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28th February 2005, 03:04 PM  #1 
diyAudio Member
Join Date: Apr 2004
Location: CA

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 ? 
28th February 2005, 03:49 PM  #2 
diyAudio Member
Join Date: Aug 2002
Location: Sofia

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. 
28th February 2005, 04:09 PM  #3 
diyAudio Member
Join Date: Apr 2004
Location: CA

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 ?

28th February 2005, 04:42 PM  #4 
diyAudio Member
Join Date: Aug 2002
Location: Sofia

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 
28th February 2005, 04:44 PM  #5 
diyAudio Moderator

Yes, either one will probably work just fine.
If you want a deeper understanding, check out the Radiotron Designers Handbook.
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28th February 2005, 08:35 PM  #6  
diyAudio Member
Join Date: Apr 2004
Location: CA

Ok thanks, I will check that out...
analog_sa: Quote:
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 12k16kohm 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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