Explain this twist in a Boss flanger to me...please

[Mark Hammer]

My attention was drawn, via MEF, to the SYNFO site yesterday, where a number of service manuals are posted  (excellent site BTW).  One of the manuals posted is that of the Boss RBF-10, one of the micro-rack/table-top units in the Rxx-10 series.

You can download the service manual with the schematic here: http://www.synfo.nl/servicemanuals/Boss/RBF-10_SERVICE_NOTES.pdf

Most of the circuit makes perfect sense to me...all except for one part.

At the output of IC1a, the signal is split, going to a mixing stage at IC6b, where the feedback signal is combined with the audio input, and to a "mystery stage", formed by IC11b.  IC11b's output finds its way to Q11, clear on the other side of the schematic.

Would I be correct in assuming that IC11b and Q11 form some kind of noise gate to supplement the other strategies (lowpass filtering, complementary pre-emphasis/de-emphasis) the unit uses? 

[slacker]

I think you're right, that's certainly what Q11 seems to be set up to do. I can't make head nor tail out of IC11B though, it looks like some sort of comparator, but I can't figure out if it's a peak detector or envelope follower, presumably it must be doing one of those things, otherwise Q11 would be switching at the audio frequency.

[armdnrdy]

Possibly a strange way to mix the clean signal with the flanged signal for maximum notch depth?

Usually this is done at the output....but with the output channel "mix" circuit in place at the output, maybe this was the way to accomplish wet/dry mixing.

[teemuk]

It's a noise gate.


Something READING the service manual would have revealed in an instance.  

[armdnrdy]

OH.....so you're one of those guys that actually reads the instructions huh!!

That just takes all of the fun out of it!! 

[Mike Burgundy]

Something READING the service manual would have revealed in an instance.  

Teehee. If I had a <insert coinage here> for everytime I was there ;P

[teemuk]

OH.....so you're one of those guys that actually reads the instructions huh!!

That just takes all of the fun out of it!! 

Well, with that good documention in hand I was just dying to know how to adjust the RT-3. 

[slacker]

Oh yeah it's there in black and white, still interested to know how it works though, might have to simulate it and see.

[Mark Hammer]

My bad.  I downloaded the file at work and printed out just the schematic to read on the bus-commute (a regular practice), without the rest of the document.

The Rxx-10 series is funny in that the circuits are really close to their pedals, but include a twist here and there to "deluxe-ify" the unit.  The BF-2 also uses pre-de-emphasis for noise reduction but the RBF-10 uses an extra op-amp stage to isolate how the dry and wet signals are treated and applies the pre-de-emphasis ONLY to the delay signal and not to the clean.  Like I say, just a little "deluxe-ification".  The CE-1 used a gate on the wet signal for noise control, but that's the only place I've seen it on a Boss/Roland BBD-based product...apart from the RBF-10.

Thanks all for the humbling lesson.  

Enduring advice: RTFM!!  

[teemuk]

...still interested to know how it works though...

Quickly viewed, it looks like the signal path is muted with the shunting transistor each time signal's average amplitude falls below a certain threshold.

This is no way unique to Boss/Roland circuits with BBD's. At least pretty much each and every "chorus" design of them does the same thing (with varying methods), that also including the Jazz Chorus series amps. If I remember right, they also had some sort of a patent for it.


There's probably some frequency selective stuff featured in the comparator, and there are with utmost certainty some RC time constants defining the attack and decay characteristics of the muting.

Simulating is a great idea.

I can't make head nor tail out of IC11B though, it looks like some sort of comparator, but I can't figure out if it's a peak detector or envelope follower, presumably it must be doing one of those things, otherwise Q11 would be switching at the audio frequency.

Nope. As said, there are several RC time constants slowing down the switching. The comparator indeed switches on audio signal's frequency, but it's output signal is heavily filtered in three stages resulting in more of less a "DC-like" drive signal with slowly varying amplitude. The purpose of this signal is merely "tracking" whether audio signal of certain amplitude exists or not. e.g. Signal not present = Q13 collector near Vcc, signal present = Q13 collector near zero volts. The real action of course is not that drastic as it is slowed down by various time constants.


This in mind, if and when you simulate, it might be worthwhile to tamper with the circuit so that RC time constants are much shorter than in normal operation. This way you can run a much shorter timeframe of simulation and you don't have to wait ages for results. 100 milliseconds is a long time in SPICE and you don't neccessarily want to simulate time constants of several seconds just to figure out how certain circuit operates by principle.

[Paul Marossy]

Enduring advice: RTFM!!  

Yeah, it works almost every time. 

[slacker]

...still interested to know how it works though...

Quickly viewed, it looks like the signal path is muted with the shunting transistor each time signal's average amplitude falls below a certain threshold.

Yeah I got that part, what I couldn't see was how the signal was being turned into DC to control it. I simmed it and what happens is with no signal C41 charges up through R74 to about 7 volts turning on Q11 muting/attenuating the signal.
Signals above some threshold trigger the comparator and it tries to turn Q13 on and off at the audio frequency, without C41 there that's what happens. However what happens with C41 connected is the output of Q13 goes instantly to near zero with just a little ripple. I assume what happens is when Q13 goes low it discharges C41 and tries to recharge it when it goes high, but due to the R74/C41 time constant it discharges much faster than it can recharge and so stays near ground. This turns off Q11 allowing signal to pass, R83/C22 slow down the transitions slightly maybe to reduce pops by fading the signal in, hard to tell on the sim. When the signal drops below the threshold C41 starts to charge up and takes about 1 second to get to 4.5 volts when it turns Q11 back on muting the signal, again there's a bit of fade out.

It's an interesting little snippet worth stashing away.