Question: JFET in simple acoustic guitar preamp

[MarkusBass]

Finally I had some time to simulate the TONE control from this preamp. This is one-knob tone control used for example in Supro Thunderbolt amp and later in BIG MUFF and BIG MUFF PI effects.
Some information is available here: http://chasingtone.com/yourguitaramp...tone-controls/ (scroll down to filter #9) and here: https://www.electrosmash.com/big-muff-pi-analysis .
The change of the capacitor from 4.7nF to 7.6nF is not that good because it lowers the middle frequency of the filter from 260 Hz to 215 Hz and also lowers the gain on the circuit by 2 dB at 100 Hz. This is not that much but may be audible. It's better to use 4.7nF.
In general the filter is not the best due to incorrect component values used (another problem of the designers). As a result you can change the gain by 10dB @1kHz but only by 3dB @100Hz. But since the owner of the guitar liked the new sound, I would leave it like that.

Mark

[Tone Meister]

The change of the capacitor from 4.7nF to 7.6nF is not that good because it lowers the middle frequency of the filter from 260 Hz to 215 Hz and also lowers the gain on the circuit by 2 dB at 100 Hz. This is not that much but may be audible. It's better to use 4.7nF.

Mark

The original 4.7n cap was compromised by heat when removing components for the original testing. It was a small cheap ceramic cap and crumbled when removed by my desoldering station. I used the 7.6n (8.2n) because that was the closest value I had on hand.

The TONE control functions to my ears like a simple low pass filter somewhat similar to the tone control on an electric guitar. Is that not correct?

I have the flexibility to make additional changes to the preamp if you would like to make suggestions for further adjustment. The guitar's owner would welcome any additional modifications that would further improve the performance.

Please make suggestions if you feel the result is worth a few more minutes of my time. I am certainly willing to try anything you suggest.

[Tone Meister]

Also, the TONE potentiometer is Alpha B500K, not B250K as I stated initially. I made this correction in post #28 (and #45) but we continued without making the correction on the updated drawings.

Perhaps that omission would skew your test results?

[MarkusBass]

The original 4.7n cap was compromised by heat when removing components for the original testing. It was a small cheap ceramic cap and crumbled when removed by my desoldering station. I used the 7.6n (8.2n) because that was the closest value I had on hand.

I understand the reason but this is not an explanation. If you wouldn't have 8.2n, would you put e.g. 100n in (because this is the only value you have)? The filter was designed for 4.7n and this cannot be changed (without changing the frequency response of the preamp).

The TONE control functions to my ears like a simple low pass filter somewhat similar to the tone control on an electric guitar. Is that not correct?

This is not correct. The filter consists of two filters: low-pass and high-pass. With the pot you can vary the frequency response from one filter to the other, or stop somewhere in between. It works like a TILT filter: when you increase the high frequency gain, at the same time you decrease the low frequency gain. In the middle it should be more or less flat. So it is not a low-pass filter only. With this filter you can add 10dB gain at 1kHz.
I could show you the frequency response of the filter (depending on the tone pot setting) but due to some problem with the forum software I cannot add attachments to my posts .

Please make suggestions if you feel the result is worth a few more minutes of my time. I am certainly willing to try anything you suggest.

I could make some suggestions but this would change your work to a series of experiments (because every new suggestion should be verified with live circuit). And this would cost you a lot of time. For the moment I suggest changing the 7.8n cap back to 4.7n (buy one).

Also, the TONE potentiometer is Alpha B500K, not B250K as I stated initially. I made this correction in post #28 but we continued without making the correction on the updated drawings. Perhaps that omission would skew your test results?

Yes, this changes slightly test results but only slightly. Instead of regulation of 3dB @100Hz now I have 4dB. This is still not much compared to 10dB @1kHz.

Mark

[Tone Meister]

I put in what I had because the circuit would not work with no cap. I will replace the 4.7n as soon as one can be ordered. Thanks Mark and Nick.

[MarkusBass]

This is the frequency response of the TONE control circuit. The mid frequency is about 200Hz (it seems to me a little bit too low) and the regulation @100Hz is not very impressive (4.3dB only).




EDIT: it looks like attachments work correctly now. I may post some additional info soon.

Mark

[MarkusBass]

The basic idea of this TONE control circuit is that there are two filters: low-pass (R13 and C21) and high-pass (C20 and R14). The TONE pot allows for balance between these two filters frequency responses.



Please note that initial schematic is slightly different: there is additional resistor in series with C21 and the R14 resistor is missing. But of course the pots play a role of this resistor.
One of my suggestions is shown on this schematic.
First, I suggest to increase the input capacitor from 0.47uF to 10uF. This changes the gain on the circuit @100Hz only by 0.3dB but in my opinion it is still worth doing (we want more bass from the circuit).
Then, I suggest to change the components to the values shown on the schematic above (47k and 10n). This will change the middle frequency of the circuit from 200Hz to 400Hz and the range of regulation to 9-10dB both at 100Hz and 1kHz. The resistor in series with the capacitor in the original circuit should be replaced with a jumper to the ground.
Here is the new frequency response of the circuit:

You can see that it is much better "balanced" than the initial filter.
But of course the most important is listening test. I'm not sure whether wider range of regulation will be beneficial but I would try it.
If the sound is not better, revert the circuit to the previous version. Just leave 10uF on the input.
Anyway, the owner of the guitar is already happy with the sound of the new version of the preamp.

Mark

[Tone Meister]

UPDATE Owner brought the guitar back to make the suggested changes. Decided to go the simple route, so I just put in the correct 4.7n cap and replaced the 0.47uF electrolytic cap with the suggested 10uF.

We did notice a slight increase in bass and gain. I was prepared to make the changes Markus suggested above, but the tone control works fine for him now, so we plan to leave it this way. The preamp sounds much better that it did stock.

Thanks everyone.