DIYstompboxes.com

DIY Stompboxes => Building your own stompbox => Topic started by: DIY Dood on May 24, 2021, 10:53:20 PM

Title: LPB-1 Unusual Version / Circuit Comparison?
Post by: DIY Dood on May 24, 2021, 10:53:20 PM
I have an LPB-1 from 1975. It's a little out of order, so I'm trying to rebuild it.

I found a circuit diagram that I drew back in 1976 of the original circuit. It's significantly (from my narrow viewpoint and limited electronic knowledge) from any other variation I can find of the LPB-1 on-line. I'm wondering about some of the differences and the effect on the output sound, etc. I've done some googling and learned a bit, but my theory is still very weak.

Questions are below the diagrams.

My Circuit:

(https://i.postimg.cc/zVJDvZt3/my-circuit.jpg) (https://postimg.cc/zVJDvZt3)

Common Circuit:

(https://i.postimg.cc/vcHMM7TV/common-circuit.jpg) (https://postimg.cc/vcHMM7TV)

Questions:

- The first difference is the 22K resistor in series in the input. Is that just to provide impedance? Or is it an R/C filter with the .2uF DC blocking capacitor? The resistor doesn't run to ground, so, the former?

- The second difference is the .2uF cap vs. the .1uF I find in the others on-line. That's on both the input and output. So it's blocking DC on the way in but I'm guessing that it's potentially filtering bass too... but with the .2uF, it's not really doing any bass filtering?

- R3 is 5.6K vs. 10K. So that's feeding more voltage to the collector for less possible gain? The common version uses 10K.

- The emitter goes direct to ground instead of through a 390K or similar resistor in the other versions. Is this a way to get more gain and "compensate" for the 5.6K resistor vs. 10K?

- R4 isn't there at all for the usual voltage divider. ??

Thanks,
Title: Re: LPB-1 Unusual Version / Circuit Comparison?
Post by: Gargaman on May 24, 2021, 11:17:29 PM
My guesses:
- The first difference is the 22K resistor in series in the input. Is that just to provide impedance? Or is it an R/C filter with the .2uF DC blocking capacitor? The resistor doesn't run to ground, so, the former?
The resistor by itself won't change the impedance; also won't make filter with the cap since there is no path to ground; I guess is more like limiting the current going in.
- The second difference is the .2uF cap vs. the .1uF I find in the others on-line. That's on both the input and output. So it's blocking DC on the way in but I'm guessing that it's potentially filtering bass too... but with the .2uF, it's not really doing any bass filtering?

Input and output capacitor blocks DC and also limit the amount of bass; generally, the greater the capacitor, more bass going in and out; 0.2uF is greater than 0.1uF, so it's filtering less bass.
- R3 is 5.6K vs. 10K. So that's feeding more voltage to the collector for less possible gain? The common version uses 10K.
- The emitter goes direct to ground instead of through a 390K or similar resistor in the other versions. Is this a way to get more gain and "compensate" for the 5.6K resistor vs. 10K?
- R4 isn't there at all for the usual voltage divider. ??
Check for transistor Feedback Resistor Bias (1st) vs Voltage Divider Bias (2nd).
 
Title: Re: LPB-1 Unusual Version / Circuit Comparison?
Post by: Rob Strand on May 24, 2021, 11:22:55 PM
The 22k resistor sets the gain and it does affect the input impedance.

It looks like they have tuned the 22k to have roughly the same input impedance as the classic version.  (After checking with spice your circuit has a lower impedance.)

When you consider the impedance of the guitar the signal gains are about the same as well.   The 22k also contributes here.

The bias point seems to be little different on your circuit although it does come down to the specific gains.    A small effort in choosing the transistor gain could match that as well.

In fact all the above depends on the transistor gain to some degree.

So in simple terms they are two different circuits doing the roughly same job.

FWIW, here's the classic version,
(https://4.bp.blogspot.com/-xvvlr0110hg/U0cTXlN-3pI/AAAAAAAAA_U/XcD8WyFOwvM/s1600/electro+harmonix+screaming+bird.bmp)
Title: Re: LPB-1 Unusual Version / Circuit Comparison?
Post by: antonis on May 25, 2021, 04:31:11 PM
+1 to what Rob said..!! :icon_wink:

Both are single BJT boosters with different bias and stage gain configuration..
RCollector / (REmitter + re) vs RFeedback / RIn (roughly)

Also, 390R + re vs 1M feedback resistor take care of beta variations..
Title: Re: LPB-1 Unusual Version / Circuit Comparison?
Post by: DIY Dood on May 25, 2021, 10:18:32 PM
Thanks. Interesting... as mine was very early in the manufacturing years. 1975 at the latest. More classic than the classic, I'm guessing.

The .22uF cap was actually two .1uF caps soldered in parallel. Must have been an on-the-fly tweak to decrease bass reduction. Maybe it was competing with the Screaming Bird for sound. I recall those being in the catalog.

Mine also sports a 10K volume POT per my check yesterday, not the 100K of the "Classic".

I'll get to rebuilding it and see what it does.
Title: Re: LPB-1 Unusual Version / Circuit Comparison?
Post by: Rob Strand on May 26, 2021, 12:33:13 AM
Quote
Mine also sports a 10K volume POT per my check yesterday, not the 100K of the "Classic".

Interesting.   I wonder if it has the 10k resistor + 10k vol put or it's another variant where the 10k pot *is* the 10k resistor, like the Rangemaster.

RG's schematic shows the 430k + 43k divider but it mentions the 1M + 100k (which is in the Beavis Audio schematic posted at the top of the thread).

The were some Maxon boosters in the 70's.  IRC they used a 50k pot at the output.  I think the divider on the base was 470k and 47k and the transistor was a 2SC828 (maybe an 2SC828R but don't quote me on it).   Maxon had a few model sort of paralleling some of the EHX offerings.
Title: Re: LPB-1 Unusual Version / Circuit Comparison?
Post by: DIY Dood on May 26, 2021, 12:19:22 PM
@ Rob Strand

"I wonder if it has the 10k resistor + 10k vol put or it's another variant where the 10k pot *is* the 10k resistor, like the Rangemaster."

I have a "spare" 10K resistor that I think might have come from one of these. But I'm not sure. I had two of these coupled and I had the 10K off the output of the first one where the POT would have been - before it fed the second one. I had minimal knowledge of electronics at the time (just slightly above that now) and no resources so I just coupled them. I probably just put it in to mimic. But, it might have come from the original circuit.

Are you thinking it would be in series to create a pot with a minimum 10K resistance... transforming it to a 10K to 20K variable?
Title: Re: LPB-1 Unusual Version / Circuit Comparison?
Post by: Gus on May 26, 2021, 12:57:07 PM
reminds me of the EM drive

https://tagboardeffects.blogspot.com/2013/01/emerson-custom-guitars-em-drive.html (https://tagboardeffects.blogspot.com/2013/01/emerson-custom-guitars-em-drive.html)
Title: Re: LPB-1 Unusual Version / Circuit Comparison?
Post by: DIY Dood on May 26, 2021, 04:13:16 PM
OK, wired it up P-T-P in the original fashion, because I lack a breadboard. LOL. It's not working.

It's passing audio through and the POT is controlling volume. However, it's actually cutting volume, not boosting it. Test conditions are a single coil guitar into an amp, physically plugged through the circuit or into the amp (no switching).

I assume the caps are OK, i have no tester other than "shorted" but they are passing the audio. I tested all the resistors and values seem good. Battery is running a little low at 8.83V, but I assume that would not be low enough to cut output this way. Polarity is correct per the diagram (negative ground).

I rechecked it all ten times. Seems to match the diagram.. I did some basic voltage measurements without an audio signal, but I'm not sure what I'm looking for. Voltages are in the diagram below.

Ideas?

(Also corrected the Pot to be 10K per the original parts, not 100K).

(https://i.postimg.cc/qg47qzBs/my-circuit-with-voltages.jpg) (https://postimg.cc/qg47qzBs)
Title: Re: LPB-1 Unusual Version / Circuit Comparison?
Post by: antonis on May 26, 2021, 04:38:15 PM
Base should sit at about 600 - 700 mV..
(as it is, B-E junction looks like 25MΩ resistor..)

Check for Emitter to GND continuity..
Title: Re: LPB-1 Unusual Version / Circuit Comparison?
Post by: Rob Strand on May 26, 2021, 07:02:30 PM
Quote
Are you thinking it would be in series to create a pot with a minimum 10K resistance... transforming it to a 10K to 20K variable?
The way you have drawn it on your schematic looks fine.   

FWIW, the circuits with a feedback resistor have a lower output impedance than the "classic" circuit so you won't lose much gain with the 10k pot.   However, you do lose output swing.      These things are what they are.  I'm only explaining the mechanics of the circuit.  I'm not saying it's a problem as such.
 
(Re your current issue, antonis is on the right track.)
Title: Re: LPB-1 Unusual Version / Circuit Comparison?
Post by: DIY Dood on May 26, 2021, 10:52:10 PM
I've checked the emitter to ground line and I'm 99% sure it's OK, but I'll check again in the morning and update.
Title: Re: LPB-1 Unusual Version / Circuit Comparison?
Post by: antonis on May 27, 2021, 06:22:32 AM
I've checked the emitter to ground line and I'm 99% sure it's OK, but I'll check again in the morning and update.

If indeed it's OK, check transistor B-E junction..
(DMM on Diode test setting..)

(https://i.imgur.com/2qaKIjf.gif)
Title: Re: LPB-1 Unusual Version / Circuit Comparison?
Post by: DIY Dood on May 27, 2021, 03:00:34 PM
I'm getting 239 emitter to base.  This is an old T05 package. I don't know for sure if it's a 2N5088 as there are no markings on the package, but it definitely came as stock in the LPB-1.

Bad transistor?
Title: Re: LPB-1 Unusual Version / Circuit Comparison?
Post by: antonis on May 27, 2021, 04:47:20 PM
Bad transistor?

Not, if it's Ge BJT..

Otherwise, yes..!!  :icon_wink:
Title: Re: LPB-1 Unusual Version / Circuit Comparison?
Post by: DIY Dood on May 28, 2021, 07:53:55 PM
I am guessing that the only way to know that would be if I could read printing on the T05 case?  Not other way to know what brand it is?
Title: Re: LPB-1 Unusual Version / Circuit Comparison?
Post by: Rob Strand on May 28, 2021, 08:14:32 PM
Quote
I am guessing that the only way to know that would be if I could read printing on the T05 case?  Not other way to know what brand it is?

You can use you multimeter like antonis's picture.   Use the diode test range on the multimeter.   A silicon transistor will measure 0.5V to 0.7V and a germanium transistor will measure 0.1 to 0.3V.  You can test either or both of the junctions BE and BC.   It's best to do that with the transistor out of circuit, especially if the circuit isn't working.   On a working (and powered) circuit you can use the *voltage range* on the multimeter and measure the BE voltage (CB usually of no use in this case).

Title: Re: LPB-1 Unusual Version / Circuit Comparison?
Post by: DIY Dood on May 29, 2021, 05:52:03 PM
 

You can use you multimeter like antonis's picture.   Use the diode test range on the multimeter.   A silicon transistor will measure 0.5V to 0.7V and a germanium transistor will measure 0.1 to 0.3V.  You can test either or both of the junctions BE and BC.   It's best to do that with the transistor out of circuit, especially if the circuit isn't working.   On a working (and powered) circuit you can use the *voltage range* on the multimeter and measure the BE voltage (CB usually of no use in this case).

Rob: OK, measured both today with the transistor out of circuit. Got 224mv  E > B and 211mv C > B. No connectivity in the other direction.

What does that tell me?
Title: Re: LPB-1 Unusual Version / Circuit Comparison?
Post by: antonis on May 29, 2021, 06:21:39 PM
Got 224mv  E > B and 211mv C > B. No connectivity in the other direction.
What does that tell me?

It introduces itself as a healthy Ge p-n-p .. :icon_wink:
Title: Re: LPB-1 Unusual Version / Circuit Comparison?
Post by: DIY Dood on May 29, 2021, 07:39:10 PM
Got 224mv  E > B and 211mv C > B. No connectivity in the other direction.
What does that tell me?

It introduces itself as a healthy Ge p-n-p .. :icon_wink:

OK, then I will have to go back through the other components and the circuit one more time.

What sort of voltage tests can I do with the circuit live other than the power voltages previously tested/posted?
Title: Re: LPB-1 Unusual Version / Circuit Comparison?
Post by: Keppy on May 30, 2021, 12:44:16 AM
If that's a PNP, then you need to run the circuit on -9v. Your schematic says your battery is backwards.
Title: Re: LPB-1 Unusual Version / Circuit Comparison?
Post by: DIY Dood on May 30, 2021, 04:08:37 PM
If that's a PNP, then you need to run the circuit on -9v. Your schematic says your battery is backwards.

I believe it's a NPN. All the diagrams I've found of the LPB-1 show negative ground. I can guarantee the transistor came out of an original LPB-1.
Title: Re: LPB-1 Unusual Version / Circuit Comparison?
Post by: DIY Dood on May 30, 2021, 07:20:06 PM

It introduces itself as a healthy Ge p-n-p .. :icon_wink:

If that's a PNP, then you need to run the circuit on -9v. Your schematic says your battery is backwards.


Ding, ding, ding, we have a winner!!!

Unfortunately I focused on the "healthy" part of Antonis' reply, not the PNP part.

Keppy's post stuck it in my face.

I dug through MY old diagrams, vs. the ones on-line, and I found one where I'd labeled B voltage as positive ground.  Rewired and it's working!

On to the tuning part of it. I will do some testing of varied components and come back with some better questions.

Just one question for right now... what transistor do you think it is if it's a PNP in an early LPB-1. Obviously not the universally referenced 2N5088. Is there something else they were known to use back then?

Title: Re: LPB-1 Unusual Version / Circuit Comparison?
Post by: Rob Strand on May 30, 2021, 07:37:50 PM
Quote
Ding, ding, ding, we have a winner!!!

Unfortunately I focused on the "healthy" part of Antonis' reply, not the PNP part.

Keppy's post stuck it in my face.
Good to see it working.  These things happen.

Quote
Just one question for right now... what transistor do you think it is if it's a PNP in an early LPB-1. Obviously not the universally referenced 2N5088. Is there something else they were known to use back then?
It's very hard to guess.  One way is to look at some EHX pedals with germaniums from the same era and see if you can find any part number.   Nothing to prove it will be that anyway.   IIRC there was an EHX parts list at some point it might even be on that.

What package is it?  Does it have any markings at all?
Title: Re: LPB-1 Unusual Version / Circuit Comparison?
Post by: DIY Dood on May 30, 2021, 10:56:09 PM
What package is it?  Does it have any markings at all?


T05. Not a mark on it. Perhaps it faded... or there was never anything there. But it's unmarked at this point.

Title: Re: LPB-1 Unusual Version / Circuit Comparison?
Post by: Rob Strand on May 31, 2021, 04:16:17 AM
Quote
T05. Not a mark on it. Perhaps it faded... or there was never anything there. But it's unmarked at this point.
Nothing obvious comes to mind.   There does exist PNP TO-5 germaniums  but there's nothing linking the possible part numbers to something that Electro-harmonix used in that era.  I actually couldn't find the Electro-harmonix parts cross-reference online.

Maybe someone else can chip in.
Title: Re: LPB-1 Unusual Version / Circuit Comparison?
Post by: DIY Dood on June 01, 2021, 09:41:18 PM
Moving ahead... I tried swapping in the standard resistors to a 10K on the feed from the battery and a 440K on the emitter to ground. I did it with a DPST switch so that I could flip back and forth. I found no apparent difference... at least that I could hear. Overall gain the same.

I also tried changing the input and output caps from .22uf to .1uF and the other way to .4uf. I did it with two switches so that I could change input, output, or both. Also didn't seem to make any appreciable difference. I was testing with a single coil (read bright) guitar, so perhaps that's affecting it.  But I was expecting it to have more effect.

It does cut treble a bit. Not a huge amount, but you can hear it when it's set to unity and you flip back and forth. Not a bad sound, but I'd like to see it just a bit more transparent. Any thoughts on how to achieve that in view of the above testing?
Title: Re: LPB-1 Unusual Version / Circuit Comparison?
Post by: antonis on June 02, 2021, 06:56:12 AM
I tried swapping in the standard resistors to a 10K on the feed from the battery and a 440K on the emitter to ground.

Presuming 10k instead of 5k6 one, gain practically remains the same 'cause major gain factor is 1M / (22k + plus signal source output impedance)

Also presuming 440R instead or 390R, gain is lowred by less than 1db..
(10k/(440+re) vs 10k/(390+re), where re = 0.025/ICQ)
Title: Re: LPB-1 Unusual Version / Circuit Comparison?
Post by: DIY Dood on June 02, 2021, 03:37:03 PM
Presuming 10k instead of 5k6 one, gain practically remains the same 'cause major gain factor is 1M / (22k + plus signal source output impedance)

Also presuming 440R instead or 390R, gain is lowred by less than 1db..
(10k/(440+re) vs 10k/(390+re), where re = 0.025/ICQ)

Thanks... as observed.

Any thoughts on the slight treble reduction? I wouldn't think the tested (.1 to .4uF) caps would be in that range. But there's definitely a little of the high edge cut off.
Title: Re: LPB-1 Unusual Version / Circuit Comparison?
Post by: antonis on June 02, 2021, 05:31:07 PM
Any thoughts on the slight treble reduction?

C1 (220nF) toghether with Q1 input impedance (in parallel with 1M divided with stage gain) form a HPF of about 70Hz cut-off frequecy (for a hFE = 200)

For C1 of 100nF, the above cut-off frequency is lowered down to 30Hz and for 400nF ( :o ) is raised up to 130Hz..
Of cource, the above mentioned -3dB cut-off frequencies are strongly dependent both on particular Q1 hFE and Collector quiescent current..
That said, HPF cut-off frequencies might be even higher (highly likely) or lower (more unlikely)..

C2 (220nF) also forms a HPF together with 100k Volume pot of  about 7Hz (considering pot fully CW and next effect input impedance much higher than 100k), so 100nF raises it up to 16Hz where 400nF lower it down to 4 Hz..
(even with next effect input impedance 100k, say, max cut-off frequency is only 32Hz - no treble loss..!!)
Title: Re: LPB-1 Unusual Version / Circuit Comparison?
Post by: DIY Dood on June 09, 2021, 11:12:48 PM
Any thoughts on the slight treble reduction?

C1 (220nF) toghether with Q1 input impedance (in parallel with 1M divided with stage gain) form a HPF of about 70Hz cut-off frequecy (for a hFE = 200)

For C1 of 100nF, the above cut-off frequency is lowered down to 30Hz and for 400nF ( :o ) is raised up to 130Hz..
Of cource, the above mentioned -3dB cut-off frequencies are strongly dependent both on particular Q1 hFE and Collector quiescent current..
That said, HPF cut-off frequencies might be even higher (highly likely) or lower (more unlikely)..

C2 (220nF) also forms a HPF together with 100k Volume pot of  about 7Hz (considering pot fully CW and next effect input impedance much higher than 100k), so 100nF raises it up to 16Hz where 400nF lower it down to 4 Hz..
(even with next effect input impedance 100k, say, max cut-off frequency is only 32Hz - no treble loss..!!)

Antonis: If I understand what you are saying:
- the C1 side is, at the most, giving me a slight bass filter at 110 or 220nf. So that's not a treble loss.
- the C2 side is, at the most, cutting at 7K, so that's not a loss in the usual guitar range (unless harmonics are the issue)

One factor I will add: The volume pot is not fully CW, it's backed off to equity volume for the back to back testing. Would that lowered pot change the filter and cause the treble loss?

Also: You mention "quiescent current." Does that imply that a higher voltage battery or PS would reduce the effect? I am running at 8.8 off the battery right now (it's a little worn down).


Thanks,
Title: Re: LPB-1 Unusual Version / Circuit Comparison?
Post by: antonis on June 10, 2021, 06:48:42 AM
- the C2 side is, at the most, cutting at 7K, so that's not a loss in the usual guitar range (unless harmonics are the issue)

7 Hz (not kHz)..
(no loss at all..)

The volume pot is not fully CW, it's backed off to equity volume for the back to back testing. Would that lowered pot change the filter and cause the treble loss?

No..
For next stage input impedance much higher than 100k, HPF cut-off frequency is always 1/2π*C2*(pot body resistance)

Also: You mention "quiescent current." Does that imply that a higher voltage battery or PS would reduce the effect?

It depends on various factors related to bias point..
Collector quiescent current sets intrinsic Emitter resistor value (0.025/ICQ) so the higher the current the lower the resistor value..
(maintaining same Collector resistor for higher supply..)

That's good for stage open-loop gain but bad for input impedance (hFE X re)..
(lower input impedance means higher HPF cut-off frequency -> bass loss ..)

Anyway, for particular circuit and 200mV lower supply, don't even bother.. :icon_wink:

P.S.
By re-reading your queries, I'm getting confused a bit..
Both IN & OUT filters are High-Pass so none of them has nothing to do with Treble loss..
(they cut Bass..)

The only item which could affect treble loss should be a high value Miller cap (feedback cap between Collector & Base..)
Title: Re: LPB-1 Unusual Version / Circuit Comparison?
Post by: DIY Dood on June 10, 2021, 10:06:59 PM

The only item which could affect treble loss should be a high value Miller cap (feedback cap between Collector & Base..)

What sort of values should I experiment with to explore that? Also, in series with R1, the 1M resistor, or parallel?
Title: Re: LPB-1 Unusual Version / Circuit Comparison?
Post by: Rob Strand on June 11, 2021, 12:21:19 AM
Quote
What sort of values should I experiment with to explore that? Also, in series with R1, the 1M resistor, or parallel?
The 22k input resistor loads down the pickups and you can loose some highs.    If you make the 22k resistor higher it will reduce the loading but it also reduces the gain.    A limitation of that circuit is the gain and input impedance are linked.

So one way around that is to increase all the resistors.   For example input resistor 47k, feedback resistor 1M changed to 2.2M, 5.6k changed to 10k.   They are only rough values as the germanium transistor leakage stuffs up the proportionality.    You might have to play with emitter resistor or collector base-resistor to get the collector voltage to be what it was before.  Also the low valued 10k volume pot means you don't want to make the collector resistor too high.   So for the parts you have you are kind of boxed-in in terms of juggling the input impedance.   By raising the resistances all the caps need to be halved - more or less.

On the classic circuit there's two components which contribute to loading:  The divider on the base and the 390R in the emitter.  The two loading components are in parallel.  The 390R contributes a loading impedance =  transistor gain * 390.   For a silicon version with a high gain transistor, say hFE = 600, 390 * 600 = 234k which is quite high.  In this case the base divider contributes more to loading 43k//430k = 39k.   So for a classic circuit with a high gain silicon transistor it is possible to raise the input impedance by using higher value resistors on the divider on the base.   A germanium version of the classic circuit isn't so easy to increase the impedance as the low gain  of the germanium transistor limits how high you can make the input impedance.  If the germanium transistor gain is 100 then 100*390 = 39k  so the transistor and the base divider contribute equally.   So even if you removed the base divider loading entirely you can't raise the input impedance above 39k.

(I've glossed over a few details but they are the main points and the main bottle-necks.)
Title: Re: LPB-1 Unusual Version / Circuit Comparison?
Post by: antonis on June 11, 2021, 05:52:24 AM
Also, in series with R1, the 1M resistor, or parallel?

1M resistor is a feedback BIAS one..
(meaning there is DC flowing from Collector to Base..)

I let considering the effect of a series cap up to you.. :icon_wink: