Zero Field Transformer With DC Servo

[burdij]

Just a guess but, it appears that the Lundahl circuit is attempting to drive the negative terminal of the transformer through the negative feedback path. Ordinarily, the positive input feedback path would attempt to keep the negative terminal of the opamp near zero volts (+- the DC offset). In this case, the negative terminal of the transformer and negative terminal of the opamp appear to be acting in a "current mode" relationship so that when the input voltage rises, the opamp attempts to move the negative terminal of the transformer toward the positive voltage direction. This would result in a net instantaneous voltage across the secondary of the transformer of zero volts, nulling the field within the transformer secondary. From EE class, I vaguely recall that the inductor represents a high impedance to the changing voltage (as opposed to a capacitor, that represents a low impedance to changes in voltage).

The capacitor C does two functions, apparently. The first is to prevent DC from flowing in the secondary. The second is that it serves as a high pass filter with respect to R (some kind of impedance mirror across the opamp is allowing them to interact, perhaps someone can elucidate). A one pole filter of this type has a rolloff of 6db per octave, I think. The reason why the value of R needs to match the DC resistance of the transformer winding is probably because the two resistances form two legs of a wheatstone bridge. When all currents and voltages are in balance, the lowest THD (center of the hysterisis curve) results. The other circuit that was referenced in a later post does not appear to have a similar function to this circuit, in that there are no impedance elements in positive feedback path.

 

[TedF]

I think you are spot-on burdij; but isn't it simpler to just think of it as a simple 'virtual earth' amplifier, ignoring the transformer completely as the impedance of the windings is small compared to the series resistance in the primary circuit?
The C acts as as a high-pass filter, yes, but it can also react (!) with the inductance of the secondary producing an unwanted response peak. :sad:

I'm sure that the 'bridge' explanation for the positive feedback components is the correct one, but as I said before, I don't find the necessity for such extra bits with the values and devices I use.... but I will certainly have a look at this.

A further interesting point is that by using ratios other than 1:1, it's possible to achieve some interesting gain/impedance matching; remembering, of course, that a 'step down' transformer is a 'step up' in current! :grin:

 

[bcarso]

I believe they really are generating a negative Z termination to cancel the winding resistance, despite how small it is.

The banter with SSLtech alludes to an Ap patent that does temperature compensation of their output transformer negative impedance drive with a noninductive winding in the trafo, essentially a copper resistor which by construction is in intimate contact with the rest of the device. That, along with using the same batch of Cu, ensures virtually perfect tracking.

UREI used to license this and used it in some products before and at least for a while after their acquisition by the evil empire :razz:

 

[Samuel Groner]

I wonder why AP has an US-patent only on this one; seems not to be too important for them.

Samuel

 

[TedF]

Temperature compensated output transformer?
I expect the sound is improved about the same as with gold plated RCA plugs. :grin:

 

[bcarso]

[quote author="TedF"]Temperature compensated output transformer?
I expect the sound is improved about the same as with gold plated RCA plugs. :grin:[/quote]

No. It is anything but audiophoolery. Actually, it has significant utility. It allows Ap generators to have very low output Z and low distortion while preserving galvanic isolation.

Believe me, if it wasn't (or weren't, if they are still doing it) useful, UREI wouldn't have bothered to use it and license it.

Without the ability to compensate for the copper resistance in some way, one is sorely limited in the amount of negative resistance drive that can be safely applied without risk of instability.

 

[TedF]

I hear what you are saying, and appreciate the technology, but at the same time I question the need to approach zero source impedance; it still smacks of oxygen-free loudspeaker cables and gold plated mains plugs.
(I know.... I'm just a bit reactionary and steeped in dusty old radio studios.)
Neve used to use a simple tertiary feedback arrangement on line output transformers; it was effective in reducing the requirement for transformer mass. They could use a core with less mu-metal and more soft iron, and control the LF distortion with feedback.

Galvanic isolation? absolutely yes.... there's nothing quite like it; if in doubt, use a transformer. :grin:

I may be reactionary, but I would be delighted to learn more about why Urei felt that they needed such sophistication in an audio output stage;
can you elaborate?

 

[bcarso]

[quote author="TedF"]I hear what you are saying, and appreciate the technology, but at the same time I question the need to approach zero source impedance; it still smacks of oxygen-free loudspeaker cables and gold plated mains plugs.
(I know.... I'm just a bit reactionary and steeped in dusty old radio studios.)
Neve used to use a simple tertiary feedback arrangement on line output transformers; it was effective in reducing the requirement for transformer mass. They could use a core with less mu-metal and more soft iron, and control the LF distortion with feedback.

Galvanic isolation? absolutely yes.... there's nothing quite like it; if in doubt, use a transformer. :grin:

I may be reactionary, but I would be delighted to learn more about why Urei felt that they needed such sophistication in an audio output stage;
can you elaborate?[/quote]

I'm likely to see Brad Plunkett for lunch sometime soon and I will try to remember to ask him. I believe the primary (no pun intended) appeal was to facilitate lower distortion and gain flatness at low frequencies---the lower output Z was secondary.

 

[cuelist]

[quote author="Samuel Groner"]I wonder why AP has an US-patent only on this one; seems not to be too important for them.[/quote]

Simple - 40% of the world market is in the US. Yes, someone could copy the circuit but with 40% of the market blocked, it would not make good business sense.

Filing for a patent elsewhere is always an option. By having a patent or even an application, you have established a "priority date" that is of great use for future filing in other countries.

Unfortunately, while there is "EU patent" of sorts, you still need to pay for it in each country. That makes it very expensive to get adequate cover in the key countries.

Hence, most people (including me) file in US only. It is enough of a deterrent for competitors.

 

[SSLtech]

[quote author="Samuel Groner"]

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: [/quote]
Well, the discussion reached a dead-end when we realised that it wasn't the answer to what we were (more accurately what I was) trying to figure out: specifically the output transformer DC feed in the LA-3 output circuit.

So while I learned of the existence of the AP patent from Brad (Hence the déja-vu!) I learned little more other than the +ve feedback was temp-compensated for good reason. I moved on to looking more at the UREI stage.

Brad, if the other Brad wants to illuminate me/us, I'd be delighted to know more!

Keef

 

[Samuel Groner]

Simple - 40% of the world market is in the US. Yes, someone could copy the circuit but with 40% of the market blocked, it would not make good business sense.

The point is that I can make a product using the patented circuitry, sell it 60% without any trouble and 40% by paying AP a fee. I guess AP would prefer to make me pay 100%.

Samuel

 

[bcarso]

[quote author="Samuel Groner"]

Simple - 40% of the world market is in the US. Yes, someone could copy the circuit but with 40% of the market blocked, it would not make good business sense.

The point is that I can make a product using the patented circuitry, sell it 60% without any trouble and 40% by paying AP a fee. I guess AP would prefer to make me pay 100%.

Samuel[/quote]

Ap might well stipulate that they be paid a fee for every unit made, regardless of where sold.

 

[cuelist]

[quote author="bcarso"]Ap might well stipulate that they be paid a fee for every unit made, regardless of where sold.[/quote]

Indeed, that's usually the case. I have just returned from US after a week of patent licensing negotiations and this point came up, as usual.

 

[Samuel Groner]

Sure, in the end it all depends if you feel the need to really sign such a contract or rather develop something else.

Anyway the AP patent expires next year, so let's start breadboarding! :razz:

Samuel

 

[bcarso]

[quote author="Samuel Groner"]Sure, in the end it all depends if you feel the need to really sign such a contract or rather develop something else.

Anyway the AP patent expires next year, so let's start breadboarding! :razz:

Samuel[/quote]

I figured it was nearly if not already public domain.

 

[Samuel Groner]

It is dated 1986, so plus 20 would make next year.

Samuel

 

[bcarso]

[quote author="Samuel Groner"]It is dated 1986, so plus 20 would make next year.

Samuel[/quote]

Is that the filing date or the issue date?

 

[Samuel Groner]

Just asked my father (who is a patent agent), and he told me that it had expired in 2003, as older US-patents run for 17 years only (and not for 20 years as in Europe).

Samuel

 

[bcarso]

It's now 20 years from the filing date, according to my Nolo Press book, which is why I asked.

The 17 year "enforceable monopoly" period starts after ther patent is issued, although this can be extended in some cases for various reasons.

 

[Samuel Groner]

Yep, they changed that.

Samuel