Zero Field Transformer With DC Servo

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Samuel Groner

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Joined
Aug 19, 2004
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2,940
Location
Zürich, Switzerland
Hi

Thought about using some zero field transformer for line inputs and mix bus amps; is there a way to implement the feedback network without a large 'lytic cap?

For reference, let's look at the Lundahl LL6404: 6404.pdf

First of all, why do we need this cap (C in the datasheet)? Does it stop the feedback trying to extend the bandwith to DC?

What about an inverting servo to the R3-noninverting input-node?

Thanks for your help!

Samuel
 
There is a little offset in there, so I think the cap is there to stop DC from running throught the secondary windings.
 
Positive feedback is something of a devil's bargain. If you are tweaking things up to compensate for the winding R you are on the verge of instability all over the place. With the cap at least you can keep the d.c. component from banging things into one or the other rail.

Note that these circuits are inherently temperature sensitive because of the largish tempco of the winding. Ap has a patent on a transformer with a noninductive winding used as a copper resistor, which employed as a component within the drive amp feedback loop accounts for the tempco. They are using this in the outputs of their equipment, and some manufacturers of other pro audio stuff have been known to license it as well.
 
Thanks for the answers. So if I understud things right, there should be no further surprises by replacing the cap with a servo? At least if the servo IC does not show phase reversal when clipping and we don't mind a few minutes till things settled.

Google spites out a tempco of +0.3937% per K for copper - would a thermistor make sense here?

..It's like déja-vu all over again...
I beg your pardon :?:

Samuel
 
[quote author="Samuel Groner"]Thanks for the answers. So if I understud things right, there should be no further surprises by replacing the cap with a servo?

Google spites out a tempco of +0.3937% per K for copper - would a thermistor make sense here?

..It's like déja-vu all over again...
I beg your pardon :?:

Samuel[/quote]

Heh. Be careful about Keith, now that he's been revealed to be a 17 foot tuba player. :green:

The thermistor---hmmm. Possibly. It would have a better chance in this circuit where there isn't much self-heating due to the signal.
 
Many of us have been around this group for years (including the unfamous membership at RO) and have seen the same topics discussed over and over again. Unfortunately, we lost many good discussions when we parted RO and they cannot be located on this one. :sad:

Respectfully, patrons should supress their dissapointment of reading reoccuring topics except when the post is about making revlog taper variable resistors using linear taper pots. In that case the poster should get an instant full frontal lobotomy. Can phpBB configured to do that?
 
[quote author="Samuel Groner"]
He's been revealed to be a 17 foot tuba player.
And you tell that to a 2.9 m Bösendorf Imperial player? :cool:

No intention of being rude, simply did not understand his pun or whatever it was.

Samuel[/quote]

Unfortunately you have to know some insider info which has still not been entirely disclosed.

Regards the Bösendorfer Imperial Concert Grand: UCLA had one in one of the classrooms, and the great teacher Natalie Limonick would teach from the keyboard many times and play musical examples.

The big piano of course has extra notes, and they provide a flap that fits over them so that pianists not accustomed to the beast will not get their references screwed up.

It was a favorite pasttime among the students to sneak up and lift the flap while La Limonick was distracted. Then she would play and the first time she plunged down the the lower regisiter she would hit a clinker and curse :green:

Forgive me if I told this story before :?
 
It's no secret that I use 'zero field' transformers all the time.
The huge advantage with 'current mode' as I call it, is that as there is virtually no voltage generated across the windings, then there can be no saturation, so the main distortion generator is removed.

I would argue with Lars with his 'zero field' name; after all the field is a function of current and turns..... sure, the field is small, but it ain't 'zero'.

I must have a look at this 'bootstrap' arrangement that he uses; I must admit that I have never found a reason for anything like that; in its raw state the distortions are minimal and predominantly second order with the transformers I use, so I leave them alone! (my transformers have DC resistances of around 40 ohms and operate with a feedback resistor of 22Kohm wrapped around a MC33078, a 1000uF cap works nicely, giving amplitude frequency response pleasantly flat down below 8Hz.)

As for replacing the cap with a servo.... yes, I'm sure it would work, but I'm equally sure that it's not worth the added complexity.
 
Sorry for the obscurity, but I'd just been talking to bcarso about the AP output transformer positive-feedback complexity. It was sort of an in-joke.

Well, the Boozey might be 2.9 meters, but how big is the player? -In my case, I really AM 17 feet tall! :green:

When I lie on my back (about the seventh Guinness) I'm reduced to about 7 feet to the highest point (mid-belly) :wink:

Keef
 
If you drink a couple of bottles of Liebfraumilch and fall on the low notes of the Bosendorf, does the LF make you go funny?
 
But I'd just been talking to bcarso about the AP output transformer positive-feedback complexity.
And why don't you discuss this here? Want to learn as well! :sad:

Thanks, Ted, for joining the discussion!
I must admit that I have never found a reason for anything like that;
The LL6404 is extremely small; I suspect that without the positive feedback, distortion wouldn't be that low.

So you are using your transformers basically as shown here?
[removed]

With different active electronics and balanced input impedances (for a line in), of course.

Samuel
 
The LL6404 is extremely small; I suspect that without the positive feedback, distortion wouldn't be that low.

Hi Samuel,
The size of the transformer is really immaterial.... if there's no flux to start with, you can have an infinitely small core! :grin:

Yes that circuit is similar to ones I use, but again, there's really no need for C1/R1 phase correction if the windings are small enough (low DC resistance and not too many turns), and with good low noise silicon junction ICs (not J-Fet!) then there is not really any need to go to the extravagence of the transistors.
Things to remember in this circuit; using phase correcting capacitance (C1) increases the system gain at HF, so noise can be a problem.
As the system works entirely on current, the ground paths around the transformer are critical.

That circuit looks very specialised.... is it some sort of bio-medical preamp? :idea:
 
Ah yes!! that makes sense now! the 'R in' is theoretical.
Yes, they would need the transistors to share the gain to keep the noise of the 301 down! :?

It's interesting isn't it.... they decided to run the buses unbalanced; they could have lost 3dB of noise and eliminated all those nasty ground-path problems by running balanced.... Ah well.... :cry:
 
Thanks Samuel,

I have a great respect for St*der; I 'cut my teeth' on the B62 and worked on numerous machines in the early days of UK Independent Radio.

We learn so many things on these forums!... I had no idea that St*der used transformer coupled balanced buses. :idea:
 

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