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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: 13 May 2021 at 11:02am
Vinyl listening - "One For The Vine" (Genesis - Wind and Wuthering) and "Starship Trooper" (Yes - the Yes Album).

Speakers: Harbeth M20 (6 ohms)

I started with Hana EL, Accession C with Enigma, Majestic (as preamp).

My test is for 'the melody level' - If music isn't melodic, you may as well forget it! - and the ambience (character and atmosphere), and my marking system is very simple.

The EL and C weren't entirely pulling it off, so I swapped to an Ortofon Black and Accession M with PSU1. That improved matters, but the ambience wasn't quite where these tracks should be.

Next up was to include the Enigma, which I'd not considered to make a difference with an Accession M. It quite clearly did, and Jon Anderson's voice appeared clearer and moved more centre-stage than it had using the PSU1. Stereo positioning was better.

I decided to use the Ortofon Black and Accession M with Enigma as a reference for the vinyl tests of the power amp. The turntable is a standard Technics SL1200 Mk2.

If my mind drifts into 'what is doing what' in the circuitry during a listening test, it is a fail!

So, it is, yet again, a fail.

I can only describe it as a treble chatter, a distortion, but not a measurable one. There is a slight emphasis on the upper percussion frequencies, which is sufficient for me to lose interest and start thinking about causes.

I am purposely using the M20's because they are a 6-ohm load. Although the amplifier is designed for 8-ohms, I cannot predict what the end-user will use.

My thought was that the Zobel network might fair better if its resistive part were closer to the minimum expected speaker impedance, and so it becomes 4R7 instead of 10R. This had zero effect on my ears but will remain anyway.

I will also re-examine the output inductor shunt resistor because I think the inductor should be more resistive at high frequencies, which is its purpose. However, some think a tweeter needs plenty of damping factor, which I think is crazy.

I suppose I can announce some good news in that the new layout seems to have improved on sibilance, which no longer gets between me and the music.

I also have to remember that it is being powered using an off the peg RS transformer, which hums the case because it is too saturated, and that has been shown to have a bearing on HF distortion (subjectively, at least). A replacement "Amplimo" 2 x 25V secondary transformer has been ordered from the Chesterfield manufacturer.

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Sylvain View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Sylvain Quote  Post ReplyReply Direct Link To This Post Posted: 13 May 2021 at 11:36am
'''
....an off the peg RS transformer, which hums the case because it is too saturated, and that has been shown to have a bearing on HF distortion (subjectively, at least). 

I notice this problem has caught many HIFI manufacturer attention and DC Blocker being marketed as a solution .........what says you please 


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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 01 Jun 2021 at 4:00am
Current feedback asymmetry

The NFB summing junction connects two resistors that form a potential divider such that the potential equals the input voltage.

The summing junction must have linearity such that the full swing of the fed back potential can be accommodated.

HERE, the DC emitter voltage is 3V, and then the fed-back signal is 1V peak and, therefore, fits.

The negative slew rate (green curve) was asymmetrical, and none of the known 'fixes' could alter the fact.

It helps to write down every function of a circuit, and in doing so, I was explaining the NFB potential divider. In particular, I was looking at the power handling of the feedback resistor, and in doing so, derived the current.

It hit me that the current exceeded that of the DC emitter current. The NFB resistor gave rise to 5mA a.c. The DC emitter current is just half of that.

Current does not divide down as voltage does. The current remains the same - how can it differ?

Candidly, I summed the currents: 7.5mA positive to -2.5mA negative. However, the divider voltage must remain the same - mustn't it? Well, how can it?

By this point, the grey matter was well into overload, and as is usual for us humans, we take a peek over the nearest shoulders to see how they did it. I noticed that a 'revered designer had done the same, but the 'run of the mill' mass-produced designs hadn't. I'd only ever heard one of those mass-produced designs (the Teleton), and it had sounded OK by me.

Doubling the values for the NFB potential divider reduced the a.c. peak current to 2.5mA, and summed gave 5mA positive to zero - no negative value!

Rerunning the transient simulator gave the red curve, which looks just about symmetrical.

(the curved undershoots of the traces are due to me taking the output after the output inductor)

slew rate asymmetry

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Sylvain Quote  Post ReplyReply Direct Link To This Post Posted: 01 Jun 2021 at 8:33pm
The attention to detail and the painstaking patience to attend and get a  solution for the reward '' revered''' premium charge ....or 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 01 Jun 2021 at 9:05pm
Originally posted by Sylvain Sylvain wrote:

premium charge ....or

No, just wanting to top up my state pension Wink
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Sylvain Quote  Post ReplyReply Direct Link To This Post Posted: 03 Jun 2021 at 8:47pm
I only pray in hope of a better considered Price fixing formula than that achieved for One Unit Proprius. I listen in stereo or 5:1 and had to multiply. 

that said, I have good  faith in the fairness attribute even for old spirit Yorkshire man could never abandon teh goodwill of his design creation. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 07 Jun 2021 at 8:18am
Roughly 4/5ths of the fed back a.c. current flows in the parallel emitter resistor to the one used for DC. It is not current feedback!

We are told that current feedback amplifiers are inherently stable, but this configuration isn't (it needs compensation), which again says, it isn't current feedback.

My ears also tell me that by making adjustments to resistor values, there is increased audible distortion.

I think I must therefore take the transient plot with a pinch of salt. It must be a case of GIGO, and I don't know where the input garbage to the simulation program occurs, and the ICE part of SPICE might be something to do with it - ICE meaning integrated circuit emphasis - this is not an IC!

Perhaps SPICE modelling is a hindrance to this design?
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