LME49811 front-end for Gilmore amp? - diyAudio
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Old 29th April 2009, 02:32 AM   #1
Artie is offline Artie  United States
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Default LME49811 front-end for Gilmore amp?

I was all set to start gathering the parts for the Gilmore headphone amp, when a little research showed that acquireing the dual-matched JFET's, (2SK389/2SJ109), might be harder than I expected. (Amazing amount of drama surrounding those parts.)

So, can I use the new LME49811 chip to replace the front-end of the class A output section? Kinda like this:

Click the image to open in full size.

Will the 9ma max output of the 49811 be sufficient to drive the pre-drivers? I realize that the output-stage gain is less than unity, so the chip will need to run from a higher voltage.

Just not sure if its this simple.
Thanks all.
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Old 29th April 2009, 10:17 AM   #2
Artie is offline Artie  United States
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Anyone?
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Old 29th April 2009, 10:39 AM   #3
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Quote:
Will the 9ma max output of the 49811 be sufficient to drive the pre-drivers?
If your drivers has a Hfe at 50, then they can put out >400mA to drive the output devices which is more than needed.
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Old 29th April 2009, 10:41 AM   #4
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Have you deliberately swapped the drivers?

You can replace the 389/109 pairs with 170/74 discretes.
Buy matched pairs or match them yourself and then thermally couple the pairs.

Cin>=1.4 * Ci * Ri+Rs / Rin >= 1.4 * 10 * 57k8 / 1k8 >=450uF

Ci could be reduced to 1u5F or 2u2F and still pass sufficient bass.
Cin @ 10uF will kill the bass response.
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Old 29th April 2009, 05:10 PM   #5
Artie is offline Artie  United States
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Quote:
Originally posted by AndrewT
Have you deliberately swapped the drivers?
Hey Andrew. I'm not exactly sure what you're asking me here. The right side of that schematic, (with the green resistors), is straight from Kevin Gilmore's drawing. The left side is straight from the 49811 app note.

I like the idea of using the 170/74 subs, but I thought those were discontinued also.

Thanks for the input cap info. I'll see if I can locate the 170/74 subs.
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Old 29th April 2009, 08:56 PM   #6
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Quote:
Originally posted by Artie



I like the idea of using the 170/74 subs, but I thought those were discontinued also.
You can find matched quads at amb.org. I think you can find them matched at borbelyaudio.com as well.

When using LME49811, I don't think you need the discrete gain stage, just emitter followers, double emitter followers, CFP or something like that.
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Old 29th April 2009, 09:00 PM   #7
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Quote:
Originally posted by Artie


Hey Andrew. I'm not exactly sure what you're asking me here. The right side of that schematic, (with the green resistors), is straight from Kevin Gilmore's drawing. The left side is straight from the 49811 app note.

I like the idea of using the 170/74 subs, but I thought those were discontinued also.

Thanks for the input cap info. I'll see if I can locate the 170/74 subs.
The left transistors did form the VAS stage so you can't use this. You have to make them emitter followers instead. Otherwise it might work.
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Old 29th April 2009, 09:36 PM   #8
Artie is offline Artie  United States
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Thanks again guys. (Gals?)

I guess this just brings me back to the idea that I should just build it as it is, before I go messing around with "mods" that I'm not sure of.
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