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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: 15 Jul 2019 at 12:31pm
I am sure we are all now suitably enlightened in the c**p that is fed through or picked up by or comes through the mains supply. 

In Order to work round many of the unregulated items (which you are attempting to do) feeding more c**p into the mains do you feel it possible that any of the usually very expensive mains regeneration units (however they are termed) could provide a part solution for this issue?




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Post Options Post Options   Thanks (0) Thanks(0)   Quote Richardl60 Quote  Post ReplyReply Direct Link To This Post Posted: 15 Jul 2019 at 12:32pm
..sorry not talking of conditioners here.. but generators?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote John1479 Quote  Post ReplyReply Direct Link To This Post Posted: 15 Jul 2019 at 11:33pm
Originally posted by Richardl60 Richardl60 wrote:

I am sure we are all now suitably enlightened in the c**p that is fed through or picked up by or comes through the mains supply. 

In Order to work round many of the unregulated items (which you are attempting to do) feeding more c**p into the mains do you feel it possible that any of the usually very expensive mains regeneration units (however they are termed) could provide a part solution for this issue?

Further to Richards comments, I gawp at the price of some of these mains products. John C emailed a picture of the StromTank audio power supply/mains conditioner seen at Munich High End this year..... Rather spectacular!  The recent posts have made me very curious as to what innovation a Graham Slee product might bring to the 'Hot' topic area of mains conditioning/filtering? 
I'm sure your not sat around looking for suggestions.... Not very 1970's. Wink



Edited by John1479 - 15 Jul 2019 at 11:35pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Richardl60 Quote  Post ReplyReply Direct Link To This Post Posted: 16 Jul 2019 at 6:34am
..or put another way do you think you would still have encountered the same issue in the 70s?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 16 Jul 2019 at 7:17pm
New Transformers Fitted

After a long wait the new transformers arrived today and were duly fitted.

These are EI laminated transformers specially made to be low-flux (low saturation - using high-grade grained-steel laminations), and split-bobbin, which combined should reduce chances of mains borne RFI influencing the subjective performance.

Pictured alongside a stripped out RS transformer you can see the new transformers have more laminations.

transformers compared

They fit in the same locations as the RS transformers.

transformers fitted in amplifier

Lead-outs are used instead of solder lugs so the lid will only-just fit above them.

transformer lead-outs

They are much lower in magnetic field noise. The left-right differences are due to differences in board layout and component tolerances, the amplifier only having 14dB NFB.

signal to noise ratio measurement

For only 14dB NFB the distortion (THD) sweep is quite impressive. The rise in the right hand channel is due to not being able to fully trim the output voltage, due to component values which need adjusting in value.

power amplifier thd distortion sweep display

Frequency response is quite reasonable at 14Hz - 38kHz -3dB, the top end tailored by the input filter to reduce slew induced distortion (SID).

power amplifier frequency response curve

Again, this must now be run 80 - 100 hours just to be sure that it returns a good subjective performance. The amplifier is driving the Harbeth M20's as these show up upper-mid sibilance more noticeably than the LS3/5A's.

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Post Options Post Options   Thanks (1) Thanks(1)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 17 Jul 2019 at 1:52am
Originally posted by Richardl60 Richardl60 wrote:

..or put another way do you think you would still have encountered the same issue in the 70s?


Radio frequency interference in the 70s meant that local radio-cab communications could "gatecrash" an evening's entertainment. Easily solved for good by slipping a ferrite bead on the input transistor's lead, or even just a small series reisistor, and maybe a small value capacitor to ground.

Some commercial power amplifiers didn't have any stability compensation, except perhaps that the "slow" output transistors gave the roll-off. In fact, capacitor coupled power amplifiers didn't need output inductors - the small inductance of the output capacitor was sufficient.

I remember my own earliest power amplifier design in 1977 not using any 'miller' compensation until my mother told me her radio in the next room was playing the same music as me...

A 47pf polystyrene capacitor between VAS collector and base solved the problem. The following year I made it into a kit and sold a few via Worksop Electronics. Everybody loved the sound.

OK, the output stage used 2N3055's which weren't very fast (so did the NAD 3020 by the way), but it was a quazi-complimentary without any precautions.

The power transformer was a Gardiner, surplus stock sourced via the famous (now gone) Bardwells in Sheffield, who had sufficient to keep the kits going for quite some time. I only stopped making them because my PCB supplier went on to "bigger things".

My next power amplifier wasn't for music, it was for fire chiefs to evacuate large residential properties (obviously they don't bother using them anymore), so I can't tell you if mains RFI made it sound worse or not. It drove up to one hundred 64 ohm speakers in parallel via lots of capacitive cable, so I know about driving low reactive impedances. The client was a Cable & Wireless company and I built around 120 of those amplifiers for them.

1990 was the year I started on the Audionics 1030 (MA230) which was fitted in a number of radio studios and also sold under the Canford Audio brand.

It had a complimentary feedback pair (CFP) output stage rather than the quazi-complimentary or emitter follower (EF). This being done to save the loss of 1.2 volts (two base-emitter voltage drops) which enabled it to reach 35 watts whilst using the low voltage TIP33A/34A power transistors (we simply could not get the C versions).

A (Trans-Tronic custom-made) toroidal transformer was used because of the 1U height, and I couldn't get the output stage to stabilise (it kept blowing) until I added an extra inductor in each collector. It measured the way "the powers that be" wanted so it went into production. Each one was soak tested a full week playing Radio 1 into them with each output taken to dummy loads.

They sounded fine, and I was allowed to have the prototype parts and spare prototype boards to take home and build my own. I wasn't allowed to have a transformer so I bought my own from RS which resulted in a little less HT, but was good enough. I used that power amplifier up until 1999. I enjoyed its sound, even though I'm sure it would fail inspection by Doug Self.

John C was far more crafty, and built his "one piece at a time" (you'll have heard the song?), and it's pictured below, slightly modded.

Audionics 1030 (MA230) power amplifier


Edited by Graham Slee - 17 Jul 2019 at 2:14am
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Post Options Post Options   Thanks (1) Thanks(1)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 17 Jul 2019 at 3:00am
Ethernet over mains:

Standards introduced 2001 - few users

Faster devices 2005 - still few users

Latest speeds 2012 - large expansion of users


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