end up  xP
Oh well 2.

Lessons learned: For some, .... can be everything about tube power amp sound, and I mean everything. Some people hear that very same thing only as a slight difference, one with almost no importance at all. Then... When objective (eg. purely technical) discussion extends to subjective issues (eg. tone) *s!!t* can hit the fan quickly.  Smiley

Amen, Brother. :tu:

PS: please somebody link to those Rocktron schematics. Thanks.

[teemuk]

ampix.org hosts a few. In fact, all I've ever seen should be found from there.

[Kaz Kylheku]

The trick to finding some of these schematics is to do a Google image search, not a regular search.

Here is one source, where someone posted them to some forum:

http://www.audunmelbye.no/wp-content/uploads/2011/04/rocktron-velocity-300-main-schematic.gif
http://www.audunmelbye.no/wp-content/uploads/2011/04/rocktron-velocity-300-power-section.gif

I found another version of the above which is very similar, but the VR2 and VR4 pots are clearly labeled as reactance. Ah, here we go, I ferreted it up. It was way deeper down in the image search. This one labels the reactance potentiometers as such:

http://www.vagus.it/images/Velocity_300_Schematics.jpg

(I already have copies of all this stuff stashed, of course. Get it while it's hot! Some dude 30 years from now will thank you that you're the only one who has it, haha.)

As for the newer schematics, I also found them in some forum. Some guy who was debugging his Velocity requested them from Rocktron and then posted them because the schematic did not match his unit exactly:

http://music-electronics-forum.com/t16420/

See the "Velocity 300 Sch (2).pdf‎" attachment.

It's a damn annoying schematic. Everything is broken into parts, and there are no clear labels about what connects to what. There are some misspellings, and the identifying legend is in Chinese. But you can easily recognize the pre and power sections.  The drive for the main transistors, which are some kind of darlington device, is an op-amp. I.e. there is no discrete differential input section like in the older Velocity; it's just a big triangle symbol.

[J M Fahey]

Well, lately the identifying labels and even some of the schematic text of *everything* is coming in Chinese.
Even datasheets, simply there is no English version available, period.
And count on designs becoming poorer everyday, specially on "famous" trademarks, since now investors own them, and don't know nor care about sound or much else, only that the product sells well, based almost exclusively on their label or looks, so they can recover sooner what they paid for it.