100 watt Guitar Amp Pedal

Continuing on my Stomp Amp theme; I have created a 100 watt (24V supply, 4 ohm load) guitar amplifier with FV-1 based DSP reverb and optional treble boost. It fits in a 1590b stomp box. Yes it really is a 100 watt amp!

red_scare

It has Gain, a single tone control, and reverb level control. The reverb room size is set by a resistive divider(R21 , R22) and can also be made adjustable. It utilizes a TPA3116D2 class D amplifier IC which can be configured for mono or stereo output.

Click Here for Large Schematic Image:

Stomp Amp 100_mini

The amplifier sounds delightful. The class D topology provides greater than 90% efficiency. This  eliminates the need for substantial heat sinking. The only penalty is that for guitar applications, pushing the amplifier to distortion does not sound so great. I use an overdrive pedal so I don’t care about this.

Update: I added a LED clipping circuit to make sure the input into the class D final amplifier is level limited(clips/distorts) before the final amplifier starts distorting

Link to Revised Schematic

 

pop_pcbred pcbHome brew laser printer resist circuit board

Completed Circuit Board 

Link To CAD FILES: https://www.adrive.com/public/QpRQMX/RED%20SCARE.zip

The single tone control is surprisingly versatile. It alters the level and center of a MID scoop. You can get really FAT all the way to bright twang all with one control. The circuit  can easily be modified to employ a more sophisticated tone stack if desired. The amplifier requires a 12-24V power supply with at least 4 amp sourcing capability. You can purchase a small lightweight switch-mode supply from Amazon for less than $20.00 that will work nicely. It is ridiculously small, lightweight, loud as hell, and sounds superb through a couple of 10″s or a single 12″  cabinet.

 

Quick Demo of built in Blue LED clipper limiter added to original prototype(you can see the LEDs flash as the input is clipped)

How To Improve Your Danelectro

So I like sparkly guitars and decided to buy one. I figured I should get the goofiest thing I could find and so I decided on a Danelectro Convertible re-issue from the late 90’s!guitar

I knew it would have issues…and it did! But fear not, this story has a great ending!

Danelectros are cheaply made guitars but do have some nice features. My convertible has a fantastic neck made of quality maple, but not too thick(good for my playing). The rosewood fretboard is almost a 1/4 inch thick and is also high grade wood. The frets are on the beefy side which I like. You need to remove the neck to adjust the truss rod but, this is easily done and you can loosen the strings but leave them on to do this. The bodies are made of masonite and plywood composites. These materials are nice in that they are stable and tolerate of temperature variation but god help you if they ever get wet! Yes they look cool, but besides the necks, the pickups are the big claim to fame of Danelectros. The lipstick type pickups have a unique sound with a scooped mid range and a very bright high end…probably from having less self capacitance and a lower winding impedance. I don’t actually like the sound for most types of music I play, so one of my primary objectives with this instrument was to get a punchier mid range without making the low end suffer or make it sound muddy. Also contributing to the sound is a tone/volume setup with a dual gang pot and a circuit I did not care much for. As shown in the picture above this is operated via two concentric knobs.

So this is what I did to make this a really well playing and great sounding guitar

1. I took the neck off and did a complete fret leveling and re-crowning. I won’t go into the details of the process here but I just started doing this and have done fret leveling on three guitars  – all with stellar results! I highly recommend that guitar players learn how to level and crown the frets on their guitars. There are lots of tutorials on youtube. It is surprisingly easy to do(you need the right tools).It takes about three hours and makes such an improvement if your guitar has uneven frets(likely). You can get everything you need from Amazon..etc.

2. I adjusted the truss rod such that the neck is ultra close to perfectly straight. I did this to get the action even along the neck while allowing for the bridge to be adjusted up higher; so it has more tension across it. It plays better and it sounds better.

After fret leveling and truss rod adjustment, this guitar plays unbelievably well. This picture shows the action from frets 8-12. It just a little higher than a credit card along the whole neck with no buzzing or weird anything!

3. Modified the the floating bridge so the strings won’t slide all over the place (problem with these bridges). I simply carved a little notch for each string along the fret wire saddle. Note that I made sure the notch is deeper on the front such that the strings don’t buzz by hitting the front of the saddle when vibrating. I did this with a little triangle and rat tail file.

4. I designed and installed a preamp that is tailored to improve the tone of the pedestrian lipstick pickup that came stock on the guitar. This preamp uses an LMV641 low noise, low power op amp. It draws less than 150uA which is amazing. The Danelectro Convertible has plenty of room to put all kinds of stuff in it, but I just opted to make this tiny amp board and use a A23 12v battery  glued to the back of the cover plate. You could use a  standard 9v or two 3V coin cells in series, etc. The jack on my convertible was already a stereo jack so I was able to use this as an on/off switch for  the preamp. The ring connection on the jack can be use connect the battery to the ground through the guitar cable plug. The preamp has a gain of two which could be adjusted if desired by changing R4 or R3. R6 and C3 replicate the load resistance and capacitance of the volume pot that would normally be present and can be adjusted to alter tone  as desired. C2 provides super high frequency roll off for amplifier stability and does not affect the tone. C6 and R3 roll off the high end just a little bit. R3 can be a POT thereby creating a typical guitar tone control. A 10k POT is a good value to use in this case. C6 could be .1, .22 or .47uF depending on taste.

After adding the preamp, the pickup sounds fantastic. It still has the lipstick high end sparkle but with a fatter overall sound. When you turn down the volume, the tone doesn’t change either because the drive of the preamp is not loaded down by cable capacitance.I love it.

This is the preamp with the battery shown in the background. A 9 volt will work also.

Preamp Schematic shown above

5. I Changed to a single volume only control. My preamp has a provision for a tone control but I opted not to use it because I didn’t want to drill another hole in the instrument.

6. The Guitar came with really nice Gotoh tuners(not sure they are original). So I did not have to upgrade these, but this is not always the case. If You have a Danelectro with cheap tuners, change them out!

img_20170129_121627101

7. I adjusted the pickup as close to strings as I could. I found this to improve the tone. While it also boosts output, that’s not why I did it.

img_20170129_121613545

      Completed Guitar

Playing the Danelectro Live

25 Watt Hybrid EL84 Tube Amp

This is my new  hybrid guitar tube amp which utilizes a solid-state input stage, DSP reverb, and solid-state phase splitter. Only the push pull, class AB output stage utilizes tubes, namely two EL84’s run at 390 volts with cathode bias. The bias uses two 15 volt zeners which creates a bias current of about 26mA. This requires almost 30 volts of swing on the grids to drive the amp to saturation. This is accomplished with a little switch mode boost converter that generates 29 volts to drive the phase splitter opamps. All of the solid-state circuitry runs off the AC filament supply for the tubes. The solid state portion is basically my stomp amp design( also on this blog) minus the final power amp, which is replaced with the phase splitter.

A couple notes about the design: Using zeners works great, but they can fail(haven’t had a problem  yet) and typically they fail by shorting(very bad for the tubes!)..so it may be prudent to parallel with 1k ohm  resistors and .1 uF caps to make them more tolerate of current or voltage spikes. I use 5 watt zeners and have yet to have one blow on me with numerous amp designs.

Also the gain distribution is not ideal. This is because of the low headroom of the FV-1 reverb IC which runs at 3.3v. This requires that there be lower gain in the first two stages than is possible – degrading noise figure somewhat. Despite this, the amp is very quiet – even with noisy un-bypassed zeners in the final bias circuit.

 

 

 

completed amp head
Front View of Amp Head
back of amp
Back View
completed amp circuit
Complete Circuit
hybrid amp circuit board
Solid State Circuit Board

 

Amp Demo:

Ultra Simple 8 pole Low Pass CW filter

I wanted to add more audio filtering to my 40 meter CW transceiver https://circuitsaladdotcom.files.wordpress.com/2015/08/qrpxcvr2.jpg  but didn’t want to put much effort into it, so I used a MAX7401, 8 pole switch cap filter IC. This IC requires just a few discretes and has an extremely low passband ripple and group delay, as it is a Bessel configuration. I have used this IC before in some of my guitar effects pedals. It has worked very well in my other designs. The knee of the filter is adjustable via a capacitor(C4). To integrate into my receiver, I simply connected the input and output of the filter across the input capacitor(C27 removed) connections to the final audio amp. It may be desirable to put a single RC low pass filter stage in series with the output of the switch cap filter to remove clk artifacts on the output.(clk is 100X greater than the roll off frequency). Performance is very good with clear tone and no ringing.

Below, I have a video demo of the XCVR utilizing the filter. This version of the XCVR is tuned via a POT connected to the microcontroller A2D converter instead of a rotary encoder. It also has a push button to shift in 5Khz increments. When the button is depressed for longer, the frequency in KHz is sounded in morse code. Only the KHz is sounded, so for 7.100 Mhz for example, only 100 is sounded in morse code. In the video you can also see my cool 3D printed Code Key. It uses rare earth magnets instead of a spring for key action. It’s a really delightful bug.

Picture of filter Daughter Board connected to XCVR

8polefilt

Filter Schematic

Simple CW filter

Video of XCVR using the Filter

As Seen On TV Crunchy Boost ….. a simple overdrive that sounds great

as seen on tv pedal

My Zombie Screamer is really good for heavier overdrive sounds but I wanted to design a simple overdrive that had a wide range tonal control(treble boost and cut), that could be used as a simple clean boost…through light overdrive….. all the way to heavier distortion. It uses junk-box type parts and is easy to get up and running. The only part that is a little unusual is the 5k tone control pot. I say this because ideally, it would have an anti-log taper. A linear pot will work fine. I like the sound and it can be tweaked in a number of  interesting ways. Decreasing R3- will increase your max overdrive level, you can go to just two diodes instead of two sets of series diodes for clipping, R10 can be made larger( softer clipping) or smaller(harsher clipping).  The tone control can be modded also. You can swap the pot connections to C6 and R8(wiper will now go to C6 and the high side connection will go to R8). This changes the curve some. Another option is to tie C6 to the top of R8 and then connect c9(you may want to adjust this value) through the tone pot(as a variable resistor). This only gives treble cut but the overall gain is higher giving more intense distortion.

Schematic:

as seen on tv2

Alternate Schematic

(more drive for low output single coil pickups)

as seen on tv3

Yet another version which is hybrid FET/Transistor

(best range of overdrive)

as seen on tv4

Surface Mount Circuit Board:

IMG_20150519_122103308

 

PWM Phaser using PIC DDS LFO and 74HC4066 analog switches as synthetic resistors

I finally got this thing working well. I am using a TL5001 PWM chip  at 48 Khz to pulse width modulate analog switches in series with 10k resistors. The average resistance seen by the all pass filter circuits is proportional to the on time of the switches. For example is the switches are only on half the time, the average resistance is 20k instead of 10k. The advantages of this approach are that there are no critical adjustments, good dynamic range, and extremely linear resistance change in proportion to control voltage. The current draw is 25mA or so…not great but better than the 50mA I started with. The LFO is a DDS sine generator using a PIC micro controller. I will include a link to the code and a working hex file. I only use a sine wave output but the code can easily be modified to support numerous wave forms and frequency ranges. The  code for this project has been modified in two ways from the DDS code described in my other blog entries. I lowered the internal clock frequency to 8 MHz from 32Mhz(saves current) and reduced the output swing of the sine wave so it did not drive the PWM chip beyond 90% duty cycle.  Originally I just use the PWM directly from the PIC with no filtering but found this created noise artifacts, so I filter the output and drive a PWM chip. Typically, phaser circuits like this use high pass filter structures at the positive input, I chose to use low pass to help eliminate the switching noise from being introduced into the output. This worked well and eliminated the need for extensive filtering. The output is very clean

One thing that is very cool about this design is that it has variable sweep range instead of depth control. So the frequency range of sweep of the phasing notches can be adjusted starting at 100Hz or so all the way up to 5KHz.

Schematic:

pwm phaser

 

Demo

Link to code: https://www.adrive.com/public/mWSkFb/PWM%20phaser.zip

New phaser with Homebrew 1×4 Vactrol and DDS LFO

I decided that I  should build a Phaser using my PIC DDS LFO. I designed a PWM version using CD4066 analog switches but this had issues. It just drew to much current and had some noise artifacts. I got it working but it became more complicated  than I liked. However, the design was easily modified to use a homemade vactrol. The vactrol uses four photcells glued together to make a square and then the LED is glued right in the center. The whole thing is then covered in a piece of heat shrink, electric tape or whatever.  Performance is excellent and the circuit is simple. Of course the DDS can be replaced by some other conventional LFO circuit. The main issue is operating the LDR’s in a good range to get even and adjustable notch sweep. With this design, the sweep is adjustable and you can even eliminate one notch by pushing its position sub audio. The sweep controls the level of illumination of the LDR’s and thereby adjusting the total range of phase shift. The LFO uses my PIC DDS circuit and generates a Sine wave envelope. The code can easily be changed to generate other wave forms. I am going to include a link to the a zip file containing all of my source code. This was all compiled using MikroC. The program is small enough to compile in the freeware version of MikroC or you can just use my compiled hex file.

More detail on the PIC DDS is available on this blog in earlier post.

Link to source code and  hex file:  http://www.filedropper.com/ldrphaserdds

Completed Phaser:

brainwash

Schematic:

LDR phaser

Etched Circuit Board:

etched phaser brd

Completed Board:

phaser board

Homebrew Vactrol:

quad vactrol