Using the THAT1646 As A Transistor Pre-Driver/Headphone Amp

[mediatechnology]

I finally had a chance to finish the Vectorboard and run a few quick tests with RMAA. My power supply connections and load returns could be a little better (clip leads) but here's the test results I got:

http://www.ka-electronics.com/Headphone ... ut_30R.htm

Here's the summary updated 12/20:

[mediatechnology]

Some pictures:



The heatsinks make nice hand warmers with dissipation of a little over 2W each at idle. With an efficiency of perhaps 2.5% I don't think I'll go for an "Energy Star" rating. Not shown on my original schematic are the 22 uF bulk bypass in the foreground.



Here we have the Vbe multiplier using a second BD139 bolted to the rear of the heatsink. I used a 27R build out rather than the 33R on the original schematic. I had 27Rs on hand and it's probably a better choice anyway. All recent tests BTW were with the 27R. Note the carbon comp resistors from the last century. On my final build I'll use good metal films and a vertical trimmer.

On levels: With my 63 ohm MDR-7506s I have plenty of drive voltage and still use 10-12 dB of attenuation with typical-level "-10" sources. For my application I will dispense with an input gain buffer to keep the signal path short.

As it is now, the whole thing is DC-coupled. The low-end totally rocks. The output Vos is about 3.5 mV (at the Re junction) on the left channel and 6.5 mV on the right. The Vbe mismatch on both pairs (NPN to PNP) is 15 mV.

I'll continue to do the buffer for those that need it: I may end up using one anyway to add polarity inversion.

[mediatechnology]

Happy New Years everyone!

I decided to do some higher level measurements into various loads. The following measurements do not take into account the external generator distortion which has been posted earlier.

At 7.6 Vrms into 270 ohms, 213 mW, 1 KHz THD reads 0.0021%.
At 1.4 Vrms into 30 ohms, 65 mW, 1 KHz THD reads 0.0039%.
At 2.0 Vrms into 30 ohms, 133 mW, THD reads 0.0093%.

The above voltages are into the actual load, at the right-hand side of the 27R build out resistor and take into account its IR drop.

A pair of 300 ohm or 600 ohm phones can thus be driven to over 20V P-P. I checked the THD up to 6 KHz fundamental and there appears to be no rise. (My measurement beyond 3rd harmonic is limited by the 20 KHz measurement bandwidth of the soundcard.)

It should be noted that voltage gain upfront will be required to produce high level drive but with low impedance phones unity gain operation of the basic circuit is reasonable.

[mediatechnology]

Thanks Roger. Don't have a finished schematic yet but the one posted is really close. The only changes are:

1) Added 22/25V bypass caps, one set.
2) Changed the output R to 27 ohms from 33.
3) Added a dual 10K audio taped pot with the wiper tied to the input.

I do think it needs an input gain stage for higher drive requirement phones.

[mediatechnology]

While going through my office I found the following article describing a simple null tester to measure low-level THD: http://electronicdesign.com/Articles/Ar ... /8188.html

This allows the generator THD, to a significant degree, to be nulled while amplifying the device under test's THD to more measureable levels. Since the article cited above has broken links to the figures I've reproduced a copy:


Image Courtesy of Electronic Design, June 21, 2004 authored by Jerry Riddick of Texas Instruments.

While not perfect, this technique allows the generator THD and fundamental to be nulled significantly to perform lower-level THD measurements. If the device under test has low phase shift and non-inverting operation, null testing with this device becomes a useful tool. The distortion and fundamental of the generator are nulled by the common mode rejection of the amplifier while the distortion of the device under test is amplified by 40 dB making measurement of it easier.

Since the THAT1646-based headphone amp has very low distortion, "almost" unity gain and very little phase shift it was a good candidate to measure. I did introduce slight attenuation, not shown, in the input leg of the instrumentation amp, a THAT1510, because the THAT1646 headphone amp does not have infinitely low output impedance and its' level is reduced by loading. This allows the CMR to be adjusted for maximum nulling. The gain of the 1510 was set to be 40 dB.

The output sample was taken at the junction of the amplifier's emitter resistors. The load was connected after the 27R build out and power measurements are with respect to the load on the right-hand side of the build out. This allowed measurements to be taken with smaller adjustments in CMR and without resistive attenuation by the build-out.

Please note that, to avoid confusion, the right hand scale of the spectral plots was replaced in an image editor with values reflecting the gain of the 1510. Thus, the -100 dB line, representing the actual THD level prior to amplification by the instrumentation amp, was in fact -60 dB with regard to the analyzer '0'. A 200 sample average was used to provide display clarity. The analyzer was operating in 16 bit 48 KHz. Please note that the 1 KHz is input that has not been fully canceled. The level of this, if nulled reasonably well, does not affect the harmonic measurements of the DUT given that the generator second and third are so far in the mud as to not matter.

The following is without load:


No output load
The fundamental has been attenuated by ~70 dB. Note the complete cancellation of generator THD which typically ranges from -90 to -100 dB.

This is with a 30 ohm load driven to 25 mW:


30 ohm load, 25 mW
Note the rise in second and third harmonic to approximately -95 dB each.

This is with a 30 ohm load driven to 65 mW:


30 ohm load 65 mW
The third harmonic has increased to -83 dB which is 0.007% of the actual fundamental. With headphones having a sensitivity of 106 dB/mW, 65 mW would be, to say the least, quite loud at approximately 124 dB SPL.

This is with a 330 ohm load driven to 150 mW representing the higher end of modern headphone impedance (not withstanding vintage AKG240s). The drive level is 20 V P-P.


330 ohm load, 150 mW, 20V P-P
Here we have an increase in second harmonic and may begin to be limited in our high level tests by the common mode range of the instrumentation amp.

Conclusion: When you don't have an Audio Precision System Two or a device that's too clean to measure with what you have sometimes you just have to make do.

[radiance]

Wayne

Good stuff! You must have a "finished" schematic for this, no?
Deserves a good layout, with ground plane and all that!

Any news on the "finished" schematic yet?

[mediatechnology]

This is the one:


A headphone amplifier made from the THAT1646 and BD139/BD140 NPN/PNP pair.

[radiance]

Thanks!

[mediatechnology]

Hey you're welcome.

I did build a two-channel unit with a dual-gang 10K input pot directly ahead of the THAT1646. Placing a pot ahead of a 1646 in a conventional line driver application is not a good idea but in this situation, using common mode drive, it's not a problem. For low-sensitivity phones and -10 inputs you might need an input gain stage but with my Sony phones at -10 it wasn't necessary with plenty of level. The thing sounded so good I thought I'd quit while I was ahead.

Use fairly large heatsinks as the idle is about 2W/ch. With this being heavy class-A efficiency is certainly not one of its attributes.

[mediatechnology]

You might find this headphone amp from National interesting using the LME49600:

http://www.national.com/an/AN/AN-1768.pdf

Datasheet here: http://www.national.com/pdf/LM/LME49600.html