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Old 25th September 2008, 02:10 PM   #1
Dxvideo is offline Dxvideo  Turkey
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Exclamation Another Class A headphone amplifier project.

Hi all,

I have just finished my second headphone amplifier projects design..
Here the schematic;
Click the image to open in full size.
This is the improved version of my old opamp driven SE MOSFET headamp..
Bias current is 85mA for each channel. So for 32R headphones it can produce at least 150mW power. And for the 600R headphones the max power will be also 150mW with +/-12V supply.
Against old design, the CCS is improved with a LM317 regulator. And the SE component is replaced with a BJT. The MOSFET design is leaved because of its gate capacitance..
At worst conditions, with just a 100Hfe of BD139 the opamp will sink/source just 0.85mA. So its enough to keep it under Class A.
I havent tested it yet. But on the simulation it works like a charm...
May you have some critics about the design please?
Thx in advance..
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Old 25th September 2008, 02:31 PM   #2
lineup is offline lineup  Sweden
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I havent tested it yet. But on the simulation it works like a charm...
May you have some critics about the design please?


No, no critic, Dxvideo ..... yet
Definitly one High Fidelity (hifi) design, in my book.


Because of
1. Regulated clean Supply
Using separate (from op-amp) Ouput Stage supply, would be overdoing it. For regulated PSU and 80-100mA amplifier.

2. High Perfomance OPA2134 Op-amp Signal quality Controller
The heart of this circuit.

3. As the Impedance OPA2134 drives is rather high (BD140 Base)
I would guess OPA2134 output never go out of True Class A operation.

4. Class A Single End Output with very high PSRR (thanks to LM317)
Normal CCS will have one resistor from supply rail, to bias the currect source.
LM317 does its own bias, same way as JFET CCS.
Only drawback with LM317 as current source, is high Saturation Voltage (guess need 2.0-3.0 Volt across).
But as the voltage out into Headphones is only a few volts,
in this case high sat is no issue.

Good designing, Dxvideo
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Old 25th September 2008, 02:40 PM   #3
Dxvideo is offline Dxvideo  Turkey
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Dear Lineup,
Thanks for the critics..
Is it better to make seperated PSUs for each channel you say
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Old 25th September 2008, 03:11 PM   #4
lineup is offline lineup  Sweden
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No.
You can use same +-12 VDC supply for both.
Remember to keep some distance from Transformer to the Op-Amps input.
At least 200 mm, I would say. Otherwise can be hum sounds.

I have used this for my HeadPhone regulated 12 VDC (7812) single supply.
Worked very well, even if my circuit did not reject power supply (lower PSRR) as well as you circuit does.
So, if I did not have problem feed BOTH channels fromsame regulator,
I would find it very, very strange if you will have.
----------------

At the regulator output you have Two Electrolytic Cap.
Can be 100-220uF. I do not know exactly.
From +pos and -neg.
This is common supply point for both channels.

From there you make separate wires to feed channels.
Or separate rails, if you use PCB tracks.
Close to the BD140-LM317 you put Two Electrolytic Cap in each channel.
Same here, I do not know exact value.
But I would try 220uF-470uF.

So to each channel goes 3 wires/rails:
V+ .... 0 Volt .... V-
From 3 points in Regulators Output.
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Old 25th September 2008, 04:26 PM   #5
jcx is offline jcx  United States
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I wouldn't recommend the shown compensation scheme, feedback C closing the loop around the Q with the high base drive Z and cable C that may reach nF seems to be asking for stability problems before tossing in the LM317 CCS undefined high frequency performance

the "usual" compensation for buffered output op amps is to connect your C13,18 feedback caps to the op amp's output, eliminating the uncertainty of the output Q and load added phase shift at high frequencies I expect these C to be much smaller in value when trimmed for good step response

the "noise gain" connection of C14,17 can be useful in some situations but I don't think this is the one of them - I would put similar size C from op amp +in to gnd to shunt RF input and give a high frequency AC gnd to the op amp when not connected to source

it is also possible to get into trouble with a high speed Q like the BD139 in an emitter follower driving a C load remember that cable
this could include oscillation at 10-100 MHz
either isolate the Q from load C with series emitter impedance like R||L or lossy ferrite bead
or high frequency terminate the Q base in a low R with a series RC from base to gnd with good hf layout
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Old 26th September 2008, 07:27 AM   #6
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Quote:
Originally posted by lineup
No.
You can use same +-12 VDC supply for both.
Remember to keep some distance from Transformer to the Op-Amps input.
At least 200 mm, I would say. Otherwise can be hum sounds.

I have used this for my HeadPhone regulated 12 VDC (7812) single supply.
Worked very well, even if my circuit did not reject power supply (lower PSRR) as well as you circuit does.
So, if I did not have problem feed BOTH channels fromsame regulator,
I would find it very, very strange if you will have.
----------------

At the regulator output you have Two Electrolytic Cap.
Can be 100-220uF. I do not know exactly.
From +pos and -neg.
This is common supply point for both channels.

From there you make separate wires to feed channels.
Or separate rails, if you use PCB tracks.
Close to the BD140-LM317 you put Two Electrolytic Cap in each channel.
Same here, I do not know exact value.
But I would try 220uF-470uF.

So to each channel goes 3 wires/rails:
V+ .... 0 Volt .... V-
From 3 points in Regulators Output.
Dear Lineup,
There is a big capacitor (1,000uF) near BD139 & opamps +power pin and another 1,000uF near LM317 & opamps -power pin. Also there 100nFs exist opamps both power pins.. I think that much capacitor is enough for a headphone amplifier..

Quote:
Originally posted by jcx
I wouldn't recommend the shown compensation scheme, feedback C closing the loop around the Q with the high base drive Z and cable C that may reach nF seems to be asking for stability problems before tossing in the LM317 CCS undefined high frequency performance

the "usual" compensation for buffered output op amps is to connect your C13,18 feedback caps to the op amp's output, eliminating the uncertainty of the output Q and load added phase shift at high frequencies I expect these C to be much smaller in value when trimmed for good step response

the "noise gain" connection of C14,17 can be useful in some situations but I don't think this is the one of them - I would put similar size C from op amp +in to gnd to shunt RF input and give a high frequency AC gnd to the op amp when not connected to source

it is also possible to get into trouble with a high speed Q like the BD139 in an emitter follower driving a C load remember that cable
this could include oscillation at 10-100 MHz
either isolate the Q from load C with series emitter impedance like R||L or lossy ferrite bead
or high frequency terminate the Q base in a low R with a series RC from base to gnd with good hf layout
Hmmm..
I am beginning to worry about my new design..
Concerning to change its compansation style;
If I put a compansation capacitor right opamps outputs then the frequency response becomes unlinear. (Tried on simulation) And also, I've replaced the output component with BD139 instead of a HEXFET in my new design because of HEXFETs gate capacitance..
If I put a capacitor there then HEXFET comes back again..

If I put another capacitor to input pin of OPA to GND then its a risk of reaching the sources capacitive drive limits! I've tried this with the first design, then I had different result with different sources. Especially with my CDP, treble details was very weak when this cap was exist.

So this is my solution,
If I replace my compansation capacitors (both on NFB line and for noise gain) with snubbers like 270R + 150p.. Does it works? Because on the simulation this solution looks like work...

And about the LM317s high frequency response.. I dont know anything about that.. But ones upon a time while I was making my regulated GC power supply, everybody mentioned bad things about the LT1083s high frequency response also. But I am using my supply for over six months and see (hear) no problem until now.. Anyway, I can design a MOSFET based CCS also.. Do you think it works better than LM317?

Thank you for the comments and advices..
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Ozgur
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