Firebottles And Groove Tubes Versus Devices That Find Their Origins in Sand

[Joe Gwinn]

On the eternal debate on tube versus solid-state amps, the following two-part article may be of interest. Electronic Design is a trade rag for EEs.

“House of Fire”: Firebottles And Groove Tubes Versus Devices That Find Their Origins in Sand (Part 1)
Why do Tube Amps and Solid-State Amps Sound Different?

http://electronicdesign.com/article/...n-Sand-Part-1-

If the ads disturb you, hit the "print" button to get a clean screen, ready for printing.

[Steve Conner]

Good article, but I do take issue with one statement:

This brings us to the statement, “If we can hear it, we can measure it!” I wholeheartedly agree. The trouble is that if our head has 130-dB dynamic range and can discriminate phase at or near phase-locked loop (PLL) levels, that 90-dB notch filter-type distortion analyzer isn’t even close.

To me, this says that the author doesn't understand the principles behind psychoacoustic masking. He's probably never tried getting a PLL to track phase really accurately, either

I found that getting a distortion analyser, even an old second-hand one, was a huge help in my hi-fi amp building endeavours. The trick is to test the amp at 10 or 20kHz. This is the worst-case scenario where the amount of feedback available for error correction is at a minimum, and it reveals all sorts of unpleasant things.

I think these are the reason why manufacturers only publish the 1kHz figure. It's easy to minimise this while still having lots of distortion at high frequencies, and an amp with a crappy sounding high end. (The classic "transistor sound".) But conversely, if you can get good performance at the top of the audio band, it'll only get better at lower frequencies.

I believe this is the reason why THD measurements on hi-fi amps fell out of favour. If they had published the 10 or 20kHz figures instead, they would have correlated better with subjective sound quality and might have been taken seriously.

It may also be the reason for negative feedback phobia. NFB wasn't the problem with transistor amps, it was shitty circuits with dreadful open-loop linearity, patched up with NFB. The low-feedback designs forced designers to get their open-loop behaviour sorted out. Douglas Self's approach (which I'm a follower of) is to take a good circuit like this, and then use NFB to make it even cleaner.

For guitar amps the distortion analyser is of course useless. It's a one-dimensional scientific approach to a complex non-linear organic system. You're not trying to get rid of distortion, you're tuning it to your taste over the whole dynamic range of the amp. I don't know of any test equipment that can help you, beyond plugging in a guitar and playing it.

Tube hi-fi gets confusing, because you can get amplifiers representative of both schools of design, and they both claim to be "accurate".

[Mike Sulzer]

... if you can get good performance at the top of the audio band, it'll only get better at lower frequencies.

Exactly. For example, in systems using three pairs of amps, low, mid and high, the solid state amps exceeded the quality of the tube amps over a period of several decaes in precisely that order . My VT 52 amp with fully regulated power supplies and a good quality op amp for a first stage hung in there as a tweeter amp for quite a while, but even it went in the 90s. Now the fancy switching things can provide a very transparent sound and essentially infinite power (and they are still evolving, I believe).

[salvarsan]

Thanks, Joe.

Honest article: very early, the author states:
The probability that this article will shatter all myths and end the discussion once and for all is absolutely nil. As creatures of habit, we like what we like. That part is subjective.

[Joe Gwinn]

To me, this says that the author doesn't understand the principles behind psychoacoustic masking. He's probably never tried getting a PLL to track phase really accurately, either.

What I found most interesting and useful were the experiments he made. His theories on what these experiments show may or may not be correct, but the test results are what they are.

I found that getting a distortion analyzer, even an old second-hand one, was a huge help in my hi-fi amp building endeavours. The trick is to test the amp at 10 or 20kHz. This is the worst-case scenario where the amount of feedback available for error correction is at a minimum, and it reveals all sorts of unpleasant things.

I think these are the reason why manufacturers only publish the 1kHz figure. It's easy to minimise this while still having lots of distortion at high frequencies, and an amp with a crappy sounding high end. (The classic "transistor sound".) But conversely, if you can get good performance at the top of the audio band, it'll only get better at lower frequencies.

I believe this is the reason why THD measurements on hi-fi amps fell out of favour. If they had published the 10 or 20kHz figures instead, they would have correlated better with subjective sound quality and might have been taken seriously.

My recollection is that 1 KHz was chosen in the 1930s, when hi-fi was being worked out in the age of tubes and spinning vinyl, and the best they could do was a few percent at 1 KHz, and even 10 KHz was a reach. So, for choosing gear, THD was a pretty good indicator, or rather was as good as any that could be tested easily. The details may be found in The Radiotron Designers Handbook, which I grew up with.

When semiconductors came along, it was suddenly easy to achieve essentially negligible THD at 1 KHz, so THD lost any power to tell good from bad.

It may also be the reason for negative feedback phobia. NFB wasn't the problem with transistor amps, it was shitty circuits with dreadful open-loop linearity, patched up with NFB. The low-feedback designs forced designers to get their open-loop behaviour sorted out. Douglas Self's approach (which I'm a follower of) is to take a good circuit like this, and then use NFB to make it even cleaner.

This sounds correct to me.

For guitar amps the distortion analyzer is of course useless. It's a one-dimensional scientific approach to a complex non-linear organic system. You're not trying to get rid of distortion, you're tuning it to your taste over the whole dynamic range of the amp. I don't know of any test equipment that can help you, beyond plugging in a guitar and playing it.

Distortion analyzers are not useless for guitar amps. Such instruments are good for annoying musicians.

Tube hi-fi gets confusing, because you can get amplifiers representative of both schools of design, and they both claim to be "accurate".

Pickups are of course nothing like this.