Resurrection: Old Parts - New Amp.

[danielwritesbac]

Originally posted by MJL21193
Hi Dan,
Where have you been? Long time no see.

I've been very busy lately so not much time for the project. I'm still in the planning stage. I have all of the parts for the amp and the power supply, just need time to put them all together.

I want to give the case a vintage look too. Similar to this:



With the full wrap around wood and the aluminum face. That Pioneer fluoro VU meter, a selector switch and a master volume control.
What do you think? Throw in some bass and treble control too?

Now that is a much needed option group for many projects.

Wooden case. . . I like this very much!

Vu Meter = cool.

I think that Bass-n-Treble need to center on about 2.2k, not 1k. I'd LOVE to see a well-done bass-n-treble arrangement.
It would also be a pretty nice improvement to add a "speaker size" (low bass kick up) switch and a cancel (bypass tone controls) switch. The low bass kick up switch could prevent bass quality issues (too much drum) when using the bass knob.
In addition to that, a "rumble filter" could perhaps be aimed at approximately 40 hz (a bit lower than normal) so that smaller speakers could survive the extra fun with grace. That's not really a rumble filter if its 40hz, but really more of a PA sort of thing that allows playing loud without the usual clipping/xmax trouble. Anyway, there's 3 fun chrome switches for the front panel.

Rustoleum has a "clear Lacquer" product that can seal up freshly sanded aluminum for a nice old-school brushed aluminum front panel that doesn't corrode. You know, that "No Parking" sign that just got mushed by the caddy? lol!

[MJL21193]

New circuit. This one's a bit different:



~10 watts class A operation with no feedback. I haven't tried it yet but it looks good in simulation. It's based on the "adjustment free" amp talked about here.

I made some changes, using the old salvaged devices.
I mock it up and see if it flies

[MJL21193]

I've acquired a pretty Luxman tuner:



And I will be building this amp in it's image.

A recent find at Goodwill has increased my stock of junk parts. I have a nice pair of heatsinks (a little thin but should be enough for this) and a hefty 30-0-30 transformer:



Heatsinks are 18cm x 11cm with 4cm fins.
I'll do 2 power supplies from the transformer, using half the secondary for each.
Should be fun once I find the time to get going on it.

[MJL21193]

Ok, proof of concept:



Piece of perf board in the middle of the photo is the front end and the output stage mounted on the heatsink. On the scope a 1k sinewave output into my 8 ohm dummy. With 4 diode drops ( no parallel 2k pot for bias adjust - yet) it's running at 30VDC and the idle current is 600mA/output, so not up to full steam yet but I have some confidence to proceed.

10k squarewave:



Perhaps a bit overcompensated with the 47pF cap. I'll try it without it or a lower value.
Not bad so far.

[MJL21193]

I revised the schematic:



Added a CCS for the VAS (eliminated the bootstrap) and changed to a Vbe multiplier.

I also split the power and will run the front end from a higher, regulated supply.

The amp (as prototyped) will run fine and stable without any compensation but I've reduced the one originally at 47pF to 15pF - this has the 20k squarewave looking good. No overshoot.

The sim is showing less than 0.05%THD at 1k, 9 harmonics. The majority is H2 and 3:



Not too bad for no feedback.

[MJL21193]

A more refined prototype:



This matches the new schematic above, with a couple exceptions: first, R6 in the Vbe multiplier is a new value - 470 ohms (Multisim always screws this up) and second, the cdom is back to 47pF. I immediately had stability problems with the 15pF cap.

Seems to be operating nicely. The next step is to play with this prototype until I'm satisfied with its performance, then I'll layout the permanent boards.

1k squarewave at 1A idle:

[MJL21193]

20k squarewave:

[MJL21193]

A bit more tinkering and some more changes. Reduced R10 to 3.6k and increased the current through the VAS by changing R2.



The prototype responded accordingly - strong, stable output and very clean clipping performance.

[Pano]

Originally Posted by 454Casull

Word of caution about pancakes: mix them only long enough so that you do not see white flour.

This is the conventional wisdom and the way I was taught and did it for years. Don't over mix the batter.

But my father used to stop often for breakfast at a diner in Tallahassee, Fla.
The cook there was famous for her pancakes. And she made the batter in a blender!

So my father started doing it that way and by golly, they were great. Light an fluffy, very tasty. Seems all wrong, but it works great. Go figure...

MJL, those cases are pretty! Waveforms are pretty slick, too. (light and fluffy)

[MJL21193]

Thanks panomaniac!

Yes, I'm not afraid to mix it up thoroughly; I don't like lumpy pancakes!
The extra baking powder helps to block the formation of gluten anyway.

I have another pancake trick I didn't mention - after you've poured the batter in the pan, put a small (or large, depends on you cholesterol count ) pat of butter in the middle. When you flip it, it sears the butter, spreading flavour and creates a handy syrup pocket...