V1.1 Actualized

Still need to add jacks and a volume pot. The power and ground buses are on the back side of the breadboard. Still figuring out layout; breadboards are a pain and I can see the appeal of designing a PCB.

V1.1

My original idea of a Sziklai pair for the output buffer was good. My abort for the first version was based on a faulty understanding of when a BJT goes into saturation. I like the pair output buffer a lot better.

There's also an interesting effect when the input to the pair goes over 1.4V: the first transistor goes into saturation and starts pulling a lot of base current. In my configuration, this would pull down the last gain resister and so function as negative feedback. I wonder if this softens the clipping at all?

(Note there should a Miller capacitor in the last gain stage too.)

V1

I felt that the paired output buffer mentioned in the previous post wasn't a good idea, although this one, which seems more sensible, isn't achieving quite the same game; final output into 20Ω is barely a milliwatt. We'll see how it works... Happily enough, the performance didn't change between arbitrary resister values, and values that I have laying around the workbench. I think that's a good sign for my design.

Turning Up the Noise

I moved quickly through the sawdust portion of this blog. Maybe I will return to it at some point. So far I don't think I've done anything that isn't obvious to someone who's spent more than 20 minutes with a router. I did learn a lesson after destroying one pickguard template (have shallow and deep pattern bits), and there was the aforementioned problem with the bridge placement. Otherwise the build went smoothly.

There's a couple more things to figure out. I've sewn the headphones into the strap, and am still experimenting with their placement. The balance is a little off; I've adjusted one of the strap buttons already to keep the body from tilting down, and will be moving the neck button up to the top of the guitar to allow it to hang better centered.

All of that will wait until I get the first version of the headphone amp: the noise portion of this blog.

As mentioned, I want to try to make a one-cell amp, mostly as an excuse to use up the bag of transistors I have in the garage. The basic design is this.

The fundamental gain component is a buffer (emitter-follower) and a degenerated common emitter with degeneration bypass and a Miller feedback capacitor. This was the result of a lot of simulation and design for something that has microamps of quiescent current, robust with respect to component values, and has high input impedance. The buffers seem necessary to chain these stages together, and a single gain transistor isn't sufficient given my low voltages. These don't chain together perfectly, even with alternating between npn and pnp transistors, as the best input offset is less than the output offset, but a spice simulation of putting three together ends up working good enough. To drive a 20Ω output I needed a paired buffer pulling 20mA quiescent. That's enough to take a 5mV peak input to 200mV. I'm sure I'll need to tweak this after soldering up the first version. Note that 2 stages would probably provide enough gain, but I want three so I can have global feedback.

The idea of the Miller feedback is to get a second or third order high frequency filter in order to cope with the overly bright tone. I've had a chance to travel with this over the last few weeks (as pictured), and have tended to zero out the tone dial on the amPlug amplifier I'm using. It's tolerable, but somehow simultaneously ringy and muddy.

As you can see in the picture above, I did some math to figure out what the gain would be as a function of the capacitors, ignoring the buffers. Naively plotting this along with some simple feedback gives the following frequency response. This matches up with the spice simulation (which includes the final output pair and a 20Ω load), with one exception.

I think there's something weird with the spice simulation, because it's reporting 70dB of gain even though looking at the waveform in a transient plot shows something like 30dB of gain. We see with three 4nF capacitors we roll off at 1 kHz nearly at a 3rd order rate (18dB/octave). Dropping the capacitors to 500pF moves the shoulder to 6 or 8 kHz which may be more reasonable. I'll have to let my ear decide. I don't know what harmonics are necessary for a string to sound good.

There are some oddities. In the theoretical plot, there is ripple as the gain is reduced.

This can be much reduced by spreading out the capacitors, for example using 800, 500 and 100 pF capacitors rather than identically 500 pF.

So far so good. However, when looking at the transient waveform simulation, weird stuff is happening. With all 500 pF caps and varying the (shunt) global feedback from 10kΩ to 5kΩ to 1kΩ we have the following at 500 Hz.

At 10kΩ we are clipped. As we decrease the gain we get some weirdness. I originally thought it was due to the phase shift from the capacitors. My theory tells me the phase shift is negligible at this frequency, but my theory could be wrong.

Plotting at 100 Hz, which should be well away from any shift, is closer to what I'd expect, but there are artifacts that make me distrust what spice is doing.

In addition, you'll note that I start the transient analysis well away from zero. This is to eliminate what looks like a capacitor charging, even though that should be dealt with by the operating point calculation that I thought spice did. Hmmm.

At any rate, the ripple never shows up in the frequency plot. But then, that plot is also always pegged at 75dB of gain regardless what size feedback resister I use. I think I'm missing something fundamental about how spice .AC analyses work.

In spite of all this, I feel like there's enough agreement between my theory and the spice simulation that I should solder this up and see what it sounds like. It's a bigger amplifier than I was hoping for (8 transistors), but it has a lot of flexibility in frequency shaping that I hope will make it fun to play with.

I'm even less skilled at soldering than I am at woodworking, if such a thing is possible, so this next stage should be interesting to watch.

More travelling

Utility

It wouldn't be much of a travel guitar if I didn't travel with it.