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percy 28th February 2005 03:04 PM

Need help understanding transformer impedance ratios and impedance matching
 
I am a bit confused about impedance ratios and actual impedances of the primary and secondary of a transformer when it comes to matching a source to a load.

An impedance ratio of 10:1 is required - for matching a 10k load to a 100k source.

Now there are two transformers both having an impedance ratio of 10:1 but one of them is spec'd as having a 600 ohm secondary and the other with a 15000 ohm secondary. My understanding is that if I put a 10k load on either of these transformer's secondary, the primary will simply reflect a 10x (100k) load to the source.

Then what does 600ohm and 15000ohm mean ? What impedance is this ?

analog_sa 28th February 2005 03:49 PM

Transformers may have the same ratios but with different number of turns in both primary and secondaries. The more turns in the primary - the more inductance. Higher inductance will allow higher impedance of the driving source within the same bandwidth - in very simplistic terms if you don't have enough primary inductance you can't get good bass from a high impedance source. If the source is low impedance high inductance is no longer essential and better results in the high frequencies may be obtained with a transformer using less wire in the primary.

Many other factors are at play when choosing the best impedance for a transformer but this can give you an idea.

percy 28th February 2005 04:09 PM

so if bass, bandwidth and fidelity are not really important - (the load is a vintage heaphone with a very limited bandwidth anyway, and the source a crystal detector) - then either of these transformers should do ?

analog_sa 28th February 2005 04:42 PM

I have no idea. The piezo source is presumably high impedance, something like megaohm? And earphones are what, kiloohms?


Ooops, sorry i see you've given this data in your first post

SY 28th February 2005 04:44 PM

Yes, either one will probably work just fine.

If you want a deeper understanding, check out the Radiotron Designers Handbook.

percy 28th February 2005 08:35 PM

Ok thanks, I will check that out...

analog_sa:
Quote:

The piezo source is presumably high impedance, something like megaohm? And earphones are what, kiloohms?
I think thats the DC resistance you're referring to. Piezo's are typically megohm and magnetics are ~2Kohm.
Just to satisfy my curiosity I measured the impedance of a piezo earplug and and also a pair of magnetic headphones I had lying around...
The impedance of the piezo drops as the frequency rises. It goes from about 25Kohm at 200hz to settle down to about 1Kohm from 5Khz onwards - like a capacitor.

The impedance of the magnetic phone rises with frequency. From about 2Kohm at 200hz to about 12k-16kohm in the upper frequency range.- like an inductor.

Acoustic measurements reveal a horrible frequency response - barely usable from 200hz to 3Khz.

;)


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