Low watt Aleph J

[jetbat]

Hi.
I have built the Aleph J from the First Watt schematic and it sounds amazing. The problem with that amp is it runs a little hot. In a cold room in the winter a couple inches off the floor, (around 55 to 60 degrees F) after an hour the heat sinks get hot enough that I can only keep my hand on them for 4 seconds. So in warmer temps it would be way too hot. I was thinking about larger heat sinks when I saw Gray's recipe for Aleph J X . He says to pick the output watts and select a power transformer based on that wattage. This would also affect the current used. So if I lowered the rail voltage I could make the amp run cooler.

I was wondering what the lower limit is for choosing the output watts. I am using high efficiency full range speakers and I do not need a lot of power. With the amp the way it is now I have the volume barely turned up to get loud volume. I would like to have only 5 watts per channel output. That would be plenty.

Would it be possible to bias the amp for 5 watts output with + -11V rails?

The figures I get from Gray's recipe for 5 watts are;
current and voltage
.8A
6.4V
target rail voltage
10.3V
transformer voltage
7.7V
transformer VA x2
33VA
double for stereo
66VA

So a 8-0-8V 80VA transformer would be a little higher voltage and have plenty of current, if this would be even possible.

Thank
Scott

[Nelson Pass]

You can pick your rails for the peak wattage you need (which
will be twice the rms figure), but you should allow an additional
5 volt margin. So for 5 watts, you need a 10 watt peak, which
means a +/-9 volt peak, so you should have at least +/-
14 volts, and a bias figure of at least 0.75 A - more if you
can get away with it.

[BobEllis]

An easy way to reduce the dissipation if you only need a few watts is to pull a pair of output devices. If you lift one leg of the Source resistors in one of the current source devices and one on the gain side along with a pair of the output current sense resistors. Then you could reinstall easily, should you want more power next winter.

Another option is to increase the power resistors (Source and current sense) from 0R47 to 0R68, which would reduce your bias ~20%.

[jetbat]

Thanks for the replies.

I ordered a transformer and I am going to try it with +/-14V rails. Until that arrives next week I am going to experiment with pulling the pairs from each channel. I will compare both to the way the amp sounds now.

Scott

[Bill Fuss]

You might also try lowering R27 to decrease the overall bias. The Aleph J is biased pretty high by Aleph standards. Stick a 100k trimmer in there if you like, preset it to around 50k.

[woody]

If you have a thick aluminum face plate on your chassis you might use it as an extra heatsink and put a couple of mosfet cap. multipliers there. That would lose you about 4v and then just turn the bias down a little. Sure would be cheaper than another transformer.

[jetbat]

BobEllis, I tried lifting those resistors but when I powered the amp, all I got was static/hiss and the speakers pushed out. Put them back everything still works. So just to make sure I lifted the right resistors, they are R17,R19,R20,and R21?

Bill Fuss, to adjust the bias on that side requires adjusting only R27? You would not have to adjust R26 or R25?

[Bill Fuss]

You obviously lifted the wrong resistors. If you lift R16 instead of R17 the whole thing will go to hell. Never do anything like this with spkrs hooked up. Always look for output offset after any adjustment or modification with a meter. You are lucky your voice coils are still intact, but they could still be damaged severely.

R27 is the bias trim resistor. Without it the amp will bias to around .63V across R16, which is around 1.3A per fet. The lower it is the less bias current you will have.

[BobEllis]

Those are the ones to pull on the schematic I have. I guess you should pull the devices instead, or as Bill points out you may have pulled R16 instead of R17.

You can try tacking something smaller across R27 - Another 68K or something less should take a sizeable chunk out of the bias. Based on the Aleph 30 Schematic with voltages that someone posted, bringing R27 down to 26K ought to get you around .4A per output device, right around your initial target. You won't need to drop the rail voltage unless you really want to.

[jetbat]

Before I lifted the resistors I double checked the schematic, layout for the pcb and followed the traces on the pcb to make sure they were numbered correctly. So I know I lifted the correct resistors.

Could I use for testing a 1000uF 35V electrolytic capacitor between the positive output of the amp and the speaker that could be removed after checking for DC offset? Or would there be an advantage to just getting some power resistors. I saw some 220W 6.8Ohm on bay.

I'm going to look into adjusting the bias and see how that sounds.