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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: 23 Sep 2018 at 9:54am
Back then (early 1980s) I knew some of it to a greater or lesser degree, whatever that was, but I wasn't sure of myself sufficiently to put my head above the parapet, and so I designed printed circuit layouts for others, then branched out into making prototype PCBs, and so on.

In doing so it allowed me to see into the minds of other engineers, their different techniques and (surprisingly to me at first) their limitations. I was in the company of other engineers, all on different rungs of the ladder (working for big organisations as well as small businesses), as well as some well-off wannabes, and being my own boss I wasn't restricted to the length of any conversation.

Although that episode didn't end all that well financially, the knowledge gained was priceless. It gave me the confidence and ability to design many of the broadcast audio circuits during my stint as a YRN senior engineer, without my earlier fears.

That was around the time that circuits such as the above were being published, and I started drawing level with what the author's were explaining. But to some degree I was also aware of what the author's were not explaining, the "disguised servo" for example, which I think, from memory, was lacking in the article.

This is not a criticism. It isn't easy writing technically for the consumption of electronics magazine readers. In fact it is quite daunting when you realise the breadth of understanding of the readership - some extremely knowledgeable - some outright beginners, and it all has to be worded so that all can grasp what it's about. In fact many are taken to task by the readers: JLH was no exception, and neither was I when my one and only (very amateurish) magazine article was published in Everyday Electronics (Jan 1980).

What this is all about is learning and understanding (or at least trying), for all of us, me included. Somebody once said "the guy who knows everything hasn't been born yet". Something we ought to realise, especially in "Hi-Fi".

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 24 Sep 2018 at 6:45am
Disc Preamp

I already spent some time a few pages back talking about the phono stage but there are two things bothering me.

1. insufficient open-loop gain
2. one sided RIAA network drive

Insufficient open-loop gain was a problem in early silicon transistor phono stages, and nearly all manufacturers around that time used a variation of the shunt-series pair (as I call it) described previously. But even with my extra transistors assisting, I have my doubts as to it being sufficient.

The active negative feedback RIAA equalisation is obviously capacitive and has to be driven. The capacitors actually charge and discharge and there are doubts whether the emitter follower is doing it symmetrically. The transistor can pull the signal up toward the positive rail, but we've learnt that a resistor cannot pull the signal down to the negative rail.

It is like a spring pulling a spring. At low frequency the impedance of the RIAA network is high, so a small value resistor will be able to pull the signal down to the negative rail.

But at high frequencies the impedance is much lower and approaching the same value. And like two identical springs in series, one cannot be stiff whilst the other is stretched.

A push-pull (pull-pull) output stage would solve that problem, but unlike the power amplifier its job is to apply exactly the same output slew rate on each half cycle - the same "speed of charge" on both positive and negative excursions - suggesting we need complimentary devices.

For voltage gain I recall a circuit from Elektor summer circuits July-August 1978, at the height of interest in audio circuits. I have kept as pristinely as possible this issue for the inspiration value it has. Article number 75 on page 71 proposes a shunt-series pair with an additional two transistors.

The first adds to the input transistor to make a two transistor cascode (stacked vertically) stage. It effectively ignores the first transistor's emitter resistance, or so we are told, and therefore the input stage has the maximum gain possible. This calculates as being around 600 instead of around 250, which shows it isn't that much higher, but it is just over 6dB so might be useful.

The next stage is basically the same as my earlier suggested circuit, except that Elektor had the habit of using a PNP as emitter follower "buffer" in the third transistor's collector load. It works in much the same way, and the bootstrap works without a bootstrap capacitor, which might be a good thing.

However, neither emitter follower fulfils our second wish list requirement of symmetrical RIAA network drive.

Even after 40 years I am reluctant to publish the Elektor circuit here. I having a commercial interest could be breaking their copyright, whereas other members should be able to legally post copied circuits.

I shall simulate the circuit with my own ideas added in and publish that instead, and point out the similarities with the Elektor circuit - if successful that is.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Dave Friday Quote  Post ReplyReply Direct Link To This Post Posted: 24 Sep 2018 at 7:09pm
Hi Graham,where did you find the Elektor article ?


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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 24 Sep 2018 at 9:08pm
Hi Barry, I bought the physical magazine in 1978 because it was "packed" with audio circuit ideas, and have kept it (along with other magazines of interest) ever since.

Today I noticed it has been uploaded to the American Radio History website, and you can find it here: https://www.americanradiohistory.com/UK/Elektor/70s/Elektor-1978-07-08.pdf

Now what a stroke of luck that is?

Scroll to page 76 of the PDF and you should find you're on page 71...
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Dave Friday Quote  Post ReplyReply Direct Link To This Post Posted: 25 Sep 2018 at 12:30am
Thanks Graham,I didn't have that copie in my library,I had to throw most of my mags away when moving around Europe! I still have a copie of "Transistor audio and radio circuits"(Mullard second edition 1972) though!

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Post Options Post Options   Thanks (1) Thanks(1)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 25 Sep 2018 at 7:12am
What you need is one of those little Netgear servers to save all the PDFs from American Radio History... that's if you've time to read them all Wink
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 26 Sep 2018 at 11:37pm
Today reloading everything to a new PC and not realising I had to map my server I called the software provider of my SPICE program for assistance. Didn't get anywhere but it was interesting to note they are not offering the program anymore so I'm a bit upset about that. The age of the support department - the same chap everytime you call - must be similar to mine. It begs the question "what the ****s up with Britain?" It seems we've all given up!

Thanks to John C for saying "have you mapped the drive?" The SPICE program is running again.

I did a bit on the Elektor circuit on the ailing old PC prior to moving over to the new PC and I wasn't impressed. Having said that I am sure the designer whoever it was hadn't the luxury of an integrated SPICE interface with a schematic program - not in 1978!

Unless you're a transistor physicist which I am not, and even if you are, it is difficult to predict how a circuit will perform, and phase margin might as well be pure guesswork even though the maths and formulas exist! I have modeled that many published and commercial audio circuits that I'm surprised any of them sold in the first place, or have lasted all that long.

And as far as measurements are concerned there is no way one lab will measure the same results as another. You might be conned into thinking electronics can be made perfect. If you think that way you have been! We are tossing about with tiny unstable lumps of planet "the dirt beneath our feet" and we expect perfection.

Yet people swear by some products. Amazing!

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