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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: 27 Jun 2020 at 2:24am
The 100uH inductor works. In fact, it advertises itself by singing along in the upper mids to high frequencies. Don't worry, the volume from your speakers will drown it out, even at low settings.

It has reduced the high-frequency current flowing in the power supply reservoir capacitors. High-frequency current instead flows in a circle round a small 2.2uF film capacitor. Any parasitic tendency of the output stage consumes the small energy store of the 2.2uF capacitor, and without the energy, the parasitic trend has little chance of being sustained.

Because CRT oscilloscope driver transistors can suffer parasitics, "tricks" were devised from research data to control the problem. Thanks to the publication of such findings, the parasitic amplitude is known to self-limit at around 1 volt. The parasitic frequency range is also given and starts to become evident around 50kHz. This is only 2.5 times the upper audio spectrum frequency of 20kHz.

The FFT display has shown disturbances in the 50kHz plus range with all variants of the amplifier. No matter what the precautions, some part of the disturbances remain. This demonstrates how ineffective all the suggested "cures" are. It indicates that an EF2 is just as bad as an EF3 (two-stage Darlington or three-stage Darlington).

The digital FFT isn't fast enough to capture the amplitude, but display "flicker" suggests peaks greater than shown. Using the power supply inductor modification, the FFT in the 50kHz plus neighbourhood stays around -80dB.

Without damping, LC filters resonate, and I am using a 100-ohm damping resistor across the inductor. The simulation indicates a small peak around 10kHz, and with 10 ohms, there is none. 100 ohms gives 13.5 times reduction in HF reservoir current, and 10 ohms only 2.25 times. I think somewhere in between, possibly 33 ohms, might be a good compromise. It might also reduce the coil's sonics - that, and some mechanical damping.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote BAK Quote  Post ReplyReply Direct Link To This Post Posted: 27 Jun 2020 at 2:42am
Are you using a separate LC filter on each channel's HT supply?

Bruce
AT-14SA, Pickering XV-15/625, Technics SL-1600MK2, Reflex M, Lautus, Technics SH-8066, Dynaco ST120a, Eminence Beta 8A in custom cabs;; Using Majestic DAC
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Post Options Post Options   Thanks (0) Thanks(0)   Quote DogBox Quote  Post ReplyReply Direct Link To This Post Posted: 27 Jun 2020 at 2:50am
Graham said:"Google MJL21193 negative resistance, and you'll be lucky to find anything to do with it." 

Well, let's see if we can brighten someone's day...! 
https://en.wikipedia.org/wiki/Negative_resistance 

And from what I can see, a few more felt that this was indeed part of the curriculum that should be understood! Wink 

I also got to find out that there is such thing as a 'Tunnel Diode'... So much I don't know... 

The quite well known electronics magazine in Australia, Silicon Chip used the same MJL21193/94 in a Class A amp they did back in '07. Interesting read..

I'm absolutely sure that this amp will have a truely wonderful song to sing when Graham's touch has been involved! 
Really interesting to (try to anyway!) follow along in the build and testing. Smile

Kind Regards,
Steve
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Post Options Post Options   Thanks (1) Thanks(1)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 28 Jun 2020 at 8:15pm
There are no secrets on the Internet, or if there are, they're not secret for long...

Advanced designers accept the existence of parasitic emitter follower oscillation. From my in-depth studies, I have to take that nobody has yet solved the problem of output BJT parasitics, but we can learn a few things about them.

1. Inductance, capacitance, and negative resistance combine to make an emitter follower have voltage gain (this is not what we want).

2. The oscillatory voltage gain is mostly short-lived or self-limited (to around 1 volt) and might be set off by a stimulus, which might be a high-frequency signal (therefore, more noticeable at low-level).

3. The "cure" used in an oscilloscope driver seems to be a 2.7-ohm power supply series resistor and a 1.8uF capacitor to ground. This is no use in an audio power amplifier (2.7 ohms in series with an 8-ohm speaker!)

4. A large value resistor in the base lead causes negative resistance to sum to positive. Still, the output impedance is increased, just like adding a resistor in series with the speaker.

5. A lossy ferrite bead placed on the base lead might roll-off the troublesome high frequency, but negative feedback can then cause its own oscillation due to loss of phase margin.

6. The theory of placing a 100uH inductor in series with the supply, and decoupling using 2.2uF (or any values in the vicinity) does not work because of the need for constant current in the voltage amplifier (here, it is achieved with a bootstrap). The amplifier boosts its bass, open-loop, that's all. It could make one think it does work, but proper simulation shows differently. Closed-loop, it measures fine! Audio analyzers can be tricked too.

7. The real solution is to use a much slower output transistor (obsolete) or to not use an emitter follower output stage.

And, on that last point, all my early power amplifier transistor amplifiers used slow output transistors. My 1990s studio amplifier design used complimentary feedback pairs. The Proprius is a "quasi" obtaining its power from a switched-mode power supply.

All my previous designs have neatly avoided the emitter follower problem, not intentionally, but it has left it until the grey matter had thought it was entering retirement!

There is a solution; however, most amplifiers have one. It's called the on/off switch.

Switch it off after use, and the reservoir capacitors should recover sufficiently for the next few hours listening session.

I'll leave you with this one:

The rear cover of the HEWLETT-PACKARD JOURNAL, April 1966 (https://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1966-04.pdf) states: "simple adjustment in bias current will often stabilize the circuit."
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 29 Jun 2020 at 9:46am
"The Grand Vizier's Garden Party (Part 2: Entertainment)" from Pink Floyd's Ummagumma, is purely percussion. In the beginning, it is normally recorded, and (I guess) features the large gong, pictured on the vinyl album's sleeve. Multiple mallet beats make it give a continuous resonant sound, but the mallet beats should also be heard. Snatches, or "samples" of the high hat and woodblock, and the resonating gong, are recorded with varying reverb and/or echo, to give the effect of distant perspective, gradually zooming-in to an up-front sound. These interplay with the regular drumkit, giving way to the snatches, and then back again.

The high hat seems meant to be bright, but not so that it hurts the ears. The work should sound acoustic even during the snatches, or sampled sections, which sound like the instrument's mike channels are being muted - switched on and off. The "cut-up" gong makes a mid-bass noise. None of it should sound like a synthesizer, because drummer, Nick Mason, is doing the track as a drum solo.

This is the sort of insight a good system, with a good amplifier, should teach, without having to refer to Wikipedia.

The amplifier was demonstrating its ability to do this after changing the power supply components. The 100uH inductor was kept, without the damping resistor, and the 2.2uF decoupling capacitor was replaced by a 680uF high-temperature electrolytic capacitor.

How long this will last is anybody's guess.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Dave Friday Quote  Post ReplyReply Direct Link To This Post Posted: 30 Jun 2020 at 1:47am
What about;trying SMP smoothing capisitors?,half wave rectification (25cps),putting a choke in the earthy side of the smoothing capisitor,and stick your rattling choke in some melted wax?


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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 30 Jun 2020 at 7:49pm
Originally posted by Dave Friday Dave Friday wrote:

What about;trying SMP smoothing capisitors?,half wave rectification (25cps),putting a choke in the earthy side of the smoothing capisitor,and stick your rattling choke in some melted wax?




Could be a great idea!

After all, I've tried everything Doug Self, Bob Cordell, John Linsley Hood, Paul Horowitz, Winfield Hill, Matti Otala, Bernard Oliver, Russell Hamm, Walter Jung, David Hafler, Huang Lin, Jack Dinsdale, Ian Shaw, Morgan Jones, Michael Chessman, Nathan Sokal, and that guy from Decca, have suggested.
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