Tube-Buffered Gainclone

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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.
 
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 said:
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 :eek:
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 said:
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 :smash: 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.
 
I would go for the unregulated one since the LM3886 has a good psrr and some regulated power supplies have problems at high frequencies, but there is people here reporting an improvement in sound as a consequence of using a regulated supply. There are schematics of discrete regulators floating arround that may suit your needs if you want it complicated.
 
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