LPB-1: What's up with these voltages?

[nee]

I was just fiddling around with the basic LPB-1 type booster. I didn't have any 430K or 43K resistors so substituted 470K and 47K. All other components remained the same. With a battery reading right on 9V I got a voltage of 4.6V at the collector. I messed around with the 390R emitter resistor and found the circuit sounded great with a lower value here - 200R. Not just louder, but... smoother and chunkier or something. 200R gave only about 2V at the collector. This voltage surged upwards when signal was put through the circuit, whereas the 4.6V version remained stable.

Can anyone comment on what was happening to the sound with the lower voltage, and why the voltage surges?
Thanks!


IanG

[Caferacernoc]

As you lower the emitter resistor you increase gain by reducing feedback. So it gets louder and "looser". The voltage probably surges because there is less control when the circuit clips and the negative feedback is temporarily defeated.
Here's more  explanation:

http://www.beavisaudio.com/techpages/HIW/hiw1.gif

[nee]

Thanks Caferacernoc.

With the basic LPB-1 circuit I'm also noticing a slight loss in high end - just a loss of shimmer and sparkle. Is that normal for this circuit? I've played around with reducing the size of the caps which - of course -  reduces bottom end to give the impression of more treble, but doesn't actually restore the lost shimmer. I guess it could partly be the project board I'm working with, plus the relatively long leads going to the pots - about 6 inches/15cm.

IanG

[ayayay!]

Yes that is normal.  To combat this, just place a buffer or buffered pedal in front of it like a Boss.  It doesn't even have to be turned on, just before the LPB.  (And it has to have a battery in it, of course.)

[Gus]

Time to do searches at the forum
Look for things like input resistance and how it loads the pickups.
Look in text books and online for that "standard" one transistor 4 resistor circuit.

The beginner project was "designed" to keep the input resistance high and have variable gain.  Looks a little like the "textbook standard" LPB but with a few refinements.

[nee]

Thanks for your pointers everyone.

Like most noobs I'm having trouble with "impedance" and "reactance" despite hours of reading. I know that, ideally, we want a high impedance input and a low impedance output, but I have no idea how to achieve this.

For example, take the basic LPB-1 circuit on one hand, and J. Donald Tillman's FET preamp on the other - schematic here:

http://www.till.com/articles/GuitarPreamp/

If I put the LPB-1 between guitar and amp with 2 normal guitar leads, my guitar signal is amplified, and the high end is rolled off slightly. Not a lot, just enough to be noticeable.

If I put the Tillman preamp between guitar and amp with 2 normal guitar leads, my guitar signal is amplified, and the high end is amplified even more - it's extremely noticeable. (Or to be more exact the high end is restored to what it would be without the capacitance inherent in the guitar leads.) Whatever - there's a big high end boost with the Tillman in circuit.

So what is the difference between the two circuits? It's not just the 3M pull-down resistor of the Tillman is it? Otherwise adding a 3M pull-down to the LPB-1 would magically restore all my top end, which it doesn't.

[aron]

The FET preamp has a lot higher input impedance as the article noted. The reason why your 3M resistor doesn't magically restore the top end of the LPB1 is because there's already an input resistor, so the 3M would be in parallel with the 43K resistor among other reasons.

[nee]

The FET preamp has a lot higher input impedance as the article noted. The reason why your 3M resistor doesn't magically restore the top end of the LPB1 is because there's already an input resistor, so the 3M would be in parallel with the 43K resistor among other reasons.

Aha! I see! So what would happen in the LPB-1 if we were to use a 10M/1M pair instead of the 430K/43K? Higher impedance, but... less current? And the effect of that would be...?

(In fact, I just tried it, and the signal is brighter, but there must be some reason higher value resistors aren't used in the original circuit.)

[ayayay!]

Did you get your answer nee?  I'd like to know this too. 

[Caferacernoc]

The FET preamp has a lot higher input impedance as the article noted. The reason why your 3M resistor doesn't magically restore the top end of the LPB1 is because there's already an input resistor, so the 3M would be in parallel with the 43K resistor among other reasons.

Aha! I see! So what would happen in the LPB-1 if we were to use a 10M/1M pair instead of the 430K/43K? Higher impedance, but... less current? And the effect of that would be...?

(In fact, I just tried it, and the signal is brighter, but there must be some reason higher value resistors aren't used in the original circuit.)

I've never seen one with values that high but there are a lot of LPB schematics out there with a 833k/100k combo.

[nee]

Did you get your answer nee?  I'd like to know this too. 

Not specifically, no, but there are a tremendous number of threads on input impedance if you do a search. The great RG Keen has an uncanny knack for explaining things very well. I'd still like to know about the effects of substituting 10M/1M for 430K/43K though.

IanG

[Gus]

If you find a good text book or good web site with the 4 resistor bias setup you might find the answer about the input resistors.

Things to look for the hfe of the transistor and the available current from the resistors used to bias the transistor at the base.