Electro harmonix lpb booster with different transistor (bc548)

[Ryanhardy]

Does anyone know if the standard circuit will work with a bc548 ? Would any changes be needed? Im pretty new to this diy stuff. Was trying to get a clean boost working out of a 4558, but it kept sounding like a dist pedal, so thought using a single bc458 might be simpler.

[Kipper4]

Welcome Ryan

What have you tried?
Show us the schematic you used. for the lpb and 4558 booster.

Boosting the input to amp will sound like a distortion if it's overdriving the amp.

I dont see why an lpb with the bc548 shouldnt work.

data sheet here

https://www.sparkfun.com/datasheets/Components/BC546.pdf

Quite a high gain jellybean.

[Ryanhardy]

I meant bc548 sorry. Pulled a bunch of them from junk. The circuit is here

[Ryanhardy]

The 4558 circuit i tried is like this (i assumed 4556 was simillar).

[Gus]

An good older thread about the lpb circuit
LucifersTrip posted voltages
http://www.diystompboxes.com/smfforum/index.php?topic=104730.msg940166#msg940166

[Plexi]

Almost any NPN work there. One with a bit gain than another: increase 390R to avoid it.
Any: 2N2222, 2N3904, 2N5088, 2N4441, BC550, BC549, even some darlington like MPSA13.

[Ryanhardy]

What if my 9volt power is actually putting out about 13 volts?

[Jeema]

More voltage should actually be better because then you will have more 'headroom' - in other words, the transistor will be able to amplify more before it starts to distort.

[Jeema]

^To clarify - what I mean by that is that you would generally need a stronger input signal to make it distort if your power supply voltage were 13 volts as opposed to 9.

[Ryanhardy]

I sounds great on battery but bad with power supply. Is this due to the higher voltage of about 15 volts or slight ripple in wallwarts?

[GibsonGM]

I sounds great on battery but bad with power supply. Is this due to the higher voltage of about 15 volts or slight ripple in wallwarts?

Depends what you mean by "bad"....if there's buzzy hum crap all the time, then it's your power supply...so you'd add a 100 ohm resistor in series w/the power supply followed by a 47u cap to ground to help filter it out...some wall warts just s*ck too, and are very hard to filter due to not being meant for this purpose.

Side note:  I don't see any electrolytic caps in your schematic above - but if there were, you'd want to make sure they were rated for more than your power supply, if you go above (the expected) 9V.   The rating should be right on the can, usually 16v, 25v etc....but occasionally you'll find they're rated lower.

[Ryanhardy]

Thanks for your reply. The caps are mylar so not sure of the voltage. This particular wallwart works ok with commercial pedals but I guess those pedals have some kind of regulation built in? The battery provides perfectly clean and loud boost but the wallwart makes the boost kind of crunchy like turning up the gain knob on overdrive pedals. Ill try your filtering suggestion.

[Ryanhardy]

I also realised, after it was working great with battery that I actually had the battery reversed.  Seems to work great though and if I change the battery around it stops working. Bizarre. Think I got lucky. The apparently there is some thing called reverse active mode with npns

[bluebunny]

I also realised, after it was working great with battery that I actually had the battery reversed.  Seems to work great though and if I change the battery around it stops working. Bizarre. Think I got lucky. The apparently there is some thing called reverse active mode with npns

Hmmm... be aware that the pinouts of the 2N5088 and BC548 are different.  You may already have had your transistor "upside-down" with the back-to-front battery?

[Ryanhardy]

No I checked the pinout before I started so I think it's just in reverse active mode with positive positive ground.

[Ryanhardy]

I solved the distortion problem when using a wallwart.  Two zener diodes, one on each lead.  Completely fixed it.  It was a fluke though as i have no idea what value they are.  I took a couple of pics hoping someone might be able to tell what their rating is.  My understanding was that diodes only allow current to flow in one direction (reducing ac ripple from wallwart).  But after reading about zener dioides it seems they allow forwards and reverse flow?





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[antonis]

Can't say from your pics bur they should have some symbols like PH 05V6 (5.6 Volts Zener diode) or BZX or PL followed by a xVy code...

Yes, Zeners allow forward and reverse current flow but with an "oddity" on reverse flow..
They stabilize the voltage drop across them according to their specific structure..
(e.g. you'll get a 5.6 V drop between cathode & anode of a respective rating zener despite the overall apllied voltage - with limits, of course..)
So, if you place a zener of, 5.6V say, in series with the wire of +15V (cathode facing to supply) you'll result in a 9.4V possitive supply..
(if you place it like a normal diode you'll only "subtract" its forward voltage drop - on forward flow they behave like a "normal" diode..)

I can't see the way you've connected the zeners on the wall wart leads but you're lucky not to blow them out because of circuit's low current need..
(Zeners are also classified of their power dissipation limits which are simply the product of zener voltage and current flowing through them..)

P.S.
Although it's theoretically OK, it's better to settle your voltage on only ONE wire (the possitive one) to avoid a "floating" load..
(with zener, or resistor, also on negative wire you "offset" your load ground level above zero Volts which is quite confusing for other circuits sharing the same GND..)

[GibsonGM]

I solved the distortion problem when using a wallwart.  Two zener diodes, one on each lead.  Completely fixed it.  It was a fluke though as i have no idea what value they are.  I took a couple of pics hoping someone might be able to tell what their rating is.  My understanding was that diodes only allow current to flow in one direction (reducing ac ripple from wallwart).  But after reading about zener dioides it seems they allow forwards and reverse flow?





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Hi Ryan,

To add to what Antonis said...diodes aren't really for 'reducing AC ripple"...we use caps and resistors for that.  An R-C filter is usually what is needed, and sometimes more than one!    Refer back to what I said about the 100 ohm resistor and cap above.

Yes, diodes act as 1-way valves, and DO have SOME reverse current flow (but not much for 'normal diodes').  Zeners are a special case, 'doped' to be operated IN the reverse direction!   

When used for regulation, you typically find  a resistor in series to limit current ("Rs") (or you may kill the zener...), then the zener in SHUNT (cathode toward "+", anode to GROUND).    The regulated voltage is taken from the R and cathode junction.  You can look this up easily on the net. 


The fact that the zeners "fixed" your problem suggests to me that you have a lot of ripple on your wall wart, so might try the 100R/47u cap (or even 100u or higher!)....and if that only does a little, add ANOTHER section like that.   

Ultimately, if you CAN'T fix the wall wart this way, it would tell me it's just not made well enough for regulation and probably shouldn't be used with pedals.   There are so many of them around, it's easier to grab a new one. 

[antonis]

I'm afraid Sir that Ryan just placed Zeners in series with PS..
(no current limiting resistor or anything else..)

So, he just stepped down final wall wart output voltage..

(maybe I'm wrong, but.... )

[Ryanhardy]

Think you are correct.  Turns out one works almost as well as two. Measuring voltage across the diode measures 7v exactly. But no idea of wattage, so it may blow. I see 1w versions can be had very cheap online. Is it possible to achieve this voltage limiting effect with a normal diode?