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Old 25th April 2006, 07:34 AM   #11
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Quite so, EC8010.

But then I would still have expected Harold Leak to have noticed some effect from that 1nF with whatever equipment he had, so what could it have been?

I have noticed on occasion a C9.C1 network used to take over capacitively so to speak from R18.R4 (using the Leak circuit designations), ostensibly to swamp other undesirable capacitive effects from wiring or whatever. It also seemed an early attempt at the phase-lead-input-lag kind of feedback equalisation as opposed to the Cdom kind (R.C over the first anode load). This Leak does rather suffer from the input ECC83 Miller effect, curtailing loop gain early at h.f. (which is why I am not in favour of using this tube for inputs - but that is another subject).

But all that does not work here. One can also frown at the relatively long lengths of wiring used in the low impedance side of the feedback where C1 is supposed to act. (You would not build your medium wave radio with such leads.) So ........

Regards and thanks for your input.
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Old 25th April 2006, 07:46 AM   #12
SY is offline SY  United States
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May I toss out a hypothesis about the 1nF based on no evidence other than the results that Johan has reported? I'd guess that it is to allow the spec sheet to show the amplifier to be flat to 20kHz. The ringing is a consequence of trying to extend the high end to make the specs be competitive. No doubt the designer felt that the sonic consequence was minimal...
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Old 25th April 2006, 08:24 AM   #13
EC8010 is offline EC8010  United Kingdom
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I've just been leafing through Kevin Spicer's "Firsts in High Fidelity" book that is a history of Leak (interesting book by the way) in the hope of picking up a clue and I also reread this thread. Spicer reports that Leak had extremely long loudspeaker cables to allow stability testing and it occurs to me that they would be a good aerial. Perhaps the purpose of that 1nF capacitor was to shunt any RF picked up by the aerial that might overload the (ludicrously sensitive) input stage and be demodulated?

Regarding SY's hypothesis, I noticed that the original TL12 amplifier was expected to be flat 20Hz - 20kHz +/-0.1dB. I would expect that to have been a very tricky measurement using 50s and early 60s test equipment...
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Old 25th April 2006, 11:17 PM   #14
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SY,

Ye-e-e-s - but the amp easily reaches 20 KHz although the loop gain is 3 dB down at about 14 KHz. (That can be moved up by decreasing input serie resistor R2 from 22K to 3,3K or so as said previously.) As said C1 does nothing until way out of the audio range. The time constant with R4=100 ohms is at 1,6 MHz! (although phase influence shows earlier, but still at quite over 100 KHz). Nevertheless a very valid remark.

EC8010,

I did not think of that. The amp is still here and I will try to see if I can induce any rf effects. I think your remark is probably close. Then I would very much like to have a look at the Kevin Spicer book. I would imagine it is out of print, but can you kindly give the publisher, please?

Also, thanks to both for not picking up on my error regarding the phase-lead-input-lag remark! I was staring blind at the present C1 and forgot that it goes to common. For input lag it of course needs to go cathode-grid instead (with a much lower value).

Regarding the +/- 0,1 dB: If one should measure at 5Vp output (about 1W into 15 ohm), such a difference would show an output of 5,06Vp; indeed a tricky measurement; if by scope not much more than the scope trace width those days. I presume a vacuum tube voltmeter with a big meter face could show that, but then how did you determine that the VTVM is accurate to that degree in the first place?! Paper started to be patient a long time ago.

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Old 26th April 2006, 04:55 AM   #15
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They had AC voltmeters in those days. VTVM technology.
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Old 26th April 2006, 07:40 AM   #16
EC8010 is offline EC8010  United Kingdom
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It would have been nice if I had got the author's name right. "Firsts in High Fidelity" Stephen Spicer. Audio Amateur Press. (2001) I looked on the audioXpress site but couldn't find it. Amazon (UK) claimed to be able to get it, but at the princely sum of 68.70! Looks like you may have to go to Alibris.
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Old 27th April 2006, 12:27 AM   #17
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Much obliged.
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Old 27th April 2006, 08:10 AM   #18
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Stone Audio (www.stoneaudio.co.uk) are still advertising the book at its original price of 24.95.
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Old 27th April 2006, 12:37 PM   #19
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Hi Johan,
Partsconnexion has it at $29.95 US here You have to click on the books tab at the bottom of the page. It's under "Vacuum Tube Audio Book".

-Chris
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Old 28th April 2006, 04:30 PM   #20
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It's a very entertaining book, well written, factual, detailed.. I recommend it highly.
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