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Author Topic: Eddie Ciletti on slew rate, op amps and output transformers  (Read 25521 times)

bruno putzeys

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Re: Eddie Ciletti on slew rate, op amps and output transformers
« Reply #75 on: December 11, 2006, 04:01:42 AM »

I might spend a thread on this but in short the only way of properly analysing an emitter follower (and hence a degenerated cec) is as a feedback loop. Redraw the transistor as a 4-terminal block with voltage inputs and a current source output. This is a transconductance amp. One end of the current source is tied to a load and to the inverting input for feedback. Feedback is unity gain so loop gain = transconductance * load impedance. Tadaa...

The reason why many don't see the feedback circuit is:
*Because they don't get that the "input terminal" is the voltage difference between b and e and the "output terminal" is the current through c and e.
*Because feedback is evil whilst emitter followers are not so emitter followers are not feedback.
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Terry Demol

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Re: Eddie Ciletti on slew rate, op amps and output transformers
« Reply #76 on: December 11, 2006, 10:54:37 AM »

Bruno Putzeys wrote on Mon, 11 December 2006 09:01

I might spend a thread on this but in short the only way of properly analysing an emitter follower (and hence a degenerated cec) is as a feedback loop. Redraw the transistor as a 4-terminal block with voltage inputs and a current source output. This is a transconductance amp. One end of the current source is tied to a load and to the inverting input for feedback. Feedback is unity gain so loop gain = transconductance * load impedance. Tadaa...

The reason why many don't see the feedback circuit is:
*Because they don't get that the "input terminal" is the voltage difference between b and e and the "output terminal" is the current through c and e.
*Because feedback is evil whilst emitter followers are not so emitter followers are not feedback.


Yes, I completely understand this  Cool

I think most designers do and it has been discussed a lot over
the last few years. It's really a terminology thing to describe a
particular topology. Smile

cheers

T
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bruno putzeys

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Re: Eddie Ciletti on slew rate, op amps and output transformers
« Reply #77 on: December 11, 2006, 04:08:05 PM »

Yeah Terry I knew *you* understood. The reason why I jumped in to make this post is because last weekend I heard a talk by a certain Tim de Paravicini. At some point he made it a point to claim that cathode followers weren't feedback, an utterance that drew applause from the (highly tube oriented) audience.

That's it, innit? Cathode followers aren't evil so they can't be feedback...

sheesh.
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maxdimario

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Re: Eddie Ciletti on slew rate, op amps and output transformers
« Reply #78 on: December 11, 2006, 07:34:29 PM »

i hate cathode followers.
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dcollins

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Re: Eddie Ciletti on slew rate, op amps and output transformers
« Reply #79 on: December 11, 2006, 08:49:58 PM »

Bruno Putzeys wrote on Mon, 11 December 2006 13:08


That's it, innit? Cathode followers aren't evil so they can't be feedback...



Reminiscent of the quiz question "If my emitter-follower has no NFB and a gain of less that one, why is it oscillating!?"

DC

Andy Peters

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Re: Eddie Ciletti on slew rate, op amps and output transformers
« Reply #80 on: December 12, 2006, 02:52:03 PM »

maxdimario wrote on Mon, 11 December 2006 17:34

i hate cathode followers.


To what message are you replying?

-a
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zmix

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Re: Eddie Ciletti on slew rate, op amps and output transformers
« Reply #81 on: December 12, 2006, 03:14:41 PM »

Shhhh..... listen..... do you hear voices???

Larrchild

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Re: Eddie Ciletti on slew rate, op amps and output transformers
« Reply #82 on: December 12, 2006, 04:41:00 PM »

Yes. "White" supremacists can be uppity about their no feedback.
But it's there.
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bruno putzeys

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Re: Eddie Ciletti on slew rate, op amps and output transformers
« Reply #83 on: December 13, 2006, 06:36:23 AM »

Terry's question about nonoversampling converters has been moved to another thread. The same post contained a question pertaining to cathode followers:
Terry Demol wrote on Tue, 12 December 2006 00:19


You might find it interesting that the cathode follower has a
reputation in the tube fraternity for sounding bad. Apparently
loss of detail ( I think).

But I'm sure you have heard plenty of tube stuff from Mr Tent and
others. Smile

(Max gladly chimed in to confirm the CF's reputation)

The same tube community also has people who know how to build cathode followers that don't sound choked. Simply cascode another triode on top of the output tube to insure constant voltage operation, and replace the cathode resistor by a current source or use a negative supply to obtain constant current operation (apart from the output current).

An alternative to the cascode is to use a pentode with g2 boot-strapped to the cathode.
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maxdimario

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Re: Eddie Ciletti on slew rate, op amps and output transformers
« Reply #84 on: December 13, 2006, 09:11:49 PM »

I agree, although if you have a good power supply with inaudible noise you can tap the plate resistor making a voltage divider: one low value resistor on top of a higher value resistor.

this will not yield as 'low' of a source impedance as in CF, but it does not have the load-dependent shortcomings of a CF.

In cathode followers the gain of the tube is being used 100% to compensate for voltage distortions due to the load. you also have asymmetrical current draw.

2-tube totem-pole-type circuits work better but are more complex and still rely on voltage feedback.

but better is to use a tapped inductor or... a transformer

this way you use the energy flowing through the tube in a more efficient and symmetrical manner, with the actual work of the tube being transferred to the load ideally.



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eddieaudio

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Re: Eddie Ciletti on slew rate, op amps and output transformers
« Reply #85 on: December 27, 2006, 03:59:05 AM »

Hello!

My apologies for taking so long to jump in on this topic, due to the holidaze...  I thought I'd take a moment to explain myself...

When I've got a box on the bench, one of the first things I do is run a sine wave through and look at the overload characteristic - no load and 600-ohm load - and then apply a square wave under similar conditions.  These two tests quickly tell me if there are any obvious problems (like a bad class AB output stage, bad caps, transformer ringing etc.).  And, since I know what the rise time of a square wave is supposed to look like, I tend to investigate when things look a little "slow," if only to satisfy my own curiousity.  

Take, for example, the 100k output pot on the 1176 (attached).  I was scoping things out when I noticed a difference pre and post pot - keeping in mind the vulnerability to cable capacitance.  UREI used a generic shielded wire from the pot wiper to the amp.  For the hell of it, I tried a higer grade, low-capacitance belden cable and realized a slight improvement.  I kept the original wire, just in case the customer objected, but he was happy and gave me other work.

In the studio, by contrast (and, you could even say, to contradict myself) I find that lots of things are too bright - or brittle - and while I wish I had an arsenal of ribbon mics to compensate, I make do with what's available.  

One day I was recording sax and chose to use a modified Pultec MB-1.  The circuit is very similar to the Altec 1566, but with negative feedback, so adding a feedback control allowed me to hear it both ways - and to demonstrate the effect feedback has on frequency response, the overload characteristic and square wave response to my studio maintenance class.

Now, understand I like the way 78-era recordings make the reeds sound and when the sax mic was opened up it was NOT what I was looking for so I tried the MB-1.  Initially, it to had too much detail, so I turned the feedback OFF and attenuated the secondary of the input transformer to compensate for the additional gain.  The "defocused" sound was an improvement to me and to those listening.

My point here is not to say whether feedback is good or bad - for opamps, it's obviously necessary.  And, for opamps, I do prefer mine to be neutral.  ALL of you have detailed how challenging that can be, both technically and psychologically.  

In the case of the UREI LA-4, for example, I think we can all agree that the 4136 is neither transparent nor does it color the sound in a complimentary way AND that almost any newer opamp would be an improvement.  Especially in this case, when a transformerless circuit degrades a squarewave like the stock LA-4 does, I tweaked it until it looked more like the input.  And, I might add, not from an expensive piece of test gear but from a $60 battery powered oscillator.

In this digital era, we have come to rely upon / expect a wider range from our sonic palette and this is the reason there are so many options from so many manufacturers.  On one hand, there is a definite need to pursue the more realistic path of high fidelity, of precise capture and reproduction.  On the other, I think simple discrete circuits without feedback have an equal but opposite contribution to make.

Our ears are all different and so pleasing them at the same time is not an easy task.  I rely on my ears, but I also take stock in what I see on the 'scope and the distortion analyzer - we've got to at least attempt to correlate these things - me in my crude way and others with their greater depth and arsenal of tools.

My hat is off to Bruno and anyone who can correlate the math with what we see via schematic and test equipment, as well as what our ears tell us.  I also apologize for whatever shortcomings my columns may have.  They are not easy to edit and I am often late in getting them in on time, making it harder to zoom out and get a better perspective (until they appear in print, when it's too late).  I am thankful to all those who take the time to read and attempt to make sense of them.  I also welcome your criticism and feedback, positive and negative.

Best wishes to all in the coming year.

sincerely,
eddie ciletti



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bruno putzeys

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Re: Eddie Ciletti on slew rate, op amps and output transformers
« Reply #86 on: December 27, 2006, 10:10:34 AM »

Hi Eddie! Thanks for chiming in!

eddieaudio wrote on Wed, 27 December 2006 09:59

I also apologize for whatever shortcomings my columns may have (...)

I've only recently started to appreciate how hard it is to strike the balance between technical exactness and readability. So far most times I tried writing something for a wide audience I got woefully lost somewhere between the facts and useful copy. So, there's no need to apologise!
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maxdimario

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Re: Eddie Ciletti on slew rate, op amps and output transformers
« Reply #87 on: December 28, 2006, 01:13:04 AM »

Quote:

Now, understand I like the way 78-era recordings make the reeds sound and when the sax mic was opened up it was NOT what I was looking for so I tried the MB-1. Initially, it to had too much detail, so I turned the feedback OFF and attenuated the secondary of the input transformer to compensate for the additional gain. The "defocused" sound was an improvement to me and to those listening.



This is what I used to think as well about 10 years ago.

with more experience in the matter, and after building some very clean sounding amps with superior tubes, I realized that detail is not that important compared to artifacts induced by NFB.

the fact that the sound goes out of focus (even though it may be a desireable 'effect') is not usually what is good about low fdbk amps..

you can build low feedback amps which are 'in focus' and only then do you realize that some of the information which the ear needs to hear to identify a sound as 'real' gets lost in higher feedback amps.

this is especially true in circuits with high 'global' or network feedback..
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eddieaudio

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Re: Eddie Ciletti on slew rate, op amps and output transformers
« Reply #88 on: December 28, 2006, 06:21:28 PM »

I am sure there are many ways to skin this cat, I am by no means advocating one approach.  A ribbon mic, like the Coles, RCA or Royer, would have been a good start, or an Equalizer...  In this instance, my preamp options were the digi-002, an LA-3 (in high sensitivity mode) or the Pultec MB-1.

Even after using the MB-1, I still pulled out alot of upper midrange when it was time to mix.  Here's a link so you can hear what I was going for.  

http://www.tangible-technology.com/ipr/AP-292/summer_06/day- 8/saveYourLove4me_proc.mp3

There are two saxes, one live and one overdubbed.  The live sax is a combo of a Royer and a Sennheiser e-609.  For the overdubbed vocal and Sax, a Neumann M-149 was used, amplified by the LA-3 and the Pultec, respectively.  The rough mix was done on PTLE running on a sony VAIO lapdog.  The verb is both real and soft - the overdubs were recorded in a reverberant space.

ec
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