What makes a good valve amplifier?

[Gabe001]

So today I connected my Olson inspired amp and the Mullard 5-10 as monoblocks, one to each speaker, just to see how different they sound. This created a ground loop, but I put up with it just for the sake of the experiment.

We're all familiar with the Millard 5-10 performance. Mine has the ef86 running as a triode with a DC supply and is effectively hum free, and pretty much performs to spec.

My modified Olson has a 6v6 ultralinear output stage, with a 6c5 and 1/2 a 6sn7 in cascade followed by a concertina phase splitter. I gave it 6db of global negative feedback. THD at 1w is 0.067% and THD + noise is 0.096%. This one is also effectively hum free, with a flat frequency response curve (same as the Millard)

The circuit designs couldn't be any more different, the valves are generations apart and yet the two amps sound completely identical.

In simple objective terms, are valve amps meant to sound the same once you remove most of the harmonics? What makes a valve amp good, or bad?

Thanks
Gabriel

[paulsherwin]

Good designs built with high quality components (particularly transformers) should sound pretty much the same.

[Ted Kendall]

Originally Posted by Gabe001

What makes a valve amp good, or bad?l

The same things that make any amplifier good - adequate power output, negligible distortion and good steady state and transient response over the required bandwidth. Unless, of course, you're dealing with an amplifier as part of a musical instrument, be it Ondes Martinot or electric guitar, where what would otherwise be classed as faults form part of the instrument's character.

[Hartley118]

Originally Posted by Gabe001

So today I connected my Olson inspired amp and the Mullard 5-10 as monoblocks, one to each speaker, just to see how different they sound. This created a ground loop, but I put up with it just for the sake of the experiment.

We're all familiar with the Millard 5-10 performance……..
.

In simple objective terms, are valve amps meant to sound the same once you remove most of the harmonics? What makes a valve amp good, or bad?

Thanks
Gabriel

Regarding ‘removing the harmonics’, they are just a way of measuring non-linearity. We don’t generally hear harmonics themselves very clearly because they tend to be masked by their related fundamental signal itself. What we do hear more clearly from non-linearity is intermodulation between signals of different frequencies because the intermodulation products are less likely to be masked by the original signals and stick out like a sore thumb.

Compared with other elements in the signal chain, amplifiers are normally paragons of virtue, so long as they are run well within their rated power output. Once you run into overload, all bets are off, different rules apply, negative feedback fails, and often a fairly crude valve amplifier with little feedback may well sound better than a more sophisticated design.

Martin

[Gabe001]

So can you measure intermodulation products or is it just a question of listening?

[Radio Wrangler]

Intermodulation products are not difficult to measure. You need a pair of decent audio signal generators, a combiner (resistor network) a dummy load and a good audio spectrum analyser. A fair amount of equipment, but available on the surplus market.

The advantage with the spectrum analyser is that you can see the different intermodulation orders and their strengths. By ear you get that something isn't right as it starts to get significant, but it is very difficult to decide just what isn't right.

Any amplifier with non-linearity will produce both harmonic and intermodulation distortion.

So what about guitar amps with loads of distortion? Yep. you would get both, except that there is oonly a single instrument playing. You'll get intermods between the components of chords, but that's part of what chords do. Distortion as part of a single instrument is OK if that's what it takes.

Where things go wrong is that hifi is much more critical, any distortion here will be between different instruments playing different parts and it makes a mess... an audible mess.

I gave my brother (guitarist) a distortion meter for Christmas one year - so he could make sure he was getting enough!

THe amplifier in my living room when run 1dB below clipping has all harmonics more than 100dB down on the fundamental. When driven by a 2-tone intermod test signal, the intermod products are all better than 100dB below the tone powers. I had to build some special test gear to measure this. The level of performance is utterly unnecessary, but I was having fun and it's difficult stopping once you get going

David

[Hartley118]

Originally Posted by Gabe001

So can you measure intermodulation products or is it just a question of listening?

Yes - by injecting two test sinewaves and looking for the sum and difference frequencies with a spectrum analyzer. But it’s generally not worth the trouble because it’s really just another way of measuring non-linearity, so that total harmonic distortion (THD) is now the standard method.

Where THD can be misleading is in an amplifier/audio chain of limited bandwidth because the higher frequency harmonic distortion products don’t get through to the output. This used to be a problem in measuring movie sound systems which used to use steep cut HF filters to reduce noise, but which therefore gave misleading (understated) harmonic distortion measurements. That led to intermodulation distortion being adopted as the standard non-linearity measurement in that industry.

Martin

[radio69]

ISTR in one of Doug Self's books; he states something along the lines of - THD is the measurement to take on an amp, and that IMD doesn't provide any additional info.
I'll have a look through my Self books, and see if I can find the quote.

To many valve-amp purists; feedback of any kind is the work of the devil !!

[radio69]

Originally Posted by Radio Wrangler

I gave my brother (guitarist) a distortion meter for Christmas one year - so he could make sure he was getting enough!

Crying with laughter!

[GrimJosef]

The modern definition of what makes an objectively 'good' amplifier was discussed by DTN Williamson in 1947 and expanded upon by Williamson and PJ Walker in their article Amplifiers and Superlatives, Wireless World, vol 58 no 9, pp357-361, Sept 1952. You can read it here https://worldradiohistory.com/UK/Wir...ld-1952-09.pdf. Amplifier capabilities have improved since then, but the basics still apply.

What makes a subjectively good amplifier varies from person to person. Some people genuinely like the sound of, for example, second harmonic distortion.

Cheers,

GJ

[Radio Wrangler]

Originally Posted by radio69

To many valve-amp purists; feedback of any kind is the work of the devil !!

Which is why Harold Leak and Peter Walker etc. bust guts reducing the high-frequency phase lag of their amplifiers so that they could employ more of it.

Someone once called the signal after the implicit subtraction process of a negative feedback system "The error signal"

Mathematically-challenged audiophiles latched onto this and said that all a feedback amplifier could produce was "amplified errors" and the rest is history.

If it had been called "The difference signal" it would have seemed less pejorative, and the case of a low distortion, finite-gain basic amplifier could be seen to have an error insomuch as falling slightly short of the gain target set by the feedback network, but simultaneously achieving a reduction in distortion, a reduction in output impedance and an improvement in flatness and bandwidth.

Measured by weight, there is an awful lot more psycho than acoustics in psychoacoustics.

There are sane, mathematically-competent audiophiles, but at the phoolish end of the spectrum they can only handle words and are suckers for honeyed adjectives and carefully-cast aspersions. They got a lot of mileage out of that use of 'error'.

If audiophools didn't exist, the engineers amongst us would have invented them, just for a laugh.

The anti-feedback business is hilarious. Feedback action is implicit in many devices and in circuits lacking overt overall loops. The amplifiers supposedly without it, actually have feedback processes acting in many places, but quite invisibly to people with limited understanding. Some of these feedback processes are undesirable, some desirable. Understanding is the key.

An audiophile-educating version of Pygmalion? "The gain in Spain falls mainly..."

David

[Gabe001]

Originally Posted by GrimJosef

The modern definition of what makes an objectively 'good' amplifier was discussed by DTN Williamson in 1947 and expanded upon by Williamson and PJ Walker in their article Amplifiers and Superlatives, Wireless World, vol 58 no 9, pp357-361, Sept 1952. You can read it here https://worldradiohistory.com/UK/Wir...ld-1952-09.pdf. Amplifier capabilities have improved since then, but the basics still apply.

What makes a subjectively good amplifier varies from person to person. Some people genuinely like the sound of, for example, second harmonic distortion.

Cheers,

GJ

That's a great article GJ. I'll need to read it a few more times to understand all of it, but it's great.

[Diabolical Artificer]

In my limited experience the output transformers seem to influence how good a valve amp is, that and a over spec'd power supply, by that I mean one capable of delivering current on big transient's. Inna nutshell good iron makes a good valve amp.

I can hear a difference between various amps I've built, though the difference is subtle and hard to define. The three I had recently to compare were a 70w Class A PP amp using 4 807's and a pair of Joe Bogatek's OPT's, a class AB2 2C34 triode PP amp using off the shelf OPT's and a little 3W 6V6 SE jobbie that used signal genny OPT's, all quite different in class, size and design but they sound very similar.

Andy.

[cathoderay57]

Hi Gabriel, from a pragmatic point of view, and on account of most peoples' hearing ability, especially those of us with advancing years and possibly having worked in noisy environments, a listening test is good enough. Detailed measurements are fine if you want to do it as a technical exercise, but for those like me it's academic. The subjective test for me is, does the stereo image sound right? That is largely a matter of getting the gain of the 2 amplifiers close and positioning the speakers to suit the acoustic characteristics of the room (that can make a significant impact on how the bass is heard) and, of course, both speakers must be in phase. The frequency response will of course have an effect, depending how much your hearing has deteriorated, similarly to when a failed tweeter is detectable when all the top end appears in one channel. I regularly use one Quad II and one 5-10 both driven from a Quad 22 control unit and, once the input levels have been adjusted on the 5-10, it being more sensitive than the Quad II, there is (to me) no audible difference in sound quality. You could always blast out some heavy metal and then you'll never know the difference anyway Cheers, Jerry

[rontech]

In my opinion it is difficult beat the later Radford 25 watt amplifiers. The design which used a triode / pentode phase splitter.

[thermionic]

My answer to your question, Gabe, would be, ‘your ears’!

If your amplifier sounds good to you, and you enjoy listening to it then why would it even be necessary to measure it’s performance? Say you did, and found that the THD came out at 1% or more - higher than you expected. Would your mind subconsciously then start picking out the faults in its reproduction? Would your listening enjoyment then be muted?

As a kid, I can remember reading in an early Maplin catalogue their description of the Maplin 50 Watt Hi Fi amplifier kit. It went something like this......’We threw away all of our technical design handbooks and built an Amp that sounded musically perfect. When we finished, we discovered we had a pretty impressive technical spec too...’

Marketing gobbledegook, maybe, but it stuck with me!

Keep experimenting and building your amps.

SimonT.

[Heatercathodeshort]

Originally Posted by thermionic

My answer to your question, Gabe, would be, ‘your ears’! If your amplifier sounds good to you, and you enjoy listening to it then why would it even be necessary to measure it’s performance? SimonT.

Throw away the measuring instruments and use your ears!

Due to my interest in record reproduction, mainly 78s, I have built many amplifiers over the years. I have one that is used regularly employing 807s in the output and a decent Williamson style output transformer. It has evolved over 60 years!

Another a commercial DICTOGRAPH has a pair of 6BW6s [B9A 6V6GT] in the output and both sound absolutely fabulous! I usually use a 10" Celestion in quite a small cabinet that reproduces excellent bass. Keep it fun. John.

[ms660]

Originally Posted by Heatercathodeshort

Throw away the measuring instruments and use your ears!

Ditto...No BS/Waffle required.

Lawrence.

[Peter.N.]

We used to sell Quad and Leak in the '50s and purely by ear I have not heard anything better especially with Lowther speakers.

Peter

[Gabe001]

Thanks for the comments. This has made quite an interesting read, and great to hear your experiences.

Would I expect a solid state amp to sound very much like my 5-10 as well?

Gabriel