Newbie Audio transformer Frequency response question

[Jumpnz]

Ok, guys, Im just about to set out building a Bobtavia as my first build. Im completely new to this stuff so bare with me if this is a dumb question. I live in New Zealand so cant get the audio transformer from radio shack that the schematic says to use, as they dont do international shipping and I have looked around and found some locally that are 1k:8R audio transformers but they have a Freq response of 300Hz to 3KHz and the one from radioshack is 300Hz to 10KHz. Will it still sound good using a 300 - 3KHz one or should I try and find another one.

Any comments appreciated

Cheers

[tubelectron]

jumpnz,

if no other specs are different from the Radio Shack Xfo, then you may find a little loss in the treble - but it is not compulsory, as specs can be different from the reality on this kind of small (and cheap) Xfo's. If this Xfo is affordable, then buy it and have a try : you will probably have a good surprise.

This kind of Xfo is a certainly a speaker line transformer (70VAC) used to carry speaker modulation on long distances, for public adress usually. You can find many sizes depending on the power to carry. As you don't need power, you can chose the smallest (and the cheapest). Any professional audio and PA shop should have that.

Best regards,

[R.G.]

Ok, guys, Im just about to set out building a Bobtavia as my first build. Im completely new to this stuff so bare with me if this is a dumb question. I live in New Zealand so cant get the audio transformer from radio shack that the schematic says to use, as they dont do international shipping and I have looked around and found some locally that are 1k:8R audio transformers but they have a Freq response of 300Hz to 3KHz and the one from radioshack is 300Hz to 10KHz. Will it still sound good using a 300 - 3KHz one or should I try and find another one.

One of Keen's Laws says to use what you can get and enjoy the music all  you can while you search for the Perfect Part.

I also encourage you to study electronics, as there is an answer for you there. It's buried pretty deeply in this case, but there is one.

The first piece is electronics knowledge about datasheets and electronic history. Manufacturers regard their published specs as a boundary condition, enforced by refused shipments to customers who test the product and by lawsuits from irate individuals. So they will tell you the bare minimum they have to, and the parts will all be within the range of the published specs. In this case, what your local supplier is telling you is that the transformers they sell will have a frequency response AT LEAST as low as 300Hz and AT LEAST as high as 3000Hz. Those numbers are highly suspect because they happen to be what the old Bell Laboratories found to be the necessary frequency response for good intelligibility of the human voice on telephone networks about 80 years ago. From this, you could project that every one of the transformers you buy with those specs will have a low frequency rolloff of no worse than 300Hz, and no worse than 3000Hz.

Second piece: the top end will almost certainly be better than 3000Hz. I used to design transformers for a living for a while, and it's darned difficult to wind a small transformer that will only go to 3000Hz. On the ones I used that were spec'ed to 3kHz, they really measured out at 22kHz. But the manufacturer will not tell you this in a spec sheet because then they'd have to measure it and refund for the ones that only went to 20kHz.

Third piece: The low end is calculable. Once you know about the standard model for a signal transformer, you realize that the low frequency spec and the impedance rating together tell you the primary inductance, and from that you can pretty reasonably calculate what the thing will do at the low end under odd circumstances.

Putting those together, I'd say use the one you can get and listen to it. It will probably be just fine. If not yell, and we'll figure it out.

[George Giblet]

RG's right on the mark.

I was going to post some examples but I found this, which is a classic case,

Look for "TTC-108 Frequency Response" about the last 20% of the page,

http://www.cliftonlaboratories.com/elecraft_k3_receive_audio.htm

The specific low and high frequency response depends on the source and load impedances.  You are more likely to see variations in the low frequency response with transformer size and source impedances.