Notes from the Test Bench
By Bruce Hofer, Chairman & Co-Founder, Audio Precision
I’ve recently returned from a trip visiting customers in Japan and China. I always like to see the different ways that customers use our analyzers out in the field. In this month’s issue, we share that perspective with you by visiting a small vacuum tube mic preamp manufacturer here in the US, who’s using his APx in some conventional and not-so-conventional ways.
Output: Electronaut and the APx
In the first of an occasional series on how our customers use their AP audio analyzers, we talk to Rob Roy M. Campbell of the Chicago-based company Electronaut about designing 21st-century tube-based audio equipment.
by Matt Bell, special to Audio.TST
Few audio equipment manufacturers are true “one-man bands” these days, but with the exception of two interns who help with assembly, musician, vacuum tube enthusiast, and engineer Rob Roy M. Campbell of Electronaut Company designs, manufactures, QCs, and ships all his company’s M63 dual-channel tube-based mic preamps himself.
Rob Roy M. Campbell working at his bench.
A decade ago, having a successful career in IT, Rob Roy found that his weekend hobby as a touring musician was beginning to encroach on his time. Dissatisfied with the guitar amps he owned back then, he decided to begin building his own and founded Electronaut.
For the first few years, Rob Roy continued to work in IT to support his new business, while at the same time perfecting the M63 preamp and repairing guitar amplifiers on the side. “The repair business is dependent on how quickly you can turn jobs around, but if the amp design was interesting, I would spend time trying to figure it out. I remember working on an old Hiwatt guitar amp—it was all beautifully made, 90-degree point-to-point military-spec wiring—and I was so in love with the design that I spent days studying it… Of course, the owner was waiting for me to finish so that he could get on with band practice. I generally steer clear of repair work now.”
Rob Roy combines his engineering acumen with a commercial awareness that tube gear remains a niche and highly competitive proposition, even in the valve-obsessed audio industry. “There are so many players producing tube-based audio gear now, from big companies to small operations like mine, that you have to work harder than ever to make your mark,” he comments. “It’s one thing to cobble a tube-based design together, but making something well designed that will also stand out in an already saturated market is a full-time commitment.”
A New Lab and a New APx
In 2009, Rob Roy began production of the finalized M63 in earnest, but soon outgrew his existing workspace, which was based in his apartment. Fortunately, a former boiler room on the first floor of his building—itself a converted furniture factory—became available and in December 2010, after Rob Roy had totally redesigned his lab, workbench, and production area, Electronaut moved in. As part of this redesign, he purchased an Audio Precision APx515 audio analyzer. “Pretty much every performance graph I’ve ever seen has the AP logo in the corner, and I wanted that gold standard, so that when I made performance claims about my designs, people would believe them.”
At work in the Electronaut lab. Rob Roy’s APx515 can just be seen to his left.
According to Rob Roy, the APx515 has already proven to be a shrewd investment, not only in terms of effectiveness, but also in efficiency. “The signal-to-noise ratio and distortion are so much lower than I can achieve with tube test equipment. And the cumbersome nature of carrying out measurements using signal generators and scopes meant that I wasn’t doing it as often as I should have.”
"It’s no exaggeration to say that the APx has completely changed the way I work. First, it’s my main quality control instrument for finished M63 preamps, ensuring that my units are consistent when they go out the door—I have an automated APx project sequence for that. Second, when I’m designing, I have the analyzer on the bench with me so that I can test circuits as I’m constructing them, make changes, and see the performance results in real time. Given that this is audio equipment that I’m making, the final arbiters in both these processes have to be my ears (using in a proper listening test), but if I’m testing by ear for eight hours straight I get tired, and that makes the final selection difficult. If you can make measurements to get any obvious problems out of the way as soon as possible, and then do the listening tests at the end of the process, you’re better off."
Rob Roy has been impressed by AP’s post-sales support. “When I was testing different high output impedance audio transformers, I ran into difficulty. I was loading the transformers in lots of different ways to see what the effects were, and noticed that the high end was always rolling off. I contacted AP, and not only did I get a response from one of their senior engineers telling me how to construct an input attenuator that would solve the problem, they turned it into one of their online help articles (www.ap.com/kb/show/314). They seemed really interested by my problem and wanted to help me figure it out. The email I got didn’t just explain how to get around the problem, it explained why it was a problem, and gave me a couple of solutions.”
The Pod Of Absence
Using the APx at Electronaut hasn’t just been about quality control and design—it has been used on a work of art. Recently, an artist friend of Rob Roy’s, Conrad Freiburg, constructed a particleboard form entitled the Pod of Absence for a performance art installation. The pod is a heptagonal (seven sided) tapered cone designed as a performance space, large enough for musicians to stand in with their instruments. When Rob Roy first saw it, he pointed out that the structure would have its own resonant frequency, and Conrad asked him if he could find out what it was.
The Pod of Absence.
“I realized I could use my APx515,” continues Rob Roy. “First I set up a microphone in front of a speaker outdoors, ran several sweeps of the microphone and speaker combination using the APx’s acoustic response measurement, averaged the results, and exported the data. That gave me a pretty good idea of the frequency response of the mic/speaker combination.”
Rob then loaded that response curve back into the APx as generator EQ, in order to remove any response errors of the mic/speaker combination from the results. Running the test again confirmed a flat response. He then moved the mic and speaker inside the pod and ran the tests again, still applying generator EQ. The resultant curve showed the response of the pod alone.
Frequency response inside the pod, using the APx acoustic response measurement. Resonance peaks are at G natural three octaves below middle C (49Hz), A below middle C (220Hz), and G above middle C (392Hz).
“The APx confirmed the pod had a resonance at exactly G natural, and also another at A. Conrad now has plans to incorporate audio analysis into future installations, and is excited about the idea of building similar pods tuned to different resonant frequencies. There is even talk of incorporating printed response curves from the APx into the installation!”
Into the Future
Rob Roy M. Campbell remains an enthusiastic advocate for the creative possibilities of thermionic emission, and feels that he’s only just begin exploring the creative possibilities afforded by combining tube technology with his APx. “I would love to make an automated tube tester for an AP analyzer, because it’s the perfect programmable instrument. I have ideas for a programmable power supply that I would build that would be triggered by an APx, so that I could do sweeps under all sorts of operating conditions, and test for consistency between tubes, and even between triode sections within individual tubes—some circuits are very sensitive to mismatches between sections. That would help me a lot.”
While it’s a safe assumption that the M63 won’t remain the only Electronaut product for too long, Rob Roy won’t say precisely where he’s heading next, other than to suggest that he’s eager to combine the best of the past with the cream of modern technology. “The general trend at the moment is to reissue classic processor designs, but as a music lover and designer, that’s not very interesting to me. I’m interested in new circuit topologies, and new ways of interpreting existing designs. There’s plenty to be inspired by in the world of vacuum tubes, and I’m going to see if I can push the envelope further.”
Sound Advice: Differential and Common Mode DC Offset with APx
Usually an analog audio signal contains just an ac waveform with no dc component. There are, however, cases where a dc voltage is present, either intentionally (microphone bias) or unintentionally (excessive op-amp offset, failed blocking capacitor or power supply, etc.). The adjustable dc offset feature on APx Series analyzers (“Sine + offset” in the generator panel) allows you generate an ac sine wave with a dc offset, to measure the effects of dc offset on the audio quality of a device under test.
APx 2-channel balanced output normal cabling, for differential mode dc offset.
The ac audio signal on a balanced line is normally in differential mode, where the polarity of the voltage on the inverted phase (pin 3 or “lo” on an XLR connector) is opposite that of pin 2 (“hi”). However, when adding dc offset to a differential ac signal, the dc voltage can similarly be applied in differential mode, or alternatively in common mode (same dc polarity on pins 2 and 3).
Level and Gain measurement generator panel, with +5 Vdc offset added.
The dc offset added to the balanced output signal on APx instruments is differential mode. The polarity of the offset setting in the APx500 software corresponds to the polarity of pin 2.
There are cases though when it is desirable to generate a differential mode ac audio signal, but with a common mode dc component. This can be accomplished on APx analyzers by using a set of special cables that swap pin 3 on channel 1 with pin 3 on channel 2, and then inverting the dc offset voltage setting on channel 2.
APx 2-channel balanced output special cabling, for common mode dc offset.
Advanced Settings dialog, +5 Vdc set on channel 1, and –5 Vdc on channel 2.
Since pin 3 on both channels carries an identical ac signal, the special cable wiring has no effect on the sine wave test signal. However, as pin 3 on channel 2 now has a positive voltage, when combined with the positive voltage on pin 2 of channel 1, we get common mode dc.
In this configuration, you’ll have positive common mode dc on channel 1, and negative common mode dc on channel 2. If you are using an APx515 or APx525 family analyzer with two output channels and desire to have two channels with the same dc polarity, split one output of the special cable and leave the other output disconnected. With an APx585 or APx586 analyzer that has eight output channels, you can have up to four discrete outputs of each polarity before adding any splitters.
APx 2-channel balanced output normal cabling, CMTST configuration, for common mode dc offset.
If you want to test ac common mode rejection ratio (CMRR) while presenting a dc offset, do not use the APx500 CMRR measurement, as the dc voltage on pin 3 will get reversed mid-test. Instead, in signal path setup, set the Configuration to CMRR in the Output Settings box and use the Level and Gain measurement. Since the CMRR configuration puts the same signal on pins 2 and 3, dc offset will normally be common mode. To do differential mode dc, attach the special cable and set negative dc offset on one channel, as explained above.
APx 2-channel balanced output special cabling, CMTST configuration, for differential mode dc offset.
Test Results: AP News & Events
AP's Northeastern US sales rep Dan Foley will be giving a seminar on the Latest Techniques for Modern Audio Testing (including Bluetooth, HDMI, and Class D amp testing) at Boston College on Jan 10, 2012.
©2011 Audio Precision, Inc.