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The SLB (Smooth Like Butter) Active Rect/CRC/Cap Mx Class A Power Supply GB

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#705
I apologize in advance if I am posting to the wrong thread.

I am working on a dual mono SLB / Alpha 20. I used Antek 22 volt 200 VA transformers (AS-2222) and built the SLBs according to the BOM. I haven't hooked up the audio boards to the power supplies yet, but did a sanity check of the SLBs to make sure everything is working correctly. I am getting something around 29.7 volts from the SLBs with no audio board load.

Does this seem normal?

Will I harm the Alpha 20 boards with this amount of over voltage?

Thanks
 
#708
Congratulations Aljordan! The SLB and the Alpha amps were meant to be no drama start up affairs. I remember the old days of white knuckle start up of some amps where you had to be ready to kill the power if smoke started coming out. Or you had to use 3 simultaneous DVMs and 3 trim pots to set offset and bias current.

Hope it’s playing sweet music now!
 
#709
xrk971 said:
Congratulations Aljordan! The SLB and the Alpha amps were meant to be no drama start up affairs. I remember the old days of white knuckle start up of some amps where you had to be ready to kill the power if smoke started coming out. Or you had to use 3 simultaneous DVMs and 3 trim pots to set offset and bias current.

Hope it’s playing sweet music now!
Click to expand...

Thanks X. It is playing sweet music, driven by your Yarra / Melbourne. This was the first time I built a dual mono amp. I am happy to say that through my 99 dB speakers, the left channel is dead quiet, and there is only a slight hum through the right channel. That is probably due to the crappy job I did soldering the input wire to those little posts (I don't have the proper plug to do it properly).

As a sanity check, all I did was first fire it up with a light bulb tester, then fired it up and left the amp on for thirty minutes to make sure there would be no unexpected surprises, then checked on the power supply that the + and - voltages for each board and channel were similar, and then checked to make sure there wasn't too much DC offset at the speaker posts. I am getting about 18 mV of DC offset, which didn't concern me.

Now for some questions:

1) I did read through the SLB thread and the Alpha 20 thread but that was at least a few months back. The thought of looking through the Alpha 20 thread again makes me want to cry, as that thing is a monster. Is there something more I should be checking on the SLB boards or the Alpha 20 boards to make sure they are functioning optimally?

2) You will notice in the picture that I didn't twist the DC power and ground lines from the power supply to the Alpha boards. Is there an advantage of doing so?

3) What the heck are those female adapters called that go into the audio inputs on the Alpha 20 boards? I'd like to order some to do it properly.

4) What do the pots adjust on the Alpha 20 boards?

Please forgive me for asking Alpha 20 questions in this thread.

Thanks,
Alan

Alpha20_SLB_DualMono.jpg
 
#711
Hi Aljordan,

1) I did read through the SLB thread and the Alpha 20 thread but that was at least a few months back. The thought of looking through the Alpha 20 thread again makes me want to cry, as that thing is a monster. Is there something more I should be checking on the SLB boards or the Alpha 20 boards to make sure they are functioning optimally?

Measure voltage across the big Source resistors on output and calculate bias current. Should be around 1.25 to 1.3A.

2) You will notice in the picture that I didn't twist the DC power and ground lines from the power supply to the Alpha boards. Is there an advantage of doing so?

Twisting it reduces emitted 60Hz/120Hz/180Hz EMI hum noise that might get picked up by other stuff like your input cables.

3) What the heck are those female adapters called that go into the audio inputs on the Alpha 20 boards? I'd like to order some to do it properly.

It’s in the BOM, but JPS64 typically specifies Molex KK 2-pin female shell and crimp connectors. You need a Molex crimp tool too.

4) What do the pots adjust on the Alpha 20 boards?

The LTP bias current and DC offset to certain extent. We realized after making the boards that you can skip all the pots and put fixed resistors there. One of the schematics in the Alpha GB thread is redlined to show what values to put where the pots are.
Click to expand...
 
#712
Do those rather long wires between the pcbs and the power fets on the heatsinks create any loss of filter performance, stability or increased noise levels?

It looks rather compromised from the point of view about optimizing the pcb layout and designing short tracks between components, etc.

… just asking.
 
#713
Those long flying leads are for the power pass-BJTs on the SLB cap Mx. as long as they don’t oscillate, the cap Mx can do its 1mV ripple job and no sonic effects are passed though to the amp.

In the Alpha Nirvana and ABBB that do have power MOSFETs remotely located, we have the snubber and gate stoppers mounted as close to the pins as possible. We have not see any detrimental effects of doing this.
 
#715
Hi X,

I assume you are talking about R131 from this schematic. On one channel I measure .584 volts across R131 and on the other channel I measure .583 volts. R131 in my build is 0R33 ohms. I don't measure any voltage drop across the other big power resistors (R132). I couldn't find the right size so I paralleled two 0.33R resistors at R132 to get me close to the value on the schematic.

I am probably screwing something up here because wouldn't that put my current at 1.77 amps across R131? (I really don't know anything about this stuff). I am using the really big DIY Audio Store chassis, and my heat sinks are not very warm.

My power supply, loaded, is putting out about 25.3 volts.
 
#718
Aljordan,

Congrats on a successful drama free power up!!
I’ve just a couple of tweaks you can try.
Turn the each SLB board 90° so they are perpendicular to the heat sinks. Doing so will position the SLB power output directly under the Alpha inputs for a very short distance your DC power wires need to travel. Also, that will enable you to better position the BJT’s on either side of the Alpha board (left and right).
Run the Alpha Board speaker return to one of the SLB ground fastons, then from SLB ground faston to the binding post. Do this for both channels, that should eliminate the faint hum.

Hope this helps, Enjoy!!!
 
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