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Voltage readings on Bal Line stage Main Board
Voltage readings on Bal Line stage Main Board
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Old 18th July 2005, 06:30 PM   #1
Zapped is offline Zapped
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Default Voltage readings on Bal Line stage Main Board

Finishing up construction of my Balanced Line Stage, I find to my dismay that Q1 and Q2 voltage readings on the Drain pin are at the full power supply level of 60vdc, not the 30vdc which Nelson's article cites. Gate readings are all zero, indicating continuity with ground. Source pin readings are 0vdc, rather than the -3.5vdc that the article cites.

I have been going back over the various resistors in the neighborhood of Q1 and Q2 to make sure I haven't screwed up there, but haven't found anything yet (it's hard to read those *&^% color code bands anyway---on my next project I am going with Vishay/Dale and their alphanumeric code to avoid any eyestrain and error).

Does anybody have an idea what might cause the 60 volt readings ?
I've checked orientation of the Q's, which appears to be correct.

Thanks for any help you can offer.
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Old 18th July 2005, 07:36 PM   #2
Alain Dupont is offline Alain Dupont  France
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Verify first the pins of the MosFets IRF610 are GDS in TO220AB version

Pins front viewed from left to right {Package up ; pins down}
Gate 1
Drain 2
Source 3
Drain 4 in upper copper part, so they must be insulated from the heatsink or support !

Are the 2 supply's +60 volts and -60 volts ?


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Old 18th July 2005, 07:58 PM   #3
Zapped is offline Zapped
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Alain: Thank you for responding. I will double check tonight (at work now), but am pretty sure the Q's are configured as you describe:

xxxxxxx IRF610
x x x
x x x
x x x As viewed from front (facing plastic)

G D S Aligned with GDS marks on silkscreen.

Yes, supply voltage is +-60. (At first I forgot that the supply pads were channel specific and I had supply voltage only on one channel; fixed that first. Then I realized my drain pin voltage was twice what it should be, and source pin voltage was not correct either.)
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Old 20th July 2005, 11:26 PM   #4
Zapped is offline Zapped
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Well I was mistaken about my mistakes. Although I have yet to find a resistor out of place, or other stuffing error, my voltages on Q1 and Q2 are actually:

gate to ground: 0 volts dc
drain to ground: -60 volts dc
source to ground: -60 volts dc

Not sure what to make of the fact that drain and source are at same potential and that its negative with respect to ground. Should I assume I've fried the transistors ? I was especially careful in storing and handling them, and tested and matched them all before putting them into the circuit. But I suppose I could have screwed up somewhere and failed to touch ground before picking up the stuffed board.

Maybe I should have placed this post in Cowanrg's thread, since he seemed to be having a somewhat similar set of difificulties. Sounds like he found a resistor error and solved his problem, however. Wish I could see a similar solution on my board.
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Old 20th July 2005, 11:31 PM   #5
Nelson Pass is offline Nelson Pass  United States
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Voltage readings on Bal Line stage Main Board
Since you are still seeing the 60V from Gate to Source, they
are not yet fried, but I wouldn't use them, as you can create
subtle Gate damage that doesn't manifest as failure for up
to 96 hours.

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Old 20th July 2005, 11:46 PM   #6
Zapped is offline Zapped
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Thank you for the advice, Mr. Pass. I will pull the Q's from the board and replace with a new tested and matched batch. (Think I'll upgrade the magnification factor on my reading glasses a little bit too, and have another, closer look at resistors in the neighborhood.)
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Old 25th July 2005, 07:42 PM   #7
Zapped is offline Zapped
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Well, FINALLY I got this thing up and running. I have created, found, and fixed more errors on this Balanced Line Stage project than on anything I have ever built. but last night I had MUSIC running through the thing into my ZenV4, driving my Magneplanars, and the sound was absolutely beautiful !! And that was just using my iPod as a source; haven't even listened to CD quality yet.

Had it cranked up as loud as I could go with my temporary 10K output attenuators; and that was pretty loud. My wife, whose first impulse in such circumstances with my other equipment is to say "can't you please turn it down a bit?" just listened in silent appreciation. So in turn, I must express my appreciation for Nelson's design and help.

For other constructors who might be as bumble- prone or inattentive as me, here is a list of what I did wrong and had to fix before I got it right.

1. transposed some R's on the power supply. Nelson's article provided the voltage reading expectations to solve that one.
2. set the rotational stops on my selector switch wrong so I was selecting poles that had nothing connected to them. Shalllcross switch design flexibility made that easy to fix by just moving the stops. On a sealed switch, you can't see the wiper contact, so I discovered the error using a continuity check with my meter.
3. Failed to realize that the drain on Q1 and Q2 makes its circuit connection mechanically through a screw to the big donut pads on the board. (incredibly stupid, I know, and right in front of my face). This may have been a factor in my crazy voltage reading on the Q's. Solved that when I took the first set of Q's out. Since I had stand up heatsinks instead of lay-on-the-board heatsinks, I just ran a jumper from the big donut pads to the drains of the Q's. Duh !
4. Didn't realize that the polarity of the output leads from the board is physically reversed from the input leads; i.e., they are not physically identical in configuration to the input leads

I am glad I persisted, however, because Nelson's ciruit is truly great sounding.

My next step is to wire up a 4 deck 5Kohm output attenuator using a Tech Labs switch and a gazillion resistors that I bought a while back, and then substitute that for the tempoary 2 10Kohm pots I am using now. I will provide some photos of the intermediary product soon, and some of the finished product when I get the Tech Labs attenuator finished and installed.

Basic design:

* 1 balanced XLR input
* 2 unbalanced phono jack inputs
* Metalman's suggested switching arrangement to ground the unused negative inputs using 4 decks of switching, 3 poles per deck. MANY THANKS to Metalman for posting this design that avoids the use of shorting pins on the XLR inputs when using unbalanced inputs instead.
* A special little auxilliary board with 4 10-turn precision pots at input, and 2 500 ohm miniture mil-spec pots at mid-circuit for gain. These 500 ohm pots have a little screw slot and are fitted flush to two holes on the front panel for occasional screwdriver adjustment as needed for gain or balance.
* A 4 deck stepped attenuator using 1% MF resistors and the Tech Labs switch. (pricey but beautiful; the switch was $135 new, plus R's, but it s military grade and guaranteed to 1 million revolutions of operating life, according to Tech Labs literature. Nickel silver contacts, etc. )
* Custom case built from approx. 1/4 inch aluminum sourced from Boeing Surplus.
* 1 balanced XLR output
* 1 unbalanced phono output

Again, thanks to the Master, Mr. Nelson Pass, for making this design available to us for our DIY fun
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