Building custom boost circuit from scratch

[Isg1315122]

I have a plan to build a booster with a 2n5401 bjt. I have been trying to build it around the design of the apollo treble booster but obviously will need to tweak the biasing of this bjt.

I have been racking my brain trying to figure out the process can you kind folks lend a hand and steer me in the right direction? I will post what i have so far.

Based on research i came up with R3 at 4.7k and R4 at 470 ohms. I then followed the steps for an emitter bias then converted to a voltage divider bias per Thevenin's Theorem to solve R1 and R2. I came up with 2.4M and 16.5M respectively. I think those are really high  and the 16.5 is way above any standard value.

First is my process and math correct? And secondly is it ok to lessen the values of R1 and R2 with the same ratio to achieve the goal of a base voltage of approx -2V?

[GibsonGM]

Hi Isg, welcome.

Have you just tried to built it from the schematic and pop in your transistor to see what happens?

[Isg1315122]

No i like torture and wanted to see if i can properly build it from just the data sheet. Seeing as the 2n5401 had a lower gain rating than anything else pnp i had i chose it. I figure to understand it i must learn the math to support it, idk i must like torture like i said.

The pic shows the differences of where i started and what i needed in the end. I just fixed my horrendous math and came up with R1 as 220k and R2 as 68k so not really all that different from the original.

[GibsonGM]

I say try it out!  Easy enough to assemble on breadboard.

The way I learned to do it was to assume a current thru the device and choose your components based on it.   I learned with NPNs but it must be exactly the same for PNP.   

Ex: assume 1mA thru transistor (collector current); the Apollo has about the same...make the collector about 1/2 supply voltage.  (Vsupply / 1mA = collector resistor), 8.2k for Apollo (almost 9k, hmmm)
set emitter to 1V, arbitrarily; 1mA also flows thru emitter....1V/1mA = 1k emitter resistor.  Apollo used 2k, added to 8.2k is like 9k and 1k...trivial difference.

You can 'pretend' your gain is about 100, for most modern BJTs it will be close to that.   So base current would be about .01mA  (our 1mA / 100...).  We can make that current "stiff", and say we want 10x that, or .1mA in the divider.   

We will want about 1V minimum bias at the base as it needs .6V or so to turn on (and other bias levels of course place the BJT in different parts of its curve)...let's give it 2V (Apollo uses 3V, hence different values).    R1 = VSupply - 2V / .1mA ....     
9V-2V/.1mA = 90k for R1
So R2 = 2V base voltage / .1mA = 20k. 
Yields an actual 1.6V, close enough
Of course you can play with this bias value....1V, 3V, 4V....to see what it sounds like.  2V to 3V appears very often.
Your 68k and 220k gives a bias of about 2.1V, again, pretty common.

There are many ways to do this, but I find this one to be easy...or just stea...borrow a design that has already been done for you      Your values are probably right, tho 10mA seems screaming high for collector, I would scale collector and base down, personally!    Try it out tho.   If it sounds like you want, then it is fine! 

[PRR]

> obviously will need to tweak the biasing of this bjt.

Why?

No.

Transistor is transistor, near enough. Depends on the circuit, and the different transistor, of course. But this circuit looks fairly robust against transistor variation. Figuring out why I say that may be instructive.

Also: list both transistor specs side-by-side. Then cross out the ones that "don't matter". Vce(sat) is small in both cases, especially at the current this plan runs, and is completely moot. Vbe might matter, except it isn't on the specs, because Vbe for Germanium is typically small compare to other circuit voltages. (Note that a simple analysis suggests the Base starts near 3V, so the Emitter starts near 3V, whether Vbe is 0.05V or 0.2V.) hFE is like 40 to 200+ for either type--- this is the big variation. About the same either type, so if one works the other has good chance of working. Also note the RR3 R4 ratio is 40 which is very similar to the low end of the hFE spread, so low hFE really may matter.

It may also be instructive to postulate a part with hFE=infinity. Does the circuit work? Why/why not?)

[Isg1315122]

I will admit im confused PRR. I made it per my math and signal passes but seems diminished and not "boosting".   Ie straight signal is louder. If the original schematic runs with about 1mA and has 4.5v at the collector and 2-3v at the base. The schematics (mine and original apollo) would be essentially the same working conditions, is that correct?

[Isg1315122]

So i built the apollo and tried the 2n5401 and i get nothing (no signal). Collector V is -1.8v
Base V is -2.3v
Emitter V is -1.7v

Again nothing gets through, and the pot does nothing to improve or change that. Collector V seems low if its supposed to sit around -4.5v.

So my build passes sound but its attenuated which maybe because thats natural when dealing with negative feedback on bjts. Is this correct?

[Jeema]

No, that ain't normal.  That circuit should have a decent amount of gain particularly at high frequencies.

I think you need to increase R1 and see if that collector voltage gets closer to -4.5v.

Breadboarding is much easier in my experience than trying to crunch numbers.

[Isg1315122]

I only tried crunching numbers to see if i can understand the process. I understand that it is easy to build it and try different transistors but then i have these problems where the build doesnt work with the new part then i left wondering if its the transistor or if its the biasing. It would be likly that the collector Resistor would need to be changed to achieve -4.5v i will try it with a trimpot later today and see if i can make it work with original parts. The only thing thats different on my build is a 82k instead ok 80 and one cap is 2.2 instead of 1.8 and the other output cap is 4.7 instead of 5.

[Isg1315122]

Doing calculations on the apollo with -8.7v source and 230k total ohms on the base yeilds .37 mA, with 2k ohm on the emitter it yields 4mA, with 8.2k ohm on the collector it yields 1mA.

The base current seems low and the emitter current seems high.

[antonis]

Plz Isg, write down final R1, R2, R3 & R4 values.. 

I suspect your bias resistor values are tooooooo high for effective base bias current..

[Isg1315122]

Doing calculations on the apollo with -8.7v source and 230k total ohms on the base yeilds .37 mA, with 2k ohm on the emitter it yields 4mA, with 8.2k ohm on the collector it yields 1mA.

The base current seems low and the emitter current seems high.

These are the apollo build and they are :
Base R's
R1 150k
R2 82k
Collector
R3 8.2k
Emitter
R4 2k

This build does not work.

My build consists of:
Base
R1 220k
R2 68k
Collector
R3 4.7k
R4 470 ohm

This bulid passes sound but at lower volume than bypassed, i just realized that i bypassed my audio interface when trying to compare in circuit and bypass. Both using 2n5401.

[PRR]

> does not work.

This is not a useful observation. At least measure DC voltages at all nodes.

[Isg1315122]

I was at work and was going from memory.

With base:
R1 150k
R2 82k i had -2.3v .03mA which is too low.

With collector:
R3 8.2k i had -1.8v and about 1mA i think i didnt write it down because it was ok

With emitter:
R4 2k i had -1.7v and around 1mA again it looked good so i didnt write it down

Based on what antonis said and seeing this i checked math and base current was low. I reworked the resistance on the base:
R1 68k
R2 1.8k this gave me -1.7v and a hair over 1mA

I went back to my numbers for collector and emitter R3 4.7k and R4 470 ohms to see if that change helped, it didnt seem to. So looking at the voltage
It was -.9v on collecter -.9v on emitter.
I put a trimpot on the collecter as variable resistor and to achieve -4.5v @ collector with emitter at 470 ohm it took about 1.4k. It passes sound seems to work ok but my bread board must be getting loose because everything wants to wiggle and signal cut in and out at the slightest touch lol.

[Isg1315122]

Its been a while sorry, so i want to rebuild the original but thats for another day. Today i came to say that as of right now i have a working circuit. If your curious about the components used they are as follows.

Base: -3.2v 1.5mA
R1 = 5.6k
R2 = 3k
Collector: -4.2v 1.95 mA
R3 = 11k
Emitter: -2.6v 1.96mA
R4 = 6.2k
Is any of this high? I think it might becand hence why i want rebuild with original values, i think i had head phones plugged in when testing the first time.

Another question however : in the pic it shows c1 and c2 surrounding a 10k resistor. It says the centered frequency. Can someone explain this a little, is it a hi/lo pass filter or is it something else. Sencondly can you make a switch with multiple frequencies. Say if you use a 5 way switch 2 pole with both common poles connected with the different value caps on the switch side. The "center" is the common pole lugs but i worry about the caps that are now "floating" open. Will that cause issues. Is this essentially a varitone switch?

[antonis]

Another question however : in the pic it shows c1 and c2 surrounding a 10k resistor. It says the centered frequency. Can someone explain this a little, is it a hi/lo pass filter or is it something else. Sencondly can you make a switch with multiple frequencies. Say if you use a 5 way switch 2 pole with both common poles connected with the different value caps on the switch side. The "center" is the common pole lugs but i worry about the caps that are now "floating" open. Will that cause issues. Is this essentially a varitone switch?

It's a Bridged-T filter..
https://www.diystompboxes.com/smfforum/index.php?topic=86199.0

You can ground caps open legs via a 1M (or higher) resistor but I think you better look for some equalizer circuit..

[Isg1315122]

So i rebuilt it with original values and it does pass sound but at less than bypassed volume. I biased the collector resistor to get -4.5 v on it. That definitely  works better and sounds more crisp than the build i figured out. 

Base: -2.5v @ .04mA
R1 150k
R2 82k
Collector: -4.3v @ 2mA
R3 4.2k
Emitter: -1.9v @ 2mA
R4 2k

Im think im gonna go with this. Now i want to mix the signal with an overdrive circuit (obsidian overdrive). Is it worth it to mix the signals with either active or passive mixing (i have opamps tl071 and 2s but  with two other circuits i really want to power everthing off of -9v the opamp would likley need more voltage)

Or should i just test one before and after the other with a bypass for each? Then is it good practice to buffer the output. I know running the effects in series is less parts, and i dont mind adding the buffer but dont know if its really needed.

[Isg1315122]

I haven't had any response in over a month. Just checking in. Im currently putting this together with an obsidean overdrive combo pedal. My main question now is, should i mix the to signals or should i run them in series, also would a buffer be a good idea or does it not matter?

[Kipper4]

So the treble booster works now right ?
Which Obsidian are we talking about ?
Pictures, links? Help us to help you.
What buffer and to what end?

Antonia has a point. I don't think I'd put Andertons Freqauncy booster mid circuit.
Find a simple rc and work your way up.

Help us to help you.
Rich

[Isg1315122]

Sorry, its an obsidean overdrive and this booster. The booster works, I wasn't planning on putting in the Anderton thing I was just curious. The buffer is the same one from seymore Duncan's booster/ buffer diy but with another 2n5401 pnp. I will post a pic of all the schematics when I get home.