Cross-Coupled Opamps

[FredB]

Does anybody know where I can find a paper detailing how to setup cross-coupled opamps such as these?

cross-coupled.jpg

I'd like to change the gain and input impedance of this circuit.

[Steve Conner]

What is it, a floating balanced line driver? If so, I've seen plenty papers online explaining the design of these circuits. There are a few different designs, and you can even buy them on a chip. The DRV134/5 from TI are popular.

A google search for "floating balanced line driver" turns all of that up. Including an analysis of the circuit you gave, a little way down this page: Balanced Line Driver with Floating Output

[FredB]

As far as I've found, it's called a "Cross-coupled Differential Line Driver" with a single ended input. I did a good bit of googling before posting, The floating balanced is a slightly different beast from the cross coupled variety of line drivers.

I have yet to find any papers covering the type in the example I posted, which is from an amp made about ten years ago. I presume before the DRV135 and similar chips were designed. Also those chips, the all in one jobs, cost a whole lot more than one dual NE5532 or TL072.

Mostly the issue is that I am having trouble getting the DRV135 spice model to work in with single rail power supply or at all really, it just starts to work then freezes up. And I had trouble finding a spice model for a ssm2142 or a THAT1646.

The cross coupled is distinguished by the feed back of both outputs being directed back to the inputs of each opamps. The floating balanced versions just feeds back the output of one opamp to the input of the other opamp.

The cross-coupled is touted as a more robust design that acts more like a transformer and can handle a shorted output. I take it that this is a out of fashion topology from back when they were building these things from discrete opamps.

In any case I got the discrete opamp example I showed to work with a single rail power supply. Who knows maybe that's a moot point because I can easily generate needed split supply from a heater winding and a voltage doubler or tripler.

***

That article is helpful, thanks.

[Steve Conner]

Like Bob Pease said in one of his articles on Spice: "If the simulation worked, but the real circuit didn't, you wouldn't be calling to complain about the simulation, would you?"

I can see these things being troublesome in simulation. The simulator would probably struggle to find the DC conditions of the floating outputs, especially if you added DC block capacitors to them. The nature of the circuit is that it amplifies its own DC offsets, and the closer to perfectly floating you get it, the closer to infinity that amplification gets. I seem to remember that Douglas Self has some more analysis of the circuit in "Self On Audio", if you want to dig deeper.

The ready-made chips are expensive, partly because they have laser-trimmed resistors for nice CMR and trim-free operation. For a fair comparison, you should include the price of 0.1% resistors with that TL074, and the labour cost of a trimpot tweak for the DC offset, or whatever. That's the only reason TI and THAT actually manage to sell any, as far as I know.

The various DRV/THAT type chips should work off a single supply, if you connect the ground terminal to Vcc/2, and also refer the input signal to Vcc/2 somehow. A DC block capacitor may not work here, because the datasheet specifies a low impedance drive for the input, at least for the DRV13x.

[FredB]

Ha! Self on Audio does mention this circuit. He calls it a Quasi-Floating Balanced Line Driver! (Pg 102)

That style has a intricate mix of both positive and negative feedback with a loading resistor on each output to ground. That's it in a nutshell. Although he does avoid going into further detail about the circuit.

Well, that's a few more tidbits.

Thanks,

Fred

[FredB]

Hey Steve,

I found what I was looking for. http://www.hpl.hp.com/hpjournal/pdfs...Fs/1980-08.pdf The relevant information is on pages 12 and 13.

Fred

[Steve Conner]

Hey! I knew I'd seen it somewhere. Thanks for the link.

I must have read that HP Journal when I was looking to buy a used THD analyzer, but I ended up buying a govt. surplus Tektronix one instead of a HP 8903.

[FredB]

The circuit in my example is based on the origingal circuit described by George Pontif in the HP Journel article.

In contrast the article written by Uwe Beis on the ESP site is based on a circuirt described by Tom Hay in thirty year old AES article, "Technology in Recording Consoles and the Impact of Transformerless
Circuitry on Grounding Technique." The Hay circuit is a simplified version of the Pontif circuit, and is the one most of the all in one chips are based on.

Do you know anybody with a AES membership that could download that article?