The positive opamp input is meant to be grounded in an inverting amplifier, when using a split supply.

It looks to me like there is an error in the diagram, since if converted to a single supply there needs to be a virtual ground between two equal resistors. Perhaps R7 is supposed to connect to minus 9V, with the midpoint being the virtual ground. The virtual ground should be connected to the opamp + input.

Measure the output of U2. I expect if wired as shown that the output is near +9 volts. Once the + input is at about 4.5 volts, the output should go near there also. When that happens, it will amplify correctly.

Hope that helps.

You will need a separate play back amplifier. The record head will be using the only amplifier you have in the unit (remember it is designed to only record OR play at any given point in time). So it will only have one amplifier, and use it for the recording function (it gets switched between record and playback by switches).

So I suggest you leave the record electronics as it is (additionally, there is an oscillator and driver for the erase head). For simplicity, don't mess with the record side.

You will need to add an amplifier for the playback head. Then head alignment will be your next problem after that.

If you want a quick "cheat", you can probably hook the playback head up to a practice guitar amp, if you have one handy (it works great for my electric piano with mag pickups).  A magnetic phono pickup amplifier is another possibility. Otherwise, you'll need some sort of tape preamp built. There should be some opamp designs available on the net for that.

The remaining challenge is that you'll need to align that 8-track head. The tracks on it may not align well with the recording head. Ideally you'd have some sort of screwable head height adjustment on the playback head (though you could mess with the record head adjustment if that adjustment is there). This is where having an amp/preamp first would help, because you should be able to hear when you have it aligned.  While the tape is moving, you should be able to hear playback hiss at the minimum.

The case of the head should be grounded to prevent AC hum. You might be able to get away without it, depending upon how bad it is. Just being bolted down to the chassis might be enough to ground it.

With two heads one will be the erase head, the other the play/record. For a looper, you'll need another head. The easiest one to add is the playback.

I put some notes together about a pink/white noise module here, if that helps:

  http://www.experimentalistsanonymous.com/ve3wwg/doku.php?id=noise_generator

It's more of a construction than a bend.

Warren.

That was my original understanding also. But different authors use different illustrations, and in this case, encouraged me to do some homework. It's always best to do your homework.

My Raspberry Pi arrived by UPS yesterday. I'll be exploring that over the next few months as I develop a print-on-demand book for it.

Merry Christmas and Happy New Year to all experimenters!

I've also added a page with a cheap and easy way test the LM3900 on a breadboard, with nothing more than:

• single 9 volt battery
• 100k resistor
• 200k resistor (or 220k)
• and a voltmeter

http://www.experimentalistsanonymous.com/ve3wwg/doku.php?id=lm3900_testing

A package of cheap LM3900s is the perfect gift for the Scrooges you know at Christmas time!

While these are not strictly "opamps", they are very similar. Instead of having differential inputs sensing voltage, they sense a difference in current instead.

Why bother with CDAs? They have a few advantages over opamps:

• very low cost (LM3900 quad ICs)
• single power supply rail (think guitar pedals and automotive)
• low voltage operation (down to 5 volts on a single rail)

These can be a little tricky to use, since unlike normal opamps, you do need to bias them. Fortunately, the biasing calculation is very simple.

This and the other design steps needed to put them to work are available from this main page for CDAs:

http://www.experimentalistsanonymous.com/ve3wwg/doku.php?id=cda_opamp

The examples there have been tested using LTspice (what a great tool that is!).  I'll likely be adding other CDA circuits over the winter.

Understanding takes time, patience and some experimentation. Our experiments confirm that our understanding is correct (or not). Sometimes actual circuits bring in other factors that we've not thought about.

Did you have a specific area that you wanted to ask about? Maybe we can help.

I have been updating the pages starting from the opamp link given earlier. There are now the opamp design procedures for:

• Inverting opamp amplifier (AC or DC coupled)
• Non-inverting DC coupled opamp amplifier
• Non-inverting AC coupled opamp amplifier

For those that have never designed their own opamp circuits before, I encourage you to do so. You don't even need a book. The formulas and the online RC filter calculator will guide you through your resistor and capacitor selections.

I recently borrowed a book from the library "IC User's Casebook" by Joseph Carr (1990). I liked it so much, that I bought it used for cheap. What I liked about this book is the simple explanations, and the description of the design process of several opamp circuits. A lot of times you are left wondering how certain component values where chosen, but the author gives clear advice in this direction.

While I have a good understanding of opamps, I found that Joseph's explanation of how the "inputs stick together" rather intriguing. He tells the reader to "try it". So when I did try it, I was a little surprised at the results. I've outline two experiments here:

http://www.experimentalistsanonymous.com/ve3wwg/doku.php?id=opamps

There is so much confusion among texts and websites alike on this important ideal opamp concept of the "virtual ground" or how the "inputs stick together".  This is what motivated the experiments that I documented above.

Try it yourself!

When you say that you hear the "play back", do you mean that you hear it at the time it is recording? Or do you mean that it will play back the recording after the recording is completed?

For looping, what you're after:

• is a record head laying down the audio to the tape
• and a playback head placed at some distance from the recording head

The playback head of course needs an amplifier, when the recording head is also using it's own (two amplifiers are required).

The amount of delay is dependent upon:

• tape speed
• distance of the playback head from the recording head

Based upon what I see in the pictures, it appears you have one amplifier (unless there is more somewhere). I only see 3 transistors, so there must be more electronics hidden somewhere else (there is usually an oscillator to erase the tape ahead of the recording head).

So for you to have a functional looper, you'll need two amplifiers that operate simultaneously. One recording, while the other is amplifying the playback head.

To control the delay, you have to be able to move the playback head, or alter the speed of the tape.

The project sounds a bit ambitious to me. But let us know if you can simultaneously playback while recording or not.

This is pretty slick.  Would be interested to hear how it sounds on guitar.

It's actually fairly quiet as an effect, except when the signal is severely clipped. Because of the rounded corners, I think it sounds like a Bluesy tube like "limiting" effect. You can hear some distortion happening, but to the non-musician, it isn't really noticeable. Essentially, it acts like a hard limiting effect, except that it is softer (I think) than a diode pair.

The shape of the curve looks relatively constant across current, right?

Yup, though I did see some DC sag at times and that may have been attributable to some A/C components in the circuit. Otherwise, I expect that it is totally flat.

It seems like you basically have a current-controlled clipping device - lower the LED current and it will clip at lower voltages.  So it might be cool to envelope-control the clipping - so you get amplitude-independent clipping, if that makes sense.

Yes, I think it makes a great controllable limiter, though an opamp compressor is probably a better way to compress general audio. For guitar signals though, it might have a great application there.

Also, to get asymmetrical clipping, can you adjust the bias of the incoming signal?

I suppose if you DC shifted the input signal, you would create asymmetrical clipping.

I think the best thing about this application is the symmetrical limiting, within a single active component. I have built JFET distortion circuits using two stages, but the problem with that is that if AC coupling is involved, the wave forms are shaped in the following stage differently. This results in the other half of the wave to clip differently. So the clipping tends to be non-symmetrical, resulting in more even order harmonics than you might want.

But the most exciting prospect in my mind is to be able to fold the signal back, as it increases above the limit (think of an "M" shape in the extreme for an "^" shaped input). But that will require a precision rectifier stage and a threshold detection, so that only the peaks reduce the LED current (and thus droop the output level in the middle). That's my goal for this winter, on some snowy day.

The proto boards on top seem a bit bad for connections, so I'll likely replace them. I did the prototype of the H11F3 circuit using this station, and I did find that I had to wiggle some connections. But still a great find.  The replacement pots have not arrived yet, but those will help a lot also.

[symmetrically]

Colin, you will remember some time ago, we discussed the various uses of the H11F3M LDR chip and that I had noticed that nice tanh(x) signal limiting curve documented in the spec sheets for the device. I've attached that chart for everyone's convenience.

What interested me in this device was the fact that for a given amount of illumination from the internal LED, the current is limited symmetrically about the axis. For example, if the LED is illuminated with about IF=10ma, the current through the FET is limited to about +/- 400uA.

To take advantage of this characteristic one has to measure the current flowing rather than the voltage. So I rigged up a circuit like the one attached to this message, where R3 represents the H11F3M (from the outputs of pins 4 and 6, the FET portion). R3 roughly represents the resistance presented by the LDR. Current through the LDR will increase the voltage seen at one half of the voltage divider, or the other (thus seeing the effects of current).

The signal appears at the input of the LDR(FET) (R3 in the diagram). As current flows through the LDR, it wiggles the voltage up and down in the voltage divider formed by R1 and R2. The schematic shows a single power supply, but I tested on a bipolar one.

The scope trace attached is a crude photo of the signal being limited by the current flow at a particular LED current flow (unfortunately I had a fair amount of RF bothering me in this protoboard experiment). As the LED is further illuminated, the limiting is reduced (the limits are increased), and with enough illumination the signal matches the input. With reduced LED illumination, the limits are reduced and the signal starts being limited, in a nice soft way. Conversely, if the signal increases in strength to the point of the limits, the signal again becomes limited with rounded corners.

There are three things I like about this idea:

• The limits are controllable by varying the LED current
• The limiting action is soft
• The limiting action is symmetrical in one stage

The last point is important to prevent generating a lop-sided signal, which leads to further even order harmonics, which tends to sound harsher.

In short, this makes a great "limiting" circuit. I also like it for the beginnings of a distortion pedal. It is too "quiet" by itself if you want guitar distortion, but in combination with the Boss SD-1, I thought is was pretty good (though I didn't have time to try it at high volumes).

What I really want to do with this now, is to take it to the next level. In theory, as the current starts to limit in the FET, the input voltage should start to spike (something has to give). I'd like to take that input voltage, amplify and use a precision rectifier on it (another opamp circuit). Then take the pulses from that, beyond a certain threshold, to further reduce the LED current as the input voltage spikes. This should have the effect of current foldback, that is sometimes seen in more extreme tube overdrive.  So as the signal goes beyond limiting, I'd like the signal start to dip, rather than be just a straight limit.

This will add some nice high order harmonics. But this will have to wait until later this winter, perhaps. So little time.. but at least I know the fundamentals are proven.