LPB-1 Mods - Give me some 'lectronic facts on what I did!

[DIY Dood]

I have an old LPB-1. It's a slightly different PNP design than most of the very early ones. The current schematic is below. I've been experimenting with mods in a plug-n-play way. I'm testing with a modern single coil (Fender) guitar, so guitar output is medium.

I need some feedback (no pun) on why certain things are behaving certain ways. My electronic knowledge is limited.

R7 was a 22K fixed resistor in the original design. I replaced it with a 250K linear pot wired like a variable resistor. I expected a bump in signal, but it really acted more like a gain control, providing sustain and a bit of dirt as I drop the resistance. It has a nice effect, keeping things clean up until it's at 60K or less and then going into overdrive mode from there to 0 ohms.

I added the R6 pulldown because I thought there should be some resistance (impedance?) there for when R7 is dropped to zero. Switching it in and out showed no differences in tone. Likewise the R8 change to a 250K pot made no difference in in tone in switched live tests.

Questions:

- As R7 is decreased in value below maybe 40K, I get the gain effect. But also get an increasing decrease in treble. Sort of a Big Muff kind of tone at 0 ohms. Is R7 acting as an R/C filter with C1?  I know I can add a series cap as a filter to boost treble (or actually cut bass), but would there be a better way to reduce the treble reduction?

- Have I caused impedance issues? Or is this layout OK?  (I'm thinking of dropping R7 to a 100K linear pot as that's all it needs for range).

- I wired R7 as a variable series resistor as that was the original design. Should I wire it more like a standard volume control with the open lug grounded?

Is there anything else I've done in my ignorance that's a bad idea? Or other things that would be a good idea?


Thanks,

[Keppy]

- As R7 is decreased in value below maybe 40K, I get the gain effect. But also get an increasing decrease in treble. Sort of a Big Muff kind of tone at 0 ohms. Is R7 acting as an R/C filter with C1?  I know I can add a series cap as a filter to boost treble (or actually cut bass), but would there be a better way to reduce the treble reduction?

The emitter-grounded transistor is a low impedance input. Having some series resistance on the base (R7) raises the impedance a bit. This:

• ...causes less current to flow, which is why eliminating this resistance boosts gain. You're increasing current into the transistor.
• ...creates a high-pass filter with C1. Reducing the resistance of R7 should reduce bass response in the circuit. You didn't mention it, so it might not be noticeable.
• ...interacts with the inductance/resistance/capacitance inherent to your guitar and cable in a way that reduces treble response. This is like the input of a Fuzz Face. This effect probably disappears if you place a buffered pedal before the LPB. This effect is why guitar effect inputs are usually 1Meg impedance. The lower that number gets, the more you lose treble.

- Have I caused impedance issues?

Yes. It's not a problem if you like the sound, but this pedal probably won't sound the same with a buffered pedal feeding it, so it's something to keep in mind, just like with a lot of fuzz pedals.

- I wired R7 as a variable series resistor as that was the original design. Should I wire it more like a standard volume control with the open lug grounded?

There's no particular reason to, unless you want it to do double duty as a pulldown resistor.

I added the R6 pulldown because I thought there should be some resistance (impedance?) there for when R7 is dropped to zero. Switching it in and out showed no differences in tone.

That's because the transistor is a low impedance input. AC current won't travel much in that resistor when it has an easier path through the transistor. You might want the pulldown resistor to prevent switch pops, but it's not needed for any other reason.

[DIY Dood]

Thanks for all the details. I don't see the #2 effect increasing the treble at all. But I must be seeing the #3 effect as there's definitely a treble drop at higher gain (lower input resistance).  I'll put something buffered in front and see how things change.

One followup question: I've generally seen people lean towards high impedance input and low output in pedals. Or so I recall. Should I run the very original EHX 10K pot on the output to minimize impedance there?  Would that be advisable for and following pedals and the amp? There didn't seem to be any sonic difference in switching output pots.

[Keppy]

Output pot differences are usually minor, and selected to taste.

[antonis]

Sorry but the above isn't neither p-n-p nor LPB1 booster..

[DIY Dood]

Ah, but it is an LPB-1. I bought two of them new in the late 70's and disassembled them myself. I couldn't afford a Big Muff Pi so I got two of these and ran one into the other... later adding clipping diodes on the output.

The only mods to the circuit (aside from the ones I mention above) are that EHX used a 10K output pot.

[Fancy Lime]

Ah, but it is an LPB-1. I bought two of them new in the late 70's and disassembled them myself. I couldn't afford a Big Muff Pi so I got two of these and ran one into the other... later adding clipping diodes on the output.

The only mods to the circuit (aside from the ones I mention above) are that EHX used a 10K output pot.

It is definitely not "the usual LPB-1". Maybe there were some variations still floating around in the 70's from before EHX settled on the final design. Not uncommon for things made in the 60's but already fairly unusual for bigger companies like EHX by the mid 70's. Or your thing could be a knock-off, is that possible? Either way, photos of the insides and outside would be very interesting.

The power is drawn for PNP, but I cannot see if the symbol of Q1 is a PNP or NPN symbol.

Andy

[amptramp]

If you use the volume control(s) on the guitar, there will be a very large difference as you bring the control up to maximum.  The guitar pickups act as a generator in series with an inductance of about 3 to 5 Henries in series with the winding resistance which is typically 5 - 15 Kohms.  The output level control on the guitar is in the range of 250 K to 1 megohm.

If your input impedance remains high, the inductance has little effect. If it is low, you have the rising impedance of the inductor as frequency goes up acting like a voltage divider with the input impedance that reduces the treble.  If the input impedance is high, the effect only occurs at the upper end of the audio range where only the harmonics live.  You can see the effect if you turn the volume control down on the guitar because that adds resistance (which is not frequency sensitive) to the input impedance of the buffer and changes the sound dramatically.

[DIY Dood]

Ah, but it is an LPB-1. I bought two of them new in the late 70's and disassembled them myself. I couldn't afford a Big Muff Pi so I got two of these and ran one into the other... later adding clipping diodes on the output.

The only mods to the circuit (aside from the ones I mention above) are that EHX used a 10K output pot.

It is definitely not "the usual LPB-1". Maybe there were some variations still floating around in the 70's from before EHX settled on the final design. Not uncommon for things made in the 60's but already fairly unusual for bigger companies like EHX by the mid 70's. Or your thing could be a knock-off, is that possible? Either way, photos of the insides and outside would be very interesting.

The power is drawn for PNP, but I cannot see if the symbol of Q1 is a PNP or NPN symbol.

Andy

Unfortunately the boxes are long gone. I'm confident that they were genuine, especially in those days. But, the guy who owned the music store I bought them from was quite the wheeler and dealer. It wouldn't surprise me if he bought some very old stock that was from the very early EHX days.

The transistor may be drawn incorrectly... I made the schematic via cut-n-paste so it's meaningless aside from the EBC markings. The transistor itself is a plain metal TO5 package - if it ever had markings, they are long gone.

[DIY Dood]

If you use the volume control(s) on the guitar, there will be a very large difference as you bring the control up to maximum.  The guitar pickups act as a generator in series with an inductance of about 3 to 5 Henries in series with the winding resistance which is typically 5 - 15 Kohms.  The output level control on the guitar is in the range of 250 K to 1 megohm.

If your input impedance remains high, the inductance has little effect. If it is low, you have the rising impedance of the inductor as frequency goes up acting like a voltage divider with the input impedance that reduces the treble.  If the input impedance is high, the effect only occurs at the upper end of the audio range where only the harmonics live.  You can see the effect if you turn the volume control down on the guitar because that adds resistance (which is not frequency sensitive) to the input impedance of the buffer and changes the sound dramatically.

Thanks. So, if I understand that, with the guitar on 10 (0 resistance, coil impedance only) and the input resistor (pot) being turned down, I'm seeing the treble reduction effect.

I'm going to ask, but I think I know the answer... there's no way to increase that input impedance without increasing the resistance and thereby reducing the very gain effect I am looking for by dropping the input resistor (pot) value, correct?

So the only solution would be to build an active treble boost circuit to compensate? I know I can cut bass easily which is sort of the direction to go, but then I just end up with both treble and bass cut (i.e. the mid-hump option).

[antonis]

there's no way to increase that input impedance without increasing the resistance and thereby reducing the very gain effect I am looking for by dropping the input resistor (pot) value, correct?

Correct for particular booster..

For a "classic" LPB1, bootstrapping could work quite well..

[amptramp]

I'm all in favour of JFET op amps as boosters.  You can get high impedance and any gain that makes sense is available.  You also get power supply noise rejection if you decide to do away with batteries.  If you want to stay with batteries, a Tillman preamp would meet your needs better than a junction transistor.