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Old 21st December 2012, 09:34 AM   #1
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Default New portable amplifier

Hello all,

As you maybe already know, I'm bulding a new headphone portable amplifier.

Yesterday night the courier bring to my door the free samples I asked for to Analog Devices, two pieces of AD8397ARDZ (the version with enhanced thermal capability) op-amp!

I asked for the free samples about a week ago, and actually didn't believe they would really ship them to me... I'm really surprised and happy.

Few LT1115, LT1677 and LT1010 should be on the road to my house too.
We'll see if they will arrive.

In the meanwhile, I ordered two differents LiPo charging, protection and step-up boards, THIS and THIS.
With these boards I can charge a LiPo battery, protect it and I should have a 5V output to power the amplifier. GREAT!
We'll see when the boards will arrive.
I already have preparred a LiPo battery, a Nokia one to which I removed the protection circuit (alredy included in the above board).

Next step will be to order asuitable enclosure and an Alps potentiometer.

So, I'm ready to start!

So, my idea now is to use one of the two AD8397 as a virtual ground (both channels paralleled), and the other one as output buffer for the two audio channels.
All the circuit will be powered at 5V, from the above step-up board.

I still have to understand how to disconnect the battery from stepup of the board when not in use: I'd like to leave the protection/charging circuit always connected to the battery, in manner I can charge it with the amplifier powered down, but I'd also like to disconnect the battery fom the stepup to save its juice.
Probably I'll have to cut few trace on the board to do it.

Currently I do not know if to use the AD8397 as a normal op-amp, or to use it a s a buffer, and preceding it with another op-amp (LT1677 or LT1115 that are on their way to me).

Any idea?
Any schematic you can throw at me?

Ciao,
Giovanni
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Last edited by croccodillo; 21st December 2012 at 09:40 AM.
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Old 21st December 2012, 02:22 PM   #2
DUG is offline DUG  Canada
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If you are ONLY using a 5V supply for the amplifiers, watch out for the output voltage swing limits vs supply voltage...not all of the devices you have listed have rail-to-rail output swing.

http://www.analog.com/static/importe...ets/AD8397.pdf
http://cds.linear.com/docs/Datasheet/lt1115fa.pdf
http://cds.linear.com/docs/Datasheet/1677fa.pdf
http://cds.linear.com/docs/Datasheet/1010fe.pdf

That should help narrow down your choices.

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Old 21st December 2012, 03:18 PM   #3
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Hello,

Thank you for the answer.
For what I have in mind, actually I only need the output device with full rail to rail swing, the driving op-amp do not need to be rail to rail.
This is a first skecth of what I'd liike to build:
Click the image to open in full size.

As you can see there's no virtual ground, but only the base circuit.
That said, the U2 will be an AD8397 device, rail to rail; its driver (now LT1677) could be any of the many audio op-amp in the market.
Any comment?
Any improvement on the circuit?

Ciao,
Giovanni
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Old 21st December 2012, 03:42 PM   #4
DUG is offline DUG  Canada
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Looks OK to me.

I looked at the data sheet for the AD8397.

I find it very funny that it is listed as a Rail-to-Rail Output device but the "fine print" states that the "Output swing to within 0.5 V of supply rails" (copied from pdf)

Further along in the text section: "When driving light loads, the rail-to-rail output is capable of swinging to within 0.2 V of either rail." (copied from pdf)

Not what I would call Rail-to-Rail Output but:

Just so you know.

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Old 21st December 2012, 04:52 PM   #5
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You can simplify the circuit and use just one dual opamp (plus second opamp to form virtual ground). AD8397 will drive low impedance headphones well. For virtual ground use LMH6642. What headphones will it drive (what impedance)?

Why you intend to use 5V DC converter? Headphone amp doesn't require stabilized DC.
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Old 22nd December 2012, 10:54 PM   #6
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Quote:
Originally Posted by enregistree View Post
You can simplify the circuit and use just one dual opamp (plus second opamp to form virtual ground). AD8397 will drive low impedance headphones well. For virtual ground use LMH6642. What headphones will it drive (what impedance)?

Why you intend to use 5V DC converter? Headphone amp doesn't require stabilized DC.
I will drive my Sony MDR-V55 headphones, 40 Ohms and 105 dB/mW.
The idea to use 5V is to use all the juice from the battery, I intend to use a single LiPo cell; that means that the voltage can go down to 2.7V before undervotlage protection trips: this is quite a low votlage, too low for a real amplifier.
Using a step-up, I will have a fixed 5V power supply until the baattery is completely flat.

You are right on using a single op-amp per channel; but I've heard that the AD8397 is not-so-special, sonically speaking, so adding a driver I can swap different op-amps and find the best combination.
Do you think is this useless?

Quote:
Not what I would call Rail-to-Rail Output but:

Just so you know.
I already know... but I need about 3.5 peak to peak to drive the headphones to a more than acceptable level, so 5V should be enough even if the AD8397 is not reail to rail.

Ciao,
Giovanni
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Old 23rd December 2012, 10:19 PM   #7
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I used single 3.7V rechargeable Li-Ion battery in portable amp with grado sr80 for long time, without feeling the need for more ps voltage. Your cans seem to be even more efficient, and also low impedance, so you can try it without 5V step-up ckt, which has unknown properties, such as output impedance. Li-Ion batteries itself are remarkable in this regard, not needing large capacitors across terminals.

Quote:
Originally Posted by croccodillo View Post
You are right on using a single op-amp per channel; but I've heard that the AD8397 is not-so-special, sonically speaking, so adding a driver I can swap different op-amps and find the best combination.
Do you think is this useless?
If you intend to put the pot at input, I would stick with one dual opamp for both channels.

I added unity gain buffer at input before pot, source was iPod 5G. See attached drawing. Omit switch S1, it is band aid for occassional use of sources with level => 2VRMS. Pot is 1K log scale (vishay sfernice P9) with series resistors to lower possible channel mismatch. Pot can be also 10K or 50K without problem. For gain of 2, R9 is 10R.

I didn't draw ps LMH6642 forms dual supply from one battery and rails are bypassed close to each opamp / each rail with 100uF tantalum and 100nF film caps.
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