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 Post subject: Determining input impedance
PostPosted: Jul Tue 21, 2020 5:18 pm 
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Joined: Nov Mon 05, 2018 3:16 am
Posts: 52
Location: Severance, Co. 80546
Hello all. Hope you are well. Since I've never been one to give much credibility to the "if it ain't broke, don''t fix it" mentality, I recently installied a low wattage amp/bluetooth device on a Philco 41-280. I used the amp because it had treble, bass and volume controls which adds significantly to quality and quantity of the input.

http://www.nostalgiaair.org/Resources/download.asp?FN=\M0013414.pdf

However, I needed to find a suitable impedance matching transformer. I happened to have a Hammond 125ASE output transformer with 10,000 ohm primary and multiple (3.2 to 16 ohm) outputs). I reversed the transformer with the speaker side facing the amp source and 10K side hooked to the volume control. I also replaced the original speaker with a small Sony bookshelf 3 way speaker. I maintained the field coil resistance with a 1500 ohm, 25 watt resistor I happened to have on hand. It worked great and the sound from a bluetooth device, like my android phone, is nothing short of astounding.

As to the impedance, the transformer selection was totally by chance, not by any technical calculation. If the transformer was used as intended it would have been easy to select the proper output impedance by simply looking at the tube data for the output tube(s).

However, how do I determine the proper impedance on the input (grid) side of a tube.

Thanks Steve


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 Post subject: Re: Determining input impedance
PostPosted: Jul Tue 21, 2020 8:01 pm 
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Joined: Nov Tue 14, 2017 5:09 am
Posts: 3074
Location: Austin, Texas
A tube grid is a very high impedance at audio frequencies. You put a resistor across the transformer winding and that is the impedance.

Jay


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 Post subject: Re: Determining input impedance
PostPosted: Jul Wed 22, 2020 11:42 am 
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Joined: Jan Thu 16, 2020 12:29 am
Posts: 779
It is all very simple.

If you want to determine the input impedance, at audio frequencies, of an input, all you need to do is get a 1kHz sine wave signal generator and a scope and a variable resistor.

Apply the signal generator via the series variable resistor to the input. Ideally the generator has a low Z output like 50 or 75 Ohms. Adjust the resistor so that the voltage at the input of what you are testing is half of that directly out of the generator. If it is a few k or more you can ignore the generator output Z. Then measure the variable resistor on the Ohm meter and you will have your input impedance.

In most cases though, with audio frequencies, its preferable that the output stage of the driving device has a low Z to overcome cable capacitances, and the input of the system has a high Z, say a few k or more.

The only time you need to worry about matching impedances is when there are few scenarios;

1) You want to transfer power, such as an amplifier output to a speaker in a tube amp, then the max power transfer will occur when you have a load impedance that matches the output impedance of the amplifier. Having said that, semiconductor based power amps don't seek to do this at all (an impedance match) they simply have a very low Z output compared to the speaker impedance.

2) When you plan to drive a long length of cable with significant capacitance and inductance per unit length, where preferably the cable impedance matches the driving device's output impedance, and that it is terminated with the same value at the input of the device you are driving. These sorts of issues are much more significant with video or radio frequencies (up to 5MHz and greater) vs audio frequencies where it is much less of a concern.

If you have a transformer secondary feeding a tube's grid with no load resistor, and the grid is not drawing current (class A bias) the impedance of the transformer looking into the primary is complicated. If you place a high value resistor across the secondary, the primary impedance can then be calculated from the transformer's impedance ratio.


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 Post subject: Re: Determining input impedance
PostPosted: Jul Wed 22, 2020 5:23 pm 
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Joined: Jan Tue 16, 2007 7:02 am
Posts: 3914
Location: Lexington, KY USA
Note, that in this sort of application, you are not matching any impedance in the audio engineering sense. That is a different thing.

You want the input impedance of the driven device to be much higher than the source impedance driving it. When driving a tube amplifier from any real-life transformer, this is no problem, as the input impedance of the amplifier is so high.

Modern solid state signal sources designed to drive earphones usually have such a low output impedance that it might as well be considered to be zero. It's well under one ohm. What is important is the expected load impedance for the output. The earphones are often rated for 32 ohms. This is the number you need to work with.

It is easy to find transformers that will present too low an impedance to such an audio source. Particularly at the bass end of the audio frequency range. It is probably best to look for transformer windings rated for 2 to 3 times the expected load impedance.

By using a winding rated for a somewhat higher impedance, you will extend the low frequency limit thorough the transformer. This is particularly helpful when the small transformer is only rated down to 300Hz.

So a transformer winding rated for use at at least 50 ohms, and up to perhaps 600 ohms is what you want to use.

Ted


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 Post subject: Re: Determining input impedance
PostPosted: Aug Sat 01, 2020 5:21 pm 
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Joined: Nov Mon 05, 2018 3:16 am
Posts: 52
Location: Severance, Co. 80546
Thanks all for the great info. i've already put it into practice.

Thanks Steve


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