rackmonkey

Re: Mic preamp input impedance and transformer impedance
« Reply #40 on: December 06, 2017, 09:53:47 AM »
Ok, it's getting less foggy..

What about this 1:10 mic transformer (150:15k) : http://www.jensen-transformers.com/wp-content/uploads/2014/08/jt-115K-e1.pdf  -  the datasheet rates its input impedance 1.4K and its output impedance to 17K using a specific test circuit shown at the end of the doc, what are these values?

This example illustrates the struggle manufacturers have had figuring out the essential attributes to communicate to consumers (which may be product designers or regular end users) of these complex beasts. Historically, manufacturers just gave nominal impedance figures, and maybe frequency response (see old UTC or Triad catalogs for evidence). The nominal impedances didn’t reflect the actual impedances reflected by the transformer in circuit, but rather the source/output impedances of the devices connected to each side (in this case, a 150 ohm mic at the Input will reflect 17k to a FET or an input tube’s grid/grid leak resistor in the front end of a mic preamp -). This was supposed to help the consumer understand the proper application of the transformer.

Jensen here is taking the “newer” approach of specifying the impedances the transformer actually reflects in circuit. So this model will “show” a 1.4k impedance to a 150 ohm Mic when in the intended circuit, and 17k to the FET or tube. They tell you in the Datasheet both the connected device impedances and the reflected transformer impedances, and usually show a diagram of the test circuit. This is the approach Lundahl and maybe Cinemag take now (IIRC). More detail for a more educated end consumer.

BT
« Last Edit: December 06, 2017, 01:02:41 PM by rackmonkey »
Whether you think you can or you think you can't, you're probably right.


ruffrecords

Re: Mic preamp input impedance and transformer impedance
« Reply #41 on: December 06, 2017, 10:51:10 AM »
Ok, it's getting less foggy..

What about this 1:10 mic transformer (150:15k) : http://www.jensen-transformers.com/wp-content/uploads/2014/08/jt-115K-e1.pdf  -  the datasheet rates its input impedance 1.4K and its output impedance to 17K using a specific test circuit shown at the end of the doc, what are these values?

It is quite simple. the source resistance at the primary is 150 ohms. Add the primary DCR to this and you get nearly 170 ohms; multiply by the turns ratio squared and this 'looks like' 17K from the secondary.

I am not sure how they get the 1.4K input impedance but presumably it is the 150K resistor in parallel with the op amp input impedance making a total of 140K which refelected to the input is 1.4K.

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

rackmonkey

Re: Mic preamp input impedance and transformer impedance
« Reply #42 on: December 06, 2017, 12:53:55 PM »
I get the impression the OP is struggling more with manufacturer specification conventions than the theory itself.  I think that’s why despite all the lucid explanations of how transformers work in a circuit, questions like this are still unanswered.

Just keep in mind that not all manufacturers are taking the same approach w/regard to these specs. When you see something nice and clean like 150:15k specified, that’s done to make it easier to communicate intended use rather than a strict reflection of the arithmetic and electrical laws at work (usually).

BT
Whether you think you can or you think you can't, you're probably right.

PRR

Re: Mic preamp input impedance and transformer impedance
« Reply #43 on: December 07, 2017, 12:32:29 AM »
> what are these values?

Transformer is 1:10 voltage ratio.

Put 150 on the front. This reflects-over to the secondary as 150*(10^2) or 150*100 or 15,000 Ohms.

BUT there are resistances hidden in the transformer. 20r in primary, so really 150+20 or 170 Ohms. Times 10^2 is 17K.

The 2.5K hidden in the secondary also comes into play, for 19.5K.

BUT the test-circuit has 150K termination. 17.5K||150K is 17.25K. I may have slipped somewhere, but 17.0 and 17.25 are all the same to me.

Newmarket

Re: Mic preamp input impedance and transformer impedance
« Reply #44 on: December 07, 2017, 04:47:30 AM »
It is quite simple. the source resistance at the primary is 150 ohms. Add the primary DCR to this and you get nearly 170 ohms; multiply by the turns ratio squared and this 'looks like' 17K from the secondary.

I am not sure how they get the 1.4K input impedance but presumably it is the 150K resistor in parallel with the op amp input impedance making a total of 140K which refelected to the input is 1.4K.

Cheers

Ian

The figures in the spec refer to TEST FIG 1 - that doesn't show an op amp. You're probably referring to the 'Typical Application' drawing ?


> what are these values?

Transformer is 1:10 voltage ratio.

Put 150 on the front. This reflects-over to the secondary as 150*(10^2) or 150*100 or 15,000 Ohms.

BUT there are resistances hidden in the transformer. 20r in primary, so really 150+20 or 170 Ohms. Times 10^2 is 17K.

The 2.5K hidden in the secondary also comes into play, for 19.5K.

BUT the test-circuit has 150K termination. 17.5K||150K is 17.25K. I may have slipped somewhere, but 17.0 and 17.25 are all the same to me.

That should be the 19.5K total you came to in parallel with 150K to give your result.

But anyway, don't we need to put the inductive impedance  of the windings at the specified frequency into the equations (vectorally) ? Given that it is impedance that is specified rather than resistance.
I can't see the actual Inductance values quoted so don't know if the figures add up then ?

ruffrecords

Re: Mic preamp input impedance and transformer impedance
« Reply #45 on: December 07, 2017, 07:09:31 AM »

But anyway, don't we need to put the inductive impedance  of the windings at the specified frequency into the equations (vectorally) ? Given that it is impedance that is specified rather than resistance.
I can't see the actual Inductance values quoted so don't know if the figures add up then ?

Manufacturers very rarely quote inductance values.   At normal frequencies the inductive reactance is so high it can be ignored and the transformer is close to ideal, simply reflecting load and source impedances.  (the mid frequency equivalent circuit doesn't include the primary and secondary inductances). It is only at very low frequencies where the inductive reactance drops  that the inductive reactance affects the frequency response.

Cheers

Ian

Edit: If you go to the DIY section of my website:

http://www.customtubeconsoles.com/diy

and select the Iron folder you will find a couple of useful pdf files in there. The first is one I created after measuring the inductance of a range of popular audio transformers. Several groupDIY members also contributed data for transformers I don't have.

You will also find a pdf of the Audio Transformers chapter of the Handbook for Sound Engineers which covers everything we have discussed here and more.

Cheers

Ian
« Last Edit: December 07, 2017, 07:47:06 AM by ruffrecords »
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

Newmarket

Re: Mic preamp input impedance and transformer impedance
« Reply #46 on: December 07, 2017, 08:38:20 AM »
Manufacturers very rarely quote inductance values.   At normal frequencies the inductive reactance is so high it can be ignored and the transformer is close to ideal, simply reflecting load and source impedances.  (the mid frequency equivalent circuit doesn't include the primary and secondary inductances). It is only at very low frequencies where the inductive reactance drops  that the inductive reactance affects the frequency response.

Cheers

Ian

Edit: If you go to the DIY section of my website:

http://www.customtubeconsoles.com/diy

and select the Iron folder you will find a couple of useful pdf files in there. The first is one I created after measuring the inductance of a range of popular audio transformers. Several groupDIY members also contributed data for transformers I don't have.

You will also find a pdf of the Audio Transformers chapter of the Handbook for Sound Engineers which covers everything we have discussed here and more.

Cheers

Ian

Yeah - the  impedance figures are quoted for 1kHz.
Just trying to make the figures add up really.
Thanks for the link - Great Stuff. I'll take a look as soon as I can.

saint gillis

Re: Mic preamp input impedance and transformer impedance
« Reply #47 on: December 07, 2017, 08:51:15 PM »
I get the impression the OP is struggling more with manufacturer specification conventions than the theory itself.

A bit both but I'm learning a lot thanks to all of you.

My basic idea was to find a good method to measure unknown transformers but I first needed to really understand what is "input/output impedance" before trying to measure it.

I have a soundcard, I have a computer with RMAA, I have a breadboard and op amps, I can simulate different kind of sources and loads, I can solder a 10n capacitor to simulate a very long cable, I can put the transformer in an input and output confirugation. I should find a kind of method...

So the 1k4 input impedance in the Jensen datasheet is still a mystery...

ruffrecords

Re: Mic preamp input impedance and transformer impedance
« Reply #48 on: December 08, 2017, 03:57:05 AM »
A bit both but I'm learning a lot thanks to all of you.

My basic idea was to find a good method to measure unknown transformers but I first needed to really understand what is "input/output impedance" before trying to measure it.
If you can measure primary and secondary inductance you have the info you need to work out the turns ratio and the low frequency performance. If you can measure the leakage inductances you can get a handle on the HF performance (you can measure primary leakage inductance by shorting the secondary and measuring primary inductance and vice versa) Primary and secondary dcr will give you a rough idea of the circuit impedance it is intended for.

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

saint gillis

Re: Mic preamp input impedance and transformer impedance
« Reply #49 on: December 16, 2017, 11:35:40 AM »
Ok let's make some practical work!

I have an small unknown transformer, 3 windings from one side, 1 winding on the other. Let's pretend the 3 windings are the primary, I'll call them A B and C, and the secondary D.



I made some measures :

DC resistance :
A - 5,1Ω
B - 5,5Ω
C - 96Ω
D - 310Ω

Inductance :
A - 35mH
B - 35mH
C - 300mH
D - 4,2H

Turns ratio (letter = primary,  D = secondary)
A - 1 : 14
B - 1 : 14
C - 1 : 5,4

Impedances measured with the technique from Pucho's thread :
A > D  -      12Ω : 2330Ω
B > D  -      12Ω : 2330Ω
C > D -    156Ω : 5200Ω


   What do you think of these measures?
   Should I also make theses measures upside down, like if winding D was the primary?
   How should I measure the frequency response? With a 150Ω source? I'd like to use RMAA and I have a soundcard with a line and a mic input, should I add some circuitry between the transformer secondary and soundcard input?


ruffrecords

Re: Mic preamp input impedance and transformer impedance
« Reply #50 on: December 16, 2017, 12:28:57 PM »
4.2 Henries is about the sourt of primary inductance for a mic pre. 4.2H has an impedance of 528 ohms at 20Hz - just about OK for a 150 ohm mic. The other inductance are much lower; the 300mH would only be good for sources or loads of around 6 ohms.

It is very small so not likely to be much good as an output transformer.

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

saint gillis

Re: Mic preamp input impedance and transformer impedance
« Reply #51 on: December 16, 2017, 01:25:57 PM »
Could be used in 5:1 as a DI output?

Quote
4.2H has an impedance of 528 ohms at 20Hz
What is the math/rule for that?

PRR

Re: Mic preamp input impedance and transformer impedance
« Reply #52 on: December 16, 2017, 02:29:43 PM »
> What is the math/rule for that?

Readily available all over the internet, or in books.

Z= I*f*6.28

4.2H * 20 * 6.28 = 528 Ohms

PRR

Re: Mic preamp input impedance and transformer impedance
« Reply #53 on: December 16, 2017, 03:21:07 PM »
Winding A or B:  0.035 Henries, 5.1-5.5 Ohms DCR

0.035H * 24Hz * 6.28 is 5.3 Ohms. If driven with a ZERO impedance source, the response will fall at 24Hz. And loading will be severe.

In most cases we want the working impedance to be 5 to 20 times higher than the DCR. This suggests around 500 Ohms and about 240Hz bass limit.

This is a 300Hz telephone transformer.

Winding D, 4.2H 310 DC Ohms, has L=DCR around 12Hz, so is a little better. The inductive reactance at say 80Hz is 2110 Ohms, so a cheap chip will drive it OK across the guitar band. The 5:1 ratio to winding C says that 1 Volt from the opamp will deliver 200mV to winding C, which is a plenty hot "mike" level.

Whether it can honestly support a Volt at 80Hz is not yet known. For best performance you want many mA of current, enough to drive the DCR alone (when the inductive reactance is very low). 1.4Vpk/310r is 5mA, which cheap chips can easily do.

Just try it.

saint gillis

Re: Mic preamp input impedance and transformer impedance
« Reply #54 on: December 16, 2017, 03:49:45 PM »
thanks for teaching, only this I didn't understand :
Quote
This suggests around 500 Ohms