It don't work!!
Guys, HELP.
I'm almost done with my A75 but am having problems. I tested the front end the way described in the article and it works fine. The problems arise when I plug in the output stage. Voltage on R7 and 8 drops to about 350 mV. No matter what I do to P1 and P2 that figure does not increase much. I get alot of DC offset at the output. Like almost a volt in un balanced mode. I also get more than 11V of AC output signal. I see nothing on the source resistors. What the
I hope you guys can help. Thanks for looking. Dinu
Guys, HELP.
I'm almost done with my A75 but am having problems. I tested the front end the way described in the article and it works fine. The problems arise when I plug in the output stage. Voltage on R7 and 8 drops to about 350 mV. No matter what I do to P1 and P2 that figure does not increase much. I get alot of DC offset at the output. Like almost a volt in un balanced mode. I also get more than 11V of AC output signal. I see nothing on the source resistors. What the
I hope you guys can help. Thanks for looking. Dinu
Are R9 and R10 still showing ~5V?
What voltage do you get at the drive terminals? AC and DC -
You won't see any voltage on the output source resistors until you bring the bias up with P3.
what are gate-source and source-drain voltages on Q3, Q6, Q9 and Q10?
Check all your connections to the output section. What are your rail voltages?
Did you disconnect the output stage and verify that the front end still works? It may have been damaged somehow.
What input voltage gives you 11V out? (is it amplifying properly?)
that ought to give you something to start with
What voltage do you get at the drive terminals? AC and DC -
You won't see any voltage on the output source resistors until you bring the bias up with P3.
what are gate-source and source-drain voltages on Q3, Q6, Q9 and Q10?
Check all your connections to the output section. What are your rail voltages?
Did you disconnect the output stage and verify that the front end still works? It may have been damaged somehow.
What input voltage gives you 11V out? (is it amplifying properly?)
that ought to give you something to start with
Are R9 and R10 still showing ~5V?
~~Yes. I get about 5.25V
What voltage do you get at the drive terminals? AC and DC -
~~When it's all connected I see .01V on -DRV and 3.2V on +DRV That's DC. They both have about 11vac.
You won't see any voltage on the output source resistors until you bring the bias up with P3.
~~yes but I see nothing even when I turn up P3.
what are gate-source and source-drain voltages on Q3, Q6, Q9 and Q10?
~~Q3 4.8 Vgs and 4.1 Vds
~~Q6 4.26 Vgs and 4.1 Vds
~~Q9 4.66 Vgs and 23.9 Vds
~~Q10 4.03 Vgs and 22.1 Vds
Check all your connections to the output section. What are your rail voltages?
~~I've gone over these several times. the P channel drains are connected to - unreg. N channel to +unreg. -Drv to gate resistors of P channel and +drv to N channel gate resistors. output is off the source of all output transistors. The reg supply puts out + and - 56V and unreg puts out + and - 47V.
Did you disconnect the output stage and verify that the front end still works? It may have been damaged somehow.
~~I've disconnected several times to see if it still works properly and it seems to me like it does. It passes all the criteria. almost 10V across the Zeners, 5V on R9 and 10, 900mV bias at R7 and 8, and I can get the DC offset to nearly zero. Plug in the output and it all falls apart.
What input voltage gives you 11V out? (is it amplifying properly?)
~~input voltage is about 320mV. I don't have an oscillator so I used an ipod with a 1Khz tone I downloaded as an MP3. I cou;dn't see any reason why this wouldn't work. Only problem is that 300mV is about all the signal I can get from the ipod. I am an amateur ya know
that ought to give you something to start.
~~Yes. I get about 5.25V
What voltage do you get at the drive terminals? AC and DC -
~~When it's all connected I see .01V on -DRV and 3.2V on +DRV That's DC. They both have about 11vac.
You won't see any voltage on the output source resistors until you bring the bias up with P3.
~~yes but I see nothing even when I turn up P3.
what are gate-source and source-drain voltages on Q3, Q6, Q9 and Q10?
~~Q3 4.8 Vgs and 4.1 Vds
~~Q6 4.26 Vgs and 4.1 Vds
~~Q9 4.66 Vgs and 23.9 Vds
~~Q10 4.03 Vgs and 22.1 Vds
Check all your connections to the output section. What are your rail voltages?
~~I've gone over these several times. the P channel drains are connected to - unreg. N channel to +unreg. -Drv to gate resistors of P channel and +drv to N channel gate resistors. output is off the source of all output transistors. The reg supply puts out + and - 56V and unreg puts out + and - 47V.
Did you disconnect the output stage and verify that the front end still works? It may have been damaged somehow.
~~I've disconnected several times to see if it still works properly and it seems to me like it does. It passes all the criteria. almost 10V across the Zeners, 5V on R9 and 10, 900mV bias at R7 and 8, and I can get the DC offset to nearly zero. Plug in the output and it all falls apart.
What input voltage gives you 11V out? (is it amplifying properly?)
~~input voltage is about 320mV. I don't have an oscillator so I used an ipod with a 1Khz tone I downloaded as an MP3. I cou;dn't see any reason why this wouldn't work. Only problem is that 300mV is about all the signal I can get from the ipod. I am an amateur ya know
that ought to give you something to start.
Yeah, I was afraid of that. I reallydidn't want to start taking apart the output stage but I guess I have no choice. Do you think that the fact that -drv shows such little voltage is a sign that the problem lies with the P channels? Well, if I build this thing enough times I'm just bound to get it right. WheI actually get to do this is another matter altogether. I just got a new puppy recently too so that's a huge drain on what was already nearly non existant free time. Thanks again for your help Bob. I'll let you know what develops.
well, if connecting the output stage causes the -drv voltage to drop to 0, it sure seems like the problem is connected to the -drv.
the puppy could actually help - it might keep you away long enough to spot a mistake. Could you have made a mistake in the feedback switching setup that you mentioned trying?
good luck!
the puppy could actually help - it might keep you away long enough to spot a mistake. Could you have made a mistake in the feedback switching setup that you mentioned trying?
good luck!
Good call on the puppy. I don't know why, but frustration at something not working properly just makes me try the same thing again hoping that, by some act of divine kindness, it will work; Trying the same thing over and over and expecting a different result. The name for that is "insanity".
Well, I'll just have to take the P channel apart. R81 is open right now. I figured I'd adjust everything first and then hook up the feedback switch.
Talk to you later,
Well, I'll just have to take the P channel apart. R81 is open right now. I figured I'd adjust everything first and then hook up the feedback switch.
Talk to you later,
Before I get back to the output stage I still have to work out a bug in one of the front end channels. I can't get the bias to stay up. Adjusting P1 and P2 brings up the bias on R7 and 8 as well as the voltage on the drains of Q2 and 5. The drain of Q6 and gate of Q10 come up very slowly after the amp is turned on. They wander up to about 39-40V and then stop. As you bring up R7 and 8 they're constantly dropping. At full clockwise, the bias eventually settles to about 350 mV. I've matched the input fets Q1,2 and 4 to 5. The 220's aren't matched but I've checked that they work. Please help.
dinu said:
OK, I am confused. Where is the 350 mV measured? R7/8?
Are you getting 5 V across R1, R6, R 9 and R10?
Do you mean Q6 Drain and Q10 Source go to -40V referenced to ground and drift towards ground? With no signal it should be around 5V below the rail.
What is the voltages on Q9/10 relative to the rail?
check Q11 -
Hi Bob,
I can't get a good grasp of wht the circuit is doing. It does different things at different times. I just turn it off and then back on again and sometimes get different results. Well, most of the time, these are the values I get. Rails are at about 57V. The most voltage I can get across R7 and 8 is about 350mV no matter how far I turn up P1 and 2. Once it all stabilizes, I see 6V across R9 and 10. R1 and 6 start at about 4.5V and slowly climb up to about 6. Q5's drain starts at about 30V from rail and slowly climbs to settle at about 10. Q9 and 10 both have the same values from their respective rails. Both drains see full 57V, the gates are at about 6.7, and the Sources are at about 10. There is no voltage across +and - DRV. I'll just replace Q11 again to make sure but I've had it out and it tested fine at about 20mA. Is it possible for a FET to be alittle broken? I don't know if that sounds dumb but is breakage in a FET always an absolute thing?. I sure hope you have some ideas Bob. The puppy's been eyeballing this thing as a new chew toy and I'm pretty close to letting him have it. (it's going to the dogs)
I can't get a good grasp of wht the circuit is doing. It does different things at different times. I just turn it off and then back on again and sometimes get different results. Well, most of the time, these are the values I get. Rails are at about 57V. The most voltage I can get across R7 and 8 is about 350mV no matter how far I turn up P1 and 2. Once it all stabilizes, I see 6V across R9 and 10. R1 and 6 start at about 4.5V and slowly climb up to about 6. Q5's drain starts at about 30V from rail and slowly climbs to settle at about 10. Q9 and 10 both have the same values from their respective rails. Both drains see full 57V, the gates are at about 6.7, and the Sources are at about 10. There is no voltage across +and - DRV. I'll just replace Q11 again to make sure but I've had it out and it tested fine at about 20mA. Is it possible for a FET to be alittle broken? I don't know if that sounds dumb but is breakage in a FET always an absolute thing?. I sure hope you have some ideas Bob. The puppy's been eyeballing this thing as a new chew toy and I'm pretty close to letting him have it. (it's going to the dogs)
no voltage across the DRV terminals means either there is no current flowing in that leg or Q11 is a dead short (Turned on completely or blown) With P3 set at the midpoint do you measure about 2.5K gate to source on Q11? With power on and p3 set fully CCW, what is the voltage across R80? (looking for circuit continuity.)
Moving on - the gates of Q9/10 should be 9.1V below the rails. Check the voltages across Z1/2.
Check also across R17-20. Any voltage there indicates a blown FET.
If the Q5 drain (and therefore Q6 Drain and Q10 source) is 10V below the rail (10V across S-D of Q6) Q10 cannot turn on when its gate is below its source. therefore you'll get no current in the Vgs multipler - you can skip that testing above.
If You have nearly 6Vgs on Q6 and you get 10V across it, there is a problem - it doesn't seem to be turning on properly.
Not sure of all the failure mechanisms, but I seem to remember NP saying that damage can be cumulative, so it sounds like they can be partially damaged.
Since you seem to have a problem with at least Q6, I'd start by replacing all of the 610s and 9610s, as well as Z1-2.
Did you add back to back zeners on the inputs to protect against static damage? If not, replace the input differential devices. They seem to be drawing too much current - as indicated by the 6V across R10. This may be due to something funky with the VAS, so you might try just replacing the To-220's first.
Replace all the semiconductors is probably not what you wanted to hear, but that's what I would do next.
Please state what the voltage reference is - I assumed that when you described the Q9/10 voltages they were referenced to their respective rail, but that is not clear. If you reference all to ground, a rail or say that Q10 source to drain is 47 volts, it becomes clear what you are measuring.
Good luck
Moving on - the gates of Q9/10 should be 9.1V below the rails. Check the voltages across Z1/2.
Check also across R17-20. Any voltage there indicates a blown FET.
If the Q5 drain (and therefore Q6 Drain and Q10 source) is 10V below the rail (10V across S-D of Q6) Q10 cannot turn on when its gate is below its source. therefore you'll get no current in the Vgs multipler - you can skip that testing above.
If You have nearly 6Vgs on Q6 and you get 10V across it, there is a problem - it doesn't seem to be turning on properly.
Not sure of all the failure mechanisms, but I seem to remember NP saying that damage can be cumulative, so it sounds like they can be partially damaged.
Since you seem to have a problem with at least Q6, I'd start by replacing all of the 610s and 9610s, as well as Z1-2.
Did you add back to back zeners on the inputs to protect against static damage? If not, replace the input differential devices. They seem to be drawing too much current - as indicated by the 6V across R10. This may be due to something funky with the VAS, so you might try just replacing the To-220's first.
Replace all the semiconductors is probably not what you wanted to hear, but that's what I would do next.
Please state what the voltage reference is - I assumed that when you described the Q9/10 voltages they were referenced to their respective rail, but that is not clear. If you reference all to ground, a rail or say that Q10 source to drain is 47 volts, it becomes clear what you are measuring.
Good luck
Hi Bob,
well,across gate to source on Q11 I get about 1.5K with P3 at half point. I also get no voltage across R 80 when P3 is full CCW. I decided I'll replace transistors just to make sure. I need to go get a couple but while I was looking I noticed that the 610's have an Rds of 1.5 Ohm not <1 as stated in NP's article. Looking in the parts box, I found some 9640's (125W, -200V Vdss, .5 Ohm Rds) I thought I might pair them with 510's. (43W, 100Vdss, and .54 Ohm Rds). I wonder if the Rds figure really matters. The other channel seems to work fine with 610's. Anyway, I'll do what little bit I have time for and let you know. Again, I can't thank you enough for the help.
well,across gate to source on Q11 I get about 1.5K with P3 at half point. I also get no voltage across R 80 when P3 is full CCW. I decided I'll replace transistors just to make sure. I need to go get a couple but while I was looking I noticed that the 610's have an Rds of 1.5 Ohm not <1 as stated in NP's article. Looking in the parts box, I found some 9640's (125W, -200V Vdss, .5 Ohm Rds) I thought I might pair them with 510's. (43W, 100Vdss, and .54 Ohm Rds). I wonder if the Rds figure really matters. The other channel seems to work fine with 610's. Anyway, I'll do what little bit I have time for and let you know. Again, I can't thank you enough for the help.
Hi all,
I'm moving along with this thing at a snail's pace. The front end seems to work fine now and am trying to deal with whatever damage I did to the output awhile back. With regards to the output, an interesting question was posed to me. Why the need for all the fets in parallel? Would much fewer and higher power fets not have worked as well. Were these high current fets just not available when the A 75 was designed? I'd love to hear if anyone did anything like this and if so, what they used.
I'm moving along with this thing at a snail's pace. The front end seems to work fine now and am trying to deal with whatever damage I did to the output awhile back. With regards to the output, an interesting question was posed to me. Why the need for all the fets in parallel? Would much fewer and higher power fets not have worked as well. Were these high current fets just not available when the A 75 was designed? I'd love to hear if anyone did anything like this and if so, what they used.
I was actually thinking of as few as 2 to 4 pairs. I saw a reply by Nelson to another post where he suggested IRF540 and 9540's. Those have a peak current of 30 amps. One could probably get away with just a few of those and a much lower value source resistor. Suggestions?
I also have to admit I tried cheating and didn't match the output fets. I used 12 IRF230-9231 pairs per channel. I was hoping that the source resistor would take care of any reasonable Vgs difference. Well, I'm still trying to figure it all out but it didn't work. So now I either pull them all out of the board and test them all or just start fresh with the output.
While I'm at it, anyone have some matched sets they'd like to sell? I'm desperate and probably a pretty easy sell. (sounds like a bad personal ad).
The battle continues.
I also have to admit I tried cheating and didn't match the output fets. I used 12 IRF230-9231 pairs per channel. I was hoping that the source resistor would take care of any reasonable Vgs difference. Well, I'm still trying to figure it all out but it didn't work. So now I either pull them all out of the board and test them all or just start fresh with the output.
While I'm at it, anyone have some matched sets they'd like to sell? I'm desperate and probably a pretty easy sell. (sounds like a bad personal ad).
The battle continues.
Current rating is only part of the issue. The biggest obstacle to going to two pairs is the heat. With 2 pairs and the design bias of 2.2A you've got about 42W in each device. with Normal thermal resistances junction to case and case to sink, your junctions will be 95 degrees hotter than the heat sinks. Makes it tough to keep them below 100C unless you keep them in the freezer. 
4 pairs should be doable, if you have enough heat sink - you'll need bigger than you could get away with for 12 devices.
One of my sets of lost outputs was due to misreading the labels I'd put on my outputs as I measured Vgs. One device with significantly lower Vgs than the rest ended up taking almost all of the current, at least for a while. I noticed that one part of the heat sink was significantly hotter than the rest but it didn't register until the smoke came out, taking several others with it. of course, I was using smaller than recommended Source resistors.
In other words, I suggest that you match your outputs. You can simply measure the voltage across the Source resistors in place as you bring it up. If you got lucky, you'll find that they are all close (within .01 or .02V). If they are way off, pick the 6 closest devices and disconnect the rest. Even if your output stage is no longer connected to the front end, it would be a good idea to match them on the sink.
4 pairs should be doable, if you have enough heat sink - you'll need bigger than you could get away with for 12 devices.
One of my sets of lost outputs was due to misreading the labels I'd put on my outputs as I measured Vgs. One device with significantly lower Vgs than the rest ended up taking almost all of the current, at least for a while. I noticed that one part of the heat sink was significantly hotter than the rest but it didn't register until the smoke came out, taking several others with it. of course, I was using smaller than recommended Source resistors.
In other words, I suggest that you match your outputs. You can simply measure the voltage across the Source resistors in place as you bring it up. If you got lucky, you'll find that they are all close (within .01 or .02V). If they are way off, pick the 6 closest devices and disconnect the rest. Even if your output stage is no longer connected to the front end, it would be a good idea to match them on the sink.
Hey Bob,
All the output fets are on the sinks and disconnected from the front end. When I did have them connected I could turn P3 as much as I liked, I still saw no drop on the source resostor. Of course, I wasn't looking at all the resistors so I bet a couple of them had a drop, and a good one at that. I would like to symplify the design as much as possible so I think I'll try the 4 pair approach. I just need to find four matched sets. It should be alot easier a task than 12.
Interesting side note, I made the swith for the feedback. With r81 open I see the output square off almost like it's clipping. I don't get why. (This is just the front end with no ouput devices).
All the output fets are on the sinks and disconnected from the front end. When I did have them connected I could turn P3 as much as I liked, I still saw no drop on the source resostor. Of course, I wasn't looking at all the resistors so I bet a couple of them had a drop, and a good one at that. I would like to symplify the design as much as possible so I think I'll try the 4 pair approach. I just need to find four matched sets. It should be alot easier a task than 12.
Interesting side note, I made the swith for the feedback. With r81 open I see the output square off almost like it's clipping. I don't get why. (This is just the front end with no ouput devices).
I remember NP saying that the front end cannot drive the zoebel. maybe that's what is causing the clipping.
If your outputs are from the same lots, then you may find that you have pretty good matching. I bought 50 of each and sets of 12 weren't great matches, maybe within .02-3, but got 6 or 7 sets of 6 within .01V.
If your outputs are from the same lots, then you may find that you have pretty good matching. I bought 50 of each and sets of 12 weren't great matches, maybe within .02-3, but got 6 or 7 sets of 6 within .01V.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.