David, thanks once again.
"The transformer impedance ratio is the square of the turns ratio. Guess
that's all you wanted to know. :-) "
Perhaps, but I most look forward to the pearls of wisdom that are usually
included.
I'm printing that one :)
During my hiatus from posting much here, I've acquired most of the gear I'd
need to play with this. (I can see a resistance decade box would be a
worthy new addition).
I'm just cogitating on a suitable way to excite the Piezo in some
near-calibrated fashion - have function and pink noise generators.
Presumably the electrical properties will vary with a change in the
substrate it's attached to and the evenness (lack of induced stresses) of
the attachment, not to mention the substrate will have it's own acoustic
properties.
It gets to being a quagmire very quickly!
k
On Tue, Feb 17, 2015 at 9:12 PM, [naturerecordists] <
> wrote:
>
>
> > Radio Shack sells piezo buzzers for a couple of bucks that are nice and
> > big. The Schallers can be had at most music stores for around $15, I
> think,
> > and there are lots of sources on the web, of course.
>
> Keith,
>
> Many piezo buzzers or beepers have a strong resonance around 2 KHz for us=
e
> as sounders. These can be damped when used as contact mics or sealed
> hydrophones, but in free air they may not have a flat response.
>
> Piezo mic elements have a rising impedance with frequency (capacitive) as
> are piezo gramophone pickups if anyone still has one. Fed into a very hig=
h
> impedance, their response rises with frequency, but using a suitable
> resistive load, this more or less flattens out. The recommended load
> should
> be specified or it can be found by testing.
>
> To match this impedance - and this also applies to capacitor mics - a
> resistor is connected across the output as with a capacitor mic bias feed
> which can be 100 MegOhms or more. A higher impedance gives lower bass
> response.
>
> With a piezo, the capacitance is higher and to match this to a medium or
> low
> impedance, you need to know the actual input impedance of the preamp whic=
h
> is rarely the same as the rated impedance for estimating responses. For a
> 150/200 ohm rated mic input the input impedance is often in the KOhm
> range.
>
> Sorry but this gets worse. :-( Mics and inputs don't have to match
> impedances, but for the lowest noise, the "noise impedance" has to match.
> This is simply the input device impedance which gives the best overall
> noise
> figure. Thus a preamp with a 2000 Ohm actual impedance input can have a
> 200
> Ohm optimum noise impedance. Hope this makes sense.
>
> Transformer impedance matching is much simpler and I don't know why
> transformers have gone our of fashion except for cost. Note input
> transformers must be mu-metal shielded as are Sowter's to minimise hum
> pickup.
>
> The other proviso is that at one side of a transformer must have a
> resistive
> load or source. For instance, a capacitive source will form a resonance i=
f
> the preamp output is high.
>
> The transformer impedance ratio is the square of the turns ratio. Guess
> that's all you wanted to know. :-)
>
> PS. If the theory fails - experiment.
>
> David Brinicombe
>
>
>
--
Keith Smith - KeithSmith.ca_Freelance Guitarist & Location Recording Servic=
e
<http://www.keithsmith.ca>
|