Shorted power supply for Aleph 2

After follow this forum for 3 month I was decided to build a Aleph 2, by using the following part to build the power supply.
Toroid : 1000Va, 40-0-40V
Bridge : 35 amp 600V
Inductor : 5mh/15amp iron core
Cap : 46000UF/75V X2 Philip

Today I was able to start testing on it, but the sad story began.
My test sequence as follow:
1) Using 2 5mh/15A inductor connected to +/- supply rail (after the bridge) follow with 46K UF capacitor. (I was intent to lower the rail voltage). Without connect to load.
-„³ Result is: Everything is O.K. except rail voltage too low, only +/-38V with respect to ground

2) I decide to remove the inductor, test it with 46K UF capacitor and connect to 1 channel of my Aleph 2 clone,
-„³ Result is: Supply Voltage stable at +/-51V, and the offset voltage measure from output of the amplifier when power up was at ¡V19 V and drop to few mV after 2-3 second. Is it normal for A2?
-„³ I¡¦ve tested both channel separately in this way, and getting the same rail voltage and offset voltage.

3) Connect both channel together with the same power supply, and try to measure the rail voltage
-„³ Result is: initially the rail voltage and offset voltage of both channel is O.K., after a few second the fuse at primary (5amp) blow, why? After check found the bridge was blow, (-ve rail 2 diode was shorted) is it because of the surplus bridge was a defective part or the inrush limit fail. (I using 2 SG27 rated at 6A/10 Ohm, in each primary rail.)

4) After disconnect the load replace the fuse (still using 5amp), and a new bridge (same type), with only the 46K UF cap.
-„³ When power up immediately the fuse blows again, and found the ¡Vve rails 2 diode was shorted. Why? What is different from sequence no.1 is the inductor act as a soft start when powering up.

Few question:
1. What is the fuse rating (at primary of the toroidal) for Aleph 2, with 240V incoming.
2. Do I need to check all my mosfet especially the ¡VVe rail module Q12-Q17? Anything¡¦s else I need to check?
3. Do I need to increase the thermistor in the primary rail.


any help will appreciate

alex
 

leroy

Member
2001-10-31 10:52 pm
Let me give you my opinion:

1. Primary fuse value depends on type of transformer. The value should be about 1,25*Inominal. If you use a toroidal transformer make sure a high I^2t rated fuse (slow blow type) is used.

2. Better safe than sorry: yes, you'd better check all MOSFET's and see if one is shorting the power supply.

3. Using two 10Ohms thermistors give you a max. primary inrush current of 240/20=12Amps. On the secondary side this will be (worst case) (240/40)*12=72Amps. So you might want to increase the primary thermistors although I don't think 72Amps for a few cycles would destroy a 35Amp bridge.

At last: don't place inductors between bridge and caps, they decrease the cycle charge current of the caps resulting in a lower RMS voltage, like you experienced. (Using inductors in a PI-filter configuration is recommended).

leroy
 
The Aleph 2 will run quite happily with a +-38V rail, albeit with slightly reduced output power.
It will run at +-51V also, delivering slightly more than rated power but...watch out for device dissipation as it will increase accordingly. Depending on how much heatsink you have on hand, your output devices could run hot. If need be, you can reduce the bias a bit to bring the heat down to match whatever heatsinking you're using. Yes, reducing the bias will change the sound of the circuit, but if it's a question of reliability you'd be better off reducing the bias.

Grey
 
Thanks leroy for your help.
Regarding the thermistor I have found one serious mistake by connecting them in parallel to obtain more higher current rating, after checked the application notes (rhopoint components) found thermistor should not connect in parallel (most of voltage will only flow to one of the thermistor), and half the resistance(only 5 ohm per rail). According to your Cal. The bridge have to sustain 144Amps when startup.
I want try to using 2 bridge connect in parallel , can you advise any advantage of it.
Can you help to calculate the main fuse rating for me ? I was using 1000Va toroidal and rate at, primary 240V and secondary 2X40V,12.5Amp per winding.

Thanks Grey,
yes I have enough heatsink to run at higher bias (4 X 300mm P3 per channel) http://www.semikron.com/databook99/13heatsi/p3.pdf , but before that what adjustment I¡¯ve to make.(like R19, R21 ¡_??).
Is a 1000Va toroidal enough to power up 2 channel of A2

Alex
 

leroy

Member
2001-10-31 10:52 pm
alexmark,

Primary fuse should be 1,25*(1000/240) is about 5Amps slow blow type. I won't recommend a higher value. Connect the thermistors in series, in case primary fuse blows use more thermistors.
The secondary fuses should be rated slightly higher than the secondary bias current.