john curl said:
Be it 0.5uH or 0.78uH, who cares? I didn't double check andy_c's calculation. The pertinent point (which you and others keep blissfully ignoring) is that a small inductance at the output only incurs a very small amplitude variation in the frequency responce at the output terminals of the amplifier when driving a reactive load.
Lets assume that the DM88's inductor is 0.5uH then. We've got a ~0.2dB ripple for the DM88 into a rective loudspeaker-simulating test load.
How audible would that be, do you think?
How much do you think that this miniscule figure would be worsened if the inductor was 0.78uH, 1uH or 2uH?
john curl said:
Please try to address the point of contention with a logical argument instead of evading with patronising red herrings.
Hi G.Kleinschmidt,
You appear to be trying to hammer home a point here.
The voltage across an inductor always leads current, and the induced voltage (error) is not greatest with steady sines but with transients, and due to crossover circuitry combined with the dynamic reactivity of loudspeaker systems.
That ~0.2dB variation is with steady sine, and clearly shows a degree of reactivity that is not necessaily related to an output choke alone, so I wonder what your point is in relation to John's 'series output choke' thread ?
There are those of us who have heard a difference, albeit in my case 35yrs ago when fitted to a monobloc that did not need a choke and driving a LS with just 1m cables, also with the older 5>10uH values as once used.
Whether I could still 'hear' the difference now I don't know, but I have no intention of taking a step backwards anyway.
Cheers ........ Graham.
You appear to be trying to hammer home a point here.
The voltage across an inductor always leads current, and the induced voltage (error) is not greatest with steady sines but with transients, and due to crossover circuitry combined with the dynamic reactivity of loudspeaker systems.
That ~0.2dB variation is with steady sine, and clearly shows a degree of reactivity that is not necessaily related to an output choke alone, so I wonder what your point is in relation to John's 'series output choke' thread ?
There are those of us who have heard a difference, albeit in my case 35yrs ago when fitted to a monobloc that did not need a choke and driving a LS with just 1m cables, also with the older 5>10uH values as once used.
Whether I could still 'hear' the difference now I don't know, but I have no intention of taking a step backwards anyway.
Cheers ........ Graham.
I think John just likes to leave away things that are not absolutely necessary. It doesn't make sense if some manufacturers try hard to achieve good slew-rate performance for instance and then they sacrifice it again by the use of an output inductor.
You may very well run into trouble connecting a capacitor to such an amp without coil.
But in real-life you don't do this - you rather connect a speaker to your amp - well at least I do !
Some might argue that real-life speakers may also present capacitive loads. But compared to even the cheapest capacitor the are far more lossy and thus less dangerous.
Regards
Charles
You may very well run into trouble connecting a capacitor to such an amp without coil.
But in real-life you don't do this - you rather connect a speaker to your amp - well at least I do !
Some might argue that real-life speakers may also present capacitive loads. But compared to even the cheapest capacitor the are far more lossy and thus less dangerous.
Regards
Charles
pooge said:
G’day pooge.
An important point made in that paper:
quote:
“It is believed that the increase in output impedance caused by the feed forward compensation in the present example is not significant enough to cause a change in performance of an amplifier with a typical loudspeaker load. Because the low-frequency output impedance is not changed, the damping factor at the lower cut-off frequency of a woofer would not be affected. In the present example, the predicted output inductance with and without the feedforward network is 4.8uH and 0.95uH respectively.”
/quote.
In other words, the need for an output inductor to maintain stability when driving capacitive loads is removed, but the output impedance of the amplifier is actually significantly worsened over that with standard compensation and an output inductor. High frequency THD performance is also potentially compromised as the output stage is removed from the feedback network at HF.
There are cons in the use of a load-isolating output inductor, but there are also performance compromises that must be made and accepted if a comparable degree of stability phase margin when driving difficult capacitive loads is to be maintained without it than with it.
Cheers,
Glen
Not promoting the technique or dismissing it.
I found another important point, that of the issue that lead compensation in the feedback network is affected by the load impedance.
The issue of RFI into the output end, although mentioned in this thread, has not been given much attention. Thus, advocates of audibility of the output coil have not, to my thinking, ruled out the RFI issue when stating that removing the coil is audible. Is it the coil or the RFI? Further, even if audible, is it better or worse. Perhaps people think RFI makes it better?
I found another important point, that of the issue that lead compensation in the feedback network is affected by the load impedance.
The issue of RFI into the output end, although mentioned in this thread, has not been given much attention. Thus, advocates of audibility of the output coil have not, to my thinking, ruled out the RFI issue when stating that removing the coil is audible. Is it the coil or the RFI? Further, even if audible, is it better or worse. Perhaps people think RFI makes it better?
Graham Maynard said:
G'day Graham.
Well I think that it is a pertinent point that is being ignored. You're right that the 0.2dB amplitude variation isn't entirely caused by the reactive component of the inductor. This further buttresses my point with regards to the (in)significance of the 0.5uH of inductance and is why I specifically asked John to estimate how much the ripple would be worsened if the inductance was larger.
Now, I can't comment on your subjective experience, other than saying that my subjective evaluations do not support them.
This is a rather pointless line of argumentation, so is why I'm sticking to the technical side of the debate.
Also, I don't take any issue with those who wish to design without the output inductance. Amplifiers can be made perfectly stable without them and that is all good and fine.
I take issue with those who persist in propagating the myth than an output inductor (even a very small one):
1) Necessarily causes sonic degradation (note that this is not necessarily inferred simply by “audibility”),
2) Omission of which comes without other design compromises,
3) That those who use output inductors due to a consideration of #2) are somewhat technically incompetent and produce inferior audio amplifiers.
Cheers,
Glen
Glen, KEEP YOUR COILS! I once lived line of sight to a big transmitter tower. I had RFI everywhere! I had to use 100% (foil) shielded coax wire, just to keep RFI in control. Even Parasound uses coils on their cheaper products for the same reason. It has to be bulletproof for the installers. However, I just talked to the chief tech at Parasound, and they have not found any problem with oscillation in my power amps. Nothing worth noting, in any case.
Hi Glen,
Ah, now I see some of what has been in your mind; your initial challenge appears to apply towards a specific design already having significant internal output impedance which would mask the effect of a series output choke anyway; maybe for a specified reason ?
Once this choke effect has been heard you realise that there is a loss of transient attack and the micro-detail of dynamic reproduction, which I class as sonic degradation, though choke values of 5uH upwards are the only ones likely to be spotted.
Maybe that 'myth' needs some numbers; which brings in LS cables too.
If I found a self design amplifier needed an output choke I would think of myself as being technically incompetent if I used a value exceeding 0.5uH rather than scrap it and try another topology, no matter what anyone else thought.
Cheers ....... Graham.
Ah, now I see some of what has been in your mind; your initial challenge appears to apply towards a specific design already having significant internal output impedance which would mask the effect of a series output choke anyway; maybe for a specified reason ?
Once this choke effect has been heard you realise that there is a loss of transient attack and the micro-detail of dynamic reproduction, which I class as sonic degradation, though choke values of 5uH upwards are the only ones likely to be spotted.
Maybe that 'myth' needs some numbers; which brings in LS cables too.
If I found a self design amplifier needed an output choke I would think of myself as being technically incompetent if I used a value exceeding 0.5uH rather than scrap it and try another topology, no matter what anyone else thought.
Cheers ....... Graham.
I'm a bit off-topic here, but there's an interesting table showing inductance measurements of various speaker wire types here. Earlier in the article they state that these measurements are on a per-foot basis. What I found interesting was that a length of about 2.6 feet of 10 AWG zip cord amounts to 0.5 uH. I didn't realize its inductance was that large! 
Thanks for the link Andy.
So a 10ft/3m lead of 10AWG would be approx 2uH.
I note that that Audioholics website has an investigation into bi- and mono-wiring.
They appear to be looking at (R based) dissipation only, and not dynamically energised (L+C) energy storage and release mechanisms.
Cheers ...... Graham.
So a 10ft/3m lead of 10AWG would be approx 2uH.
I note that that Audioholics website has an investigation into bi- and mono-wiring.
They appear to be looking at (R based) dissipation only, and not dynamically energised (L+C) energy storage and release mechanisms.
Cheers ...... Graham.
Hi Pooge,
--- The issue of RFI into the output end, although mentioned in this thread, has not been given much attention. Thus, advocates of audibility of the output coil have not, to my thinking, ruled out the RFI issue when stating that removing the coil is audible. Is it the coil or the RFI? Further, even if audible, is it better or worse. Perhaps people think RFI makes it better?---
Due to a typing fault, I erased what I was going to post yesterday about the issue of RFI. You exactly express my thinking, but much better than I can do in English.
I retain from experience that listening judgements can be easily fooled, quite often interpreting harmonic and inter-modulation distortions as "better", more detailed, and the like…
Former readers of Electronics World are aware of Cyril Bateman's works published circa 1997. From private correspondence with Cyril, I know he has recently continued his investigations on the interface between the amplifier output and its load. He had collected a great amount of data and has done many simulations, all show that RFI are not innocuous and have to be taken seriously : they can even be responsible of amplifier failures.
Lets have a quick look at what is the load of a loudspeaker for an amplifier
- a Zobel network, capacitor + resistor
- a wounded output coil
- some lengthes of wire
- capacitors, inductances, power resistances in case of passive crossovers
- voice coil(s)
At HF, all these components are far from what they are at audio frequencies, their behaviour is governed by parasitic values which imply that they suffer from all kinds of LC resonances. The whole circuit presents complex impedances at high frequencies. Just think that a two-way speaker can be measured at more than 500 Ohm at 1 Mhz ! Cables just complicate the matter, so we can expect some voltage due to RFI and EMI just waiting at the output of the amplifier to enter the circuit, with both inter-modulation and demodulation through non-linear active devices.
What role the damped output coil can play in all this ? that's the question. Numbers given by measurements have more significance here than listening experiences.
--- The issue of RFI into the output end, although mentioned in this thread, has not been given much attention. Thus, advocates of audibility of the output coil have not, to my thinking, ruled out the RFI issue when stating that removing the coil is audible. Is it the coil or the RFI? Further, even if audible, is it better or worse. Perhaps people think RFI makes it better?---
Due to a typing fault, I erased what I was going to post yesterday about the issue of RFI. You exactly express my thinking, but much better than I can do in English.
I retain from experience that listening judgements can be easily fooled, quite often interpreting harmonic and inter-modulation distortions as "better", more detailed, and the like…
Former readers of Electronics World are aware of Cyril Bateman's works published circa 1997. From private correspondence with Cyril, I know he has recently continued his investigations on the interface between the amplifier output and its load. He had collected a great amount of data and has done many simulations, all show that RFI are not innocuous and have to be taken seriously : they can even be responsible of amplifier failures.
Lets have a quick look at what is the load of a loudspeaker for an amplifier
- a Zobel network, capacitor + resistor
- a wounded output coil
- some lengthes of wire
- capacitors, inductances, power resistances in case of passive crossovers
- voice coil(s)
At HF, all these components are far from what they are at audio frequencies, their behaviour is governed by parasitic values which imply that they suffer from all kinds of LC resonances. The whole circuit presents complex impedances at high frequencies. Just think that a two-way speaker can be measured at more than 500 Ohm at 1 Mhz ! Cables just complicate the matter, so we can expect some voltage due to RFI and EMI just waiting at the output of the amplifier to enter the circuit, with both inter-modulation and demodulation through non-linear active devices.
What role the damped output coil can play in all this ? that's the question. Numbers given by measurements have more significance here than listening experiences.
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