Letters to WTF: What kind of rectification am I supposed to use with this power transformer?

I was helping someone with an amp build over Telegram (chat app) yesterday when this question came up. He had in fact been trying to use a diode bridge with a center tapped transformer with both the center tap and the bridge grounded. He released the magic smoke from his transformer, though there were a couple of other issues that may have contributed to this.

When I was starting out, I had some confusion with power transformers and rectifiers, too. Probably like many others, I started with small solid state circuits, where center tapped transformers are rare. Once I started building with tubes, the secondary ratings of center tap transformers were another source of confusion. So here’s a by no means complete rundown of transformer configurations.

1. My transformer doesn’t have a center tap and I want a full-wave rectified DC output.

You want to use a diode bridge (figure 4-8). This is four diodes arranged to rectify both positive and negative phases of the power transformer’s AC output. Your ground will be taken from the bridge, NOT THE TRANSFORMER. This ground at the junction of the diodes creates a return path for current that ‘switches’ with the changing phase of the secondary’s AC output.

2. My transformer has a center tap and I want a full-wave rectified DC output.

You want to use a “conventional” full-wave rectifier (figure 4-5A). This requires only two diodes (solid state or a rectifier tube). Your ground is taken from the center tap of the transformer (which is then the return path for current). Many center tapped transformers are rated as the full end-to-end secondary voltage. For example, a 300VAC center tapped secondary would actually provide 150VAC into a conventional full-wave rectifier. You’ll sometimes see the same transformer listed as 150V-0-150V.

Here’s a great clarification of what’s going on with full-wave bridges and conventional full-wave rectification.

How much voltage do I get?

With either of the above, the unloaded DC output into a capacitor-input filter is approximately the AC output from the secondary times the square root of two, minus the voltage drop across the diodes (minimal for solid-state, can be considerable for tube rectifiers). Into a choke-input filter (unloaded, ignoring diode drop), the output will be approximately two times the square root of two divided by pi (about 0.9) of the AC output of the transformer secondary.

3. My transformer secondary has a center tap, but I want a bipolar power supply.

Here you can combine the center-tapped transformer and the aforementioned bridge style rectifier. See figure 5.1c here. This creates two separate full-wave rectified voltages, one positive and the other negative with respect to the center tap. If you read a lot of TubeCAD, you see bipolar tube circuits pretty regularly.

4. My power transformer is 300VAC (150V-0-150V) center tapped, but I want 400VDC!

Another way to combine the center tapped transformer and bridge rectifier is to ignore the center tap altogether. Do not connect it to ground; just SAFELY tape it off and tuck it away. Now you have basically a non-center tapped transformer and you can treat it like number 1 above. Note that current capacity in this configuration is typically half of what the transformer was originally rated for.

5. My power transformer is 120VAC without a center tap and I want 300VDC!

To achieve this, you can use a voltage doubler (see figure 4 “Delon circuit). This requires two diodes and two capacitors. Because the capacitors will see large pulses from the diodes and will be supplying the rest of the circuit continuously, they need to be a fairly large value. But because each only sees half of the supply voltage, their voltage ratings are a little more relaxed in comparison to what is required in a filter. The unloaded DC output into a capacitor-input filter is approximately twice the AC voltage from the transformer secondary times the square root of two. Current capacity must be de-rated at the output voltage by a factor of at least two.

6. My power transformer has dual matching secondaries and no center tap. What do I do?

This is common with toroidal power transformers in particular. You can wire the two secondaries in parallel (making sure the polarities are matching) and use a bridge rectifier like number 1 above. The AC output of the transformer will be the same as either secondary by itself (and current capacity will be doubled). You can also wire the secondaries in series by connecting a positive and negative from each secondary (not the positive and negative from the same secondary!). This creates a center tap at the junction. The AC output end-to-end will be twice the AC output of a single secondary if the secondary is not grounded (see number 4 above). If you ground the secondary you created, you can use a rectifier like number 2 above.

Letters to WTF: Why do “neutral” amps sound different?

One of the issues is that although we can measure a heck of a lot about an amplifier, we can’t express those measurements in ways that are easy to digest and interpret for the average reader. We end up with data that is easy to misinterpret or take out of context. Part of the blame lies with consumers, who want to compare and contrast specs as a replacement for firsthand experience, and part lies with marketing departments that are being deliberately selective or unintentionally incomplete with what they publish.

THD is one of the classic examples of this. The THD as a percent often appears in marketing specifications, but it is not nearly so often accompanied by the all-important context. To make practical sense of a THD specification, you need to know what kind of signal was applied during measurement (frequency, level, single sine wave, multiples for IMD, program material, etc) and at what level the output was measured (voltage or wattage). Even when that information is provided, judging % THD across devices doesn’t give you a good comparative idea of the sound unless you know the harmonic makeup of the distortion spectrum (very rarely provided by manufacturers). All 0.X% THD measurements are not created equal in terms of your listening perception.

Power output is another example of specs that tell less than they should if we want to make a judgement on numbers alone. Like THD, power output requires context in the form of % THD at the measured level, the load used to measure (reactive load, resistive load, etc), and the signal used to measure. This information is not often given and so apples-to-apples comparisons that would allow someone to say with confidence that “these are equally neutral amplifiers” really is not possible. To make matters worse, most of these measurements are interactive, making expressing the results in a practical way more difficult.

While we hope manufacturers would include this kind of information in specs as we shop, it just does not happen. A big reason for that is the average person just doesn’t care. The responsibility falls to the niche press that reviews products and is able to measure them with a consistent method. But there’s a diminishing return on the amount of work it takes to measure and share all the data needed to give a clear picture of how something sounds. Even if the measurement work is done, it takes a certain amount of technical knowledge and experience to interpret it, so we end up with catch-all terms like ‘warm’ or ‘neutral’ to paint the picture. This audiophile language is as much a shorthand for all the measurements that aren’t being done as it is flowery jargon.

TL;DR It takes a huge amount of work to measure and express the findings if you want to capture all there is to say about how something sounds. Few are willing to do it, so we have words like ‘neutral’ that are part subjectivity and part experienced evaluation by trained ears. Even then, it’s not a perfect substitute for hearing something yourself.

Guest post at Audio Primate: Pocket Class A

done with headphones

Click here to read my review of the Pocket Class A tin amp headphone amplifier over at Audio Primate.  This is a solid state project, but a very fun build that performs surprisingly well for the low cost.  In principal, it is like a shrunk down version of my El Estudiante amp. Board came courtesy of XRK971 (the designer) on diyaudio.com.

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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