source for cost effective mic preamp input transformers

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CAlbertson

Member
Joined
Aug 19, 2013
Messages
8
Location
Redondo Beach, California
A beginner question:  What are people using for microphone input transformers in tube pre amp builds.  I am NOT looking to build my dream pre first.  I'l start with some economical parts.  I hate to ask if Edcor is any good.

I've built a few audio amps, have lots of experience, an engineering degree and so on.  But I don't know the market for microphone xformers. 

Actually I'd like to eventually develop an ability to build audio transformers.  I want to build a test bed preamp.  So I can start with just "whatever".
 
Ebay or

http://uk.farnell.com/oep-oxford-electrical-products/a262a3e/transformer-1-1-6-45-6-45-150-6/dp/1172344

+ mu metal shield.
 
As you are in the USA then Jensen and Cinemag would be my primary recommendations for tube mic preamps. They are not cheap but they are very good.

The big problem with cheap transformers for tube mic pres is that they usually skimp on primary inductance and screening. Primary inductance determines low frequency performance. You can get by with reduced primary inductance provided your source is a modern SS condenser mic with an output impedance of 50 ohms or less but you will notice a loss of bass with higher impedance sources like dynamic or ribbon mics.

Screening is important too. The mic pre transformer is the most sensitive magnetic component in the pre and if not properly screened it will pick up any extraneous magnetic interference.

Cheers

Ian
 
I think the classicapi prices are not bad , I don't think you want so cheap that you can tell it's cheap
A list of transformer manufacturers in the doc section is a good idea [ don't have it yet ] but I'll start it .
 
bruno2000 said:
How many, what type are you looking for?
Best,
Bruno2000

Yes, I should have said that.  Just one system.  But  what I want to build is a test bed system,  Eventually I'll have the ability to try out different transformers by swapping them.    I want what I call a "modern recording front end"  That means it's purpose is to drive a commuter audio interface

I'm thinking of placing the XLR jack, phase, pad and phantom switches and all the associated passive parts inside some aluminum box.  The XLR and switches, LEDS are all are at one end of the box and they poke through a large oval opening in the amp's front panel.  The other end of the box is connectors for phantom power in and audio out.    Now to find a box large enough for any transformers I might use.  The Hammond die cast boxes look good but the side walls are not at 90 degrees.

In the end I think I'd like four channels but I can start with one and then two  It will all likely go inside a repurposed computer server or and old router chassis about 4U tall.

One construction method I thought of is to make the front end boxes exactly the same size as a 3.5 inch disk drive.  Many computer chassis have front loading slots that are machined to fit 3.5 inch drive.

Mechanical design and construction is always the harder part

 
If you are in an learning experimental stage, you can use transformers from an old mixer.  These can be economical and relatively well made. 

But in the end (unless you need a lot of them) a couple of Jensens may be just as inexpensive compared to the hassles of finding an appropriate ratio for tubes in an old mixer (1:8, 1:10) because most of the mixers use 1:4 TO 1:5 ratio for transistor inputs.

If you are interested in the practicalities of lower cost vs better transformers the usual most obvious and measurable weakness of lower cost transformers is  overloading and distorting or passing a diminished signal at low frequencies (IE Maximum input level and frequency response). 

 
perhaps a bit tangential but, I provided a transformer to CA (OP).
small core, high step-up, from pa mixer CA quickly ran a sweep and provided it to me, i'll try to attach...
something seems wrong thou as I would have expected a lack of low end as Bruce has indicated--CAs results show good low end and significant drop above 5k...
Is the  1:15 ratio V or Z? I thought it was more like 1:10 voltage ratio.
response is acceptable for speech weird dip though
mic%20input%20trans%20freq%20%20plot.jpg
 
Here is my test setup:

An HP oscillator in parallel with a AC VTVM.  I adjust the output to 0.1 VAC RMS.  This connects to the primary side.
I happen to have an identical AC VTVM that is connected to the secondary side.  The load on the secondary is minimal, almost an open circuit as VTVMs are about 11 Meg Ohms.  The two VTVMs are long out of calibration and show about a 2% voltage difference

I swept the oscillator and wrote down the voltages.  My spread sheet converted volts to dB using 20log(v/v)  I used the volts I got at 1KHz as the zero point.

Some other points..  The DCR on the primary is about 37R and the secondary about 4.6K.  THere is an electrostatic shield that was connected to my workbench ground. 

One more point, the frequencies are just "close", the oscillator has a mechanical dial and the calibration has about a 10% error.  I shoud have used my frequency counter in the circuit but I was got lazy.  I did look at the sine wave on my scope and it looked good (but it has to be about 10% THD before I can detect distortion by eye.)

Here is some of the raw data..
10Hz  .1V  1.5V
100Hz  .1V  1.5V
1KHz  .1V 1.5V
10K .1V  2.1V
(I saw this big jump at 10KHz so I went back to got this
2KHz .1V  1.55V
4K  .1v  1.65v
(then go back)
20K  .1v  3.0v
25k  .1v  1.4v
30K  .1v  .8V
35k  .1v  .54v
40k  .1v  .3v
50k  .1v .2v

I can run other tests if some on would like.  I'd like to understand this this.
 
I think the reason that you might be seeing the Low end transferring well is that you are using reasonably low levels (-17dBu/-20dbV).  These are reasonable levels for most microphones.

Usually you see the distortion come in at higher levels (300mv on the little beyer's for instance) but they are all different.

As far as the mid and high end, it is interesting.


I note that the test setup did not have the transformer terminated and this can lead to ringing, especially in cheap trafo's.  I also notice the rolloff starting at 4K, this could be parasitic capacitance to ground operating with high input impedence (what is the generator impedence?). Then the response curve heads up, this could be some interwinding capacitance causing signal to transmit from primary to secondary capacitively (although with the electrostatic shield I don't know if that would happen) or it could be ringing ( I don't know what a VTVM is, but I am going to assume it is a scope (I. E. it has low capacitance and no time constants we need to worry about).

A first step might be to take a look at the schematic from the mixer you pulled these from and see how the designer terminated the transformer.  If you don't have the schematic or the PCB, then try some sort of resistor load or Zobel to terminate the secondary.  For a 1:15 transformer start with values from 225K to 450K across the secondary. Possibly in parallel with a 50pf to 200pf ca

Before messing with that, do make sure that the source impedance is appropriate (I.E. Looks like a microphone). 150 to 350 ohms is typical.  If it is too high you can get there in various way (step down trafo, preamp, pad).  If the source impedance is too high, and the secondary in unterminated, then the capacitive effects will be pretty dominant.
 
VTVM=vacuum tube volt meter---very high Z, loads circuit to be measured minimally.

this xfmr came from solid state mixer, npn front end I believe. maybe 33k termination, have to check

i dont think there is an electrostatic shield just ground lead to the core. I have a few more of these maybe a CJ style chop up is in order.

edit: does anyone else recall PRR's saying re: mystery transformer, dcr of winding is roughly 1/10th its ac Z?
 
Well if there is no electrostatic shield, then think of the transformer as a coupling capacitor (which with a path to ground on the secondary side is a high pass filter).  So that part explains the rising response on the high frequency end.  This can be controlled by putting some load on the secondary which will raise the frequency at which that High Pass takes place.  This is actually a part of signal transformer design because the excitation current varies inversely with flux (so not they transfer less at high frequencies) so the designers set it up so the transformers work magnetically at low frequency and capacitively at high frequency... which is a design that requires the load and source impedances to be as within design parameters to work well.  But unterminated the frequency where the capacitive coupling comes in is probably too low (so you get the nasty rise).

As far as the dip is concerned, I don't know but I suspect that you need to lower the impedance of the input to match a mic.  (I think it works like more current, more flux, more transfer, so you get more current by having a lower resistance which is done both by terminating the transformer with a resistor and by lowering impedance of the source) .

Start by just terminating the transformer the way it was in the mixer.  33K termination on a 1:15 step up would reflect a Z of 146Ohms (33000 divided by turns ratio (15) squared... so 33000 divided by 225). 

It matches the mic as opposed to briding it, which I guess is fine (but not common these days).

In any case, maybe CA can measure it with some load across the secondary.

PS:  I am sort of new to this transformer stuff, if I have my head where the sun don't shine then those that know more feel free to chime in.

 

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