[sdiy] Flanger???

Gene Stopp gene at ixiacom.com
Thu Jun 27 03:07:39 CEST 2002


Hi list....

Still going to go forward with my dual tape deck flanging experiments. As
you may remember the unfortunate thing I have to deal with is the fact that
my two identical tape decks (TEAC 2340) do NOT have a pitch adjustment.
Therefore in the interest of not splaying open the guts of the machines I
have decided to vary the frequency of the AC power source to one of the
decks. This means I have to make a tuneable inverter.

I tried the following on my workbench:

Tri/Square LFO on breadboard

into

Old clunky home-built stereo power amp

into

Big 33 VAC transformer (secondary)

Primary side (110 VAC) side of transformer

into

Power outlet




Since my power amp does not have a bridge-mono mode, my LFO has
complimentary triangle wave outputs (i.e. normal and inverted) going to the
left/right inputs of the amp, and the transformer winding across the two
hots on the amp speaker terminals.

I plugged a bench utility light (60W incandescent) into the power outlet,
and a DVM set to AC V in parallel. The LFO was created such that the min
frequency was about 50 Hz and the max was in the 80-100 Hz range. The output
level is determined by an amplitude pot, so I can tweak the output voltage.

Powered up everything, and turned up the amplitude pot. The lamp started to
glow, and when I reached 120 VAC it was a nice normal brightness. I let this
all sit (as I chuckled in delight) and once in a while I would feel the
temps of the output transistors on the amp. Not bad!! A little over room
temp, but that was about it!!

Feeling adventurous, I turned down the frequency pot to minimum to get a
feel of how things might look inside a European home. Then I backed it back
up, but I kind of overshot (oops!) and I heard a buzz from somewhere
(transformer? bulb?) and the light went out. Rats.

Powered down again. Checked everything. Powered up again. My scope was
monitoring amp output on channels 1 and 2 (to show complimentary triangles
plus any potential amp clipping), and things looked OK there. Still no AC on
the lamp side, and of course no light.

Took out the transformer and measured DC ohms on primary and secondary -
about 0.9 ohms on secondary, and about 1 ohm on primary. I know I hosed up
the transformer, but not sure why yet.

Anyway, that was my excitement for the weekend... anybody have comments on:

a) Are power transformers *that* sensitive to frequency when running at
load?
b) Or did I just pick a crummy transformer?
c) It was about 4" cubic, so I would estimate in the 5-10 amp range. Is that
too small for inverter purposes?
d) Any experience out there with driving power transformers backwards like
this?

Best Regards,

- Gene




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