Greetings. I've run into a problem with the amp board power up (everything up through the power supply building and testing has gone fine). In a nutshell, I can't get more than about 68mV of bias, well short of the 500mV the build guide calls for. I have the standard 5.1V Zener diodes installed, and can certainly obtain some higher value Zeners, but that reading seems awfully low. I wonder if I should be looking someplace else for the source of the problem.
I'm very much a noob when it comes to building amps, though I have put together three of the Amp Camp Amps up to now. The main difference here is that I sourced all the parts myself for this build since the store was out of the kits. I thought I followed the BOM very closely, though it's possible I sourced an incorrect part. I'm going to go back and check my parts against the BOM now as a first step.
Any help is greatly appreciated,
J
I'm very much a noob when it comes to building amps, though I have put together three of the Amp Camp Amps up to now. The main difference here is that I sourced all the parts myself for this build since the store was out of the kits. I thought I followed the BOM very closely, though it's possible I sourced an incorrect part. I'm going to go back and check my parts against the BOM now as a first step.
Any help is greatly appreciated,
J
there is 3K3 and 5V6 (up to 6V8, if you have those) mod
obviously you need that
you can also search for picodumb mod, or simply use 3 series leds, cathodes pointing down
red or green
obviously you need that
you can also search for picodumb mod, or simply use 3 series leds, cathodes pointing down
red or green
Hi J -
Before swapping out parts, could you confirm a few things, please. I interpreted from your posts in the PSU thread and above that you may want to "just get it working" in the most simple manner before going too far.
1) Did you determine "which way" the pot turns to increase your bias? CW is not necessarily an increase in bias current. Note from page 1, patience is a virtue. Generally, it can take quite a few "full" turns to get the current to start flowing, and then just a wee bit of a turn can move things more than you may expect. As an analogy, you may have been starting from a point where the faucet was off (no current was flowing through the part) - then it took what may have been perceived to be A LOT of turns to get a reading. The valve is now open. Then, each quarter turn might move things more than you may expect to gradually increase the flow (current).
2) If you've figured out #1, is the bias pot turned all the way "to the click"? Most multiturn pots will have a (to me) very subtle click when the wiper has reached its end point of travel in either direction.
If neither 1 or 2 are true, then I wouldn't suggest swapping parts just yet.
If 1 and 2 are true, then I'd suggest simply changing out the part to a 6V1 Zener, and my guess is that all will be well. HOWEVER, what I don't think anyone has mentioned yet is that if you need to go that route...
Turn the bias pot back FULLY (until it clicks) the opposite way (lower the bias) before powering up the amp again after your part swap.
Before swapping out parts, could you confirm a few things, please. I interpreted from your posts in the PSU thread and above that you may want to "just get it working" in the most simple manner before going too far.
1) Did you determine "which way" the pot turns to increase your bias? CW is not necessarily an increase in bias current. Note from page 1, patience is a virtue. Generally, it can take quite a few "full" turns to get the current to start flowing, and then just a wee bit of a turn can move things more than you may expect. As an analogy, you may have been starting from a point where the faucet was off (no current was flowing through the part) - then it took what may have been perceived to be A LOT of turns to get a reading. The valve is now open. Then, each quarter turn might move things more than you may expect to gradually increase the flow (current).
2) If you've figured out #1, is the bias pot turned all the way "to the click"? Most multiturn pots will have a (to me) very subtle click when the wiper has reached its end point of travel in either direction.
If neither 1 or 2 are true, then I wouldn't suggest swapping parts just yet.
If 1 and 2 are true, then I'd suggest simply changing out the part to a 6V1 Zener, and my guess is that all will be well. HOWEVER, what I don't think anyone has mentioned yet is that if you need to go that route...
Turn the bias pot back FULLY (until it clicks) the opposite way (lower the bias) before powering up the amp again after your part swap.
I've ran into a problem biasing the board on the right hand side. I am not getting any readings from .47R resistor but i am seeing DC offset measurements. Mosfets are not heating up. I reflowed all the components and still the issue persists. Left channel biased just fine. I am using 6.8 zeners with 3k3 r7,8. I'm getting around +-26V from PSU. I tested jfets before installing and thought I was careful to install in the correct position. I set both variable resistors to 2500 midpoint before installing. I am using a Variac and when testing both channels, left is working and right is cold. Nothing seems to be burning. I'll include some pictures. Some of my wiring colors are off but i tried to be consistent. Appreciate the communities help! Thank you in advance.
At the midpoint on the pots your Vgs voltages are still lower enough that the mosfets may or may not turn on. When they are fully in, they should be in the 4V range.
If you have dc offset and you have no voltage across the 0.47 resistor then the other mosfet is already starting to turn on and you should measure voltage across the 0.56R resistor. Try turning the 'bias' pot to get some voltage across the 0.47R which should begin to lower the dc offset. If that works, then alternately adjust the pots to increase bias current while keeping DC offset low.
If you have dc offset and you have no voltage across the 0.47 resistor then the other mosfet is already starting to turn on and you should measure voltage across the 0.56R resistor. Try turning the 'bias' pot to get some voltage across the 0.47R which should begin to lower the dc offset. If that works, then alternately adjust the pots to increase bias current while keeping DC offset low.
Thank you Dennis for the quick reply.
You were right. I had to crank the bias several turns before I started to see a reading. At first I measured .57 resistor but it was at 0 but after several good turns everything seems to be back on track. I appreciate your help.
You were right. I had to crank the bias several turns before I started to see a reading. At first I measured .57 resistor but it was at 0 but after several good turns everything seems to be back on track. I appreciate your help.
So I've set bias at 500mV on both channels and the offset as close to 0. Offset is steady on both channels but when i have a multimeter on each channel i am seeing one channel steady at 500mV and the other oscillating between 300 and 500mV. Then after some time goes by, the channel that was oscillating is steady and the other oscillates. Sometimes both are steady. Temperature of heatsinks have been below 55C and everything seems to be going well.. is this something I should be concerned about?
So i shorted the RCA inputs. I checked all the zeners and they are all around 6.74V (these are 6.8V zeners so within acceptable range).
I did observe one channel oscillating again and I saw the zener oscillate too. I have a suspicion the problem is with the toroid but i don't know how to make sure. This is an Antek AS-3218 and it shipped with a dent in it. Everything seemed to work fine so I didn't question it... Could it be possible that it is faulty? How would I verify it? I am assuming that these voltage drops are not normal.
Hmmm, actually, i am noticing that i have it plugged into an isolation transformer with a 600va toroid, perhaps that is the bottle neck, will plug directly to mains and test
I did observe one channel oscillating again and I saw the zener oscillate too. I have a suspicion the problem is with the toroid but i don't know how to make sure. This is an Antek AS-3218 and it shipped with a dent in it. Everything seemed to work fine so I didn't question it... Could it be possible that it is faulty? How would I verify it? I am assuming that these voltage drops are not normal.
Hmmm, actually, i am noticing that i have it plugged into an isolation transformer with a 600va toroid, perhaps that is the bottle neck, will plug directly to mains and test
Ok, it's plugged straight to mains, i have a voltmeter on the rectifier bridge and I'm getting a reading of 21-22V and it seems stable during the voltage drops. So perhaps it was naive of me to think it was the toroid due to the PSU cap bank i now doubt that the toroid could be the issue.. i started it up again measuring the output from PSU and this time everything is running smoothly so far but i am still monitoring. I'm still a bit concerned and will report back if i witness anything.
Should my expectation be that if the unit is on for 8+ hours that the voltage on .47R is stable or does it fluctuate +-20mV ok? Offset also fluctuates a bit. One channel seems more steady than the other. When starting from cold one channel lags the other and experiences voltage drops, eventually both become more or less steady. Cause for concern?
Fluctuation of 20 mV over 8 hours is expected, but depends on how long after startup the first measurement was taken. With zeners you will experience changes based on ambient temperature.
Voltage drops however are not good. Check the soldering of the zener and both 10k (or 3.3k) resistors for that channel.
Voltage drops however are not good. Check the soldering of the zener and both 10k (or 3.3k) resistors for that channel.
Alright, parts on order. I will replace the zeners and resistors. I checked and I'm using 1.3W zeners and will replace with 1W. Will also replace the 3k3R resistors for good measure and report back next week, thank you.
zener wattage is irrelevant, as long is sufficient
in this case , 500mW ones are good enough
more powerful ones are just going to be colder (somewhat) due to bigger body/slightly better dissipation of heat
now, even that is irrelevant, few much bigger heat points ( thus changes in operative values) being present in system
in this case , 500mW ones are good enough
more powerful ones are just going to be colder (somewhat) due to bigger body/slightly better dissipation of heat
now, even that is irrelevant, few much bigger heat points ( thus changes in operative values) being present in system
That's good to know zen mod, thank you.
I'm hoping the issue is with the zeners and resistors, could voltage regulation issues be related to something else?
I'm hoping the issue is with the zeners and resistors, could voltage regulation issues be related to something else?
Do you have the top cover in place on your amp? Is your amp in a place where there are temperature fluctuations or air currents? Changes in temperature of components, especially JFETs, caused by air currents or ambient temperature changes will cause fluctuations in current (source resistor voltage) and offset.
I do have the top cover in place. I had a fan overhead in my latest tests, which may have produced those results but only in one channel. I'll test again without the fan on. I was initially testing in a non climate controlled garage workshop - I'm in los Angeles and even summer nights are quite warm, but I've only seen one channel display these intermittent fluctuations. I'll try reflowing the solder on all connections and testing again before the new components arrive. Thank you so much.