Music Electronics

I'll be developing various circuits such as guitar fx pedals guitar amps, preamps, hifi power amplifiers and other stuff. Talk to me if you feel like, I'm quite a bit of a geek so what ever project you have going on or what ever question you may have, I will probably be interested. It's a secondary blog, so I can't follow back with this one. Any finished, build and fully tested circuits posted on here will be clearly marked as being so. Anything else I post on here is merely design reference or inspiration for projects.

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I’m moving!

This blog will no more be updated.

I made this WordPress instead.

[legalmexican]

science side of Tumblr explain why the ride home always takes less time 

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Since side of tumblr does not agree with the premise of your question

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LTspice for Windows can run on Mac

If you use a mac for your electronics design you have probably tried to install LTspice on your mac. Like me you’ll find that it’s quite different from the version that runs in windows! No menus and you have to be able to remember some syntax to make your simulations. This sucks. 

So now i just installed the windows version on my mac, and it works just fine!

Go give it a try, or ask me for the LTspice IV.app file to avoid having to make it your self. 

And here comes the best part. Running the windows version in Wine gives about 40% faster simulations than with the OS X version! This may or may not vary from system to system, but one thing is sure, wine does not ruin simulation performance. 

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New Toshiba BJT’s

TMBT3906/TMBT3904 are bjt’s in SOT23 package and they seem to have quite ideal specs for low distortion amplifiers. They can only stand 50Vce, but often ultra low distortion calls for cascode stages anyway, so you can keep these safe from high voltages that way. 

As of now they are only available from digikey.

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OPA1622 headphone amp.

I've made a headphone amplifier with this op amp, and I'll share all of the design files soon, so you guys and girls can make one your self. It's a brilliant little op amp btw, buy or order samples directly from TI, it's not that expensive. I'll even have some spare pcb's that I'll sell cheaply if any one is interested. (Let me know) It fits in a relatively inexpensive extruded hamond box. Powered from usb or any nominal 5v supply. Isolated so you don't have to bother with ground loops.

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The actual art

As I’m struggling with the issues of current feedback in power amplifiers, it’s starting to dawn on me why Douglass Self is doing it the way he is. 

He really is achieving the needed performance with a remarkable simplicity and low cost. 

One obvious issue is that the current feedback amplifier will typically require a very low value resistor from the output to the negative input. - In the 100R to 2k2 order. This is because the open loop gain is heavily dependent on this resistor. The lower its value, the higher the open loop gain. And in a 200W amplifier you’ll end up having almost about one watt of power dissipation in your feedback resistors! This is pricey and might cause thermal distortion at lower frequencies. I presume the cheapest way to do it is to use a few resistors in series to create the desired value, but this is still very expensive with thin film or metal film resistors. 

I always knew that making something with a high price-performance ratio is the actual art. But mastering it is a whole other thing. 

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Prober open loop gain in current feedback amplifier

Here is my latest circuit and what looks like a succesfull attempt at getting the open loop gain above 100dB. 

View larger

I’ve taken the output stage out of the equation because it really doesn’t have much to do with this part at all. So for practical tests I use an LME49610.

I’ll have to experiment a little with C3 and C4. They increase open loop gain, but not at lower frequencies, and they might introduce low frequency distortion as they can’t really be “large enough” - They’d have to be above 1000u to work well at low frequency, which is just not practical. 

I’ve also connected the frequency compensation capacitors back to the input rather than just to GND for some negative feedback on the gain stage. On final versions I might make room to populate both options. Which option works best might depend on the performance of the output stage. 

EDIT: for get about C3 and C4. It doesn’t work as soon as your output signal value is even remotely close to the rails. 

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Current feedback - It’s not that difficult at all!

A current feedback has an input that consists of a unity gain buffer and two current mirrors in the configuration you see here: 

The buffer marked Buf1 has a quiescent current running in through +V terminal and out -V terminal. This current is mirrored so the same current runs from Q3 collector to Q4 collector. 

When +IN is higher than -IN, a current will flow out of the -IN terminal (this is the error current, hence the name current feedback). This current causes the current in terminal +V to be higher than the current in terminal -V. This in turn causes the upper current mirror to source more current than the lower one sinks. This difference in current is flowing to capacitor Cdom and so the voltage on Cdom rises and the output voltage follows and rises too. And so +IN > -IN = Vout rises, and vise versa. And so when you connect the output to the negative input you get a circuit that functions very much like a conventional opperational amplifier. 

This also explains why the feedback resistor value from out to -IN affects the bandwith of the amplifier. A lower resistor value will cause a higher feedback current to flow at the same error voltage and so the Cdom charges faster and voltage rises faster. 

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Have you seen the new Runoffgroove circuit, "Thunderbird"? What are your thoughts on the approach they went for this time?

It seems like a complex and expensive approach. But it’s probably a good circuit. And in its own right simpler and less convoluted to make than many of their other circuits. I’ve only ever build one ROG circuit, but I generally think who ever designs those circuits are full of interesting ideas. But I very much prefer that they now have chosen circuit that can be reliably replicated without having to individually trim the bias of J-fets that has more or less gone out of production. That seemed a little unnecessary. 

The LT1054 seems like an odd choice though. Isolated DC/DC converters with +/- outputs aren’t very expensive, and they have the benefit of eliminating the ground loops when using a wall wart for powering pedals. I think I would have gone for that option, especially for a DIY optimised circuit. 

Edit: 

I just checked, and sure enough, from my supplier, a Murata isolated +/- 12V 42mA DC/DC converter is cheaper than a LT1054! Go figure… I’d strongly recommend using a isolated converter. For any pedal, actually, regardless of wether a negative supply is needed, since it removes ground loops from using wall warts for powering it. 

Oh and what is up with those tantalum caps? What’s wrong with using ceramic multilayer capacitors?

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Another rule of thumb

5ps propagation delay per 1mm of PCB trace. 

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Another headphone amp

PDF here

It’s a no budget solution, USB powered and it has a Baxandall volume. I made it because it bothers me that most computers headphone output has pretty decent performance when it’s unloaded and at max volume. And perform horribly at any other condition. So this allows the system volume to be turned to max (where it should be) and then you adjust the volume with an amplifier with adjustable gain, rather than with a noisy variable attenuator. 

Oh, and I’m making it with KiCad. 

[micdotcom]

Watch: Ahmed Mohamed speaks out about being arrested 

[markredito]

:’)

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What a brave kid! No doubt he'll reach the top of electronics engineering!

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The fear of every electronics engineer

My boss have decided that I’m going to use a different schematic capture and PCB layout software. 

From now on I’ll be using KiCad, presumably also privately. I will of course, when I get around to share all my libraries, also share my Eagle libraries, but I probably won’t be making any new Eagle libraries.

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Half/H bridge dead time

If you’re not opting for one of the IR gate drivers that takes care of all this for you, here’s how to do it. 

You might for example want to use an iCoupler gate driver like ADuM3224: 

It has quite remarkable specs along with two fully isolated gate drivers, 43ns typical propagation delay, <5ns delay matching between the two fully isolated gate drivers! In other words, very ideal for class D. And not too expensive either. 

And this tiny bit of logic will take care of the dead time: 

This diagram is just conceptual, you’ll of course have to add the necessary driving circuitry, but I just added the mosfets for clarity.   

NEW!! 

Try a simulation of the circuit Click Here

It’s made with faldstad circuit simulator, it requires nothing but javascript and is free to use! 

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Simple, discrete, low drop out voltage regulator

This is great for when ever you need a tiny amount of power from a high voltage rail, cost optimisation or high reliability applications.

It will regulate to roughly the voltage of the zener. 

If you have tight tolerances for your supply voltage but still need the high input voltage of the discrete solution, just put a regular integrated linear regulator after it. 

Edit: I forgot that stability might not be trivial to any one who would need this circuit. This regulator of course need a miller cap on the upper most p-channel mosfet.

1n between gate and drain will typically work fine. If you expect high voltages here it might be a good idea to use a NP0 rated ceramic cap  

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Balanced class D and PCB layout

I’m developing a class D amplifier, and I have currently solved the problem of keeping the common mode output voltage zero at all times. I’ll write up on that later. Here’s something that i happen to find more interesting. It’s the general idea that a differential amplifier does not need to draw any ground current at all! 

This makes for much simpler and better pcb layouts since there is no need to deal with splitting the audio ground and keeping the switching ground the same. (yep that shit is actually possible. You make slits in the GND plane, and place ceramic capacitors across it. The switching currents will pass through the caps, but the LF audio signal will not!) 

Here in stead you make all decoupling between the two power rails and keep ground completely out of the picture on the output side. So the output filter capacitor (C3) is not connected from ground to output, it’s connected between outputs like you see here: 

And yes, this is the basic topology used for my class D amplifier. The only thing not shown here is how i keep common mode output under controll. It’s really not that complicated, I’ll show you if there’s interest. The output stages consist of  IRS20957 driver and IRF6665 mosfets. The inputstage is simply an op amp for which I haven’t yet settled for a specific one, and two comparators. And then an integrator for keeping the common mode output level at zero. 

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New tool! This is awesome. Accompanied by a 200$ stereo loupe, anyone can solder almost anything. The future really is now.