Body
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 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