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1970s Design Indulgence

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Post Options Post Options   Thanks (1) Thanks(1)   Quote BackinBlack Quote  Post ReplyReply Direct Link To This Post Posted: 22 Jan 2023 at 4:58pm
As is now obvious I have handed the Amp on to Ifor.
I thoroughly enjoyed listening to it, despite the unplanned interuption due to my wife breaking her leg. Fortunately my wife can now manage the stairs a couple of times a day, so at least the lounge is back to normal.
During my stewardship the Amp had almost 200 hours of active time, most of that from Internet Radio and local NAS. Dedicated listening sessions totalled about 30 hours.
My overall impressions of the Amp are very positive, I found the current adjustment to be quite stable after about 15-20 minutes warm up, very little drift thereafter, the meters remaining at 21, only rising slightly after a louder listening session. My speakers are moderately sensitive and with no crossovers, present an easy load.
The stereo image is particularly stable, but doesn't seem to have the width beyond the speakers I recall the Proprii presenting. Speech is reproduced with no sign of sibilance. Whilst my speakers have good bass extension, they do not produce powerful bass as you might expect of a 4.5 inch dia cone. I do however feel that the bass is not quite as powerful as it was with the Proprii. On tracks such as Roger Waters' Amused to Death the Amps' ability to present the sound effects and dynamic scale was impressive.
Overall an impressive Amp with no quirks. It doesn't have quite the bass power or wide soundstage the Proprius have, nonetheless just a pleasure to listen to.
It has prompted me to rethink my current set up. I shall have to give serious thought to a new pair of speakers. Although I like much of what Full Range drivers offer, their shortcomings are becoming more of an annoyance these days, particularly with such a capable amp as the Retro.
My Thanks to Graham for all his work and tutorials in developing this and the fine range of HiFi Systems Components.
The question now is where the Retro Amp fits in John Cs plans for the future.

Ian
Just listen, if it sounds good to you, enjoy it.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote morris_minor Quote  Post ReplyReply Direct Link To This Post Posted: 24 Jan 2023 at 5:26pm
Originally posted by Graham Slee Graham Slee wrote:

Originally posted by ICL1P ICL1P wrote:

Does it have a name? 

Yes, several. None of which can be repeated here Wink
I had a bit of fun with this . . . 


Bob

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Accession MC/Enigma, Accession MM, Reflex M, Elevator EXP, Era Gold V
Solo ULDE, Novo, Lautus USB and digital, Libran balanced, CuSat50
2 x Proprius + Spatia/Spatia Links
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Fatmangolf Quote  Post ReplyReply Direct Link To This Post Posted: 24 Jan 2023 at 5:33pm
Groovy baby!
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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Post Options Post Options   Thanks (0) Thanks(0)   Quote morris_minor Quote  Post ReplyReply Direct Link To This Post Posted: 24 Jan 2023 at 6:14pm
Originally posted by Fatmangolf Fatmangolf wrote:

Groovy baby!
The Austin Powers vibe is strong in this one, Luke LOL
Bob

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Accession MC/Enigma, Accession MM, Reflex M, Elevator EXP, Era Gold V
Solo ULDE, Novo, Lautus USB and digital, Libran balanced, CuSat50
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Fatmangolf Quote  Post ReplyReply Direct Link To This Post Posted: 24 Jan 2023 at 7:26pm
I sense a family connection but daddy wasn't there.
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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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 27 Jan 2023 at 9:29am
I remember working in one particular R&D workshop on "class-A" amplifiers.

Massive mains voltage fluctuations were unheard of 26 years ago in the "post-industrial" north.

Most of the new designs (I was NOT the designer) looked suspiciously like those in Elektor.

I always wanted to ask Mr Gisberts how current could flow out of a node and back into that node when, as we know, current can only flow positive to negative (the opposite of electron flow).

But this company insisted their designer (not Mr Gisberts) was right, and it would be my fault if things didn't work!

Increasing bias to "class-A" - high bias in other words - was a frightful experience. The trimmer preset was advanced while measuring the output stage current until it reached the value the designer had determined.

Very shortly, the current would die, indicating the loss of a fuse, or both HT fuses.

I would then replace the fuses, back off the bias, switch on, and the fuses would blow again.

That would be followed by me desoldering and demounting each transistor in-turn, and testing each one, until I had a set of blown or welded transistors, and a set of working ones. The working ones may have been two or three in count.

The circuit would be rebuilt, and the whole cycle would repeat, at which point the boss man was informed, who would get angry and go away to phone the designer.

A fix would be presented - often involving the change of a capacitor (actually two because this was "class-AA") and that said to me that the damned thing was unstable in the first place!

I'd then rebuild the amp, and go through the motions again. In most cases, the desired high-bias current could then be reached, but woe betide me if I went a tad further! Because it would blow the lot again!

There was no denying that the designs were unstable (unless you were the boss man), but as long as the high bias was reached, the amps were classed as class-A and went into production.

So, what would happen if the mains voltage increased? Well, obviously, the bias would increase with it. And if the bias increased sufficiently? Then what?

I'll let you work that one out.

This retro amp or whatever you might call it, is stable, and increasing bias beyond that stated will not instantly blow it.

But I guess by putting temptation in the way, the fact that the final trimming presets are available to the customer, then it could be tempting to wind them up into high bias.

As this design is stable, all winding the bias up does, is make the output stage hot, because there's more current flowing in it. Extending Ohms law to include power, more current times the HT voltage means more power being dissipated by the output transistors.

The heatsinks get rather hot, and so does the transformer, and should it get to a chassis temperature of 60C, the bimetal thermal switch operates cutting the mains. The enclosure then cools gradually to below 45C and then switches back on.

I have not catalogued how long it takes to cool back down, but I suspect it could take maybe a hour or so.

But why put temptation in the way?

Well, it isn't temptation at all. On a 207V mains supply, which the standards say the amplifier must work at, the trimmer presets will have to be set nearly fully clockwise to reach 20 on the meters.

Now, on 254 volts, the trimmers have to be set nearly fully anti-clockwise, or the meters will read more than 20.

And on 230 volts, somewhere in between.

Now, on switch on, the meters will indicate less than 20 because the thermal conditions take a few minutes to stabilise, so you either wait or play music anyway, and check the meters after say 30 minutes.

So what does this say about amplifiers without this adjustment?

If an amplifier has been set up in the factory to its optimum at the rated 230V, then at 254V it is over biased and at 207V it is under biased.

This is assuming the amplifier has the usual unregulated power supply - most have.

And, incredibly, such an amplifier sounds just as great in reviews no matter where the review took place. Hmm, quite incredible! Actually, it's a load of bollocks!

But, you see, if you don't have either this accessible trimmer and a meter, then you don't know where the bias is - whether it's optimum or not.

I believe McIntosh use a microprocessor to achieve this end (good on them!), and if an amp has a regulated power supply, the bias will remain as set in the factory.

Anyway, I hope this post educates those who perhaps didn't know anything about output stage bias.
Not simple enough for Google-Bot to understand...
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 27 Jan 2023 at 3:47pm
John and I are having discussions on how he takes the amp forward, and my thoughts are that he'd be better off making it a DIY only item.

My view is that the existing case design ought to be ditched along with the meters, leaving the output stage current setting to the constructor to suit their own supply voltage.

It can then be simplified further by shoving both channels on a single PCB with ground spine, thus getting round the stereo-loop problem.

And then it only requires a large heatsink midships, so could be housed in a Takachi Electric Industrial MB Series Silver Aluminium Enclosure, 350 x 210 x 90mm, which the constructor can source from RS.

So, if John is up for that, I'll build one and see how it goes.

(could take a while though)

----------------------------------------------------

As regards the line stage/preamp, I'd already thought about making that as a DIY project, and shoving the thing into the same type enclosure as above.

It could use an aux supply from the power amp or the 30V nominal output from a PSU1, with which the prototype stages published on here a few pages back were powered.

With the state of British industry right now (and possibly for some time to come) this is possibly the best and only option.

What do I mean by the state of British industry? Take a look around. Do you see any?



Edited by Graham Slee - 27 Jan 2023 at 3:58pm
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