Question: JFET in simple acoustic guitar preamp

[Tone Meister]

Have an Epiphone Chet Atkins here with complaint of "thin sound that is mostly treble with little bass response" (amplified) and a string balance problem - some strings sounded strong and others were very weak. Play test confirmed both problems.

Has a Shadow SH-091 under saddle transducer connected to an unidentified preamp that has volume and tone controls. The transducer was intermittent and the proprietary bridge saddle was broken on one end. My best efforts failed to duplicate that saddle with bone because the stock saddle was far too thin. So I replaced the transducer & saddle with stock parts and that cured the intermittence and string balance problems.

The instrument produces reasonable volume acoustically, but when plugged in the bass response is very thin, as if it is rolled off. Pickup connected straight to amp input, bypassing preamp, and bass response is much fuller albeit at a much lower volume. So I checked the preamp and found the K30A N-Channel JFET (out of circuit) had very high resistance between Gate-Source and Gate-Drain with the meter's positive lead on the gate. Resistance was 5.7 Mohms both directions.

Can this high resistance cause the "rolled off" bass response with this preamp?

Unknown Preamp


Stock Shadow Saddle Compared to Typical Bone Saddle

[nickb]

A JFET G to S and G to D should test as a diode so put your meter on the diode test range and try both ways.

If that is OK, put it back and measure the S,G and D voltages when operating

[bbsailor]

Under the saddle pickups typically need a very high impedance buffer or amplifier to prevent the on board volume/tone controls and the cable capacitance from loading the output and reducing the bass response. Look up on the web “Tillman buffer” to see a good way to buffer your under the saddle pickup and isolate it from all external loading.

Joseph J. Rogowski

[Tone Meister]

Under the saddle pickups typically need a very high impedance buffer or amplifier to prevent the on board volume/tone controls and the cable capacitance from loading the output and reducing the bass response. Look up on the web “Tillman buffer” to see a good way to buffer your under the saddle pickup and isolate it from all external loading.

Joseph J. Rogowski

Did you miss the part where I am troubleshooting the PREAMP? This is the stock arrangement from Epiphone, but no schematic is available.

[Tone Meister]

A JFET G to S and G to D should test as a diode so put your meter on the diode test range and try both ways.

If that is OK, put it back and measure the S,G and D voltages when operating

Tests fine as a diode
Battery measures 9.6 Vdc

With the battery installed and a cable plugged into the output jack to activate the circuit:
4.85 Vdc across the dropping resistor
4.85 Vdc at the S
Measures 0.00 Vdc at the G
Measures 0.685 Vdc at the D

So this one's bad then?

[Jazz P Bass]

Here is a project jfet preamp.

http://www.rason.org/Projects/jfetamp/jfetamp.htm

It's a good read.

[nickb]

What was your negative lead connected to when you did the measurements ? I suggest you put it on the negative terminal of the battery.

Any idea what happened to the image of the preamp in post #1? I was going to take a good look at it to see if I could guess the circuit. That said it should not be too hard to work out as there's only a few components and I think that would be the easiest strategy.

PS: Here is the Tillman buffer Joe mentioned. There's a good chance what you have is similar http://www.till.com/articles/GuitarPreamp/

[Tone Meister]

What was your negative lead connected to when you did the measurements ? I suggest you put it on the negative terminal of the battery.

Any idea what happened to the image of the preamp in post #1? I was going to take a good look at it to see if I could guess the circuit. That said it should not be too hard to work out as there's only a few components and I think that would be the easiest strategy.

PS: Here is the Tillman buffer Joe mentioned. There's a good chance what you have is similar http://www.till.com/articles/GuitarPreamp/

I'm not interested in building a buffer for this guitar, all I want to do is verify this preamp is either working as intended or that it has a problem. This preamp has too much gain to be the Tillman, in my opinion.

Negative lead was on the battery during voltage measurements. I can still see the original photo in post #1, but I'll post a few more.

[Leo_Gnardo]

Is there enough pressure on the pickup? I've run across this kind of thing, you assemble, string up & it sounds sort of OK but not robust as it should. Give the saddle a good hard push down onto the transducer. Sometimes you feel it "pop" into place then everything's OK. Worth a try, costs nothing, takes a couple seconds.

[Helmholtz]

Tests fine as a diode
Battery measures 9.6 Vdc

With the battery installed and a cable plugged into the output jack to activate the circuit:
4.85 Vdc across the dropping resistor
4.85 Vdc at the S
Measures 0.00 Vdc at the G
Measures 0.685 Vdc at the D

So this one's bad then?

Not necessarily. JFets are typically symmetrical, i.e. source and drain are interchangeable and cannot be identified by measurement of the component. Which terminal is used as source or drain depends on the circuit.
If your source is actually used as the drain, the measured voltages look fine for a common source amplifier.

The circuit examples posted should help you to identify and draw up your circuit.

[Tone Meister]

Is there enough pressure on the pickup? I've run across this kind of thing, you assemble, string up & it sounds sort of OK but not robust as it should. Give the saddle a good hard push down onto the transducer. Sometimes you feel it "pop" into place then everything's OK. Worth a try, costs nothing, takes a couple seconds.

The peg holes are slotted and there is adequate break angle across the saddle. The saddle bottom is perfectly flat and is fully inserted into the slot and mated witht hte transducer. I worked on the peg hole slots and flattened the saddle bottom myself. I feel the mechanical issues were fixed with the replacement transducer and saddle.

Now, either this preamp is performing as intended with regard to frequency response, or it isn't. There are no dropouts and no gross string imbalances. Simply put, the instrument sounds more full if the preamp is bypassed and plugged straight into an amplifier and the amp's gain increased to compensate. When the preamp is used, the response is more like a high pass filter is being used and the highs are subdued.

[nickb]

Thx for the pics. For some odd reason the original one wasn't showing up and now it is again

What Helm said is true about the symmetry of JFETS but I'd think it quite unusual for the board to be made that way. Still it should work. As the DC is OK I think I'd focus on the pots or bad connections next. If you can do it, a signal generator instead of the pickup and scope will solve this in no time.

The point of the Tillman circuit is that this one is going to look very similar. It will be a kind of road map to help you trace it out.

[bbsailor]

Have an Epiphone Chet Atkins here with complaint of "thin sound that is mostly treble with little bass response" (amplified) and a string balance problem - some strings sounded strong and others were very weak. Play test confirmed both problems.

Has a Shadow SH-091 under saddle transducer connected to an unidentified preamp that has volume and tone controls. The transducer was intermittent and the proprietary bridge saddle was broken on one end. My best efforts failed to duplicate that saddle with bone because the stock saddle was far too thin. So I replaced the transducer & saddle with stock parts and that cured the intermittence and string balance problems.

The instrument produces reasonable volume acoustically, but when plugged in the bass response is very thin, as if it is rolled off. Pickup connected straight to amp input, bypassing preamp, and bass response is much fuller albeit at a much lower volume. So I checked the preamp and found the K30A N-Channel JFET (out of circuit) had very high resistance between Gate-Source and Gate-Drain with the meter's positive lead on the gate. Resistance was 5.7 Mohms both directions.

Can this high resistance cause the "rolled off" bass response with this preamp?

Unknown Preamp


Stock Shadow Saddle Compared to Typical Bone Saddle

Maybe this is more relevant. Other people have has similar problems on the same guitar as yours.
See this web link to see reviews of an active pre-amplifier that they used to solve their problem.
https://www.allparts.com/PU-6413-000...ne_p_3114.html

Joseph J. Rogowski

[MarkusBass]

I suggest drawing the schematic - exactly what Nick suggests. This is very simple circuit and you can do it in 5 minutes. 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

[MarkusBass]

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.