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Old 20th June 2008, 01:18 AM   #111
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Default JFET Guitar Preamp

The first picture file was too big. Attached is a "smaller" picture file of a similar JFET preamp I designed about a year ago. I mounted it in an American Standard Stratocaster.

Big sound from a little circuit.

Please let me know if you have any questions on how to adjust the tone or the gain of the Hugsley Guitar Preamplifier.

Thanks, JP Hugsley
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File Type: jpg hugsley_jfet_preamp.jpg (76.7 KB, 1759 views)
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Old 20th June 2008, 03:40 AM   #112
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Alright! Now I can make one complete!

Yes, I'd like to adjust the tone. Is it simply a matter of trim pots? I know that would make it a little larger but I have plenty of room.

My Strat originally came with a preamp, but it was defective. I bought it anyway (got a big discount) knowing I could build one for it myself. What I had in mind at the time was an opamp design, but happily I have succumbed to the discreet component design and, well, here we are! So I have a cutout for the amp and even a battery slot in the back with its own cover - very accessible.

Great job, Mr. Hugsley! Thanks a Million!
And thank you, Javier, for your help as well!

Ken
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Old 20th June 2008, 02:42 PM   #113
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Default JFET Guitar Preamp

Hi Ken,

If you do use trim pots for a versatile tone control, then try using two 100KOhm potentiometers (pots). The Treble control would take the place of R5 and R6 with the CW (or, top) terminal of the pot conected to C2, the wiper connected to C5 and the CCW (or, bottom) terminal connected to the junction of C3 and R7.

For Bass control, you must wire the pot as a "variable resistor" by soldering the wiper terminal to the CW terminal of the pot (so you end-up with a two terminal device) and use it to replace R7. With a pot value of zero Ohms you have full Bass-Cut, with a pot value of 100kOhm (full-CW) you have full Bass-Boost.

Like I said, the tone control is virtually that of a Fender Deluxe Reverb, only it has been scaled to use 100KOhm pots rather than the standard 250KA Ohm pots. You would have better control with audio-taper pots but linear tapers will work too (a bit "touchy" on the control though; e.g. the Bass-Boost will be near maximum with R7 at half value).

If you decide to just use resistors, remeber that:

With R5=0 Ohms and R6=100KOhms you have full Treble-Boost.

With R5=100KOhms and R6=0 Ohms you have full Treble-Cut.

With R7=100kOhms, you have full Bass-Boost.

With R7=0 Ohms, you have full Bass-Cut.

Ther is NO magic to the values that I have chosen so feel free to experiment with other values if you like.

More information: The 33p, C6 capacitor functions as a high-frequency roll-off cap. You can "cut" the high-frequency content of the preamp by making the value of C6 larger. Also, if you find that the signal level from your pickups is overdriving your JFET, you can increase the value of R12 to act as a voltage divider. In other words, if you make R12 equal to 470KOhm and R1 equal to 470KOhm you will cut the input signal level from your pickups by about 6dB (or, by "half").

Please let me know if you have any questions. Good Luck!

JP Hugsley.
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Old 20th June 2008, 06:15 PM   #114
Nordic is offline Nordic  South Africa
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Hi JP, looking forward to giving it a try....
Attached please find a PCB for a through hole component version.

Regards
Nico

Click the image to open in full size.
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File Type: zip hugsleypre.zip (96.5 KB, 411 views)
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Old 21st June 2008, 01:23 AM   #115
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Default JFET Guitar Preamp

Hi Nico,

Wow, you work fast! That's great! I'm very happy that you found the design interesting enough to create a PC Board.

Thanks so much for giving me credit on the design. I hope you enjoy it and please let me know if you have any questions.

Take care,

JP Hugsley
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Old 21st June 2008, 07:36 AM   #116
Nordic is offline Nordic  South Africa
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Do I need to make any major changes to use J309...? All I have at the moment...
At least the pinout is the same...


http://www.datasheetcatalog.org/data...child/J309.pdf
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Old 21st June 2008, 04:56 PM   #117
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Default JFET Guitar Preamp

Hi Nico,

Yes, there would be quite a difference in using a J309 instead of a J201 but it may still be worth experimenting with. The difference would be due to the fact that the IDSS of the J309 is between 12mA and 30mA, and VGS-off is between -1.0V and -4.0 Volt.

For arguments sake, lets say you "characterize" your J309, as described in my previous postings, and find that it is a middle-of-the-road device with an IDSS = 20mA and a VGS-off = -2.5V. Using the rule-of-thumb where Vgs = 1/2(VGS-off) = -1.25V and guessing that the Id would be 8.0mA (this is only a guess, you should verify all the specifications of your device as described in previous postings) you would then determine the resistor values to bias your common-source amplifier.

1. Rs = VGS/Id = (1.25V)/(8.0mA) = 157 Ohms or 150 Ohms

Using the rule-of-thumb that VRd = 1/3(Vcc) = 1/3(9.0V) = 3.0V;

2. Rd = (3.0V)/(8mA) = 375 Ohms or 390 Ohms.

Assuming that the values of IDSS and VGS-off are correct (these were only guesses) then the Rs and Rd values just calculated would bias-up your amplifier in the linear region for proper opearation.

However, there are two main differences that you would encounter:

1. The gain would be a little "low" for the Hugsley preamp. However, due to the realativley low output impeadance of your J309 amp as compared to the J201 amp, this may not be as big a problem as one would first think.

With a J309, you would get a gain factor of about 2. The J201 in the Hugsley preamp will give you a gain factor of about 6. However, due to the differences in output impeadances, you may still get a percieved signal that is reasonable (similar) at the output of the emitter-follower stage with the J309 (again, you would have to know the specifications of your J309 to calculate the difference).

Of course, you could always modify the EQ so that the attenuation is not so great but, in doing so, you may have to alter the "Fender Deluxe Reverb" topology a little (not really a problem). Experiment if you like, you may just like what you find better than the Hugsley values.

2. The battery-drain would be too high for practical use. With an Id of 8.0mA (The Hugsley preamp, with the J201, has an Id = 0.1uA) the battery would last no longer than 60 to 70 hours. This may not be a problem for you if you have easy access to your battery but it would not be very practical for a comercial product (it may end-up as the "number-one" customer complaint in your service department).

I would suggest that you evaluate your J309 as described in previous postings (for fun and practice) and then "solder-it-in" with with the proper biasing resistors.

Final pont: Although a fine JFET, there is nothing "magical" about the J201 or in how it is applied in the Hugsley circuit. Remeber that, as long as you qualify the differences and quantify the coresponding bias resistor values, you should have little or no problem in making this little preamp work with a variety of JFETs.

I would emphasize, however, that you must be aware of the trade-offs when dealing with various parameters like gain, battery-life, noise and transfer characteristcs in order to compensate effectively for optimal performance.

Keep me posted on how things turn out.

Good Luck, JP Hugsley
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Old 21st June 2008, 07:52 PM   #118
Nordic is offline Nordic  South Africa
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Very valuable, thread, I intend to go through it from the start with a calculator tommorrow...

I see that from a battery life point of view the original choice is much better... I had some J309 left from my rogic buffers, in which they sound unbeatable... Not such a big issue... I want to build it into my friend's guitar. He recently killed his pickups, got some new ones but did such a bad job wireing it up that none of the knobs work etc... I will build this plus a dynaComp compressor into the guitar...

The guy is realy good, just not rich... although he lately have been doing lead guitar for some commercial stuff and even a movie..
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Old 26th June 2008, 12:17 AM   #119
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Hi guys,

Mr. Hugsley:
I wrote a program to calculate the various figures and values around the FET stage of your preamp using the "Square-Law" method. If I still had my O-Scope . . but I don't so I'll have to rely on this method.
Anyway, while I was writing the program a couple of questions came to mind.

1. In your example you set the Drain voltage to 5 volts but for my values you used a Drain voltage of 6. I know that both are within the 1/2 to 2/3 range recommended but, well wouldn't closer to the 1/2 mark be better to get an even swing, positive and negative, of the input? Is there a reason I'm missing for the varied Drain voltage and if so, how is it determined.

2. While I will build both stages of your design for my guitar, I have a friend who only wants the first, FET stage for his. I know that the gain for it needs to be "loaded down" to get within the range I want. On my test run I used the 10k you recommended and it worked fine. How can I determine the value I need to achieve a particular gain? Again, if I still had my Scope it would be easy to experiment and set.

Any light you could shed on either of these would be greatly appreciated.

Thanks,
Ken
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Old 27th June 2008, 07:27 PM   #120
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Default JFET Guitar Preamp

Hi Ken,

There are essentially two reasons why I suggest the VD (Drain Voltage) of your JFET be biased between 1/2 and 2/3 your supply Voltage and both have to due with maintaining a symmetry in the output-voltage "swing" above and below your "bias point" (or, quiescent point):

1. The bias point will be "raised" by the value of Vgs (Vgs is equal to the Voltage "across" the source resistor (Rs)). In other words, if the Vgs (bias point) of your JFET is 1.0 Volt (Rs = 1.0V), then the mid-point between the voltage "across" the JFET (VDS) and the Voltage across your Drain resistor (VD) will be 5.0 Volts when a 9.0Volt supply is used.

Rs = 1.0V, VDS = 4.0V and VD = 4.0V for a total of 9.0V.

This would give you perfect symmetry of output voltage swing both above and below the bias point IF the JFET were perfectly linear (which, of course, it is not). However, as can be seen graphically in both the "Transfer Characteristic Curve" and the "VDS/ID Family of Curves" which are supplied in the JFET data sheet, the JFET is not perfectly linear.

This leads us to the second reason why I suggest you bias the JFET for a VD of between 1/2 and 2/3 the supply voltage.

2. You will notice in the "VDS/ID Family of Curves" that the spacings between the constant Vgs curves are not "equidistant". In other words, the difference (or, DELTA) in ID (Drain Current) will be greater as the Vgs moves incrementally from VGS-off (or, "pinch-off") to 0.0 Volts.

What this means is that there will be a non-symmetry about the bias point. The negative going voltage at the output of your JFET amplifier will swing more negative in value than it will swing postive in value (this can easily be seen on an oscilliscope with a "large-amplitude" signal just before "clipping"). For this reason, you will require a little more negative going voltage "headroom" with your amplifier than you will positive going "headroom" if you are to avoid un-equal "clipping" with large-signals.

What this all means is that, in order to acheive equal-headroom, you will need the bias point to be greater than 1/2 the supply voltage to compensate for both VRs (or, Vgs) and the non-linear voltage swing inherent in your JFET amplifier.

Although greater than 1/2 and less than 2/3 supply voltage is a good "rule-of-thumb" and will work well without verification, it really is best to verify your bias point with an oscilliscope as you suggest.

Concerning the "Square-Law" equation: by it's very nature, this equation is non-linear and, although used succesfully in the calculation of Id in a JFET, it must be remebered that it is only a simplified approximation that is usually within 10% of the real value.

On the bright-side of all this non-symmetry (or, non-linearity), this artifact is what gives rise to what is know as "even-harmonic-distortion".

Remeber that term because the manifistation of that particular non-linearity has been debated among audio enthusiasts as actually "enhacing the tone" of an amplifier (I hope I didn't open too big a can-of-worms with that last statement

Concerning using just the first-stage of the preamp design; that's fine, experimenting with the design to best fit your application is what this exercise is all about.

To calculate how to lower your gain with a loading resistor is a straight-forward process using a variation of the voltage-divider formula.

Using the first stage of the Hugsley Preamp as an example, the output impedance is simply the Drain resistor value of 43 kOhm (although not exact, it is a good approximation) and, if you may recall, the gain of the first stage was about 6. If I want to lower the gain by half (or, to 3) then I would "load-it-down" with a 43 kOhm resistor after the AC coupling capacitor.

Other values can be calculated in the same manner using the voltage divider formula:

Vout/Vin = Gain = (RL/(RL + RD)).

Where RL is the load resistance and RD is the Drain resistance.

Remeber to only load the amplifier after the AC coupling capacitor.

I hope this helps,

JP Hugsley
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