SSM2142 driving transformer ?

[mobyd]

I have often used SSM2142 and similar devices for balanced long line drivers without any problems. Are there any issues I should be aware of using these to drive 600/600 transformers (Edcor PCW600/600 for example) in situations where galvanic isolation is required. The SSM2142 spec sheet warns against any passives between chip and load but no specific mention is made of transformers.
Cheers
M

 

[abbey road d enfer]

The main problem is the 2142 has a fixed output impedance of 50 ohms, which is not conducive to good low-frequency THD. Most transformers like to be driven with a very low impedance (in fact their best performance is achieved when driven with negative impedance).

 

[emrr]

50 ohms sounds rather fine for many things.  Depends on the transformer you use, and the effect you desire.  Some transformers will ring if driven with too low of an impedance.  You find use of build-out resistances between opamp and transformer in many cases.  As always, you'll have to try it and see. 

 

[abbey road d enfer]

If a transformer rings when driven with very low Z, the proper way to compensate is a Zobel network at the secondary, because increasing the source impedance will inevitably impair LF THD.
Increasing the source impedance for getting rid of a ring is a quick and dirty solution that actually reduces the stimulus; a Zobel combined with low source Z allows using the transformer at optimum performance.

 

[emrr]

see 'Audio Cyclopedia', transformers chapter. 

To the OP, the question of passives between chip and load; how inclusive is that?  Wire, and it's various effects (capacitance)? 

abbey road d enfer seems to usually prefer a clean-and-modern-approach, I live with a studio full of glorious sounding 1940's and 1950's broadcast equipment driving transformers with semi-matched loads, or at most, bridging source Z's (50 to 600 counts as bridging).  Again, try the circuit and fiddle about yerself, to see what suits.  Are you tracking clinical classical or acoustic music, or making punk rock records?  Or doing simple broadcast work in a large physical plant with fixed parameters?