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Simple Class A Headphone Amp Using THAT1646
Simple Class A Headphone Amp Using THAT1646
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Old 6th June 2012, 09:02 PM   #21
Telstar is offline Telstar  Italy
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Quote:
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?
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Old 7th June 2012, 11:57 AM   #22
mediatechnology is offline mediatechnology  United States
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Quote:
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, 03:44 PM   #23
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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.
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Old 24th September 2012, 07:36 PM   #24
mediatechnology is offline mediatechnology  United States
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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 20th February 2017, 01:36 PM   #25
mediatechnology is offline mediatechnology  United States
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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:

Simple Class A Headphone Amp Using THAT1646

Dual Class-A "DCAO2" Headphone and Line Amplifier Schematic

Simple Class A Headphone Amp Using THAT1646

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 20th February 2017, 11:22 PM   #26
xrk971 is online now xrk971  United States
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Simple Class A Headphone Amp Using THAT1646
Nice looking amp. Great job!
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Old 21st February 2017, 11:26 AM   #27
mediatechnology is offline mediatechnology  United States
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Thank you!
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Old 4th November 2017, 09:35 AM   #28
xrk971 is online now xrk971  United States
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Simple Class A Headphone Amp Using THAT1646
Hi Wayne,
I just built your amp, unbeknownst to me, it was the one from this thread. Prasi did a nice layout and I made it pretty simple without hardly any extra rail smoothing caps.

I posted your schematic in another thread, and Prasi was kind enough to do the layout:
Click the image to open in full size.

Here is the layout he made:
Click the image to open in full size.

The stereo version is here:
ESP HPA

There is even a version that looks more integrated to avoid some PCB fab houses that think you are trying to make 2 boards for the price of one:
Click the image to open in full size.

Gerbers here.

Here is my amp made from PCBs from EasyEDA, using a simple 7815/7915 regulated PSU from a 10w trafo and a 10mF//0.47R//10mF CRC. i am running about 100mA bias current and uisng 2R2 emitter resistors:

Click the image to open in full size.

And here is a quick measurement with a Focusrite 2i4 USB sound interface. Driving 1.00Vrms into 50ohms. Well, this amp has the distinction of being the lowest distortion amp I have measured. It's below the limits of the 2i4 (-130dB). Anyhow, it sounds really nice.

Click the image to open in full size.

Thanks for a super design. I will probably make a few to use as low distortion instrumentation amps. i will also use one for a low distortion line preamp for my power amp for speakers.

I can highly recommend this amp - very simple to make. A small handfull of cheap parts (under $2) and a $5 THAT1646 driver,
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Last edited by xrk971; 4th November 2017 at 09:40 AM.
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Old 4th November 2017, 10:25 AM   #29
mediatechnology is offline mediatechnology  United States
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Thank you for the update and the really cool layout. Prasi did a great job. I appreciate the design credit on the layout.

It nice to see interest in this design. I sold out of the original boards a couple of years ago.

The THAT1646 design was also used in the Foote Control Systems mastering compressors to drive the transformer outputs. Up to about 100 mW it provides very low distortion.

For those that needed higher power and sub-Ohm output impedance I replaced it with this one: Dual Class-A II Line Output and Headphone Amplifier

This is the current board minus output transistors:

Simple Class A Headphone Amp Using THAT1646


Though not as simple as the original it's the same topology as the THAT1646 driver (with the THAT1646 replaced by LME49720s) and has either class-A open loop, class-A closed loop and class-A/B closed loop at Pout >100 mW. It's able to produce several watts for difficult-to-drive 'phones.

Simple Class A Headphone Amp Using THAT1646


For those interested in the design you can follow it here: Dual Class-A II Line and Headphone Output Board Documents - Pro Audio Design Forum

If you are really into headphone listening you might consider using my new "elliptic EQ" for bass-to-mono crossfeed. On material that needs it, the low-end "focus" improvement on 'phones is significant. Elliptic Equalizer Boards and Kits

Thanks again for the update. Its great to see interest in this "different" use of the THAT1646.

Last edited by mediatechnology; 4th November 2017 at 10:31 AM.
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