Signal transformer question

[IG81]

Hi,

I was not sure where to post this thread, as it mainly pertains to a signal transformer question, but will have a tube amp application and I figure there should be many knowledgeable people in here for my question.

With that said, I have a paire of Cinemag CMMI-5C step up transformers. They are rated to operate with a secondary load of 39kohm, which requires a driving impedance of 150ohm. I know that using lower impedances will quickly degrade the performances of the transformer, but what about higher impedances?

My application is to boost signal prior to my tube amp's input. It's an RCA/Victor console amp I resotred, but the thing has only 6dB of gain and therefore needs strong input signals if I hope to achieve it's full ~10-12W into 8ohm. Since I plan to use it with a CDP and passive pre, 6dB gain is not enough.

The input impedance of my tube amp is approximately 367kohm. The CMMI-5C being 1:5 step-up, reflects that as a ~14.7kohm load on the primary, wich is easily driven by a CDP. I did subjective listening tests and it generally seems fine, no distortion, and seemingly intact frequency response.

Using such a transformer with lower impedances would kill the HF response due to inductance, but higher impedances, as is my case should not cause a decrease in performance the way I see it. Does that sound correct? Are there issues you can see with what I'm doing?

Thanks!

IG

[sreten]

Hi,

It will be interesting to see an experts reply .....

If you are using it at higher voltage levels than its designed for then I
understand things balance out. e.g. if its a 1:5 microphone transformer
then this would well with higher voltages and impedances, the core (sic)
point being the designed normal operating flux or current in each winding.

/Sreten.

[janneman]

I think SY is the resident expert about transformers in general and the Cinemags in particular. You may want to ping him.

jd

[SY]

Actually, Steve Eddy more than me.

In any case, having the load higher than nominal will often cause ultrasonic ringing. If it sounds OK to you, then proceed no further, If you want to tweak and have some fun, you can optimize things by driving the input with a square wave generator and (if needed) a series resistor to pad the source impedance to match the source impedance of your CDP, then attach a series RC network (Zobel) to the transformer secondary. While looking at the secondary voltage with a scope, adjust the R and C values to optimize the square wave response. As starting values, I'd try 47k in series with 100pF.

[IG81]

Originally Posted by SY

Actually, Steve Eddy more than me.

In any case, having the load higher than nominal will often cause ultrasonic ringing. If it sounds OK to you, then proceed no further, If you want to tweak and have some fun, you can optimize things by driving the input with a square wave generator and (if needed) a series resistor to pad the source impedance to match the source impedance of your CDP, then attach a series RC network (Zobel) to the transformer secondary. While looking at the secondary voltage with a scope, adjust the R and C values to optimize the square wave response. As starting values, I'd try 47k in series with 100pF.

If there was ringing, I could not hear it, but I'll definitely test for it, because it cannot be a good thing to have around, wether you hear it or not.

The Cinemag datasheet already shows a Zobel in their test network, optimized for their rated impedances though. I'll look into this...when I get my O'scope working...nearly every one of my projects is getting stalled by this lately.

Thanks for the contribution!

IG

[Wavebourn]

I have no experience with this particular transformer, but microphone input transformers when made optimal (and I believe Cinemag transformers are) are not good for higher voltages and resistances. 150 Ohm primary would have too low inductance causing roll-off and saturation on low end of frequencies.

I would suggest to go on Edcor website and buy a couple of transformers under $10 each for levels of signals you need.

[IG81]

Originally Posted by Wavebourn

I have no experience with this particular transformer, but microphone input transformers when made optimal (and I believe Cinemag transformers are) are not good for higher voltages and resistances. 150 Ohm primary would have too low inductance causing roll-off and saturation on low end of frequencies.

I would suggest to go on Edcor website and buy a couple of transformers under $10 each for levels of signals you need.

As for signal level, with my CDP's 2V output, I'm slightly under the maximum input signal level of +10dBu for my particular model, so that aspect should be OK. As for higher impedances on their own, I would not know right now.

Thanks for your input. I'll have to do some actual testing when I repair my scope, those tips from all of you guys will give me stuff to look for.

Edit - I might just check out some of these Edcor you suggested, they are pretty cheap! Thanks for the suggestion

Cheers,

IG

[sreten]

Originally Posted by Wavebourn

I have no experience with this particular transformer, but microphone input transformers when made optimal (and I believe Cinemag transformers are) are not good for higher voltages and resistances. 150 Ohm primary would have too low inductance causing roll-off and saturation on low end of frequencies.

I would suggest to go on Edcor website and buy a couple of transformers under $10 each for levels of signals you need.

Hi,

Higher voltages I agree would cause saturation, but I cannot see the case
for when this is combine with higher impedances. If the currents are the same
the voltages do not matter, a transformer is a current operated device IMO.
Capacitance of some sort would be the major issue into higher impedance.

rgds, /Sreten.

FWIW I think you have to chuck nominal loading out of the window and
re-zobel with a far higher resistance + capacitor to keep currents down.

[Wavebourn]

Originally Posted by sreten

a transformer is a current operated device IMO.

Sure. And current depends on frequency.

What inductance can you expect from 150 Ohm primary of a microphone transformer?
Can you calculate it's impedance on low end of an audio band?
Can you imagine what happens to signal if it's source is loaded on such an impedance?

[sreten]

Originally Posted by Wavebourn

Sure. And current depends on frequency.

What inductance can you expect from 150 Ohm primary of a microphone transformer?
Can you calculate it's impedance on low end of an audio band?
Can you imagine what happens to signal if it's source is loaded on such an impedance?

Hi,

The primary impedance is reflected from the secondary, its not fixed.
In this case its around 1.5kohms not 150ohms, bass will be extended.
The problem area AIUI will be the treble end.

rgds, /Sreten.