Question: JFET in simple acoustic guitar preamp

[nickb]

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

"Pulled every value"? You need just one new resistor 2.2M, 4.7M or 10M.

EDIT: You can't leave out the resistor as it's piezo pickup

[Helmholtz]

"Pulled every value"? You need just one new resistor 2.2M, 4.7M or 10M. Actually you don't even need that as the pickup is DC coupled. Just leave it out - I should have realized that sooner...

A piezo PU is a capacitor, a capacitive source. A 1M termination often results in a lower corner frequency of around 100Hz. Higher R- values lower this frequency. But I would not totally omit a DC ground reference for the gate.

[nickb]

A piezo PU is a capacitor, a capacitive source. A 1M termination often results in a lower corner frequency of around 100Hz. Higher R- values lower this frequency. But I would not totally omit a DC ground reference for the gate.

Thanks. I had already realized that and had corrected my error (see post). You must have got in really quickly!

[g1]

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

For some reason, some of us are luckier than others with the attachments:



Nickb: totally guessing, but I thought when he said 'pulled every value' he may have meant he got them all ready/on-hand.

[MarkusBass]

For some reason, some of us are luckier than others with the attachments

Thanks G1 - it's magic. Or, maybe you are using different software, e.g. Firefox on Vista ? I use Chrome on Windows10.

Mark

[52 Bill]

I'd love to see a photo of the foil side of the original pc board.

[Tone Meister]

I'd love to see a photo of the foil side of the original pc board.

[Tone Meister]

"Pulled every value"? You need just one new resistor 2.2M, 4.7M or 10M.

EDIT: You can't leave out the resistor as it's piezo pickup

Yes, every alternate value you listed in the second drawing, which totals 7 different values that aren't in the original circuit, including two different pot values. Sometimes I think my comments get lost in translation here on this board.

I absolutely plan to install the 4.7Meg input resistor and swap the wires first, but I'm open to modding the circuit exactly as Nick has drawn. Just gotta find the time to get back to the bench and do it.

[nickb]

Yes, every alternate value you listed in the second drawing, which totals 7 different values that aren't in the original circuit, including two different pot values. Sometimes I think my comments get lost in translation here on this board.

I absolutely plan to install the 4.7Meg input resistor and swap the wires first, but I'm open to modding the circuit exactly as Nick has drawn. Just gotta find the time to get back to the bench and do it.

But you don't have to use those values. Just go with what is already there but with the added resistor. Using the alternate values is optional if you want to get better cable drive. Try the easy route with the existing values first. If you find that the sound is fine you're done. If you find it's bit muddy with some (long) cables then go for the alternate values.

PS: Hmm.. Maybe that is what you meant. I'm just anxious for you not to do any unnecessary work, that's all.

[Tone Meister]

PS: Hmm.. Maybe that is what you meant. I'm just anxious for you not to do any unnecessary work, that's all.

Actually, that was exactly what I meant. But the parts are in a cup in case I need to go further. Thanks Nick.

[Tone Meister]

OK, it took about 20 minutes to make the changes from Nick's drawing. I want to point out a few things first.

The .47/50 electrolytic cap at the output was missing the decimal point on my drawing, and as a result in Nick's drawing too. I left it as it was.
The 4.7n cap was compromised and I replaced it with an 8.2n (measured 7.6n) because that's all I had.
Had to cut one trace to access the points in the circuit where the changes occurred

There is a huge, very noticeable difference in the output AND frequency response. The treble is moderately extended (to my ears) and the bass is quite full as compared to the stock circuit. The owner plays a Taylor T5 on stage into a pedalboard, so it's doubtful he'd ever use this on stage. But if he did, I feel sure it needs no further modification since he isn't driving long lengths of cable, and his pedalboard has input and output buffering.

All in all this was a simple cure to a major issue(s) with this guitar. It is night and day from when he dropped it off last month. Thanks for all the contributions.

EDIT TO ADD: I ended up using a 4.7M input resistor rather than the 2.2 M on the original drawing

[nickb]

Glad to hear it all worked out.

Here is the final version should anyone ever come across this thread again.


Preamp-2-version-C.pdf

[MarkusBass]

Congratulation. The guitar has finally a preamp that should be there from the very beginning. I'm sure Epiphone, as well as other owners of the guitar would like to know the details of the modification. I have no idea how the Epiphone designers could make such a big mistake. The previous version could possibly work with a magnetic pickup but not with piezo. It is hard to compare the preamp to the preamp used in Gibson Chet Atkins guitar (which is extremely good but also quite complex).
And the changes that were needed were quite straightforward. The biggest change was to cut the track between the slider of the VOLUME pot and the gate of the JFET transistor. The rest was easy.
I'm just not sure about the change of the capacitor from 4.7n to 7.6n. This could slightly influence the frequency response of the TONE control (I do not have time to simulate it). It would make sense to check it and if the value of the capacitor is important, then change it to the old value.
I'm curious what the owner of the guitar is going to say now .

Mark

[Tone Meister]

I'm just not sure about the change of the capacitor from 4.7n to 7.6n. This could slightly influence the frequency response of the TONE control (I do not have time to simulate it). It would make sense to check it and if the value of the capacitor is important, then change it to the old value.
I'm curious what the owner of the guitar is going to say now .

Mark

The tone control is more useful now, and the 7.6n (8.2n) value was all I had on hand so the result is very much an accident. The owner brought his entire acoustic guitar rig by and was blown away with the difference in overall sound. His stage rig has much more dynamic range and wider frequency response than my little test amplifier - the highs are very crisp and detailed now and the bass is full without being too much. From what I heard both playing myself and then listening as he played, the guitar sounds like it should have from the factory and has plenty of gain.

[Tone Meister]

For the sake of posterity in case someone else wants to perform this mod.