From: "Tony (remove \"_\" from email address)"
Subject: Re: Free "TENS" Circuits
Date: Mon, 16 Sep 2002 11:25:18 +1000
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On Sun, 15 Sep 2002 16:15:48 GMT, Watson A.Name wrote:
>In article , "Tony (remove
>\"_\" from email address)" talked about...
>> The NPN version of the circuit satisfies the "no power rule" anyway, so there's
>> no problem. I've used similar circuits before, and find they work quite well.
>> The only slight gotcha (as mentioned on the site) is that the common base input
>> impedance of the amplifier is way too high to damp the speaker electricall. So
>> you need lots of mechanical damping, eg place the speaker at the end of a tube
>> tightly packed with fibre filling.
>> One other suggestion - it's messy and not very stable to have to select the 3M3
>> resistor to suit the gain of the transistor. It's better to delete it, and
>> instead put a 1M resistor from C-B. Then you can use any transistor.
>With the 1M resistor from B to C, there is still some dependence on
>transistor beta, altho it's less than with the 3.3M.
Quite true, but it's relatively minor, and I only claimed that it allowed you to
use any (small signal) transistor, which it can, just like any other
stabilisation circuit. In fact the stabilisation performance is very good,
considering the simplicity.
> Also the negative feedback from C to B reduces the gain somewhat.
The 1M does shunt the 10K output, reducing the gain by 1%, but there's no
negative feedback effect in the passband due to the long time constant (47
>If you put a speaker at the end of a tube, even with damping material, the
>tube will reduce the bandwidth because it's a half wavelength at some
>frequency. That'll give a peak at that freq.
Where the tube is a half-wavelength, the total path difference is a whole
wavelength, which leads to a similar signal on both sides of the cone, and a
DROP in response, not a peak as suggested. And the assertion assumes an open
tube (it might also be closed, or lead into a cavity). Also, the sound might not
Although I didn't go into any detail, my point was that a heavily damped tube is
one of the few possible ways to add the much-needed mechanical damping. A
practical design would need to account for all the other parameters as well. For
the quoted example of on-axis sound and an open-ended half-wavelength tube, the
resonant peak would be reduced further, and the octave below would be
re-inforced, effectively extending the bandwidth considerably with a little