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1970s Design Indulgence

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Richardl60 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Richardl60 Quote  Post ReplyReply Direct Link To This Post Posted: 13 Aug 2018 at 2:57pm
thanks not easy to see on my phone!
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Graham Slee View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 13 Aug 2018 at 6:31pm
Originally posted by Richardl60 Richardl60 wrote:

if I understand this correctly you are having a pre amp power supply and power amp power supply?

If so will you have twin mains feeds to avoid one interfering with the other?


That would be the case if it was split into preamp and power amp separates but in an integrated you try to cover all bases the best you can.

Take for example full power into 4 Ohms both channels driven continuously. This results in 5.3V of ripple (100Hz) on the collector of the BD139. In fact the 68 volts will drop slightly too. It would have to dip by as much as 10V or a little more including the 5.3 volt ripple to defeat the preamp voltage regulator. A 2 Ohm load per channel continuously might see a little ripple getting through, but with a real world musical source you'd be into serious clipping before then, and you'd simply not drive it that hard. However, at full output continuously into 2 Ohms the fuses are likely to blow.

In normal use, even hitting maximum power on transients, and even with awkward speakers, this isn't going to happen. Even into 4 Ohm speakers with impedance dips.

Propper routing of power supply component wiring is also important, and the take off points from the reservoir capacitors circled here is the correct method...




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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 14 Aug 2018 at 5:30am
This long running blog-by-diary article has I suppose become an amp design in public, and one that is starting to show little resemblance to a 70s design except for it having a single rail supply which necessitates output capacitors (so shameful?) and it also has a "singleton" input transistor in its power amp.

Here you may note the condescending use of the word "singleton"? I would agree. Actually the word is more usually somebody's surname. I don't use it but I know two people who do. I think it indicates a certain arrogance, don't you?

Unfortunately such arrogance exists in hi-fi amplifier design circles.

I know an author who mocks the single transistor amplifier input stage in such a way, whilst using the very same in his moving coil head amp. Perhaps I missed a trick? I know I'm not god's gift to anything, or as the Two Ronnie's sketch went: "I know my place"...

It is supposed to be guilty of causing distortion in the one, but not in the other. What amount of distortion do you think is audible? The golden ears think 0.01% is... whilst speakers produce considerably more... Yes you can hear 0.01% distortion through up to 10% distortion!? Wink

Moving on, and I remember 70s integrateds usually featured scratch and rumble filters. Do you remember them too?

I never found any improvement using them. Scratches remained scratches but the highs were dulled; and as for rumble, I never heard any from my BSR MP60...

I always thought them a waste of components, and that was in the 70s, and so I shall be leaving them out now.

The other thing was the loudness switch. Bass and treble was boosted whilst the mids were attenuated so that the loudness in "Phons" imitated the "Fletcher-Munson" curves.

You would turn the volume down low to prevent causing a disturbance (remember the days...?). But in increasing the bass by using the loudness control it made little difference seeing it's the bass that permeates through buildings!

Better to use headphones for late night listening. We know however that some headphones are rather fussy about their driving impedances, and getting back to distortion, as above, headphones being less distorted than speakers (erm, some are) will let you hear more of that "singleton" distortion you don't hear using speakers.

And then to get headphones to switch the speakers off, or doing it manually, needs switch contacts, which are expected to switch the heavy load of impedance dipping floor-standers, so the precious metal plating will soon arc away... does this increase the amp's distortion? Too right!

And this is the reason why we make separate headphone amps!

So, indulgence in 70s design it may be, but I have my limits.
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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 Aug 2018 at 7:13am
I often get asked if my products are compatible with one amp or another, and the answer is invariably yes, but I have to check which involves visiting the manufacturer's website and checking specifications.

I regularly find myself amazed by the sales methods such pages use, and the way descriptions are manipulated to make the customer believe they're looking at something vastly superior, and I know from what I read around the web, and from what customers say otherwise, that they are taken in.

The magic ingredient "class-A" is so often namedropped to describe a design that is so obviously class-AB, and it is also used in a way where if challenged by advertising standards they can show it's true!

How?

What they are actually describing is the voltage amplifier stage, which is always class-A whether they design it, I design it, or "the man in the moon" designs it.

There, it doesn't seem so special anymore does it?

There are two sections in a power amplifier (other than class-D etc)...

To drive a loudspeaker:

1. The voltage has to be made large enough.

2. The large voltage has to have sufficient current.

Voltage x Current = Power

The voltage amplifier section uses transistors in "voltage gain mode".

The current amplifier section uses transistors in "current gain mode".

The voltage amplifier has to have sufficient small current so the current amplifier can multiply it to produce power, and so it always operates in class-A.

The current amplifier has an upper transistor and a lower transistor: the upper transistor pulls the speaker voltage up, the lower transistor pulls the speaker voltage down. It is (wrongly) named push-pull (you can see it's pull-pull).

The upper and lower transistors are actually made up of two or three transistors: the first has some of the current gain, the second more, and so on.

This arrangement is called a Darlington (or super-beta). A Darlington can be two transistors in one package or two discrete transistors. Some power Darlingtons have a built-in emitter resistor which saves the designer having to work out a suitable value, so his work was done by the semiconductor manufacturer, not him.

The current amplifier is always biased-on to a greater or lesser degree. This makes it class-AB.

If it wasn't biased on it would be class-B and it would distort badly.

If a current amplifier is biased-on sufficiently that it draws as much current as would be needed to produce between one third to one watt in a loudspeaker, the manufacturer claims pure class A.

But it is still class-AB!

I hope this gives some insight into what manufacturers claim? I shall be using all my knowledge to make the design I'm working on here sound like it's just as ground-breaking as the salesmen do for theirs.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Richardl60 Quote  Post ReplyReply Direct Link To This Post Posted: 15 Aug 2018 at 1:32pm
I recall many of the Japanese manufacturers coming up with various descriptions of their latest config names in the late 70s; ultimately does it matter what the sales pitch says?   If it sounds good and they have used some new means of making it work great but if it sounds mediocre to awful it will sound the same whatever the sales pitch?

Does true Class A exist; I recall the original Krell KSA range being stars at pure class A when launched in the early 80s I think?  



Edited by Richardl60 - 15 Aug 2018 at 1:33pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Fatmangolf Quote  Post ReplyReply Direct Link To This Post Posted: 15 Aug 2018 at 9:04pm
Thanks again for the educational content. I'd never questioned the term push-pull, you are right that it must be pull-pull.



Edited by Fatmangolf - 15 Aug 2018 at 9:05pm
Jon

Open mind and ears, whilst owning GSP Genera, Accession, Elevator EXP, Solo ULDE, Proprius amps, Cusat50 cables, Lautus digital cable, Spatia cables and links, and a Majestic DAC.
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Graham Slee View Drop Down
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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 Aug 2018 at 5:51pm
The prototype power supply is now built and being made ready for testing. Once that's successful the prototype power amplifier board can be tested, and if OK, the design can move into its next phase.


Note that the tag posts are arranged so that each ground (and supply) is hard wired to its individual star point (star earthing).
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