RM Treble Booster & Pull Down Resistor

[Ben Lyman]

If I add a 2M2 pull down resistor to both the input and the output, is it going to change the impedance or affect the pedal in any negative way? I want to be able to have some other effects before or after it and also be able to use it directly into an amp without any other pedals. I also don't want it to pop or cause my amp to "ventriloquism" pop. Here's the scheme I'm using
http://fuzzcentral.ssguitar.com/rangemaster/rangemasterschematic.gif

[GibsonGM]

It will increase the input impedance....might make the circuit brighter sounding...you can increase the input cap accordingly to add more bass (altho, it's a T booster, lol)...can you try it on breadboard to see just how MUCH it changes things?   That, I can't tell you offhand, but you could sim it with LT or something... 

I'd try 1M, that should be fine... higher = more noise, no benefit.   1M in || with the bias resistor and Q impedance...so it will raise the input Z.

[Ben Lyman]

Thanks Mike, it's on the BB now and I don't notice any bad side effects to the tone, I have not tried it with any other pedals yet. I have heard that these can cause your amp to pop because of the way the output is set up with no volume pot. At the Fuzz Central site there is pictured an option to use a 1M at both in and out but I thought maybe a 1M on the output might make it behave weird with other pedals.

[induction]

It will increase the input impedance....might make the circuit brighter sounding...you can increase the input cap accordingly to add more bass (altho, it's a T booster, lol)...can you try it on breadboard to see just how MUCH it changes things?   That, I can't tell you offhand, but you could sim it with LT or something... 

I'd try 1M, that should be fine... higher = more noise, no benefit.   1M in || with the bias resistor and Q impedance...so it will raise the input Z.

It will lower the input Z, not raise it. But it will probably be unnoticeable,  tone-wise.

[Ben Lyman]

Yeah, tonewise I don't hear anything changing. I guess my main concern is having a big resistor on the output...
does that mean it becomes a pedal with "high output impedance"?
That is bad, right?
as it is (check my linked schematic above) it has a very low output impedance decided by where you set the "Boost" knob, but never higher than 10k, all according to what I read at geofex "Techno o/t RM"
So, adding a 1M from output to ground (+9v) is... ummm... I really don't know what I'm talking about...              
should I do it or not?

[GibsonGM]

It will lower the input Z, not raise it. But it will probably be unnoticeable,  tone-wise.

Yeah....      lowest values limiting the R and all.   As Ben found, didn't change anything - so, go for it if you want! 

I am not 100% sure how it works on the output side...1) I think it's not needed as a following pedal should "have its crap together" and have its own input pulldown and

2) are we in the same situation that I choked on above?  If you had say 600 ohm output Z on the thing, and add 1M....would the 600 limit the output impedance to values LESS THAN that?   I.E., 600 in parallel with 1M?  599 ohms?

[induction]

I am not 100% sure how it works on the output side...1) I think it's not needed as a following pedal should "have its crap together" and have its own input pulldown and

2) are we in the same situation that I choked on above?  If you had say 600 ohm output Z on the thing, and add 1M....would the 600 limit the output impedance to values LESS THAN that?   I.E., 600 in parallel with 1M?  599 ohms?

Output Z is conceptually similar to a series resistor that the output signal passes through, but an output pulldown is not a series resistor. It's a resistor to ground in parallel with the input Z of the following stage. So the output pulldown has the effect of essentially reducing the input Z of the following circuit, ie. loading down the output of the circuit it's part of.

But yes, if it's big enough, the effect will be small. 'Big enough' is defined by the output impedance of the circuit and the input impedance of the following circuit.  Calculate how much the output Z/input Z voltage divider will change by adding the output pulldown if you want to be more quantitative.

Edit: And don't forget to include the effect on the frequency response by modeling the output Z/input Z voltage divider as a high-pass filter (since the output Z is at least partially capacitative reactance).

[thermionix]

Thanks for starting this thread, Ben, because I've been wondering the same thing.  I had to do input and output pulldowns on my RM to stop switch pop.  I used 1M for both.  Don't know if there are any ill effects because I've only heard it this way.

[Ben Lyman]

I've been tinkering with it and the 1M on input had no ill effects. However, a 1M and even up to a 2M2 at output had some drastic side effects. Not so bad with my Electra on either side of the RM but my Tone Bender did not like it at all, gated and farting, slow to even respond at all right after switching either effect on. Really Cheap Compressor had issues with it too. The best sound was with only 1M on input and RM first in line after guitar. I can make all the pedals work in any order but there is a certain sound that I liked best personally: Guitar, RM, Comp, MKII. Also sounded good to me with: Guitar, Comp, RM, MKII.

Here's something off topic about my RM on the BB right now, I got a 3-way toggle on the input caps, on-off-on.

1. Middle (off) is connected to input in front of stock .005uF so always on.
2. Switch to left connects the middle to a .022uF (plus the still connected .005)
3. Switch to the right is a .0068uF (plus middle .005uF)

It sounds so badass with the MKII after it that it should probably be built into a single box with 2 footswitches.
But I wonder if it's better to switch the 2 extra caps on the other side of the stock input cap and leave them connected at the input so they can drain through the pull down?

[GibsonGM]

I am not 100% sure how it works on the output side...1) I think it's not needed as a following pedal should "have its crap together" and have its own input pulldown and

2) are we in the same situation that I choked on above?  If you had say 600 ohm output Z on the thing, and add 1M....would the 600 limit the output impedance to values LESS THAN that?   I.E., 600 in parallel with 1M?  599 ohms?

Output Z is conceptually similar to a series resistor that the output signal passes through, but an output pulldown is not a series resistor. It's a resistor to ground in parallel with the input Z of the following stage. So the output pulldown has the effect of essentially reducing the input Z of the following circuit, ie. loading down the output of the circuit it's part of.

But yes, if it's big enough, the effect will be small. 'Big enough' is defined by the output impedance of the circuit and the input impedance of the following circuit.  Calculate how much the output Z/input Z voltage divider will change by adding the output pulldown if you want to be more quantitative.

Edit: And don't forget to include the effect on the frequency response by modeling the output Z/input Z voltage divider as a high-pass filter (since the output Z is at least partially capacitative reactance).

That's sorta kinda basically how I've always viewed it - the 'block' that I've had has been what to do with caps in the way and things like this. 

I suppose we treat them as frequency dependent resistors, eh??  Capacitive reactance - so we need to consider the frequency spectrum, looking at only 1 frequency would only be a snapshot...like many, I've simply always stuck a 1M in there and been fine, ha ha...

[R.G.]

That's sorta kinda basically how I've always viewed it - the 'block' that I've had has been what to do with caps in the way and things like this. 

I suppose we treat them as frequency dependent resistors, eh??  Capacitive reactance - so we need to consider the frequency spectrum, looking at only 1 frequency would only be a snapshot...like many, I've simply always stuck a 1M in there and been fine, ha ha...

Let's remove the block, at least partially. When you mess with a mixture of capacitors and resistors, you have three situations, depending on frequency.
-There are frequencies so high that the capacitors all have impedances much lower than the resistors, in which case you can consider the caps to be short circuits.
-There are frequencies where the capacitive reactance is much larger than the resistors, so the capacitors dominate the response and the resistors don't matter much.
-Then there is the place where the caps have reactances similar to the resistances, say, within a range of 10:1 to 1:10. At those frequencies, both the resistors and caps have to be included to figure out what is happening.

In general, we want input and output caps to be of a size that does not matter, usually big enough that the resistors tell you input and output impedance. The output cap on the rangemaster is this way; but the input cap on the rangemaster is unusual in that it's sized "too small" to deliberately choke off some of the input bass. That's how it gets to being a "treble booster".

After that, adding resistance in series with inputs increases input resistance and decreases the signal that gets amplified by voltage divider effect. Adding input resistors in parallel decreases input resistance by shunting signal to ground. If there is a cap there, this functions as a "pull down" to make the cap's leakage not float the input up while it's switched open by bypass switching. A pulldown resistor is sized to be too big to hurt the input impedance much, but much smaller than the cap's DC leakage, which may be hundreds of megohms.

Pulldown resistors on the output do the same trick to prevent popping due to capacitor leakage, and also add some loading to the output. This can be significant if the output stage has a noticeable output impedance itself. For simple common-emitter stages, the output impedance is approximately the collector emitter for audio frequencies, and also affects the transistor's gain, so adding load on the outside of the collector output cap can lower gain, just like the load being driven by the collector can.

[Kipper4]

Sorry to but in but I think I just had an epiphany.
I googled common emitter and common collector, looked at the wiki pages quick and realised the
common emitter has no R between E and Gnd and vice versa for the common collector.....
True?

[duck_arse]

But I wonder if it's better to switch the 2 extra caps on the other side of the stock input cap and leave them connected at the input so they can drain through the pull down?

draw the circuit. caps all commoned to the left, hanging terminal on the right will float and pop-on-switch. you need a pull-popper resistor for each 'empty' capacitor terminal.

[R.G.]

Sorry to but in but I think I just had an epiphany.
I googled common emitter and common collector, looked at the wiki pages quick and realised the
common emitter has no R between E and Gnd and vice versa for the common collector.....
True?

It started that way, and that's why they're named that.

Transistors are three-terminal devices, but have two "ports" - an input and an output. An electrical input has to have two terminals to make input voltage appear or current to flow; likewise, an output has to have two terminals.  So if a transistor has an input and an output, and only three pins, one of the pins has to be common to both input and output, and that's how those configurations got names. A common-emitter circuit conceptually uses the emitter as common to both input and output, for instance, and has no emitter resistor.

In the real world, we use resistors in the emitter leads of common emitter amplifiers as a way to provide feedback. A resistor that's common to both input and output by definition is feedback, so that's handy.

We also use resistors in the collector leads of common-collector circuits for reasons unrelated to their function as common collector circuits. Because the bipolar transistor's collector impedance is so high, putting a resistor in series with it doesn't change the function much, but it can do other things like reducing the voltage on the collector for lower power dissipation, or providing isolation from a possibly noisy power supply.

There's a third circuit type: the common base amplifier. Given that the circuit is properly biased for DC conditions, you can put a BFC to ground on the base and force input signal into the emitter. The current gain of this circuit is slightly less than one, but the voltage gain can be very large, limited by the voltage and impedance on the collector side.

A circuit's operation as common base, common emitter, or common collector is more a matter of where the signal goes in and where it comes out; the terminal not used as an in/out is "common".

There's even a combination circuit. If you use resistors in both collector and emitter, it acts like a common emitter (signal at the collector) and a common collector (signal at the emitter) at the same time. Since these are opposite phase, the circuit is used as a phase splitter, as in the univibe.

[Ben Lyman]

Here's how I have it now, should I turn the switch around? or add a 2M to each extra cap connecting them to the Q1b? or NOT turn the switch, and add a 2M to each extra cap going to ground? If they pop a little it's not the end of the world, as it's just a toggle that can be set to the desired tone and left there.



Maybe this is better?


or this... probably makes no difference, huh

[duck_arse]

from the hanging free ends to some reference. doesn't matter which side is empty, doesn't matter where the switch is. a 2M from switch common//R1 to each C3 and C4 should cover all bases.

the electrons will find you, Mr Lyman, they are like that.

[Ben Lyman]

Ahh, I see Duck, thanks. So as long as I have the fat resistors on the hanging cap ends it doesn't matter which way. cool. I suppose I can even solder two fat resistors directly to the switch terminals, outside lugs connected to the middle lug, then run the hook up wires from those lugs to their respective locations on the perf

[Kipper4]

Thanks R.G
Love the explanation.
I recognise the combination configuration from my recent foray into the magnavibe. The so called concertina splitter.
Appreciate your input.
Rich

[Ben Lyman]

Ya, thanks RG and everybody. I can understand maybe 1% of what you all said but every little bit of knowledge helps add to the pile inside my brain
Through trial and error I think I got it working in a way I like, it sounds great. It's pretty cool to have a giant pile of old Ge PNP's sitting here to play around with. I could probably crank out 2 Tone Benders and 2 Rangemasters everyday as long as I'm using these pre-painted Tayda boxes and there is no LED, DC jack or extra other stuff, these old school circuits are so easy!

[GibsonGM]

Thanks, R.G. - the block is smaller now     It is as I suspected but was not sure of, he he he...