Thanks David!
That's very helpful and much appreciated.
Keith
On Fri, Dec 28, 2012 at 7:18 AM, Avocet <> wrote:
> > Over here (Canada), Radial seems to have cornered the DI market,
> > using real Jensen's. Are there any cheaper alternatives that
> > actually work?
>
> Keith,
>
> In the UK, Sowter have cornered the quality transformer market but
> they tend to be quite large. Sowters were standard in the BBC in
> analogue days. The specs are on:
> http://www.sowter.co.uk/pro-audio-transformers.php
> but I expect other manufacturers are similar.
>
> What you pay for is good screening with a mu-metal can. Outdoors you
> could easily get away with unscreened transformers.
>
> > I'd also be interested in knowing how to specify a transformer for
> > specific source impedance's. Would a different ratio work better on
> > peizo's? It also seems the isolation spec is where the cheaper
> > competition falls down.
>
> Matching is a simple square law. Isolation is important with long
> balanced or floating (no centre tap) lines, but not for simple
> impedance matching. You can use any winding or part winding as an
> input or output.
>
> The problem with peizos is that their impedance rises with falling
> frequencies, giving you a choice of input level versus LF response.
> The same applies to electrets, and the solution with them is a very
> high impedance amplifier, which will add hiss.
>
> You don't want the transformer itself to load the circuit, but just
> to reflect the input impedance. The impedance tolerances of a good
> transformer are high. The design impedances are usually specified, but
> doubling to halving these will still give a good spec.
>
> To show the ratios look at:
> http://www.sowter.co.uk/specs/3195.htm
> This has a centre tapped low impedance side and a 1:7 ratio. It will
> work both ways, so using the centre tapped side as an balanced output
> into 200ohms, the input impedance would be 200 X 7 squared or
> 9800ohms. Just using one winding as a floating output, the impedance
> ratio quadruples, giving about 40Kohms input into 200ohms.
>
> The squares law with a 10:1 transformer gives an impedance ratio of
> 100, matching 20000ohms into 200ohms. If your actual mixer or recorder
> input is 2000ohms, that will match 200,000ohms. My guess from the spec
> on:
> http://www.sowter.co.uk/specs/9610.htm
> is that the response at 100Hz would still be within 1dB.
>
> The multitapped Sowter 3678
> http://www.sowter.co.uk/specs/3678.htm
> gives a choice of 16200oms, 5000ohms or 1800ohms into a 200
> ohms input. Using an intermediate tap (4 to 5) would also give you
> 15300ohms into 200ohms.
>
> David
>
> David Brinicombe
> North Devon, UK
> Cogito cogito ergo cogito sum - Ambrose Bierce
>
>
>
>
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>
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>
--
Keith Smith
Keith Smith Trio, Northern Lights =96 Altai Khangai - www.keithsmith.ca
Photography - www.mymountains.ca
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