well,
it depends a bit on how your power supply is build but if you´ve got abs dc offset then one half of the power supply is using more power than the other half (because of the resistors to ground at the output) causing the two secondaries to be loaded unequally wich can cause hum.
William
it depends a bit on how your power supply is build but if you´ve got abs dc offset then one half of the power supply is using more power than the other half (because of the resistors to ground at the output) causing the two secondaries to be loaded unequally wich can cause hum.
William
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Got it, William! Thanks!
I spent the day yesterday tweaking and measuring voltages and currents and adjusting the trim pots. First, I changed R12 and R34 back to 1k3 and was able to measure AC Current Gain at 50.5% on each side of the amp. I then changed the McMillan Resistors from 3k3 to 10k on each side of the amp. I also moved Q6 from the back panel of the amp to the heat sink directly over one of the output transistors. I double checked to make sure it was electrically isolated from the chassis when I moved it. Absolute DC Offset now starts at 14v. With Q6 on the heatsink vs on the back panel of the chassis, it fell to near zero volts more quickly. Within 5 mins it fell to 6.2v, by 10 mins it was at 3.2v, and it reach 0.41v at 20 mins.
The amp ran all day (largely with the cover on) and abolute DC Offset dropped to about -2v, relative DC offset stayed < .01v, and bias current continued to fall as the amp heated up. After a few hours, I readjusted Absolute DC offset to near 0v, and bumped the bias back up to about 7.5A. The power supply is providing about 20.4v at 7.5A, and measurements of AC Current Gain stayed at 50%.
I then drove the amp with my preamp and a 70Hz tone for a while using my dummy load. On the 8-ohm setting, those resistors really got hot this time! With my preamp delivering 9.1v to the input, I was able to swing 27.8v into 4 ohms (just about 96w) and 38.3v into 8 ohms (just about (91w). I made these measurements with my DMM. Putting the preamp at its max volume, output topped out at 100w into both 4 and 8 ohms. The resistors got really hot and the heatsinks ran noticeably cooler as power went out through the speaker terminals.
Playing some music, it sounded very nice (though its hard to evaluate with only a single speaker).
Before I shut it down, I re-measured everything and it was spot-on. Power supply voltage was holding at 20.4 or so, bias current was holding steady at about 7.5A per transisor bank, AC Current Gain still measured 50%.
Then I powered down and let it cool. Now when I turn it on, it behaves strangely with respect to temperature. The heat sinks on the output side of the PCB (Q2 and Q11) run extra hot - I'm measuring 66c on the screw the holds the transistors to the sink. These sinks heat up VERY quickly. On the Current Source Side (Q1 and Q10), they take MUCH longer to heat up than the output side. After 5 minutes, Q1 and Q10 are still pretty cool, but Q2 and Q11 are running hot. Absolute DC Offset now takes much longer to come down to zero v, but still starts at 14v when cold.
After an hour of running, Q1 and Q10 run about 58c (just about the same as before), but now Q2 and Q11 are running about 66c (hotter than before). Absolute DC Offset is running about 0.5v by this time.
I never noticed the two side of the amp running at different temperatures or warming at obviously different rates before. AC Current Gain is still 50% and all transistor banks are measuring the same 0.420v across the source resistors (about 7.5A at 20.4v). All of the measurements look great except the temperature differential. Playing music still sounds good, too.
Is this normal behavior? Any ideas what is going on? Did something get damaged while I was measuring output power into my dummy load?
Thanks!
I spent the day yesterday tweaking and measuring voltages and currents and adjusting the trim pots. First, I changed R12 and R34 back to 1k3 and was able to measure AC Current Gain at 50.5% on each side of the amp. I then changed the McMillan Resistors from 3k3 to 10k on each side of the amp. I also moved Q6 from the back panel of the amp to the heat sink directly over one of the output transistors. I double checked to make sure it was electrically isolated from the chassis when I moved it. Absolute DC Offset now starts at 14v. With Q6 on the heatsink vs on the back panel of the chassis, it fell to near zero volts more quickly. Within 5 mins it fell to 6.2v, by 10 mins it was at 3.2v, and it reach 0.41v at 20 mins.
The amp ran all day (largely with the cover on) and abolute DC Offset dropped to about -2v, relative DC offset stayed < .01v, and bias current continued to fall as the amp heated up. After a few hours, I readjusted Absolute DC offset to near 0v, and bumped the bias back up to about 7.5A. The power supply is providing about 20.4v at 7.5A, and measurements of AC Current Gain stayed at 50%.
I then drove the amp with my preamp and a 70Hz tone for a while using my dummy load. On the 8-ohm setting, those resistors really got hot this time! With my preamp delivering 9.1v to the input, I was able to swing 27.8v into 4 ohms (just about 96w) and 38.3v into 8 ohms (just about (91w). I made these measurements with my DMM. Putting the preamp at its max volume, output topped out at 100w into both 4 and 8 ohms. The resistors got really hot and the heatsinks ran noticeably cooler as power went out through the speaker terminals.
Playing some music, it sounded very nice (though its hard to evaluate with only a single speaker).
Before I shut it down, I re-measured everything and it was spot-on. Power supply voltage was holding at 20.4 or so, bias current was holding steady at about 7.5A per transisor bank, AC Current Gain still measured 50%.
Then I powered down and let it cool. Now when I turn it on, it behaves strangely with respect to temperature. The heat sinks on the output side of the PCB (Q2 and Q11) run extra hot - I'm measuring 66c on the screw the holds the transistors to the sink. These sinks heat up VERY quickly. On the Current Source Side (Q1 and Q10), they take MUCH longer to heat up than the output side. After 5 minutes, Q1 and Q10 are still pretty cool, but Q2 and Q11 are running hot. Absolute DC Offset now takes much longer to come down to zero v, but still starts at 14v when cold.
After an hour of running, Q1 and Q10 run about 58c (just about the same as before), but now Q2 and Q11 are running about 66c (hotter than before). Absolute DC Offset is running about 0.5v by this time.
I never noticed the two side of the amp running at different temperatures or warming at obviously different rates before. AC Current Gain is still 50% and all transistor banks are measuring the same 0.420v across the source resistors (about 7.5A at 20.4v). All of the measurements look great except the temperature differential. Playing music still sounds good, too.
Is this normal behavior? Any ideas what is going on? Did something get damaged while I was measuring output power into my dummy load?
Thanks!
Last edited:
Hi,
if you´ve got a positive absolute DC Offsett then the positive side will dissapate Ibias x (Vpower supply / 2 -V abs dc) and the other side will dissipate I bias x (V supply /2 - V abs dc offset) wich is a very big difference at 14V startup voltage. 6 V x 7.5A = 60 watts, 34Volts x 7.5A = a lot more.
So it is logical that the positive side heats up a lot slower. 14V is a lot.
What resistor did you use between output and ground?
Maybe putting Q6 on the other (colder?) heatsink would be better.
William
if you´ve got a positive absolute DC Offsett then the positive side will dissapate Ibias x (Vpower supply / 2 -V abs dc) and the other side will dissipate I bias x (V supply /2 - V abs dc offset) wich is a very big difference at 14V startup voltage. 6 V x 7.5A = 60 watts, 34Volts x 7.5A = a lot more.
So it is logical that the positive side heats up a lot slower. 14V is a lot.
What resistor did you use between output and ground?
Maybe putting Q6 on the other (colder?) heatsink would be better.
William
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Hi William,
Thanks, this is making me feel much better! I was wondering if this was connect to absolute DC offset - I didn't fully understand what it meant before.
The output to ground resistor (R1/R4 on the PCB) is 78-ohms made from 5 x 390ohm 1% 1w all in parallel. Perhaps I'll move Q6 to the other side and/or reduce the 10k value for the McMillan resistors to something closer to 6k.
Thanks, this makes me feel MUCH better!
Eric
Thanks, this is making me feel much better! I was wondering if this was connect to absolute DC offset - I didn't fully understand what it meant before.
The output to ground resistor (R1/R4 on the PCB) is 78-ohms made from 5 x 390ohm 1% 1w all in parallel. Perhaps I'll move Q6 to the other side and/or reduce the 10k value for the McMillan resistors to something closer to 6k.
Thanks, this makes me feel MUCH better!
Eric
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Funny, I was thinking about this same arrangement. The way its wired now is really just an artifact of the way I decided to orient the PCB within the chassis. If I rotate the board 90 degrees, then I'll have the same arrangement that you do.
From what I have observed, it looks like the McMillan resistors control both the initial DC offset (high resistance leads to higher initial values of absolute DC offset) and the rate at which the absolute DC offset declines. Using 3k3, the DC offset fell pretty quickly - within 20 mins, it was at zero. Using 10k, the DC offset falls much more slowly.
I think I'll stack two 3k3 in series for a total value of 6k6. If this does not work, perhaps I'll explore moving Q6 to a small, external heatsink in an effort to keep it thermally isolated from the changing temperatures inside the chassis.
From what I have observed, it looks like the McMillan resistors control both the initial DC offset (high resistance leads to higher initial values of absolute DC offset) and the rate at which the absolute DC offset declines. Using 3k3, the DC offset fell pretty quickly - within 20 mins, it was at zero. Using 10k, the DC offset falls much more slowly.
I think I'll stack two 3k3 in series for a total value of 6k6. If this does not work, perhaps I'll explore moving Q6 to a small, external heatsink in an effort to keep it thermally isolated from the changing temperatures inside the chassis.
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I'll see how it behaves with a value of 6k6 and having Q6 on the main heat sink first. If DC offset falls to zero within 15-20 mins, I'm not going to worry about it any more. Its rather fascinating to study how the behavior of this amp changes (voltage, current, temperature) and how each change affects the other settings...
I don't think coupling it to ambient temperature in this case is such a bad thing. Its for my basement theater, which keeps a pretty consistent temperature all year round (maybe a cold as 65-66F in winter and as high as 69-70F in summer).
I don't think coupling it to ambient temperature in this case is such a bad thing. Its for my basement theater, which keeps a pretty consistent temperature all year round (maybe a cold as 65-66F in winter and as high as 69-70F in summer).
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On another note, I have the opportunity to bring my amp into the EE lab on campus this week and hook it to a function generator and a scope. Other than using a sine wave as input and looking for symmetric waveform and clipping behavior on the output, is there anything else that I can easily look for while I'm there?
Eric
Eric
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OK, I've changed out the McMillan resistors to 6k6 and still have Q6 on the main heat sink. With the input shorted and no load on the output, I get initial Absolute DC Offset ov 8.0v, this drops to 4.8v within 60 seconds, and by 5 mins is at 2.8v. By 15-20 mins, it is less than a volt (all with the top plate off). I am satisfied with this behavior. With the top plate on, the amp will only warm up faster to reach steady state, so the offset should decline a little faster.
Tonight I'll perforate the top panel and include holes just above the trim pots and get some wires out for measurement and "final" adjustment (for now, anyhow).
With square waves as input, I presume I'm looking for fairly regular square waves on the output - I've seen some examples posted in the threads so I'll see if I can go back and dig any out for comparison.
I think I'm almost done with this one. It'll soon be time to shift my focus to speaker construction and see how the amp and speakers mate with one another... Just love another project ;-)
Tonight I'll perforate the top panel and include holes just above the trim pots and get some wires out for measurement and "final" adjustment (for now, anyhow).
With square waves as input, I presume I'm looking for fairly regular square waves on the output - I've seen some examples posted in the threads so I'll see if I can go back and dig any out for comparison.
I think I'm almost done with this one. It'll soon be time to shift my focus to speaker construction and see how the amp and speakers mate with one another... Just love another project ;-)
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Of course, adding holes to the top of the chassis changes the temperature characteristic of the amp. As I result, I had to make adjustments again to Absolute DC Offset over the course of an hour or more.
Now, DC offset starts at about 12v, drop to 4v within 5 minutes, and is about 1.7v within 20-25 mins. I'm probably being too detail oriented at this point, but its almost maddening how EVERYTHING you change seems to affect this setting...
Now, DC offset starts at about 12v, drop to 4v within 5 minutes, and is about 1.7v within 20-25 mins. I'm probably being too detail oriented at this point, but its almost maddening how EVERYTHING you change seems to affect this setting...
Hi Guys.
Does anybody now if there is still a source, that has any Aleph-X PCB for the High Powered Version, that is described on Eris Homepage:PassDIY Aleph-X 100w amplifier construction notes, parts, & mosfet matching ?
Or is it maybe possible to order them somewhere?
It would be so great if anybody could find some!
Ben
Does anybody now if there is still a source, that has any Aleph-X PCB for the High Powered Version, that is described on Eris Homepage:PassDIY Aleph-X 100w amplifier construction notes, parts, & mosfet matching ?
Or is it maybe possible to order them somewhere?
It would be so great if anybody could find some!
Ben
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OK, I just screwed something up and I don't know what it is...
I was measuring and tracking the change of Absolute DC Offset over time when I realized (duh!) that the cover was off of the chassis. I had several wire lead-out so I could make measurements with the lid on - I had one pair for measuring bias on the Q10 bank, one pair of leads for measuring rail voltage (on the negative side), and Q6 is on wire leads outside of the chassis.
Typically Absolute DC Offset starts at about +13v and then begins dropping toward zero. The amp had been on for ~20 mins and things were largely warmed up, DC offset was about 5v at the time. I place the lid on top of the chassis and this is when things went strange. As soon as the lid was in place and I looked back at the meter for Absolute DC Offset and it changed from +5v to nearly -15v within a few seconds. The only thing that I can think of is that one of my lead out wires somehow touched the grounded chassis, but I don't see how because they were taped down.
Both power supply rails are measuring + and - 20v, each individual output mosfet on the heatsinks are running at close to normal bias (source resistor on each fet is dropping just over 0.4v). I have replaced Q6 thinking I damaged it, but there is no difference - absolute DC offset now starts at -15v and climbs as time goes by (exactly opposite of how it used to behave).
When I adjust VR2, it makes no difference in absolute DC offset at all any more. VR1 and VR3 still adjust bias in a predictable manner. When I pull the shorting pin from the RCA input, Absolute DC Offset doesn't change at all now (It used to drop by a few volts).
Any guesses what is going on here? I can't seem to find the problem. There was no smoke, no pops, no crackling, just very strange Absolute DC Offset behavior now.
I was measuring and tracking the change of Absolute DC Offset over time when I realized (duh!) that the cover was off of the chassis. I had several wire lead-out so I could make measurements with the lid on - I had one pair for measuring bias on the Q10 bank, one pair of leads for measuring rail voltage (on the negative side), and Q6 is on wire leads outside of the chassis.
Typically Absolute DC Offset starts at about +13v and then begins dropping toward zero. The amp had been on for ~20 mins and things were largely warmed up, DC offset was about 5v at the time. I place the lid on top of the chassis and this is when things went strange. As soon as the lid was in place and I looked back at the meter for Absolute DC Offset and it changed from +5v to nearly -15v within a few seconds. The only thing that I can think of is that one of my lead out wires somehow touched the grounded chassis, but I don't see how because they were taped down.
Both power supply rails are measuring + and - 20v, each individual output mosfet on the heatsinks are running at close to normal bias (source resistor on each fet is dropping just over 0.4v). I have replaced Q6 thinking I damaged it, but there is no difference - absolute DC offset now starts at -15v and climbs as time goes by (exactly opposite of how it used to behave).
When I adjust VR2, it makes no difference in absolute DC offset at all any more. VR1 and VR3 still adjust bias in a predictable manner. When I pull the shorting pin from the RCA input, Absolute DC Offset doesn't change at all now (It used to drop by a few volts).
Any guesses what is going on here? I can't seem to find the problem. There was no smoke, no pops, no crackling, just very strange Absolute DC Offset behavior now.
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Curious - must be a lose connection somewhere. When I started up this morning, things were fine. I had shut down and restarted a number of times last night and it made no difference.... Guess I've been doing too much removing and re-insertion into the PCB and things have gotten jostled around a little. I suppose its a good thing that I'm rebuilding the chassis so I have a new chance to make sure all of my connections are more secure.
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