Why can't BJTs be used instead of JFETs in phasers?

[Mark Hammer]

Regular bipolar transistors are often used as if they were voltage-controlled resistors in things lije autowahs.  Given the narrow scope and availability of those JFETs preferentially used for phasers, you'd think that we'd turn our attention to the vast universe and availability of bipolars.

So why do we not do that, and why can't we do that?  It's a sincere question, in search of understanding.

[Scruffie]

The Pearl PH44, googling for the schematic, it seems you own(ed) one.

[ElectricDruid]

So CMOS switches, LDRs, and OTAs are not enough options for you, Mark?!

[Fancy Lime]

So CMOS switches, LDRs, and OTAs are not enough options for you, Mark?!

Confucius says: never use an OTA for anything that you can do just as well with a BJT.
I read that somewhere on the internet. True story, trust me.

On topic: no idea, been wondering the same thing in the past but since I am not super into phasers, I did not pursue the topic.

Andy

[Rob Strand]

One of the last pedals I could be bothered tinkering with was in fact a BJT-based phaser scheme.   The motivation was  exactly the reasons you mentioned.    IMHO, it's very much something which might work out.

So this is how far I got:

The idea is to plop BJTs into the common all-pass stage, like the MXR.   The emitters go to a common DC voltage.  Each collector goes the all-pass caps.   The bases have a series resistor and that goes to the LFO.   The base resistors can be used to "match" the BJTs, although the default matching isn't too bad anyway.

The differences between BJTS and JFETs are:
1) The effective CE resistance depends on the base-current not the gate voltage.
     The resistance depends on 1/base-current.
2) The BJTs have an undesirable CE voltage which depends on the base-current.
3) The BJTs stop working like controlled resistances for large CE signal swings. 

The CE voltage gets modulated by the LFO and is a source of ticks from the LFO.   

Some additions/changes to the JFET circuit to combat the non-ideal quirks of the BJT were:
-  buffered and/or separate bias for the emitters to prevent the LFO base-currents causing ticks in the audio via current injection into the vcc/2 line
-  a buffer and low-pass filter at the output of the LFO to help reduce ticks cause by the modulation of the BJT's collector-emitter voltage feeding-through into the audio.   A higher-order filter, say second order, might help.  The  cut-off frequency is set as low as possible.    The key idea here was if the CE voltage was varied slowly we would not hear it.     The thing we hear is the harmonics and idea of heavy filtering is to push those harmonics down to the point were we can't hear them.

One very interesting thing is the non-linearity of the BJT.    The MXR JFET circuit uses non-linearized JFETs; no RC networks on the drain-gate.    Surprisingly the BJT non-linearity was quite similar to the non-linearized JFET.

Where  the basic idea has issues, at least in theory, was for high signal levels.   Here the transistors no-longer operate as controlled resistances, they kind of 'let go'.  For this problem I had a few strategies:

- just let it be.   See what it sounded like without doing anything.
- reduce the input a bit and add some pre-emphasis/de-emphasis.   
   Same idea used on MOSFET based phasers.
- come-up with elaborate schemes to get around it.

I spent quite some time on the last point.  One conceptually simple idea was to use a PNP in parallel with an NPN as an attempt to make the resistance control more symmetrical.  The bases need to be driven by opposed signal.  There were quite a few other schemes I tried as well.  None were perfect and many had possible issues with over tweaking not working in the real world were parts have tolerances.

So basically I concluded the "let it be" scheme needed to be built-up to see if it was OK as is (at least for guitar/bass).   Then I'd take it from there.   But that's as far as I got.  So I left it for a rainy day and moved onto other things.

A hanging issue is to make sure the LFO voltage is shaped so the control profile matches that of a JFET.

Forgot something:
- You might need to add a large cap in series with the collector to block the DC on the transistor.

[ElectricDruid]

If you go with a "ladder filter" scheme like Moog for your allpass filter, you get to the Tau Pipe Phaser:

http://jhaible.com/legacy/tau/jh_tau.html

The actual ladder allpass network looks like this:

http://jhaible.com/legacy/tau/jh_tau_sch_page2_allpass.pdf

I don't claim to understand the detail of it, but presumably the differential drive allows you to run small signals through it and cancel out the much larger modulation by the LFO, like the Moog lowpass.

[R.G.]

Rob nailed it with the list of things that can bite you.

You really need to keep signal voltage across the collector-emitter down to the 25mV range to get distortion under a percent or so. The collector sees an offset voltage dependent on base current, and that "pollutes" the 25mv-ish audio, and has to be dealt with by making the control signal's change rate slower than is audible, somehow. And you have to feed it a control >current<.

You can handle some of these by making the bipolar that does the changes be part of a differential stage. It helps by cancelling out the control signal's effect on the collector voltage. If you're using a differential stage, you can get cancellation of the worst parts of that. A differential stage with emitter current controlled by the control current fits the bill. You can clean up the output funnies by having the collectors feed a current mirror. Then you diffamp the two collectors...
and you're 90% of the way to the 3080 OTA. The Moog ladder filter is another variant with stacked diffamps and capacitors to get a cascaded RC phase shift filter in current mode, with linearizing feedback.

I guess the bottom line is that you can use a bipolar as a controlled resistor in a (for instance) phaser. You just have to work hard at getting around its limitations.

[Mark Hammer]

Thanks, R.G. and Rob.  And yes, Tom, those devices ARE enough options for me.  Like I say, I asked purely out of curiosity.  And yes, Scruffie, I still have my PH44, prompted mostly by Osamu Hoshuyama's enthusiasm for it.  Here's his analysis of how BJTs are used to form a simple OTA in the PH44.

Here's his take on using BJTs in the manner I was musing about.  Not a good idea, it seems.

[Gus]

I wonder if you could stack two of the same NPN or PNP transistors in series. The issue will be diving both. Maybe CV voltage driven series resistors with taps going to the bases?

[digi2t]

All I know is that I have a loaded clone project of the PH-44, and I haven't been able to get it to work right. It's sitting in the "stuff I'll try to fix later" box right now. According to a Japanese site on this, it makes mention that there's a mess o trannys that need careful matching. Like just about... all of them.

Or....

I bodged the trace of Marks unit.

Also, idiot me, I never thought to take the voltages from the original. (Face palm!)

[Rob Strand]

Here's his take on using BJTs in the manner I was musing about.  Not a good idea, it seems.

That's pretty much the same configuration.  Interesting how people think  I remember finding a MOSFET configuration by that same author which was very similar to one I was playing with.

As for it being good or bad I'm not so sure.  It's not ideal but it would be very convenient, especially if it just works out.

Having played with transistors like this in the past the LFO ticks are a pain.    However the new thing I was going to try was filtering the LFO.   To me that is something that could push it toward the usable zone.  If that isn't enough a high-pass filter in the audio path might help, you will need some high-passing anyway, but the LFO low-pass filter is where the benefits seem to lie.

The signal level limitation may or may not be an issue.    That's something that needs to be tried.

As for better schemes the whole build your own OTA thing is something to consider.

I do remember a diode based phaser which used 20 1N4148 diodes in series for each stage.   IIRC it was a pro-quality phaser, not a pedal.

[Rob Strand]

I bodged the trace of Marks unit.

There's a schematic on "the other" forum  .

[Mark Hammer]

One of the things to take into consideration, when looking at different phaser designs is their intended application.  When I would speak to Mike Irwin about this and that design, he would point out their susceptibility to distortion and the degree to which distortion was gradually introduced with increasing signal amplitude, versus clean-until-threshold-then-audio-garbage.  The qualification is the difference in amplitude between guitar signals and synth signals.  Oscillators can put out rather large signals, so attention to clipping characteristics, and gain-staging within circuits (e.g., padding down followed by gain recovery), may be a bigger deal for synths than for guitars with generally much lower signal amplitudes.  Osamu is also a synth guy, rather than guitar guy, so he's thinking in terms of typical VCO amplitudes as well.

[StephenGiles]

Here's his take on using BJTs in the manner I was musing about.  Not a good idea, it seems.

I do remember a diode based phaser which used 20 1N4148 diodes in series for each stage.   IIRC it was a pro-quality phaser, not a pedal.

Which one was that Rob? Either my memory is going or I just haven't seen it!!

[digi2t]

Here's his take on using BJTs in the manner I was musing about.  Not a good idea, it seems.

I do remember a diode based phaser which used 20 1N4148 diodes in series for each stage.   IIRC it was a pro-quality phaser, not a pedal.

Which one was that Rob? Either my memory is going or I just haven't seen it!!

The Hi Fli is a perfect example of diode based phasing. 240 diodes to be exact...

I bodged the trace of Marks unit.

There's a schematic on "the other" forum  .

I've seen those, and those are trace schematics. I had a bit of a hard time following them because of how they're broken up. I was using a factory schematic, which I found had a few little differences compared to Mark's unit, so I penciled them in (so to speak). I also cleaned it up so it's readable (open in a new window to see it all).



To be honest, I'll have to plug it back in and listen to it to define what exactly isn't right with it, but IIRC the phasing sounded like shite, and the speed control was way too fast. I'll dig it out tonight and report back.

[Mark Hammer]

Notwithstanding any differences between the trace and the actual board, I'll draw attention to two aspects of this circuit that align with some other things noted in earlier posts:

1) A 571 compander.  The only time one sees compander chips in phasers tends to be in those >4-stagers using OTAs, where "coddling" is required.

2) The "peak" indicator is not tied to the input level, but rather to the feedback path, which is where you'd absolutely want to keep levels under control, particularly since they are compressed and not-yet expanded at that point.

Both of these speak to the importance and difficulty of managing distortion in OTA-based phasers - the "orchids" of the modulation-pedal world.

[Scruffie]

To be honest, I'll have to plug it back in and listen to it to define what exactly isn't right with it, but IIRC the phasing sounded like shite, and the speed control was way too fast. I'll dig it out tonight and report back.

IC15 looks quite oscillatey, according to the other site schematic, R99 is 470k, which should slow it down quite a bit.

[digi2t]

To be honest, I'll have to plug it back in and listen to it to define what exactly isn't right with it, but IIRC the phasing sounded like shite, and the speed control was way too fast. I'll dig it out tonight and report back.

IC15 looks quite oscillatey, according to the other site schematic, R99 is 470k, which should slow it down quite a bit.

Oh, thanks Scruf, I hadn't noticed that before. I'll put it on the "to do" list.

[amptramp]

I have a Univox U-65 amplifier that uses a phase shift oscillator driving a BJT as a tremolo, working against 68K resistors from the input.  It works and although it is difficult to hear non-linearity in a tremolo, it should be suitable for a phaser.

[Rob Strand]

Which one was that Rob? Either my memory is going or I just haven't seen it!!

I can't actually remember the name because it was a one-off obscure unit but the "Hi Fli" unit  digi2t posted is probably it.   I remember the schematic had a note about the diodes being a module.  I've got the schematic but it's on another computer.  (If I had my stuff I'd post all the info.)