[The Quality Guru]

Quote:

Originally Posted by kevin gilmore Yes Sir! kodak slr/n not 14n re-installing photoshop cs now on new machine.

I'm so proud of you!!

 

[rickcr42]

hmmm........

looking at the initial schematic .........


BDT preamp meets servoed Melos !

 

[eric343]

I got this from Rudi:

The small triangle on the bottom, is an op-amp ( or at least is deglind as an op-amp ) It take the signal from the output and put it rotated of 180 deg. to the Fet input. So this is a pure feed-back.Don't ask to me or to Gillmore, use goggle and your mind and when you learn on feed-back, answare to me.
I know the intention of Gillmore was only to control the working point of power stage, but he made the worst feed-back. ( same asin all his designs )
Even the input tube have feedback, not intentional feed back and much hard to explain but the way the tube is polarized make an internal feed back effect. Since the cathode voltage is not fixed but oscillating in opposed phase to the input grid, it control the input lowering the gain ( this is good ) but creating many harmonics at high frequency ( tipical commercial artificial sound )
I'm surprised how so simple topics ( at level of first year of any tech school ) are unknown from almost all the forum users. This explain to me why Gillmore is so popular in this forum.
Since I'm really disgusted I'm unsubscribing to the 3d. I you have more questions please mail directly to me mail@rudistor.com

 

[kevin gilmore]

quote
BDT preamp meets servoed Melos !

DING DING DING. We have a winner.

(damm eric beat me to it oh well...)

Melos (at least some versions) were also servo'd output.
Melos used the tube filament as the current sink. Novel but
not so good at very low frequencys as the audio modulated
the tube. And the +/-6 volt output of the melos is certainly
not enough voltage swing for high impedance headphones.

Bet you can't guess who uses this same exact output stage
without the servo but with a huge electrolytic in the signal
path instead...

Could it be the same person that obviously does not understand
what an integrator is and how audio frequencys cannot pass thru
such a device? Could it be the same person that thinks that the
cathode of the input tube goes up and down with audio level and
therefore causes massive internal feedback??

By this definition absolutely every active device out there with
gain has massive internal feedback. Oh wait a triode input tube
with the cathode bypassed to ground with another large electrolytic
must not have any feedback.


A complete and accurate description of the 6ar8 follows. It is
a beam steering device. The current that flows thru the tube
is absolutely constant with or without any audio signal applied.
Much as a Oscilliscope tube the audio signal causes a deflection
in the electron beam from one plate to the other. It is a pure
class A device the current thru the tube is constant at all times.
This is why there is no miller effect. This is why there is no
distortion due to the S curve either.

This is the diy section and a healthy discussion of alternate
methods of amplification should be encouraged. No single
amplification device without feedback is perfect. Adding feedback
can but does not always improve things.


Found a few old (really old) 6ar8's and benched up a version
today. Works remarkably well. For those people that want a low
distortion completely open loop design you might just like this.

 

[rickcr42]

I have been "toying" with a couple of the BDT tubes and what is really cool is how you can control the volume of more than one stage (channel) simultaneously by varying a single control voltage !

Not my original idea but something i find very cool as an alternative volume control for say 5.1-7.1 surround channels and can be totally bypassed when not needed so any detrimental effects ,and so far there are none,can be taken "out of circuit" when listening to two channels.

It seems a simple two button remote volume control can be whipped up for adding "chairside volume control" .another very cool application.

Very exciting gain stage topology.The first "new thing" in a while for me.

 

[kevin gilmore]

quote
I have been "toying" with a couple of the BDT tubes and what is really cool is how you can control the volume of more than one stage (channel) simultaneously by varying a single control voltage !

You bet. Have any idea how much a 6 channel balanced step attenuator
with caddok resistors costs...

 

[drewd]

Quote:

Originally Posted by eric343 I got this from Rudi: The small triangle on the bottom, is an op-amp ( or at least is deglind as an op-amp ) It take the signal from the output and put it rotated of 180 deg. to the Fet input. So this is a pure feed-back.Don't ask to me or to Gillmore, use goggle and your mind and when you learn on feed-back, answare to me. I know the intention of Gillmore was only to control the working point of power stage, but he made the worst feed-back. ( same asin all his designs )

I hesitated at first to jump into this particular argument, particularly since I've seen some of Rudi's other posts, but I can't help it here. Kevin's servo is nothing more than the classic example of an opamp integrator! All that it does is pass the integral of the input voltage!

Vout = - 1/ (RC ) * ∫Vin dt

Son of a gun - you aren't going to get any audio frequency information out of that! The servo simply tracks the slew rate of the output of the amplifier and generates a voltage in response to it. That's not feedback, not the way that we think of it. But I do remember learning about integrators in my first year of electrical engineering. And there's a wealth of information on Google about integrators - it should be fairly apparent just what value it has as an alternative to a traditional feedback design.

What Rudi described was an inverting amplifier. Most definitely not the same thing, although a very common opamp configuration (and easily confused with an integrator).

-Drew

(Edit) Removed a hideously embarrassing arithmetic error!

 

[eric343]

Yeah, I told him that (via PM) in reply to the message I posted, and then repeated myself when he still wouldn't believe me.

Kevin -- It appears that I'm not the only one that needs to re-read Horowitz and Hill.

 

[eric343]

Rudi has changed his opinion a bit on the nature of feedback in this amp.

have built scientific instruments for many years, teached and written books, so I know what is the mean of the kevin's design opamp ( I used simple words with you since I dont' know your skills ). I use "DC servo" on many instruments but not in audio, since in real word a zero is never a zero and a silence is never a silence ( I use your terms ) the opamp always act as a feed back generator ( as secondary effect I admit ).the ammount of feedback should be small but not negligible from my point of view . I believe to talk about hi-end and not "basic audio", and in my hi-end way nothing is to small to be negligible
From my poimt of view the the advantages of this totpology ( no output caps ) are less then disadvantages, and in a deep analysis it must be considered as feed-back circuit.

About input stage. The original article is interesting and well written, and I agree with author when he say the circiut works in a similar way to long-tails. Well I consider long tail as a internally feed back topology ( but on this I admit is possible to have different poit of views, since the feed.back is not so evident).

Obviously kevin and me are designing in different way, he is a skilled diyer using the component manufacturer databooks standard circuits and well known topologies, very good for industrial production, less for hi-end research. (IMHO)
I use my experience, my lab and my finance to go deeply and found smething different.

rudi

 

[Nik]

I cannot understand why is so impossible to accept that there are (as in any other thing in this world) DIFFERENT kind of views... and not necessary one wrongl an one right, one good and one bad.

Exactly like the discussion if there is better the tube or the SS amp, who know? DIFFERENT kinds of mentality and taste. Point !

What is better tha classical music or the jazz ... ??? (other example).

If I think to my profession (classical music) how many school of classical guitar there are today, I cannot find one totally bad or one totally perfect...

Best!
Nicola

 

[Glassman]

electronics is not kind of magic, it's purely objective.. all your examples are purely subjective matters.. this is a serious difference.. I think we're not against new ideas, but we first need them explained and vindicated..

 

[Nik]

Quote:

Originally Posted by Glassman electronics is not kind of magic, it's purely objective.. all your examples are purely subjective matters.. this is a serious difference.. I think we're not against new ideas, but we first need them explained and vindicated..

"Purely objective" (???)

Good!

So were is the PERFECT project for a headphone amp? Do you know wich one is and we (all the heafiers) finally will adopt this one and the forum will not have more any sense to be. Beteween all the designers in the world noboby have found yet?

What is better a Pass project or Hiraga projects... or, or, or ...

Best!
Nicola

 

[Glassman]

I was talking about design, which is of course objective, not sound because how it sounds to each one is highly subjective.. personaly I'd like to know more about the inherent negative feedback in the input tube of this particular amp, Kevin says there is no such thing, Rudi is of different opinion, I'll be more then happy to finaly clear things up..

 

[Nik]

Quote:

Originally Posted by Glassman I was talking about design, which is of course objective, not sound because how it sounds to each one is highly subjective.. personaly I'd like to know more about the inherent negative feedback in the input tube of this particular amp, Kevin says there is no such thing, Rudi is of different opinion, I'll be more then happy to finaly clear things up..

So, you agree with me... do you think that Gilmore and Rudi will find the same position about the feedback? I think no, because even in design each one have his own opinion and experience. Do you know something in this world tha is objective? I found one person that dislike the Mechelangiolo "Pietà" in San Peter (Rome), believe it ???

Best!
Nicola

 

[Glassman]

heh, no I don't

tell Rudi to write his explanation here, it shouldn't be that hard I think.. coz this is not subjective, it's not like Rudi's tubes always show signs of negative feedback while Kevin's don't, hope you understand

I think Kevin is [silently]not[/silently] God and that there may be aspects even he don't know of, who knows, but first I need the explanation and after I understand it I'll become a believer.. that's all.. for now I trust Kevin, you should understand