Aleph 2 HELP

I am building a pair of Aleph 2s and upon power up ran into major problems. After about 20-30 minutes, POP, the Source pin on one of the Q6-11 current source MOSFETs fried!

Prior to the POP, I ramped up the voltage using a VARIAC with no smoke. I plug a tape deck into the input and a 30yr old speaker in the output, but I heard no music. At one point during the ramp of voltage the speaker produced a high pitch whine.

At full voltage, my rails were about +/- 46V. It was at full voltage for about 20-30 minutes before the POP, but producing no music. I probed a few spots and voltage drops seemed fine. At the end before POP, the current source heatsinks were noticeably warmer than those of the output MOSFETs (although they were barely warm).

Since the incident I have re-check my outboard power supply (it is working fine). I tried powering up at low VARIAC voltages (with the one blown MOSFET), but with immediate power supply hum and blown fuse.

I have powered at low VARIAC voltages the other amp and have done limited probing. Here's my results, with the VARIAC dial at about 27 VAC:
1) Voltage rails at +/- 10.1 VDC.
2) Voltage drop over 4.75K R17 is 7.4 volts.
3) Voltage drop over Z5 is 8.7 volts.
4) Voltage drop over 221ohm R11 is 4.9 volts.
5) There is NO voltage drop over the 1ohm R40-45.
6) There is NO voltage drop over the 1ohm R46-51.

I have made a few minor variations to the basic Aleph 2:
1) Power supply is outboard.
2) Power supply has a Pi filter (8 - 24,000uF caps, with 2 - 2mH air-core 16ga inductors per channel).
3) Each power supply cap is by-passed with a 220uF .
4) Signal boards are wired point to point on a sheet of teflon.
5) The 220uF capacitor C9 is connected to the emitter of the bipolar, instead of to the OUT+.
6) ZTX-450 bipolars are used instead of MPSA18.
7) All 220uF electrolytic capacitors are by-passed with a 1uF polyester film caps.

Does anyone have any ideas what is wrong?

At what minimum power supply rail level should the amp work and minimize blowing the other amp, while checking?

Thanks!
 
Markl.,

the mods you've made can't be the cause of your problem.

I would check and recheck the wiring from the main board to the fets. If there's no current going through them something must be wrong with your current source.

7.4 volt is way to much over 4k75 at 10 volts. Are you shure the transistor is the right way around?

william
 
Correction ...

My mistake, I should have said C7 instead of C8 from my previous post. What did you use for C7? 0.001uf or 0.01uf? If I'm not mistaken, if you have C7 connected across B & C of the bipolar, the value should be 0.001uf. (somebody please back check me!).

This may not be the problem. I'm just trying to see if I can help.
 
C7 is a 0.001uF Orange Drop across b-c of the bipolar.

Q5, the ZTX-450, is wired with the collector connected to the 1.5K ohm R18 and the 0.001uF C7; base to the 0.001uF C7, 681 ohm R21, and 1K ohm R20; and the emitter to the 0.47 ohm R22-27, the negative side of the 220uF C9, the 1.0 ohm R40-45, and the Drain of Q12-17.

My assumption on the pin-out of the ZTX-450 (viewed from the top) with the flat side on the bottom and curved side on the top is (from left to right): e, b, c.

Thanks!
 
The pin-out orientation of Q5 looks right to me.

You probably just overlooked and did not mentioned it in your post but do you have Q5 collector connected to R28-R33? Same for Q4 collector to R34-R39.

Also, double check the wiring position/connection of the G, D, S and sense on the output board for both Current Source and Gain Source. The "sense" line on the output board should go to the 1K resistor (R20) on the main board ... and work from there.

Did you make the board yourself?
 
At 1/3 power Vgs across all FETs should measure about 4V. (or it did on both my '4' and '5')

If this is not the case please do not apply full power .... something is wired incorrectly.

The amp will make music well below the rated rail voltage but you do not need to try this, just use your variac and a multimeter ..... if the voltages are not correct, don't try anything else.

I agree, pin-out of ZTX is as you describe. Something else is clearly wrong.

cheers, mark :confused:
 

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fcel:

The collector of Q5 DOES connect to R28-33, I just forgot to mention it.

I made the board myself. It is NOT a PCB, just a sheet of teflon with point to point wiring, using component leads when ever possible. When necessary solid silver wire (18 or 24 gauge) in teflon tubing was used. I made a graphic depiction of the board in Visio, before I wired the physical board.

Looking at your http://web.vip.hr/pcb-design.vip/aleph-2.html schematic showed some differences (other than MPSA18 and C9 connections), these are:
* My boards have the 1K ohm R20 connect to the Source of all current source Q6-11 MOSFETs, while your schematic has it connected to only Q6. Which is correct?
* My boards have the 221 ohm R15 connect to the Source of all gain stage Q12-17MOSFETs, while your schematic has it connected to only Q12. Which is correct?
* Are these the "sense" connections you referred to?
 
MarkL,

Yes, those are the "sense" line that I was talking about. I had a feeling that you have your output stage incorrectly wired.
Yes, the 1K and 221 ohm resistor are connected to the FIRST transistor only.
I guess in a couple of days, you'll also be a happy owner of a Aleph 2 clone! Keep us posted!
 
An email from NP, confirmed "fcel" assertion of only single connections from R20 and R15, NOT connections for every MOSFET. NP did not think that multiple connections would cause my blown MOSFET or other problems, but it would not be optimized. NP suspected a bad MOSFET.

I disconnected my extra "sense" connections and powered the non-blown amp to +/- 15VDC rails, but I still have problems:

1) Voltage rails at +/- 15.0 VDC.
2) Voltage drop over 4.75K R17 is 11.4 volts. (????)
3) Voltage drop over Z5 is 9.2 volts. (OK)
4) Voltage drop over 221ohm R11 is 5.0 volts. (OK)
5) There is NO voltage drop over the 1ohm R40-45. (PROBLEM)
6) There is NO voltage drop over the 1ohm R46-51. (PROBLEM)
7) Voltage at Q12-17 Drain is -0.1 volts.
8) Voltage at OUTPUT+ is -0.1 volts.
9) Voltage at Q5 Collector/current source 221 ohm R28-33 is +0.5 volts.
10) Voltage at current source "sense", 1K ohm R20, is -0.1 volts.
11) Voltage at gain stage"sense", 221 ohm R15, is +15.0 volts.
12) Voltage at C6/gain stage 221 ohm R34-39 is -12.2 volts.

Any other ideas? Suggestions for VOM probing?
 
No reading across R40-51 tells me that there is no return path for the current to get back to the main board and so I still suspect that there is some other problem on the output boards.

Double check the connections(locations) of G, D, S and sense of the output board to the connections (locations) of the G, D, S and sense of the main board -- for both the Current and Gain output board. The 2 output board connections should almost looks like mirror image of each other.

Since you have done P2P wiring, how about a picture showing the interconnection of the main and output boards. I'm not sure exactly how it can help but I always said a picture is worth a thousand words.
 
An email from NP, confirmed "fcel" assertion of only single connections from R20 and R15, NOT connections for every MOSFET. NP did not think that multiple connections would cause my blown MOSFET or other problems, but it would not be optimized. NP suspected a bad MOSFET.

I disconnected my extra "sense" connections and powered the non-blown amp to +/- 15VDC rails, but I still have problems:

1) Voltage rails at +/- 15.0 VDC.
2) Voltage drop over 4.75K R17 is 11.4 volts. (????)
3) Voltage drop over Z5 is 9.2 volts. (OK)
4) Voltage drop over 221ohm R11 is 5.0 volts. (OK)
5) There is NO voltage drop over the 1ohm R40-45. (PROBLEM)
6) There is NO voltage drop over the 1ohm R46-51. (PROBLEM)
7) Voltage at Q12-17 Drain is -0.1 volts.
8) Voltage at OUTPUT+ is -0.1 volts.
9) Voltage at Q5 Collector/current source 221 ohm R28-33 is +0.5 volts.
10) Voltage at current source "sense", 1K ohm R20, is -0.1 volts.
11) Voltage at gain stage"sense", 221 ohm R15, is +15.0 volts.
12) Voltage at C6/gain stage 221 ohm R34-39 is -12.2 volts.

Any other ideas? Suggestions for VOM probing?
 
I think you can just click on the icon (below) to email me. That's what I did to email you.

You should still try to post the picture on this thread - even though I myself was never successful in posting any pictures on this forum - because some members of this forum might be able to "see' what is the problem right away even if I don't.