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1970s Design Indulgence
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Fatmangolf View Drop Down
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Joined: 23 Dec 2009
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Points: 8305 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Fatmangolf Quote  Post ReplyReply Direct Link To This Post Posted: 20 Jan 2023 at 8:37pm
Ah well.
Jon

Open mind and ears whilst owning GSP Genera, Accession M, Accession MC, Elevator EXP, Solo ULDE, Proprius amps, Cusat50 cables, Lautus digital cable, Spatia cables and links, and a Majestic DAC.
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Points: 2294 		Post Options Post Options   Thanks (0) Thanks(0)   Quote ICL1P Quote  Post ReplyReply Direct Link To This Post Posted: 21 Jan 2023 at 5:27pm
We need a map of active UK members locations. Any of them who have not followed this thread, and I’m sure many haven’t, won’t know that they could be a link in “the chain”.
Ifor
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Points: 15005 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 22 Jan 2023 at 9:40am
This is the simplest high performance vintage transistor power amplifier in my experience.

Vintage? The first valve amplifier was invented by deForest in 1912. 35 years later the transistor was invented by Shockley. The first output transformer less transistor amplifier was published in 1956 by Jimmy Huang  Lin. This transistor amplifier appears 67 years on. Of course it's vintage.

It uses just six transistors per channel, and is powered by a simple mains transformer non-regulated power supply. The transformer being of the "Westinghouse" E-I lamination type (a frame transformer).

40W Final diagram

At switch-on the output capacitor draws current, which is used for supplying the bulk of the negative going load current.

Methods of slowing this switch-on charge to reduce the loudspeaker "thump" can easily result in too much LF phase shift, which at worse, can result in "motorboating" (low frequency oscillation).

Although some may find the switch-on "thump" undesirable, it does more to upset the nerves than it does to damage a loudspeaker. A loudspeaker unable to handle Pink Floyd's "Get Your Filthy Hands Of My Desert" (from "The Final Cut") for example, at normal volume, is simply rubbish, and the switch-on excursion here is very similar in cone displacement.

Output relays to charge via a dummy load can be used if desired, but the circuitry is cumbersome, and relay contacts in the output path increase distortion, which worsens as the contacts wear.

(as King David noted, "all is vanity and vexation of the spirit")

The output capacitor does cause some non-linearity on negative going output, and this is because it draws top-up current from the HT via the positive going side of the "push-pull", and during that top-up the driving impedance must obviously change.

Anything which changes one half of the signal results in a second harmonic, and as it isn't part of the input signal, is therefore distortion. This distortion happens at around -80dB on steady-state sine wave testing and contributes approximately 0.01% to total harmonic distortion. As such, it should not be heard.

However, as the HT current is also asymmetrical, ground current disturbances are inevitable and these cause problems with any multi-channel amplifier contained inside one enclosure.

Power currents can be contained by separated ground wiring but the input stage grounds must join at some point. Where they join, current can flow from one channel to the other via that join. Therefore, a disturbance on one channel can inflict its disturbance on the other channel.

A trunk and branch approach to input stage grounding would have been better than the fully separated boards this design ended up with.

As such, the stereo image is not as precise as two monoblocs can produce.
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Points: 2294 		Post Options Post Options   Thanks (0) Thanks(0)   Quote ICL1P Quote  Post ReplyReply Direct Link To This Post Posted: 22 Jan 2023 at 2:44pm
For one reason and another the amp only made it onto the shelf this morning. The thump was nowhere near as loud as I expected; much quieter than when I forget to turn the volume down on the Proprii before powering them up after a power cut. The meters have worked their way up from 10 to 19 since I switched it on. I think Ian told me to set them at 21; did I remember correctly?

Now for some music.
Ifor
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Points: 15005 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 22 Jan 2023 at 2:58pm
Originally posted by ICL1P ICL1P wrote:

For one reason and another the amp only made it onto the shelf this morning. The thump was nowhere near as loud as I expected; much quieter than when I forget to turn the volume down on the Proprii before powering them up after a power cut. The meters have worked their way up from 10 to 19 since I switched it on. I think Ian told me to set them at 21; did I remember correctly?

Now for some music.

Play some and then see where the meters point.
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Points: 2294 		Post Options Post Options   Thanks (0) Thanks(0)   Quote ICL1P Quote  Post ReplyReply Direct Link To This Post Posted: 22 Jan 2023 at 3:02pm
20

RPi (MoOde), Majestic, 1970SDI
 
Damon Albarn - Everyday Robots
The Specials - Encore
The Beatles - Abbey Road
Hollie Stephenson
The Good, The Bad & The Queen - Merrie Land
R.E.M. - Out Of Time
Caravan - In The Land Of Grey & Pink
The Cranberries - No Need To Argue
Pete Atkin - The Master Of The Revels (compilation)
Pink Floyd - Wish You Were Here
Fat Mattress
Cream - Strange Brew: The Very Best Of Cream
The Stranglers - Black & White
Talking Heads - Little Creatures
The Tubes - Remote Control
Tom Jones - Reload


Edited by ICL1P - 26 Jan 2023 at 6:46pm
Ifor
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Points: 15005 		Post Options Post Options   Thanks (1) Thanks(1)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 22 Jan 2023 at 4:42pm
The power supply has changed a bit since the last diagram. It no longer has the centraliser ("DC filter") because it no longer has a toroidal transformer. The mains fuse is in a little drawer in the IEC connector and is 2A anti-surge. The case is mains earthed and only connects to 0V at the input grounds, as is the practice for EMC. The transformer is about 135VA.

40WPC power amplifier power supply
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