Impedance matching

[Alkis]

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?

[sreten]

Hi, So what is the useful purpose of the input transformer ? , rgds, sreten.

[Alkis]

It's a stepup line input, sreten.

[Sch3mat1c]

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

[DF96]

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.

[Alkis]

Thx, but the question remains.
Does the source takes into account just the inductance of the primary of the input transformer?

Cheers!

[DF96]

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.

[Alkis]

Thanks DF96.

[jlsem]

wideband audio

Oxymoron.

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

John

[Sch3mat1c]

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