Tube Amps / Music Electronics
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|10/20/2000 3:13 PM|
||Re: Technology of Phasers and Flangers|
I could be completely out to lunch on this, but is it possible that due to an open or shorted speed pot the speed of the LFO is always all the way up? EH stuff sometimes had fairly extreme settings available and I seem to remember that my Polyphase sounded ring mod-ish with the speed pot dimed.
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|10/20/2000 4:41 PM|
I will say this about Rob's SS. The sweep range is probably faster than other SS that do not ring modulate. I did notice that SS that do not ring modulte sweep at a slower speed when the pot is fully CCW. I compared it to a Univox Phasor that copied the SS design almost exactly (difference was the paint job) and the sweep range for it was very linear and very controllable. To me it was very desirable. Again, I did not have enough time to dive into it either.
So much to do, so little time to do it, I think a lot of Ampagers say the same thing?????
|10/20/2000 8:35 PM|
||Chips and ring modulation|
1) The NE571 is pin for pin replaceable with the NE570, and has slightly worse noise specs (like anyone would notice in an analog delay!). So, if you find a local distributor with 571's but no 570's, you are at liberty to sub.
2) ANY...I repeat....ANY modulated effect whose LFO speed can be brought up to, or currently can oscillate in, the audio range, can produce vaguely "ring modulation" type timbres. It won't be *exactly* the same as a true ring mod, but it will have enough of that rubber band kind of tone to suit most tastes. In most cases (and LFO circuits), the range of possible LFO rates is set by a single capacitor. Reduce that capacitor value, and the range of oscillation will be increased by a proportionate amount. So, if your LFO is spec'd to range between .1 and 10hz (100:1 is common), simply cut the cap value by 10 and it will now sweep from 10-100hz. Since the lowest rate of sweep will likely be unsatisfactory, one would ideally want to be able to switch between cap values. Some modulated pedals already have this feature, but usually it involves switching to a cap value that permit super slow modulation (e.g., with units that offer chorus and flanging).
Those Small Stones that seem to possess this capability probably have rate-setting caps that are a bit on the low side of their rated value, yielding higher possible sweep rates (e.g., as high as 20hz). I have an old MXR rackmount Digital Delay, and it has a pull-switch function to increase the LFO rate by either 10 or 100 (forget which), giving ring-mod type sounds. Of course the nice thing about being able to do it with a 4-knob modulated effect is that you often have control of the balance of modulated vs straight signal, so you can mix the "boing" to taste.
Once again, this will work for vibrato, tremolo, phase shifters, flangers, chorusses automatically swept filters and anything else where the sound goes back and forth at some predetermined rate.
|10/22/2000 6:45 PM|
I tried replacing the pot and it did not seem to change at all. Mark, the fact that you say "vaguely ring mod type timbres" makes me think that is what is happening. Should I now bug with this rate-setting cap? Is it still possible for the LFO chip to go bad in this way, or would it just not oscillate at all? I feel that if I had a quick walkthrough of how to eliminate possible causes I could figure out the problem.
|10/23/2000 8:45 PM|
My first question would be "Is there ANY change in sweep rate when you vary the pot position?".
If there is, then the LFO is obviously working as an LFO. It just isn't "L" enough. If you know which cap is setting the frequency range for the LFO, you can monkey around by getting some smaller value caps, and touching their leads to the relevant contacts on the board, and listen to hear whether the sweep rate slows down (best done by setting it to a default fast rate).
There are several issues of the SS. At least one which uses 6 OTA chips, and at least another that uses 5 such chips (CA3094). I'm looking at the 5-chip issue at the moment (found on JD Sleep's web-site) and there are 2 caps which look like the culprits. Not knowing how to use an OTA as an oscillator, I couldn't say which one, but 2 isn't bad odds, especially since adding another cap won't blow anything up. The larger cap (33uf) looks like the better candidate, given its location and value.
Is it possible there is a 3.3uf cap in there by accident? If so, that would explain a lot. If your slowest sweep speed right now is bubbly fast (rather than rubber-band sounding or chime-ey), then there is a pretty good chance you may have stuck the wrong value in there (lousy decimal points that rub off!!).
Just a stab in the dark on my part, though. Get a second opinion.
|10/23/2000 10:42 PM|
Yes, there is a change in sweep rate. The lowest setting is still real fast, I hear nothing that sounds like a phaser even at its highest speed. BTW, I haven't actually messed around with the caps yet. Everything looks original, so I'm still suspecting the LFO chip. But they are hard to find, so maybe I will monkey with that cap.
This sounds like an ignorant question, but I have a Fluke DMM with all your regular functions plus freq. counter (Hz). How could I use it in this situation, or can I?
|10/24/2000 3:30 AM|
OK. It seems like your LFO chip (which is not a dedicated LFO chip but simply an OTA configured to produce a low frequency triangle wave) is generally behaving correctly, if you get variations in sweep rate.
A few questions to clarify things for me, though, since I may have lost track.
1) Is this one of the 6-chip or 5-chip models?
2) Is this a homebrew or a stock unit?
3) If homebrew, did you socket the chips or solder them directly?
4) If homebrew, is it on a PC board or perfboard?
5) If homebrew, can you vouch for the values of the caps?
6) If stock, do you know if any mods or repairs were done to it?
7) Does the color switch function?
Let's start with these, and troubleshoot from there.
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