1Wamp is a one Watt small guitar amplifier based on a JFET guitar pre-amp, the Big Muff Pi tone control and the LM386 power amplifier. This portable amp is an open hardware project designed by ElectroSmash using only free and open-source tools.
The preamp stage with two J201 transistors is designed to give a tube-like sound, the BMP tone stage is able to produce a big range of tones and the LM386 output stage can drive any kind of speaker, from headphones to a Marshall 2x12 cabinet.
The circuit can be broken down into 5 simpler blocks: JFET Input Stage, Tone Control, JFET Booster, LM386 Power Amplifier and Power Supply:
The functionality is simple: The input stage isolates the amp from the guitar, keeping the signal quality and avoiding tone sucking. Then the tone control shapes the frequency response adding more bass/treble to the mix. The JFET booster will recover the signal after the tone control and prepare it to the Power Amplifier stage that delivers up to 1W.
The aux/mp3 input adds any line level input signal to the guitar sound, allowing to practice with metronome/mp3/youtube backing tracks.
What is a pre-amplifier?:
A pre-amplifier is the part that precedes power amplifier (JFET Input Stage, Tone Control and JFET Booster). It prepares signal for further amplification or in other words, the pre-amplifier is just a circuit that does some preparing for the signal, usually coloring and integrating small gain stages with good impedance matching, tone controls and filter elements. This part does not generate any power to drive a speaker.
What is a power amplifier?:
The LM386 power amplifier block will amplify and deliver the voltage and more important the current to drive the load (headphones/speaker).
The this stage will define the wattage of the amp, and should just deliver power without adding any colour to the signal.
There are 3 options in the store:
If you have any question, contact us:
The power stage provides 9 volts to all the circuit stages, giving also protection against reverse polarity connections and additionally it filters the power line to remove any noise.
The cut-frequency of the filter is defined by R15 and C10 and C11 and can be calculated following the equation:
So, any humming noise over 0.7Hz will be removed by this filter.
There is more info about this filter and how to remove hum from LM386 designs in the Layout Section.
The input stage is JFET pre-amplifier based on a Common Source Class-A amp with a high input impedance and medium output impedance. The JFET preamps became very popular in guitar circuits because they are simple, easy to build and able to deliver warmth tones.
There are several famous guitar pedals that use this circuit topology:
- The Tillman Amplifier, it uses a R2=6.8K and R3=2.2K, famous for giving a tube-like sounds with 6dB of gain and asymmetrical clipping.
- The Fetzer Valve is popular as a stand alone booster and as a building block in larger circuits. It is based in a white paper by Dimitri Danyuk "Triode Emulator". It proposes the use of a carefully chosen source resistor on the JFET amplifier to mimic a vacuum tube sound. It uses R2= 10K and R3=1K, giving a high gain of 14 dB.
- Low gain JFET preamp: The Tillman and Fetzer valve introduce a big amount of gain and sometimes could make difficult to get clean tones. If you prefer natural clean sounds and light overdrive R2= 2.2K and R3=1K can also be used, giving soft tones and mellow saturation.
The whole 1Wamp circuit includes 2 of this JFET booster stages, so you can also combine this topologies as you like; Low Gain + Low Gain, Fezter + Low Gain, Tillman + Fetzer, etc...
How to Calculate RS and RD in a JFET Common Source Amplifier:
Intro - Background
The source resistor RS (R3 or R7 in 1Wamp) and the drain resistor RD (R2 or R8 in 1Wamp) define all the important parameters and sound of the pre-amp. There are plenty of ways to design and calculate the components for a JFET amp: graphical methods, mathematical, experimenting... sometimes misleading and confusing. Here we describe the simplest way (in my opinion) to design and calculate RG, RS and RD for this particular amp called "Self Biased Common Source JFET Amplifier".
Fixed values: A JFET transistor has 2 important fixed parameters:
This parameters are indicated in the datasheet with big tolerances. In the J201 for example VGS(off) goes from -0.3 to -1.5V and IDSS could be anything from 0.2 to 1mA. Note that this values change from one transistor to another:
There is a JFET tester designed by RunoffGroove that can accurately measure the IDSS and VGS(off) in a JFET. From our experience, measuring one hundred reliable J201 JFETS we can average this values to:
How to calculate RS and RD in 5 steps:
The Midpoint Bias method sets the transistor to the midpoint of its transfer curve, allowing maximum headroom (the drain current swings with maximum span between IDSS and 0 without clipping):
note: sometimes this number is also approached to VGS= VGS(off)/4
4. RS = VGS/ID
VGS= VG - VS
VGS= (IG*RG) - (IS*RS)
VGS= (0*RG) - (ID*RS) = IS*RD
5. RD = [VCC / (2*ID)] - (RS/2)
If we want VD to seat in the middle point of its maximum excursion we have to take VS into consideration.
note: Sometimes in order to simplify maths, some designers take the midpoint bias point as VD=VCC/2. You can also use it, there is not a big difference in the end:
The gain of this mid-point biased JFET amp will be defined by the equation:
These 5 steps can be summarized in the following image:
In this ideal mid-point biasing, the gain of the amp is limited by the values of RD and RS which in turn are limited by the intrinsic characteristics of the JFET (IDSS and VGS(off)), to trim the gain the value of RD can be reduced/increased. The midpoint is often traded for higher gain; changing the value of RD will modify the gain, but the clipping will be more prone to happen.
To finish this notes about the JFET biasing, it should be said that there is some mysticism biasing the JFET, the values of RD and RS can be tuned by ear for the best sound or following complex mathematical analysis trying to replicate the tube sound.
Some designers prefer not to follow the ideal mid-point biasing and make their designs with some variations, to see that we will study the Tillman, the Fetzer Valve and the Low Gain Fet:
J201 Tillman pre-amp:
There is no simple formula to calculate ID and VGS. you will need to solve a system with 2 ecuations and 2 variables (ID and VGS):
Resolving we get the values:
First of all, we have a system with 2 equations and 2 variables:
Developing the square binomy
Now the real values can be substitued: IDSS = 0.7mA and VGS(off) = VP = -0.8V
Resolving thie quadratic equacion (you can use any online calculator, or make the maths)
We get 2 possible solutions, ID= 0.73mA or 0.18mA. ID<IDSS so the first solution can be discarded.
AV= - ((gm*RD)/(1+gm*RS))
AV= -((0.8*6.8)/(1+0.8*2.2)) = -5.3/2.7= 1.9 times or 5.8dB
These results show that in the Tillman amp, the drain current ID is below IDSS/2 (0.3mA) and also VD is displaced (4.5V). These will make the amp to have a low amout of gain and the clipping will happen easier in the positive semicycle of the signal. The designer was not looking for a high-gain design and due to the popular acceptance of this circuit seems that it also sounds good.
J201 Fetzer Valve
Summarizing the fetzer valve, Runoffgroove uses 2 equiations derived from Danyuk Triode Emulator paper to calculate RS and RD:
Rs = 0.83 * |Vp| / Idss = 0.83 * 0.8 / 0.7 = 0.9KΩ approx. to 1KΩ
Rd = 0.9 * (Vcc - 2*|Vp|) / Idss = 0.9 *(9 - 2*0.8) / 0.7 = 9.5KΩ approx to 10KΩ
Using a similar approach that in the Tillman amp we can calculate the bias points:
In the Fetzer amp, the value of ID is very close the IDss/2, allowing a nice big span of this current. The value of RD also makes VD close to its ideal value. In this point the transistor has a big amount of gain and seems that replicates the behavior of a vacuum tube.
J201 Low Gain Fet:
Following once again the maths of the Tillman amp ID and VGS can be calculated:
The current in the Low Gain JFET amp is designed to be ideal (IDSS/2), and the VD point is intentionally shifted from the mid point biasing, it will make the signal to clip in the positive semi-cycle easily. The LM386 does not need a high level input so placing this amp/buffer before it make it sound pretty good.
Amplifier Input Impedance:
These JFETs have the advantage over bipolar transistors of having an extremely high input impedance along with a low noise output making them ideal for use in amplifier circuits that have very small input signals.
The JFET input impedance can be considered infinite and only the value of the gate resistor (R1) will define the total value of it
ZIN= ∞ // R1 = R1 = 1MΩ
The rule of thumb is to interface the guitar to an input impedance that is at least 1MΩ, it will keep the signal uncorrupted as foundations for the next stages avoiding tone sucking.
The passive tone control is Big Muff Pi style, following a classic simple and effective design that generates a great variety of tones.
This topology is a combination of a high pass filter (C1R5) and a low pass filter (R4C2) that are mixed together by a linear potentiometer POT1. The cut-off frequencies of both filters are designed so that their interweaving effect introduces a middle frequency scoop/notch at 800Hz (see the graph below) when the potentiometer is set to middle.
1Wamp Tone Control Frequency Response:
Find below the frequency response, showing from blue to red all values of the tone potentiometer:
The green response line has the tone pot set at mid point showing the 800Hz scoop. There is an overall 7dB loss and at the notch the loss is about -10dB in total at 800Hz. The blue and red colour curves have the tone at full bass/treble respectively.
There are some tips using BMP based tone controls:
This second JFET stage is identical to the first one, it is designed primarily to recover the volume loss of -7dB during the passive tone stack and to introduce more harmonic content to boost the tube sound.
R6, R7 and R8 are the gate, source and drain resistors. Their functionality is exactly the same as the R1, R2 and R3 resistors in the Input Stage. If you want to learn more about this resistors and how to calculate them check the Input Stage section.
LM386 Aux/mp3 input
This auxiliary input mixes the guitar signal with an additional line level input from a laptop/mp3/music player just before the power amplifier. Doing this it is easy to practice with backing tracks, metronomes or drum bases.
An Aux/MP3 player is low impedance source. Placing a buffer in this input would be perfect (adding circuit complexity), but you may be able to work without them.
The LM386 has a 50K input impedance, so using summing resistor of any more than 50k is not ideal as the input voltage would be cut in half when using 50k resistor (neglecting the booster stage impedance).
The volume of the aux input is adjusted from the external device, for circuit simplicity there are not pot to reduce the input level. Note also that the mp3/aux input is directly fed into the LM386 and using high distortion levels the aux input might suffer some distortion. This is the little price to pay for having a simple and practical direct input.
Additionally, this input can also be used to connect a second guitar or a bass guitar with a jack to mini-jack adapter.
The main power amplifier stage is based in the LM386 audio integrated amplifier. It is popular choice for low powered guitar amplifiers due to its low quiescent current and ability to run on 9V. It has been used in a lot of popular amps like the Smokey Amp, the Little Gem, the Ruby Amp, the Marshall MS-2 and the Noisy Cricket.
The POT2 Volume potentiometer limits the amount of signal into the LM386. As the Volume is increased, you will start getting nice breakup.
Why POT3 and R16 are mounted in parallel?
The range of values of PCB mounted potentiometers on the market is limited, finding 1KΩ pots with the correct footprint and shaft length could be difficult. Placing 50K potentiometer (easy to find) and 1KΩ simple resistor in parallel will create a 1KΩ potentiometer:
The resulting potentiometer will be logarithmic instead of linear, but with a high gain amp like this it is not bad to have a more sensitive pot in the fist half of the potentiomenter. However if you are able to find a 1K potentiometer you can place it instead of the parallel combination.
LM386 Voltage Gain Calculation
The potentiometer (1K as explained above) placed between pins 1 and 8 to adjust the gain from 41 (28dB) to 200 (46db) following the general LM386 voltage gain formula taken from the datasheet:
Where Z1-5 and Z1-8 are the impedances between the respective pins. Note that Z1-5 internal resistance is 15KΩ and Z1-8 is 1.35KΩ.
If you want to read more about how to calculate the voltage gain, you can read the Voltage Gain Calculation in the Ruby Amp.
The output power of the LM386 is too high for a pair of headphones. An average pair of headphones only need 1mW (into 32Ω) for a reasonable good volume. Depending on the LM386 model, it is able to deliver ap to 1000mW (1W), so in order to reduce this output power below the threshold of pain a series resistor is placed. This auxiliary load will take part of the power giving to the headphones the righ amount of it.
Frequency response of the output attenuator:
Placing an output attenuator will modify the frequency response, reducing the amount of bass into the headphones.
The maximum input voltage that will make the output clip is 8Vpp/41=195mVpp, any voltage over this level will clip the output creating distortion tones. The Volume potentiometer (POT3) is placed to attenuate the incoming signal below this 195mVpp (clean tones) or over it (crunch-distortion).
The design and layout of the power supply in audio circuits with a pre-amp and power-amplifier stages is critical. If the power from the battery/adapter jack is given to the circuit following a wrong path, the noise of the power-amp could be added to the pre-amp stage, resulting in a positive feedback and creating noise or oscillation commonly called motor-boating. To avoid this, the power should be connected following this block diagram:
Rules to minimize noise in audio circuits:
Star grounding: Ground is the common reference potential, commonly determined to be zero volts. This ground is the shared return path from the power supply to all the stages of the circuit.
In the above image the center of the star ground is indicated, from this all the green ground tracks for the different subsystems (input jack, led, input stage, tone control, booster, power amp, output jack). These tracks flow in parallel and with similar lengths minimizing common noise.
The most important DC bias points of the 1Wamp are shown. It can be useful for troubleshooting:
note: Your values sould be all shifted a bit up or down, depending on your power supply levels. For this graph Vcc is ideal and equal to 9V.
- To get this voltage values, the POT1 (Tone) is in mid position, POT2 (Vol) is at maximum value and POT3 (Gain) is at minimum value.
- VD1 voltage is slightly lower than the calculated in the Tillman pre-amp section (7.8 VS 7.4). This is due to the loading of the tone control.
Using an oscilloscope the most important points of the amplifier are measured: input signal (TP1), after the first JFET stage (TP2), after the tone control (TP3), after the second JFET stage - input of LM386 (TP4) and output signal.
This waveforms were taken using different JFET topologies: with 2 Tillman JFETS, with 2 Fetzer Valves and with 2 Low Gain JFETs:
It might seem that the guitar signal at the input of the LM386 is too low (63mVpp) but its not a problem, the LM386 does not in fact require high level waveforms at its input, in fact signalss over 195mVpp will be clipped as seen in the Clipping Section.
The last two graphs show the output of the amplifier when the gain is set to its minimum and maximum value. The output signal goes from clean to light asymmetric overdrive.
- For all the images, the input signal is a 1KHz sinusoid with 200VPP, comparable with an average guitar signal.
- The POT1 (Tone) is set to mid, the POT2 (VOL) gain to max and the POT3 (Gain) varies from min to max in the last 2 images.
The components for the 1Wamp are easy-to-find through hole parts with the minimum number of references. There is plenty of alternatives and replacement parts altough the J201 JFET is a bit tricky.
|1Wamp Bill of Materials|
|C1||1||6.8n||CAP, FILM, PET, 6.8NF, 100V, RAD||R82EC1680Z350J|
|C2||1||10n||CAP, FILM, PET, 0.01UF, 100V, RAD||R82EC2100DQ50J|
|C3,C4,C6||3||47n||CAP, FILM, PET, 0.047UF, 100V, RAD||R82EC2470DQ60J|
|C5,C11,C12||3||100n||CAP, FILM, PET, 0.1UF, 100V, RAD||R82EC3100DQ70J|
|C7,C8,C9,C10||4||220uF||Aluminum Electrolytic Capacitor 16V||REA221M1CBK-0811P|
|R12||1||10R||TH Metal Film, 10R, 1/4W, ±1%||MF25 10R|
|R13,R15,R16||3||1K||TH Metal Film, 1K, 1/4W, ±1%||MF25 1K|
|R3,R8||2||2K2||TH Metal Film, 2.2K, 1/4W, ±1%||MF25 2K2|
|R2,R7,R11||3||6K8||TH Metal Film, 6.8K, 1/4W, ±1%||MF25 6K8|
|R5,R9,R10,R14||4||22K||TH Metal Film, 22K, 1/4W, ±1%||MF25 22K|
|R4||1||47K||TH Metal Film, 47k, 1/4W, ±1%||MF25 47K|
|R1,R6||2||1M||TH Metal Film, 1M, 1/4W, ±1%||MF25 1M|
|Pot1,Pot2,Pot3||3||50K||Potentiometers 9MM 50K V/ADJ||RK09D113000D|
|CON1, CON3||2||6.35 jack||Neutrix 1/4 ST Chrome Conn PCB
|CON2||1||3.5 jack||Jack 3.5 mm st - CLIFF 4 POLE, PCB
|CONN4||1||2.1 jack||DC Power Conn 2.0mm PCB Jack||KLDX-0202-AP-LT||MJ-179PH|
|D1||1||1N5817||Schottky Diodes Vr/20V Io/1A||1N5817|
|D2||1||LED||Standard LED - TH 3MM LED WHITE||VLHW4100|
|D3,D4||2||1N4733A||Zene Diode 5.1V 1W ZENER||1N4733A|
|8pin dip socket||1||8Pin socket||IC Sockets 8P DUAL||4808-3000-CP|
|Plastic Knobs||3||6.35mm knob||Knob 6.35 mm shaft size.||450-2023-GRX|
|SW1||1||toggle switch||Toggle 3-Pin SPDT ON-ON||612-100-A1111|
|9V Battery Conn.||1||Battery Clip||9V Battery Contacts 26AWG||121-0526/I-GR|
|Nylon Spacer||4||m3x16||Spacers M3 x 16mm nylon||R30-1611600|
|Batt. optional clips||2||-||4.8X0.5MM||TAB38250568|
|* Additional Comp.|
|R2, R7||2||10K||TH Metal Film 10K, 250 mW, ± 1%||MF25 10K|
|R3, R8||2||1K||TH Metal Film, 1K 250 mW, ± 1%||MF25 1K|
Capacitors: Electrolitics, Film, Ceramics ...?
For big values (>1uF) use aluminium electrolitics, for medium size (1nF - 1uF) use Film and for small values (<1nF) use ceramics. If you are not sure about using ceramics or film somewhere go for film; film capacitors are generally preferred over ceramics in audio path applications. There is a great article about capacitors for audio by BeavisAudio.
Fetzer Valve Amplifier by RunoffGroove.
Tillman Amplifier by Donald Tillman.
Teemuk Kyttala Solid State Guitar Amplifiers, the Holy Scripture.
JFET Basics by by Kenneth A. Kuhn
Common Source JFET Amplifier by Electronic Tutorials.
Starground by Lighthouse Electronic Tutorials.
JFET Preamplifier Circuits by Mike Martell.
Smokey Amplifier by ElectroSmash.
Ruby Amplifier by ElectroSmash.
Spice model for the J201 JFET Transistor:
* J201 with VGSoff=-0.8V and IDSS=0.6mA
.MODEL J201 NJF (VTO=-0.8 BETA=0.9375M LAMBDA=2M IS=114.5F RD=1 RS=1
+ CGD=4.667P CGS=2.992P M=.2271 PB=.5 FC=.5 VTOTC=-2.5M BETATCE=-.5
If you want a JFET with a certain value of VGSoff and IDSS, just change the model so:
VTO = VGSoff
BETA = IDSS / (VGSoff^2)
All credits to stm in diystompboxes.com
Spice model for the 2N5457 JFET Transistor:
* 2N5457 with VGSoff=-1.6V and IDSS=3.3mA
.MODEL 2N5457 NJF (VTO=-1.6 BETA=1.29M LAMBDA=2M RD=1 RS=1 CGD=6E-12
+ CGS=2.25E-12 KF=6.5E-17 AF=0.5)
Spice model for the MPF102 JFET Transistor:
* MPF102 with VGSoff=-2.5V and IDSS=6mA
.MODEL MPF102 NJF (VTO=-2.5 BETA=0.96M LAMBDA=5M RD=1 RS=1 CGD=1.54248P
+ CGS=2.567P PB=1.49 KF=7.90591F AF=499.953M)
Thanks for reading, all feedback is appreciated
Some Rights Reserved, you are free to copy, share, remix and use all material.
Trademarks, brand names and logos are the property of their respective owners.