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Old 4th February 2006, 01:59 PM   #11
Bazukaz is offline Bazukaz  Lithuania
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Thanks ,
How could i use a NPN transistor and a LED/Zener to make a css ?


Regards ,
Lukas
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Old 26th April 2012, 03:34 PM   #12
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Default ESL Headphone Direct Drive Solid State Amp

Stumbled on an ESL headphone amp circuit in a TI application note I thought might be of some interest.
I attached a copy of the relevent pages in case the link disappears in the future...hate it when that happens.
http://www.ti.com/lit/an/snaa046/snaa046.pdf
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File Type: pdf snaa046_pg4-6.pdf (153.5 KB, 55 views)
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Old 26th April 2012, 06:06 PM   #13
Calvin is online now Calvin  Germany
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Hi,

that ones on for quite a time ;-)
What I find very annoying is the fact that there are a couple of failures in the schematic.
More and more I get the impression, that circuits presented in application notes never made it even to the breadboard, but are just intellectual baby spoo.
It is for at least 20 years now possible to cut and paste correct schematics into a document.
Why are there still major faults in many and even simple schematics of global players like TI, LT et al?

jauu
Calvin
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Old 26th April 2012, 08:01 PM   #14
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Quote:
Originally Posted by Calvin View Post
What I find very annoying is the fact that there are a couple of failures in the schematic.
More and more I get the impression, that circuits presented in application notes never made it even to the breadboard
Hello Calvin,

I must admit I didn't give the circuit more than a quick glance since it is not really of interest to me at this time. But, I posted it since I figured others might be in search of such a circuit. If it is not too involved, and you have the time, you might describe the required corrections to the schematic to achieve a working circuit.

A 60 second look and I noticed:
1) the -15 VDC supply was mislabeled as +15 VDC
2) the balance resistor R11 is missing from the schematic
3) the +OUT line fails to connect to the drain of Q2
4) Q1 & Q2 most likely IRF720, not IRF72

Are these the type of things you are talking about?
or are you saying there is something fundamentally wrong with the circuit.

I know National used to provide PC board layouts and even demo boards for some circuits in their application notes. This provided some level of confidence that the circuits would work as described. But, this was certainly the exception rather than the rule.

Last edited by bolserst; 26th April 2012 at 08:25 PM.
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Old 26th April 2012, 11:14 PM   #15
Bazukaz is offline Bazukaz  Lithuania
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Hi,

Several years ago I have built successfully a rather simple direct drive amp.
The op amp was driving a high voltage FET in class A. Current source was made by using another FET and string of LEDs.
The amp is lying somewhere in the garrage but schematic seems to be lost.
I could dig my archives if anybody is interested in.

BTW - Calvin - a long time

Regards,
Lukas.
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Old 27th April 2012, 03:13 AM   #16
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So, is anybody into making electrostatic headphones as well?

Wachara C.
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Old 27th April 2012, 08:32 AM   #17
Calvin is online now Calvin  Germany
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Hi,

the list of corrections are what I found also.
In addition one should check the wattage of the drain resistors R8, R9.
Since the circuit runs in class-A, just 3W will be low.
My gut-feeling says, stay well clear and enjoy the fireworks.
Again, since running in class-A the Pot R11 takes on the current of both MOSFETs, hence it wattage needs to be observed. But in any case is it a rather bold idea to run loads of currents over the wiper of an pot.
Stability of the amp needs also to be observed. Adding external gain stages and include them into the feedback loop and driving capacitve loads a local loop (here C1) is almost always required. It might be, that a modification of the local loop is required to get the amp stable.
One might want to check what is happening at the amps outputs when starting and shutting down the amp.
The Bias generating resistor-networks values are weird. Here a Bias voltage of just 200V results, which must lead to gross distortions as soon as the signal voltage reaches or superseeds these 200V. A correct coltage would be >=400V or at least as high as the ESL-panel allows for.
R15 could be much smaller, or omitted with alltogether and optionally R16 could be raised in value up to ~1Meg.

jauu
Calvin
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Old 28th April 2012, 05:20 AM   #18
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I have messed with these types of circuits in spice and on the bench.

I have built a small version to prove the concept.
But I haven't made one large enough to drive an ESL nor have I finished my ES headphones yet.

Here are a few designs that I posted a while back,

All Discrete Class-A Headphone Amp

I drove a Piezo element to very high volumes on a 170v supply and quite cleanly as well,I might add.



jer
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Old 28th April 2012, 08:49 AM   #19
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Quote:
Originally Posted by geraldfryjr View Post
I have messed with these types of circuits in spice and on the bench.

I have built a small version to prove the concept.
But I haven't made one large enough to drive an ESL nor have I finished my ES headphones yet.

Here are a few designs that I posted a while back,

All Discrete Class-A Headphone Amp

I drove a Piezo element to very high volumes on a 170v supply and quite cleanly as well,I might add.



jer
You might find this interesting. Go to page 4

http://www.edn.com/file/15429-101404di.pdf?force=true

I believe 2 junctions are missing at the + and - 1000V inputs and the junction of R23 and the output should not be there. Scale this back to +/- 400 and it might be about right. Of course you need a total of 4 (2 inverting) for stereo push/pull.

G
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Old 28th April 2012, 10:02 AM   #20
PChi is online now PChi  United Kingdom
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Thanks for the links to the National Semiconductor / Texas Instruments Application note and to the EDN article. I agree that stability could be an issue driving the capacitive load.

It's a pity the semiconductor companies don't have the time to prototype the application circuits properly.
A rather weak excuse for the schematic errors is that they can be redrawn by the documentation / marketing department to conform to a 'company standard' so cut and paste may not be an option. Inter department rivalry and politics can also cause grief.

I am currently working on the design of a discrete high voltage direct drive amplifier that uses the capacitive load for frequency compensation. I am in the design stage and still need to prototype the circuit. I am using a push pull class A output stage using stacked N and P channel MOSFETS running in class A with a quiescent current of about 10 mA. Supply voltage about +/- 950 V. Peak single ended output voltage about +/- 920 V.
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