Tube-Buffered Gainclone

sorenj07 · 31st May 2008, 07:37 PM

Hi, first post in this neck of the woods

I've got a 2x30V 300VA Hammond toroid kicking around. Would it be suitable for building a gainclone using the standard LM3875T chips? I usually see secondaries around 18-22V being used. I would probably have a rail voltage of around +- 42V. On the other hand, I could wire the dual primaries in series to put out +-21V or so, but this seems a little low... Speakers used would be 8 ohms.

Does anyone happen to know the input impedance of the LM3875 by the way? It'd be useful in designing the cathode follower which would feed the volume control.

Greg Erskine · 31st May 2008, 07:43 PM

http://www.national.com/ds/LM/LM3875.pdf

sorenj07 · 31st May 2008, 08:06 PM

Am I right in assuming, then, that 42V will be ok into 8 ohms since it's well under the maximum rating of 80V or so? I'm new to these "solid state" things

I assume I should try to mount them to a heatsink, right?

ionomolo · 31st May 2008, 10:01 PM

Quote:

Originally posted by sorenj07Am I right in assuming, then, that 42V will be ok into 8 ohms since it's well under the maximum rating of 80V or so? I'm new to these "solid state" things I assume I should try to mount them to a heatsink, right?

Yes, you need a big one! there is an application note on heatsinking national amplifiers and some information on the datasheet.

sorenj07 · 31st May 2008, 10:06 PM

All right. Do you think some CPU style heatsinks with a mounted fan would work all right? I have no clue about dissipation ratings and the like, but I will take another look at the datasheet. It's just so long

Nordic · 31st May 2008, 10:33 PM

Quote:

Originally posted by sorenj07
All right. Do you think some CPU style heatsinks with a mounted fan would work all right? I have no clue about dissipation ratings and the like, but I will take another look at the datasheet. It's just so long

Not a chance.... They dissipate about 60W with fans blowing like mad. If at all possible issolate the heatsink from the case, then mount the chip useing only a little thermal grease... but you are going to need a much sturdier sink than that...

sorenj07 · 31st May 2008, 11:25 PM

There are some interesting-looking options for the tweaker market..
http://www.newegg.com/Product/Produc...page=1&bop=And

Either some of the more crazy ones, or I start looking for massive blocks. Pity, I figured that these chips wouldn't need that much sinking.

DC Dave · 31st May 2008, 11:55 PM

Quote:

Originally posted by sorenj07
Am I right in assuming, then, that 42V will be ok into 8 ohms since it's well under the maximum rating of 80V or so? I'm new to these "solid state" things I assume I should try to mount them to a heatsink, right?

Wait a minute.

That is going to be to high. 42+42=84V. The 80V maximum referred to is the sum of the + and -. You are going to want a lower voltage for that chip.

Zigis · 1st June 2008, 12:12 AM

With this trafo you can use regulated PS, to regulate voltage down.
LM338 regulated snubberized PSU for audio amplifiers

sorenj07 · 1st June 2008, 05:58 PM

Hey, looks perfect!

What do you make of using the following schem for the signal part?
http://www.geocities.com/rjm003.geo/...io/diy_gc.html
specifically, this:

Or should I just use the textbook example from the datasheet, under "Typical Application"? Anyone have any recommendations?

31st May 2008, 07:37 PM	#1
sorenj07 diyAudio Member Join Date: May 2006	Tube-Buffered Gainclone Hi, first post in this neck of the woods I've got a 2x30V 300VA Hammond toroid kicking around. Would it be suitable for building a gainclone using the standard LM3875T chips? I usually see secondaries around 18-22V being used. I would probably have a rail voltage of around +- 42V. On the other hand, I could wire the dual primaries in series to put out +-21V or so, but this seems a little low... Speakers used would be 8 ohms. Does anyone happen to know the input impedance of the LM3875 by the way? It'd be useful in designing the cathode follower which would feed the volume control.

31st May 2008, 07:43 PM	#2
Greg Erskine diyAudio Member Join Date: Jan 2002 Location: Sydney/Australia	http://www.national.com/ds/LM/LM3875.pdf __________________ Greg Erskine

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31st May 2008, 08:06 PM	#3
sorenj07 diyAudio Member Join Date: May 2006	Am I right in assuming, then, that 42V will be ok into 8 ohms since it's well under the maximum rating of 80V or so? I'm new to these "solid state" things I assume I should try to mount them to a heatsink, right?

31st May 2008, 10:06 PM	#5
sorenj07 diyAudio Member Join Date: May 2006	All right. Do you think some CPU style heatsinks with a mounted fan would work all right? I have no clue about dissipation ratings and the like, but I will take another look at the datasheet. It's just so long

31st May 2008, 11:25 PM	#7
sorenj07 diyAudio Member Join Date: May 2006	There are some interesting-looking options for the tweaker market.. http://www.newegg.com/Product/Produc...page=1&bop=And Either some of the more crazy ones, or I start looking for massive blocks. Pity, I figured that these chips wouldn't need that much sinking.

1st June 2008, 12:12 AM	#9
Zigis diyAudio Member Join Date: May 2005 Location: Riga, Latvia	With this trafo you can use regulated PS, to regulate voltage down. LM338 regulated snubberized PSU for audio amplifiers

1st June 2008, 05:58 PM	#10
sorenj07 diyAudio Member Join Date: May 2006	Hey, looks perfect! What do you make of using the following schem for the signal part? http://www.geocities.com/rjm003.geo/...io/diy_gc.html specifically, this: Or should I just use the textbook example from the datasheet, under "Typical Application"? Anyone have any recommendations?