Many thanks for the fast and detailed reply!
mediatechnology wrote:Are you going to use a linear supply or a switcher? The switchers worked great...
Yes, I think initially at least this makes a whole lot of sense. If I get the amp working well and with the features I want I might build a regulated supply with a monster torioid - if only for the added weight and bling
mediatechnology wrote:Other than unused components that might not be needed for an unbalanced input, the only real difference in the TO-220 low power and TO-3P 10W version are the size of the emitter resistors.
Oh, I see - so the headphone amp board is perfectly usable for a power amp as is? I'm probably still leaning towards a new layout though, since I want to add some functionality - more on that below.
mediatechnology wrote:Those resistors could be located off-board - the ideal place would be at the speaker terminal with individual leads to the emitters.
I might use housed resistors mounted to the outside of the case, again for the bling! (Yes, the bling factor
is important!)
mediatechnology wrote:If I were going to lay out a board specifically for the dual Class-A 10W I'd put in some variable voltage gain. I prototyped a "Self-style" shared gain volume control, balance trim, mono and optional Width circuit that I still have on the bench. I can't recall the exact range but I think it was -inf to +15 dB. It may have been +20. I was able to get pretty decent tracking and "feel" with linear controls. I favored this approach for my own use since I felt it would be quieter than attenuate, then amplify to get the voltage swing needed to hit 8-10 Watts.
This is the most essential thing I'd like to see added, though I'm thinking of using a stepped attenuator (DIY) rather than a linear pot - not because I think it will "sound better" but because I want repeatable gain - and here I have to expand a bit:
The reason I got interested in class A amplifiers was because I got interested in measuring loudspeaker driver performance. I started looking for amplifiers with very low distortion and high linearity, with capacitance free outputs, and soon realised class A was the way to go, preferably single-ended. I got excited by the relative simplicity of most class A amps I looked at and realised that building one would not only be a nice way to get my own test & measurement amplifier, but that it would be a great little project in many other ways. I actually
like the fact that they idle hot and and need big heatsinks and I look forward to (eventually) working on the case design as much as the electronics. The heat is a feature, not a drawback!
But if I'm going to be able to reliably compare individual speaker drivers I need to be able to set the same output level repeatably, so stepped attenuators actually
make sense here (as opposed in a "they sound better" sense) and is the perfect excuse to add some extra bling (again).
mediatechnology wrote:I'll re-visit that circuit and scan the sketch I did.
Thank you, that would be great!
mediatechnology wrote:In any case, if you want to do a layout then go for it.
I'll have a stab at it, if you can get me a sketch with the variable gain stage included. Apart from locating the resistors off-board, and (maybe) connecting the power transistors directly to the PCB, are there any other layout pointers you can give me? Like where things need to be kept well apart, which connections need extra copper, and whether it's worth having a ground plane?
mediatechnology wrote:Having the output devices directly soldered to the board would be a big time saver.
I'm temped to keep them off-board to make the case design more flexible, but I'll bear this in mind.
mediatechnology wrote:FWIW I found this heatsink source while I was doing this a year or so ago. HeatsinkUSA
http://www.heatsinkusa.com/
Those are great prices, thanks for the tip! I'm based in the UK though, and I already have some plans for how to keep this thing cool, which probably won't involve conventional heatsinks... More on that later!