Balanced electret mike wanted
I have seen and built many unbalanced electret mike preamps but
would now like to build a phantom-powered balanced electret mike. To date I have
not been able to find any circuits in previous issues.
Ideally, the circuit would be able to mount inside an XLR or
6.5mm jack plug. It may also be possible to create a circuit that would enable
an external battery if phantom power is not available but still remain balanced.
(D. P., via email).
Have a look at the
Lapel Microphone Adaptor project in our January 2004 issue and the add-on
phantom power circuit in the February 2004 issue (Circuit Notebook).
How to dispose of used etchant
A few months ago, in a flash of optimism and enthusiasm, I
bought a beginner’s Riston PC board kit, with a little tank, PC boards, sachets
of developer and etchant - from a major electronics component retailer. I made
my transparency, as needed and then found that I had a problem.
Being environmentally and OH&S conscious, I began looking
for suitable plastic bottles to put the (as yet unmade) solutions into. Not
wanting to use old milk or juice containers or pasta sauce glass bottles, I
found that something that should be relatively simple to find wasn’t.
In addition, with no MSDS (Material Safety Data Sheets), I am
unsure whether I will have to store the solutions indefinitely or if there is
some way to suitably deal with the wastes. Nowadays, you can barely walk without
tripping over talk of environmental or personal safety, yet I found this an area
to be curiously lacking such advice. (C. G., via email).
We have never been
able to get a definitive answer on this question. For hobbyist use (ie, small
volumes) the only practical method is to massively dilute the etchant and then
flush it down the sewer. Alternatively, if your council has a regular pickup of
paints, pesticides etc, use that.
Power supply design and earthing
After browsing through a number of SILICON CHIP
articles on amplifiers, mains-operated circuits and the like, I have a few
questions.
Most of the articles use 6.4mm spade lugs for earthing. Could a
3.2mm ring terminal not be used or is there a specific reason for avoiding ring
terminals; ie, low current capability, weaker structure, etc?
When rectifying AC to DC in the power supply modules, the
approximate output voltage with a bridge rectifier and smoothing capacitors is
ACV x 1.414 = DCV.
I’ve noticed that many of the SILICON CHIP
circuits don’t seem take into account the 0.9-1.4V drop from the diodes. Is this
because it is only meant to be a rough approximation?
If so, would it be damaging to a circuit if the stated voltage
in the article was used.
For example, if the article power supply has rails of ±25V but
the real life voltage was ±23.47, would it be damaging to use an actual ±25V.
I realise this 1.53V difference would be negligible but for a
higher-powered circuit, the voltage difference would be in the order of
4-5V.
You can use alternative connectors to
spade lugs. 3.2mm eyelets are also OK.
Power supply voltages are always approximate, because the
transformer output does vary with load.
Normally, the diode drop does not contribute to much voltage
loss and the drop varies with load in any case. This drop is small compared to
the transformer output voltage variation with load; ie, its load regulation.
The transformer secondary voltage can also be 10% higher or
lower depending on mains voltage (line regulation).
Amplifier supply voltages are also given as approximate values
and may vary significantly due to the transformer regulation reasons. So a ±25V
supply versus ±23.47V is not a problem.
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