Mixed feedback drive circuits for audio output transformers

abbey road d enfer said:

Not much has changed in the last 50 years, except the occasional use of amorphous lams. Modern xfmrs do not behave very differently than vintage ones. Electronics have changed a lot though; in particular 600 ohm termination, that was a cause of stress on the output stages, is unnecessary.
Acceptability of distortion is a personal matter; someone's clean is another's dirty.

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Didnt know transformers where perfected so many years ago, still the new transformers sound really clean to me, im still trying to figure out what people mean with trafo sound?

user 37518 said:

Didnt know transformers where perfected so many years ago, still the new transformers sound really clean to me, im still trying to figure out what people mean with trafo sound?

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Transformers were very useful when electronic gain stages were expensive and noisy, and they are useful for interfaces, but modern solid state is generally superior (IMO).

Transformers have difficulty with high level low frequency signals.  One of my utility patents is related to mitigating this transformer limitation

JR said:

US05509080 Roberts

04/16/1996 Bass clipping circuit. This circuit combines a simple clamp diode with a Baxandall tone control circuit to provide frequency selective (bass only) clipping.

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In connection with designing a line of fixed install mixer amps, where the constant voltage (70V-100V) outputs were fed through transformers, with additional step down transformers for each speaker, saturating those transformers with too much bass, was a real Amplifier killing problem. (Saturated output transformers can load amplifiers unnaturally).

Since those same customers love to boost the heck out of their bass signal, on top of standard loudness contours, so managing bass levels was a real problem in that market.

My invention was a slick diode clipper built into a baxandall bass tone control circuit so just the bass boost leg gets clipped when too hot. Since the HF signal is unmolested, this clipping is not as audible as you might expect.

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Modern transformers using good media (Deane Jensen was big on making quality transformers with good metallurgy) with adequate iron and copper to avoid saturation in normal use, should sound neutral.  Push them too hard at low frequency and they might reveal any weakness there.

JR

user 37518 said:

Didnt know transformers where perfected so many years ago, still the new transformers sound really clean to me, im still trying to figure out what people mean with trafo sound?

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Three things: elimination of VLF noise because the LF response of transformers is limited, smoothing of HF because bandwidth is limited, and distortion due to core saturation that some think is euphonic.
These effects are present in variable doses, depending on construction. The best xfmrs have very little of each (=very transparent), cheaper ones may have too much of one or several.

1831 transformer shown below,

looks like Faraday copied the WE 111C, 

probably more than one way to distort a transformer, impedance mismatch, saturation,

some distortions probably sound better than others, don't know which, have not done those types of experiments,

Neve consoles sure sound good, they use nickel core input transformers with small cores,  is that the reason? would have to wire up some Lundahl's into a Neve and see what happens, don't have the coin for that experiment, probably been done somewhere,

Douglas Self explains quite well the concept of mixed feedback in his book, basically it creates a negative resistance which counteracts the primary resistance and thus lowers distortion, quite an ingenious scheme. However I have only seen this when opamps work in inverting mode, and not in non-inverting.

user 37518 said:

Douglas Self explains quite well the concept of mixed feedback in his book, basically it creates a negative resistance which counteracts the primary resistance and thus lowers distortion, quite an ingenious scheme. However I have only seen this when opamps work in inverting mode, and not in non-inverting.

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Check this
https://groupdiy.com/index.php?topic=49634.msg762968#msg762968
Output stage in the north-west corner.

abbey road d enfer said:

Check this
https://groupdiy.com/index.php?topic=49634.msg762968#msg762968
Output stage in the north-west corner.

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Thats great! thanks for the link abbey, it also uses NFB instead of PFB.

user 37518 said:

Thats great! thanks for the link abbey, it also uses NFB instead of PFB.

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There's a combination of both; NFB is necessary for opamps stability.
NFB comes from 2 paths: from the tertiary winding andat LF and MF, and from the opamp's output at HF via capacitor.
But there is PFB from the 10R resistor that senses current in the xfmr's primary.

abbey road d enfer said:

There's a combination of both; NFB is necessary for opamps stability.
NFB comes from 2 paths: from the tertiary winding andat LF and MF, and from the opamp's output at HF via capacitor.
But there is PFB from the 10R resistor that senses current in the xfmr's primary.

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Do you happen to know if its patented? or is it free for all?

user 37518 said:

Do you happen to know if its patented? or is it free for all?

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I "invented" it, but it's very likely someone used time travel and made copies 20 years earlier. I didn't patent it because the commercial value is negligible compared to the cost of patenting efficiently (i.e. patenting in 30+ countries and hiring lawiers who enforce the patent). There was a time when you had to have xfmr ins and outs if you wanted your products accepted in the broadcast market, but today a $2 chip does as good and is accepted (because the white-coat guys are not there anymore).
So yes, it's free.