Body
Ole,
Current amplifiers are not a really a good choice to drive speakers
anyways as high impedance will get more and more voltage with a CCS.
Better to use voltage amps biased with current sources. Dynamic
speakers present a varying impedance bandwidth so tweeters will begin to
clip, unless the speakers are electrostatic where the impedance rises
as freq drops inverse to dynamics. Current amps are a good choice for
some preamp circuits though which are driving a less reactive much more
constant impedance and as you claim can then leverage the bandwidth of
transistors. I am having an issue with a current pre design right now
that has huge bandwidth on the bench. On a scope impulse response is
flat from several Hz to almost a mHz however when used for recording
real world signals rather than a sweep the low end is much thinner
sounding than it should be. I will try a diff power supply to see if
massive current reserves will make a difference.
Thanks for the probe link, I will read in a bit.
Interesting enough that audio is normally considered to low freq for
skin effect to matter as most RF engineers will attest but silver plated
oxygen free copper makes a big difference in sound quality. The old
fashioned wire wrap is often very good for audio as it is silver plated
copper and very thin and very inexpensive. Good way to make
interconnects that rival those expensive ones for a few bucks. Coax is
not really good for audio either as presents too many non linearities.
Audio is a weird duck as to an RF engineer it looks like but a small
notch however there are some really big differentials in that small
window concerning how materials react likely having to do with the long
wavelengths and the (phase?) change around 10kHz where radio emission
begins. I imagine you can explain this much better than I just did and
I don't know the proper terminology of where AF starts becoming RF which
is much lower then the formal claim of about 100kHz.
On 11/23/2018 6:27 PM, [email protected] [EVGRAY] wrote:
>
>
> Hi Mick,
>
> Here is an article about oscilloscope probes: 'The Secret World of
> Oscilloscope Probes'
> (http://www.dfad.com.au/links/THE%20SECRET%20WORLD%20OF%20PROBES%20OCt09.pdf).
> It's not only the compensation network that matters. For minimum
> reflections the damping has to be done along the length of the cable.
> Just look at those thin coaxial cables on scope probes. I guess they
> would sound good as audio signal cables if used with the compensation
> network correct adjusted. It does require some extra amplification to
> compensate for the damping.
>
> Regards
> Ole
>
>
>
> ---In [email protected], <mkjekyll@...> wrote :
>
> 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, onielsen@... <mailto:onielsen@...> [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] <mailto:[email protected]>,
>> <mkjekyll@...> <mailto: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
>>>>
>