Letters to WTF: What is distortion?

This is a loaded question, but here’s a short take that I hope is easy to understand and sufficiently device-neutral.

Distortion is any change in the input signal found at the output, aside from making the signal “bigger”. We usually talk about and measure it as harmonics (Total Harmonic Distortion or THD).

Harmonic distortions are integer multiples of the original signal frequency found on the output. The 2nd and 4th harmonics are octave musical intervals. The 3rd and 6th harmonic are a musical fifth interval and the 5th is a major third interval. The 7th harmonic is a minor seventh interval (at least that one is easy to remember).  There are harmonics above the 7th, but you don’t really see them often in competently designed equipment.

Generally, tubes produce more harmonic distortion than solid state. There are reasons (single-ended operation, lack of negative feedback) and exceptions to this (push pull amps with NFB). The harmonics they produce tend to be monotonically decaying and dominated by even orders (musical octaves and fifths). This is part of what earns them the reputation of sounding ‘full’ or ‘organic.’ There is literally more musical content (even if it wasn’t intended by the original producer).

Transistor amplification generally uses negative feedback to reduce the amount of harmonic distortion in an amplifier. This is in part because transistors can (they achieve much higher gain) and partly because they must (transistors without feedback are inherently less-linear voltage gain devices). This feedback, up to a point, increases high order harmonics and beyond about 20db applied reduces all harmonic distortion. While this sounds like a free lunch, it isn’t. Lots of gain to be used in negative feedback means more stages that each contribute non-linearity and high feedback amplifiers have ugly clipping characteristics.

In the end, there is no one best device. Amps are the culmination of a series of design decisions that each have trade-offs. Unfortunately the trade-offs become the stereotypes that are bandied about among enthusiasts and you end up with ideas like ‘tube amps are full of distortion’ or ‘solid state is lifeless.’


New page on headphone output stages


I posted a new page on power output stages with a headphone focus (though it can be applied to speakers as well). My aim is to eventually try all the variants.  Between the Estudiante MOSFET hybrid, Papa Rusa parafeed SE, Bad Hombre differential, and Luciernaga series-feed SE, I’m part way there. Been working on ideas for an all-tube OTL lately.  Hopefully I’ll tick that box soon!

The Joy of Local

RCA ST 2I’ve mentioned here and there the clubs and local organizations I’m involved in (Wisconsin Antique Radio Club, represent). I also try to get to local DIY meet-ups and sip and spins wherever possible. Getting to know and network with experienced designers and builders was and is one of the best parts of the hobby for me. Having someone you personally know and who personally knows you (and your experience and capabilities) is an important resource as you get started in DIY. A lot of the knowledge, especially in our hobby, doesn’t live online and the perspective of an ‘old timer’ is far more insightful than a website (my own included). So get out there and make friends as a newbie.  Most of us in the hobby do it for the passion and fun of it and are happy to share thoughts and suggestions with others.


In addition to getting to know other DIYers, being involved in local clubs puts you in touch with local vendors. Online stores are a great way to find that specific part or tube without a hunt, but part of the fun of DIY is scrounging and working with what you find. Unless you are from a big city, there probably aren’t any “DIY tube amp builder association” meetings in your area, so look for HAM or radio clubs or meets, too. Chances are good that there is at least one local organization that you can join and start attending events. Many small vendors are willing to make quite a trip to offer their parts and tubes and meeting these local businesses is an adventure in itself.

The NOS matched RCA carbonized ST envelope tubes pictured above came from the swap meet I attended this past weekend.  The price would make you jealous.  Since joining  my local radio club, I’ve started buying almost all my tubes at the monthly meets. Dave at Electric Guru Parts House is my local go-to.  Join your local club and find your own!





DIY Cables: braiding video essentials from the interwebs

If you’re into headphones and DIY, here’s a nice intro to DIY cables, the styles, and the math involved.

Audio Primate

So if you’ve ever looked at aftermarket cables, you’ve probably seen about a million braids. The most common is the quad braid, usually seen in a round variety, but I do happen to own a flat braid quad cable that I bought in the UK. I’ve not made any of my own cables yet, but think it is about darn time I start. Honestly, I feel bad asking a friend to do it for me. It’s lazy, and unnecessary.

So for braiding, we should have a number of wires in multiples of 4. The reason is because this is what you’ll need to make a balanced cable. When you see cables that only have 2 wires and are balanced, it’s because the wires have multiple enamelled cores that they are splitting out, so essentially the wires are still working in multiples of 4. Even if you are going to use…

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Another Muchedumbre lives

I finished another Muchedumbre build with some slight variations.  This has two outputs and two inputs (easily switchable back to the 1+3 arrangement).  The power supply CLC filter uses all motor run caps instead of a mix of motor run and electrolytic. Other than these small tweaks, it is built as designed.

The wood apron is a very nice piece of walnut with a lot of prominent grain motion and color variation and the panel is inset rather than sitting on separate interior spacer boards. This is going to live a very happy life in Madison, WI.