Re: [EVGRAY] J ² = 1

Body

Hi Ole,

Same concept we use when designing a zobel network for a transformer in
a mic preamp.  I suppose this magic device sits near the speaker.   I
have a hard time believing this would do much with most dynamic speakers
as the mechanical impedance and reactance makes a square wave hard to
recognize.  On an electrostatic speaker though one can see a pretty good
square wave however that is one huge capacitor compared to some trimmer
so likely would require a tunable inductor in line with the step up
transformer.  Of course I am referring to using a laboratory mic for
looking at the square wave not just a probe across the speaker terminals.


On 11/22/2018 11:29 AM, [email protected] [EVGRAY] wrote:
>  
>
> Hi Andreas,
>
> Yahoo doesn't like to transfer the complete messages today even though
> the delay isn't too great. I completed the message history from the
> picture and hope it gets through.
>
> To make your own BNC feed through get one male and one female 50 Ohm
> connector and an inductance free 50 Ohm resistor. Then connect them
> all in parallel. Instead of one 50 Ohm resistor perhaps use two100 Ohm
> resistors in parallel. The resistors must be able to dissipate the
> power at the max voltage across them. With a low impedance current
> shunt this voltage won't be great. The 50 Ohm feed through must be
> placed as close as possible to the input of the scope's input
> amplifier which means the BNC input terminal of the scope.
>
> For high speed oscilloscope probes the inner conductor of the coax
> cables are made of resistive wire to dissipate energy along the wire.
> This reduces reflections when the signal reaches the ends of the cable.
>
> I know of a compensation device like used in oscilloscope probes (i.e.
> like 1:10, 1:20, 1:50, 1:100 etc.). But this device is for audio
> signals. He tells that when it's correct adjusted the sound picture
> has depth end everything placed correct. He got the idea from
> oscilloscope probes where you want to see the signal on the screen
> like how it is at the probe tip. Never mind how it looks along the
> signal path as long as the end result is correct. That's what the
> compensation is for. I.e. when the compensation is correct adjusted an
> input square wave looks like a square wave on the screen. Without the
> compensation the corners of the square wave are round. With too much
> compensation the corners becomes pin shaped.
>
> Regards
> Ole
>
>
>
> ---In [email protected], <freeen2012@...> wrote :
>
> Hi Ole,
>
>>     On 22 Nov 2018, at 18:51, onielsen@... <mailto:onielsen@...>
>>     [EVGRAY] <[email protected] <mailto:[email protected]>>
>>     wrote:
>>
>>     Both ways can be used. For low inductance of a current shunt let
>>     the return wire go close to and in parallel with the shunt
>>     resistor to minimize any area for the magnetic field.
>>
>>     Here the return wire is twisted around the current shunt.
>>
>>     Also the 50 Ohm feed through is necessary. Without it a lot of
>>     reflections will happen unless the scope itself has a 50 Ohm
>>     input. As scope inputs usually are 1M Ohm the signal w ill be
>>     reflected due to the great impedance mismatch from 50m Ohm to 1M
>>     Ohm and the signal will drown in noise. 50m Ohm to 50 Ohm reduces
>>     the reflections a lot.
>>
>>     Thanks for the pic (illustrates it well). I already have such a
>>     50 Ohm feed through but never really understood what exactly it
>>     does and therefore have never used it. I understand it has an
>>     impedance of 50 Ohm like the coax cable to prevent reflexions,
>>     yes, but how does it transfer the potential difference to be
>>     measured (over the shunt) to the 1M Ohm impedance input of the
>>     scope. If we had to build such a 50 Ohm feed through what would
>>     we have to do (what is it composed of)? This might help me to
>>     understand how it works. 
>>
>>     Thanks a lot,
>>
>>      Andreas
>>
>