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Old 24th October 2011, 05:05 PM   #1
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Default Newbie - Schematic Help

Hi!

I'm attempting to cobble together a headphone amplifier for my wee brother. He has a pair of KNS-6400, which aren't terribly efficient.
This circuit uses a single (dual) AD8656, which seems very capable of fulfilling the requirements. Powersupply consists of 5 AAA batteries in series, which are fed through a voltage regulator.

It's my first electronics project and I've tried to keep it as simple as possible, while making sure the design is battery-friendly and somewhat portable.
Apologies in advance if the schematic seems a bit of a mess! Some things aren't quite there yet, but hopefully you'll be able to navigate through it.

I'm looking for some feedback before I finalize the schematic and order the necessary parts - is the general layout OK? Does it make sense? Have I missed something vital?
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Old 24th October 2011, 06:05 PM   #2
Mooly is online now Mooly  United Kingdom
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A few points... I have corrected the R/H channel so you can see.

The bias network can be shared between the two channels with no problems.

The feedback resistors (that set the gain) were incorrectly connected. The cap I have added reduces the gain of the opamp to unity at DC (so size the cap depending on the feedback resistors). If you are not sure ask.

The volume control was incorrectly wired.

The cap on the non inverting input prevents it being biased so removed it.

R4 would be (say) 1K with R5 and R6 equal in value and around 47K to 100K

You don't (imo) need the regulator... it loses vital voltage across it and you need every bit you can get on battery. Batteries are a clean low impedance supply anyway.

On a practical level the choice of opamp is critical to make the most of the low supply. I haven't looked the one you mention up

If the diode is meant to be an LED it's back to front. Use a high efficiency one and run at low current of under 1ma again to maximise battery life.
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Old 24th October 2011, 06:16 PM   #3
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OK... had a quick look at the data sheet and can see it's a CMOS opamp with a 5 volt supply limit. That's why you added a reg.

The absolute max ratings show 6 volts so it might be possible to use 4 batteries and if needed add a series diode (shottky type with low forward volt drop) to lose a few hundred millivolts.
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Old 24th October 2011, 07:01 PM   #4
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Thank you so much! It looks a lot better now (and simpler!).

I'm a bit confused about the feedback capacitor. I I do want gain, do I remove the capacitor? What happens I I keep the capacitor and change the ratio of the feedback resistors to increase the gain?

Yes, that's one of the reasons I added a regulator (a low-power LDO type). Another was that I'd recieved the notion that a battery source was "noisy" and that a regulator could clean it up a bit. I also thought that the opamp had to be supplied at 2.7 V OR 5 V and not just within the range... Oops!

Which leads to a question: will the sound change dramatically as the supply diminishes? As far as I can tell from the datasheet, it wouldn't be noticeable. But if it does and is, I'd rather supply it a constant voltage.
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Old 24th October 2011, 07:35 PM   #5
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Pretty brave for a first project. No idea how fussy the AD8656 is, its 28 MHz GBW is a little bit higher than what most audio opamps sport. Given it's a CMOS part, ESD precautions seem like a good idea.
As we can expect around a nF from a typical headphone cable, the RC snubber suggested in the datasheet may be worth implementing.

A pair of Schottkys from the noninverting input to supply and ground (normally nonconductive) for overvoltage protection should at least be considered. And what about thermal considerations? A wee little SO-8 package better get some groundplane to dump heat into if it's supposed to drive cans with authority.

For a first project, I'd normally consider something like a gain-hacked TDA2822M on a 9V block more appropriate.

BTW, as far as I can tell from measurements, KNS-6400s aren't that insensitive. About 34 ohms at 1 kHz, and around 100 mVrms for 90 dB SPL.

Oh, and the part about batteries being "low impedance"... well, not terribly. (LSD) NiMH AAs maybe, but non-rechargeable AAAs? (Near-empty ones even?) 9V blocks with their teeny tiny AAAA cells are even less exciting, some don't even hit 200 mA short-circuit current. A 'lytic of several 100 F definitely is a good idea.
Rechargeables can be noisy during discharge, see here, but that will also taken care of by the 'lytic.
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Originally Posted by ElectricApe View Post
I'm a bit confused about the feedback capacitor. I I do want gain, do I remove the capacitor? What happens I I keep the capacitor and change the ratio of the feedback resistors to increase the gain?
You can keep the values of the resistor and capacitor to ground constant (let's call them Rg=R2 and Cg) and vary the value of the feedback resistor (Rf=R1). Aim for 1 / (2 Pi Rg Cg) ~= 2..5 Hz. Cg is supposed to be transparent at audio frequencies, but since it blocks DC, it reduces amplifier gain to unity and hence reduces offset at DC.
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Which leads to a question: will the sound change dramatically as the supply diminishes? As far as I can tell from the datasheet, it wouldn't be noticeable. But if it does and is, I'd rather supply it a constant voltage.
Unless the OP starts clipping or its distortion behavior degrades significantly, there shouldn't be any audible change. You'd probably run the batteries flat first.

Last edited by sgrossklass; 24th October 2011 at 08:00 PM.
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Old 24th October 2011, 08:18 PM   #6
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Thanks! You guys are a tremendous help. I wish I'd posted here sooner! Instead I got caught up reading datasheets, which for some unfathomable reason is rather addictive.

I hadn't considered the temperature, which does raise a concern. I might try a DIP op amp if that type runs cooler, or give the project you linked a shot.

As for batteries, I was going to use NiMH AAA/AA. The discharge curves appear satisfyingly flat until about 1-1.1 V, at which they take a nosedive. Such behaviour would be beneficial, I think. I decided against 9V batteries because a) I don't want to buy/build another recharger , and b) graphs seemed to suggest they generally perform worse.

I couldn't find any measurements that show what the 6400 would require at 110-115 dB (loud, but he listens to classical from time to time). I had been using the table at NwAvGuy: More Power? to get an estimate (the efficiency is listed at 96 dB/mW in the manual), which seemed to suggest a fairly large power requirement.

Thanks again!

Last edited by ElectricApe; 24th October 2011 at 08:35 PM.
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Old 24th October 2011, 09:10 PM   #7
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Re: batteries, of course you can also use 6 AAAs or AAs instead of a single 9 V block. AAs in particular get a little big though - but would also power the circuit for many hours in return.
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I couldn't find any measurements that show what the 6400 would require at 110-115 dB (loud, but he listens to classical from time to time).
Since we know what they need for 90 dB SPL, math will do fine. About 1Vrms or 30 mW for 110 dB. Even the little gain-hacked TDA2822M seems to be able to deliver that without breaking into sweat. (You do want the 20 dB gain version though, else it'll be very noisy.)

BTW, I rarely ever listen louder than about 90 dB for fortissimo, it starts getting painful at this point. Admittely I'm not exactly a loud listener in general.
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Old 25th October 2011, 02:15 AM   #8
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Updated the schematic (still a bit of a mess, I'm afraid) with your suggestions (hopefully I implemented them not entirely stupid-like). Added snubber networks with some approximate values found in the datasheet for the AD8606, which I've switched to from AD8565. It's slower, which is nice, and just a bit noisier.
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Old 25th October 2011, 06:34 AM   #9
Mooly is online now Mooly  United Kingdom
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As to power requirements and what's really needed. Have a look at post #2 here where I did some real measurements on a pair of Sony 'phones.

GERMANIUM Single ended Class A Headphone Amp.

The AC voltage gain of your circuit is R1+R2/R1 so using standard values if R2 were 22k and R1 were 2K7 then the gain is around 9.1

The value of C1 needs to be large enough not to affect the low frequencies. Not going to low for R1 means that C1 can be smaller. A small 22uf cap would give a cut off point around 3hz.

The volume control should be in the 10 to 47k range.

On a really practical level I don't know how successful this will all be... it will definitely work, the question is whether under real conditions the circuit can put enough voltage across the load (the 'phones) given the low supply voltage.

Also when selecting opamps look at the packages available. The AD8656 originally mentioned appeared to be available in tiny SMD outline only which I suspect you don't want.
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Old 25th October 2011, 10:28 AM   #10
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I'm not sure why I put the pot at 100 Ohm. Thank you for noticing!

I'll take a look at the R1/C1 values and then I'll probably order the parts. Not that I'm not heeding your advice, but I'm curious to see how this circuit works out, even if the result is poor.

I've a friend with adapters and steady soldering hands, so I might give the (8606) SMD a try.

Thanks again! You guys are terrific.
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