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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: 18 Nov 2018 at 6:41am
is this what the people below are thrashing around at graham - I have no idea whether any of these claims carry any validity?

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 18 Nov 2018 at 10:32am
Originally posted by Richardl60 Richardl60 wrote:

is this what the people below are thrashing around at graham - I have no idea whether any of these claims carry any validity?



By default the input phono socket "earthy" side (the common connection) is connected to mains earth via the chassis. Adding one of these cables creates another loop, probably harmless, but it won't improve anything. Connecting it to PCB ground connections will guarantee faulty operation. It will sound different, and as I've tried to teach, different doesn't automatically mean better.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 19 Nov 2018 at 12:52am
For those interested in my references for the earlier article on The Problem With Stereo (and multi-channel amps) please refer to the following:




At time of publication of these journals I was aged 6 and 9 respectively, and only know of them by following the references shown in later published designs, and only being able to view them since David Gleason did the arduous work in compiling the data at American Radio History dot com (https://www.americanradiohistory.com/Scanning-for-American-Radio-History.htm)

I trust David gets more interest than the 2 or 3 readers I get here on a good day. The Internet started for the purpose of sharing scientific documents, engineering being a science, and in searching a book you look through the index, and if a page exists it is in there.

Google on the other hand do not index pages such as these, and so they may as well not exist. I found it difficult throughout my career finding good information, and that looks set to continue for others. I trust somebody is getting the benefit from all this research? (other than copycats that is)

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 19 Nov 2018 at 8:59pm
The two power supplies (two windings on one transformer) addressed the problem described earlier.

Also because the transformer is 50V instead of the previous 48V one, there was more HT voltage available (about 2.8V), and 100 watts per channel into 4 ohms was squeezed out of it at just under 1% THD+N (both channels driven). Likewise into 8 ohms 55 watts per channel was recorded, again just under 1% THD+N.

Here however, I'm using two less reservoir capacitors, with 14,000uF per channel, instead of 28,000uF shared.

Distortion was roughly the same as before but with no interaction it was easier and quicker to set up.

I'm still rating it at 50 into 8 and 75 into 4, because that keeps its swept THD 20Hz - 20kHz under 1%, in fact it was nearer 0.5% at 20kHz and fell below 0.1% under 3kHz to around 0.045% at 75W into 4 ohms, and 0.035% at 50W into 8 ohms.

It might be possible to up the HT a little more to obtain a better 8 ohm 4 ohm balance.

As for the sound: a bit thin on initial switch-on but after a few minutes warm up it is definitely my "house sound".

The stereo image is also much better. All of this goes to show how spot-on Tobey & Dinsdale were 53 years ago!

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Dave Friday Quote  Post ReplyReply Direct Link To This Post Posted: 19 Nov 2018 at 11:09pm
Hi Graham,why does c 10 go to the output stage and not to the  chassis? And who are Tobey and Dinsdale?
Ta.
Found Tobey and Dinsdale (Wireless World 1961)


Edited by Dave Friday - 19 Nov 2018 at 11:39pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 20 Nov 2018 at 6:51am
Originally posted by Dave Friday Dave Friday wrote:

Hi Graham,why does c 10 go to the output stage and not to the  chassis? And who are Tobey and Dinsdale?
Ta.
Found Tobey and Dinsdale (Wireless World 1961)


Hi Barry, the way it's drawn it looks like a decoupling cap, but C10 serves the bootstrap connection.

And for further clarification, the bootstrap is the AC version of a 'constant' current source.

The capacitor feeds the output signal to a resistor split (same as saying a potential divider, but that's not its purpose as such here).

So R9,15,17 appear in parallel with the load (because supply and ground are the same with AC), and on positive swings where C10 meets them, the voltage is pulled (shoved) upwards maintaining the same voltage drop across R10. And if R10's voltage drop is always the same, then so is the current, it's constant.

But it's only constant down to the turnover frequency of C10 vs the parallel value of R9,15,17. If it went lower it would cause bass boost and low frequency instability.

By making current constant here it linearises T2's gain, which otherwise would vary depending on the signal's position in its cycle.

Another way of doing constant current would be to use two transistors and two resistors as shown in numerous power amp schematics. Sometimes the reference transistor is replaced by a LED, or a diode chain. These are only really suitable in a DC input and output coupled design.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Dave Friday Quote  Post ReplyReply Direct Link To This Post Posted: 20 Nov 2018 at 10:32am
Thanks,I'm going to have to read your explanation quite a few times ( always have/had problems with "bootstrapping ").
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