[sdiy] Power supply for Big Briar Etherwave Theremin

[Mike Bryant]

Possibly because the hand sensor isn't properly insulated and hence had to have a low impedance return to earth at 50Hz.


-----Original Message-----
From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of Roman Sowa
Sent: 06 July 2022 15:04
To: rburnett at richieburnett.co.uk
Cc: synth-diy at synth-diy.org
Subject: Re: [sdiy] Power supply for Big Briar Etherwave Theremin

"Why didn't they just" is a question that pops up in my mind everytime I see any kind of equipment.

Roman

W dniu 2022-07-06 o 15:47, rburnett at richieburnett.co.uk pisze:
> As that document says, the Theremin requires a proper AC ground for 
> the RF return currents so that it is able to react to the capacitance 
> of hands around its antennas correctly.  For this reason the AC power 
> adapter has one of its output lines connected to mains earth (ground.)
> 
> The document goes on to say that if the Theremin is also grounded via 
> an audio amplifier/mixer/whatever then you might get a low-frequency 
> ground-loop and experience some unwanted mains hum (>.<)  Which begs 
> the question... "Why didn't they just AC ground the Theremin via a 
> reasonable sized capacitor?"   A suitably chosen value would be 
> insignificant in series with typical antenna capacitances 
> (pico-Farads) and yet still present a sufficiently high reactance to 
> limit the flow of low-frequency 50/60Hz hum currents.
> 
> -Richie,
> 
> 
> 
> On 2022-07-06 14:14, Peter Pearson via Synth-diy wrote:
>> I looked at Didier's pic and yes the center pin specifically says 
>> "earth ground".  I'll admit that I got out in the weeds here, however 
>> the ebay link I had sent earlier also says GND on the center pin and 
>> has the added benefit of already being a DIN plug.
>>
>> https://www.ebay.com/itm/303827582131?hash=item46bd88fcb3:g:9D0AAOSwe
>> uFf6gew
>>
>>
>> I fail to see how this is a worse option than cutting a barrel plug 
>> AC adapter and wiring it up yourself.
>>
>> On Wed, Jul 6, 2022 at 4:55 AM Roman Sowa <modular at go2.pl> wrote:
>>
>>> Exactly, it must be "grounded" in a sense of "earthed", and not 
>>> "center-tap".
>>> If you connect center-tap transformer there, it will not be 
>>> grounded, it will be floating. And also it will be very hot, because 
>>> ground pin on the socket is shorthed to one of AC pins.
>>>
>>> You need a link from earth protect prong in your AC outlet to one 
>>> end of your supply.
>>> It all becomes so clear when you look at power input schematics on 
>>> page 4
>>>
>>> Do what Gordon says
>>>
>>> Roman
>>>
>>> W dniu 2022-07-06 o 08:05, Peter Pearson via Synth-diy pisze:
>>>>
>>>
>> https://manuals.fdiskc.com/flat/BigBriar%20Etherwave%20Service%20Manu
>> al.pdf
>>
>>>
>>>>
>>>
>> <https://manuals.fdiskc.com/flat/BigBriar%20Etherwave%20Service%20Man
>> ual.pdf>
>>
>>>>
>>>> Read page 8 of the service manual where it says - and I'm quoting
>>> here -
>>>> "*WHY IS THE POWER ADAPTER GROUNDED?*"
>>>>
>>>> On Tue, Jul 5, 2022 at 7:04 AM Gordonjcp <gordonjcp at gjcp.net 
>>>> <mailto:gordonjcp at gjcp.net>> wrote:
>>>>
>>>> On Thu, Jun 30, 2022 at 02:41:07PM -0400, Peter Pearson via 
>>>> Synth-diy wrote:
>>>>>
>>>>
>>>
>> https://manuals.fdiskc.com/flat/BigBriar%20Etherwave%20Service%20Manu
>> al.pdf
>>
>>>>
>>>
>> <https://manuals.fdiskc.com/flat/BigBriar%20Etherwave%20Service%20Man
>> ual.pdf>
>>
>>>>>
>>>>> The schematic is here.  A DC supply won't work.  The
>>> theremin
>>>> might be
>>>>> fussy about grounding though, so you'll definitely want a
>>>> center-tap on
>>>>> your power adapter.  I wouldn't recommend trying to cheat a
>>>> barrel jack AC
>>>>> power adapter in there.
>>>>
>>>> If you look at the power input circuit, there are only two 
>>>> connections.  There's nowhere for a centre-tap to go.
>>>>
>>>> It's a simple transformer fed into a pair of half-wave
>>> rectifiers,
>>>> same as the various crappy modular PSUs that are kicking
>>> around.
>>>>
>>>> Everyone is making this waaaaaaaaaaaaaaaaaaaaaaaaaaaaaay more 
>>>> complicated than it needs to be.
>>>>
>>>> A 15V transformer will be fine.  It doesn't need a centre tap.
>>>
>>>> Anything over 200mA will be fine.
>>>>
>>>> --
>>>> Gordonjcp
>>>>
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