Line input transformer for unbalanced to balanced conversion

[gootee]

Hi okapi,

For unbalanced to balanced conversion, I also like the circuit shown in Figure 6-54, in Walt Jung's "Op Amp Applications Handbook" (as long as it is driving balanced high impedances).

Chapter Six can be downloaded by right-clicking on the URL below and selecting "Save Target As".

Chapter 6 (3289 kB):

http://www.analog.com/library/analog...h6_final_I.pdf

Figure 6-54 is on page 67 of the PDF file, and is labeled as Page 6.65.

As he says in the accompanying text, if you use an eight-resistor 20k film-resistor array, the circuit can be built with only eight components. And note that the OP275 dual opamp can run on up to +/-22V.

The circuit is a simple "Inverter Follower" differential line driver. But it has some very nice performance features, especially considering its low cost and low complexity.

You should probably also look at the circuit in Figure 6-56, on page 69 of the PDF file. You could either build something similar with discrete parts (with tightly-matched resistors), or just use the Analog Devices SSM2142 (or something similar).

[KSTR]

Returning to Okapi's original question, there is also a simple and cheap (and good) all-in-one solution.

One can combine the balancing with the bridging, using the copper-compensated zero-field topology (simplifed schematic is attached). This allows for small and affordable xfomers, like the small Neutrik or Lundahl types, but even a $5 model might easily suffice for a chip-amp performance level. Only thing which would be not on top level is CMRR, but this can be significantly compensated for with an arrangement of two equal xformers in anti-parallel/anti-series.

I've done this circuit with small-signal op-amps with success, it should work equally well with any power op-amp, given proper compensation for stability if required (e.g., a series-RC accross the xformer might be needed to get a stable HF gain for a decompensated op-amp). The op-amps run in inverting mode with only little common-mode voltage on their inputs which helps linearity. Also impedances can be very low. As in every true bridge, GND is only a reference voltage (Vs/2) and does not carry load current, therefore a single supply can be used and the ref. voltage can come from a simple divider. The divider can by bypassed to the supply end with lower supply rejection from the chips, which is the neg rail for LM3886 IIRC. The series cap after the xfomer is neccesary to keep DC away from the xformer and it gives unity gain for DC which keeps output offset low.

But if you want to go for a single IC symmetrizer, consider the THAT 1600 series. Those will need a buffer in front of them, as will the SSM counterpart. However, the simplest/cheapest circuit would still be a buffer, followed by an inverter.

Note: Forget about the high CMRR of the IL300... not true (saw the CMRR vs. freq. graph too late...)

- Klaus

[okapi]

thanks to everyone for their replies and sorry i let this thread lie dormant for so long. the first week back was a busy one.

i am still integrating a couple of the posts 10 21 22

but it is obvious my question needs some clarification at this point.

immediately i have some assembled lm4780 chip amps running in parallel that i would like to create a balanced signal for so that i can bridge them. i am interested in seeing what advantages more power will bring to my system (if i can reduce CMRR at the same time, great). depending on how this works, i may then build a bridge parallel chip amp using LM3875 chips (6 chips/chan, two groups of three running in parallel).

i may now alter this plan and begin experimenting with some of the different methods proposed in this thread for balancing a signal and use only two chips. i expect this should help me with the posts i don't fully appreciate and lead to more questions.

thanks again.

[okapi]

positives and negatives for creating balanced signal from transformers and SS devices

transformer
+ high CMRR
+ simplicity, no other components
+ increases the input impedance of the amp??
- cost, this can be offset by using a cheaper transformer in a zero field topology but this requires the amps be used in inverted mode??

Op Amp or SS solution
+ low cost
+ good CMRR performance although maybe not quite as good as the best transformers??
- the devices tend to have a low input impedance and require a buffer (ex THAT 1600 series)??
- complexity, devices require a power supply and OP Amp designs require more parts (not necessarily the case for SS solutions like the DRV134 that needs only three parts including itself).

are there performance measures in which the SS solutions outperform transformers?

corrections and comments are welcome especially on the double ?? sections.

related question. what is the best position for the balancing device? near the amp or the source?