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Old 6th June 2012, 10:02 PM   #21
Telstar is offline Telstar  Italy
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Originally Posted by mediatechnology View Post
I estimate the output impedance (it's open loop) to be <2 Ohms. It can be run with or without build-out resistors into large capacitive loads. 30 Ohm loads can be driven to the 100 mW range with decent performance.

I've run it on supplies as high as +/-18V or as low as +/-9V. Things head south around +/- 6V. I also included a soft clipping circuit which tracks the rail voltage.

As expected for class-A, the current drain is huge. I use 85 mA Iq/channel typically so the totals run about 200-225 mA.
Do you think it can work for 2W 500mA into 30 Ohm with little adaptation?
"The total harmonic distortion is not a measure of the degree of distastefulness to the listener and it is recommended that its use should be discontinued." D. Masa, 1938
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Old 7th June 2012, 12:57 PM   #22
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Do you think it can work for 2W 500mA into 30 Ohm with little adaptation?
It would require much larger output devices and heatsink along with some additional current gain to produce that level of output (500 mA) with the same linearity.

The principle of "mirroring" the Vbe multiplier and driving the outputs in common mode (offset by Vbe and -Vbe) could be used for higher currents, but this board could only be used as a pre-driver.
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Old 21st September 2012, 04:44 PM   #23
udailey is offline udailey  United States
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Gary Galo's article shows a DC servo for the 1200 that might be a nice add on to your circuit to eliminate any offset.
purchase LDRs anytime Also try my Resistor Replacers or LDR based Input Selector Email me.
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Old 24th September 2012, 08:36 PM   #24
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Thanks for the post.

I've used a similar technique using a simple integrator to offset null a THAT1240, 1243 or 1246.

Having said that the typical output offset I'm seeing around 0-5 mV isn't really a problem but anyone wanting to add it to the circuit could certainly do so by driving the input line receiver's ref pin with the servo.
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Old Yesterday, 02:36 PM   #25
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Default Dual Class-A II Headphone and Line Amplifier

I wanted to post an update to this project after a long absence.

The original design posted here using a THAT1646/DRV134 wound up not only in headphone amps but wound up being used as line amps in mastering and transfer facilities.

After selling out of the original run of boards I added features. I kept the original biasing scheme, added gain, added the option of overall NFB for outputs >>100mW and "0Ω" (40mΩ) Rout, and sprinkled in more jumpers.

The result is this:

Click the image to open in full size.
Dual Class-A "DCAO2" Headphone and Line Amplifier Schematic

Click the image to open in full size.
Dual Class-A II, DCAO2, Headphone and Line Amplifier PC Board

More information can be found here: Pro Audio Design Forum &bull; View topic - Dual Class-A II Line and Headphone Output Board Documents and here: Dual Class-A II Line Output and Headphone Amplifier
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Old Today, 12:22 AM   #26
xrk971 is online now xrk971  United States
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Nice looking amp. Great job!
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