Drive an audio transformer from an AOP. Good or bad idea?

[mudevilo]

Of course, there is nothing in the academic definition of an opamp that prevents it, or make it particularly suitable to a specific task.
Driving a xfmr implies a few constraints, such as current capability commensurate with the xfmr characteristics and output expectations. Low offset is to be considered, since it can result in doing without teh dreaded output capacitor.

Is it a good idea to do without the last output capacitor before transforming?
Couldn't even the slightest current in the transformer generate noise?

 

[Newmarket]

Is it a good idea to do without the last output capacitor before transforming?
Couldn't even the slightest current in the transformer generate noise?

I don't think noise is the issue with dc / transformer ? Tell me if I'm wrong here - not particularly a analogue audio (yes - that's the correct spelling ) tx bod although have used a few in DIY interface boxes and that has worked out well

 

[abbey road d enfer]

Is it a good idea to do without the last output capacitor before transforming?
Couldn't even the slightest current in the transformer generate noise?

DC current into xfmr does not result in noise, but rather in possible distortion.
My output stages combined absence of coupling capacitor with a global NFB from an auxiliary winding. As a result whatever distortion resulting from the very low DC current was cancelled by global NFB.

 

[Cqwet Dbdfte]

Transformers offer:
+Galvanic Isolation,
+Noise free gain,
+Bandwidth limiting.

- Costly,
- Size,
- Weight,
- Magnetic field susceptibility (double shield, orientation, and location reduces this).
Cost is the major obstacle, otherwise all good.

 

[abbey road d enfer]

Transformers offer:
+Galvanic Isolation,
+Noise free gain,
+Bandwidth limiting.

- Costly,
- Size,
- Weight,
- Magnetic field susceptibility (double shield, orientation, and location reduces this).
Cost is the major obstacle, otherwise all good.

You forgot distortion.

 

[vintageampfixer]

Interesting thread.

Transformers don't add anything because they provide step up or down of voltage with equivalent step down or up of current. Ignoring losses for simplicity, the voltage multiplied by the current on the primary winding must be the same as that on the secondary. In reality, transformers only add losses.

Thus how you drive a transformer rather depends upon the load connected to the "output" or secondary of the transformer. It's perfectly possible to drive a small audio transformer with just an op amp, although you may find the finished circuit's capability to drive a complex load - for example a long piece of cable - isn't what you'd hoped for. In the output amplifiers in Neve desks (and indeed the same was true in Audix desks) Rupert Neve made a huge play of "driving the transformers with small power amplifiers"; at least, I think that was his wording in a somewhat lengthy interview. The reason for doing that was to preserve the desk's sound quality as much as they possibly could when the mixer moved from the lab into a real studio or outside broadcast setting.

Audix desks used something like an NE5534 driving a complementary pair of medium power transistors which at the time might have been BC301/BC303. I don't recall what components Neve used although that isn't really the point.

What I'm saying is that the load presented to the transformer secondary (the long piece of cable, the cascaded inputs, the mismatched impedance, etc.) is effectively passed back through its primary to whatever is driving the transformer. This makes the circuit driving the transformer critical and perhaps the article originally mentioned missed out the wordy explanations and went straight to the punchline!

 

[john12ax7]

You forgot distortion.

Does that go under pro or con?

 

[Khron]

Does that go under pro or con?

Yes

 

[abbey road d enfer]

Does that go under pro or con?

It does.

 

[moamps]

Transformers offer:
+Galvanic Isolation,
+Noise free gain,
+Bandwidth limiting.

- Costly,
- Size,
- Weight,
- Magnetic field susceptibility (double shield, orientation, and location reduces this).
Cost is the major obstacle, otherwise all good.

+ perfect balancing and unbalancing of the signal (if the transformer is well designed)

 

[Newmarket]

Transformers don't add anything because they provide step up or down of voltage with equivalent step down or up of current. Ignoring losses for simplicity, the voltage multiplied by the current on the primary winding must be the same as that on the secondary. In reality, transformers only add losses.

Not really. Whilst the "noise free gain" is in some ways not completely true since there a corresponding increase in source impedance (approx square of turns/gain ratio) and the effect of that depends on the following stage - hence optimised transformers for a given case.
But in most cases - ie where power transfer is not the objective - the trading of current for voltage is not an issue. Voltage transfer is the point. So yes - transformers lose power - since no physical implementation is 100% efficient - but in most instances that is not important or relevant.

 

[ricardo]

Transformers don't add anything because they provide step up or down of voltage with equivalent step down or up of current. Ignoring losses for simplicity, the voltage multiplied by the current on the primary winding must be the same as that on the secondary. In reality, transformers only add losses.

You forgot LF Distortion. To combat this, Calrec/Lundahl had special circuits and in this Millenium, AP have one too.

I've designed mini power amps to drive line level transformers at Calrec but for test gear. In my time, the broadcast mixing desks used the circuit in the Lundahl application note.

I've never figured out if Ken Farrar dreamt it up before Lundahl or was it the other way round.

 

[abbey road d enfer]

You forgot LF Distortion. To combat this, Calrec/Lundahl had special circuits and in this Millenium, AP have one too.

The problem with this circuit is that the current sense resistor must follow the primary resistance variations with temperature to provide optimum distortion compensation.
In the AP output, it is done by including in the transformer a special uncoupled winding made of the same material as the primary.
My choice of a tertiary winding makes this unnecessary, and the circuit is not more complex.
Bill Whitlock (member CMRR) may chime on it, as he was stakeholder in the AP/Jensen collab.

 

[Matt Syson]

The 'negative impedance' approach to transformer distortion reduction was the subject of a German patent many years ago and indeed Studer and others used the principles. Before or after Ken Farrar of Calrec (RIP Ken) may never be known. Indeed Calder Recordings may never have happened if the inspiration of capacitor Microphones had not been an article in Wireless World.
Audix used a 40371 /40372 pair of transistors on some of the line outputs as well as the BC301/303.
Ken Farrar did quite a bit of lateral thinking and came up with a tuneable Wein Bridge oscillator that only needed a single gang pot for tuning, On top of his work on Ambisonics and the Soundfield microphones.

 

[abbey road d enfer]

The 'negative impedance' approach to transformer distortion reduction was the subject of a German patent many years ago and indeed Studer and others used the principles.

It is intyeresting to note that Studer chose a slightly different approach in their later products. They combined near-zero impedance drive with global NFB from a tertiary. Probably because they understood the limits of zero-Z drive alone.

 

[JF]

for example a 1:2 transformer (step up 6db) makes a 600 Ohm LOAD look like 50 Ohms so a small 'power amplifier' is necessary.

How so? The load goes with the square of the ratio - 600R load with 1:2 would be 150R.
https://en.wikipedia.org/wiki/Transformer

But let's be realistic - we don't have 600R loads out there. This is loooong gone, modern line inputs are in the 10k area.
Mic inputs can be down to 1k, standard in the 2k area but more and more I see 3k and higher in datasheets. (RME 3,4k, Midas 10k!) This makes sense to enable condenser mics for high output without distortion.
Microphones don't use step up transformers and you only have about 10mA over the phantom power line anyways. (OPA1692 could be used and is able to send 50mA - way more as you have available)

Many modern OPA can output 50mA (OPA1612) - 18V peak in 600R would need 30mA.
Just take care you have no DC at the output and you should be fine to drive a transformer.

 

[Cqwet Dbdfte]

Distortion is not necessary bad, sometimes induced on purpose, called "tone". Jensen lists distortion numbers for their pieces.
Everything has losses, and noise, except maybe nuclear materials that can put out power for a very long time.
A nice transformer is made by SRS. A bit expensive.
https://www.thinksrs.com/products/sr554.html
Just add a RIAA filter; -)

 

[abbey road d enfer]

Microphones don't use step up transformers

Just to show I can be a little pedantic, many dynamic mics have step-up xfmrs.

Many modern OPA can output 50mA (OPA1612) - 18V peak in 600R would need 30mA.

10mA from phantom P48 would reduce it to about 6Vpk or 4Vrms. Still plenty.

Just take care you have no DC at the output and you should be fine to drive a transformer.

It is some kind of challenge in the context of a single rail supply. The simplest solution is a coupling cap, that many frown upon.
Alternatively, one could use an active V+/2, at the cost of increased current demand.

 

[abbey road d enfer]

Jensen lists distortion numbers for their pieces.

They are the best case figures, with zero-ohm drive. In practice, it may be difficult to achieve the same results. Now if one wants "tone" or "colour", it's easy to add a resistor in series.

 

[Cqwet Dbdfte]

They are the best case figures, with zero-ohm drive. In practice, it may be difficult to achieve the same results. Now if one wants "tone" or "colour", it's easy to add a resistor in series.

"Zero" is more of a abstraction than a reality. A strong OP amp with negative feedback can make a drive impedance very low, but adding some series resistance protects against bad loads.
Obviously massive feedback has its own issues, like suppression of even order harmonics.
Another plus for a transformer is it can be used as a phase splitter, with the correct winding config.