Robben Ford Discussion

[Aeolian]

Daved, I wonder if you folks have every tried one of these. After a long and involved technical discussion with the fellow who designed it at the AES Furman booth, I decided to give it a try.

One of the things Andy Fuchs did was to design the power supply of his take on Alexander's amps following his experience in esoteric audio. Where the power supplies are much more immune to the wall supply than typical vintage circuits. And my understanding is that Robben's amps actually have a modification to Alexander's standard design to make them a bit saggier (Although they certainly don't sound saggy to me! Rather, very stiff and solid).

Anyway, even with the high speed switching diodes and distributed capacitance in the Fuchs, I was able to tell the difference with the PF Pro. Having a few gigs with it under my belt, I can see the consistency and feel improve. There are always those nights when the rig just seems to sing and there are those nights when you struggle with it. The PF Pro seemed to pull it up to a good night regardless of where I was playing.

I know that you use a Variac to level the input voltage to a set point. In previous threads we have discussed Red Suede's AR-15 that automatically sets itself to a fixed output voltage regardless of the incoming voltage.

If there is a wide variation in input voltages, these devices can help to level the playing field. The Furman designer felt that they were very appropriate for situations such as festivals where generators are used and the voltage can fluctuate all over the place.

What the Power Factor does (and the technology is in a little sealed can) is lower the effective impedance of the AC into the amp. This means that the amp thinks it is plugged into an optimum supply. Even when there may actually be extension cables, small gauge wire in the walls, aging connections, aging breakers, and any number of other things that interfere with the current flow from the power station to the amp.

It can be plugged into the Variac or something like the AR-15 so that it gets a constant voltage, but it should be the last thing before the amps AC input.

There is a big difference between this and some surge suppressor/noise filter. Surge suppressors typically use Metal Oxide Varistors to shunt the spikes off to ground. These can cause noise and raise the supplies impedance. Most coil filters in rack power strips are only effective at very high frequencies. The Variac you use has the effect of being a better filter as the transformer is made to work at 60 Hz and not at higher frequencies.

What the Power Factor does is let the amp behave consistently, regardless of where it's plugged in. That is the "feel" of the amp is consistent. A great amp becomes like an extension of the guitar and your fingers. And you play it just like you play the guitar to get the tone that you want. Typically, between the acoustics of a venue, and the power supply, it sometimes feels as if you have different gauge strings on your guitar. What sounded great last night, suddenly sounds no where near, even though everything is set the same.

It seemed silly to spend $300 on something when I already had a very well designed power supply in my amp, but it's a keeper. Like the Furman guy said, he'd never had a guitar player use one and then give it back.
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There are no such things as wrong notes, there's only the look on your face.
My Stuff: www.stevekirbymusic.com

[JackD]

I picked one up early last year. I plug my amp (GDS 100w) and my rack (with a TC 2290) into it. In addition to the improvement in sound, I feel much better protected using the PF Pro.

[Ian Hurtt]

If you get a surge, does this die a heroic death to save the gear or can it be reset?

[Aeolian]

It's not really a surge suppressor. There may be an MOV at the input end but that's not it's purpose in life. There are some devices from folks like PS Audio that have more effective clamping circuits without the artifacts of MOVs or sacrificing themselves. But the incidence of killer surges is vastly overstated.

The PF is a storage device that provides instantaneous current to the amps mains input. This is relatively easy with DC, every amp has a storage cap or two after the rectifier to both smooth out the AC ripple and provide a current reserve to the active circutry. Car stereo nuts put huge capacitors on their power supplies so that those humping big amps have something to work with when the kick drum hits. The generator and car battery cannot supply current quickly enough for the amp when that transient hits. The resistance from the source to the amps output transistors is too high.

To a lesser degree, the same thing happens in the clubs we play in. Between the transformer on the pole and the outlet our amps is stuck into, there are all kinds of wire and corroded connections adding resistance. Over an average, the voltage gets though. But when the amp needs some right now, as when Robben gets his finger under the string and does a chicken pickin pop, all the amp has to go on is whatever is in the internal filtering capacitors. By the time the power at the AC plug recovers from the drain caused by that notes attack, the moment is past.

What the PF Pro does is compensate for all that resistance in the AC line (since it is an AC source and the current draw is modulated by the amps responding to music, the term impedance is used). This allows the amp to behave as it was intended, and consistently, regardless of how much resistance there is in the AC line.
_________________
There are no such things as wrong notes, there's only the look on your face.
My Stuff: www.stevekirbymusic.com

[diatonicdude]

Aeolian,

we can always rely upon you to give a great in formative post, no matter what the subject. Also "hats off" to you for the relaying of your knowledge of electrical engineering (respect from a fellow electrical engineer ), and in particular your analogies like Robben's current hungry "chicken popping". Loved it

All the best,

DD

P.S I have repaired many a device over the years when people were going to throw them to the trash seeing the blackening of the pcb, as all that has happened is that the MOV has short circuited, causing itself to result in an eventual open circuit (change in resistance proportional to the pd increase). A few pence (cents) for the varistor & use of a soldering iron and their electronic baby is up and running as new (that's unless some other components have not also taken the brunt of the transient!!!
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Music is the universal language; speak it with emotion, listen with a passion.