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1970s Design Indulgence

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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: 25 Jul 2018 at 10:40am
Originally posted by BAK BAK wrote:

Originally posted by Graham Slee Graham Slee wrote:

Part 2: (what shall we call part 2?)

How about "Real Input Impedance" and "Feedback concerns"


OK, we can call it that.

For the few who are following this topic I will continue by commenting on circuitry of real products in the 70s, in the hope we can learn how we can use these early circuits to good effect in perhaps an amplifier project?

One of the big problems with some 70s products was the use of "brilliant ideas" and how they impacted on the sound.

Often it was the case of using technology for technology's sake rather than for sound quality (in my opinion). And at least one British firm was guilty of doing so. I shall not mention who, but they have grown into a massive multi-million pound turnover retail and manufacturing outfit. Which just goes to show the general ignorance of the hi-fi buyer and those who recommend by way of review.

An excellent exposé site on the subject is by Paul Kemble: http://www.angelfire.com/sd/paulkemble/soundindex.html

I also "blame" "the adventures of Doug Self" or was he simply regurgitating the ideas of the era for the delectation of Wireless World readers?

All I can say, equipped as I am today with my copy of Number One Systems "EasySPICE", is some of what we bought in the 70s (and into the 80s) was to a degree, a result of "anal retentiveness"...

No wonder the transistor has taken such a battering. But if used properly it can trounce all which valves can throw at it. But the damage has been done and valvee people will have none of it.

Yes, I think we need to make a transistor amplifier "the proper way" to enable "the few" to truly appreciate the transistor's capabilities.

So let us now begin to consider what's required.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote morris_minor Quote  Post ReplyReply Direct Link To This Post Posted: 25 Jul 2018 at 12:24pm
Without meaning to divert this thread - which I've tried to follow, mostly without success - what would the difference be between "solid state" circuits and "transistor" circuits?

You say you need to make a transistor amp the proper way (really make one?? For sale??) - so how would this differ from the Proprius?
Bob

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 25 Jul 2018 at 1:09pm
Solid state can refer to transistors as well as integrated circuits such as op-amps (operational amplifiers). Transistors are therefore solid state. And integrated circuits contain a quantity of transistors all etched/deposited onto the same piece of silicon, along with any small passive components necessary, as well as diodes and zener diodes required for reference voltages.

Originally posted by morris_minor morris_minor wrote:

You say you need to make a transistor amp the proper way (really make one?? For sale??) - so how would this differ from the Proprius?


Why not? After taking a look under the lid of other 70s amps it shouldn't present much difficulty. And I have also produced one amplifier kit in the late 70s; a studio power amp which actually got used in quite a few studios; and I was R&D dogs body for an amplifier manufacturer (in fact two).

The Proprius is a bit of a unique design (erm... Bruce showed me a Dynaco who'd already done a similar voltage amplifier stage - it was news to me, honest!)

A 70s power amp could take many forms and is something I may look into in this topic, illustrating as I go.

Could even do the entire thing as a kit... Wink

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Post Options Post Options   Thanks (0) Thanks(0)   Quote morris_minor Quote  Post ReplyReply Direct Link To This Post Posted: 25 Jul 2018 at 2:30pm
Thank you Graham - I shall follow this thread with renewed enthusiasm! 
Bob

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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 Jul 2018 at 6:55am
I thought it may be of interest to show you the circuit of my late 70s kit amp...

Graham Slee Kit Amp

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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 Jul 2018 at 8:41am
The amplifier was often used with a 40V transformer, which gave 56V DC on load, because the main supply decoupling and reservoir capacitors were difficult to obtain above a 63 volt rating. Even so it would do nearly 90 watts into 4 Ohms and 45 watts into 8 Ohms. Run at 68 volts on-load (using a 48 volt transformer) it could swing in excess of 20 volts, delivering more than 100 watts into 4 Ohms and over 60 watts into 8 Ohms. There is in fact an error in the drawing between what's shown and the actual kit: T2, R12, 13 and 14 was two signal diodes and a small value trimmer all placed in series which did the same job. Thermal feedback to stop the output transistors going into thermal runaway was achieved by glueing the diodes to the output transistors with Araldite (one apiece), in a similar way to many commercial products.
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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 Jul 2018 at 9:46am
Savvy readers will note the output capacitor and also the lack of a differential input. As it uses an output capacitor it doesn't really need such precision control over its output mid point, as long as it gives the full expected output it doesn't matter if there's a few millivolts offset. Some may argue the offset increases distortion, but by how much? Often such argument is about distortion we would never hear even using high class headphones instead of a loudspeaker.

The output capacitor can cause distortion however, but only if it acts on the audio signal. A value of 1000uF was quite common in the early days, which had a turnover at 20Hz using 8 Ohms. For 4 Ohm use it was doubled up to 2200uF. It is debatable whether distortion can be heard at 20Hz because 20Hz is below what most speakers can do, and if they can, we have a job hearing it.

But phase shift relative to other frequencies might fudge timing, and 20Hz only comes back into phase at 200Hz. Perhaps nowadays we should use 10,000uF (or two 4700uF) and that would take the capacitor's effect way out of range.

DC coupling removes this problem, but adds the possibility of extreme loudspeaker damage should the amplifier "tip over" (if the output offset goes wrong). You would think that servos could overcome this problem, but I've never known it!

Just a little bit of instability seems to affect some amplifiers, blowing a pre-driver, and setting off a chain reaction ending in speaker drive unit burn out.

Therefore my preference is for output capacitors and I never heard anything untoward on a friend's Cambridge P50 driving Tangent RS4's on some really low bass. The P50 used output capacitors.

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