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
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
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.
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.
still4given said:
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
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
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
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
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
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
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
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.
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.
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
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
No voltage on Source resistor.
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
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
Hi Steve,
No, I didn't move anything around. I did measure the the resistor and it reads the same as all the rest. It's very weird. Nothing gets hot or anything, as a matter of fact, the heatsinks barely get warm. I fed the same signal to all three channels and used banana plugs on a speaker and switched back and forth between the three channels and couldn't hear any difference. If I had a scope maybe something would show up but I can't hear it.
For now, I'm satisfied.
Thanks for the help,
Terry
No, I didn't move anything around. I did measure the the resistor and it reads the same as all the rest. It's very weird. Nothing gets hot or anything, as a matter of fact, the heatsinks barely get warm. I fed the same signal to all three channels and used banana plugs on a speaker and switched back and forth between the three channels and couldn't hear any difference. If I had a scope maybe something would show up but I can't hear it.
For now, I'm satisfied.
Thanks for the help,
Terry
Hi,
I am interested in your comment re bass cf Soundcraftsman. Could the soundcraftsman have a bit of bass lift or has it got a higher input impedance? I'm looking for excuses because I want to believe that the dual LTP has good bass but I fear others have reported a similar view to yours.
Secondly you report 8 * 15mF in your amp. Is this total for all three channels or per channel or per rail or (now getting silly) per rail and per channel?
I have a little 110w into 8R amp that has 21mF per rail per channel and that is a big bass improvement over 10mF.
Yours has a much higher current capacity.
regards Andrew T.
I am interested in your comment re bass cf Soundcraftsman. Could the soundcraftsman have a bit of bass lift or has it got a higher input impedance? I'm looking for excuses because I want to believe that the dual LTP has good bass but I fear others have reported a similar view to yours.
Secondly you report 8 * 15mF in your amp. Is this total for all three channels or per channel or per rail or (now getting silly) per rail and per channel?
I have a little 110w into 8R amp that has 21mF per rail per channel and that is a big bass improvement over 10mF.
Yours has a much higher current capacity.
regards Andrew T.
still4given said:
Terry,
you can download small scope programs for free that allows you to use your pc and microphone entrance of the soundcard as a simple single channel oscilloscope.
If you have a way of generating a sinoidal signal for the entrance of your amplifier, with help of your computer and such a program you are able to check the output signal.
jacco
AndrewT said:
I'm not sure what you mean by protection on the VAS, do you mean output current limit implemented by gate voltage limiting? This is implemented at least in a short term fashion by the gate-source diodes on the output devices.
Even without this protection, if you short the output and manage to blow the output devices the VAS will get away unscathed. This is because MOSFETs -- unlike BJTs -- the gate is isolated from the rest of the device.
Hi all
re post 12 & 14;ignore my bass response request. I got my threads mixed up SORRY.
VAS protection is a 2 transistor ccs wrapped around the VAS transistor. One extra transistor and an emitter resistor.
I think VI limiting applied to BJT is exactly equivalent when applied to FET output, because it is driver drive limiting that is applied.
When VI limiting is applied across the drivers to limit current in the output it effectively shorts the driver base connection to the output connection. All the current feeding through here comes from the VAS. The VAS will be running hi current and hi voltage.
But the VAS current varies either side of the preset bias and in extremes could range fron zero current to two times bias under normal operating condition. It follows that the VAS protection should limit at 2.2 to 3 times bias current to allow normal driving into low loads. This is easily achieved by splitting the emitter resistor say 40% and 60% and detecting volts drop on the 40% end nearest the Vrail. The current sink using ccs automatically limits itself to bias current combined with hi voltage and should be designed to survice a short circuit with hi input signal.
If all that is unclear look up Leach (low tim), he shows the circuit modification and describes the operation. It is commomly adopted in many designs (I have it in all my Crimson).
regards Andrew T.
re post 12 & 14;ignore my bass response request. I got my threads mixed up SORRY.
VAS protection is a 2 transistor ccs wrapped around the VAS transistor. One extra transistor and an emitter resistor.
I think VI limiting applied to BJT is exactly equivalent when applied to FET output, because it is driver drive limiting that is applied.
When VI limiting is applied across the drivers to limit current in the output it effectively shorts the driver base connection to the output connection. All the current feeding through here comes from the VAS. The VAS will be running hi current and hi voltage.
But the VAS current varies either side of the preset bias and in extremes could range fron zero current to two times bias under normal operating condition. It follows that the VAS protection should limit at 2.2 to 3 times bias current to allow normal driving into low loads. This is easily achieved by splitting the emitter resistor say 40% and 60% and detecting volts drop on the 40% end nearest the Vrail. The current sink using ccs automatically limits itself to bias current combined with hi voltage and should be designed to survice a short circuit with hi input signal.
If all that is unclear look up Leach (low tim), he shows the circuit modification and describes the operation. It is commomly adopted in many designs (I have it in all my Crimson).
regards Andrew T.
Steve, I know this is an old thread, but did you ever check the Resistance of VR1 on ESP project 101. Have a hard time and would help.
Thanks