Audio Transformer Inductance

[PRR]

> That sounds huge for mic source impedances

1,884 ohms at 20Hz. A common rule is to load 200 ohm mikes in 2K. If taken literally, this barely fails.

However the winding was probably designed for very low bass THD rather than simple impedance. Iron distorts bad. Less if self-impedance is high. Push-pull power output transformers often have low-level bass to 5Hz, not for the 5Hz, but for decent THD at 50Hz.

I agree that inductance (or lack of it) is interesting. But only one point of interest.

 

[CJ]

every meter i have used has given different readings for inductance,

but if you use the same meter, you get a idea of how different iron compares to each other,

the gen rad bridge allows for a signal generator input, so you can measure inductance at any frequency, and also adjust the signal level,

most hand held bridges measure at 1 k hz, so the bottom end inductance is not well represented,

 

[ruffrecords]

I agree that inductance (or lack of it) is interesting. But only one point of interest.

Agreed. It just seems fairly fundamental to me. If the inductance is too low, all the other points are fairly... pointless. And I did not have an easy means of measuring it and now I do. This is just a first step. I make no claims to any expertise in transformer design but I like to be able to get a broad feel for a device despite the manufacturer's lack of detailed info.

Inductance and DCR are fairly easy to measure. What would be. in your view, the next most important parameter to measure? Leakage inductance would be quite easy simply by shorting secondaries.

Cheers

Ian

 

[AZ999]

i just found this email from 2007, primary inductance of Cinemag CMMI-7(C) is ~ 30H, meassured (bridged) by David from Cinemag.
Btw, original Redd47's input transformer had 15H of primary inductance.

That sounds huge for mic source impedances.

Did he happen to give you sec L as well?

Sorry, back then i didn't even know what leakage inductance is. If this would really help you i can ask about it.
Here are some parts of David's replies you or others might want to read and discuss. Parts about hum-bucking winding, THD at 20Hz and amorphous/tape cores seem very interesting to me. I deleted what i feel is too personal to post without asking:

"Transformer distortion is primarily a function of source impedance, load impedance, core characteristics, and winding design.  It is independent of whether the source is solid state or not.  However, most solid state amplifiers are run at lower voltages for best slew rate and (hence) bandwidth.  "
"From what I have seen from ..., they are using dual bobbins hooked up in a hum-bucking mode.  They will reject hum fairly well without mu-metal shielding.  However, there are a number of performance trade-offs with this design.  We do not use that with output transformers because it takes a lot of mass in the lamination stack to enable them to work at the power levels which are required."
"Twisting leads helps, of course, by canceling out common mode signals being picked up by them.  (In effect, the wires act both as an antenna and a transformer secondary.)  "
"We have looked at amorphous cores and tape cores for professional audio.  We do not like them very much.  As noted, output levels are not as good.  Also, look at THD data at 20Hz.  You might assume that this is not important because the human ear is not very accurate at such a low frequency.  Wrong.  Small errors in the signal cause all sorts of higher frequency problems (i.e. harmonic distortion and inter-modulation distortion) which fall at the beginning of the range that our ears are most sensitive to."

 

[MatthisD]

Ian, if you were to get your hands on an OEP tx, I would love to see some data there... They seem to be in the same proce range as edcor, but slightly higher quality (and a bit cheaper). I do love that edcor is so small, their facebook page has some great pics of the facility and the employees seem to take pride in what they do.

Their datasheets are quite detailed compared to others, like for this 600ohm 1:1 they give the Lam type, core, along with set parameter inductance.
http://www.oep.co.uk/pdf/A262A6E.pdf

I've got three different OEPs and access to an inductance meter so I'll measure them and post the readings. The Z300 type are the ones that I'm most interested in, it doesn't seem to clip as nastily as the others. They don't list the Lam or core type for it but by the looks of things its an E type 50% Ni. for the 600ohm 1:1, DC of each winding is listed as 30ohms so it could be bifilar wound and might be useable for some circuits. The problem is that its €22.67 (+vat) and Canford dont have it. I have the 10k version so I'll try and test it today.

 

[Mzaar]

most hand held bridges measure at 1 k hz, so the bottom end inductance is not well represented,

An exotic knick-knack (DMM) which measures inductance at 100 hz  :

http://www.ebay.com/itm/LCR-Inductance-Capacitance-Digital-Multimeter-Meter-RCL-/300418629699?pt=LH_DefaultDomain_0&hash=item45f2588043

 

[ruffrecords]

most hand held bridges measure at 1 k hz, so the bottom end inductance is not well represented,

An exotic knick-knack (DMM) which measures inductance at 100 hz  :

http://www.ebay.com/itm/LCR-Inductance-Capacitance-Digital-Multimeter-Meter-RCL-/300418629699?pt=LH_DefaultDomain_0&hash=item45f2588043

But unfortunately it can only measure up to 20 Henries.

Cheers

Ian

 

[abbey road d enfer]

I agree that inductance (or lack of it) is interesting. But only one point of interest.

I don't understand that. To me it's like saying the capacitance of a capacitor is interesting, but only one point of interest.
Inductance is the primary (no pun) parameter when designing a transformer-based circuit. Indeed immediately followed by others.
Designing as in assessing the various parameters in order to meet requirements dictated by the desired performance and chosen topology, not as in browsing the pages of a catalogue and choosing one that looks suitable.

 

[lassoharp]

I have a medium quality LCR that measures at 1K and 120Hz  and up to 200H and it always been a question as to what true L is at the lower end.  Still seems useful enough to sort what's what and gave relatively good accuracy for small EQ inductors.

 

[ruffrecords]

I have a medium quality LCR that measures at 1K and 120Hz  and up to 200H and it always been a question as to what true L is at the lower end.  Still seems useful enough to sort what's what and gave relatively good accuracy for small EQ inductors.

I agree 100% with that. My LCR meter does 100Hz, 120Hz and 1KHz so it is pretty much the same. I find the measured inductance of iron core based transformers increases at the lower frequencies. I also tested some of the Carnhill EQ inductors and found that they were pretty much identical at 100Hz and 1KHz, presumably because they are ferrite cored.

Cheers

Ian

 

[ricardo]

What would be. in your view, the next most important parameter to measure?

The maximum level at some low frequency.

Taken as 3% thd (mainly 3rd) when I was talking to transformer people in Jurassic days.  With matching source & load.

This parameter actually scales nicely with frequency (unlike inductance).  So 3% thd @ 0dBu 100Hz will be -6dBu @ 50Hz, -12dBu @ 25Hz etc.

 

[PRR]

> like saying the capacitance of a capacitor is interesting, but only one point of interest.

The flaws of capacitors are small. DCR and leakage are a thousandth of reactance at most frequencies, value shift with signal level rarely 10% and often <0.1%, THD in sensible circuits <<1%.

Audio inductors have DCR more like 10%, core loss only a bit better, 2:1 change of inductance with reasonable signal levels, THD may approach 10%.
_________________

> next most important parameter

After weeding out the lame ones, I'd put it between a source and load and look for limits and anomalies. There's way too much funny-stuff in iron-core audio to focus on any one measure.

 

[CJ]

phase shift is fun to look at,
happens at low and high end,

you can measure leakage inductance by shorting the sec and measuring pri mH,

but the effects are visible without the measuring, find the res peak of the transformer,

some transformers run flat unloaded, some not,

others have small bumps in the mid-range that contribute to their sound,

 

[abbey road d enfer]

> like saying the capacitance of a capacitor is interesting, but only one point of interest.

The flaws of capacitors are small. DCR and leakage are a thousandth of reactance at most frequencies, value shift with signal level rarely 10% and often <0.1%, THD in sensible circuits <<1%.

Exactly what I say. Nominal value is the most.

Audio inductors have DCR more like 10%, core loss only a bit better, 2:1 change of inductance with reasonable signal levels, THD may approach 10%.

Does it make these parameters the primary design parameters when you go to a transformer manufacturer an dorder a custom-built part? No. The primary design parameters in a transformer are power and LF response, which, in combination determine the nature and size of the core and the inductance. And after that, you deal with the idiosyncracies. You know it well.

> next most important parameter

After weeding out the lame ones, I'd put it between a source and load and look for limits and anomalies. There's way too much funny-stuff in iron-core audio to focus on any one measure.

I can't believe such an non-scientific answer comes from you. You must be in a guitar-playing streak. 
BTW, how do you "weed out the lame ones" without looking, directly or indirectly, at the inductance?

 

[CJ]

western electric 111c, is that right, or 117c?

anyway, bass response is 1% thd at 1 hz,

not really, but if you want to hear the effects of more henries at 20 hz, that is the best out of all the iron i have scoped.

it was hard getting readings at 2 hz, need a tracer scope like you see on ben casey re-runs,

W.E, huge chunk of core material, no lams, just a spool just like you get when you buy the metal, welded shut, cloth wrapper, and precisely balanced windings for rejection,

but climbing the poles to get the iron is a real pain, 

 

[lassoharp]

huge chunk of core material

something unobtanium?

 

[trobbins]

What would be missing if just the old Williamson type method was used for OT primary inductance, based on 5V at mains frequency and a current reading.  It's an easy technique to standardise on for that parameter at least for OT's.

 

[MatthisD]

I've got three different OEPs and access to an inductance meter so I'll measure them and post the readings.

OEP Z3003e primaries in series for 1:1
Using handheld meter
primaries 65.5H, secondaries 65.5H

Using HP 4262A meter
at 120Hz - 82.6H
at 1k - 45H
at 10k - no reading, it will only read up to 10H at 10k.


OEP Mic input 1:6
Using handheld meter
Primaries 5.83H

Using HP 4262A meter
Primaries at 120Hz - 10.5H, at 1K - 6.9H
Secondaries at 120Hz - 318H



Some others measured with handheld meter
Beyer 1:10 Mic input
Primaries - 5.43H

Beyer 1:15 Mic input (tiny sized canned model)
Primaries 4.38

Lundahl LL1540
Primaries - no reading
Secondaries  - 117.6H and 117.3H for each winding

Jensen JT-123-S 1:1 (600ohm)
dual primaries in series - 9.43H
dual secondaries in series - 9.43H

 

[ruffrecords]

Thanks for those readings, I'll add them to the document.

Cheers

Ian

 

[lassoharp]

OEP Mic input 1:6
Using handheld meter
Primaries 5.83H

Using HP 4262A meter
Primaries at 120Hz - 10.5H, at 1K - 6.9H
Secondaries at 120Hz - 318H

That's interesting.  Looks like 4262A routinely gives higher L numbers.  Assuming handheld reading is @120Hz, what do we do?  Interpolate or simple avg?

A-10 falls smack in the middle on handheld. (pri)