Hey All,
Put on your "Double E" thinking cap and give me your opinion on this.
Now I was weened on audio electronics in the late 70's, and for the past 40 years or so I've designed, built and repaired a lot of stuff, but I just ran upon something that baffles me.
A guy brought me a powered sub-woofer to fix, and there's something about the power supply that I don't get.
Look at this schematic, the transformer has two secondary's, the top one is a single ended higher voltage that feeds the power amp, (which is also a strange design).
But look at the bottom secondary, a lower voltage "center tapped" winding that feeds two typical bipolar zener supplies off the same bridge rectifier, each one supplies a few opamps (I guess the designer thought it was more cost effective to double the parts count rather than using bigger parts, like a 1N4744 which has twice the current of the 1N5245, and 1/4W 330 ohm resisters instead if 1/8W).
But look at the center tap, it is fused to the chassis ground (what?).....
So if some over-current event happens and blows the fuse,
IF all the components are still good, but the supply's no longer reference to the center tap,
THEN in theory, does this become a "dual voltage" supply instead of "bipolar"?
That is, does the negative rail become 0V, and the chassis ground become +15V, and the positive rail become +30V?
OR IF a zener on the negative rail is shorted, THEN the negative rail become 0V, the chassis ground becomes 0V (due to the shorted zener), and the positive rail becomes +15V?
OR IF a zener on the positive rail is shorted, THEN the negative rail becomes 0V, the chassis ground becomes +15V, and the positive rail is also +15V (due to the shorted zener)?
Am I missing some obvious "good design practice" here, or does this look as goofy to ya'll as it does to me?
JohnR
Put on your "Double E" thinking cap and give me your opinion on this.
Now I was weened on audio electronics in the late 70's, and for the past 40 years or so I've designed, built and repaired a lot of stuff, but I just ran upon something that baffles me.
A guy brought me a powered sub-woofer to fix, and there's something about the power supply that I don't get.
Look at this schematic, the transformer has two secondary's, the top one is a single ended higher voltage that feeds the power amp, (which is also a strange design).
But look at the bottom secondary, a lower voltage "center tapped" winding that feeds two typical bipolar zener supplies off the same bridge rectifier, each one supplies a few opamps (I guess the designer thought it was more cost effective to double the parts count rather than using bigger parts, like a 1N4744 which has twice the current of the 1N5245, and 1/4W 330 ohm resisters instead if 1/8W).
But look at the center tap, it is fused to the chassis ground (what?).....
So if some over-current event happens and blows the fuse,
IF all the components are still good, but the supply's no longer reference to the center tap,
THEN in theory, does this become a "dual voltage" supply instead of "bipolar"?
That is, does the negative rail become 0V, and the chassis ground become +15V, and the positive rail become +30V?
OR IF a zener on the negative rail is shorted, THEN the negative rail become 0V, the chassis ground becomes 0V (due to the shorted zener), and the positive rail becomes +15V?
OR IF a zener on the positive rail is shorted, THEN the negative rail becomes 0V, the chassis ground becomes +15V, and the positive rail is also +15V (due to the shorted zener)?
Am I missing some obvious "good design practice" here, or does this look as goofy to ya'll as it does to me?
JohnR
It does look goofy to me. A choke before the bridge? Maybe they were concerned it the output switching was putting hash back to the mains.
I admit, strange fuse. It would blow if either rail shorted to ground. It would not if one rail shorted to the other. I think. A bit rusty too.
I admit, strange fuse. It would blow if either rail shorted to ground. It would not if one rail shorted to the other. I think. A bit rusty too.
Is the power amp fed from the upper secondary a class D one? So, choke L2, in conjunction with the three capacitors, is the RFI filter.
Best regards!
Best regards!
Fine to re-design a product that was built to a price point. That was not the OPs question.
These power supplies are definitely curious. There is no ground in the power supply evident, for the power amp, so the signal must be capacitor coupled on both signal and ground. Putting the fuse in the negative rail is curious but not out of the question.
The real problem is F2 on the CT ground of the transformer
It doesn't actually protect anything and if it blows output voltages now are totally on the balance in current draw between the +ve and -ve halves of the two parts of the supply outputs, the current through either the +ve or -ve side of the zener may exceed the dissipation rates of the zeners! An invitation for lots of other stuff being fed from that supply to blow up! It just depends on the balances.
Personally, I'd replace that fuse F2 with a piece of wire (10A at least). And put an adequately rated (but not highly over-rated) fuse on the hot lead of the mains cord into the box!!!!
The real problem is F2 on the CT ground of the transformer
It doesn't actually protect anything and if it blows output voltages now are totally on the balance in current draw between the +ve and -ve halves of the two parts of the supply outputs, the current through either the +ve or -ve side of the zener may exceed the dissipation rates of the zeners! An invitation for lots of other stuff being fed from that supply to blow up! It just depends on the balances.
Personally, I'd replace that fuse F2 with a piece of wire (10A at least). And put an adequately rated (but not highly over-rated) fuse on the hot lead of the mains cord into the box!!!!
Remember, fuses blow for a reason, so instead of thinking what happens if F2 blows ( it is not on the output so a speaker short can't blow it.) think about what a short on either low voltage rail would so. with those 330's there, really only a cap or bridge failure is going to pull an amp. I sure would not put a bigger fuse in it. We don't have a picture of the mains, but I am sure there is a fuse on it.
But that was not the OPs question. He is not trying to re-engineer the amp, just fix it and found it curious. Personally, I would have put fuses on the two primaries, but I am not designing to a price point.
But that was not the OPs question. He is not trying to re-engineer the amp, just fix it and found it curious. Personally, I would have put fuses on the two primaries, but I am not designing to a price point.
Shunt regulated supplies can have advantages in low current applications. Although the output voltage is less tightly controlled Zener supplies can offer excellent noise performance and they deal with superimposed transients better as the Zener intrinsically acts as a clamp. In other words you get a super clean supply. Absolute supply voltage accuracy and slight drift are of no consequence in typical opamp based circuitry.
I have used very similar type supplies in my amp by choice.
The fuse is a bit odd as it provides limited protection against a few fault scenarios only.
The 330 ohm resistors limit currents to 75mA max even in case both zeners failed short.
Ironically, if you took out the fuse altogether, that thing would still work, because the zeners form a virtual ground and opamps don' t have a ground pin anyhow ...
Ironically, if you took out the fuse altogether, that thing would still work, because the zeners form a virtual ground and opamps don' t have a ground pin anyhow ...
OP asked for opinions ... he didn't limit them to what's broken.
If F2 blows then there's no reference for the + and - on the regulated parts of the supplies other than whatever the load and regulators balance out at. So, if there's a fault on any of the 4 rails who knows where it will end up!
But what's going to cause F2 to blow? About the only think I can see would be for C28 or C29 to fail. A fault further down stream will be limited by the current through the 330R dropper resistors on the problem leg. If that 15V leg shorted, the max current would be under 100mA. Not enough to blow the fuse.
If F2 blows then there's no reference for the + and - on the regulated parts of the supplies other than whatever the load and regulators balance out at. So, if there's a fault on any of the 4 rails who knows where it will end up!
But what's going to cause F2 to blow? About the only think I can see would be for C28 or C29 to fail. A fault further down stream will be limited by the current through the 330R dropper resistors on the problem leg. If that 15V leg shorted, the max current would be under 100mA. Not enough to blow the fuse.
Well, glad others scratched their heads on this fuse thing, I may be old and gray, and these new-fangled designs sometimes confuse me, but there is something just not right with that....😵
Now, the amp is equally confusing, it's a class H design of sorts...
That 100VAC rectifies out to about 140VDC (think +-70V with a floating ground) running a (somewhat) basic AB amp at low power,
but when it hits a given threshold it starts flip-flopping some power mosfets, and magically makes it like +-140V...
I really have not found issues with the power supplies, but right now I'm working through some toasted silicon in the power amp.
I'll see where this all goes after I get some parts in...
JohnR
Now, the amp is equally confusing, it's a class H design of sorts...
That 100VAC rectifies out to about 140VDC (think +-70V with a floating ground) running a (somewhat) basic AB amp at low power,
but when it hits a given threshold it starts flip-flopping some power mosfets, and magically makes it like +-140V...
I really have not found issues with the power supplies, but right now I'm working through some toasted silicon in the power amp.
I'll see where this all goes after I get some parts in...
JohnR
Did you physically confirm that F2 is wired the way it shows in the schematic? Maybe it's a goof like a thermal fuse gone astray?
a higher voltage monorail feeding the main power implied one of two things, a quasi amplifier with output coupling capacitor, or a bridge tied load type....these are the types that can use the monorail, i do not see anything unusual at all...
the 8A L2 was a common mode ac filter used in some Denon designs of the old...nothing unusual there either....i use that trick too....
the 8A L2 was a common mode ac filter used in some Denon designs of the old...nothing unusual there either....i use that trick too....