THAT1646 Current Booster and Transformer Driver

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mediatechnology
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THAT1646 Current Booster and Transformer Driver

Post by mediatechnology » Sat Jun 28, 2008 3:53 pm

I've discovered an interesting connection for the THAT1646 that lends itself to applications that it was not originally intended for. What I've found is a way to make the outputs swing together rather than in anti-polarity.

I contacted THAT with a question as to how one would float the outputs up to some common mode DC potential. An example where this would be useful is an A/D driver where the inputs typical bias to 2.5V. (This particular application may be the subject of another thread someday.) Gary Hebert explained how this might be done using the OutSmarts(r) common mode inputs.

The outputs of the 1646 normally swing in opposite directions to provide differential line driving capability. It was explained to me that by coupling the sense lines together (pins 2 and 7) a common mode DC reference can be used to bias each output up to 2.5V. Drive at pin 4, the "differential" input, called differential because it produces a differential output, still causes the outputs to operate in anti-polarity. Drive at pin 4 causes one output to "yen" while the other one "yangs."

There are some applications where it's desirable to use the 1646's high-current outputs to drive a single-ended load. As normally applied the 1646's outputs cannot be made to swing together. Or can they? Then it hit me: Why not drive pins 2 and 7 with signal, with the "differential" input grounded, to make them swing together in tandem? We'll call this "common mode" output drive, or Vcm drive. Once you're able to do this the outputs can be current-boosted by a parallel connection. They both "yen" and "yang" together.

Here's the basic tested circuit:

Image

It can drive 20 volts peak-to-peak into 100 ohms. That's 200 mA p-p! The 1646 barely breaks a sweat.

Rather than measure the distortion and performance of this stage I decided to drive a difficult transformer load and measure that. Roger Foote and I have played around with driving transformers differentially and they work great.

Here's a differential drive circuit Roger's used in the stereo Pico Compressor:

Image

I decided to try single-ended drive with the new current-boosted topology and to make it really difficult on the 1646 using a 150 ohm primary. Here are the results:

Image

The circuit sounds really good with the Edcor WSM600:600.

Now you'll notice that the opportunity to drive the transformer in push-pull exists. That circuit will use two 1646s in a current-boosted differential configuration. I should be able to get approx +30 dBm from +/- 15V supplies. I hope to have that one going in a little bit.

I want to thank Gary Hebert at THAT for his help in applying the 1646 in a way it was never intended. It's a very versatile part.
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mediatechnology
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Re: THAT1646 Current Booster and Transformer Driver

Post by mediatechnology » Sat Jun 28, 2008 4:00 pm

Part two:

Checked the distortion with RMAA. The generator residual is typically -105 to -100 second and third; I'm using the analog generator and analyzing the result with RMAA. The drive level was adjusted to correct changes in output level with transformer impedance ratio and normalized to +4 dBm. Being lazy I didn't want to have to pad the soundcard input so these aren't ultra high-level measurements.

The image below is a single-ended Vcm drive current boosted with a 600 ohm primary connection:

Image

The image below is a single-ended Vcm drive current boosted with a 150 ohm primary connection. Note the rise in third which was confirmed to be the transformer.

Image

The next two are with dual differential current boosted drive using two THAT 1646 and the "normal" differential input. The first one is with a 600 ohm primary connection:

Image

This one is differential drive with a 150 ohm primary connection. Using +/- 18V supplies this one will drive a secondary loaded in 600 ohms to +27 dBm. Unloaded we can almost squeeze +30 dBv.

Image

What I can conclude is that most of the distortion I'm measuring is the Edcor WSM600:600, which is pretty good, and the generator. In all cases using a 150 ohm primary connection raised the third harmonic by about 10 dB. This rise was not measured in the primary circuit, it is transformer-generated. At 1 KHz we're not seeing a performance penalty with the Vcm-driven single THAT1646. Now with two we pickup +3dB of power for a 6 dB increase in cost. Our slew rate goes from 5-6V/uS to 15 typically.

BTW: The SE current-boosted 150 ohm primary configuration is what I've really listened to and it sounds really nice. I think that third may add some sparkle.
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Re: THAT1646 Current Booster and Transformer Driver

Post by Jeffin90620 » Sun Oct 04, 2009 11:45 am

I notice that your schematic doesn't show any Common Mode Offset Reduction in the output.

Did you omit it for clarity, or just ignore it? Or does this off-book use eliminate it?

Regards,

Jeff

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Re: THAT1646 Current Booster and Transformer Driver

Post by Jeffin90620 » Sun Oct 04, 2009 12:09 pm

Hhmmm... I am concerned about saturation of the transformer's core. In one application I have (a 150:150 Ohm model from Pico), the DC resistance is only 12 ohms, so it wouldn't take much of an offset differential to drive a considerable DC current through the primary.

Still and all, the transformer is rated to 1.5 watts and I am only outputting 30 mW, so I suppose there wouldn't be a whole lot of distortion added.

Jeff

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Re: THAT1646 Current Booster and Transformer Driver

Post by mediatechnology » Sun Oct 04, 2009 12:52 pm

Hi Jeff -

Thanks for joining us!

Image

The 1646 in the current-boosted application shown above using "common mode drive" (sense inputs driven - both outputs tied together) does not have OutSmarts enabled. The outputs do not have the high common mode gain that they do with OutSmarts on. In this connection, the typical output offset is about 4 mV with +/-15mV the maximum. Assuming the worst-case 15 mV and a 12 Ohm primary the offset current will be about +/-1.3 mA.

If differential drive is used in the figure below, the common mode offset, by virtue of it being common mode, drives both ends of the transformer primary equally. (Outsmarts is also off because pins 2 and 7 are open.) No net current will flow from CM offset. In this configuration, differential offset is a concern. With typical differential offsets also being 4 mV typical, +/-15mV max, the primary offset current will also be low - about +/-1.3 mA - or with typical values of 4 mV less than 300 uA.

Image
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Re: THAT1646 Current Booster and Transformer Driver

Post by Jeffin90620 » Sun Oct 04, 2009 1:27 pm

raf wrote: That transformer you mention, from Pico?
Here it is: http://www.picoelectronics.com/surface/ ... rtNo=83065

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Re: THAT1646 Current Booster and Transformer Driver

Post by Jeffin90620 » Sun Oct 04, 2009 1:40 pm

raf wrote: At 150 Ohms, it gets more critical. I use 600 Ohm primaries in my builds.
I have chosen a 150 ohm primary to minimize the supply voltage requirements in the driver (to minimize power dissipation at higher temperatures).

I had passed on the 1646 in favor of a dual rail-to-rail opamp (buffer and inverter to generate a differential output), but am growing concerned that the inductive load may cause stability problems.

Now I am thinking of a discrete power booster circuit built around an opamp and adding that to the prototype PCB as an alternative.

Jeff

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Re: THAT1646 Current Booster and Transformer Driver

Post by Jeffin90620 » Sun Oct 04, 2009 1:42 pm

raf wrote:As a reference, the new stereo offering from FCS uses the Carnhill transformers and have a 3dB down point of 9Hz.
My application is an intercom circuit for cargo airplanes, so I don't need to go that low. The Pico transformers were chosen because they have stock on units that meet the extended temperature ranges required for aircraft.

Jeff

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Re: THAT1646 Current Booster and Transformer Driver

Post by mediatechnology » Sun Oct 04, 2009 3:15 pm

Jeff - I drove a 150 Ohm Edcor primary with no problems using the 1646. The output produced up to +24 dBm.

Is this 28V single-supply?
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Re: THAT1646 Current Booster and Transformer Driver

Post by Jeffin90620 » Sun Oct 04, 2009 3:21 pm

The nominal supply voltage is 27.5V, but it has to operate down to 17V.

To minimize power dissipation (so it will work at higher temperatures), I am trying to keep the supply at 7 volts.


Jeff

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