I want to make a pre-amp / headphone amp using discrete components that must be able to cope with a shorted output. Not just as a precaution, but because 1/4" jack used by headphones causes a short when being plugged in. I don't want to use any overload protection schemes if it will compromise sound quality like putting a resistor on the output to limit current.
How about using many transistors in parallel in the output stage so that each transistor will not be overloaded, and all the transistors in parallel reduce the output impedance to something very low like <1 Ohm. I've attached image of what I mean. Not sure how many transistors are needed to reduce output impedance. The value of the resistor (if the transistors used are TO-92 with 500mA max current and 15v supply) would be 15/0.5 = 30 ohm. Is this correct and do you think it will work ok?
Also, is output impedance reduced when global negative feedback is used?
How about using many transistors in parallel in the output stage so that each transistor will not be overloaded, and all the transistors in parallel reduce the output impedance to something very low like <1 Ohm. I've attached image of what I mean. Not sure how many transistors are needed to reduce output impedance. The value of the resistor (if the transistors used are TO-92 with 500mA max current and 15v supply) would be 15/0.5 = 30 ohm. Is this correct and do you think it will work ok?
Also, is output impedance reduced when global negative feedback is used?
Attachments
You may use my output stage, it is not a secret anymore, I've published it several times already on the forum. However, if you age going to run production you can't use it without my permission.
It is made of pair of complementary operational amplifiers where one transistor is a difeerencial cascade is replaced by diode. Why diode? Because impedance of feedback loop when voltage gain is =1 is low, and because breakdown voltage of a diode is higher than b-e junction of transistors. As I said, both opamps have 100% voltage feedback (through diodes), so voltage gain is =1. But when output is shorted feedback by voltage stops working ("closed" diodes), so input transistors have 100% NFB by current, output transistors amplify it up to safe level (actually, a bit higher than possible peak current on nominal load).
It is made of pair of complementary operational amplifiers where one transistor is a difeerencial cascade is replaced by diode. Why diode? Because impedance of feedback loop when voltage gain is =1 is low, and because breakdown voltage of a diode is higher than b-e junction of transistors. As I said, both opamps have 100% voltage feedback (through diodes), so voltage gain is =1. But when output is shorted feedback by voltage stops working ("closed" diodes), so input transistors have 100% NFB by current, output transistors amplify it up to safe level (actually, a bit higher than possible peak current on nominal load).
Wavebourn said:
Anatoliy, unless you have already patented your stage, anyone can do anything they want with it. The sole significant exception is in the U.S., where you have up to one year after public disclosure to file. Even so, anyone can use the published method up until the patent issues. If a commercial vendor wants to give you a royalty when you have no legal right to demand one, they of course can. But they are not legally required to do so. If you want to protect your ideas, keep them to yourself, or execute an agreement with every user to keep them secret. BTW, making and selling a product that comprises the idea is the same as publishing it, unless you have a secrecy agreement with each buyer.
Sheldon
Sheldon;
thank you for the patent law related lesson;
actually, I am for a freedom of flexible usage of electronic schematic solutions, like in case of Free Software Foundation. When you use some free software you have to follow some rules mentioned in the special file. Does not matter if it is protected by law, or just a jentlemen - like agreement, but I want the same with my electyronics ideas, so no crooks can patent what people are free to use as an air, or water, or whatever.
thank you for the patent law related lesson;
actually, I am for a freedom of flexible usage of electronic schematic solutions, like in case of Free Software Foundation. When you use some free software you have to follow some rules mentioned in the special file. Does not matter if it is protected by law, or just a jentlemen - like agreement, but I want the same with my electyronics ideas, so no crooks can patent what people are free to use as an air, or water, or whatever.
...and patent it?Sheldon said:
Wavebourn said:
And I have no problem at all with that.
Wavebourn said:...and patent it?
If you mean by this question, can someone else patent it? The short answer is no.
No one can take your published invention and file a claim on it. Even with the U.S. one year grace period, they would have to show that they invented first and made a filing within a reasonable period - not something they invented years ago and were prompted by your publication to file.
One technical addendum I should add to my prior comments on unissued patents: The publication of a provisional patent can allow for royalty claims from its date provided the patent issues with substantially the same invention claims.
Sheldon
For headphone amp (+/- 15 V rails), I have used the simple expedient of a 50-ohm or 75 ohm resister in series with the output. So far I have neither heard, seen or measured any adverse effect from this. Dead shorts appear to do no harm.
It's class-A so it's running full-out all the time anyway.
This may not be a suitable means in your case.
It's class-A so it's running full-out all the time anyway.
This may not be a suitable means in your case.
sam9 said:
Hi, I didn't want high output impedance for same reasons you don't want high output impedance from a speaker amp. Low output impedance for better damping factor, to give tighter bass. Some headphones may be designed to sound best from high output impedance (120 ohm is the official spec I believe). But most high end headphones work best from low output impedance like a loudspeaker does.
Hi,
it's a while since I looked at the topology, but I seem to recall that a CCS sinked, single ended ClassA output stage when bridged to a second like stage is immune to output short circuit.
The pair of low voltage stages can drive good current for typical headphone impedances from low voltage supplies and thus keep the ClassA dissipation manageable.
it's a while since I looked at the topology, but I seem to recall that a CCS sinked, single ended ClassA output stage when bridged to a second like stage is immune to output short circuit.
The pair of low voltage stages can drive good current for typical headphone impedances from low voltage supplies and thus keep the ClassA dissipation manageable.
XEAGLEKEEPER said:Is'nt there such a thing as a non-shorting headphone jack? Just asking.
If that's not feasible, here's a simple one that seems like it would be appropriate:
http://sound.westhost.com/vi.htm
Sheldon
Sheldon said:
A V-I limiter is a bit of an overkill for a headphone amplifier though. The easiest way to make a bullet-proof complementary emitter follower output for driving headphones with a high typical impedance, is to just use a pair of cheap but excessively robust transistors such as a BD139/BD140 pair with nothing more than a current limiting resistor in series with each collector.
Cheers,
Glen
G.Kleinschmidt said:
You guys know much more than I do about this stuff, so I mentioned this one to see if it would be appropriate. The article discusses the problem with VI limiters and loudspeakers. But that's a situation where the amp may be running near it's current limit under normal operation. I don't guess that would be the case with a headphone amp. So there should be a wide margin between the current max for the headphone and a dead short. That should make it easy to design a current limiter like this. Might be overkill, I don't know, but it only requires a few parts. Seems like it would be completely out of the circuit (except for a small sense resistor) under normal operation. No? Just curious.
Sheldon
Somehow I fail to see how a well constructed jack can short the amp output???
The jack insertion may well short the "whatever to be driven", in this case the headphones, but the amp output??
OTOH- most headphone outputs I have seen, does have a series resistor.. even just a few ohms is miles away from a dead short.
I'll admit I haven't paid much attention to the headphone market the last decade, but most if not all high end HP's use to be at least 100 ohms or more ( except ESL derivates of course..), which means that 5-10 ohms have no effect but a small attenuation. The psychological effect might be somewhat different, though.....
The jack insertion may well short the "whatever to be driven", in this case the headphones, but the amp output??
OTOH- most headphone outputs I have seen, does have a series resistor.. even just a few ohms is miles away from a dead short.
I'll admit I haven't paid much attention to the headphone market the last decade, but most if not all high end HP's use to be at least 100 ohms or more ( except ESL derivates of course..), which means that 5-10 ohms have no effect but a small attenuation. The psychological effect might be somewhat different, though.....
hi
I can't see why headphone amplifier need shortcut protection.
It is not like with power amps,
where you have two wires that you attach to output terminal
which by mistake could get into contact and cause shortcut.
Headphones are connected using some kind of plug into a jacket.
Teleplug 3-pol, 6.3mm or smaller Stereo Plug, 3mm.
Using this would make it practically impossible to cause a short.
The whole idea seems to me a bit unnecessary.
Advice:
Use standard plugs for connection:
Female socket in chassis and Male plug for phones.
By the way a question:
Does not your headphones come with such a plug at end of cable?
If we will discuss power amplifier shortcut protection
I may bother my head a bit more.
Regards
lineup
I can't see why headphone amplifier need shortcut protection.
It is not like with power amps,
where you have two wires that you attach to output terminal
which by mistake could get into contact and cause shortcut.
Headphones are connected using some kind of plug into a jacket.
Teleplug 3-pol, 6.3mm or smaller Stereo Plug, 3mm.
Using this would make it practically impossible to cause a short.
The whole idea seems to me a bit unnecessary.
Advice:
Use standard plugs for connection:
Female socket in chassis and Male plug for phones.
By the way a question:
Does not your headphones come with such a plug at end of cable?
If we will discuss power amplifier shortcut protection
I may bother my head a bit more.
Regards
lineup
![image005285[1].jpg](/community/data/attachments/70/70024-c0530b31ab208f0cac8854fb78e91fea.jpg)
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