[ARC5] Reverse engineeing the BC 456
J Mcvey
ac2eu at yahoo.com
Mon Mar 20 11:27:41 EDT 2017
I was sent the schematic of the simple circuit with the electret, resistor and transistor (thanks!).
The varying voltage on the input side from the action of the amp could easily oscillate.
One of the ways my circuit will combat this is regulating the preamp to 12 volts, which will give it immunity for the voltage swings produced by the emitter follower.
The same isolation could be applied by perhaps regulating the base drive to even a lower voltage, like 5v. I'm surprised that the electret didn't self-destruct with essentially 24 volts on it.Most are rated for 3.3 to 10 volts. They are happy with 2 V.
I bread boarded the circuit and it does work.It also worked well with 5v in the drive circuit which should eliminate positive feedback.The Q point voltage was very low about 2V ( drawing near max current). Adding a resistor across the BE allowed for Q point adjustment.The thermal stability is probably not too good so setting the resting voltage in the middle , will allow for drifting high or low without distortion.
simple is good!
On Monday, March 20, 2017 8:01 AM, David Stinson <arc5 at ix.netcom.com> wrote:
Please note that the some of these "simple" circuits
turned out to have flaw. With no base path
to ground, the mic element could go Hi-Z
and the little preamp could start "motorboating."
I kind of expected others would refine and
post about it but no one ever did as far as I know.
I haven't gone back and addressed the problem
yet. Will eventually.
Dave S.
----- Original Message -----
From: "Mark K3MSB" <mark.k3msb at gmail.com>
Subject: Re: [ARC5] Reverse engineeing the BC 456
> Took me about an hour.
>
> Here's the contents.
>
> (25 Mar 2016) [ARC5] Carbon mic replacements
> (27 Mar 2016) [ARC5] Carbon Mic Replacements
> (29 May 2016) [Milsurplus] Carbon mic booster ?
> (30 May 2016) [Milsurplus] Carbon mics, more.....
> (30 May 2016) [Milsurplus] Aw Heck (Was: SIMPLE Carbon
> Microphone Sub)
> (30 May 2016) [Boatanchors] SIMPLE Carbon Microphone Substitute
> (22 Jun 2016) [ARC5] More on Bashless T-17 Microphone
> Resurrection
> (16 Sep 2016) Re: [ARC5] Microphone Element
>
> I'll send you and others that have expressed interest in
> private email.
>
> 73 Mark K3MSB
>
>
> On Sun, Mar 19, 2017 at 2:45 PM, J Mcvey <ac2eu at yahoo.com>
> wrote:
>
>> Mark,
>>
>> that sounds like a lot to wade through.
>> I was just curious to see the conclusion/circuits made, if
>> any. Don't
>> spend a lot of time doing a compilation, especially since it
>> wasn't what
>> you expected.
>> My next venture will be to tone modulate the BC456 to verify
>> what input it
>> needs to get to 100% modulation.
>> I'm waiting for some pl-68's to arrive so I can do that.
>>
>> When I'm done with this project, it will have ample modulation
>> to be heard.
>> The goal is to participate in some of the AM "military" nets
>> on 75 meters
>> as well as CW.
>>
>>
>> On Sunday, March 19, 2017 1:50 PM, Mark K3MSB
>> <mark.k3msb at gmail.com>
>> wrote:
>>
>>
>> J
>>
>> I kept all the emails for that those threads as I need to get
>> my BC-456
>> working here. I tried it a few times for test, and it wasn't
>> impressive,
>> but then again I didn't really work on it that much for AM.
>>
>> There are probably 50 to 75 emails in those threads; I'll try
>> to put them
>> all in a pdf for you (and anyone else that's interested).
>>
>> Mark K3MSB
>>
>> On Sun, Mar 19, 2017 at 1:12 PM, J Mcvey via ARC5
>> <arc5 at mailman.qth.net>
>> wrote:
>>
>> Got a link to that subject?
>>
>> Meanwhile, I've been doing some baseline tests on what
>> amplitude is
>> produced by a carbon mike.
>> Here's what i found with a "whistle test" ( whistling a tone
>> in the mike)
>> power supply 18VDC and a load resistor of 300 ohms . This
>> simulates the
>> 60ma current limit supplied by the BC456.
>>
>> After several tries, the best amplitude from the T-17 was
>> about 1.12 volts
>> p-p.
>> Doing the same test with an old telephone carbon element
>> yielded over 3
>> volts p-p.
>>
>> I suspect that the T-17 is supposed to be more like the
>> telephone element
>> and that's why the modulation is so weak.
>>
>> The electrets I've tested only give me about 70-100mv for the
>> same test.
>> Could have sworn I seen a lot higher outputs than that.
>>
>> looks like this is going to need an amp with an emitter
>> follower to work.
>>
>> I'm working on the premise that 4v-pp will definitely
>> over-drive the
>> modulator. What say you folks?
>>
>>
>>
>>
>> On Sunday, March 19, 2017 8:34 AM, John Hutchins
>> <jphutch60bj at gmail.com>
>> wrote:
>>
>>
>> Chris -
>> there was many recent emails, last year, on
>> milsurplus at mailman.qth.net
>> on this subject.
>> Schematic and parts list.
>> Hutch
>>
>> On 3/18/2017 2:45 PM, Chris Bowne wrote:
>>
>> Many years back someone had a simple circuit published in ER
>> of an
>> electret element mic and I believe a source follower low
>> impedance output
>> stage with appropriate gain ahead of it to provide more than
>> ample drive
>> for the various WW2 sets that were designed to be used with a
>> T-17 carbon
>> mic, maybe someone can supply the particular issue to look for
>> it in. If
>> you can't fit all the components in the T-17 case, why not
>> just have them
>> in an in-line tube of appropriate size on the mic cable?
>>
>> Sent from my iPhone
>>
>> On Mar 18, 2017, at 13:46, J Mcvey via ARC5
>> <arc5 at mailman.qth.net> wrote:
>>
>> My idea to to modulate the bc456 with a 'level shifted"
>> electret mike hit
>> a snag.
>>
>> The mike input transformer has ratio of 1:10 primary to
>> secondary which is
>> terminated with about 3000 ohms which means that the primary
>> is looking to
>> match a 30 ohm device?
>>
>> That explains why the modulation is so poor. My T-17 likes to
>> be around 2K.
>> It also precludes using the electret output directly because
>> the internal
>> fet needs a drain resistor of 2- 5k to provide the gain,
>> requiring a medium
>> Z to match.
>> Bummer...
>> Nothing is ever simple...
>>
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