Thanks, David.
I'm sure you're familiar with these:
http://www.jensen-transformers.com/db.html
-the classic, and best of the passive D.I.'s used all over the music
business.
They're designed for 10Kohm magnetic guitar pickups, but do wonders on
peizo crystals measuring around 2Kohm DC, too.
I have one DIY version (genuine Jensen) and it's the best, by far.
At 120CAD, I thought ART might be as good for a dual unit. Sadly they miss
the mark on noise/hum performance.
The $20 versions of these things are absolute junk -not surprising when the
bare Jensen transformer sells around $75 in small quantities
Over here (Canada), Radial seems to have cornered the DI market, using real
Jensen's. Are there any cheaper alternatives that actually work?
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.
Post Thought: I shouldn't damn the ART just yet. My initial opinion came
from an experience with the PMD 661 and high preamp gain. As I work though
my noise troubles...
Thanks again,
Keith
On Wed, Dec 26, 2012 at 5:57 PM, Avocet <> wrote:
> **
>
>
> > Yes, there has always been nonsense talk about "impedance matching".
> > However, as we don't use dynamic mic's and input transformers so
> > much
> > any more, it doesn't affect my sleep. It was much worse 30 years
> > ago.
>
> I've been sitting on my hands trying not to do a bit on impedance
> matching, but I've failed. :-)
>
> Summary: With source and destination impedances between 50ohms and
> 2000ohms, cables with around a 150ohm impedance work fine up to
> several hundred metres.
>
> I go back to when phones had a handle on the side and carbon mics.
> When they went fizzy or weak, you had to shake the handset or hit it.
> Your handset mic had to generate all the signal energy to go long
> distances down the single wires strung along the "telegraph poles" to
> the distant earpiece. The impedance of the wires was about 600ohms,
> and this impedance is still used in the definitions of various 0dB
> levels.
>
> Impedance matching generally has three elements; source, transmission
> line and receiver. A mismatch at either of these junctions will
> reflect back some of the signal energy instead of passing it on. With
> most current audio recording methods, the impedance of a short mic
> cable is not important, but long cables produce a reflection at any
> point of impedance mismatch which produces a time shifted reflection
> which can affect the response. For instance a long cable driven from a
> high impedance will look like a capacitor.
>
> As above, with source and destination impedances between 50ohms and
> 2000ohms, cables with around a 150ohm impedance work fine up to
> several hundred metres.
>
> Modern powered mics are often low impedance output but are sometimes
> specified to run into 1000ohms or greater. You get a power mismatch
> here, and a reflection is sent back from the far end of the cable but
> the original mic voltage is hardly affected. In practice any noise
> increase is small.
>
> Running a higher impedance mic, say 50Kohms, into a low impedance
> input will cut the voltage in proportion and the energy by the voltage
> squared.
>
> There is a parameter called "noise impedance". You won't find a simple
> definition for this but it is the ideal load impedance to get the
> lowest signal to noise ratio. It is not critical, but many mics are
> designed to run into a higher impedance than they show and and this
> generally gives the optimum noise figure.
>
> Microphone transformers can give an optimum matching for the lowest
> noise, but they are out of fashion these days. I use transformers on
> "affordable" electret mics which have an output impedance of around
> 30Kohms and match well into low impedance cables and mixer. They also
> give me a floating source which minimises pickup and losses from 100+
> metre cables.
>
>
> David
>
> David Brinicombe
> North Devon, UK
> Cogito cogito ergo cogito sum - Ambrose Bierce
>
>
>
--
Keith Smith
Keith Smith Trio, Northern Lights =96 Altai Khangai - www.keithsmith.ca
Photography - www.mymountains.ca
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