Those BPR resistors look similar to the Fukushima/Futaba MPC resistors that some builders have used as source/emitter resistors. I don't expect any issue with using them.
The only thing I would caution is that since the BPR are radial with 9mm pitch, they don't physically 'drop in' the R1/R2 spots on the PCB and you need to be careful bending the leads to fit them in.
Have fun.
The only thing I would caution is that since the BPR are radial with 9mm pitch, they don't physically 'drop in' the R1/R2 spots on the PCB and you need to be careful bending the leads to fit them in.
Have fun.
I've been banging my head against this problem, hope someone can point me in the right direction...
I've got one channel installed and am trying to set bias and offset. with bias set to 0 volts, I can get offset to zero no problems. However, once I set bias past around .28v, I can no longer decrease offset to zero. As I increase bias, the offset seems to hit a floor and will no longer decrease. There are plenty of counterclockwise turns available on P1, but turning has no affect on the Offset. This effect increases in severity as I increase bias. Once I set bias to around .5v, the offset is also around .5v and will not go down even as I make several turns counterclockwise with P1.
Can someone point me in the right direction on how to troubleshoot this? I was very careful to check all resistors when I ordered the parts, and I double checked each part as I installed everything. I am planning on installing the other channel circuit board this afternoon, just to see if it exhibits the same behavior.
Voltage across Z1 is 4.5v, Z2 is 5.9v. Q1 and Q2 read 4.5v. Temp on the MOSFETS is around 39c
Thanks in advance for any tips!
I've got one channel installed and am trying to set bias and offset. with bias set to 0 volts, I can get offset to zero no problems. However, once I set bias past around .28v, I can no longer decrease offset to zero. As I increase bias, the offset seems to hit a floor and will no longer decrease. There are plenty of counterclockwise turns available on P1, but turning has no affect on the Offset. This effect increases in severity as I increase bias. Once I set bias to around .5v, the offset is also around .5v and will not go down even as I make several turns counterclockwise with P1.
Can someone point me in the right direction on how to troubleshoot this? I was very careful to check all resistors when I ordered the parts, and I double checked each part as I installed everything. I am planning on installing the other channel circuit board this afternoon, just to see if it exhibits the same behavior.
Voltage across Z1 is 4.5v, Z2 is 5.9v. Q1 and Q2 read 4.5v. Temp on the MOSFETS is around 39c
Thanks in advance for any tips!
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check the voltage accross the 3w .56 resistor. That is the bias for the 2nd mosfet. The two JFETs are supposed to be zip tied or shrink wrapped so they share the same temp.
I will check that , thanks! The JFETS were zip tied together, i just removed it to double check their part numbers...
Voltage across the .56 resistor is .67v
Voltage across the .56 resistor is .67v
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@jbholman56
I’m assuming you’re giving plenty of time between adjusting pot labled bias and then adjusting the pot labled offset?
I’m assuming you’re giving plenty of time between adjusting pot labled bias and then adjusting the pot labled offset?
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I've let it sit for as long as 20 minutes with no change in this behavior... At low values of bias, the offset tracks fairly quickly (within 2-3 minutes it seems to settle down).
The 5.1V zener diodes, Z1 and Z2, set the maximum bias voltage to the mosfets. 4.5V across Z1 is too low, so it's best to replace it.
Just to be clear since it seems the term "bias voltage" is used on the forum to describe all sorts of things, I am referring the the voltage at the mosfet gate.
Just to be clear since it seems the term "bias voltage" is used on the forum to describe all sorts of things, I am referring the the voltage at the mosfet gate.
@jbholman56 : 4.5V for Z1 is kind of low even if it's a 5.1V zener. One possibility is that's it's current starved. Can you please answer ZM's earlier question about the resistor values you used for R7 and R8? (I'm referencing the schematics from page 1: https://www.diyaudio.com/archive/gallery/data/500/F6_DIY_SCH.gif ) Also, can you confirm the Z1 and Z2 values you used? (You mentioned a 5.9V measurement for Z2)
For an explanation for what Dennis is looking at, read post #27 by ZenMod few years back. The general recommendation has been to use a smaller value resistor for R7, R8; for example 3.3K ohms. If they are 10K ohms, then the zeners are current starved.
Direction of bias pot is immaterial as long as you know which direction to turn them. If you are watching the voltages on your DMM, you know which way to turn them 😉 .
Best,
Anand.
Direction of bias pot is immaterial as long as you know which direction to turn them. If you are watching the voltages on your DMM, you know which way to turn them 😉 .
Best,
Anand.
I will replace the zener with a known good 5.1v, as well as reverse the direction of P2. if that does not fix the issue, i will research the zenmod link provided and revisit those resistor values (I am using what was in the BOM). Thanks for all your suggestions, i will try this tomorrow morning and let you know the results...
Thanks again, Brad
Thanks again, Brad
Gonna chime in again in favor of using three small green LEDs instead of the 5.1V Zener. Their negative temperature coefficient lets the amp reach a stable temperature sooner, making it easier to set the final bias current and offset.
The LEDs also have better voltage regulation than most Zener diodes.
The LEDs also have better voltage regulation than most Zener diodes.
And they glow green, which honestly looks pretty neat on top of the benefits listed above.
😎
Don't risk messing up the pot just to turn it around, as Anand says, just twist the knob whatever way makes the reading respond.
😎
Don't risk messing up the pot just to turn it around, as Anand says, just twist the knob whatever way makes the reading respond.
Physically reversing the whole trimmer pot won't make a difference. Check the resistance across R7 and R8 as suggested.
You can use this calculator to see what resistors you can parallel with your 10k's to get the current that you need. For instance, a 10k and a 5k give you 3.33k. It doesn't have to be 3.3k on the dot. Just a good amount lower than 10k.
You can use this calculator to see what resistors you can parallel with your 10k's to get the current that you need. For instance, a 10k and a 5k give you 3.33k. It doesn't have to be 3.3k on the dot. Just a good amount lower than 10k.
Reducing the value of R7 is a good idea to increase the current, but I do not believe the zener is current starved if R7 is 10k. If the PS voltage is 23V and the zener is current starved (not conducting), the voltage across the 5k trimmer would be closer to (5k/15k)x23V=7.7V, not the measured 4.5V.
I think it is most likely the zener has a voltage tolerance of of 10% and the zener installed is at the lower end of the allowed tolerance. So replacing the zener is likely still required.
I think it is most likely the zener has a voltage tolerance of of 10% and the zener installed is at the lower end of the allowed tolerance. So replacing the zener is likely still required.
I replaced the zener, and am now good to go! I measured the replacement zener before installing, and meant to post measurements on both the old and new, but I cannot find the old one. If I can find the old one I will measure it and post what I find out.
I will read up on the R7 issues you mentioned before proceeding with installing the second channel, in case I want to try those suggestions.
Many thanks to everyone! Brad
I will read up on the R7 issues you mentioned before proceeding with installing the second channel, in case I want to try those suggestions.
Many thanks to everyone! Brad
New 3.3k resistors and 6.8v zeners are on order. One real plus for me is that Mouser is in the Dallas/Fort Worth area, so I can get parts two days via USPS, and sometimes the next day!