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Old 29th June 2015, 10:57 PM   #1
nzlowie is offline nzlowie  Australia
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Default Simplicity or complexity?

I’ve decided to try a valve amp in my system, I currently have a Peachtree Audio Inova driving some beautiful speakers designed by Troels Gravesen. Having built a few amps and speakers over the years I’ve decided to go with a Dynaco ST-70, and here is the question…

The ST-70 is a very simple design and I believe in the theory that the less we put into the signal path the better, how true is this really? Amplification is simply making the audio voltage larger…. The response of our hearing is a long way from flat so why introduce a whole lot of complexity into the signal path to make perfectly flat frequency response?

Would love your thoughts on this please, also any opinions on the Dynac ST-70. Sometimes (most of the time) the written audio reviews are just a whole lot of dribble written by people who obviously get paid by the amount of words written.

Many thanks. Dave
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Old 30th June 2015, 02:29 AM   #2
Keit is offline Keit  Australia
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Encyclopeadia-sized books could be written on this, and have indeed been written.... And it isn't really about frequency response, it's about non-linear distortion. Getting a good frequency response in tube amplifiers is mostly about spending money on the output transformer and being sensible about coupling capacitors.

While the ear is certainly anything but flat, you need an amp reasonably flat for the same reason you need accurate colour reproduction in photography (the frequency response (colour sensitivity) or the eye is not much better than the ear) - so the reproduction is realistic. But it isn't critical, and getting it flat is easy anyway.


While having as few parts as possible in the signal path seems common sense, it is not anywhere as simple as that. Here's just a few points:-

1) A single vacuum tube (or transistor) generates mostly even order distortion. This distortion is not wholey unpleaseant to the ear but it does muddly the sound, make it difficult to identify individual instruments, and makes listening to music tiring.

This even order distortion can be largely cancelled out put using a second tube as an active load (ie instead on just using a load resistor).

However, an active load can, if you aren't carefull, introduce problems of its own - eg hum.


2. A pair of transistors used in cascade can be arranged to cancel out their individual distortions, by arranging their emitter currents in proportion to signal level - as the second one gets an inverted signal. For decades I used to think that this could not be done with vacuum tubes. But I learnt on diyAudio (one should learn one new thing every day if you keep your mind open) that indeed it can be done - by introducing a bit of resistive attenuation between the two stages, in conjunction with carefull tube selection, biasing, and anode load values.

3) An amplifier with three stages must alsys distort more than one with one or two stages - as each stage contributes. Actually, not necessarily. With three stages you have a lot more gain ad so you can make negative feedback a lot more effective, so distortion ends up less.

4) Push pull output and long tail pair operation offers cancellation of distortion.


5) Even with identical circuit configuration and tubes, one amplifier can clearly sound much better than another. That is, if some factors on how to choose component values were not understood by the designer of one amplifier (or the other got it right by chance/luck/experimentaion). Some of these factors are very subtle - eg the choice of resistors in series with grids. But theses subtle factors can add up, and sooner or later a badly designed amp will make it's defects known on particular music passages.


6) With more complexity, it becomes possible to work individual tubes less hard - good for tube life and good for sound quality.

Over time, a few amplifiers have become respected for impeccable performance. Generally, these are also the more complex - eg the General Electric "88-50" (a modified version of the Williamson.

Importantly, you should note that, for any given number of tubes, there are a limitted number of "sweet-spot" circuits that give the best possible performance for that number of tubes. For example, while the GEC 88-50 gives about the best that 6 tubes can do, the Quad II is one circuit in which 4 tubes can do really well. But a Quad II is not quite as good as the more expensive GEC 88-50. And since that best possible performance improves as the number of tubes (ie the sweet spot circuit complexity) increases, the real decision to make is this: "How much time or money do I want to spend?"

You get what you pay for. What you pay for in parts, what you pay for in design time, what you pay for in expertise.

Last edited by Keit; 30th June 2015 at 02:57 AM.
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Old 30th June 2015, 02:50 AM   #3
rayma is offline rayma  United States
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Quote:
Originally Posted by nzlowie View Post
I’ve decided to try a valve amp in my system, I’ve decided to go with a Dynaco ST-70.
Good choice, but make sure that your speakers will be happy with it. Can you borrow a tube amp to try?
What kind of driver stage do you plan to use?
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Old 30th June 2015, 03:05 AM   #4
Keit is offline Keit  Australia
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I should also point out that, while a more complex amplifier should, and ought, to sound better than a simple one, it is quite possible for a simple amplifier designed by a master to sound better than a complex amplifier designed by a fool.
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Old 30th June 2015, 03:19 AM   #5
nzlowie is offline nzlowie  Australia
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Thanks guys
So the Dynaco is a good amp then? As for the source it's all .wav files and I'm looking at DAC options now. Have also posted in the DAC forum, looking at a DDDAC 1794.

I've built a bit of audio gear over the years, is it still an option in the modern digital world?

I love the satisfaction of building gear and can't justify spending $3-5K per component...

Cheers
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Old 30th June 2015, 07:40 AM   #6
Keit is offline Keit  Australia
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Quote:
Originally Posted by nzlowie View Post
I've built a bit of audio gear over the years, is it still an option in the modern digital world?

I love the satisfaction of building gear and can't justify spending $3-5K per component...

Cheers
If you look at it strictly objectively, well, up to the late 1970's, building your own amplifier, if you had the skills, was easily justified - it was not that difficult to build something that was better in performance than than what you could buy in a hi-fi shop for the same money. The cost of parts back then to build an amp matching the famous British brands was a LOT less than the cost of the factory products.

That advantage has long gone. There's a lot of crap on the market, but but the good stuff can now be purchased for much less than the parts cost.

If you look at it strictly objectively, well, solid state in the hands of competent engineers gives impeccable performance that is not easy to duplicate with tubes. And solid state keeps on working without any change in sound quality for years and years.... Yous can buy low cost integrated circuits which, with very few external parts, offer perfomance that top discrete-circuit engineers only dreamt about 30 years ago.

But I still build with tubes. I also do a lot of restoration work on gear made in the 1950's. I like using crafsman skills in my workshop. Like you I find it it satisfying. Very satisfying in fact. And a tube amplifier attracts interest and appreciation from visiting friends that a solid state job never does.

Building good tube amplifiers is not a cheap hobby. If your priority is saving money, go to a discount store and buy something factory made. But what really satisfying hobby is cheap? My wife paints in acrylics. She spends thousands of dollars on materials etc for that.

Satisfaction, and the challenge, why we do it. That's why we have diyAudio.

Last edited by Keit; 30th June 2015 at 07:54 AM.
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Old 30th June 2015, 09:28 AM   #7
DF96 is offline DF96  England
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Quote:
Originally Posted by nzlowie
The ST-70 is a very simple design and I believe in the theory that the less we put into the signal path the better, how true is this really?
First you have to decide what you mean by signal path. Most people who use this term don't understand it. For example, some of them claim to remove capacitors from the signal path by using a DC servo instead. They don't realise that the capacitors setting the DC servo time constant are very much in the signal path, but now the signal path is much more complex than it was.

Good designs are often deceptively simple. They look simple at a superficial level, but a careful analysis (i.e. reverse engineering) will show that everything has been carefully designed to fit together. This is far from simple. People who don't realise this may believe that the design sounds good because it is simple; in reality it sounds good because it is well designed.

Two ways to achieve poor sound (while remaining blissfully convinced that the sound is good):
1. oversimplify - minimise the number of components "in the signal path", in the false belief that as everything distorts then less must be more
2. overcomplicate - solve all known audio problems by adding a new sub-circuit for each problem, use four valves where one would do, add lots of solid-state bias and load circuits, replace all voltage amps by SRPP, replace all cathode followers with White CF, use DC coupling and then have complex level-shifting circuits etc.

Quote:
Amplification is simply making the audio voltage larger…
True, but you would be surprised how many people believe something different from this.

Quote:
The response of our hearing is a long way from flat so why introduce a whole lot of complexity into the signal path to make perfectly flat frequency response?
Getting a reasonably flat frequency response does not require complexity. The reason we want (well, some of us want!) a flat frequency response from our audio system is that the air (the medium which comes between us and the performers at a concert) has a reasonably flat frequency response. Those of us who want to reproduce sound require a flat frequency response; others have other aims, but sometimes don't realise or admit this.
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Old 30th June 2015, 09:35 AM   #8
nzlowie is offline nzlowie  Australia
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So... does the ST-70 fit the category
"it sounds good because it is simple; or it sounds good because it is well designed or is it not good after all?
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Old 30th June 2015, 12:56 PM   #9
Keit is offline Keit  Australia
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Quote:
Originally Posted by DF96 View Post
.... overcomplicate - solve all known audio problems by adding a new sub-circuit for each problem, use four valves where one would do, add lots of solid-state bias and load circuits, replace all voltage amps by SRPP, replace all cathode followers with White CF, use DC coupling and then have complex level-shifting circuits etc.
Very true. Also true that some folk add complexity to solve problems that were never an issue or never even existed at all in the first place.
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Old 30th June 2015, 01:34 PM   #10
GoatGuy is offline GoatGuy  United States
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The tension is always between perfection and practicality. You with the ST–70 are choosing conventionally and wisely. Practical and reasonably perfected. Pentode section on the 7199 as voltage amplifier. Triode section as phase inverter. Push-pull final. Ultralinear tap on transformer for screen grid drive. 82 pF in series with 13 kΩ for oscillation / stability between stage 1 and phase inverter. All very simple.

Sounds very conventional?

Consider: the use of a small-signal pentode is by definition equivalent to the more-complicated-on-a-schematic cascode stage. (I'm going to get some flak here, but the analogy is true.) The mechanically shaded screen grid decouples the dynamics of amplification from influencing the control-grid's acceleration gradient. Just like cascode's 'upper stage' decouples an amplification triode's plate swing influence on amplification (and plate-grid capacitance). Etc.

So, in theory, you could substitute other topologies to effect similar ends. But why? The stage 1 pentode will work well, not being asked to deal with too large a voltage swing from the incoming signal.

The triode phase inverter is about as conventional as one might imagine. Because the triode is driven with such a high signal, we cannot expect brilliant linearity out of it. Makes for consideration but not concern to me. It'll work, and well. After all, any 2nd or even harmonics it adds might be viewed as “warmth”. As DF96 sez… 'others have different aims'.

Perfection? Well, in the end, it is not. But it is minimalist, and it has been engineered for reliable operation, practical affordability. As KEIT mentions, so much of a modest amplifier's performance depends on the quality of the output transformer, and investing prudently in the coupling capacitors. Nothing crazy, nothing crâp. After all, there really are only 2 ea. 0.1μF capacitors in the signal path. (The tiny ones are all high-frequency suppressors, and apart from being tiny (which implies HIGHLY linear), aren't even dealing with the critical sub–3500 Hz audio band.)

You could have a bit of fun experimentation swapping out one channels' worth of these for some other 0.1μF cap; then a little A-B switching might give your ears a bit of a learning experience.

Anyway, again: good choice. You will probably want to invest in an equally simple, modest and reliable preamplifier "and knobs" set. You know: knobs for switching sources. Knobs to adjust volume, tone contours. Linearity contours. Tiny blue LEDs. Glitz!

GoatGuy
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Five things matter in audio · LED underlit tubes, gold connectors bearing bright plastic ID bands, speaker cable thicker than your thumb, a hundred pounds of granite under the turntable, and those little spikey-things under the speakers.

Last edited by GoatGuy; 30th June 2015 at 01:36 PM.
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