New JFET guitar preamp project
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 16th June 2008, 09:15 PM #101 xavmdq   diyAudio Member   Join Date: Mar 2008 Hi Ken: If I understand Hugsley explanation the idea is to fix Id (and fixing Id you fix Rd and voltage gain). To achieve that we need to evaluate Idss. The steps are: - Obtain Idss and Vp (or Vgsoff) and select JFET with Idss between 200uA and 600uA (Id ~ Idss/3 to Idss/2). - Id=100uA, then Rd = (Vdd/2) / Id (for a Vout = Vdd/2) - From transfer curve or using equation Vp*(1-sqrt(Id/Idss)) (with 10% error) obtain Vgs. - Finally Rs = - Vgs/Id - Voltage gain is aproxx: Av ~ 2 * (Idss / Vp) * (Vgs/Vp –1) * Rd I think Rg=1Mohm is not necessary if you use magnetic pickup. A piezo pickup need Rg because (Hugsley please correct me if not) have more impedance (infinite resistance). The idea of this preamp is to use between pickup and the rest of circuits (tone/vol/effects). Preamp act as impedance isolator (very high input impedance , low to medium output impedance) The next circuit tells you if necessary a DC blocking output capacitor. For example, if go to amplifier directly, I think is not necessary (the amplifier have input capacitor). Sorry for my english and I hope this helps. Javier
 16th June 2008, 10:21 PM #102 sepulchre   diyAudio Member   Join Date: Jun 2008 Hi Javier, Please pardon my ignorance but I do not know some terms. I know : Id is drain current, Idss is drain current at saturation, Vgs is voltage gate to source, and I understand Vp or Vgsoff. But what exactly is Vdd? Is it supply voltage? Maybe you can help me calculate Rd and Rs. I want Id = 100uA and Gain = 2. My supply is 9 volts Idss = 590uA Vp = -0.82 volts If you can tell me the values you calculate then maybe I can better understand the equations. Thanks! Ken
xavmdq
diyAudio Member

Join Date: Mar 2008
Quote:
 But what exactly is Vdd? Is it supply voltage?
Sorry Ken, yes, Vdd is supply voltage, 9V.

Quote:
 Maybe you can help me calculate Rd and Rs. I want Id = 100uA and Gain = 2. My supply is 9 volts Idss = 590uA Vp = -0.82 volts
The problem is that you can't fix gain because is a consecuence of fixing Id and the desire output operating point. Then I'll make step-by-step how to obtain Rd and Rs from Id and Vdd:

1_ Id = 100uA then Rd = (9V/2) / 100uA = 45Kohm
then Rd = 43kohm or 47kohm (more common)
2_ You can recalculate Id with the normalized Rd:
Id = (9V/2) / 47Kohm = 95.75uA
3_ Using transfer curve of your particular JFET (Id vs Vgs), obtain Vgs. I'll use the equation because I don't know your JFET. Then:
Vgs = -0.82V*(1-sqrt(95.75uA/590uA)) = -0.49V
4_ Rs = -Vgs/Id = 5114ohm. The normalized value is:
Rs = 5.1kohm
5_ Voltage gain:
Av = 2 * (Idss / Vp) * (Vgs/Vp –1) * Rd or
Av = 2 * sqrt(Id * Idss) / Vp
Av = 27
Is to much gain but I use JFET equations (with 10% error).
Av depends on Rd-Id (or Rd-Vgs), then we must reduce Rd or Id to reduce gain. But reducing Rd, Vout and Id increases.

Now, if you want to obtain Rd and Rs fixing Av and Id:

Rd = Av * Vp / (2 * sqrt(Id * Idss)) = 3376ohm
then Rd = 3.3kohm
Rs is calculated in the same way:
Rs = 5.1kohm
But now, Vout is Vdd-Id*Rd=8.67V!! (no swing for pickup signal).

Then, we can only fix 2 variables: Id-Av , Id-Vout , etc.

I think we must increase Vout (don't use Vdd/2) to decrease gain. But we must have room to signal and this depend on input signal level.

Well Ken, I'll wait JP Hugsley response to see what he has to say.
Javier

 17th June 2008, 11:53 AM #104 sepulchre   diyAudio Member   Join Date: Jun 2008 Hi Javier, First, thank you for your help. You obviously have a better understanding of this than I. The equations you have shown here are what I was looking for, and your step by step explanation is Very helpful! About the gain: I'm sure you are right. In his last post Mr. Hugsley used another resistor, from output to ground, to drop the gain from a figure of about 7 down to the level I needed - about 2. That's how the first JFET I saw lowered the gain (by Donald Tillman). It's all academic anyway because I'm sure that the output resistor will not be used when the second stage of the preamp is integrated with the first. As you say, we'll see what Mr. Hugsley has to say. Thanks again, Ken
xavmdq
diyAudio Member

Join Date: Mar 2008
Hi Ken:

Quote:
 About the gain: I'm sure you are right. In his last post Mr. Hugsley used another resistor, from output to ground, to drop the gain from a figure of about 7 down to the level I needed - about 2. That's how the first JFET I saw lowered the gain (by Donald Tillman).
Right! I forget the effect of output resistance (Ro) and the gain I calculated is with a bypass capacitor from source to ground.
The equations are (I used Avb for gain with bypass capacitor and Av without bypass capacitor):

With source capacitor:
Avb = (Rd//Ro) * 2 * sqrt(Id * Idss) / |Vp|
|Vp| means absolute value of Vp (0.82V in your FET)

Without source capacitor:
Av = 1 / [1/Avb + Rs/(Rd//Ro)]

In Hugsley circuit Ro >> Rd , so gain is little affected (may be will be more affected by the following stage).

Now, with Ro, you can adjust Av:
Ro = 1 / [(1/Av - 1/Avb)/Rs - 1/Rd]

Ro = 14.5kohm

Correction from my previous post: you must use bypass capacitor at output if you want to adjust gain using Ro and input impendance from following stage.

Another thing I forget in my previous post:
Not only Idss is important, Vp too because will limitate voltage input. I don't know the level given by magnetic pickup but the max. input voltage peak to peak, Vippmax, will be |Vp| or less (|Vp| if Vg=0V at operating point). For your particular JFET, Vippmax ~ 0.8V. The "exact" value can be calculated using:
Vippmax = 2 * |Vgs - Vp|

I hope the circuit will be more clear to you with this explanation.
Javier

 18th June 2008, 03:08 AM #107 sepulchre   diyAudio Member   Join Date: Jun 2008 Hi Guys, You are both excellent teachers; I've learned pretty much all I need to know (I think, lol). Your explanations are very clear and concise and I'm confident that they will work for optimizing pretty much any JFET amp stage. The only question I have is about Mr. Hugsley's equation for obtaining Id: Id = IDSS (1 ? (Vgs/ VGS-off))**2 Is the question mark after the 1 a square root symbol? Maybe it wouldn't translate to the forum here? Other than that I'm ready to see the second stage. You both have my thanks, Ken
 18th June 2008, 02:16 PM #108 xavmdq   diyAudio Member   Join Date: Mar 2008 Hi Hugsley and Ken: Hugsley, I think the confussion was because with option 1 (theoric) you start from Vgs=Vp/2 and with option 2 (experimental) you start fixing Id (making a balance between consumtion and linear operation). I started fixing Id directly (with both methods). I checked some datasheets and I saw that the linear part of transfer characteristic is at 1/3*Vgs to 1/2*Vgs like you mention (and this Vgs values corresponds with Id values in the 2/3*Idss zone). But low Vgs implies high Id. I think we have to make a balance between desired Id value and selected Vgs point, taking into account the linear zone. When using option 1: Selecting Vgs = Vp/2, Id will be: Id = Idss/4. And if we want Vgs = Vp/3, Id will be: Id = Idss * 4/9 Some points: 1_ I saw you use for Vd a Vdd*2/3 instead of Vdd/2. In this point there is no rule, right? This depends on output signal swing and gain? 2_ I agree with you about output bypass capacitor (OBC). I say to Ken that OBC is necessary if the following stage don't have a bypass cap. 3_ Gain: Using your calculated values (and without Ro): Avb = (Rd) * 2 * sqrt(Id * Idss) / |Vp| = 28.83 Av = 1 / [1/Avb + Rs/Rd] = 5.9 I derive gain equations from JFET equation and using the following definition: Avb = (dId / dVgs) * Rd And for your original circuit: Avb = (Rd) * 2 * sqrt(Id * Idss) / |Vp| = 22.87 Av = 1 / [1/Avb + Rs/Rd] = 7.44 Obviously, these equations are not accurate because are derived from JFET equation and don't take account of Rds (dinamic resistance between Drain and Source). Ken: I saw correctly Mr Hugsley equations: is a minus. Id = Idss * (1 - (Vgs/ VGS-off))**2 If you fix Vgs: Id = Idss * (1 - (Vgs/Vgsoff))^2 And if you fix Id: Vgs = Vgsoff*(1-sqrt(Id/Idss)) Both equations are equivalent. Please Hugsley continue with following stage. After exams I'll post the idea I have to the new design (I want to transform TL082 based equalizer to JFET based).
JP Hugsley
diyAudio Member

Join Date: Apr 2008
JFET Guitar Preamp

Hi Guys,

I have attached a schematic of the complete "on-board" guitar preamp. I will also try and post a picture of another on-board guitar preamplifier that I did about a year ago (the file may be too-big for DIY). Notice that the preamplifier circuit board is attached to the back of a standard Fender, 250KA potentiometer. This made for relatively easy mounting.

As you can see from the schematic, the guitar preamplifier is composed of three basic circuits: 1) The self-biased, J201, common-source amplifier, 2) A virtual copy of a standard Fender Bass and Treble tone control and, 3) The emitter-follower.

The J201 Amplifier.

We have gone over this aspect of the design in detail so there will be nothing mentioned about it here.

The Tone Control

This is essentially the Bass and Treble tone control of a Fender Deluxe Reverb guitar preamplifier. There is a slight difference in capacitor values due to impedance-scaling and the resistance values were chosen so as to emulate the Bass and Treble potentiometer "settings" that I prefer. The overall result is a slight Bass-boost (about 6.0dB) and a larger Treble-boost (about 10 dB) with the mid-frequencies (centered around 1KHz) "scooped-out" (by about 3 dB). These settings give the guitar a kind of "Bell-Chime" tone that really helps generate a "liveliness" to the overall sound. I realize these are subjective statements but you will just have to hear and judge for yourself.

The tone control topology used in this design is that of "attenuation" only; no "boost", only "cut". That is why you need gain in the first stage of your preamp; the tone control will cut the perceived gain of your J201 preamp stage significantly (a "cut" of nearly 12 dB overall).

By changing the resistance values in the tone circuit you can cut and boost a wide range of frequencies. You have the ability to "set" a tone that you like best. Then again, by adding a Bass and Treble potentiometer to your design, you can emulate the front-end of a Fender Deluxe Reverb right in the body of your guitar.

The Emitter-Follower

The Emitter-Follower is nothing more than a buffer that isolates the tone control and gain circuitry of the preamp from the un-predictable impedance of your guitar cable and guitar amplifier input. By using a 2N5088 transistor, you will get an affective output impedance of less than 200 Ohms. Other NPN devices can be used (e.g. the 2N3904 and 2N4401); these devices will have slightly less performance concerning input and output impedances but I doubt anyone could really tell the difference (maybe someone).

Conclusion

The battery-drain of this design is less than 180uA. That translates to a battery-life of more that 3,000 hours for a healthy 9 Volt battery.

I'm sure you will have some questions so please feel free to ask. I have had some pretty good results using this kind of topology for an on-board guitar preamplifier. I hope you do as well.

JP Hugsley
Attached Images
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 19th June 2008, 02:16 PM #110 JP Hugsley   diyAudio Member   Join Date: Apr 2008 JFET GUitar Preamplifier Attached is a picture of the similar JFET preamp design mentioned in the previous posting. __________________

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