> 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
Message: 9800ohms.
Subject: 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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