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

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

Post by 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
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:

Image
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:

Image
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:

Image
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.

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

Post by radiance »

raf wrote: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?
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mediatechnology
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Re: Using the THAT1646 As A Transistor Pre-Driver/Headphone Amp

Post by mediatechnology »

This is the one:

Image
A headphone amplifier made from the THAT1646 and BD139/BD140 NPN/PNP pair.
radiance
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Re: Using the THAT1646 As A Transistor Pre-Driver/Headphone Amp

Post by radiance »

Thanks!
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Re: Using the THAT1646 As A Transistor Pre-Driver/Headphone Amp

Post by 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.
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mediatechnology
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Re: Using the THAT1646 As A Transistor Pre-Driver/Headphone

Post by 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
lofi
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Re: Using the THAT1646 As A Transistor Pre-Driver/Headphone Amp

Post by lofi »

will you allow me to pretend that i understood the above? my bear ringer head phone amps dying on me, so i need to make about 7 amps. this looks ideal. i will be using the +4 outs on the back of my ada8000, and am assuming the above is for 1 channel, so i need two for sterio? also whats the 1/25V thing next to the 139 on input.

sorry for the stupid questions,

Iain
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mediatechnology
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Re: Using the THAT1646 As A Transistor Pre-Driver/Headphone Amp

Post by mediatechnology »

Hi lofi

Yes there are two of these for stereo and +4 should be plenty of level for it. If you need a balanced input you might put a THAT1240 or 1246 ahead of it. This unit takes two big heatsinks and is a power hog due to it being heavy Class-A. But the distortion is very low.

The 1/25V is the capacitance and voltage of the C directly above it. I ran out of room and the value wound up being next to the BD139. That C reduces the noise from the Vbe multiplier and softens the turn-on current.

The input on my prototype is driven from the wiper of a 10k audio taper level control. Under normal circumstances the THAT1646 would need to be driven from a low impedance source. In this application, using "common mode drive," a low source impedance is not mandatory. But, the internal 5 K (+/-20%) input impedance of the 1646 does load the pot slightly deviating the taper. It's not really enough to matter.

Thanks for posting Ian. Let us know how things work out.
lofi
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Re: Using the THAT1646 As A Transistor Pre-Driver/Headphone Amp

Post by lofi »

i am not intending to use pots as its studio use only and i control the mix for each head phone and hence the level.

seems i need 1243 for the balanced input (15V gives 24dBu head room) and then a 1646, and this is per channel.

off to do some maths and flex paypal methinks

Iain

thanks for all your work on this Wayne!!! much appreciated
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Re: Using the THAT1646 As A Transistor Pre-Driver/Headphone Amp

Post by mediatechnology »

Ian you are correct that a 1243 is a good choice for line interfaces since it equalizes the clipping points of the driver and receiver.

In this case you may still have considerable gain to throw away and may need more than even a 1246 can provide for line-level inputs. You might still consider putting a trim or fixed attenuator between the line receiver and 1646 to keep the average levels feeding the box up. Otherwise it may be too loud unless the inputs are pulled way back. Most of my listening tests were from -10 sources and I had more than enough drive for the 'phones I was using.

Wayne
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