Alignment Advice.

[Joe]

600Ω, as far as I know.

- Joe

[AlanBeckett]

That's what the 204C Manual says.

Alan

[Mark Hennessy]

Pity - most audio gear I use has 600 ohms as an option, but defaults to low-Z. In audio terms, 600 ohms went out with the dinosaurs.

With that in mind, assuming it's a pure 600 ohms at those sorts of frequencies, we could re-think the numbers (I'll get a chance later this morning). Alternatively, a buffer between the Marconi and the test circuit could be built - I wonder how good the LM386 is at ~1MHz

[Joe]

Don't worry about it, Mark. I only started playing about as Al had asked about the inductance. It does what I want it to whatever the numbers are.

Cheers

Joe

[AlanBeckett]

In audio terms, 600 ohms went out with the dinosaurs.

What's perceived wisdom these days?

Alan

[Mark Hennessy]

What's perceived wisdom these days?

Alan

Current practice is low-Z out, hi-Z in. Typically less than 50 ohms out and more than 10k in. It's been this way for decades.

Matched impedances aren't useful for audio as transmission line theory doesn't apply over practical lengths. All it does is rob you of 6dB of headroom and oblige you to use distribution amplifiers to feed a signal to more than one place...

We still have to mention it because of the occasional bit of legacy gear that we might want to use.

Of course, for digital audio things are quite different

Mark

[Mark Hennessy]

Don't worry about it, Mark. I only started playing about as Al had asked about the inductance. It does what I want it to whatever the numbers are.

Cheers

Joe

I wasn't sure why you needed to know, but I guess it's a useful learning exercise if nothing else.

If curiosity still bites, try connecting the loop across the output - you'll now have a high-pass filter with Zout forming the resistance. You might need to add to Zout, depending on the values. Ideally, we want a nice "passband" at the high frequencies, then a clear "rollover", and the -3dB point. If you can observe these, then we can calculate L.

It's what we're doing this morning at work, by coincidence...

[Joe]

Connected like that the amplitude goes down to 5.6V (p-p) at 410 kc/s.

- Joe

[Mark Hennessy]

Hi Joe,

I reckon that's 233uH. Not a million miles away from the earlier result. Assuming you got the full 8 divisions at higher frequencies, and that the HP is pretty resistive, then this is probably the most believable result so far.

If I was clever, I could substitute this new value into the earlier circuit, taking the 600 ohms into account, and prove that it was right there as well. But that would probably involve drawing phasor diagrams or similar. I'll leave that to the real experts

[Skywave]

Seems I may have opened a can of worms by asking about that loop Joe! Sorry if that has caused you a headache or two, but questions about basic components (e.g. capacitors & coils) in R.F. ccts. will always induce a technical response - which should come as no surprise, since Radio, per se, is a technical subject. However, thanks for your reply.

Anyway, before I develop my question about that loop, am I correct in understanding that you are using it with a transistor radio which has its own ferrite rod antenna (albeit giving a somewhat dubious performance on account of its breakage) ?

Al. / Mar. 13th., 2013 //