Mic preamp input impedance and transformer impedance

  Hello, I'm trying to have a better comprehension of transformers impedance ratio.
 
In this page http://www.jensen-transformers.com/transformers/mic-input/ mic transformers seem to have a 150 Ohm input impedance.
And here for instance http://www.jensen-transformers.com/wp-content/uploads/2014/08/jt-13k7-a.pdf the jt-13k7-a's imput impedance is rated at 1.5K.
Is input impedance of a transformer different from the value in the impedance ratio?

I'm no expert but I think you have to differentiate between load impedance and input impedance.
The Microphone has an ouput impedance of about 150 Ohm. So I think that is the load impedance. The input impedance of the ampliefer should be about 10x higher than the load impedance so the microphone doesn't get loaded down.  That's were youre 1.5k impedance comes from.
I hope that is right. Please someone correct me if I'm wrong.
Also, there is alot more to this but I'll leave that to the more experienced members.

Thanks, also I found this method to measure transformers' input impedance : http://www.crystal-radio.eu/entrafometing.htm

Do you think the measurement indicates load impedance or input impedance?

The microphone is the source impedance.

The input impedance of the preamp is the impedance of the preamp input circuitry (the load impedance) reflected down through the input transformer.

If I measure the input impedance of a jt-13k7-a with the technique explained here http://www.crystal-radio.eu/entrafometing.htm will I obtain something close to 150 ohm or1k5 ?

The nominal impedances (like "150R:15k") of a transformer are only virtual, they describe conditions under which the transformer is optimized (e.g. regarding inductance/frequency response).

It has no impedance of it´s own, it just reflects/projects the source impedance to the load to get optimum noise conditions (read up on OSI- optimum source impedance)and the load impedance to the source to properly load the microphone. Both in a way depending on the square of the transformer winding ratio -> 1:7 winding ratio gives 1:49 impedance ratio etc.

And then there is DC-resistance, but that´s another story...

Transformers get their name from their property to "transform" impedance between primary and secondary.

Since the winding turns ratio predicts a voltage drop (or increase) and power is relatively constant, so current moves to keep input power and output power in balance.

The simple rule of thumb predicts that impedance transforms between primary and secondary varies with the square of the turns ratio (this can be proved by power equations. If you raise the voltage the current drops to keep constant power).

A typical mic transformer could have 10X turns ratio (and that makes my math easier) so impedance transforms by 10^2 or 100x.  So a nominal 150 ohm transformer will look like a 15k ohm source at the secondary... Likewise a 150k termination at transformer secondary will look like 1.5k at the primary.

There is additional confusion about actual winding impedance, but this is mainly about turns ratios.

JR

So this has no sense : http://www.crystal-radio.eu/entrafometing.htm ?

saint gillis said:

So this has no sense : http://www.crystal-radio.eu/entrafometing.htm ?

Click to expand...

Yes it has sense. It says: "The input impedance depends on the load resistor."

Ok, but the result of the measurement gives us a value called "input impedance of the transformer" that is independent of the load resistor.

Without any intention to hi-jack this thread, but regarding the impedance ratios stated by the manufacturer, for example 150:600, that only means that the transformer is optimized to work at those impedances right? What are the differences for example between a 10K:10K transformer and a 600:600 transformer, the turn ratios are the same but what about the number of turns ? or the core?

I've read this http://www.lundahl.se/wp-content/uploads/datasheets/PSW_WhitePaper_Download_Chapter_6.pdf.

It says that 2 problematic things in a transformer is leakage inductance and internal capacitance.

You can minimize leakage inductance by interleaving more the primary and secondary windings, and that you can minimize internal capacitance by less interleaving.

When the load impedance is high, leakage inductance is less a problem, and when the load impedance is low, the internal capacitance is less a problem.

if you ignore the secondary of a transformer, then you can say that the primary is just an inductor.

this inductor has an impedance determined by the number of turns and the core that the turns are wrapped around.

core size and type of steel used will determine inductance and therefore the impedance by the formula XL=2 pi f L

the higher the inductance, then the higher the impedance, but there is a catch.

in an inductor, the impedance will depend on the frequency across the inductor.

usually a transformer will have an impedance rating at a certain frequency.

reflected impedance is associated with the ratio of the transformer, not the inductance of the pri/sec.

so you have two types of impedance associated with a transformer,  the first type is associated with the enviroment you are working in, ie, 150 ohm mic, 600 ohm line, 10K bridging,

usually for voltage transfer we want the transformer primary to have about 10 times the impedance of the source,

so a 600:600 might have a Reactance of 6000 ohms at 20 Hz, (so we need 4.7 Henries of pri inducance)
and a 10K might have 100K ohms of Reactance in order not to lose too much signal. (so we need 80 Henries pri ind for he 10K:10K transformer,) this is so we do not load down the source as it usually can not supply much current, so we need hi resistance(inductance)

this Reactance will depend on he turns and the core. a 600:600 will have fewer turns than  he 10K;10K, etc.

Reactance is just impedance related to what kind of device is creating it, Cap or Coil. both are measured in Ohms.

as mentioned above, reflected load will be a function of the load and the turns ratio,

1:1 with RL at 1K reflects 1K, 
1:2 with RL at 1K reflects 2^2 = 4 so 1:4 Z ratio reflects 250 Ohms
1:4 with RL at 1K reflects 4^4 =16 so 1:16 Z ratio reflects 62.5 Ohms
1:10 = 1:100 Z = 10 ohms reflected
1:20 = 1:400 Z ratio = 2.5 ohms reflected

usually the sec of a signal transformer sits with a very Hi Z input circuit, for tubes at least,
just as you want the source to go into a 10 times load, you want the sec to go into a 10 times load, usually no a problem with tubes, so you might have a 1: 10 xfmr reflecting 100 times a 150 ohm mic, or 15K, into a grid of many megohms, so no loss of signal would be expected.

sometimes there are resonant peaks associated with a hi ratio xfmr, in which case you will see a resistor stuck on the sec  to dampen this out a bit. this comes at he expense of signal current, but usually this current is in the 60 K Hz range so we do not care.

CJ said:

if you ignore the secondary of a transformer, then you can say that the primary is just an inductor.

this inductor has an impedance determined by the number of turns and the core that the turns are wrapped around.

core size and type of steel used will determine inductance and therefore the impedance by the formula XL=2 pi f L

the higher the inductance, then the higher the impedance, but there is a catch.

in an inductor, the impedance will depend on the frequency across the inductor.

usually a transformer will have an impedance rating at a certain frequency.

reflected impedance is associated with the ratio of the transformer, not the inductance of the pri/sec.

so you have two types of impedance associated with a transformer,  the first type is associated with the enviroment you are working in, ie, 150 ohm mic, 600 ohm line, 10K bridging,

usually for voltage transfer we want the transformer primary to have about 10 times the impedance of the source,

so a 600:600 might have a Reactance of 6000 ohms at 20 Hz, and a 10K might have 100K ohms of Reactance in order not to lose too much signal.

this Reactance will depend on he turns and the core. a 600:600 will have fewer turns than  he 10K;10K, etc.

Reactance is just impedance related to what kind of device is creating it, Cap or Coil. both are measured in Ohms.

as mentioned above, reflected load will be a function of the load and the turns ratio,

1:1 with RL at 1K reflects 1K, 
1:2 with RL at 1K reflects 2^2 = 4 so 1:4 Z ratio reflects 250 Ohms
1:4 with RL at 1K reflects 4^4 =16 so 1:16 Z ratio reflects 62.5 Ohms
1:10 = 1:100 Z = 10 ohms reflected
1:20 = 1:400 Z ratio = 2.5 ohms reflected

usually the sec of a signal transformer sits with a very Hi Z input circuit, for tubes at least,
just as you want the source to go into a 10 times load, you want the sec to go into a 10 times load, usually no a problem with tubes, so you might have a 1: 10 xfmr reflecting 100 times a 150 ohm mic, or 15K, into a grid of many megohms, so no loss of signal would be expected.

sometimes there are resonant peaks associated with a hi ratio xfmr, in which case you will see a resistor stuck on the sec  to dampen this out a bit. this comes at he expense of signal current, but usually this current is in the 60 K Hz range so we do not care.

Click to expand...

I think I get it now, so basically a 600:600 ohm transformer will reflect impedances the same way as a 10k:10k  but depending on the source and load conditions, its better to use a 600 ohm rather than a 10k ohm and viceversa? so if one should consider something like the thevenin equivalent at the output of the transformer, the impedance of the transformer itself would be in series  with the load ? . In which case suppose that the source impedance of an equipment is really low as to neglect it on the secondary side, and I use a 10k:10k transformer to connect it to say an interface with a 10k input impedance, would it produce a 1/2 voltage at the load at a certain frequency due to the 10k impedance of the trafo and the 10k of the load, just like a voltage divider would ?

What about the output of a power amplifier, wouldnt you want a transformer with the same impedance of the load, to transfer the max. amount of power?

sounds like you got it, usually there is only one primary and one secondary with audio transformer work, so you do not need complex math to solve impedance questions,

now with an output transformer you want the pri Z to be close to equal the power tube's Dynamic plate resistance in order to transfer max power. 

so the condensed version goes like this:

primary inductance should not drag down the transducer or signal you are trying to amplify,

turns ratio will depend on what type of amplification device you are trying to activate.

CJ said:

sounds like you got it, usually there is only one primary and one secondary with audio transformer work, so you do not need complex math to solve impedance questions,

now with an output transformer you want the pri Z to be close to equal the power tube's Dynamic plate resistance in order to transfer max power. 

so the condensed version goes like this:

primary inductance should not drag down the transducer or signal you are trying to amplify,

turns ratio will depend on what type of amplification device you are trying to activate.

Click to expand...

Thanks CJ, BTW is there a book or something that explains this more in depth? I've seen the Transformers META but theres a lot of books listed in there, I dont know which one addresses this specific topic.

So what is the value the guy is trying to measure here : http://www.crystal-radio.eu/entrafometing.htm  ?

> the result of the measurement gives us a value called "input impedance of the transformer" that is independent of the load resistor.

No.

Have you tried it yet?

Yes, with 33K, but I didn't try to change the value...

user 37518 said:

Without any intention to hi-jack this thread, but regarding the impedance ratios stated by the manufacturer, for example 150:600, that only means that the transformer is optimized to work at those impedances right? What are the differences for example between a 10K:10K transformer and a 600:600 transformer, the turn ratios are the same but what about the number of turns ? or the core?

Click to expand...

A transformer is basically a pair of coupled inductors. The impedance of an inductor varies with frequency getting smaller as the frequency gets lower. This impedance forms a potential divider with the source. So, for a 150 ohm source like a mic, the impedance of the inductance should be no more than 150 ohms at 20Hz if we want the bass response to be no more than 3dB down at 20Hz. So this determines the number of turns necessary.

Also, the impedance of an inductor is directly proportional to the inductance, so a 10K transformer needs an inductance 10,000/150  = 67 times that needed for a 150 ohm one ( for the same bass response) which requires a lot more turns if the same core is used.

Cheers

Ian