Hi everyone. First off, I'm going to apoligize for my lazyness and for not matching my diodes and transistors from the start. But here is the scenario. I finished assembling my leach amp boards. The first one, I tested and it worked great. However the second board I tested had a bad DC offset problem (300mA). I checked the two boards and they are identical. (yes, I took a multimeter to each and every resistor, + the caps all are the same values).
My question is... can not matching produce this bad of a DC offset, or should I be looking for something else? If it is due to not matching, should I start with the diodes or the transistors?
Thanks in advance,
Wes
My question is... can not matching produce this bad of a DC offset, or should I be looking for something else? If it is due to not matching, should I start with the diodes or the transistors?
Thanks in advance,
Wes
check all Ds and Qs
Leach amp is 100% feedback for DC and in no way can the unmatching of the devices make such a bad offset.
Leach amp is 100% feedback for DC and in no way can the unmatching of the devices make such a bad offset.
D's and Q's?
I assume diodes and transistors. Should I just make sure all are correct, or do I need to check and make sure all are working properly. If the latter is the case, how can I do this simply?scratch:
Thanks,
Wes:
I assume diodes and transistors. Should I just make sure all are correct, or do I need to check and make sure all are working properly. If the latter is the case, how can I do this simply?scratch:
Thanks,
Wes:
Raise the PS voltage applied to the board. 12v is probably not high enough to bring everything into sufficient operating range.
12 V not enough??
This is interesting because 12 V seemed to be fine for the other channel. 🙁 This is just to test the boards (they aren't connected to the output transistors yet.) Is this still not high enough?
However, I can't seem to find anything different about the two boards. To my knowledge they are identical. Every component on the boards is brand new, so I can't imagine anything would be burnt out or broken.
I'll try your suggestion the next time I can get access to the scope, but in the mean time, is there any other way I can check for what might be wrong? Are there suggestions about which parts I should start checking first?
Thanks Guys,
Wes
This is interesting because 12 V seemed to be fine for the other channel. 🙁 This is just to test the boards (they aren't connected to the output transistors yet.) Is this still not high enough?
However, I can't seem to find anything different about the two boards. To my knowledge they are identical. Every component on the boards is brand new, so I can't imagine anything would be burnt out or broken.
I'll try your suggestion the next time I can get access to the scope, but in the mean time, is there any other way I can check for what might be wrong? Are there suggestions about which parts I should start checking first?
Thanks Guys,
Wes
wrl, if you just take two transistors (the input transistors) out of a bag and solder them in you will expect 50-70 mV offset (times the DC-gain).
+- 12 V seems little (for a Leach original). Have you done any calculations so you know the working conditions of the amp?
+- 12 V seems little (for a Leach original). Have you done any calculations so you know the working conditions of the amp?
If the output transistors are not connected, there should be
no voltage present at the output terminal. You'll have to bite the bullet and connect at least one pair of output transistors and a load. The temperature sensing diodes will have to be connected properly as well.
The conditions under which you are attempting to test the board are not the normal operating conditions, at all.
It would be helpful to have a variac to slowly bring up the power supply voltage, or to put light bulbs in series with the power supply, in place of the fuses. These will limit current if a serious
fault condition exists, and may limit damage.
--Damon
no voltage present at the output terminal. You'll have to bite the bullet and connect at least one pair of output transistors and a load. The temperature sensing diodes will have to be connected properly as well.
The conditions under which you are attempting to test the board are not the normal operating conditions, at all.
It would be helpful to have a variac to slowly bring up the power supply voltage, or to put light bulbs in series with the power supply, in place of the fuses. These will limit current if a serious
fault condition exists, and may limit damage.
--Damon
I was just following the test procedures as per Prof. Leach's web sight. He says:
"The circuit boards should be tested with a bench power supply before they are installed in the chassis...
Several additional tests should be performed on each circuit board before it is disconnected from the power supply. These are as follows: ...
With the input signal generator disconnected, the dc voltage at the output should not exceed 100 mV, preferably less than 50 mV. Mismatched + and - power supply voltages can affect this."
I was wrong before, as the output signal is loaded with two 100 ohm resistors. An observable DC voltage was present at the output of both boards. From what I understand this is "normal" so long as it doesn't exceed 100 mV. I understand that these conditions are far different from the operating conditions of the amp, but they are the test procedures Prof Leach suggests. I am hesitant to connect the rest of the board if I can't be sure the boards are working correctly.
BTW, the amp will be used to drive 2 electrostatic panels. I assume that DC offset will affect these in much the same was as it does conventional drivers.
"The circuit boards should be tested with a bench power supply before they are installed in the chassis...
Several additional tests should be performed on each circuit board before it is disconnected from the power supply. These are as follows: ...
With the input signal generator disconnected, the dc voltage at the output should not exceed 100 mV, preferably less than 50 mV. Mismatched + and - power supply voltages can affect this."
I was wrong before, as the output signal is loaded with two 100 ohm resistors. An observable DC voltage was present at the output of both boards. From what I understand this is "normal" so long as it doesn't exceed 100 mV. I understand that these conditions are far different from the operating conditions of the amp, but they are the test procedures Prof Leach suggests. I am hesitant to connect the rest of the board if I can't be sure the boards are working correctly.
BTW, the amp will be used to drive 2 electrostatic panels. I assume that DC offset will affect these in much the same was as it does conventional drivers.
Not quite right. You can test the driver without the output transistors BUT you must change the feedback point (R17) to either one of the collectors of Q12 or Q13 (doesn't matter which one).Damon Hill said:
This advice is good if you have the first emitter follower mounted (Q14, Q15). If you short the C12 you will have a DC feedback from the first emitter follower (not only at very high frequencies)
If you have 100 mV as a steady offset voltage no normal speaker will brake but since you will use electrostatic speakers with transformers you should examine how much these transformers can take. Since the amp has gain of 1 at DC I think 100 mV is rather much. I have worked for less than 50 mV and I would have been pleased if it was 10-30 mV!
I'm with Damon, I suspect that under "normal conditions" the offset will (almost) disappear. This "functional test" is meant to verify that you have no big problems. Connect the output pair and recheck (without speaker). If still there, do some measuring. Try to follow the offset voltage through the feedback chain back to the input. Is the input stage in balance etc. Much more intelligent than to replace/match a lot of stuff without knowing what the problem is.
Jan Didden
Jan Didden
The offset will not go away! but I agree with janneman that you should measure all voltages (and calculate currents) and judge if everything is normal. You should use normal supply voltage when you measure.
Thanks guys, I'll crank up the power on my bench supply and start measuring. I'm a little hesitant to start poking around once I have the transformer and caps hooked up, so I'll start with this.
Thanks,
Wes
Thanks,
Wes
ask Dr. Leach
You can send Dr. Leach an email. He almost always returns emails about building the leachamp. He's seen about every problem that could occur over the years. Chances are he can tell you exactly what you need to know.
Austin
You can send Dr. Leach an email. He almost always returns emails about building the leachamp. He's seen about every problem that could occur over the years. Chances are he can tell you exactly what you need to know.
Austin
Hmmm?
Is there a better way to get his attention? I've sent him several e-mails in the past (for purchasing boards etc.) and he has never actually responded. 🙁
Is there a better way to get his attention? I've sent him several e-mails in the past (for purchasing boards etc.) and he has never actually responded. 🙁
Prof. Leach is indeed responding but sometimes he has other things to do I suppose. My email was sent two months ago and he replied yesterday.
Doh!!! Oh well, hopefully I won't have killed myself with this thing before he responds. 🙂 I'm surprised he still had your address after that long.
More tests, less results
Ok, so I made it back to the lab today and ran some more tests.
First: I cranked up the supply voltage, and no the offset didn't get better. In fact it got worse. It was 400 mV at +-20V.
Second: From what I observed, the amp is balanced. There is no DC difference between the two traces from the input stage to the secondary stage. Nor is there any between the traces from the secondary stage to the output stage. (From what I understand this means that the amp is balanced right?)
Third: With +-15V the offset was 340 mV. This was true at the output and at the end of the feedback path. So from what I gather, this means that there is no current flowing in the feedback loop.
Forth: Some interesting details. With equal voltages on each rail (i.e. +-15v) the signal is a clipped sign wave with the top more clipped than the bottom. If I change the voltage to +15 and -13 then the response is normal (i.e. amplified sign wave) but there is still over 100mV of offset at the output. The minimum DC offset was observed at +15V and -10V.
I know that this is a lot of info, and most of it may be completely useless as I don't have a lot of experience with active components. I am thinking I can only hook up both boards to different power supplies but with equal volatges and compare different points on both them to hopefully isolate the problem.
Does anyone have suggestions about what else I should look for or better trouble shooting techniques?
Thanks guys I know I'll get this figured out eventually,
Wes
Ok, so I made it back to the lab today and ran some more tests.
First: I cranked up the supply voltage, and no the offset didn't get better. In fact it got worse. It was 400 mV at +-20V.
Second: From what I observed, the amp is balanced. There is no DC difference between the two traces from the input stage to the secondary stage. Nor is there any between the traces from the secondary stage to the output stage. (From what I understand this means that the amp is balanced right?)
Third: With +-15V the offset was 340 mV. This was true at the output and at the end of the feedback path. So from what I gather, this means that there is no current flowing in the feedback loop.
Forth: Some interesting details. With equal voltages on each rail (i.e. +-15v) the signal is a clipped sign wave with the top more clipped than the bottom. If I change the voltage to +15 and -13 then the response is normal (i.e. amplified sign wave) but there is still over 100mV of offset at the output. The minimum DC offset was observed at +15V and -10V.
I know that this is a lot of info, and most of it may be completely useless as I don't have a lot of experience with active components. I am thinking I can only hook up both boards to different power supplies but with equal volatges and compare different points on both them to hopefully isolate the problem.
Does anyone have suggestions about what else I should look for or better trouble shooting techniques?
Thanks guys I know I'll get this figured out eventually,
Wes
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