NEW SCHEMATIC - Class A PNP Germanium Boost - Thoughts?

[darron]

I thought I'd like to share this with you guys, as people are always asking about making simple boosters:






So basically it's a PNP booster with room for customisation. It is designed to run with ground hooked up to negative, so it can daisy-chain with your common pedals or run on a battery as is. It's PNP so it can accept just about anything common that you put in like AC128, OC44/OCxx etc. etc.

The knob adjusts the GAIN of the pedal, not the volume before or after boost.

So what's going on? Usual deal. There's an input cap - changing the value of that pretty much changes the character of the boost. A massive 100n-220n (0.1uF-0.22uF) lets pretty much all bass through and is very creamy. Around 30nF if a good mid value.

The 500K is a bias trimmer (there's no point in having this as an external pot).

The 240r is there to protect the transistor a tiny bit from spikes and limit over-hot signals. The 240r with the 220p in theory may help regulate the bias noise down a bit too. The 100n and 220uF are only input filtering and can be eliminated or adjusted to whatever you want. The 5K lin pot gives a nice sweep range for almost linear to full boost potential.


The only problem that I've found is that they can often be a little bit too noisy for me. I use a 500k multi turn trim pot, so maybe there's noise there? I wouldn't imagine noise coming from the gain pot as it's a dead short at full gain. If anybody has some suggestions then I'd love to hear, but otherwise let me know any other thoughts and how you go with it!


Cheers

Darron

[alfafalfa]

I guess you just have to find a germanium transistor that's less noisy !

The first oc44 I used in my rangemaster clone sounded like the surf of the ocean.

Alf

[darron]

I guess you just have to find a germanium transistor that's less noisy !

The first oc44 I used in my rangemaster clone sounded like the surf of the ocean.

Alf

that's very true. i search through and find ones i like and often take out ones i don't (usually noise problems). it seems the quieter they are though the less boost that you get out of it. that's using all sorts of hfe from 40-300. so far the main responses have been NOS ac128s, new UK ac128s, oc44/cv3007s and the best results have been from the new ac128s. the introduced noise is almost at the same level for the minimal gain setting compared to the maximum gain (full 9v), except the signal noise goes up proportionately of course. i bet a guru can look at the circuit that i use and tell me exactly how to get it down. then some people will probably tell me that pnp does not belong in a negative ground world....

[amptramp]

I would add a couple of germanium diodes in series with the 500K pot so that the changes in Vbe with temperature are matched with the variation in bias with temperature.  As it stands, the circuit will work at the temperature it is set for but the bias variation with temperature would not be compensated.  This may be OK for your own use at home or in a studio, but for gigs where you could go from Phoenix to Fairbanks temperatures, you will want some temperature compensation.  One diode would be attached between the lower end of the 500K pot and ground with the cathode on the bottom and the other diode would be attached between the upper end of the pot and the junction of the 500K pot and the 1N5819/capacitors/gain pot with the cathode on the bottom.

Even el cheapo transistor radios usually had this type of temperature compensation for the output stage since they did not want to sacrifice gain by using an emitter resistor to stabilize stage bias characteristics against temperature.  This design does not have the bias point feedback that a Fuzz Face has from biasing the first transistor's base from the second transistor's emitter.

[mac]

darron,
I built a booster using a high gain 2sb176 leaking 100ua. I let the leakage feed the base, there is no biasing at the base, so I put a 25k pot from C to Vcc to set collector voltage. Gain is controlled in the same way, a pot at the emiter.
It is not noisy at all... I mean the transistor is quiet, any noise is due to the fact that... it is a booster!

mac

[caress]

try a reverse biased diode between the base/emitter junction (cathode to base)

[darron]

thanks for your input guys. i think i'll have to move away from the pcb and back to breadboard....

there's not much to reference for negative ground stuff. i just moved house last night so i'll see how i go... i'm hoping that someone might want to build it and let me know how they go too (:

[andrew_k]

there's not much to reference for negative ground stuff. i just moved house last night so i'll see how i go... i'm hoping that someone might want to build it and let me know how they go too (:

I have the breadboard out at the moment for the first time in months. There's a pile of various PNP Ge transistors nearby. It was clearly meant to be! I'll have a look tonight or tomorrow and let you know if I find anything interesting.
Thanks for sharing 

[darron]

andrew! good to hear that you are still alive! please do let me know. then we'll have to have a get-together. like i said, i just moved, but soon i'll have a new music/pedals room setup at my new place in dandy

going to japan on the 9th though.... ten days. should be cool.

if you like it, then let me know and i'll send you a pcb

[andrew_k]

Can I give you a shopping list for Akihabara? So many nice components!!  I'll email you the details of the only decent surplus place I found there too

[Zzagar]

Darron,
Thank you for sharing this build! It is an EXCELLENT booster! Ultra clean and I actually noticed no added noise from the transistor. I used a 100nf at the input and a 220 resistor instead of the 240. I'm excited to experiment more! I know they say PNP negative ground circuits are a no-no in most circumstances, but I always enjoy not having to compensate for the typical PNP positive ground. Thanks again!

[darron]

hey Zzagar! i'm reallyglad thatsomebody was able to make useof this! it's pretty cool and there's been no probs running it with negative ground asit'sgot heaps of filtering and the diode keeps the juice for the pedal.


the 220 won't make a difference and the 100n will be really nice and fat

thanks for building it.. made my day (:

[Ben N]

I wonder if the newer manufacture Russia germs are any better in terms of noise and consistency. I'm waiting on a box of them now... Darron, does you gain pot crackle? What kind of specifically germanium mojo does it have?

[darron]

I wonder if the newer manufacture Russia germs are any better in terms of noise and consistency. I'm waiting on a box of them now... Darron, does you gain pot crackle? What kind of specifically germanium mojo does it have?

i've got a box of those that have just arrived, i need to pick them up... i shall find out >: D

yeah.. that gain pot is a 'crackle okay' style knob haha. putting a cap (22uf?) on the wiper to ground will probably fix that, though i haven't tried it.

or, you can give it full gain by eliminating the gain pot (short straight to full 9V+) and put a simple voltage divider at the end of the circuit for volume control. i don't mind the crackle though.


i'll find out about the new russian transistor thing, though the ones i have coming in are output trannies that draw 50ma!

[darron]

here are some pictures from over the years.... they will all be in the pictures thread too:

[earthtonesaudio]

You could move the 220p cap to the other side of the 240 ohm resistor (so it now connects directly between base and ground) for more effective filtering.
By switching to "noiseless biasing" you might eliminate some of the noise.  Though I would suspect it's mostly due to individual transistor quirks.

[darron]

thanks for your help earthtones. i really want to get this as good as i can. it is something simple though.

i originally had the 220p cap on the other side, i might try moving it back there. thanks (:

i was mucking around with noiseless biassing, but with the values i was using i couldn't get the thing to fire up at all! it wanted too much current. maybe i'll try a smaller trim pot (10k / 50k... maybe even 100k?) tied to a cap and then a small value bias resistor probably not much larger than 100k as a guess. it's been a while since it was on breadboard. i did the simple bias to save space on the pcb as you can see it's a very tight build the way that i did it.... i doubt it's the problem too but it is probably the second thing i'd look at, other than the transistors.

i'm glad somebody reported back that their one was quiet! i hope some people can start building these or integrating them into another circuit like putting them before a fuzz with a mid value cap

[askwho69]

Hi daron. . .  can you email it to me the schematic? i really love your booster and its the one i need . .  i bought a ac128 in a local  electronic store yesterday and it was a vintage store with so much old stuff that still there hehehe . . .

cheers

A2

email is askwho69@yahoo.com

thanks a lot again

[darron]

Hi daron. . .  can you email it to me the schematic? i really love your booster and its the one i need . .  i bought a ac128 in a local  electronic store yesterday and it was a vintage store with so much old stuff that still there hehehe . . .

cheers

A2

email is askwho69@yahoo.com

thanks a lot again

done!

i'm selling these pedals, but if anyone wants the schematic which has been heavily revised and got LOTS of good feedback then just message me as i'd be happy for people to have a play of them!

[askwho69]

wow youre a wonderful person! thanks for the schematic! i really love your pedal it make me dreaming all day to have this. . . i make it today and feedback later tomorrow!!

more power i will advertise your pedal to our country! so that they will buy to you