If for some reason, the output sticks at one voltage, while there is input, often the NTP will draw several hundred mA trying to correct it. A resistor between the emitters of the NTP has been suggested to help with this, but I like this idea better. Using a Jfet here is a more complete solution. The 2SJ74 here limits the NTP current to 10mA. This should also help keep the transistors from heating up during overload moments and causing memory distortion. Of course, it's best to avoid overdriving in any case.
R17 and R18 should be calculated for the closest match the DC impedance of the Jfet at whatever quiescent current you operate the NTP at. Here it is about 2mA, and according to simulation the Jfet impedance is about 26 ohms.
- keantoken
R17 and R18 should be calculated for the closest match the DC impedance of the Jfet at whatever quiescent current you operate the NTP at. Here it is about 2mA, and according to simulation the Jfet impedance is about 26 ohms.
- keantoken
Attachments
Also, I've been looking into PCB ordering ordering options. OurPCB was recommended to me, and they will send you a first board to test and verify. 3 ten square inch boards go for 54$, but I could buy 10 for 64$. Two channels of this amp will fit on a single 2.5"x4" board, with room to spare for signal controls.
I haven't gotten around to further troubleshooting yet, since there have been other things I needed done first. However I'm worried by the long wire between output stage and input stage grounds. I simulated several nH there and there was oscillation. This is something I will definitely remember for my next PCB session
- keantoken
I haven't gotten around to further troubleshooting yet, since there have been other things I needed done first. However I'm worried by the long wire between output stage and input stage grounds. I simulated several nH there and there was oscillation. This is something I will definitely remember for my next PCB session
- keantoken
Alright, as it turns out I have a severe case of projectitis. I'm so busy procrastinating for one project that I never get any other projects done.
I figured I should post here to let everyone know what progress I HAVE made.
I wasn't able to test the prototype further, which I think I will do soon. However, I have made several attempts at a PCB. Attached is my current one. I'm pretty sure this is the most compact it can be, although different capacitor placements can be made. It should probably be expanded a little so as to be easier to service.
For me this has to be an experiment, since I've never made a PCB. So I would appreciate advice...
As for the schematic, notice the addition of J1. This works better than a resistor for overload protection, just remember the Jfet's CCS current should not be below the Rush's bias current, or very near it. This isn't included in the PCB.
- keantoken
I figured I should post here to let everyone know what progress I HAVE made.
I wasn't able to test the prototype further, which I think I will do soon. However, I have made several attempts at a PCB. Attached is my current one. I'm pretty sure this is the most compact it can be, although different capacitor placements can be made. It should probably be expanded a little so as to be easier to service.
For me this has to be an experiment, since I've never made a PCB. So I would appreciate advice...
As for the schematic, notice the addition of J1. This works better than a resistor for overload protection, just remember the Jfet's CCS current should not be below the Rush's bias current, or very near it. This isn't included in the PCB.
- keantoken
Attachments
Ken, that looks nice. Thanks for the link. I had a few PCB's sitting around that I could Manhattan on top of but they've been consumed by the pile that slowly levels out toward the center of my room...
I was told OurPCB has a prototype policy where they'll send you a board to test out before doing the whole production. So if I ordered several PCBs, they would send me a sample and if things didn't work out, I could cancel the order. This would speed things up quite a bit. Am I out of my mind?
- keantoken
I was told OurPCB has a prototype policy where they'll send you a board to test out before doing the whole production. So if I ordered several PCBs, they would send me a sample and if things didn't work out, I could cancel the order. This would speed things up quite a bit. Am I out of my mind?
- keantoken
Does anyone else think it looks like a spaceship?
Added a jumper to change between voltage and current feedback mode; I'm curious to see the affect after reading this thread:
http://www.diyaudio.com/forums/vend...lifiers-revealed-much-more-4.html#post2192006
The current feedback resistor doesn't do much in voltage feedback mode. It can be bypassed if it's not used. There is some sort of standard for series resistors in headphone amps, but I've talked to people and I've been recommended the values of 0, 10, and 120 ohm series resistors. I figure I'll just let the builder decide on their preferences.
- keantoken
Added a jumper to change between voltage and current feedback mode; I'm curious to see the affect after reading this thread:
http://www.diyaudio.com/forums/vend...lifiers-revealed-much-more-4.html#post2192006
The current feedback resistor doesn't do much in voltage feedback mode. It can be bypassed if it's not used. There is some sort of standard for series resistors in headphone amps, but I've talked to people and I've been recommended the values of 0, 10, and 120 ohm series resistors. I figure I'll just let the builder decide on their preferences.
- keantoken
Attachments
It sure looks like m/s Voyager blueprint alright, captain K. Btw, could you point me to Seven of Nine and T 'Pol cabins please?
I'm hoping I just figured it out.
I measured with the ohm meter across the higher MOSFET's S-D junction, which read 7 ohms.
Running so much current, you'd think it would get hot. But no, it's only 7 ohms so it doesn't even get warm. Instead, the other MOSFET gets hot because it has to sink all the current!
I fried one of my precious P55NE's...
Well, let's try that again...
- keantoken
I measured with the ohm meter across the higher MOSFET's S-D junction, which read 7 ohms.
Running so much current, you'd think it would get hot. But no, it's only 7 ohms so it doesn't even get warm. Instead, the other MOSFET gets hot because it has to sink all the current!
I fried one of my precious P55NE's...
Well, let's try that again...
- keantoken
Well that was short-lived. The buzzing was louder this time.
The MOSFET I replaced no longer measures 7 ohms S-D.
I just realized, my MOSFETs are both conducting to ground through the heatsinks.
Moved to a smaller heatsink, which allowed for those nylon (?) isolators.
Voltage reads fine, and the MOSFETs are running at reasonable bias, but now output is -8V with 20mV of ripple. Now I must fix a problem somewhere else...
- keantoken
The MOSFET I replaced no longer measures 7 ohms S-D.
I just realized, my MOSFETs are both conducting to ground through the heatsinks.
Moved to a smaller heatsink, which allowed for those nylon (?) isolators.
Voltage reads fine, and the MOSFETs are running at reasonable bias, but now output is -8V with 20mV of ripple. Now I must fix a problem somewhere else...
- keantoken
Member
Joined 2009
Paid Member
I took a break and constructed the Allison on my shiny new power amp tesbed (heatsink with original drivers and outputs mounted, enough for stereo). Subtle detail is there which was obscured by my Sony amp... Probably mediocre AB arrangement.
I will replace the questionable devices but I think I will reconstruct the whole circuit on my new testbed; I fear I made a fatal error in the grounding scheme.
- keantoken
I will replace the questionable devices but I think I will reconstruct the whole circuit on my new testbed; I fear I made a fatal error in the grounding scheme.
- keantoken
Last edited:
Have you tried the hand matched germaniums as comparators in the Allison?
VBE something like 0.25 might allow for much tighter output voltage gap.
I know old NPN and PNP packages don't look much alike... But they were
cherry picked closest available compliments for your specific application
in mind. Very best junk (Tanner Electronics) I could lay my hands on...
VBE something like 0.25 might allow for much tighter output voltage gap.
I know old NPN and PNP packages don't look much alike... But they were
cherry picked closest available compliments for your specific application
in mind. Very best junk (Tanner Electronics) I could lay my hands on...
Last edited:
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.