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chorus effect


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1/13/2003 11:59 AM
Andy
chorus effect
does anyone have a schematic for a good chorus effect that uses only tubes?, if not, is it possible to use a solid state chorus with tube amps without sacrificing the tube sound?
 
1/13/2003 2:10 PM
Mark Hammer

Being concerned about "tube tone" when using a chorus is a waste of your time. Note the following:  
 
1) People like tubes because of their high input impedance which assists in retaining bandwidth.  
2) people like tubes because of the way they respond to transients and pick attack and the smoother-sounding distortion that comes with that.  
 
Now note the following:  
A) Because of their inherent limitations, bandwidth in (analog) choruses is usually limited to keep signal quality and noise under control. Whatever bandwidth advantage you might gain having other devices based on tubes is essentially lost in the chorus context because of what the chorus needs to have (a BBD).  
 
B) BBD's have a limited amount of dynamic range/headroom, and generally have to either have a compander or an input level control to keep the signal from overdriving the BBD (which does NOT sound good when it happens). So there is no sonic advantage gained by having tubes involved in a chorus effect.  
 
The best way to retain the tone of your tube amp AND use a chorus is to stick the chorus in the effects loop of the amp, after the preamp (so you can overdrive the preamp with transients to your heart's content). Purists will argue that tube tone comes from the power stage, and while they may be right this is about as close to having an all-tube signal chain with chorus as you can get. If you want it purer than that, your best bet is to mic up the amp and run THAT through a chorus....or buy/build a Leslie.  
 
Depending on the amp and the type of effects send signal it puts out (instrument or line level), you may need to be either picky about the chorus chosen or build some type of interface so that it can be used in that position.
 
1/13/2003 11:20 PM
Andy

the issue is that I'm building an amp, and want to include a chorus circuit in it, but I also want to have an all-valvestate circuit (except the regulated, current limiting power supply). is there any way to clone a BBD with tubes to achieve the same effect? I'm sure it would take a lot of tubes if in fact it were possible, but that is of no concern. I'm making my dream amp and complexity/cost is really not an issue.
 
1/14/2003 2:25 PM
Mark Hammer

Op-amps have 8 pins and many BBDs have 8 pins. That's where the resemblance ends. Don't let the size of the chip fool you. BBD's have a LOT of transistors in them. Duplicating this is certainly technically feasible and may even have "better" tonal quality given the likely size of the caps (BBDs use a network of FETs and teeny weeny caps to store analog samples; the caps leak). If your interest is in having the ENIAC of guitar amps, be my guest (the ENIAC was one of the first working computers; as a tube-based computing device, it took up an entire room or two and needed a round-the-clock staff on hand to regularly change tubes). Even if cost is not an issue, I suspect you'd like something that could at least fit in the back of a flatbed truck and only require one or two people and a winch to hoist.  
 
The only place where tubes MIGHT make a difference to a chorus is the input and output stages, and perhaps the filters. Certainly the modulating LFO would not benefit at all from being tube-based, and as noted the BBD and clock circuitry would be difficult if not impossible to replace with a tube-based circuit. If your intent is to be a purist, let it be known that you will eventually HAVE to have a significant part of your signal path be silicon if you want a chorus effect.  
 
From an entirely different path, part of what chorus effects do is modulate the pitch. This vibrato effect can also be done with allpass filters and several older Magnasonic amps and a few others used this approach in their all-tube amps instead of volume-modulated tremolo. Each all-pass stage can probably be achieved with half a 12AX7, and it only take 2 of them to produce audible warble. Try one out or check around for posted sound samples and see if it is what you want. Tube-based vibrato IS feasible, is easiy integrated into the amp, and is a desirable effect. Not chorus, though.  
 
Tubes are neat, but don't approach them like some religious truth. There are places where they add value, and other places where all they add is heat and cost and size. Recognize the difference.
 
1/14/2003 4:31 PM
Mike Burgundy

I think RG has tinkered with a tube-based "univibe". That's already going too far for some, but it's a lot more workable than replicating a BBD. A quick look at a Panasonic datasheet tells us the commonly used MN3007 is a 1024-stage bucket-brigade, with two FETs per stage. Replicating with tubes this would take at least 1024 12AX7's and a bloody big fan for cooling.  
Now that's a sure winner in a pissing contest.... ;)
 
1/14/2003 10:51 PM
Andy

that being said, do you know of any high-quality bbd's with extended dynamic range so I'm not attenuating and then boosting my signal (not as inefficient as 1000+ tubes, but still bad) so much? I was already thinking of making the input and output stages of the chorus tube based, and I can clearly see from some of the chorus circuits that the lfo can be solid state with no effect on tone.  
 
andy
 
1/15/2003 9:15 PM
Mark Hammer
"Quality" is not an issue since there are only 3 manufacturers of BBD's and I think all have gone out of the business of doing that.  
 
That being said, the different series of BBDs produced vary with respect to their supply voltage requirements. The first ones in the 70's needed higher supply voltages, which is why battery-powered flangers and choruses (or at least those that functioned for any length of time on less than 2 9V batteries) didn't emerge until close to 1980 or so.  
 
Download the document called BBDementia.zip from my site at hammer.ampage.org and you will find most of the datasheets for what's available out there. Some of the files archived in BBDementia are bum files, but the overwhelming majority are OK. You'll see that the chips vary in S/N ratio and headroom. Typically, the higher the supply voltage range, the more headroom. There is NOTHING on the market that is more than maybe 6db different in headroom than anything else, so don't bother looking for any holy grail.  
 
Please keep in mind that cramming that many storage capacitors on a chip that size is very difficult. The ability to have something "left in the bucket" at the end of the chain depends on the leakage properties of those caps. If anybody had made a BBD in 40-pin DIP size, with big stinking caps inside, I suspect you'd have your ideal right there, but I know of no BBD that has ever been produced that was NOT an 8 or 14/16-pin DIP.  
 
Given the way in which design is pretty much compelled by the BBD, differences between commercial units often have to do with the degree of lowpass filtering provided to keep clock noise out. Plan on having minimum 2 poles of LP filtering before and after the BBD, though many high quality units are unabashed about having 3 or more poles before the BBD and 6 poles of LP after the BBD. And sometimes that is even WITH a compander.  
 
NOTE: I know of no one who has ever sought to "tube-ify" a chorus. My gut feeling is that heat will likely make the BBD behave strangely given the essential role that teeny caps play in this process. It is also important in chorus/flanger design to keep the leads from the clock circuit to the BBD as short as possible. I can see difficulties in layout and heat dissipation arising. Tubes are just a little too warm and big for the duties you have planned for them, and keeping them away from the critical solid-state components will mean a big unit. I mean decent lowpass filtering will already put you up to 3 or 4 dual triodes. If what you want out of this is to be able to scoff at those who said it couldn't be done, then have at it. If it's tone you want, expect to get minimal bang for the buck.
 

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