Help me analyze the Brian May Treble Booster

[mlabbee]

Okay, I've built this and I like it, but I have only a rudimentary understanding of how it works.  I've read the explanation of the Rangemaster and it has gotten me part of the way there, but this circuit adds a lot of components that aren't in the Rangemaster - so here's my analysis and my questions:

Capacitors:
C1 - I have no idea what this does - filter the power supply, perhaps?
C2 - this is the coupling capacitor - keeps DC out of the transistor base.  Also acts as a high pass filter (in combination with the transistor).
C3 - no clue . . . is this for "noiseless biasing?", i.e., is C3 bleeding off high frequency to keep it out of the base to reduce noise?
C4 - again, no clue. I've seen this cap on lots of transistor circuits, but have never seen an explanation of what it does.
C5 - output coupling, a bit more filtration.

Resistors:
R1 - pulldown resistor?  If so, I usually see values of 1M or more - why is this one only 120k?  Would increasing the value of R1 affect the sound?  I get some popping in my build.
R2/R3 - voltage divider - feed the collector resistor 4.5V (biases the collector?)
R4/R5 - This is where I get really confused.  It seems like R2-R4 and R5 act like a voltage divider to bias the base.  If so, why not simply join R3 and R4 into 1 resistor?  How does splitting them and sending the input into the split affect the circuit?  
R6 - biases the emitter
R7 - biases the collector.

Add an R8?  I want to add a pulldown resistor after C5 - any reason why this would be bad?

I've read that putting a booster in front of this circuit sounds good - how about incorporating an IC input buffer?  I've used the non-inverting op-amp buffer (AMZ - http://www.muzique.com/lab/buffers.htm) on other circuits and been happy . . .

Add a low pass filter at the front to knock down some noise?

I plan on breadboarding some of these options just to see, but if any of my ideas are known to be non-functional, I'd appreciate some forewarning.  

Help!

[bioroids]

Hi!

I know a couple of those!

C4 is an emmiter bypass capacitor. It shunts signal at the emmiter to ground, thus elimitating negative feedback of the stage, thus increasing the gain

C1 is in fact to filter the power supply - i would use a higher value

C3 im not sure, but i think it is a low pass filter to tame the noise a little - could be wrong

C2 and C5 coupling caps

R2 is not exactly part if the voltage divider, it forms the low pass filter together with C1 to filter the power supply

R3 and R5 together bias the base of the transistor

R4 again i'm not sure, but may be there to help define the frequency of the low pass filter with C3

You can and R8, like 1M to help reducing pop noises.

R1, i'd use 1M resistor for pulldown, but may serve other purposes, i don't know

Please anybody correct me if I got something wrong!

Luck

Miguel

[mlabbee]

Miguel: Thanks for the great input!! That cleared up just about everything for me - especially your explanation of C4. I knew I had seen a similar structure before and your comments reminded me that you do the same thing with tubes and the Real McTube write up explains exactly how it works.

It looks like the only dangling question is the role of R4 and C3 - I was thinking it might be some kind of filter,too, but I can't quite figure out how.  Anyone?

[Alex C]

This is great stuff, thanks guys!  

Alex

[Diego A]

Ok it´s been a while since anybody reading this topic, but i really need to know what resistor should i replace for a pot to control the amount of treble boost. I´ve build a couple of circuits very similar to this and just added a pot at the end like a volume control, but i would really like to have a treble pot!

   If anybody reads this in 2019 i would appreciate the help.

[Jeema]

I think you would actually need to add an additional pot to control the amount of treble boost as opposed to just replacing a resistor with a pot.

The key to circuit is C3, which appears to be part of a positive feedback loop that controls the amount of treble that's fed back into the input.  So you would probably want to add a pot there in series (just to the left or right of C3) so that it would block off the feedback loop when the pot is turned up.

Not entirely sure what value you'd need though...

[Diego A]

Thanks a lot!, i will try it n my experiments, i´ve made a lot of builds but i´m not from the electronics world so i´m starting to understand the basics and maybe who knows... some day i´ll design my own stuff.

[PRR]

It is not a "treble booster". It is a Bass Cut plus a Boost.

The *only* filtering is C2 against the input impedance. Ignoring an important detail, figure about 18k Ohms. 4n7 against 18k cuts everything below 2KHz. Leaving "treble", then boosts that.

Because the resistances *must* set happy DC conditions on the transistor, the best path is to change C2. This is far too large for a variable capacitor. (Oh, Rob has one on the shelf, but he ain't letting go of it.) Suggest a 3- or 4-way switch with several caps on it.

[Diego A]

ok thanks for the explanation, so i guess that´s the only way to go. I wish it could be a better way to set the tone on this thing, it is possible to add a tone stack to the circuit at the end of the signal path?

[antonis]

Of course but not so easily done..
(meaning not just throw a tonestack configuration..) 

As Paul told you right above, C2 "restricts" some frequencies, so you first of all have to enlarge its value..
(doubling its present value should halve present cut-off frequency - from 2kHz to 1 kHz, ten times bigger cap should divide it by 10 - from 2kHz to 200Hz, etc..)

Depending on your tonestack specific choice, you might need a buffer between C5 & tonestack input and a buffer/recover amp between tonestack out & OUTPUT..
(tonestack input should load Q1 Collector resistor 'cause it will be placed in parallel, effectively lowering Q1 Gain - tonestack out should also be loaded by whatever impedance comes next when simultaneously exhibit a significal signal loss due to its "passive" nature.)

To make long story short, you'll alter the whole circuit design.. 

[amptramp]

R1 should be much higher if you want to see much treble.  The low input resistance combined with the inductance of the pickup will kill off a lot of the treble and there is no need for such a low value - C2 should be very high resistance and R1 could go up to 22 megohms if you want.  If you never add true bypass, R1 is not needed.

[Diego A]

Of course but not so easily done..
(meaning not just throw a tonestack configuration..) 

As Paul told you right above, C2 "restricts" some frequencies, so you first of all have to enlarge its value..
(doubling its present value should halve present cut-off frequency - from 2kHz to 1 kHz, ten times bigger cap should divide it by 10 - from 2kHz to 200Hz, etc..)

Depending on your tonestack specific choice, you might need a buffer between C5 & tonestack input and a buffer/recover amp between tonestack out & OUTPUT..
(tonestack input should load Q1 Collector resistor 'cause it will be placed in parallel, effectively lowering Q1 Gain - tonestack out should also be loaded by whatever impedance comes next when simultaneously exhibit a significal signal loss due to its "passive" nature.)

To make long story short, you'll alter the whole circuit design.. 

Thanks a lot guys, i´m learning so much with your knowledge. By the way, something weird is going on with some of my breadborded circuits, i get no output and when i check the components with the probe the signal flow stops on a resistor, at one tip i got signal but nothing at the other, then i check the resistor with my multimeter and is working fine. Do you have any idea of what´s going on?

[antonis]

You obviously refer on series resistor, don't you..??

Check between the leg with signal and the tip where resistor should be connected (not on resistor's actuall leg..)

[Diego A]

You´re right, series resistors but the signal dissapears after that, i´ll send a picture of the circuit



Last year i´ve build this one with PNP germanium and sounds amazing, now i wanna do it with NPN silicon (i changed the orientation of eletrolitic caps and power source of course). I get no signal after the 100k resistor

[antonis]

Do you get your measure with ATTACK pot fully upwards..??

[Diego A]

Do you get your measure with ATTACK pot fully upwards..??

Yes i did

[antonis]

Do you get your measure with ATTACK pot fully upwards..??

Yes i did

Fair enough for practically zero signal... 
(100k right leg AC shorted via 25μF cap..)

Leave 100k resistor along, together with ATTACK pot on arbitrary saetting, and check for signal on Q1 Collector / Q2 Base..
If it's OK, check on 470R/8k2 node..

[Diego A]

Do you get your measure with ATTACK pot fully upwards..??

Yes i did

Fair enough for practically zero signal... 
(100k right leg AC shorted via 25μF cap..)

Leave 100k resistor along, together with ATTACK pot on arbitrary saetting, and check for signal on Q1 Collector / Q2 Base..
If it's OK, check on 470R/8k2 node..

  Many Thanks Antonis!!, i´ll check that as soon as i get home.

[loolirer]

howdy
i did one myself with the same circuit but everytime i do chords or strum too hard, it has some popping noises in the background
any help?
i tried switching the R1 for a 1M but the boost just got weaker (solved the popping a little tho)