GT ViPre Rise Time

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kafka

Well-known member
Joined
Sep 7, 2006
Messages
77
Location
Maryland, USA
I was wondering if anyone had some insight into the electrical principle that's used in the GT ViPre rise time control. I haven't seen a schematic - I doubt I ever will. I was originally thinking that this might be just a variable global negative feedback control. But, from the block diagram, it looks like it controls something entirely in the 2nd differential gain stage. Has anyone seen the inside of one of these boxes who would be able to shed some light?
 
I'm not sure. Looking at the block diagram, it specifies that this is a totem-pole (i.e. SRPP) gain stage. I'm now wondering if it just changes the value of the cathode bypass cap on the bottom tube.
 
While LPF is the more common terminology, rise time, IMO is a useful metric to characterize rate of change for audio impulses and isn't level dependent like slew rate. Slew rate limiting is a little like amplitude clipping when you run out of power supply. As long as a circuit is rise time limited (LPF) to a point below where it would run out of slew rate, it will never encounter slew rate limiting and the associated distortions from effectively clipping and losing NF at extreme rate of change. It obviously needs enough power bandwidth to reproduce the full audio frequency range.

Consumers are more familiar with slew rate, so rise time hasn't gained much traction with consumers, who buy what they think they understand.

JR
 
I've listened but not dissected the vipre.

It does sound like slew rate limiting, not rise time.

The distortion characteristic changes dynamically in a way that would suggest a starved stage.

But that's only a quick listen.
 
Hi Group:
I did some test on the Vipre some years back when I was designing the Nash Audio MPA-60.
I can't go into a lot of detail how the MPA-60 does it, but here are the Vipre harmonic scans. I don't have the freq response scans available but the rise time control just seem to roll the top end off as you changed to the slower ranges. Tracing the rise time circuit IIRC, seem to change
primary taps on the output transformer. The build quality was so poor that tracing the circuit was next to impossible without taking the whole unit apart. It was a rats nest. BTW none of the Vipre techniques were used in the MPA-60. I wanted to make sure that I was not infringing on any of the GT techniques.


50 dB Gain
Vipre50dBgain.jpg


Fast

VipreFast.jpg


Med Fast

VipreMedFast.jpg


Med Slow

VipreMedSlow.jpg


Slow

VipreSlow.jpg


Hope this helps.
RonL
 
I said it before: I don't think there's anything really exciting about the Vipre. I much prefer a simpler tube preamp. The built quality of the unit I reviewed looked fine to me, it contained an incredible number of high quality large foil capacitors, probably polypropylene. But the design as a whole looked somewhat convoluted. There wasn't much *usable* variation, either. Even in its fastest setting it seemed kind of "slow" and "antique".

An A-Designs MP-2 or a Presonus ADL600 sounds better to my ears, plus they're stereo and less expensive. GT's new Supre looks like a better deal, too, although I haven't heard it yet, so I can't say anything about its sound.
 
It may or may not be a good value.  It sure is expensive.  As a DIYer, that aspect of it is probably somewhat irrelevant to me.  If there were some interesting design principle going on, it might be fun to understand it better.

I'd be surprised if it turned out to be a simple LPF.  Current limiting seems like a possibility here.  I've found a few sources in the hi-fi world discussing the principle, but nothing really astounding.

The overly-maligned MB S.O.B. used a current limiter on the phase inverter as another master volume.  This was just a CLD with a VR on the cathode of the phase inverter.  Obviously nothing that extreme would be desirable in a mic pre.  It doesn't sound great at extreme settings, but it can be useful when carefully tweaked.  It's not directly relevant, but it does give some food for thought on the concept of current limiting when thinking about any tube design.
 
I was under the impression that the rise time circuit actually controlled the amount of sag in  the circuit. LIke when you use different rectifier tubes in an amp.  ???
 
Interesting.  So it looks like the totem-pole is almost a textbook current-source loaded grounded cathode amplifier.  Here's some information on this configuration.  Notice that the output is taken directly at the cathode of the top tube, rather than at the anode of the bottom tube.  And the rise-time control introduces some kind of feed between the two balanced halves.  Do you have a scan of the meter board circuit?  The block diagram mentions that the current source is located on the meter board.

Pucho - can you describe how the rise-time switch itself is configured?  Does it just connect a resistor between the two stages?  Is it possible to get a close-up photo of it?

Thanks so much for sharing.
 
Here's a photo of the inside front panel from the top.
The rise time switch is marked as is the 6502A.
Note it is sandwiched between copper foil.
Also note all the current source adjustments on the right... Yikes!

insidefront.jpg



RonL
 
Ron, are you sure about where you indicated the rise time switch??  All of my ViPre switch's are (L to R in your photo) Rise Time, Meter Select, 1dB Gain, 5dB Gain, Trannie/Inpedence and Input Select.  The 6502 is only in circuit with the DI input.

Michael
 
rlaury said:
Your right. I fixed it.
Here's a close up of the switch.

switchdetail.jpg

Hey, Ron.  Thanks for putting that up.  It looks like the rise-time control is just a 5-way switch selecting a cap to put between the two stages.  Is that 3rd red wire on the bottom connected to the switch, or something else?

Kind of a neat idea.  It reminds me of the "cut" knob that's showing up in the power amps of a number of guitar amplifiers these days.
 

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