Mixed feedback drive circuits for audio output transformers

[hoangduongo]

Hello,
Does Studer use the same principal Mixed feedback drive circuit in their Balanced out units? for example this one:
ftp://ftp.studer.ch/public/Products/Miscellaneous/Analog_Audio_System_Components/Manuals/Single_Components/1.915.904.pdf
If yes, what's the point (pros & cons) of the BC327/337 transistors after the opamp, and why is there a resistor inside the transformer block?
Thanks

 

[Bo Deadly]

If yes, what's the point (pros & cons) of the BC327/337 transistors after the opamp, and why is there a resistor inside the transformer block?

I don't know if it's mixed feedback but it is balanced drive. The transistors are just wired as diodes for biasing and the resistor looks like it's just a "buildout" resistor to limit current through the primary (help with load capacitance maybe).

 

[abbey road d enfer]

Hello,
Does Studer use the same principal Mixed feedback drive circuit in their Balanced out units? for example this one:
ftp://ftp.studer.ch/public/Products/Miscellaneous/Analog_Audio_System_Components/Manuals/Single_Components/1.915.904.pdf

Yes, it's a combo of voltage  positive FB (via R131, R132, R129) and current Negative FB, via a built-in current sensing element (pins 5 & 9 of the transformer) to the inverting input.

If yes, what's the point (pros & cons) of the BC327/337 transistors after the opamp,

It's an old trick; using diode-connected transistors similar to the output ones allows perfect adjustment and temperature compensation of the idle current, particularly when a low quiescent current is desired (here ca. 1.5mA). I've seen it used in portable applications where current consumption is to be minimized.

 

[user 37518]

Yes, it's a combo of voltage  positive FB (via R131, R132, R129) and current Negative FB, via a built-in current sensing element (pins 5 & 9 of the transformer) to the inverting input.

Why is positive FB used?

 

[abbey road d enfer]

Why is positive FB used?

It is used to minimize distortion due to the transformer saturating. Transformer saturation increases the current for a given voltage; if an image of this increased current is fedback in adequate proportion, distortion is minimized.

 

[user 37518]

It is used to minimize distortion due to the transformer saturating. Transformer saturation increases the current for a given voltage; if an image of this increased current is fedback in adequate proportion, distortion is minimized.

Cant the same results be achieved using plain old negative FB?, Also, IC111 after the input trafo has positive feedback, looks more like a Schmitt trigger if you ask me, whats the purpose of this?

P.S. Seems like positive FB is used more often than I thought, I thought only oscillators, schmitt triggers and multivibrators used it, except of course of bootstrapping. Most books cover negative FB configurations, where can one read about these positive FB techniques?

 

[CJ]

Oscillators use  + FB.

 

[user 37518]

Oscillators use  + FB.

Yeap, thats what I said in my post

 

[CJ]

i had an Uncle named Hartley Colpitts,  he had a degree in Advanced Ignorance.  ;D

 

[abbey road d enfer]

Cant the same results be achieved using plain old negative FB?,

Yes, see
https://groupdiy.com/index.php?topic=49634.msg762968#msg762968
upper left of page 1.
It is also possible to use a tertiary winding on the xfmr. IMO the Studer method does not have a definitive advantage over the NFB methods.

  Also, IC111 after the input trafo has positive feedback, looks more like a Schmitt trigger if you ask me, whats the purpose of this?

That's correct. PFB here actually makes the impedance of the current-to-voltage converter negativeinstead of zero. That allows compensating the DC resistance of the windings, so the input signal sees a perfectly linearized impedance.

 

[moamps]

Also, IC111 after the input trafo has positive feedback, looks more like a Schmitt trigger if you ask me, whats the purpose of this?

That's correct. PFB here actually makes the impedance of the current-to-voltage converter negative
instead of zero. That allows compensating the DC resistance of the windings, so the input signal sees a perfectly linearized impedance.

It looks to me as usual "zero field"  configuration.
http://www.lundahl.se/wp-content/uploads/datasheets/7101.pdf

 

[abbey road d enfer]

It looks to me as usual "zero field"  configuration.
http://www.lundahl.se/wp-content/uploads/datasheets/7101.pdf

Not exactly. The difference is that the actual impedance the transformers sees is negative instead of zero (or near-zero). It has the advantage of making the reflected Z at the primary almost zero (by compensating the reflected DCR of the secondary and the primary's DCR). This way, the actual magnetizing voltage is nulled, when in a standard ZF configuration it is not zero. Since the magnetizing voltage is non-linear, nulling it results in a perfectly linear (resistive) the input impedance. Input current is perfectly linear, and its image at the secondary is also perfectly linear, resulting in a perfectly linear output voltage.
Typical ZF inputs have to ensure that the xfmr does not saturate, even if the constraints are much lower. The "Studer" type of inputs allows one more order of magnitude of distortion to be compensated.
Indeed, the compensation cannot be perfect, otherwise there is a risk of oscillation, but still a notable improvement can be noted.

 

[moamps]

Not exactly. The difference is that the actual impedance the transformers sees is negative instead of zero (or near-zero)...

Thanks. But how it's achieved because I don't see any significant difference in two circuits topology?

 

[abbey road d enfer]

Thanks. But how it's achieved because I don't see any significant difference in two circuits topology?

There is positive FB from the output to the non-inverting input, so it forces the inverting input to move in opposition with the incoming signal.

 

[moamps]

There is positive FB from the output to the non-inverting input, so it forces the inverting input to move in opposition with the incoming signal.

Identical as in the schematic for lundahl's 7101 transformer application I posted.

 

[abbey road d enfer]

Identical as in the schematic for lundahl's 7101 transformer application I posted.

Agreed, but you wrote: "It looks to me as usual "zero field"  configuration."
I must say I didn't look at the Lundahl app, but it is not a "usual "zero field"  configuration". The standard implementation of ZF is just an I/V converter.

 

[JohnRoberts]

Sadly nowadays people use transformers because they want the distortion, while in the past, design engineers did the right thing, working to reduce transformer distortion to vanishingly small amounts. 

"Iron"-ic    8)

JR

 

[CJ]

What, you like the Aja album?

 

[user 37518]

Sadly nowadays people use transformers because they want the distortion, while in the past, design engineers did the right thing, working to reduce transformer distortion to vanishingly small amounts. 

"Iron"-ic    8)

JR

Perhaps you and CJ can shed some light into this, but I dont know how much distortion can you get from a high end Jensen or Lundahl transformer in normal operating conditions, I used to think transformers distorted the signal, which may be true for some ancient transformers but the modern trafos are clean, at least to my ears.

 

[abbey road d enfer]

I used to think transformers distorted the signal, which may be true for some ancient transformers but the modern trafos are clean, at least to my ears.

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.