EHX lpb-1- NOISE

[Muzzatron5000]

Hi,
I have gone back to basics and made a couple of LPD-1 point to point circuits.

They sound OK. But I noticed it really increase the noise of my vol and selector switch.

Is this normal? or could I have a ground issue.

[PRR]

Welcome.

> it really increase the noise of my vol and selector switch.

It has a gain of about 20. Logically any noise (hiss, hum, wi-fi chatter) in front of it will be 20 times bigger.

[anotherjim]

An abnormal amount of noise from the guitar controls is mainly caused by DC leakage from the circuit to the input. However, that takes something like a bad/leaky input cap (150nF) which is unlikely with a film cap type although not impossible.

As PRR said, the noise could be normal and if you haven't boxed it up yet, inevitable.

If touching the strings is unusually noisy, you could have a ground issue. Don't rely on the enclosure to ground the in and out jacks.

[Muzzatron5000]

Yes ok. I thought as much. But wanted to rule out anything I couldn't think of. I will check the cap anyway. :-) Thanks

[antonis]

>Off-topic ON<

360R & 830K values can't be found even in E192 series

>Off-topic OFF<

[pacealot]

3.6-multiple values are most definitely in the E24 series — maybe not common, but I have quite a few 3.6K and 36Ks in my resistor stash, plus I do think one of my early '70s LPB-1 plug-in units has an original 360Ω emitter resistor, where the 390Ω is the usual prescribed value.

8.3, on the other hand... 

(not to mention the implication of an "830K" and a 100K replacing the standard 10-to-1 ratio of 430K/43K or even 470K/47K — that's more worrying to me...)

[antonis]

not to mention the implication of an "830K" and a 100K replacing the standard 10-to-1 ratio of 430K/43K or even 470K/47K — that's more worrying to me...

It raises Base voltage, hence Emitter voltage, about 150mV (in relation to 10/1 ratio) while Emitter resistor is lowered by 8%, resulting into over-doubling Collector current.. (880μA instead of 430μA, for V_BE=650mV)
(BJT almost bottomed out..)

[Muzzatron5000]

Hey so - you guys think the schematic I was using had incorrect values?

I am guessing this Beavis Audio version might have the correct values then.??

[GibsonGM]

I've seen varying schematics for it, Muzz...(welcome by the way!)

The ones I  build tend to use a 430k/43k divider on the base for bias.  Seems more in the realm of what the originals had...

[antonis]

Hey so - you guys think the schematic I was using had incorrect values?

Not necessarily "incorrect"..
More close to "odd enough"..

I am guessing this Beavis Audio version might have the correct values then.??

"Correct" strictly depends on what you aim to..
e.g.Bevis Audio version is correct from Collector bias and input impedance point of view (at least for such a simple CE stage..) but it violates stiff bias configuration rule..
(R4 current should be 10 times larger than Base current, at least..)

On the other hand, by scaling down R2/R4 (e.g. 470k/47k) for larger bias configuration current, you also scale down input impedance by almost(*) the same ratio..

(*)"Almost" reflects the "constant" (unaffected by voltage divider values scaling) transistor input impedance [(h_FE + 1) X (390 + 55(**)] which is always set in parallel with R2//R4 so it "spoils" the scaling a bit..

(**) 55R here is an Emitter "hidden" resistor which is inversely propotional to Collector current and should be taken into account whean dealing with Emitter resistor values of comparable size..

P.S.1
In case you scale down R2/R4, you might have to scale C1 value up by the same, give or take, amount..

P.S.2
Try a 680k/68k and 150nF (C1 & C2) to get your own LPB-1 clone..

[duck_arse]

Hi,
I have gone back to basics and made a couple of LPD-1 point to point circuits.

They sound OK. But I noticed it really increase the noise of my vol and selector switch.

Is this normal? or could I have a ground issue.

also welcome, muzza. would you be so kind as to post a photo of one of your built noisy boards? just for my personal curiosity.

[Muzzatron5000]

Oh wow! thanks so much for your replies.

antonis - Ill give that a try and see if it reduces some noise. :-) Thanks


Here is a pic (but feel like I am embarrassing myself here).
I realize that big yellow cap is OTT but was what I had.

[antonis]

2W power rating for 100k resistor is overkill..

1μF output cap (the big yellow one) is overkill also..
(unless output should be connected to a low value, say less than 5k, imput impedance circuit..)

P.S.
Sepereate input from output wires and cut all those stranded wire unsoldererd whiskas..

[GibsonGM]

It'll be great, you can run it on 630V for maximum clean headroom!     You'll get better, don't worry.  We all started messy, and sometimes frustration can cause it to re-appear here and there. 

I'd clean up as much as you can (whiskas)...pry out the little pieces of solder around the terminal strips so they don't come loose and short something.  If you can get it, a lighter gauge of wire makes building easier, as will tinning the pieces before assembly.  As long as it's bigger than the diameter of a resistor's leg, you're good to go on these 9V circuits.    Sometimes a high gainer is just noisy...holding the transistor with a heat sink while (quickly) soldering it may help (or maybe that's just mojo...).  The long legs you've left on it surely dissipated heat (good for now as you learn).

Do you have a pic of the back of the switch?

[Muzzatron5000]

Yeah OK. Is there a Hack of the Month award?? 

I was doing that thing where you are half way between bread-boarding and soldering.  Being impatient i guess.

What I need to do is clean it up, re-solder, with new values and see how it goes then.

But the main problem  was really vol pot was going "schhhhk' and pup selector clicking-popping, when the pedal was on.

[antonis]

What I need to do is clean it up, re-solder, with new values and see how it goes then.

"Heavy" metal termninals, like Turret posts or Pot lugs, should be pre-tinned during soldering..
(they need more heat and time than caps or "delicate" semicondutor devices legs..)

Just a soldering tip..