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

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

Post by jonasn » Tue Oct 13, 2009 1:08 pm

Hi all,

This is my first post on this forum. I have however been reading here for a while, and would like to thank you all for great inspiraton and education, and especially mediatechnology for this fantastic amplifier.

I decided to build this headphone amplifier, and are appx midways in the build now. The actual amplifier is working, and initial sound tests seems very promising.

For some applications, I will need a bit more gain than the unity provided standard, and thought about different ways of doing this, andcame up with an idea about using another that chip in a way other than intended.

The idea is to use the THAT1246 balanced receiver, but configured for unbalanced input, and using the internal resistors to make it switchable between 0dB or 9,5dB gain.

I want to keep the compleete amp DC coupled if possible, and in the 0dB setting the ofset is clode to 0mV, but in the 9dB setting I get more that 200mV. But this is taken care of by a 15Mogm resister to the negative voltage. this brings the ofset of the intire amplifier down under 20mV which should be low enough.

Please coment on the idea, and I will post pictures of the amplifier when it gets closer to compleetion.
HP Gain.JPG
That1246 HP gain
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mediatechnology
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Re: Using the THAT1646 As A Transistor Pre-Driver/Headphone Amp

Post by mediatechnology » Tue Oct 13, 2009 1:27 pm

Very clever idea. Thanks for joining us and posting.
I want to keep the complete amp DC coupled if possible, and in the 0dB setting the ofset is close to 0mV, but in the 9dB setting I get more that 200mV. But this is taken care of by a 15M Ohm resister to the negative voltage. This brings the ofset of the entire amplifier down under 20mV which should be low enough.
I'm puzzled by the large amount of offset. The bias current is about 900 nA worst-case and I can't see how that much Vos is developing. (The non-inverting Ibias is returning through the pot's wiper though and that may cause noise problems. You might want to load the wiper with a shunt R to ground in case the wiper lifts one day.)

For the 9.5 dB position (prior to it being tied to the 15M) was the 6K simply open or was it switched around to be in parallel with the 12K? For the 9.5 dB position you might want to have pins 1 and 3 linked by the switch. That puts 6k||12k in series with a few gig Ohms at the non inverting input. The bias current will develop about 3-4 mV across that ~4k Ohm assuming the pot is at minimum.

Let me know what you find.

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

Post by jonasn » Wed Oct 14, 2009 1:32 am

I left the 6K input pin open, you're right it is way betterto parrallel it to the input in the high gain setting.
Now with this configuration Vos is below 15mV, no matter the setting of the pot or gain switch.

I do however need to add some RF filtering on the input, as it seems pretty sensitive. any good ideas?
HP Gain2.JPG
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mediatechnology
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Re: Using the THAT1646 As A Transistor Pre-Driver/Headphone Amp

Post by mediatechnology » Wed Oct 14, 2009 4:32 am

OK, glad that worked out. For RFI filtering you might consider a series R and capacitive shunt ahead of the pot at the RCA jack. With the inverting input of the op amp unaccessible its difficult (impossible?) to put it in the feedback loop.
Read Damore v. Google and ask yourself the following questions:
Are Googlers really the people you want in charge of your search results, browser, data or operating system?
Would you be comfortable riding in a Google self-driving car knowing that Googlers might want you dead just because they don't like you?
Would you trust a Googler with your children?

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

Post by radiance » Sat Jun 01, 2013 3:06 pm

How will this amp behave with a 300? headphone ??
I'm now using a 37? headphone and This works OK.

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

Post by mediatechnology » Mon Jun 03, 2013 5:41 pm

A 300 Ohm headphone load should be fine.

The board/circuit is also a line driver and can swing up to +21 dBu with +/-15V supplies.
Read Damore v. Google and ask yourself the following questions:
Are Googlers really the people you want in charge of your search results, browser, data or operating system?
Would you be comfortable riding in a Google self-driving car knowing that Googlers might want you dead just because they don't like you?
Would you trust a Googler with your children?

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

Post by radiance » Tue Jun 04, 2013 2:20 pm

Thanks !

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

Post by mediatechnology » Tue Jun 04, 2013 2:31 pm

You're welcome. Driving 300R loads won't make it break a sweat.

You might need some voltage gain however depending upon the sensitivity of the phones.
Read Damore v. Google and ask yourself the following questions:
Are Googlers really the people you want in charge of your search results, browser, data or operating system?
Would you be comfortable riding in a Google self-driving car knowing that Googlers might want you dead just because they don't like you?
Would you trust a Googler with your children?

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

Post by radiance » Thu Jun 06, 2013 3:16 am

I see....
I have this headphone amp already build in my mixer, so no modifications are possible...it's all done.
So far it has been great with various headphones, definitely loud enough.

Since I'm moving to a small apartment I think I have to revert to mixing on headphones instead of monitors, so I'm investigating quality headphones like for example the Senheisser HD 800. This is a serious headphone. In a lot of reviews of this headphone I read that people needed to upgrade their amps to get the most out of it.
I might build another one like for example this > http://sjostromaudio.com/pages/index.ph ... dphone-amp


I know I'm kinda entering audio fool territory here ....;-)

gstarey66
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Re: Using the THAT1646 As A Transistor Pre-Driver/Headphone

Post by gstarey66 » Thu Feb 23, 2017 9:03 am

Hello,

Had built several of these for myself and family, very happy with the resulting sound.

Do have a question though. As I have a brother that would like a little more volume with the headphones he is using, couldn't I just change out the 1246 chip for the 1243 chip and have 3 DB less loss?

THAT 1246 chip has a gain of -6, the 1243 chip has a gain of -3. Signal to noise ratio is 1DB worse, but doesn't seem like much of a penalty.

Any feedback would be welcome.

Regards,
Greg

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