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 »

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

Post by lofi »

this is studio live room phone amps, so the more i can push them the better ... within reason that is :D

prob go for a resistor rather than a pot when i have an idea about how insane these are ...

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

Post by mediatechnology »

Most HP amps have about 12 dB of gain for use with -10 sources. This one is unity gain (an op amp up front would fix that) but I found that with the Sony 'phones I was using I had plenty of drive level even with -10 sources.

I don't know what kind of phones you're using but the older AKG-240s (at 500 ohms) were very hard to drive. (The newer ones are 50 ohms IIRC.) This one will swing the voltage for 500 ohm cans but it excels at lower levels/higher currents typically required for 16-200 ohm phones.If you build a board I would leave pads for a level pot. If you're driving vintage AKG-240's you'll need gain.
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Re: Using the THAT1646 As A Transistor Pre-Driver/Headphone Amp

Post by lofi »

tis going to be dt100's (16 ohm) but i get what you mean, so pads will be left, thank you sir

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

Post by mediatechnology »

I used Sony MDR-7506 'phones at 63 ohm and 106 dB/mW. Plenty of gain for my ears.

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