DIY Audio: Seek and You’ll Find in Santa Clara

This week, while visiting Santa Clara, CA-based HSC Electronic Supply, I came across a book display featuring a few of our best-selling titles: Loudspeaker Design Cookbook, Glass Audio Projects, and Loudspeaker Recipes.

Book display at HSC Electronic Supply in Santa Clara, CA

If you happen to be in the Santa Clara area, I encourage you to visit HSC. It has a wide variety of electronics components, audio parts, and test equipment. Audio enthusiasts will be particularly excited to browse HSC’s superb stock of tubes.

Tubes at HSC Electronic Supply

HSC Electronic Supply of Santa Clara
3500 Ryder Street
Santa Clara, California 95051
Mon – Fri: 8 am – 7 pm, Sat: 9 am – 5 pm



audioXpress May 2013: Special Focus on Tubes

As always, audiophiles around the world are eagerly awaiting the yearly “Glass Audio” special edition of audioXpress magazine. This year’s issue won’t disappoint, and we’d like to preview it here for you.

In addition to the regular columns and features, the May 2013 issue will feature a special section comprising interesting and informative tube-specific content. That’s right, we’re packing in an extra 16 pages for the glass audio enthusiasts out there!

The content slated for publication includes articles on the following topics:

  • An article by Bruce Heran about his “Poddwatt” Series II stereo integrated valve amplifier
  • An article by Pierre Touzelet about the “SC-OPT,” which is a self-compensated output power transformer for valve amps
  • Richard Honeycutt’s “Hollow State Electronics” glass audio column
  • An article by B. Kainka about an FET amp with valve sound
  • And more!

Want to get involved? Sponsorship and advertising opportunities are still available. Find out more by contacting Peter Wostrel at Strategic Media Marketing at 978-281-7708 (ext. 100) or Inquire about editorial opportunities by contacting the editorial department.

Differences in Amp Sound: What’s the Truth?

Back in the 1960s, after crossover distortion was tamed in the better solid-state amplifiers, many serious audiophiles remained convinced that tube amps sounded better than any solid-state ones. Then in the 1970s, after Matti Otala and others had demonstrated the effects, cause, and prevention of slewing-induced distortion, the percentage of serious audiophiles who preferred tube amps may have declined slightly, but it certainly did not drop to zero.


The first tests to determine the cause of the difference in sound, if any, were mostly performed by engineers, who concluded that since the frequency response and distortion performance of the best tube and the best solid-state amplifiers were comparable, there must not be any difference (see Photo 1). Thus began the so-called “Great Debate.” I believe we are now in a position to put this debate to rest.

Photo 1: Tubes or transistors? A listener’s preference is a subjective choice.

First, I must affirm, as mentioned in a previous article, that subjective amplifier judgments are, by nature, individual perceptions. Every perception is a fact to the person perceiving. Thus, if you tell me you can hear a difference between two amplifiers, I have to believe you. If I can hear no difference, then perhaps you are a more critical listener than I am. If neither I nor any of a dozen other skilled listeners can hear a difference, then you have indeed heard a subjective difference, but since the perceived difference is not a shared reality, we cannot say there is an objective difference. If you cannot prove in a properly designed double-blind test that you can repeatedly hear a difference, then we must conclude that the difference you do hear does not proceed from physical, and therefore, determinable, causes.


In last month’s article, I discussed blind and double-blind testing. Both assume that the equipment under test is presented in a pair: two amplifiers or two speakers and so forth are being compared. In a blind amplifier test, the test subject (i.e., the person providing opinions about the equipment under test) does not know which amplifier is playing at a given time, in order to avoid the effects of extraneous variables (e.g., manufacturer preferences or finish details). Long ago, experimenters found that test operators (e.g., set-up technicians or other persons involved with the experiment) would sometimes unintentionally provide clues via facial expression or body language that could give away the identity of the equipment under test. Thus, double-blind tests, in which no one involved in the test operation knows which piece of equipment is being used at a given time, were developed. Double-blind tests are the gold standard for any tests involving human perception.

Some people do not like double-blind tests, believing themselves to be immune to hidden biases. However, there is no evidence to prove that even the best of intentions enable a person to avoid subconsciously tilting his/her test responses in a preferred direction. (Perhaps a Vulcan could?)

Resistance to double-blind testing is not limited to those who believe tube amplifiers sound better, although some of those listeners still offer objections to the elaborate test protocols. Some people firmly believe that any well-designed IC-based amplifier sounds better than any hollow-state amplifier and still claim that double-blind testing is unnecessary for them. Perhaps the most convincing for me personally was an individual who told me he made a change that was expected to make an amplifier sound better, but he was surprised to find that it sounded worse. No details of the test protocol were given. Of course, this amounts to hearsay, and thus cannot be considered scientific evidence.


In 1977, the British magazine Hi-Fi News and Record Review published an article by Jean Hiraga detailing sensitive distortion measurements made in Japan on a variety of tube and solid-state amplifiers that operated well below clipping. The article was reprinted by audioXpress in the March, 2004 issue. This article showed conclusively that differences in the distortion spectra of excellent amplifiers do exist, but no effort was made to correlate these measured differences with perception.

In the March, 1980 issue of High Fidelity magazine, “The Great Ego-Crunchers: Equalized Double-Blind Tests” by Daniel Shanefield was published. This article directly addressed the perception issue. Shanefield mentions the division of audiophiles into “golden ears” who insisted that they could hear differences in amplifiers and “nonbelievers,” who constitute the majority. He makes the statement: “In the next few years (after 1970) several small-circulation magazines espoused the golden-ear point of view, though they often disagreed with each other about which components were truly excellent and even changed their minds drastically from issue to issue.” Shanefield’s conclusion was that audible differences among good-to-excellent amplifiers do indeed exist, but that if the frequency response of all amplifiers under test was equalized to be flat within 0.25 dB, no perceptible difference remained. He includes the somewhat startling detail that when he compared three Dynaco 400 samples, frequency response differences of a few tenths of a decibel did exist, and the amplifiers did sound different. His experimental protocol ensured that the amplifiers were operated well below clipping. Shanefield’s tests were subsequently replicated by several members of the Boston Audio Society.


In October 1991, David Clark of DLC Design presented the paper, “Ten Years of A/B/X Testing” at the 91st Convention of the Audio Engineering Society. A/B/X amplifier tests compare an unknown amplifier “X” with two known amplifiers, “A” and “B.” The test subject’s goal is to determine whether “X” is “A” or “B.” For example, an excellent amplifier can be used as “A,” and a medium-grade amplifier as “B.” The test subject can switch at will among “A,” “B,” and “X,” but the switches may be connected so that position “X” is actually amplifier “A” (see Photo 2). If the subject can reliably identify “A” is “X,” then clearly the difference between “A” and “B” is perceptible to him. If not, we cannot conclude that there are physical causes for the differences that some listeners perceive under less-controlled conditions.

Photo 2: A flick of the switch enables test subjects to switch amplifiers. Sometimes “X” was the same amplifier as “A” or “B.”

A/B/X tests are usually double blind, but they do not require the equipment’s brand/model under test be kept from the listeners, since neither the listener nor the test operator knows which is “A,” “B,” or “X.” Test subjects are not permitted to communicate with each other.

In an A/B/X test, the listener chooses when to flip the switch, allowing whatever amount of time he feels is needed to properly identify the unit under test as “A” or “B.” A test may span several listening sessions, if the listener so chooses, or may be finished quickly if the listener is confident he has determined the identity of “A” or “B” as “X” in a short time.

A/B/X testing excels at finding perceptible differences, if any exist, but is not designed to establish levels of accuracy or preferability. The A/B/X test itself was compared with long-term listening as a method of identifying a calibrated 2.5% total harmonic distortion (THD) component that was added to a musical signal. The Audiophile Society acted as the “golden ears.” The Southwestern Michigan Woofer and Tweeter Marching Society (SMWTMS) acted as the “engineers.” Neither group could identify the distortion at a 5% confidence level in long-term listening tests. However, using A/B/X testing, the SMWTMS not only correctly proved the audibility of the distortion in 45 min. of testing, but also correctly identified a lower amount of distortion. In the complete series of tests, THD was found to be audible at 4% using big-band jazz music, 2% using flute music, and 0.4% using a sine wave. The spectrum of the harmonic distortion was not specified in Clark’s AES paper (AES Preprint 3167).

Clark added a note in his 1991 paper, based on a private conversation with Thomlinson Holman (the “TH” of THX). Holman has found that a number of professional power amplifiers do distort audibly when driving highly reactive loads (e.g., some theater speakers) when playing explosive movie sounds. The cause could well be that under such severe load conditions, the amps’ power supplies experience instantaneous drops in voltage. This condition would be easy to identify using an oscilloscope.


Audio professional Richard Clark (note: this is a different Clark), originally a believer that different amplifiers sound different, set up a $10,000 challenge: anyone who, by listening only, can identify which of two amplifiers is which, under rules he has established, will receive the prize. The rules can be found at They primarily include minimum-quality levels for participating amplifiers, level matching, and so forth, all of which are essential to the validity of any comparative test.

In the years since the challenge was first offered, most large groups have obtained accuracy  of  49–51%, which are essentially the results one would expect to occur by chance. Smaller groups have never gotten more than 60% correct. In any statistical sampling, small test groups are more likely to deviate from chance results. As the test population is increased, test results converge toward a specific value. For a random process, a larger test population is likely to converge toward chance results. The fact that Clark found more nearly chance results when measuring larger groups is itself a strong indicator the test subjects’ responses were random, not ordered as would be the case if there were perceptible differences between the amplifiers being tested. These are averaged scores for the groups. No individual has ever reached 65% correct. These results do not permit us to say no person can ever hear differences between two good amplifiers, but they do strongly indicate that any such differences must not be very robust (see Photo 3).

Photo 3: Listeners must often determine for themselves what they hear (e.g., if this sound plotted here is actually clipping on transients).

No test such as David Clark’s or Richard Clark’s can ever show that there are no perceptible differences in amplifier sound. Proving the non-existence of anything is philosophically problematic because we can truthfully say only that any experiment did not find such-and-such a thing. However, we cannot perform all possible experiments. As an analogy, we cannot say there are no white crows, because we cannot look everywhere at once. If we postulate the nonexistence of white crows, the person who finds a single example will prove us wrong. So far, however, no person has demonstrated publicly in a scientific fashion that he can reliably distinguish between the sound of two good amplifiers with identical frequency response and low noise driven below clipping. And yet the perception remains that there are real differences in amp sound. As a consultant in acoustics and sound/video system design, I regularly encounter people who assume I must have a vacuum-tube stereo system “because everyone knows they’re better.”

It is true that almost all tube power amplifiers have a very slight high-frequency rolloff (tenths of a dB), but no test has convincingly shown that such a small deviation from flat is perceptible (see Photo 4). (If it is, and makes the sound better, should we all add inexpensive RC low-pass filters to our amplifiers to improve the sound so they sound like tube amps?)

Photo 4: One factor could be whether or not the output transformer affects the sound.


There are still serious researchers who are trying to find the elusive ingredient to tube sound. In October, 2011, Shengchao Li of Potomac, MD, presented a paper, “Why Tube Amps Have Fat Sound While Solid-State Amplifiers Don’t,” to the Audio Engineering Society’s 131st convention in New York. The paper was reviewed by two qualified anonymous reviewers. Li begins from the assumption that tube amps do indeed sound different. He proceeds to explain that the differences arise from output-tube nonlinearities, amplifier output impedance, and output-transformer nonlinearity resulting from the core material’s B-H curve. These nonlinearities, Li says, interact to reduce the low-frequency output of the amplifier under some conditions. The speakers, whose nonlinearity is worst at low frequencies, thus have less signal at those frequencies and thus produce less distortion. In this manner, some low-frequency output is traded for reduced low-frequency distortion. The first two mechanisms suggested by Li have been discussed in earlier Hollow-State columns. The third may be significant in low-feedback amplifier designs, but in more typical Williamson designs, transformer nonlinearities are largely compensated by negative feedback. Certainly all three mechanisms will be exacerbated at levels approaching or exceeding clipping. It would be instructive to see if a low-feedback tube amplifier could be identified in A/B/X testing, and just what level of overdriving is necessary to permit objective identification of any amplifier.


Let us now step bravely into the hornets’ nest. After four decades of testing by a number of very capable scientists, no evidence has been published that shows any objective difference among the sound of good-to-excellent audio amplifiers operated well below clipping, if the frequency responses are equalized within 0.25 dB of flat. Does this indicate there is no “tube sound”? It does not, for several reasons.

First, almost no home-music listener (or even recording studio engineer) equalizes the amplifiers flat within 0.25 dB. Virtually no speaker pairs are matched that closely all across the passband. And a flat response is not always everyone’s first choice. Hi-Fi systems of the 1950s usually had “scratch” and “rumble” filters to remove the sound of artifacts on vinyl recordings. These typically applied a 3-dB/octave high cut  above 8 kHz, and low cut below 80 Hz, respectively. Presumably they were included because equipment manufacturers found that their customers wanted and used them: at least under some conditions, listeners did not prefer flat response.

Second, a significant number of audio amplifiers are made for instrument amplification. The distortion used intentionally with electric guitars is well-known. I have also played with professional keyboardists who preferred tube-type Hammond organs because of the “growl” they produce at high volumes. This growl comes from distortion—largely intermodulation distortion—in the tube power amplifiers. A smaller percentage of audio listeners, but still a non-negligible number, prefer some distortion even in their music reproduced from recordings. Naturally, the distortion spectrum  is quite important to such people. As shown in a previous column and in Hiraga’s 1977 tests, that spectrum is very different for a tube amplifier versus a solid-state amplifier. It is a safe generalization to say that most people prefer tube distortion over the distortion of a solid-state amplifier. Most probably prefer triode tube distortion specifically.

Third, there are conditions under which distortion, even if not desired, occurs in an audio chain. It can happen when a person is listening to music at high levels, especially if low-efficiency speakers and/or underpowered amplifiers are being used. I believe this occurrence is the rule rather than the exception for many serious music listeners. Not only rock music, but also symphonic music, pipe organ music, and big-band jazz require a lot of amplifier power in order to play through typical home-stereo speakers at realistic levels without clipping. Aside from amplifiers for music listening, another place where “tube sound” is much hyped is in microphone preamps for audio recording. In this application, it is not uncommon for the microphone to put out surprisingly high voltages. Capacitor microphone cartridges, especially, are almost immune to clipping, so when exposed to high sound pressures, they can produce output voltages close to 1,000 times their normal output levels. Under these conditions, preamp clipping is pretty much inevitable. Again, the distortion spectrum is very important, and most recording engineers prefer the spectrum provided by tube pre-amps.

So in this columnist’s codgerly opinion, there is indeed a “tube sound” under some conditions, and many excellent technicians, engineers, and audiophiles find it preferable for specific applications. It is not, however, scientifically accurate to claim that hollow-state amplifiers are better or worse than—or even perceptibly different from—solid-state ones for all applications.

Purely audio considerations aside, many audiophiles prefer the aesthetics of a “warm,” artistically designed (perhaps handcrafted) amplifier over the usual “high-tech” appearance of most solid-state amplifiers. And many of us enjoy the “legacy” feel of the equipment setup when operating vacuum tubes are visible. These are valid reasons to buy hollow-state equipment, if it appeals to you.


If you are still not satisfied about the topic of “tube sound,” you are invited to take part in an online test. The test has two parts. In the first part, you will be invited to use quality headphones (please, not computer speakers) to listen to a pair of .wav files of a short clip played by the lead guitarist of Darrell Harwood and the Coolwater Band. One file was made by recording directly to digital from the guitar, then playing the recording through a solid-state power amplifier adjusted to produce an acceptable amount of distortion for proper artistic effect. The other file uses the same digital recording, played through a tube amplifier adjusted in the same way. Both recordings were made using the same speaker and cabinet, and were recorded using a type 1 calibrated measurement microphone with a tensioned stainless steel diaphragm. The same physical setup was used for both recordings, and the levels of the recordings were carefully matched. You may play the files as many times as you wish, and will then be requested to send an e-mail stating which file, “A” or “B,” sounded better to you. If you choose to include your opinion as to which is the tube-amp and which is solid-state recording, please do that as well. The purpose of this part of the test is to illustrate the sound of the different distortion spectra of tube versus solid-state amplifiers.

The second part of the test consists of three .wav recordings of a brief clip of classical music. There is a reference clip made directly from the CD, a clip of the selection being played through a McIntosh hollow state power amplifier, and a clip of the selection being played through a commercial solid-state power amplifier.  Both of the “amplifier” recordings were made using an Evenstar Pro Peregrin studio monitor speaker, recorded using the measurement microphone described above, with the same physical setup. No equalization was applied to either amplifier, and the amplifiers were operated well below clipping. The reference (“X”) straight-through recording is identified, and you are asked to listen to the “A” and “B” recordings, determine which sounds more like the reference, and e-mail your results. The test can be found online at It will be available until the end of 2012. Instructions for participating are included on the website.


Hollow-State Amps & Frequency Response

“Glass audio” has been growing in popularity among average audio enthusiasts for the past decade. Music-loving consumers worldwide enjoy the look and sound (i.e., the “warmth”) of tube amps, and innovative companies are creating demand by selling systems featuring tubes, iPod/MP3 hookups, and futuristic-looking enclosures. I suspect hybrid modern/retro designs will continue to gain popularity.

Many serious audiophiles enjoy incorporating glass tubes in their custom audio designs to create the sounds and audio system aesthetics to match their tastes. If you’re a DIYer of this sort, you’ll benefit from knowing how amps work and understanding topics such as frequency responses. In the April 2012 issue of audioXpress, columnist Richard Honeycutt details just that in his article titled “The Frequency Response of Hollow-State Amplifiers.”

Below is an excerpt from Honeycutt’s article. Click the link at the bottom of this post to read the entire article.

Early electronic devices were intended mainly for speech amplification and reproduction. By the 1930s, however, musical program material gained importance, and an extended frequency response became a commercial necessity. This emphasis grew until, in the 1950s and 1960s, the Harmon Kardon Citation audio amplifier claimed frequency response from 1 to 100,000 Hz flat within a decibel or better. Although today, other performance metrics have surpassed frequency response in advertising emphasis—in part because wide, flat frequency response is now easier to obtain with modern circuitry—frequency response remains a very important parameter …

Just which factors determine the low- and high-frequency limitations of vacuum tube amplifiers? In order to examine these factors, we need to discuss a bit of electric circuit theory. If a voltage source—AC or DC, it doesn’t matter—is connected to a resistance, the resulting current is given by Ohm’s Law: I = V/R. If the voltage source is of the AC variety, and the resistor is replaced by a capacitor or inductor, the current is given by: I = V/X where X is the reactance of the capacitor or inductor. Reactance limits current flow by means of temporary energy storage: capacitive reactance XC does so via the electric field, and inductive reactance XL stores energy in the magnetic field.

Figure 1 – The values of reactance provided by a 0.1-μF capacitor and a 254-mH inductor, for a frequency range of 10 to 30,000 Hz (Source: R. Honeycutt, AX April 2012)

Figure 1 shows the values of reactance provided by a 0.1 μF capacitor and a 254 mH inductor, for a frequency range of 10 to 30,000 Hz. Notice that capacitive reactance decreases with frequency; whereas, inductive reactance increases as frequency increases.

Click here to read the entire article.

Simple Circuits: Turn a Tube Radio Into an MP3 Amp

Want to give your MP3 player vintage tube sound? You can with the proper circuits, an antique radio, and a little know-how. In addition to generating amazing sound, the design will be an eye catcher in your home or office.

Here I present excerpts from Bill Reeve’s article, “Repurposing Antique Radios as Tube Amplifiers,” in which he provides vintage radio resources, simple circuit diagrams, and essential part info. He also covers the topics of external audio mixing and audio switching. The article appeared in the May 2012 edition of audioXpress magazine.

Manufactured from the 1930s through the 1960s, vacuum tube radios often contain high-quality audio amplifiers at the end of their RF signal chain. You can repurpose these radios into vintage, low-power tube amplifiers—without marring them in any way or detracting from their original charm and functionality as working analog radios.

Wood-cased radios have especially good sound quality, and the battery compartments in antique “portable” radios (like the Philco 48-360 or the Zenith Transoceanics) provide perfect locations for additional circuitry. When restored properly, large furniture-style radios that were built for “high fidelity” (like the late 1930s and early 1940s Philco console radios) can fill a room with rich beautiful sound.

Simple Circuits

The simple circuits described in this article perform two functions. They mix an external line-level stereo signal (typically from an MP3 player or computer) and reference it to the radio’s circuit. They also use the radio’s on/off knob to switch this external signal to the radio’s audio amplifier.

There is not one circuit that will work for every antique radio. (Original schematics for antique tube radios are available on the web But the circuits described here can be adapted to any radio topology. All the parts can be ordered from an electronics supplier like Digi-Key, and the circuit can be soldered on a prototyping printed circuit board (such as RadioShack P/N 276-168B).

External audio mixing

Figure 1 and Figure 2 show some examples of circuit schematics that mix the line-level stereo audio signals together (almost all tube radios are monophonic), while providing galvanic isolation from high voltages within the radio. Figure 1 shows an inexpensive solution suitable for most table-top radios.


Figure 1: An inexpensive circuit for mixing an MP3 player’s stereo audio signals safely into an antique radio. None of the component values are critical. (Source: B. Reeve, AX 5/12)

These radios have relatively small speakers that are unable to reproduce deep bass, so an inexpensive audio transformer (available from on-line distributors) does the job. I picked up a bucket of Tamura TY-300PR transformers for $0.50 each at an electronics surplus store, and similar transformers are commercially available. Alternatively, the Hammond 560G shown in Figure 2 is an expensive, highquality audio transformer suitable to high-fidelity radios (like the furniture-sized Philco consoles). A less expensive (and fine-sounding) alternative is the Hammond 148A.

Figure 2: A high-fidelity circuit for mixing external stereo audio signals safely into an antique radio. (Source: B. Reeve, AX 5/12)

I use Belden 9154 twisted, shielded audio cable for wiring internal to the radio, but twisted, 24-gauge wire will work well. An 8′ long audio cable with a 3.5-mm stereo jack on each end can be cut in half to make input cables for two radios, or you can use the cord from trashed ear-buds. You can route the audio cable out the back of the chassis. Photo 1 is a photograph of a 1948 Philco portable tube radio restored and used as an MP3 player amplifier.

Photo 1: A 1948 Philco portable tube radio restored and repurposed as an MP3 amplifier. (Source: B. Reeve, AX 5/12)

Audio switching using the radio’s on/off knob

After creating the mixed, radio-referenced signal, the next step is to build a circuit that switches the voltage driving the radio’s audio amplifier between its own internal broadcast and the external audio signal.

Figure 3 illustrates this audio routing control using the radio’s existing front panel power knob. Turn the radio on, and it behaves like the old analog radio it was designed to be (after the tubes warm up). However, if you turn the radio off, then on again within a few of seconds, the external audio signal is routed to the radio’s tube amplifier and speaker.

The circuit shown in Figure 3 uses a transformer to create the low voltage used by the switching circuit. There are many alternative power transformers available, and many methods of creating a transformerless power supply. Use your favorite….

The next photos (see Photo 2a and Photo 2b) show our additional circuit mounted in the lower (battery) compartment of a Zenith Transoceanic AM/shortwave receiver. Note the new high-voltage (B+) capacitors (part of the radio’s restoration) attached to a transformer housing with blue tie wraps.

Photo 2a: The inside view of a Zenith Transoceanic AM/shortwave radio restored and augmented as an MP3 audio amplifier. b: This is an outside view of the repurposed Zenith Transoceanic AM/shortwave radio. (Source: B. Reeve, AX 5/12)

The added circuit board that performs the audio re-routing is mounting to a 0.125″ maple plywood base, using screws countersunk from underneath. The plywood is securely screwed to the inside base of the radio housing. Rubber grommets are added wherever cables pass through the radio’s steel frame.—Bill Reeve

Click here to view the entire article. The article is password protected. To access it, “axreeve”.