John Curl's Blowtorch preamplifier part II

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In the amp I speak about (similar to JC designs, with 4 pairs of ON-Semi MJL1302/3281 output devices) the Zobel network is 10 ohm + 33nF, placed on PCB board near output terminals. The Zobel acts above 400kHz.

What in this case are the reactive load limits for stability? Not necessarily heavy load, I've seen amps that oscillate on one polarity of peak output in a narrow range of light but mostly capacitive load but are stable with more load over a wide range of angles.
 
Not necessarily heavy load, I've seen amps that oscillate on one polarity of peak output in a narrow range of light but mostly capacitive load but are stable with more load over a wide range of angles.

I have seen and measured some amps behaving the same way you are describing, they were non-symmetrical designs with single differential input or old-style singleton input. They usually started to oscillate only on one polarity, and they've had non-symmetrical SR and step response. The amp I speak about is a JC topology:

PA4 power amplifier
 
I have seen and measured some amps behaving the same way you are describing, they were non-symmetrical designs with single differential input or old-style singleton input. They usually started to oscillate only on one polarity, and they've had non-symmetrical SR and step response. The amp I speak about is a JC topology:

PA4 power amplifier

Does using them balanced have any bearing on this? I just wondered if there were any cable/speaker combinations that were problematic. I know that for a known very benign load some (many?) amps could have the output coil removed.

That's a nice simple design no unnecessary clutter.
 
I have seen and measured some amps behaving the same way you are describing, they were non-symmetrical designs with single differential input or old-style singleton input. They usually started to oscillate only on one polarity, and they've had non-symmetrical SR and step response. The amp I speak about is a JC topology:

PA4 power amplifier

Can we talk about the lead length on your chassis to shield bypass capacitors? :) (Just kidding nice work!)
 
I have seen and measured some amps behaving the same way you are describing, they were non-symmetrical designs with single differential input or old-style singleton input. They usually started to oscillate only on one polarity, and they've had non-symmetrical SR and step response. The amp I speak about is a JC topology:

PA4 power amplifier

Nice Pavel ....
 
PA4

The amp I speak about is a JC topology:
PA4 power amplifier

Very nice amp, Pavel...but why no billet chassis? (ducks and runs...) Sweet work, you could hang a locomotive from the front panel...how much you selling them for?

Someone was talking about a SWTPC Tigersaurus recently: http://www.swtpc.com/mholley/RadioElectronics/Dec1973/RE_Dec_1973_pg44.jpg
Oddly enough I was looking in my junk...I mean supply boxes last week and found a Tigersaurus main PCB! If I remember correctly they would nuke an expensive set of outputs in a heartbeat, making the people I built them for very upset. Does anyone know if the rumor that one set Walt Jung's house on fire is true?

Howie

Howard Hoyt
CE - WXYC-FM 89.3
UNC Chapel Hill, NC
www.wxyc.org
 

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Hello Howard, thanks for your kind words. Three pieces were hand made 2 years ago and customized to individual orders. Now, audio is only a hobby for me and I am not selling the products. I still own the prototype, and it is my best power amplifier.

Single billet chassis would be nice, but to expensive for me :).

Why not share the design then? :p Just kidding. It's your choice of course, but it looks like a nice design.

I have had a similar concept in my mind for some time. Although not with mosfets, but bjt's and jfets at the input. Perhaps mosfets as drivers. Anyway, still just a sketch in my head.
 
I've some 50 foot ling mike cables I built 8 years ago from parts express, red in color. I don't have any data on the wire itself, not even a sample of the cable to determine pitch.

After the holidays I'll be able to perform some experiments on one or two of them. I've a scan I'll attach in the next coupla minutes with some test details and concepts..

Bummer, pencil doesn't come out with enough contrast.

Upper right, just a depiction of twisted pair showing pitch and centroid spacing definitions.

Upper left, a graph of inductance vs coil circumference for close packed single layer wire on a non conductive non magnetic form. In the limit of very large form, there is no inductive shift. As the form becomes smaller, the pitch coupling will alternate between maximum enhancement and maximum cancellation, the growing sine modulation of the inductance. It gets larger as the coil circumference gets smaller because there are more places where the pitches line up.

In the middle is a depiction of a coil of wire, magnetic field lines generated by a solenoidal current, and I've drawn in a cylinder of material to disturb the reluctance path of the field lines. I set a limit of 10 centroid spacing gap between the conductor and the cable to reduce the possibility of coupling to the centroid gap generated dipole field, "near field" disturbances. This test is designed to determine if any solenoidal field is present, as the conductive object will fight time varying flux. A positive change here would indicate that Ed's premise is more likely to have some credence. A negative result here would mean there is no solenoidal field being generated.

Lower right is the bog standard L and R vs frequency relationship.


jn
Converted it to pdf that help me to more easily read it
 

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Let's look at PMA's 'simplified' schematic and add to it with the simplest input stage power supply buffer that is useful:
 

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There are many useful additions to this schematic, but we would lose the 'forest' for the 'trees' if we include everything at once. I picked the easiest and most efficient power supply buffer as an example of one the best that you can do with minimum loss of power supply voltage. We can also do more exotic and costly power supply buffers, but this simple example is pretty darn good.
 
Your picture shows exactly same circuit that is used in the PA4 amplifier. With BJT (yellow), for the same reason - low voltage drop.

Another VERY important add is Vbe multiplier modification (red circle). With this mod, the thermal stability of idle current is perfect. Without, it was not.
 

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Thanks PMA, but I hope to discuss each 'addition' to the 'simplified' schematic of yours in detail, in order to show people something that they can 'add on' themselves, whatever their design. If we wanted to show a complete schematic, then the Parasound A21 or even the 20 year old Parasound HCA1200 would be even more complete, but intimidating to most of the usual participants of this website.
So getting back to my 'addition' of the power supply buffer, it should be pointed out that it has VERY LOW voltage drop across it. This is important so that if only one pair of power supply voltages is available, then the voltage drive to the output stage will not be effected too much.
It is NOT a perfect buffer, but it is a pretty good one. The limitation imposed by finite beta of the follower devices limits the bias resistor to perhaps 1K. Therefore the time constant created by the cap to ground and the bias resistor is fairly high.
Still, it is adequate for almost any amp. I listen through one, with my cable TV, every day of the week. I even drove my DW Sasha speakers with a similar amp for a few weeks, without any trouble.
But for some amps, we can even do better. (more later)
 
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