Aleph A30 - troubleshooting tips

[Stabist]

Originally posted by mpmarino

I am building an 'a30' with a 'a3' output section using brians boards. In other words, The a30 will be short 2 OP fets per channel . Do I need to mod anything in the front end or can I run the front end per the a30 schematic? I am thinking I can, as long as I am willing to play with r13brian(r19pass).

Thanks in advance,
Marc

I have in memory that A3 has 125W dissipiation/channel and A30 "only" 100W ...
So I think you will not gain lower heatsink temp with that ...
But if you are in it because of higher bias per device - then that's something else ...

[fcel]

I'm going to change subject here and ask a couple of questions on the mini-A amp. This is based on the signal and power supply boards by BrianGT.

All test performed with 75W light bulb connected in series on the primary side of transformer.

Sitiation:
1) Each channel is functional if the other channel is not connected to the shared power supply board (with CRC). Output DC is about 30mV and there is no hum. Light bulb is very dim.
2) With both channels connected to the shared power supply board, DC offset is about 150mV and there is a loud hum. Light bulb is still kind of dim.
3) For both of the above situation, all other measurements are about the same, except for the DC output which is high.

Questions:
Before I remove the light bulb, I wanted to get some feedback from this forum. Would the DC output comes down and the hum go away after the light bulb is being removed? I think I have tried all possible way of wiring the power supply ground wire but it doesn't make any difference. May be this is not a grounding issue?

Note:
The rail voltage is plus and minus 16V and I'm using IRFP240.

[fcel]

No response?

I guess everybody did the start-up one channel at a time and then power up both channels when each individual channel are confirmed working properly. Anyway, here's my update ....

I took the plunge and power up both channels WITHOUT the light bulb. Both channel DC output drop down to earth level and the loud hum is gone. My mini-A is in fact working properly. Have played music through it for a couple hours already.

I'm still curious though ...
1) Why such a high DC output and loud hum when a light bulb is connected in series on the primary side? Voltage measurement at pertainment points (as per Pass schematic) are right on the spot except for the DC output, which is approx. 4 to 5 times higher when a light bulb is connected.
2) And where is this loud hum coming from when a light bulb is connected in series?

[jleaman]

Are you saying you get no or you do get hum when light bulb is connected in series ?

My mini a's hummed at first then i changed cables and volume pot.. I'm running every thing in passive mode with my tube buffer from the dac and i had some humage lastnight but stupid me put the volume pot right ontop of my cd player to witch has a SMPS inside well i figured that out asap..

[fcel]

I get:
1) Loud hum
2) High DC output on both channel (1.5V)
ONLY when BOTH channels are connected to the shared power supply and WITH the light bulb connected in series on the primary side.

As described in my previous post, I have found out that the amp is actually working properly. I was curious as to why it's doing what it does- again, as described in my previous two posts.

[SteveA]

I bought 4 sets of the Mini/A30 boards from BrianGT. I stuffed one set as Mini's and three sets as Aleph 30's. I finished an A5 about a year ago where I learned (courtesy BDP) how to make the voltage measurements prior to installing the output boards/devices.

On all eight boards I see the ~9V drop across Z5, ~4.1Vgs on Q3, ~3.9Vgs on Q1,Q2. On seven boards I see ~4V drop across the 392R resistor (R18), the last board (an A30) I measure a drop of ~2.7V. Seven of the boards would seem to be good to go. Is this too low (ie, do I need to chase down the offending part(s))?

This resistor "sits" between the -V supply and the drain of Q2 and would seem to define the voltage drop across that resistor (I measured 2.7 volts between these points as well). What would cause the drain to be 1.3-2.3V too negative?

Thanks,
SteveA