John Curl's Blowtorch preamplifier part II

[gpapag]

Proper timing.🙂
A Christmas present for demanding participants
http://cp.literature.agilent.com/litweb/pdf/5950-3000.pdf

Happy holidays
George

 

[gpapag]

If the above link doesn't work for you, use the one below:
http://www.home.agilent.com/agilent/redirector.jspx?action=ref&cname=AGILENT_EDITORIAL&ckey=1000075407%3Aepsg%3Adow&lc=eng&cc=GR&nfr=-11143.0.00

George

 

[a.wayne]

Merry Xmas guys , best of the season ..........

 

[FrankWW]

Merry Christmas!

Play this loud:

Haendel Hallelujah from Messiah , Collegium 1704, Václav Luks - YouTube

 

[a.wayne]

I did , it dimmed the lights in the room .... 🙂

 

[Rick Miller]

Merry Christmas everybody.

 

[scott wurcer]

Thats my kind of amps ......🙂

PS and max Iout don't necessarily guarantee unconditional stability. I was intending to address the no output coil designs. I wanted to see some and a summary of their limitations, there has to be some point where reactance is an issue.

 

[john curl]

A good design allows for no obvious added inductance. Of course you have the wiring to the output of the case, AND you don't expect anybody to do anything STUPID, like put the precise worse case value of capacitance directly on the output of the power amp internally. Almost anything can at least be made to ring, if not oscillate.
However, decades ago, I was shown in a personal demonstration by a colleague that even 2uH could be audible, so I elected to lower its value as far as I could.
And THEN the Taiwanese designers had succeeded in using NO obvious output inductor in one of their prototypes, and we progressed from there.
The ONLY seeming drawback that I have found is that super fast designs that could have 500V/us or so, only to have 150V/us or so. This seems to be the tradeoff. I think that Nelson Pass would agree with me, as he did it first.

 

[mikelm]

John Linsley Hood back around 1980 designed & published and amp with no o/p choke but he used instead a 0.22 ohm resistor.

Just to be clear about our definitions are we currently discussing amps that have no added choke as well as no added resistor ?

 

[PMA]

Yes, only Zobel. My SR = 120V/us. Inductance of wires from pcb board to rear panel terminals, about 0.2 - 0.5 uH, is enough.

 

[jcx]

there are a couple of usages of "unconditional stability" - the Bode swept gain criteria - more suitable to tube amps than SS

and "unconditional load" stability - for any passive load

 

[PMA]

Of course, they can be met. Just avoid overcomplicated designs. Fight for ppm is a way to hell.

 

[john curl]

I have a total measured inductance of about 0.4uH, over 100V/us.
I have done over 500V/us with 2uH in the past. Actually, it is NOT usually REAL INDUCTANCE, like a coil, but the internal rate of change in gain with frequency, which mimics real inductance.

 

[john curl]

Instead of 'usually' I should have put 'only'. Too late to fix.

 

[a.wayne]

Think 1 pr represents much , try these on for size ... 🙂

 

[jacco vermeulen]

See the pretty girl in that mirror there ? (anger management)

 

[Jay]

Oscillation detector?

Yes, only Zobel.

Zobel to flatten impedance

I usually omit zobel, or use minimal value of capacitor (22nF). If this zobel is to flatten impedance, isn't it speaker designer's decision to give one (on the tweeter) or not? Giving one in amplifier output seems a bit of gambling.

Zobel or resistance, at amplifier or at speaker?

At speaker, we can put the component on the right driver. For example, critical is to put resistance in series with woofer and not with tweeter. I have many flat/foil inductors but I think I will never buy such coil again. Low DCR is a good think when too sensitive tweeter is an issue, but I found extra resistance in series with woofer is a good thing sound wise.

Zobel for oscillation protection

I don't know if it is effective to ground the VHF signal, to protect oscillation from happening, so how about a led indicator that is lit up when enough current is flowing thru the zobel resistor. By this, amplifier user is helped to avoid giving bad loads to the amplifier. Why it is not done in commercial amps, what is the issue?

ADD:

I forgot there's already speaker protector?

 

[mikelm]

The amps internal Zobel should be as close to the amps o/p as possible to be most effective.

LED in zobel 😱

 

[mikelm]

Zobel to flatten impedance

I usually omit zobel, or use minimal value of capacitor (22nF). If this zobel is to flatten impedance, isn't it speaker designer's decision to give one (on the tweeter) or not? Giving one in amplifier output seems a bit of gambling.

Zobels in amps & speakers serve different purposes - one cannot replace the other.

 

[PMA]

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.