Input transformer for tone shaping

[Tubetec]

I recently removed 10x Beyer miniature 1200 ohm 1:1 transformers from a vintage mixer ,

The specs can be found in the Beyer page in the documents section ,

It seems the frequency response can be varied to some degree by adjustment of the matching impedence ,
it changes the shape of the HF curve ,

I was thinking of trying them in conjunction with dynamic mics , with an adjustable load impedence across the secondary ,
switched passive HPF would also be handy to compensate for proximity effect .

They give an example of the frequency response of a step up transformer with various values of load and additional capacitance in the Beyer documentation
How do I scale the impedences to get a similar effect from a 1:1 1200 ohm ?

 

[ruffrecords]

I would suggest you feed it into a high input impedance amplifier like a FET input op amp for example, then you can safely forget about its loading effects.

Assuming it is a 1200;1200 mic transformer then it will probably be flattest loaded with 1200 ohms and gradually create an HF peak as you increase this value. It is difficult to tell how this peak will change with load resistor value. Definitely time for an experiment and a series of graphs showing the results.

Cheers

Ian

 

[CJ]

I am afraid you will not get much variation with a 1:1.

So that means you probably will as experimental results often contradict book theory.

Sounds like something a politician would say, covering both sides of the story so they can't he wrong,

 

[PermO]

I think that's the same transformer I use in my instrument line amplifier, I have a bunch of those.
I also have the 200 Ohm variant.

I bootstrapped a germanium transistor in front of it.

I played with the termination a bit and setteled with 68pF and 200K, too much HF damping does not sound good to my ears.

 

[Tubetec]

The SSL2 has a balanced hi-z fet input, 1Mohm ,
20k line and 1200 ohm mic in .

With typical mic inputs on audio interfaces you have no way to adjust for proximity effect before you hit the convertors , resulting in a boomy signal to tape ,
A high pass filter across the transformer followed by the load adjustment should give variable bass roll off ,
if we get some additional top end sparkle at higher loads all the better .

Can I place coupling caps of the appropriate value in both transformer primary and secondary , bypassable with jumpers to set the slope?

With the peanut sized transformers probably makes sense to do an inline XLR adapter .
tricky to fit a real pot inside an XLR barrel , preset adjustables dont stand up .

 

[PermO]

https://www.technicalaudio.com/pdf/Beyer_Dynamic/Beyer_Dynamic_input_transformers_1976_oct_cat.pdf

These example graphs are great for seeing what's going on, but they are all for the 200 Ohm version.

I would love to know how to get a graph like this for the 1200 Ohm version.

I do have an old Philips tonegen and a slightly wonky old analog 70MHz scope.

I am using the transformer as an output transformer to drive pedals, pedals don't need a lot of gain so this works fine, my train of thought was the 1200 Ohm secondary from the transformer would be much like a pickup from an instrument.
It's a bit of a guess, but I think most pedals have an input in the 600K -1M range, so I decided to terminate for 1M.

In the end it works really well, Rhodes piano into pedalboard sounds a lot better with this buffer amp in between, without it I'm losing some low end and I get a strong peak in the midrange that is impossible to "fix" with an EQ, must be an impedance thing. So with the buffer amp the piano sounds a lot more balanced, no bass loss and no weird peaking in the midrange.

These little transformers will handle line level at 0dB, they will crap out at +5 / +7 dB depending on how bass heavy the material is.

 

[Tubetec]

TR/BV 351001061 are the ones I got ,

Whats amazing is the higher ratio transformers have secondary dc resistances of upto 13 kohm ,
thats a lot of turns of very thin wire on a very tiny bobbin .

An averaged white noise signal on the FFT can give a good idea of frequency response, its easy enough to set up in REW ,

 

[PermO]

Mine are the TR/BV 310 001 061 same transformer, different mounting, mine have PCB pins.

 

[Tubetec]

I found this reference to a transformer based HPF but the schematics arent there ,
https://www.electronicdesign.com/te...sformer-improves-high-pass-filter-performance

 

[PermO]

A less intrusive HPF

Yup, these should be perfect for this at mic level.

I think a measurement setup is the only way to figure these exact values I have not come across a way to do a simple calculation to get these values.
I don't have a whole bunch of small pF capacitors... plenty resistors.

Will an old ajustable disc capacitor work for this, like the ones you find in old HF equipment ?
I know a dumpstore that sells these, but they are not cheap.

Something like this ?

https://www.etsy.com/nl/listing/153...MIvPfzpa6FhwMVpKqDBx3glwCKEAQYBCABEgLm8_D_BwE

And a linear 1M multiturn pot ?

 

[Tubetec]

The tuning cap even from a cheap portable AM/FM radio might work ,
Something like a 500 pf tuning cap will only go down to maybe 50 pf minimum ,
as theres still some capacitance even with the plates of the cap fully disengaged ,

https://www.ebay.ie/itm/262638838802

The AM section in that cap varies from 5pf to around 125pf ,

At best were only getting around 4 db of boost , max , so a single turn pot should allow precise enough control .
A pot with a plastic body might have less incidental capacitance than a big old fashioned metal one .

 

[Tubetec]

Another good article on the subject of transformer HPF here ,
dealing with the low end bump an undersize value coupling capacitor causes , this bump can be reduced by adding some series resistance .
https://sound-au.com/articles/audio-xfmrs.htm

With a few different values of coupling cap for the HPF , the load adjustment pot and the variable capacitor to adjust the HF bump , you get a small but useful range of adjustment from a few passive components .
Of course it wont work like a conventional eq with a given boost vs pot rotation and the elements will interact .