Body
Ole,
Yes the current buffer at the output end of the amp decauples the
voltage amp from speaker reactance and is required and the more current
the better all power amps have a basic currrent amp at the output. Most
amp designs are hybrids. As you say with audio there are so many
octaves and the impedance and reactance are non linear into a cored
inductor, specially one that is a motor also reacting to air loading
modes. A DC motor will speed up to destruction with unlimited voltage
so what happens when the impedance goes super high or the speakers are
not connected with an ideal pure CCS amp?
Bass at the lowest impedance needs the most current. Highs like fast
non slew rate limited voltage. Wrapping a design in loop feedback to
keep it stable is really not good sounding and lazy practice but
sometimes required. I use a curve tracer to match parts. Local
degenerative feedback not an issue sonically but when it's wrapped
around a whole circuit looped from out to in, yuck, at least have the
individual parts nested.
One of the biggest problems with audio designing is we cant test what is
really going on wideband with music only guess or listen. Something
that looks great with pink noise or impulse responsed can in practice
not sound so good. Why many afficianados like SET designs because it's
just one single ended tube without all the garbage, while not so good
for rock and roll or stunning dynamics for less bombastic music the
imaging can be superlative.
For audio designs I really think Nelson Pass is solid, big respect
here. In pro audio we like a shitload more watts on standby as every
extra frequency divides power but for home gear can't go wrong with
Nelsons designs and they are cheap and fun to build.
http://www.firstwatt.com/faq.html
Also a really good sound resource is Rod Elliot
http://sound.whsites.net/index2.html
This article goes into some of what we are talking about:
http://sound.whsites.net/project56.htm
On 11/24/2018 4:18 AM, [email protected] [EVGRAY] wrote:
>
>
> Hi Mick,
>
> The person here: http://nov79.com/amr/cont.htm claims that current
> amplifiers are the best way to drive speakers. Citing from page two:
> "The current amplifier drives the speakers. It has no effect upon the
> signal. It is the perfect black box which allows the voltage amplifier
> to drive a heavy load as if it were no load at all."
>
> The only problem is for mass production as the two PNP transistors and
> the two NPN transistors must be matched. For low signal the solution
> is buying the transistors as IC packages as they're matched by using
> the same crystal for all four transistors. The person describes three
> sizes of amplifiers.
>
> This could be used for making a cheap differential probe or current
> probe amplifier for scopes. The conventional ones are pretty expensive
> to buy.
>
> "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."
> To cover the hearing range of human ears the amplifier has to cover
> around seven octaves. That's pretty much. A human eye sees only one
> octave. A RF transmitter may be even more limited.
>
> Regards
> Ole
>
>
>
> ---In [email protected], <mkjekyll@...> wrote :
>
> 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, onielsen@... <mailto:onielsen@...> [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] <mailto:[email protected]>,
>> <mkjekyll@...> <mailto: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
>>>>>
>