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.


New 300B kit by Elekit (via diyaudio.com)

Note I have no affiliation with Elekit other than being an admirer of what they do for DIY audio hobbyists.

Elekit’s kits seem to hit attractive price points for what’s included (tubes, transformers, components, and a chassis) and the quality of the documentation. VK Music (Canadian importer of Elekit) just announced a new 300B amplifier on diyaudio.com here. Although details are sparse, we can maybe glean some ideas from the specs and the previous incarnation (Elekit TU-8300).

Specs for TU-8600:

• Tube Set : 12AU7 X 2 + 12AX7 X 1 + 300B X 2
• Now compatible with low to high impedance headphones
• Frequency response (-3dB) : 10Hz – 80kHz
• Max. output (THD 10%) : 8.3W + 8.3W (Input voltage : 250mV r.m.s)
• Residual noise : 42uV rms (IHF-A)
• Power consumption : 80W when no signal; 80W at max. output

The previous TU-8300 used MOSFET regulation for a B+ of 375V and because the new amp is rated for the same power, I think we can assume the new version operates at about the same voltage with a similar bias (-60V). Rather than two stages of 12AT7 on the input, the new design uses two 12AU7 and one 12AX7 triode per channel. So how are these arranged?

Let’s reverse engineer the numbers for an educated guess. We get full output of 8W from a -60V biased tube with an input of 250mVrms. If the 250mVrms (0.7V peak to peak) fully drives the output tube’s -60V bias (120V peak to peak), we have a gain of about 160x. That could be two successive stages of 12AU7, but then what is the 12AX7 doing? We appear to be using one half of the 12AX7 dual triode per channel. It would be an odd choice for a buffer stage, but it’s a plentiful tube and would make sense in that regard.

The other likely explanation is that we have some feedback at work and the 12AX7s are used for voltage gain. Maybe this is a grounded cathode 12AX7 into a 12AU7 SRPP. In terms of driving the Miller Capacitance of a 300B, this seems like a plausible arrangement. Gain would be in the 400-500x neighborhood, but feedback would knock this back down to the 160x overall and lower the output impedance.

Lastly, this amp includes a headphone output. Maybe the extra triodes are employed in some kind of follower specifically for the headphone section. A 12AU7 white cathode follower seems like a potential candidate. Whatever it is, I’ll be anxiously awaiting the manual and schematics to see what Mr. Fujita has come up with!

If you’re headed to the LA Audio Show (June 2nd-4th), the amp will be on display in the VK Music booth.

UPDATE 9/15/17: Here’s the first review I’ve seen, courtesy of Wall of Sound

Speaker sensitivity ratings and amplifier power

10 * log (power) = decibel

10 ^ (decibel / 10) = power

10db increase (10x the power) is perceived as twice as loud

Most desktop-size speakers are in the mid 80s db/W @ 1m sensitivity wise. We’ll call it 85db for the sake of calculating stuff. The sensitivity rating means that with one watt of power, you’ll get 85db of sound at one meter away. For reference, 80db is pretty loud. It’s about the level of a running garbage disposal or an alarm clock. You can listen at 85db for eight hours before you start risking hearing loss; this is also the sound level at which OSHA will fuck your shit up.

For nearfield listening, there may be less than a meter between you and the speakers. If you halve the distance, you can add 6db to the sensitivity rating. Now with the same speakers you’re getting 91db at half a meter with one watt of power. You should probably turn it down a touch to protect your hearing (2 hours at 91db is the maximum recommended duration). Every halving of the power deducts 3db, so one quarter of the power (0.25W) gets you back down 6db to a non-litigious 85db. If you want to listen at 80db (which is comfortably loud, believe me) you only need around 100mW.

Aren’t decibels fun?

This goes to say that you do not need a whole bunch of power for nearfield listening, even if the speakers have a low sensitivity rating. And if you have high sensitivity speakers in your “main rig”, a single-ended low wattage amplifier works there, too. Say you have speakers rated at 95 db/W @ 1m and like to listen around 80db. If you listen at one meter, you only need 32mW. If you listen at two meters, you need just 125mW of power.

The above discussion of power and decibels does not take into account dynamic headroom. It’s always good to have some power in reserve for music dynamics. Or for cranking it when OSHA isn’t paying attention. I try to have at least 10db to spare (10x the power) over what I expect my average listening levels to be. If you didn’t fall asleep while I fapped around with decibels and logarithmic math, you noticed that average, safe listening levels (80-85db) need only a fraction of a watt with average sensitivity speakers nearfield or high sensitivity speakers at a regular distance.  Ten times more power is just a couple watts and will often get you pretty comfortable listening levels with headroom to spare.

Just make them some high quality watts.

Back at it finally!  This is an excerpt from the first speaker amp write-up I’m doing for the site.  Happy Cinco de Mayo!

Something for beginners

Pete Millett’s Starving Student was one of the first amps I ever built completely from scratch. Unfortunately, the 19J6 tubes have become rare (or at least no longer dirt cheap) due to all the bright eyed DIYers scooping them up to build amps. I think the world needs another <50V tube amp for beginners, so I’m designing one. Like the original, it’s an oddball tube with a MOSFET buffer and an off-the-shelf power brick (same brick, in fact).

Millett is one of my personal tube heroes. This is a tribute.  Full write up coming soon (and parts values subject to change once tested).


I love you litte Weller choo choo


Sometimes people ask about how much money they need to invest to start playing around with DIY tubes and audio. The answer is probably less that you think.

I have been using the puny 23W Weller pencil iron for several years. You can buy this thing at literally every hardware store on the face of the earth. The tip is replaceable. Sometimes the little iron struggles with larger buss bars or binding posts, but its never given up. It’s my little orange choo choo iron.

Why am I thinking about this now?  I bought a 45W iron recently and immediately found it to be almost too hot for small sockets and parts. Little choo choo is not going to the tool box graveyard yet.

Why DIY?


If someone told me they hiked to the summit of Mt Everest, I’d think they’re pretty amazing. If they told me that their first thought when they got to the top was how much money they spent to get there, I’d probably think they’re not quite so amazing. If they told me they actually took a helicopter to the top instead because it was cheaper, I’d be sure that they’re an idiot.

DIY requires an investment in tools and parts, but more importantly, it requires an investment in time and patience. Your first project is not going to be the Mt Everest of projects, just like a hiker’s first backpacking trip shouldn’t be up the side of the Himalayas.  Like every hobby, excellence takes patience and practice.

It’s very tempting to compare the cost of building something yourself to the cost of buying something that’s commercially made. As you’re starting out in DIY, you won’t win this battle. The more you practice, the more you’ll refine your finish and design theory, and the more of a return you’ll see on the time you’ve invested. But price-points still do not capture what DIY is about.

Craftsmanship is what you pay for with high-end, high-dollar, boutique design. The craftsman that wields it didn’t buy his or her experience with money. He or she built experience and knowledge by making mistakes and a lot of crappy products first. When you buy commercial, you are paying for the mistakes it took for the craftsman to become exceptional.  DIY is about having the courage to overcome those mistakes yourself instead.

P.S. Yes, I know helicopters don’t fly to the top of Mt Everest.