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 » Mon Apr 27, 2009 6:13 am

raf wrote:I need to re-pitch the output device pads... I used 0.1" thinking they were TO220, they are, what TO126?
They are an insulated TO-126 on a 2.28 mm pitch. 0.1" may be close enough.

Thanks Roger and Doug.
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Re: Using the THAT1646 As A Transistor Pre-Driver/Headphone Amp

Post by mediatechnology » Mon Jul 20, 2009 11:35 am

Roger -

I think it would make a good class-A transformer driver. The gain is ever so slightlyless than unity. Any required gain would need to be ahead of the 1646. The SR is only 8 V/uS or so. A 150:600 transformer would likely be the best option.

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

Post by mediatechnology » Mon Jul 20, 2009 9:07 pm

Sam Groener's improved 2520 is only 10 V/us: http://www.sg-acoustics.ch/analogue_aud ... index.html

In some of my earlier tests I had measured the SR of the 1646 using common mode drive around 4V/us with the BD139/BD140 followers. After we got boards I re-checked it and it is indeed around the expected 8 V/us.

The 1646 itself is typically 15 V/us measured differentially so 8 V/us is about right. If a 150:600 Ohm transformer is used the SR (neglecting transformer) will about double due to voltage gain.

One advantage the 1646 with discrete buffers has is fairly low offset compared to the 990 and 2520. I'm typically seeing 4-6 mV.
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Re: Using the THAT1646 As A Transistor Pre-Driver/Headphone Amp

Post by JR. » Tue Jul 21, 2009 10:25 am

That slew rate seems adequately fast for valid audio inputs.
[edit- removed useless rant]

JR

PS: A couple of decades ago, I pondered coming up with a measure, not unlike THD to isolate and measure slew rate related nonlinearity. Simple THD measures at high slew rate will be thwarted by limited gain bandwidth. I gravitated toward two-tone (or more) IMD as a work around for the GBW limit issue, but this reduces to pass-fail test if nonlinearity suitably low at 20 kHz. Note: this test will not be very rewarding for properly rise time limited circuitry that can't be slew limited with valid input signals. Catch-22 is it could only measure results on unprotected circuitry.

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

Post by mediatechnology » Tue Jul 21, 2009 1:36 pm

Yeah John you're right. 8V/us ought to be fast enough. At 4V/us it was still fast enough but would've required some "explaning" to the audiophools. As a headphone amp some people seem to think they need 50 V/us to swing 1-4 V p-p.

It's like having a car that can do 150 miles per hour that you only drive through school zones.

I'm glad I re-measured though. Nothing made sense as to where I was "losing" that extra 4 V/us. The output devices (BD139/BD140) aren't that slow.
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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?

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

Post by JR. » Tue Jul 21, 2009 4:44 pm

It is the unfortunate reality when selling anything to consumers, their perceptions matter. Since they don't have the experience and judgement to decide how much slew rate is enough, they desire more.

One guy who should know better (active on gearslutz) still insists on uber high slew rate, silly 100v/usec stuff, but it's easier in some market niches to argue for that than against it.. Thats why I escaped from that market but still have twinges from time to time..

I know i could make some kick ass product, I just don't like wrestling in the mud with the customers about what they hear...

JR

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

Post by mediatechnology » Tue Jul 21, 2009 4:50 pm

I would like to put some voltage and current gain behind the 1646 and see how well it drives a speaker.

I was in the shop last night moving stuff around and decided to give the headphone amp a listen after a few months away from it. Maybe its the headphones themselves but after about an hour of listening and remembering how good it sounded I had to pull myself away from it and go to bed. Odd, 'cause I almost never use phones. I never finished cleaning up.
Read Damore v. Google and ask yourself the following questions:
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Would you trust a Googler with your children?

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

Post by JR. » Wed Jul 22, 2009 12:44 pm

If it sounds good listen too it... 8-)

JR

Bruno2000
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Re: Using the THAT1646 As A Transistor Pre-Driver/Headphone Amp

Post by Bruno2000 » Tue Jul 28, 2009 2:57 pm

Are these boards available? Will they run off of +&- 9V? I want a couple!
Brno2000
The large print giveth, and the small print taketh away - Tom Waits

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

Post by mediatechnology » Tue Jul 28, 2009 3:57 pm

Bruno -

They will run off +/-9V but do have a heavy 70-90 mA current requirement. I hope you weren't thinking about batteries. The efficiency is around 2%.

Dunno about boards. We did the protos and may want to change the heatsink orientation and add connectors.

Wayne
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?

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