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Old 20th February 2004, 12:11 PM   #901
Cheong is offline Cheong  Singapore
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Default Input Impedance

In the original 1969 article, JLH suggested using a FET source follower to increase the input impedance but that would also increase the harmonic distortion. Are there any other simple ways of increasing input impedance but without the penalty of increasing harmonic distortion?
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Old 20th February 2004, 12:36 PM   #902
mikelm is offline mikelm  England
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Default Re: Input Impedance

Quote:
Originally posted by Cheong
In the original 1969 article, JLH suggested using a FET source follower to increase the input impedance but that would also increase the harmonic distortion. Are there any other simple ways of increasing input impedance but without the penalty of increasing harmonic distortion?
You could try increasing the the value of the two resistors that together in paralell form the i/p resistor and also you could look around for an i/p transistor with a higher gain.

Other than that, as JLH says, it might be better to decease the o/p impedence of the preamp.

mike
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Old 20th February 2004, 12:57 PM   #903
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what if you just use a fet as the input transistor? i have used a mosfet there without any problem.
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Old 20th February 2004, 01:18 PM   #904
Geoff is offline Geoff  United Kingdom
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Cheong

The revised input stage that I included in Post #624 (Page 42 of this thread) increases the input impedance of the 1969 version to 100k. Is this high enough to meet your needs?

Geoff
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Old 20th February 2004, 02:38 PM   #905
Cheong is offline Cheong  Singapore
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Quote:
The revised input stage that I included in Post #624 (Page 42 of this thread) increases the input impedance of the 1969 version to 100k. Is this high enough to meet your needs?
Thanks Geoff, it certainly helps.
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Old 26th February 2004, 07:25 PM   #906
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I would like to know how to calculate the heat given off in the individual transisitors in the output stage of a class-A amp such as the JLH. I assume that the following is known (i) the PS voltage (ii) the standing current (iii) the circuit uses two BJTs such as the 2N3055.
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Old 26th February 2004, 10:37 PM   #907
mikelm is offline mikelm  England
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the full voltage rail to rail x the standing current = Total power in each channel. given that the o/p is sitting 1/2 way between the rails the total power will be split equally between the two o/p devices.

cheers

mike
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Old 5th April 2004, 11:50 AM   #908
rclarke is offline rclarke  Australia
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I haven't noticed any postings here for quite some time. Is it still active or has Millwood successfully killed it?
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Old 5th April 2004, 01:01 PM   #909
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Originally posted by rclarke
I haven't noticed any postings here for quite some time. Is it still active or has Millwood successfully killed it?

by your post, I certainly have failed,
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Old 5th April 2004, 07:21 PM   #910
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After the loss of our mentor John Linsley-Hood, I came back to see what had been happening here, only to find that one fanatic is still hitting us between the eyes with his egotistical keyboarding.

Millwood. When an ill person makes the effort to freely post information and to help others (on a day when he might be feeling less unwell) your unthinking, but still quite deliberate, comments come over as being particularly galling.
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In view of the length of this thread, and the now widely spaced photographic postings that were made showing some very fine examples of JLH amplifier construction, is there any way that these could be separately archived for future inspiration, or somehow be edited to a new thread ?
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The UK Electronics World magazine is running an 'In Memoriam' special edition in the June 04 issue which should be on the shelves during May. Unfortunately many readers of this thread will be unable to obtain a copy.
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POWER !!! One of the curious aspects of pure class-A is that the output devices dissipate less heat when more energy is being driven into the loudspeaker.


Cheers ............ Graham.
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