Body
Ole,
Looks like feed forward compensation, sorry wish I could read Danish but
looks like something fun to try. Current amps have high bandwidth but
in my experience more dc offset from the common emitter input.
On 11/23/2018 1:33 PM, [email protected] [EVGRAY] wrote:
>
>
> Hi Mick,
>
> The compensation network sits at the input end of the amplifier. As
> for oscilloscope probe compensations they can be placed at either end
> of the probe cable. As long as the output signal of the speaker is an
> amplified copy of the input signal at the source the compensation does
> its job fine. For an audio amplifier the signal at the speaker
> terminals should be an amplified copy of the signal at the source. If
> placing the compensation at the output end a lot of power is wasted.
> The compensation has its own battery powered amplifier to keep the
> signal level unchanged. The principle is shown in figure 4 here:
> https://ipaper.ipapercms.dk/TechMedia/AktuelElektronik/2007/13/?page=6.
> The compensation (roll-off correction in the article) is inserted in
> the signal chain as shown in figure 3. Figure 2 shows the signals
> without the compensation. His version is switched in steps while
> oscilloscope probes use a variable capacitor.
>
> It is told that by putting resistance in series with the speaker the
> sound is improved. This makes a voltage amplifier act more like a
> current amplifier.
>
> There is an old B&O speaker with high damping that sounds very good
> but wastes a lot of power. It requires quite much power just to
> operate that model at moderate level because of its high damping. I
> don't know if it is damped by resistance or if it is mechanical damped
> or both. A good way of driving a speaker is with a current amplifier
> like used in scopes for their high bandwidth. Of course the output
> current must be greater for driving a speaker. Current amplifiers can
> be made quite simple if using matched pairs of transistors and the
> bandwidth can be something like 40MHZ for standard transistors. This
> is fast enough to give a very good replication of the finer details of
> an audio signal.
>
> Regards
> Ole
>
>
>
> ---In [email protected], <mkjekyll@...> wrote :
>
> 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, onielsen@... <mailto:onielsen@...> [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] <mailto:[email protected]>,
>> <freeen2012@...> <mailto: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
>>>
>