Hi Tony,
I'm curious about your drum test CD, did you plot the FFT of that as well? we can try to repeat the initial transient a few time to see that.
also we must be aware that with simulation, and FFT, because it's only approximation. We might have to look into this further.
on your post no #413 , you mentioned :
"I then differenced the signal at the output of the coil with a suitably delayed version of the input signal to obtain an error signal"
I'm not so sure about this measurement setup, maybe you can shed a light for me, I'm don't have much experience with simulators, how to get the suitably delayed version of the input signal that have the same group delay /phase angle characteristic as the output ??
not with additional "inductor" I presume
I'm curious about your drum test CD, did you plot the FFT of that as well? we can try to repeat the initial transient a few time to see that.
also we must be aware that with simulation, and FFT, because it's only approximation. We might have to look into this further.
on your post no #413 , you mentioned :
"I then differenced the signal at the output of the coil with a suitably delayed version of the input signal to obtain an error signal"
I'm not so sure about this measurement setup, maybe you can shed a light for me, I'm don't have much experience with simulators, how to get the suitably delayed version of the input signal that have the same group delay /phase angle characteristic as the output ??
not with additional "inductor" I presume
Cellardoor said:
My description of the drum hit waveform above was a little simplistic. I should have written: The initial transient of many of the drum hits was very much like a quarter sine put through a filter. The second bit looked like the second quarter of the sine with additional harmonic content. The nearest periodic signal I could come up with (needed for the FFT analysis) was a rectified sine.
Hope this helps.
Tony
Hartono said:Hi Tony,
I'm curious about your drum test CD, did you plot the FFT of that as well? we can try to repeat the initial transient a few time to see that.
also we must be aware that with simulation, and FFT, because it's only approximation. We might have to look into this further.
on your post no #413 , you mentioned :
"I then differenced the signal at the output of the coil with a suitably delayed version of the input signal to obtain an error signal"
I'm not so sure about this measurement setup, maybe you can shed a light for me, I'm don't have much experience with simulators, how to get the suitably delayed version of the input signal that have the same group delay /phase angle characteristic as the output ??
not with additional "inductor" I presume
Hi Hartono,
The delayed signal was generated by simply adding a duplicate generator circuit which had a slight phase delay in the source oscillator. I also had an RC filter running from the main signal and setup to generate the same delay. I used both these as the subtracted reference with very similar results.
Cheers,
Tony
em....
"The delayed signal was generated by simply adding a duplicate generator circuit which had a slight phase delay in the source oscillator"
I don't see how this can make a same delay with the output, the delay is frequency dependent , right ?
if your signal only contains 1 frequency at all time, that's true
but not with rectified signal
"The delayed signal was generated by simply adding a duplicate generator circuit which had a slight phase delay in the source oscillator"
I don't see how this can make a same delay with the output, the delay is frequency dependent , right ?
if your signal only contains 1 frequency at all time, that's true
but not with rectified signal
MikeBettinger said:
It says that the input filter isn't good enough to block RF from a nearby AM radio station when the amplifier is operated with the input unterminated and connected to someone’s finger. This says nothing about the performance of the output stage, and nothing unexpected about the ground reference and feedback.
I can pick up AM Radio National on my TV by unplugging the AV leads and poking my finger on the audio input.
Audio amplifiers are not designed to amplify RF, so, given enough signal, some part of the circuit inevitably 'detects' the AM signal by a non-linear response to the RF and the resultant LF stuff gets amplified as audio.
Cheers,
Glen
Hi Conrad,
making inductor using "lumped"(your term here) carbon composition as the core is like having conductor for the core. Maybe that explains the problem.
Inductor wound on the conductor, will not have significant inductance.
if the core is steel or magnetic material, that's different story.
Cheers,
Hartono
making inductor using "lumped"(your term here) carbon composition as the core is like having conductor for the core. Maybe that explains the problem.
Inductor wound on the conductor, will not have significant inductance.
if the core is steel or magnetic material, that's different story.
Cheers,
Hartono
Hartono, you bring up an interesting question. I'm quite weak at magnetics, but I wonder if there's a difference between having the resistor inside vs. outside? The resistor is entirely non-magnetic, but the current passing through it will certainly generate some magnetic field. IMO, it's still the measurements that matter. The physical contents of the black box are irrelevant, though I may be very lonely in this viewpoint. The final configuration I installed used some nice turned Delrin coil forms about 0.9" diameter, with the resistors on the outside and perpendicular to the axis of the coils. These were mounted in free air at the amp output terminals using very heavy leads. The coils alone measured nearly perfect, about 89 degrees phase shift. For the moment they're 3 uH.
Cellardoor said:
OK, whenever it's convenient. I think it is important to remember that the composite input-signal+error is showing the error produced by a rectified wave added to a non-rectified wave. I'm not sure what that tells us. But if we could see the spectrum of the rectified wave with and without the coil, that could show us something.
Jan Didden
Hi Conrad,
Ideally, the resistor should not be close to the inductor.
I wouldn't put anything signal path and signal component close to inductor, at higher power the magnetic field is quite intense.
you have plenty access to measurement which is nice
if you image in 3d space, the radiation field will cross the spiral groove of the resistor at differing depth.
It might be negligible with spiral groove resistor , but there is a "slight" difference
I guess the field is stronger inside, especially pronounced if the diameter of the coil is smaller.
bottom line, even outside, we still need to have some distance from the inductor it self.
Cheers,
Hartono
Ideally, the resistor should not be close to the inductor.
I wouldn't put anything signal path and signal component close to inductor, at higher power the magnetic field is quite intense.
you have plenty access to measurement which is nice
if you image in 3d space, the radiation field will cross the spiral groove of the resistor at differing depth.
It might be negligible with spiral groove resistor , but there is a "slight" difference
I guess the field is stronger inside, especially pronounced if the diameter of the coil is smaller.
bottom line, even outside, we still need to have some distance from the inductor it self.
Cheers,
Hartono
I remembered that Doug Self at one time also researched this coil thing, and found yesterday the paragraph in his book 'Audio Power Amplifier Design Handbook' . For those who don't have it, he addresses three issues:
1 - Mutual coupling between coils. Short story: if you keep them 3-4 inches apart, and don't make a point of alighning them exactly, crosstalk will probably be below -100dB;
2 - Depending on the wire length and thickness of the coil material, even with thick wires a coil's resistance could be enough to cut the amp damping factor in half and cause appreciable power loss in loads less than 4 ohms.
I wonder whether this could also cause audible differences between coiled and coilless amps?
3 - Influence on ringing etc. He notes that ringing and overshoot with a coil is observed at the coil-speaker node, NOT at the actual amp output (before the coil), so in his opinion such ringing and overshoot says nothing about amp stability but just illustrates what happens when you excite a reactive network.
He notes that the most important factor determining the ringing and overshoot is the rise-time of the input test signal. I parafrase: the transient response measured in this way depends critically on the input signal rise time which can be manipulated to give the results wanted.
From the graphs he provides, it looks that with an input signal rise time down to 20uS there is almost no overshoot and ringing. I wonder how that translates into audio bandwidth. 20kHz has a period of 50uS so with a very, very rough estimate one could say that a 20kHz wave has a 25uS rise time, but it is faster at some parts of the wave because the rise time at zero crossing is faster than at the amplitude extremes. Nevertheless, testing with fast signals may make it look bad but doesn't seem to be relevant for audio.
Bottom line (for me):
- It is clear that the test signal has an major impact on the results and that audio-band limited test signals are appropriate;
- Simple resistive effects of the coil can cause audible differences due to lower damping and power loss in freq areas where the load dips to a low value.
Jan Didden
1 - Mutual coupling between coils. Short story: if you keep them 3-4 inches apart, and don't make a point of alighning them exactly, crosstalk will probably be below -100dB;
2 - Depending on the wire length and thickness of the coil material, even with thick wires a coil's resistance could be enough to cut the amp damping factor in half and cause appreciable power loss in loads less than 4 ohms.
I wonder whether this could also cause audible differences between coiled and coilless amps?
3 - Influence on ringing etc. He notes that ringing and overshoot with a coil is observed at the coil-speaker node, NOT at the actual amp output (before the coil), so in his opinion such ringing and overshoot says nothing about amp stability but just illustrates what happens when you excite a reactive network.
He notes that the most important factor determining the ringing and overshoot is the rise-time of the input test signal. I parafrase: the transient response measured in this way depends critically on the input signal rise time which can be manipulated to give the results wanted.
From the graphs he provides, it looks that with an input signal rise time down to 20uS there is almost no overshoot and ringing. I wonder how that translates into audio bandwidth. 20kHz has a period of 50uS so with a very, very rough estimate one could say that a 20kHz wave has a 25uS rise time, but it is faster at some parts of the wave because the rise time at zero crossing is faster than at the amplitude extremes. Nevertheless, testing with fast signals may make it look bad but doesn't seem to be relevant for audio.
Bottom line (for me):
- It is clear that the test signal has an major impact on the results and that audio-band limited test signals are appropriate;
- Simple resistive effects of the coil can cause audible differences due to lower damping and power loss in freq areas where the load dips to a low value.
Jan Didden
G.Kleinschmidt said:
Hi Glen,
Of course, under normal operating conditions, Q1 is not a common base amplifier, but with regard to the NFB loop, you might consider it as such. Just follow the signal path (in case of a blameless amp). The FB signal enters the base of Q2, then it is split into two parts, one part goes via the collector to the current mirror, the other part goes via the emitter resistors to the emitter of Q1 and from there, via the collector of Q1, to the VAS input. (please, note the italics)
Or to put it in an other way, what would you do with the input when measuring/simulating the characteristics of the NFB loop? Leave it open? No. You connect it to GND or at least terminate it with a resistor, equal to the impedance of the pre-amp. Now, it should be clear that Q1 is acting as a common base amplifier, that is, with regard to NFB signal.
Cheers,
janneman said:
Apologies for quoting myself ...
The last point above suggests an interesting test:
One amp has a coil of, say 5uH. Another amp has a dummy coil which has the same resistance as the first one but not the inductance. Then A/B the two amps.
The dummy coill is contructed as follows: take the same wire and same length as the coil and fold it through the middle. You now have a twin wire connected at one point. Now wind it into a coil starting with the connected end. This will give you a bi-filar wound coil with close to zero inductance, or at any rate orders of magnitude less than the first coil, but the same resistance. And same heating effects etc.
Jan Didden
estuart said:
Hi Edmond.
Hmmmm....If I was simulating or measuring the charachteristics of the NFB loop in any meaningfull way, I would not leave the non-inverting input open, but I would not connect it to AC ground either - I would connect it to a necessary signal source.
How about we just call it a common-base amplifer with regards to DC NFB? With no input signal applied, the NFB servos the output voltage to near zero volts (+/- the diff amp pair offset), so yes, Q1 acts as a common-base amplifier. The base is returned to DC ground via a resistor after all. It isn’t returned to AC ground though (except to really high frequencies well above the audio band when the RF bypass cap takes over), so the common-base classification is superfluous as far as amplifying our audio signals is concerned, IMO.
Cheers,
Glen
G.Kleinschmidt said:
To all: my apologies for being off topic.
Hi Glen,
Okay, I have overlooked the fact that you simulate the characteristics of the NFB loop in a different way, probably with a gain probe. Do you? Doing this with Micro-Cap is possible too, but a little bit clumsy (duplicating the circuit). Therefor, when I'm lazy, I disconnect the FB resistor from the output and inject a signal into the FB network in stead of into the input (and of course taking care of DC operating point).
But how would you measure the loop characteristics of a real amplifier?
Do you have a hardware gain probe?
G.Kleinschmidt said:
Cheers, Edmond.
PS: I'll be back in the evening.
estuart said:
OK, for a real amplifier, I disconnect the feedback resistor (Rf) from the output and short it to ground, so the amplifier is running open loop with the impedance as seen by the inverting input to ground unaltered. I then wire-in an external op-amp servo amplifier which stabilises the DC operating point of the amplifier. I then connect an audio oscillator to the input and measure the gain and the phase of the output with respect to the input signal on a dual trace oscilloscope (with the amplifier driving a suitable dummy load of course).
Then all I have to do is to adjust the frequency compensation for the desired phase margin at the gain crossover frequency.
Easy-peasy
Cheers,
Glen
john curl said:
Hi John,
Those were reasonable questions about how you came to think coils sound bad.
I'm disappointed that you chose not to expand on your experience. You certainly don't hesitate to expand on your 40 year old experiences with ECAD. When you make sweeping generalizations like "coils sound bad", you should expect people to ask you for a bit more detail.
It appears that you just want free reign to make pronouncements and then not be questioned or challenged on it.
Bob
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