Single or dual differential?

[Steve Eddy]

Originally posted by hitsware
My point is that whether you have 1Watt or 1kWatt (well maybe alittle exageration ) you need the same gain for a 'comfortable' response from the nob. The other 999 Watts is overhead. As enlightened as I consider myself, I don't want to turn up something 3/4 of the way for normal listening. Just has a bad "feel" ........ But that's just me. (and customers in the past)

Well if it feels good, do it.

Personally, knob position has never been an issue for me. With the possible exception of instances where there's so much gain that knob positioning is excessively limited and I find it hard to dial in a "just right" setting.

se

[Bricolo]

What are exactly the problems that GNFB intoduces?

Unstability due to phase shift (neg fb becoming pos fb) is obvious

Mr Curl told about TIM and IMD poroblems, could you developp?
Is there anything else?

[hitsware]

http://melhuish.org/audio/article5.html

Here's 1 issue .........

[fdegrove]

Hi,

Tube amps are highly unstable with feedback, depending primarily on the transformer! IIRC the Williamson had around 16dB of global NFB and was acknowledged by the designer to be on the verge of instability; this at a time when global negative feedback was regarded as the cortisone cure-all of audio, just as NONE is now!

This is all true and specifically true for the Williamson circuit which is, by design, on the brink of stability.

Hugh's analysis is quite correct although, IMO, it leaves out some important factors.
A triode has build in feedback by nature. How else would one be able to create a device with reasonable gain and still maintain some linearity?
This, however is local feedback, not global, and as such can react much quickly timewise.

The point of feedback encompassing the OPT is very real indeed and as a matter of fact OTL amps can tolerate much higher levels of GNFB which are much needed to reduce the Zout of the amp in order to be able to interface correctly with a common speaker's impedance.
IOWs, it wouldn't work without it and the same is true for most common sand amps.

By ommitting the OPT you can use more NFB but it doesn't have to include the input stages.

For tube amps using OPTs it would be advisable to use local NFB and distortion cancellation techniques, something that's certainly not all that easy to implement but it can be done.
One way is the use of cathode feedback winding(s) on the OPT but there are other ways.

What are exactly the problems that GNFB intoduces?

GFNB is always running behind on what it's trying to correct; it's a kind of "Back to the Future" artefact.
It may seem to work statically but dynamically it sucks big time.
Same goes for most measurements made that are not dynamic, they don't reveal what's going on in "real time".

Cheers,

[hitsware]

>GFNB is always running behind on what it's trying to correct; it's a kind of "Back to the Future" artefact.

Yea that is a BASIC flaw (though in practice seems not to show much). Same thing as 'bootstrapping' (in reverse). The transit time of the signal through the amp makes either sorta nebulous.
Only really shows up (on my humble equipment) in recovery or settling time, but has audible implications beyond that.....mike

[fdegrove]

Hi,

Yea that is a BASIC flaw (though in practice seems not to show much).

The faster the response time of the loudspeaker the more it becomes obvious...

Take FR speakers with low moving mass, ELSs or ribbons and you'll quite likely hear the effect.
Most other, more conventional speakers, are way too slow.

Hope this helps,

[hitsware]

>Believe me, if tube designers could use 60dB of NFB like many SS amps, they would!

Good one ! Thanks
You use single differential right?
Your amps are well regarded on
the FR driver forum..........mike

[Steve Eddy]

Originally posted by hitsware
http://melhuish.org/audio/article5.html

Here's 1 issue .........

I've some serious reservations about that article.

By Ohm's law, the current in a speaker voice coil is the amplifier output voltage divided by the speaker impedance. Therefore, the current can only be linearly related to the voltage if the speaker presents a purely resistive load to the amplifier.

Excuse me? Reactive circuits violate Ohm's Law? Last I looked, current is linearly related to voltage in reactive circuits with only a relative phase change between current and voltage.

When, as is usually the case, the speaker impedance varies with frequency, amplitude, voice coil temperature etc., these dynamic impedance fluctuations translate directly into distortion of the driving current, as I = V / Z.

I = V/Z looks like a linear relationship to me. So where does he get this notion that current can only be linearly related to voltage if the load is purely resistive?

The negative feedback often used in power amps is totally blind to all of these problems, and with the reliance upon "damping factor" (electromagnetic-mechanic damping at resonance), huge back-EMF currents (at high voice coil velocities near resonance) are dumped back into the output of the power amp...

Huge back-EMF currents? Back EMF is greatest at and near resonance, which is where impedance is highest and by Ohms Law, current is lowest. So where are these "huge back-EMF currents" coming from? Hell, many loudspeakers' impedances exceed 100 ohms near resonance. There will be FAR more current in the circuit at the impedance minimums above and below resonance.

What's this guy talking about, Mike?

se

[Fred Dieckmann]

"If I'm just trying to get attention, then you and Curl with your remarks such as these are just playing into my hands and drawing even more attention to me.

Who's looking for attention here? These comments, instead of simply going on about what it is you wish to discuss, seem to be saying "Hey, don't look at Steve, look at me!"

Go start your own thread then. I can't that any of your post have anything to do with differential input stages, or can't you get any response to your post accept by crashing threads where you have no knowledge of the topic? Be a man and stand on your own two feet instead of clinging to people's coattails till they get tired of you dragging things to a halt. I guess I could respond to people via Email instead of dealing with the constant threadjacking......... most of us are sick and tired your thread wrecking. I (and many others) are extremely sick of you ****ing Mr. Curl off in the middle of his giving extremely useful input on a topic. When you run somebody of his caliber off you are going to be even more unpopular than you are now.


Wake up and smell the coffee!

[hitsware]

I=V/Z is a linear relationship BUT
the Z of a driver (or any reactance)
is (by definition) not. Something to
do with 'rate of change' ........mike