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1970s Design Indulgence

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 15 Dec 2018 at 4:55am
Bruce, as you know, there is no euphoric moment at switch-on which seems to wet the pants of some manufacturers...

The way I test everything is to tell myself it sounds garbage and only when I have to strongly disagree with myself is the point where the job's done.

I am completely sure that most who are into hi-fi judge a product by how "technical" it sounds. And from what I hear at hi-fi shows it all seems to be about how much thunder there is in the bass, and how bright it will go in the treble (using a handful of records that fit the bill).

The same people scorn tone controls, but many of them buy amplifiers which are tonally massaged. How do I know? Well I thought it would be obvious: the Queen's Award for Industry based on the sheer amount of goods exported outside Britain went to a manufacturer who refused to publish specifications; and when Stereophile discovered its extremely challenged high frequency response coupled with its weedy output they called it a stroke of genius!

I once turned down an install because the project leader insisted on the system having a steep cut-off at 8kHz! I do have quite a few musician type friends (John C being the nearest) who were quite shocked about that one.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 15 Dec 2018 at 5:01am
And here's how contrived it gets...

Originally posted by Graham Slee Graham Slee wrote:

...and when Stereophile discovered its extremely challenged high frequency response coupled with its weedy output they called it a stroke of genius!


But I remember Michael Fremer of the same magazine slamming Bose, saying "no highs, no lows, that's Bose".

It just goes to illustrate what I mean about the hi-fi press.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 15 Dec 2018 at 5:20am
Also I guess it all depends on how well connected you are, and seeing fans boost such people up on Wikipedia, you also find out the family lineage. I'm not suggesting for one moment that our great monarch or her parliament are corrupt, just that some are well-connected!!!

Now, as for me, being from up-north, the son of a coal mine worker, not even having a Cambridge or London Polytechnic education, so not belonging to the club, all I do is donate to Jimmy Wale's website...

More fool me.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Richardl60 Quote  Post ReplyReply Direct Link To This Post Posted: 15 Dec 2018 at 7:40am
Marketing Graham, never been a fan and responsible for many of the worlds consumer ills.

Bose appears to be held as a pinnacle product by some of our best friends too something I could never understand as far back as the 70s hifi shows in which it appeared an imposter.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Richardl60 Quote  Post ReplyReply Direct Link To This Post Posted: 15 Dec 2018 at 10:15am
Oh one other thing you forgot at shows other than bass and treble used to be HOW LOUD DO THEY GO...!  
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 17 Dec 2018 at 11:51am
The "Bax Diode" plus Power or Performance?

This title might look like a contradiction in terms, but in audio performance is measured by many factors in addition to that of power.

Generally more power results in more distortion, and that is reduced by additional circuitry to the point that the amplifier can only exist with global negative feedback. This therefore suggests that the amplifier isn't very linear without it.

The circuit for the power amplifier described here can exist without negative feedback, although it isn't advised, but it is a lot more linear without it than an amplifier which has to use complexities to achieve low distortion.

We have on offer 50 watts. This is accomplished using only two transistors for voltage gain, and four transistors for current gain.

It isn't as efficient as a modern amplifier circuit because the 57V p-p voltage swing required for 50 watts (into 8 ohms) requires 70 DC volts. If we consider the losses which are basically due to the Vbe's encountered in every amplifier we can estimate that we'd need 60 DC volts in a modern configured amplifier. There are transformer-rectifier losses too, but another 5 volts would see them off.

A 70V supply would easily see 60 watts being attainable, but only 50 watts here. This being due to the "elasticity" of the bootstrap pulling up, and the DC bias losses of the simple voltage amp pulling down.

Perhaps we could "sell" this amplifier using other means than specification? Unfortunately that requires the seller to be "well-connected" either by family lineage or his peers. Some sell that way.

But here we have to sell on what the product offers. Can we sell this old school 70s amplifier on nostalgia? Maybe, but if it led to jaded ears it might be short lived.

So we have to try and make this old design perform so well in actual listening terms that it becomes accepted.

I was aware from my listening tests that it had a couple of objectionable traits. 1. the upper treble appeared odd; and 2. the highs also sounded a little false. OK, that's suitably vague: odd and false! Whatever could I mean?

It would be easier for me to explain the cures.

By odd I mean displaced, as if disconnected from the rest of the music. Capacitors are a necessary "evil" in a simple design and the global NFB contains a 220uF electrolytic to block DC. At some point inductance replaces capacitance and the larger the capacitor the lower the frequency where that happens. I honestly thought 220uF would be OK. At 63 or 100  volt rating it should be low enough in dissipation factor to do well.

So what do you do with a suspect electrolytic? Bypass it of course! I have found the best type of bypass capacitor to use in NFB is polypropylene of the high pulse variety, and the largest one available here was 10nF, which starts to "take over" at around 3kHz and is of low enough impedance to fully shunt the electrolytic at 30kHz.

That left the "false highs" to deal with. This was obviously high frequency distortion, and I was proved to be right on this one.

As high fidelity matters less to the semiconductor industry - young folk listen to music on their phones - I predict expensive to make PNP power transistors will be phased out leaving us with no other choice than to revert to quasi-complimentary output stages.

Conveniently the 70s design uses a quasi-complimentary output stage, but inconveniently its crossover distortion, although being of short duration each cycle, does mean it affects the highs (not so much the lows).

We can tweak it as low as we can, but there is still that odd-harmonic (3rd harmonic) which manifests itself more than it should considering how low in level it is. Will removing it improve the sound? Read on.

It is the sharpness of turn-on of the complimentary feedback pair in the lower half of the output stage, which compared to the emitter follower pair of the upper half, makes a step we can't fully adjust out no matter how much or how little standing current we give it.

Shaw found an answer by placing a diode in series with the complimentary feedback pair, but it had to handle the full output current of the amplifier. This arrangement is OK at low power and is in-fact used in the ubiquitous NE5534 op-amp. Peter Baxendall found another answer by placing a transdiode in the emitter of the driver transistor (T5 here) shunted by some resistance. A transdiode is simply a transistor with collector tied to base and so a diode should be able to take its place. And this is what most do. The diode has only to handle the driver current so a 1A 1N400X diode will suffice.

I gave it a try and was able to tweak out the 3rd harmonic down into the noise floor. Hooray!

But it steals 0.6V from the available voltage which ups the THD+N at 50 watts. Boo!

It also has some adverse effect on distortion at different frequencies which are up and down as the distortion sweep scans.

It isn't the perfect answer but it helps greatly at normal listening frequencies by reducing the 3rd harmonic below objectionable levels, and makes it more akin to a valve amp's distortion.

In specifying output power, the proper way in doing so is to quote the power (both channels driven of course) into the rated load (8 ohms here) for distortion not exceeding 0.1% 20Hz - 20kHz, and at the specified supply voltage.

The 0.6V diode drop steals 0.6V from our output swing, so 50W is now 47W, and by rights that is what this amplifier should be specified to do. Even though it will do 54W at just under 0.1% THD+N at 1kHz, it cannot be rated as 54W because distortion rises above 0.1% at the frequency extremes. Neither can it at 50W with the diode in place.

But the diode removes the false sounding highs, so the question remains: power or performance?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote BackinBlack Quote  Post ReplyReply Direct Link To This Post Posted: 17 Dec 2018 at 2:42pm
3W short, not really significant is it? Why not specify the amp at 45W and make it the new magic output power standard. 
For me I would choose performance every time.

Ian


Just listen, if it sounds good to you, enjoy it.
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