LDR VTL5C3 Response Time?

[tbryanh]

For the VTL5C3 http://optoelectronics.perkinelmer.c...S_vtl5c3c4.pdf

Isn't the response time slow? I am sure the ear responds much faster.

It seems the response time is similar to some settings that can be found on digital delay effects boxes, etc.

It seems that the VTL5C3 might not be usable.

Has anybody experimented with these or have any comments?

[Mark Hammer]

Usable for what? AFAIC, that is one damn fast LDR, going by the specs.

Though the "ear" (the brain, actually) responds faster than most technology, "fast enough" is really all that maters, and you need to define/specify what the application is to know what fast enough needs to be. Doesn't mean you are necesarily wrong, but I think you need to qualify what you said/proposed.

Note, for example that some people find the slow response time of some optoisolators/vactrols actually adds to their usability. For instance,if you have a sidechain of some sort, eliminating envelope ripple often requires a more sophisticated and complex rectifier circuit coupled with various filters. If an optoisolator with a slow decay/settling time is used, however, the slow settling time accomplishes much of the same thing as filtering and extra rectifying without necessarily paying for it in attack time.

If the VTL5C3 is being used for a tremolo circuit, the response time is more than adequate, and if it is being used for a phaser or other LFO-modulated effect, it is certainly more than sufficient.

Again, there may be an application where it is too slow, but you need to spell out what that is.

[tbryanh]

What I had in mind was using stomp switches to select presets and expression pedals to adjust the selected presets.

The presets and adjustments to the presets would occur during an instrumental, not between instrumentals, so it appears that an instantaneous reponse is required for most situations.

[Matt T.]

The response times are the same as the VTL5C1 which is what (I thought) many people used for channel switching...isn't that what the SLO uses?

I'm not very experienced or knowledgable on this but I did notice that the VTL5C3 has only a 75dB dynamic range where the VTL5C1 has 100dB dynamic range.

The '3' also only has 10M OFF resistance vs. 50M for the '1'...not sure if that's an issue or not.

[tbryanh]

I was looking at the VTL5C3 because it can handle 250v
The VTL5C1 can handle 100v.
The VTL5C2 can handle 100v.

I did notice the difference in the dynamic ranges as you mentioned.
I am not sure if dynamic range is an issue for setting and adjusting presets.
It seems to me that it is not.

[Mark Hammer]

Okay. Now that I know the intended application, I understand better.

Be careful what you wish for, because you might just get it. Near instantaneous switchover can sometimes either creat an audible pop or create the illusion of one by produicing sharp contrasts. I'm not sure what the exact switchover time is, but on virtually every commercial stompbox that uses FETs for switching, the abrupt change in FET resistance thatserves as the "switch" has a bit of changeover/lag time added to it by means of a cap, such that the resistance change is not as instantaneous as is technically feasible. It is on the order of milliseconds, but it exists and doesn't seem to perturb musicians. So, in theory, despite what the human ear is capable of, the very brief changeover time provided by the Vactrol you have selected should be darn near inaudible.

[R.G.]

For the VTL5C3 ...
Isn't the response time slow? I am sure the ear responds much faster.

Define "slow" (as Mark points out). Human vision-to-physical response time is nominally about 100-200 milliseconds - 1/10 to 1/5 of a second. The human eye takes "snapshots" about every 1/24 second. You may be able to hear a transient that only lasts 50uS, but I doubt that the human ear could tell the difference in a loudness change that took 1mS as opposed to 10mS.

What I had in mind was using stomp switches to select presets and expression pedals to adjust the selected presets.

The presets and adjustments to the presets would occur during an instrumental, not between instrumentals, so it appears that an instantaneous reponse is required for most situations.

As noted above, this is not true. Instantaneous is faster than the human can perceive the changes. Incredibly fast guitar players can zip along at 220 to 240 beats per minute. That's as much as 3 per second, or 330mS. The VTL seems to be about as fast as the time between notes.

I was looking at the VTL5C3 because it can handle 250v

Controls for tube guitar amps? If you'll block the DC levels from your controls with suitable capacitors, you can probably use either. The highest signal voltages in a guitar amp in most cases is the signal at the plates of the phase inverter. This is comparable to the bias voltage, because anything more is grid clipping on the output tubes. That's around 50v peak, 100Vp-p

I am not sure if dynamic range is an issue for setting and adjusting presets. It seems to me that it is not.

I am sure. It's not.

[AdmiralB]

Be careful what you wish for, because you might just get it.

Not terribly germaine to this thread, but there's a site out there describing the use of a Vactrol element as a replacement for the cell in a Fender tremolo roach. The guy did a bunch of research and concluded that the Vactrol cells would react a lot faster, and he swore by the results.

One of my beefs with the roach circuit is the lack of intensity in most applications. So I cut up a VTL5C1 and rolled the cell with a neon bulb and put it in a Bandmaster.

Boy howdy, did I get intensity. But I also got what amounted to a square wave. It sounded terrible.

[tbryanh]

Wow, thanks for the fantastic responses.

To summarize

1. The Vactrol is good because the part that is in the audio path is an actual resistor, a light dependent resistor. So, it cannot harm the tone like silicon (transistor), metal oxide (FET), etc. can.

2. Typical signal levels are 100v p-p, so a Vactrol rated for 250v should be heavy duty and last a long time.

3. We are using the Vactrol to control circuits, i.e. we are not puting an audio signal to the LED, so dynamic range is not an issue.

4. A fast response time can create a clicking noise, and in "equivalent" FET circuits, capacitors are used to slow the switching speed, so the response time of the Vactrol is potentially ideal and a capacitor will probably not be needed to slow it down.

5. There is the ongoing issue that the Vactrols available in small quantities are two-terminal resistors and not three-terminal potentiometers such as what Mesa Boogie uses.