John Curl's Blowtorch preamplifier part II

[ThorstenL]

Hi,

Originally Posted by VladimirK

I am afraid I disagree with you at this point. So called choke-input power supplies inevitably make amps intended for jazz and vocal, one should forget about punch and drive with rock music. This conclusion is now an axiom among russian audiophiles (equipment users)

I do not agree.

First, the "punch" and "drive" in Rock is almost invariably heavy compression (trust me, I know, I used to sit at the mixing desk for an East German Hard Rock band, our FX rack had more compressors than the audiences had Secret Police informers I have seen even worse in the West).

Moreover, I am unclear what a "so-called choke input supply" is. It either IS a choke input supply and thus draws current via the rectifier throughout the whole cycle or it is a capacitor input supply, even if only a small value capacitor (a few uF) is used upfront, or it is a capacitor input supply with a LC filter after. It cannot be "so-called".

In fact, compared to capacitor input supplies true choke input supplies deliver by far superior regulation and thus less supply sag than conventional Capacitor input supplies. The problem is that chokes needed for this are nearly as massive the mains transformers.

So at least technically speaking in the correct application the axiom of your countrymen is wrong. In fact, I have encountered many axioms among audiophiles worldwide which are often directly contradicting to each other (so by definition one group must be wrong) and equally often contradict reality.

Originally Posted by VladimirK

from technical point of view, I can not imagine how amp could pump 60A peak current to load during high-level bass notes (maybe many Farads lytics reservoir)

Well, most speakers voice coils will near instantly vapourise with a real 60A. So given that I am not trying to make exploding wire magnetisators but Amplifiers for listening to music I am not worried about delivering 60A. If we assume a Speaker with 1.6 Ohm DCR for the Voice Coil (two 4 Ohm drivers in parallel) we need a minimum of around +/-100V Rails for Solid State Amplifiers and deliver pulse power of 6KW.

For speakers with a more normal Impedance (say 3.2 Ohm for two paralleled 8 Ohm Drivers) the requirements for the rails in terms of Voltage and the peak voltages are simply ridiculous.

I cannot imagine any such situation under domestic or even studio conditions, but as I remarked elsewhere, I don't have an imagination.

Originally Posted by VladimirK

Nelson Pass repeated maybe hundred times, that amps are intended for applications. The one best for rock will never be best for jazz.

I find that saying such things is meaningless unless we include the Speakers. Once we do I partially agree, insofar that the more compromised the amplifiers and Speakers are the more true I fond this "If they are good at one music they are not with another".

I have found at least one system (there is a second one but I do not want to be falsely accused of bearing the rules and promoting products I have involvement with so I shall not mention that) that does perform very even handed and is probably the best I had in my long terms for Jazz, Classical, Rock, Pop and even (classic) Rap and Solo Pipe Organ. It is also absolutely superb with movies... Dynamics were explosive and realistic, except for movies, where they are hyper realistic.

In this case we are talking about 15" Tannoy Monitor Red's in Corner enclosures (some might call them corner horns, but strictly speaking they are not - measured at 97dB/2.83V/1m and posessing a 16 Ohm rated impedance - thus a real 100dB/1W/1m). Lower cutoff was somewhere a bit below 30Hz.

They where driven by 300B SE Amplifiers which I optimised for clipping / overload behaviour in addition to linearity inside the intended power range. This amplifier could be driven with peaks that are at least 6dB above the rated 7 Watt without sounding subjectively distorted or even compressed. The output stage incorporated a kind of "expansive variable quiescent current" regulation scheme, that reduced effective bias voltage and increased anode current more the more the amplifier was over driven (grid current to the 300B)...

The Amplifier of course used choke input supplies (not so-called ones) and further LC filtering, no electrolytic capacitors and relatively low values of power supply capacitance and additional noise cancellation schemes which incidentally also opposed any power supply modulation by the Amplifier (they cancelled > 26dB any disturbance on the power supply lines, no matter if mains noise or signal modulation).

Incidentally, most of these design techniques can be found among others in 1930's WE Theatrical Amplifiers and Telefunken Public Address amplifiers, they are nothing new, nothing I invented or claim as my original work. Most of these techniques cannot be ported directly to solid state gear, however it is possibly to learn from them and get similar results using different circuitry.

I eventually planned to make an 18 Watt version of this Amplifier using one of these Super 300B's, but stuff came up and it never happened. In the fullness of time I developed other design techniques that allow me more power with a similar overload handling using standard 300B's and I also have worked on other Amplifiers and a variety of speakers, but that, as they say is another story.

Ciao T

[BV]

Originally Posted by VladimirK

I am afraid I disagree with you at this point. So called choke-input power supplies inevitably make amps intended for jazz and vocal, one should forget about punch and drive with rock music. This conclusion is now an axiom among russian audiophiles (equipment users), and from technical point of view, I can not imagine how amp could pump 60A peak current to load during high-level bass notes (maybe many Farads lytics reservoir could help) Active resistance of the choke plays almost no role, but its L affects charging pulses drastically.
Nelson Pass repeated maybe hundred times, that amps are intended for applications. The one best for rock will never be best for jazz.
I am lucky, since any rock is not my kind of music, and I can implement "slow" and high Zout power supplies, but extremly cleaned from any HF noise.

Sorry, but it is IMO completely untrue, both from empirical and technical point of wiev..
Amplifier are intended to amplify, independent to genre of music.,Try to think about about role of coil before filtering capacitors (coils "behind" filtering capacitors are technical nonsense..) . It will only lower peak output voltage by some hundreds mV, but it will lower ripple, too. Or You are using PSU without filtering capacitors??

[Joshua_G]

Originally Posted by Tony

why should they?

I have no clue why fuses have impact on the sound. All I know, empirically, is that they do.

[Joshua_G]

Originally Posted by ThorstenL

...
I find all this fuse hubbub and kerfuffle about fuses distracting and feel a bit like the sorcerers apprentice.
...

Well, you are right about there is no use talking about fuses.
The reason I looked at the fuse is that in all my other gear there are Hi-Fi tuning fuses installed. So I wanted to see the value of the fuse in that CDP. Seeing that it's a propriety fuse, I decided to let it be. Out of curiosity, after the burning in, I'll try to replace it, though I doubt if any other fuse will make any improvement. I learned to trust your design and parts choosing decisions.

[PMA]

Originally Posted by VladimirK

This conclusion is now an axiom among russian audiophiles (equipment users), and from technical point of view, I can not imagine how amp could pump 60A peak current to load during high-level bass notes (maybe many Farads lytics reservoir could help)

You often play into short circuit??

Technically there is no problem to monitor supply rails with A/D converter and memory (even behind the fuse, if someone mentions fuse non-linearity as a source of problems). Same for output voltage of the amplifier playing into real speaker. I do it regularly.

It is nice to care, but some audiophile reasoning and explanations are driving me mad

[StereoEditor]

Originally Posted by Wavebourn

John;
I respectfully disagree with you here.

No problem, and my apologies for the tardy response.

Originally Posted by Wavebourn

If the amp has output impedance that changes with frequency it is much more useful to compare result with straight line than with some kind of standardized curve that also depends on output resistance of the amp, even if it is absolutely frequency independent.

If you look at the frequency response graphs I publish in Stereophile - for example, see figs.1 & 2 at Music Reference RM-200 Mk.II power amplifier Measurements | Stereophile.com - I do publish the response into resistive loads ranging from 2 ohms to 8 or 16 ohms, as well as into the simulated loudspeaker.

Originally Posted by Wavebourn

Also, dips and peaks on impedance curve of loudspeakers are not directly translated into dips and peaks on sound pressure curve.

I don't follow, Surely it's just a matter of Ohm's Law?

John Atkinson
Editor, Stereophile

[Pano]

Originally Posted by john curl

.......... AND for good measure I stuck a 10uf film capacitor across the power supply line where the line cord connects.

That's a big cap. Often over an inch in diameter or one side in the box shape. Wouldn't it be fairly low impedance at 60Hz?

Do you put it across hot and neutral, or to ground? Have you found any differences there?

[VladimirK]

Guys, I propose to differ between AB amps and A amps, and to think about whether LC filter (choke input) could really give benefits for AB amp.
I do not use AB, I use A, and even more, a load connected in parallel to active device. My case is just the best application for LC power supply filter. But I find reasonable to go with consequent RC-RC-RC-RC, simpler and could be even better.
L and C are chosen from definite requirements, in particular, for definite current consumption from power supply. If the last is strongly varying like in AB amp case, how LC can be used effectively?
Consider bass note lasting 1s, with average current to load 10A. That means 10 C (coulomb) charge is transferred to load during 1s. Typical lytics reservoir lets say 0,1F, would require 100V voltage drop in order to provide such charge. This simply indicates, that in this examplary case output current must be provided by increased current consumption from PS, not from lytics.
If one use L taken for 2A consumption, what will be effect of this L for 10A demand?

[scott wurcer]

Originally Posted by StereoEditor

I don't follow, Surely it's just a matter of Ohm's Law?

John Atkinson
Editor, Stereophile

John, I think Wavebourn is talking about the acoustic SPL out of the loudspeaker does not necessarily correlate to the dips and peaks of the voltage on the terminals, which is determined roughly by Ohms law (with complex variables). I never examined it myself so I don't know how much non-minimum phase behavior this voltage/current can exhibit but I would surmise it is small, while the SPL can have measurable non-minimum phase behavior.

[PMA]

Originally Posted by VladimirK

Consider bass note lasting 1s, with average current to load 10A. That means 10 C (coulomb) charge is transferred to load during 1s

??