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Old 10th November 2012, 10:01 PM   #1
Alkis is offline Alkis  Greece
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Default Impedance matching

Hi guys,

If I'm not wrong, since the secondaries of the input transformer are open, the reflected impedance to the primary is practically infinite and the source sees only the inductance of the primary which must be around 5K at 100H. Is that right?
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Old 10th November 2012, 10:07 PM   #2
sreten is offline sreten  United Kingdom
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Hi, So what is the useful purpose of the input transformer ? , rgds, sreten.
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Old 10th November 2012, 10:17 PM   #3
Alkis is offline Alkis  Greece
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It's a stepup line input, sreten.
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Old 10th November 2012, 10:45 PM   #4
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A secondary loading resistor is commonly used to dampen the transformer, improving response markedly.

At RF, reactance (Miller effect, phase shifts) and losses contribute a non-infinite impedance. Given the shortcomings of transformers (and DHTs, for that matter), they're of almost no use for wideband audio applications.

Tim
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Old 11th November 2012, 01:32 PM   #5
DF96 is offline DF96  England
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Transformer-coupled amps like that were used back in the olden days when triodes had very low mu. The voltage gain from the transformers was a significant part of the total voltage gain of the amplifier. When higher mu valves became available people switched to R and C instead for coupling, except where grid current would be an issue such as Class B PA output stages.
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Old 11th November 2012, 08:56 PM   #6
Alkis is offline Alkis  Greece
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Thx, but the question remains.
Does the source takes into account just the inductance of the primary of the input transformer?

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Old 11th November 2012, 08:59 PM   #7
DF96 is offline DF96  England
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Yes, to a first approximation. The second approximation would have to include any grid input impedance of the valve (which in reality won't be quite infinite), transformer losses etc.
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Old 11th November 2012, 09:13 PM   #8
Alkis is offline Alkis  Greece
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Thanks DF96.
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Old 12th November 2012, 08:43 PM   #9
jlsem is offline jlsem  United States
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Quote:
wideband audio
Oxymoron.

What is it you want that is beyond the capabilities of a transformer or DHT?

John
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Old 13th November 2012, 03:55 AM   #10
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Hardly.

Given that "audio" is already quite wide (three decades), "wideband" implies an amplifier for audio purposes whose bandwidth is still wider, perhaps 4 or more decades (7-70,000Hz, say), or alternately, the same bandwidth but with much flatter gain or phase demands.

Making a transformer that goes beyond 5 decades is ridiculously difficult (it can be done, but why would you want to?), and even then, transformers that are flat enough for [regular] audio purposes are pricey to begin with.

There are much more demanding applications, mostly towards instrumentation purposes, where flatness of only, say, 1dB would be completely useless. 1dB is a very high tolerance to put on a quality audio amp.

As for DHTs, none have performance (perveance, gain, distortion, efficiency, let alone cost) near heater-cathode types. If you're already going to make a tube amp, you might as well not shoot yourself in the foot with inferior devices.

Tim
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Last edited by Sch3mat1c; 13th November 2012 at 03:58 AM.
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