Setting the bias on an ESP P101

[still4given]

HI guys,

On another thread, setting the bias on my P101 came up. It was suggested that I measure across the souce resistors and set the bias by adjusting to .09V. The P101 that I built is a three channels. I had a problem with one channel when I put it together due to a errant drop of solder causing a bridge across two diodes which took out the VAS transistor. I replaced the transistor and corrected the solder bridge. One of the output MOSFETs read a little different than the others but I only had continuity from source to drain so I went ahead and installed it and the channel seemed to play fine.

Well, I said all of that to say this. When I took readings on the source resistors last night to try and set the bias, I noticed that one of the source resistors had a 0.00V reading. all the others showed a slight Voltage reading and I was able to adjust them to .09V.

Is the 0.00V reading an indication that the associated MOSFET is bad? The channel sounds OK. Is it possible for the channel to sound OK with one bad MOSFET?

Thanks, Terry

[AndrewT]

Hi,
Can't solve this problem but offer some advice to help next time.
ESP does not fit protection on the VAS. It is only one to92 transistor and 2 resistors. You could retro fit it, either hard wired or on a tiny piece of stripboard. Each time you overload the output you risk losing VAS and outputs again.
regards Andrew T.

[Tekko]

Sounds like your mosfet device isen´t letting any voltage at all through, change it and try again and see if its the same.

[stevebeccue]

Originally posted by still4given
HI guys,


Well, I said all of that to say this. When I took readings on the source resistors last night to try and set the bias, I noticed that one of the source resistors had a 0.00V reading. all the others showed a slight Voltage reading and I was able to adjust them to .09V.

Is the 0.00V reading an indication that the associated MOSFET is bad? The channel sounds OK. Is it possible for the channel to sound OK with one bad MOSFET?

Thanks, Terry

It is possible the MOSFET is bad. (Just remove the other and
see if it still biases up.) Even if it is good though, you don't want it switching on and off, in class B. You would like the two MOSFETS well enough matched that both are on at idle so the amp remains class AB.

I just finished three off the P101 amps. I hope you are as pleased with your result as I am.....

Steve

[still4given]

I had ordered replacements for this board when I first had trouble with it. I just received them last week. I was hoping I wouldn't need them and would keep them around for something else. I will stop being a cheapskate and install them.

I have been using that channel for my subwoofer so I haven't really noticed if it was working equally with the other two.

So far I am very pleased with how it sounds. It is better than my Haflers and maybe not quite as good as my Soundcraftsmen. It is as detailed as the SC but a little less bass athority. That is why I thought I would try the higher bias.

By the way, did you use Rod's method for setting the bias? If so, what was the voltage reading across the resistors when you had the pots backed all the way down? Mine was about .67. I set it to 2.67.

How's the lower end on yours? Mine is very detailed but not quite as strong and my Soundcraftsmen S-860.

Also, what rail voltage are you running on yours?

Thanks, Terry

[stevebeccue]

I did set it up as Rod suggested, monitoring current. I did
deviate from his schematic values though, and made the
source resistors quite small (.1 ohm). I then check the
cutoff voltage on the MOSFETS (I had 32 of them) and matched
them on each channel. I biased it up, and then just checked
there was some voltage across the source resistors. I never
even checked the pot value..... I can check this accurately
when I get home if you have an interest.

The bass on my amp seems quite good. Although I will have to move the "system" from the garage to the living room for a critical listen; I'm sure the garage floor will have a MUCH larger effect than the flattness of the amp....

The amps set up for 8 ohms have +-74 volt rails. The amps set up for 4 ohms (my kids...) have +- 67 volt rails. I have added thermal shutdown, sensed on the heat spreader, to the power supply...... (kids you know...)

Steve

[still4given]

Hi Steve,

Thanks for the reply. I don't mean to say the amp has weak lows, because I don't believe it does. It just isn't quite as strong as my Soundcraftsmen.

I'm using a 45-0-45 800vA toroid so I'm only at about 64VDC rails. I used 8 each, 80v 15000uf filter caps though so I was hoping for a little more authority in the low end. It's nothing that can't be taken care of with EQ. It runs the sub fine.

Thanks again, Terry

[jacco vermeulen]

Hi Terry,

the best thing to do after soldering is to check the pcb with a magnifying glass, and clean it.
I may be nearing my midlife crisis soon, but even with my first stuff i had a hard time seeing if nothing was on a pcb that did not belong there.

I blew a couple of Mosfets by touching them, seems i have a natural talent for collecting statics.
Nowadays i use a digital Weller soldering device that is grounded, the connecting grounding collar attached to my wrist.
On top of that i have my feet on a special non-static rubbery kind a rug while i solder.
And i never ever touch Mosfets with my bare hands again, i handle them with plastic plyers.

You could check for a bad soldering joint, cold joints often cause bad connections at low voltages.
With proper signal voltages on them a cold joint on a Mosfet may
function, but with very low voltages not, 0.09 volts is kind of low.

I hope you can get the problem fixed, shame of such a nice amplifier.

[still4given]

Well as far as the solder bridge goes, I had tested the boards before I installed them on the heatsink with low voltage first and then rail voltage. I even set the bias while they were off the heatsinks. The solder bridge happened when I was installing everything in the case. A drip must have fallen off the soldering iron sometime during that process. It landed on the front of the board between two diodes where it really couldn't be seen without pulling the board out of the case. Just a little bad luck and maybe a little carelessness.

Tonight I pulled the board out and retested all the MOSFETs. They all read the same. Even the same as all those on the other two boards. I'm still only reading 1.0 mV on that one particular source resistor while the others are reading approx 9.0 each. I put it back together the way it was and did quite a bit of listening while switching back and forth between the three channels. I can't hear any difference between them. The channel with the low reading on the resistor also checks a little low at the output but only slightly. It read 7.6mV and the other two read 10.3mV each. I think I'm going to just leave well enough alone. It sounds great.

Blessings, Terry

[gearheaddruid]

Hi Still, Have you swapped the mosfets around and measured the voltage across the resistors? Does the low voltage stay on the same resistor? If so, I would be double checking the value of said resistor. If it moves with the mosfet, you have a weak device.
Here is a trick I use to check the sharing of current among outputs: run a tone into the amp at about a 10-15 watt level. ( This is easier with a dummy load, if not make it a lower frequency so you do not drive neighbors, dogs, wife, ect, crazy) While the amp is operating measure the A.C. voltage across the resistors. I try to get them all within 10 percent. I then check again at higher power levels.

Hope this helps. Regards, Steve