Super Simple Ring Modulator Using H11M1 Optocoupler As Mixer

The Link below is for the circuit diagram for a good performing Ring Modulator, but instead of using a diode “ring” it uses a simple optocoupler as an unbalanced mixer. This works for two reasons, one; the modulation drives the optocoupler LED and is electrically isolated, eliminating modulation leakage through the mixer. Two, a simple differential amplifier is used to pass the original signal from 100%, all the way to nulling it out entirely, by means of the mix POT. The frequency mixing occurs because of the highly nonlinear nature of the switching action which generates the sum and difference components.

This simple design rivals  the performance of a well designed balanced mixer. The optocoupler is driven by a triangle wave. The  drive level  is important. By setting the drive correctly, the triangle wave peaks are rounded off  gently by the LED turning on inside the H11M1. Another LED is put in series with optocoupler to give frequency indication and the output of the oscillator is tailored to do this. It will not work correctly without the voltage drop of the LED. I just grabbed a junk box one. Depending on your LED you may need a different value for R13.  Alternatively, you can shift the level down by tweaking the oscillator values some, if you  do not want the LED.

The impedance loading the output of the H11M1 is important. If it is to high or to low, the signal will be distorted. The key is to minimize the voltage across the optofet as it is transitioning from off to on. The point being that the differential amplifier resistor values are not arbitrary and need to be adhered to.

Finally, it  is important isolate the oscillator from the the rest of the circuit or the modulation tone can leak through to the output from stray leakage. You can tell this is the problem when you disconnect the drive to the mixer but you still hear the modulation tone.

This is just a basic circuit, I am working on a really enhanced version that will do far more – so I will keep updating


The Very Cool MRF49XA ISM band Transciever

A few years ago, I remember seeing a highly integrated radio transceiver for 433 MHz and 915 MHz. It was produced by Integrated Associates, which then was bought by Silicon Labs. Recently, I found the same part sold by Microchip as the MRF49XA. It is a really cool part, requiring only a crystal and some decoupling caps, if you use the balanced loop antenna as I have in my design. If you use a standard 50 ohm antenna – you will need a few passive components to provide impedance matching and balanced to unbalanced conversion at the output. The radio uses a DC conversion topology and requires no IF filter circuits. It also provide antenna tuning, and internal TX/RX switching etc. Microchip has a very good data sheet which explains how to use the device.

In the picture below is a little USB transceiver I designed to demo the device. I will post a demonstration video showing a couple of these in operation. You can see from the picture, there are not many parts at all (there are two decoupling caps on the back)! The challenge was writing all the code to configure and operate the radio circuit and create the USB link. I created a generic HID USB device which requires no special drivers in windows. I ended up building a complete library of routines to operate the radio and configure its frequency, power, network ID, baud rate, etc…. The device works very well and with this antenna has a range of about 200 -300 feet. Much better performance can be had with a better antenna but this requires more board space and or the matching components discussed above.


more to follow…

Make your Transistor act like a triode

I have thought for some time that by adding shunt/shunt  feedback to a bipolar transistor or mosfet (a JFET also but only AC coupled), that it would emulate a Triode tube with respect to the output I/V curve. So I decided to simulate it and check.  These devices, when biased for normal operation, have a output response that becomes independent of the supply used. This is the same for a Pentode tube like a 6L6, EL84, etc, where the screen grid has a constant bias allowing the plate voltage to vary significantly without changing the current flow through the tube.  By adding shunt/shunt feedback, which in this case is a feedback resistor from the collector to base of a Darlington device, the output I/V behavior becomes constrained by the biasing of the base and therefore affected by the changing voltage seen at the collector.

Shown below is a simple amplifier circuit, simulated in SPICE, using a Darlington  but without the feedback. You can see from the I/V(current  is the Y axis and voltage the X axis) curve that at a couple volts or so, the device is in the active region and as the voltage increases the current stay mostly the same.  The current through the device becomes independent of the voltage across it – like a Pentode or saturated mosfet.

Trans cirIV_trans

Now lets add the shunt/shunt feedback as shown below. You can see with the new circuit, that as the device is turning on, there is a small but highly non-linear region. This is because the supply voltage is below the point of actually biasing the device all the way on. As the supply voltage is increased, the device is biased on but the current through the device now is dependent on the supply voltage because as the supply voltage changes so does the base bias and therefore the current flow through the collector circuit. The alters the I/V behavior the device to act like a Triode where the output I/V behavior is much like a simple linear resistor.

triodedarlington         IV_TRiode

I used the Darlington in the manner in my Battery Amp with excellent results.

Variation of Battrey Amp Schematic with Demos and new layout

I modified the circuit of the battery amp to put the volume control before the first stage like the original FET version of the amp (one of the first posts). It really doesn’t sound that different  – It just allows you to have really high output input sources without overdriving the first stage. It is very easy to modify the original board layout to do this.  On the final stage, I also modify some values because of the altered gain distribution.

Here Is The New Schematic showing what needs to be modified:

if you compare to the last version – its not that different.

Here is  jazz demo with some compression from my simple opto-compressor and the amp set with a little mid size  room reverb and mid way settings on the Bass, Treble and Presence. The guitar is a cheap beater electric that cost me $150.00. It is always fun to see if you can good sound out of crappy guitars!

Note: In the this schematic and layout below,  I changed one of the select lines for the FV-1 to use a different reverb algorithm – not a big deal – I just like it  a little more than the original.

Here is a new board layout that requires no modification and reflects the new schematic exactly:

Here is the schematic as shown above but without any references to modification:

New Amp Demos

Here is the first demo of my new battery powered amp  described in the last few posts – more demos to follow this week.

The tracks will all be recorded on a Tascam handheld recorder with the built in  MICs @ 1 foot from the speaker.

Demo 1 : No reverb – jazz tone on floating pickup arch top – simple chord melody

Demo 2: Old beater electric guitar, small room reverb, lot of treble, bridge pickup and noodling around on B minor. As a side note: I am also using  some compression with the newest opto-compressor described in a previous  post.

Some Possible Value Mods for the Simple Opto Compressor

There is another builder/designer Jon who has experimented with a number of my compressor circuits. He recommends a lower value for R1 of around 22k instead of the 100k in the schematic. This will lower the max compression a little, but will help with stray capacitance roll-off of high end.  It should work fine.  It will load the guitar pickups a little with heavy compression. Also the diode peak detector is biased in a more linear range by changing R8 and R6  from 470k to 47k and C7 from 1uF to 10uF. I think these values will work a little better.

Over my choices are:  100K for R1,  47K for R6 and 10uF for C7

More Amp Updates – Revised schematic – Very minor mod to PCB – this should be it!

I have one guitar that belts out a huge signal from the neck pickup. I was able to  make the first stage just start to distort. To correct this,  I have changed R5 to 1K and R3 to 4.7k  also I have un- bootstrapped R4 and directly connected to ground. I thought the bootstrapping was a good idea – I must have eaten too many Twinkies that day.  C30 and R29 are still optional but could be used to give treble boost or hi/lo gain boost etc.

If you have already obtained the earlier board rev, you need to cut the trace from the emitter of Q1  going to R4 and ground that side of R4 to the ground plane.

I am posted a link to the new board rev here and obsoleting the schematic and layout in the old post. All I did was eliminate this trace and add a couple of wire connection pads to allow making R29 a POT for variable gain. If you have the old board it will work just fine – with the mod described above.

Updated Schematic:

Updated Layout:

Couple other minor notes: C16 may need to be increased to 22pF or so if you have a sluggish crystal –  had one out of five that needed the bigger cap. Also with the mods above you may find that C21 should be increased to .22uF or more depending on the range and level of treble boost desired.

Important component value changes in the Battery AMP

I think the amp has better head room for high output guitar pickups and better overall balance in the tone, when I attenuate the bass a little and remove the treble boost. This means making C3 1000pF instead of 1uF and eliminating R29 and C30. The  schematic below reflects these changes.

New schematic with minor changes in values:

Fortunately the layout remains the unchanged!

Gain distribution is tricky in this amp, partly because of the low voltage supply and the need to not over drive the FV-1.  Some may have trouble with the first preamp being overloaded with hot signals. Increasing the emitter resistor R5 in the first stage and lowering it in the last R24 may help with this. The overall point here is that there is lots of room for adjustment.

Portable Battery Amp – thru-hole version Schematic and layout link

Took me awhile, but here is the new design. It’s very similar to my original design I am still  using the FV-1 DSP chip for reverb which sounds great – but you can leave it out. I moved the position of the volume control after the first preamp and this helps the loading of the guitar and improves the noise figure a bit. I added a presence control in the negative feedback loop between the final amp the driver stage – this really adds some nice high end sizzle – if that’s your thing. The topology including: the fender type tone stack and the output feedback from the speaker back to the driver is much like a classic tube amp and to my ear has a nice sound. One big change was the replacement of the JFET preamp stages with Darlington transistors. You can make your own out of two generic npn’s or use another than the one I specified. Alternatively, you can use a Mosfet like a 2N7000. This may require some adjustment of the biasing. The reason I changed this was that the JFET biasing from device to device was fussy and so my schematic biasing values did not always work out correctly. The Darlington biases very consistently and I love the sound. Another reason was just for the fun of it ( don’t usually use the Darlington much).

Lots of options to adjust the tone stack, treble boost  in the first stage, the feedback loop on the final and the reverb tone shaping. I am sure if someone puts some effort into it, they can dial in some further improvements. The FV-1 has multiple selectable effect programs. The last reverb program is hard wired( all three pins pulled high) but these traces are on the bottom and can easily be cut. There are ground connections right next to these pins so that  one can cut a given trace and connect the pin to ground,  changing the program.  The FV-1 has a good data sheet and explains this in more detail.

The final amp is the TDA7396 which is capable of cranking out up to 65 watts(2 ohm speaker) – it is easy to work with and current in production. It works well with 10 – 16 volts and in this design is intended to be used with a generic SLA 12 volt battery or  a 12-14 volt @ 3 amp supply.

There are only a few surface mount parts, the PMOS FET I use for polarity protection, the FV-1 and the 3.3volt regulator. The polarity protection can be left out or a rectifier can be used instead. Other 3.3v regulators can  be used also. If you leave off the FV-1 – you don’t need the regulator at all or the two reverb controls, and it will just work as is.

the controls on the AMP are: Volume, Treble, Bass, Reverb room size, Reverb level and Presence.

For the speaker I used a $22 Jensen 8″ MOD 4 ohm. I highly recommend this speaker – its cheap and sounds just right for this amp.

Go here for the correct schematic and layout:

Schematic:   (updated in later post – do not use)

Link to the expresspcb layout: (updated in later post – do not use)

Prototype Images (the posted board artwork is slightly different than what is in this image because of corrected errors):




New Amp Completed


New Amp Showing Back


14.4V Drill Battery Power Pack

Sound Clips: (coming this week)