ccs question on mini A

[jeapel]

thanks all these infos will be great help

[Nelson Pass]

Originally posted by Brian Donaldson
A friend and I listened for a few hours, tweaking bias, current gain, and rail voltage (with a variac), and the biggest improvement was upping the rails. It seemed to open the sound and deepen the soundstage. We couldn't really tell a differece in the sound of the bias or gain, but most people seem to prefer the sound of higher bias.

Mosfets just love high voltage and high bias.

[jrsun]

Originally posted by Brian Donaldson
the biggest improvement was upping the rails.

Brian,

from how many volts to to how many wolts ?

My recently finished A-2 is 51 volts unloaded ! Is it high enough in your opinion ?

[GRollins]

I have done a bit along these lines. Since I began with tubes, higher voltages are normal to me. The idea that a high rail draws out and flattens the center of the S-shaped gain curve is no mystery to tube folks.
Solid state people tend to think in terms of current. (You'll note that there is an inherent prejudice here--tubes do voltage easily, so tube people tend to think voltage is everything. Solid state people have current to spare so they preach the gospel of high current and ignore voltage.) Current, particularly if you leave it on all the time (class A) is a nifty thing to have.
So who's right? Both are. When putting a circuit together--or voicing it afterwards--you've got three parameters to play with: Voltage, current, and the amount of heat the device can safely dissipate before self-destructing. It's a good idea to leave some elbow room on all three, but once you establish a region where your circuit will operate reliably, you're free to adjust voltage against current and see how you like the results.
I put an Aleph-X on the bench and ran a relatively small signal through it--something like 1W. Then I ran the rail voltage from something pretty lean, perhaps 12V, on up to 20 or 25V. The distortion dropped dramatically. It's not hard to see why. If you look at the spec sheet for a MOSFET, you'll see that there are a number of things that change with voltage. As an example, the capacitance of the device drops, which makes it easier to drive, which in turn means that the front end has better control over what's going on. And, of course, the front end is behaving better itself, so it's a win-win proposition. The downside manifests itself in price (bigger transformer, caps, and heatsinks), and more heat (potential reliability problems and your air conditioner hates you). Just keep your wits about you and you'll do fine.

Grey

[Brian Donaldson]

It's funny I never thought about it, but voltage is the one parameter everyone considers static. Everyone is obsessed with hitting the exact rail voltage. I just didn't want to be under and loose potential output wattage.

When I first built my Alephs, I used some super meaty 60V ct 30A trannys (or something outrageous like that) that I found in the local surplus shop and fed them with a 22A variac. You know me, if there's a knob, I'll play with it. I also used pots for R21 and R19, and scaled up the output current sense resitors because my speakers are 4 ohm.

But I think Grey's right. Up the voltage and you decrease distortion and add headroom. (or you end up building a 250W amp instead of 100w and only use the first 100W)

This thinking makes me lead toward building my AlephX with bigger rails and slightly underbiased for its voltage, or maybe rely more on the Aleph ac current gain.

Oh, and thanks again Nelson.

[GRollins]

I'm not saying that high bias isn't effective. It is. The trick is not to allow yourself to be blinded to the 'other' parameter by group-think lemmings following a narrow-minded vision of what's important. As much of both as you can stand is the ticket. Just don't burn up your output devices.
Big heatsinks are a really good idea.
Or water cooling.

Grey

[winslow]

Water cooling and larger heatsinks is what I need for my car project.

I have been wondering this- what does bias effect the most sonically? Bass, highs, width? I'm toying with the idea of upping the bias on my Mini-A after I get more heatsink under it...but it would probably only go from 1.1A to 1.7A, and a ton more heat.

I would like to do a current source amp to play with on my horns in the car at some point.