Question: JFET in simple acoustic guitar preamp

[Tone Meister]

Back in from the road, so gonna take a closer look at this circuit again.

The reason for the "thin" bass is usually a low input impedance of the preamp. In case of the preamp I see 220k resistor on the input. It should be in range of 3-5 MOhms. But to tell what is wrong with the preamp we need to see its schematic. And this is just 5 minutes of work. Please do it.

Mark

With a battery installed and a cable in the output jack, the input of the preamp measures 1.27MOhms

BTW, two month ago I redesigned a preamp for Gibson Chet Atkins guitar (modern components and SMD assembly) but it was completely different from what you have. It was much more complex.

Yep, this is an Epiphone and built more cheaply. Neither the Gibson nor the Epiphone Chet Atkins sounds all that good in the stock configuration. I'm gonna make an attempt at drawing the schematic.

[MarkusBass]

With a battery installed and a cable in the output jack, the input of the preamp measures 1.27MOhms

I think you should know that such a measurement should be performed without a battery and without a cable in the output jack. It's because in this case you have voltages from the circuit and no one knows what actually you are measuring. Also, to make the measurement correct you should lift the input resistor from at least one side. If the input resistance is 1.27M, it's to low. It should be at least 3.3M. But most probably you didn't perform the measurement correctly.

I'm gonna make an attempt at drawing the schematic.

Yes, please.

Mark

[nickb]

I think you should know that such a measurement should be performed without a battery and without a cable in the output jack. It's because in this case you have voltages from the circuit and no one knows what actually you are measuring. Also, to make the measurement correct you should lift the input resistor from at least one side. If the input resistance is 1.27M, it's to low. It should be at least 3.3M. But most probably you didn't perform the measurement correctly.
Yes, please.

Mark

You can't do it with a ohmmeter anyway. You need a network analyzer. You can get a rough idea using a scope, a signal generator and a resistor say 100K ohms. Set the genny to 1KHz and 100mV, Put the resistor in series with the preamp input. Power up and measure the volts with the scope on either side of the 100K resistor wrt ground, call that V1 and V2, checking there is no DC. If no scope try 400Hz and ACV on meter.

Calculate Zi = R * V2/(V1-V2)
.

[Tone Meister]

CORRECTED SCHEMATIC

[nickb]

I think that one connection at the bottom of the volume control may be wrong. I've redrawn it to how I think it really is (could be wrong) and also a suggestion of you how you could modify it to get a much higher input impedance and isolate the pickup from the controls in way that should be easy to try. If you like it, you can, it you wish, improve the cable drive capability by using the new alternate values in brackets.

We can tweak the basic idea if needed. Or you can do the MarkusBass route as I'm sure he will have done a nice job.

[MarkusBass]

I think that one connection at the bottom of the volume control may be wrong. I've redrawn it to how I think it really is (could be wrong) and also a suggestion of you how you could modify it to get a much higher input impedance and isolate the pickup from the controls in way that should be easy to try. If you like it, you can, it you wish, improve the cable drive capability by using the new alternate values in brackets.

Nick,

This is great job. The original version does not look as a preamp that can be used with a piezo transducer. It looks like a preamp for a magnetic pickup. I think that the second version is very good because one can easily convert the non-working version of the preamp to one working correctly. Most probably this can be done just by exchanging few wires (and adding one resistor). I would suggest one change; the input resistor R9 (2M2) has still low value. I would increase it to 3M3 or even to 4M7.

We can tweak the basic idea if needed. Or you can do the MarkusBass route as I'm sure he will have done a nice job.

I cannot show my version of the preamp for this guitar because attachments do not work for me (how did you manage to attach the pdf file?). My version is quite complex. It is SMD version (0603 packages - not that easy to solder manually), has 6 inputs (one for each string separately), has TONE control (which actually is BASS control), has TREBLE trimmer, which can be replaced with regular pot if needed. There is also a VOLUME pot.
I think that Nick's second version of the preamp is the best and cheapest way to go. It may need some adjustments but most probably can be assembled on the original board.

PS: my preamp can be temporarily seen here:https://zapodaj.net/42c09d6a3aa3f.jpg.html

Mark

[Tone Meister]

Thanks fellas. Be reminded this pcb has limited real estate. I have this preamp working correctly but with the same complaints. This was installed from the factory, so if the preamp isn't suitable for a piezo pickup then the designers at Epiphone are the ones who put forth the poor design.

I was hoping for a simpler solution with some component value changes.

[nickb]

Nick,

This is great job. The original version does not look as a preamp that can be used with a piezo transducer. It looks like a preamp for a magnetic pickup. I think that the second version is very good because one can easily convert the non-working version of the preamp to one working correctly. attachments do not work for me (how did you manage to attach the pdf file?).

PS: my preamp can be temporarily seen here:https://zapodaj.net/42c09d6a3aa3f.jpg.html

Mark

I used "Manage Attachments". There is a suggestion going around that file size might have something to so with the problem.

[Tone Meister]

Alzo, you left off the 47uF electrolytic cap in my original circuit

[nickb]

I was hoping for a simpler solution with some component value changes.

You''re serious???

I given you about the simplest solution there is. There are two tiny extra components to fit and two wires to move. I'm sure there will be no difficulty in finding room.

PS: Actually just one extra resistor as you can use the cap I overlooked

[MarkusBass]

Thanks fellas. Be reminded this pcb has limited real estate. I have this preamp working correctly but with the same complaints. This was installed from the factory, so if the preamp isn't suitable for a piezo pickup then the designers at Epiphone are the ones who put forth the poor design.
I was hoping for a simpler solution with some component value changes.

I'm not sure what you mean by "limited real estate" (I may have some language problems ). But you are right; it's hard to imagine how the designers at Epiphone come up to such a poor design.
But as I said before - most probably the existing board can be converted to a correctly working version just by exchanging few wires and adding one resistor. This is not a difficult task. You could help yourself by posting a photo of the bottom side of the board and describing the connections to the preamp on the top side.

EDIT: Nick is right. You need to move few wires and add one or two components. What can be simpler than this?

Mark

[Tom Phillips]

...This was installed from the factory, so if the preamp isn't suitable for a piezo pickup then the designers at Epiphone are the ones who put forth the poor design...

If all the facts are correct then, YES, Epiphone totally screwed up. The circuit connects the piezo pickup directly to a passive tone & volume control circuit stage which excessively loads down the piezo output signal thus causing the problem explained earlier in this thread. This is not the only case of a musical instrument manufacturer lacking the proper electronics / physics knowledge.

[Tone Meister]

If all the facts are correct then, YES, Epiphone totally screwed up. The circuit connects the piezo pickup directly to a passive tone & volume control circuit stage which excessively loads down the piezo output signal as explained earlier in this thread. This is not the only case of a musical instrument manufacturer lacking the proper electronics / physics knowledge.

The guitar is stock Tom, and this preamp runs off a 9 volt battery.

I have reposted a corrected schematic above. I had failed to terminate the volume pot, the TONE pot was incorrectly labeled as B250K, and I failed to place a value on the single electrolytic cap at the output. I do hope I have eliminated all the mistakes.

I have put together a parts list from Nick's drawing and am ready to proceed with the circuit tweaks once you have a chance to reassess my diagram corrections.

[nickb]

..and I have likewise reciprocated with an updated schematic (above). BTW I'm fine with 4.7M or 10M on the input.

[Tone Meister]

..and I have likewise reciprocated with an updated schematic (above). BTW I'm fine with 4.7M or 10M on the input.

That ROCKS Nick! I pulled every value so I should be able to piece it together quickly. I'll post back here in the morning (US) with a report.

Thanks a million