Design critique: headphone amplifier - Page 1
Forum section: electronics / projects

author: microbug
author_rank: Frequent Contributor
timestamp: February 07, 2016, 01:13:14 pm
content: For the most recent schematic, please find my latest post.I finished the design for my headphone amplifier (schematic attached). Before I lay out the board, can you check it for me?Two questions:- Can I feed the two parts of the op-amp with the same fake ground, or would that introduce crosstalk?- Do I need to bother with hysterisis for the low-battery comparator? The circuit won't use much current (35mA max) so voltage drop should be small especially with the (relatively) massive 18650 battery I will use.Thanks!

author: Dave
author_rank: Super Contributor
timestamp: February 07, 2016, 01:30:52 pm
content: The gain will be 2.5 or 3, not 1.5 or 2.You need a low voltage cutoff, to avoid draining the battery too much and destroying it.

author: HAL-42b
author_rank: Frequent Contributor
timestamp: February 07, 2016, 02:05:13 pm
content: 1 - Separate the switcher ground and the opamp ground, connect them at single point only.2 - Ferrite beads, common mode chokes etc. etc. (aka. That switcher is making me nervous.)2 - Try two different values of caps at opamp input, one electrolytic and one ceramic. Make sure they don't ring (as per Jim Williams app note 47).http://cds.linear.com/docs/en/application-note/an47fa.pdf3 - There is no provision for charging of the battery. You are planning on replacing batteries all the time? Sounds like a hassle.

author: microbug
author_rank: Frequent Contributor
timestamp: February 07, 2016, 02:15:15 pm
content: Thanks to Dave - I have corrected those errors. I changed the gain to either 2 or 4, after reviewing the max output of my phone vs what my headphones can take. I stuck a BQ29700 in for lithium battery protection.1 - done now (I'll upload a new schematic when I've corrected the other problems)2 - Will do3 - I assume you mean at the op-amp's power supply, not the inv/non-inv inputs?EDIT: I see what you mean, I didn't realise I had left the values of the decoupling caps the same. I've changed them to one 47µF electrolytic and one 100nF ceramic, and will make sure they're properly laid out as per that app note. I've also substitued some of the 10µF ceramics for 47µF electrolytics (all of them have 100nF ceramics too so ESR shouldn't be a problem).4 - I'll stick an MCP73831 in there since I have a few anyway. This thing shouldn't need charging often though: at 50mA, it would take 68 hours to deplete the 3.4Ah NCR18650B that I'll use.

author: microbug
author_rank: Frequent Contributor
timestamp: February 07, 2016, 02:39:21 pm
content: OK, current schematic attached. I haven't put in the battery charger yet but that's easy enough. I put inthisferrite bead (600 ohm version) on the output of the LDO and a choke on the output of the switcher - I don't know what inductance to use, any suggestions?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 07, 2016, 03:20:46 pm
content: 1. No designators or net names... so I can't even call things out for critique.2. No battery protection. If this is a Li Ion or related battery, you don't want to discharge it below 3V. (Maybe a fuse too, in case the converter or load shits the bed, or something?)3. Low battery indicator literally makes things worse, and if you don't see it, well... yeah.A battery management IC would be safer. (Ideally, one with integrated charge controller so you can maybe run the whole thing from, like, a 5V USB charger cord or something?)4. The LDO probably isn't necessary. The battery is almost noiseless, so you're only filtering against whatever noise the converter produces (which shouldn't be much worse than the LDO itself), plus switching noise. The switching noise is near 1MHz, which the LDO says will be around -40dB, but it looks like a capacitor up there. You could do just as well with an LC filter of 0.5uH and another 10uF, for example (which will be cheaper and smaller; and besides, you already have the other 10uF). Note that this does nothing about possible common mode noise, which you'll have to address in the layout (keep the switcher well away from the signal path).5. Not just the input coupling caps, but all caps in the signal path (which includes the output caps, and the coupling caps for the inverting input pins) should be low-piezo types. Which really stinks for SMT construction, but if you don't mind using tantalum, that'd do a fine job. Otherwise, SMT films suck for a number of reasons (they're still large; they're expensive, and although it's not a problem here, they do absorb flux/moisture, a problem for sensitive circuits). But if you have to use through-hole films, that's hardly the end of the world.6. The input and output should be referred to the "virtual ground", not power supply ground. This way you only need one divider, and you don't get a power-on/off "pop". Or you could reconfigure the switcher so it makes symmetrical e.g. +/-5V rails (which isn't hard to do, using a dual winding inductor), but that's dumb. As long as you don't need common ground between things*, it's fine.*Which is actually a good point, if you add a e.g. USB charging connector -- charging it from the same computer it's connected to would short out the negative rail, making a bipolar supply better.7. A make-before-break switch is preferred for the gain setting. But even better: you can swap the cap-to-GND with the resistor, and switch a resistor in parallel with the existing resistor, needing only a DPST switch.As shown, with a break-before-make (most common) switch, you'll get a pop as it goes to unlimited gain for a few milliseconds and probably makes a nasty output voltage.8. More gain settings would be better, too. 1.5 and 2 is hardly 2dB different, not even worthwhile for headphones. One or two 10dB steps (~3x gain per step) might be helpful, to deal with widely varying sources. Note that attenuation might even be desirable, which you can't implement on the feedback resistors. (You could with an inverting configuration.) A good old fashioned volume control would be the most universal, of course.9. The op-amp only delivers ~40mA (as measured from 15V supplies; unknown if that's supply independent), which might not be enough for very loud signals into 32 ohm headphones. A buffer/driver/power amp device might be better suited, at least if high dynamic range is desired.I think that covers about everything, HTHTim

author: langwadt
author_rank: Super Contributor
timestamp: February 07, 2016, 04:21:20 pm
content: it is not all opamps that like to drive the capacitance of a cable without a bit of resistance on the output

author: microbug
author_rank: Frequent Contributor
timestamp: February 07, 2016, 04:26:56 pm
content: langwadt - I have added a 100 ohm resistor on the op-amp outputs. You're right, the datasheet for the OPA1612 recommends a small resistance on the output.T3sl4co1l - thanks for the feedback. I think you might be working from the original schematic - the (now second) most recent one (in my last post) has some improvements. The latest schematic is attached.1. I'll add designators.2. Already fixed in most recent schematic.3. I know the battery low LED won't help. Originally it was there in lieu of a protection IC so I know when to recharge, but now that there is one so the LED isn't useful. I have removed it.4. OK, I have removed the LDO and added that LC filter. I removed the choke since the LC filter should do the job. I'll keep your post in mind when laying out.5. I don't mind tantalum: I've put tantalum caps in.6. I have put a SPDT switch in for the power switch, to make it only charge when off. I have made the inputs and outputs referenced to the virtual ground, so that should solve the problem.7. I have switched the cap to GND and the resistor, and added the DPST gain setting. Thanks - I hadn't considered the pop but it would be there.8. The gain as of last post was 2 or 4, but that's only 3dB. I'll make it 2 or 8.9. This amp is really only for my 80 ohm DT770 headphones which benefit from a bit of a boost. I'm no audiophile and don't want to deafen myself; my phone will be fine for driving 32 ohm headphones.Question: for 7, what is the advantage of switching the cap to GND with the resistor? Also, do I have enough filtering now?Thanks to everyone for helping!EDIT: just realised GNDA and GNDPWR aren't connected. Updated schematic with fix attached.

author: dom0
author_rank: Super Contributor
timestamp: February 07, 2016, 04:50:14 pm
content: If you drive a headphone from a high-impedance output, make it 120 ?. The designers of the transducers either assume a => 0 ? drive or ~120 ?. The sound of them changessignificantlywhen varying drive impedance.This is pretty much indepedent of the headphones, it was easily observable in Beyerdynamic DT-770 Pro (my main phones), various no name and brand name in-ears and a Sennheiser I borrowed for testing.SW?B (get your ref designators straight please)willpop your ear, unless it is guaranteed make-before-brakebothways.Already fixedIt should be noted that there are many ways around using so many capacitors in the signal path. I tend to see that as a sign of poor design. With modern op amps the offset voltages are so low that there is no need for additional LF roll off (neither in- nor out-loop) other than an input coupling filter.

author: Lightages
author_rank: Unknown
timestamp: February 07, 2016, 04:55:35 pm
content: This 120 ohm output impedance requirement fallacy needs to stop. This has been discussed before. The lower the output impedance, the better. If one insists on having 120 ohms, make it a switchable resistor on the output so it can be bypassed.

author: dom0
author_rank: Super Contributor
timestamp: February 07, 2016, 05:00:06 pm
content: orororyou could read the complete paragraph I wrote, not just a number.

author: Lightages
author_rank: Unknown
timestamp: February 07, 2016, 05:11:21 pm
content: I did read your post, and it isn't clear if you are advocating 120 ohms or not. So I made my position clear.

author: microbug
author_rank: Frequent Contributor
timestamp: February 07, 2016, 06:17:41 pm
content: Latest schematic attached. I removed the output decoupling caps, but I don't think I can remove C7 and C16 because the output is biased at fake ground. I changed the output resistance to 47 ohms, since it seems that lower is better. If there is proof otherwise, let me know.EDIT: I have replaced the unpolarised symbols with polarised ones for the tantalum caps.EDIT 2: 10nF tantalum caps don't seem to be available so the input decoupling caps will have to be film.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 07, 2016, 06:32:43 pm
content: Quote from: microbug on February 07, 2016, 04:26:56 pmYes, in the time I took to write that, well...QuoteThanks!QuoteHmm, 100nH isn't very much. The capacitance after is quite a bit larger than necessary, too: C9 and C16, when just C13 will do. And having just the one chip, there's not really any need for extra bypasses (C8, C15).QuoteYou'll be hard pressed to find one as small as C4/C14 I think, but those impedances can be made lower too, or the additional LF bandwidth simply allowed.QuoteBy switching the value of R6/R13 instead, you can put the switch to ground, minimizing possible noise introduction, say if the switch is on wires and mounted to a panel. Probably a very small issue indeed here, but hints at an underlying method: the switch should be on the least sensitive nodes in the circuit, if possible. Putting it on the input pin side (where C5/C17 are now, or the same side of R7/R16) would introduce more sensitivity to noise.QuoteHow well does that work in KiCAD? What does it end up calling it? Does it compile as a net bridge? Is one name preferred? Are they aliases somehow?In Altium, that generates a net naming conflict error; in others, the net is a property of the schematic rather than of graphical objects, and cannot be misnamed (e.g., Multisim).It's also odd that VGND bypass is asymmetrical (only C18 to GND, but nothing to +V); and R6/R13 can be returned to VGND, so they have the same reference as the input and output.And R5/R12 can be smaller, since C4/C14 will probably be larger, which will reduce the voltage offset due to input bias current. You might want to keep the input coupling capacitors, but strictly speaking, beyond that, you might not need C5/C17 and C11/C19 at all.Also, D3 should probably be moved to +BATT so the resistor drops less voltage, and the resistor should be larger so as not to blind the user, and to save a few mA.Tim

author: microbug
author_rank: Frequent Contributor
timestamp: February 07, 2016, 07:46:34 pm
content: QuoteJust checking you had noticed the new attachments! Thanks for helping.QuoteI meant to write 500nH as originally suggested. I removed those extra caps, thanks.QuoteThe smallest widely available tantalums I can find are 100nF, so I'll make the impedance 100k.QuoteI've removed C5/17/11/19.QuoteThanks for clarifying, I've sorted it out now. I upped the resistances in the feedback loop by 10× since there isn't a cap there to cause trouble at startup now.QuoteThey compile as a net bridge. I just checked by generating a net file, and the name is GNDA, presumably because that was the first one. During layout I'll separate the power from the audio, and do a fill in each, connected by one point.QuoteFixed.QuoteDoneNew schematic attached. I re-annotated it since several components were removed.EDIT: Schematic now more organised.

author: jeroen74
author_rank: Frequent Contributor
timestamp: February 07, 2016, 09:16:10 pm
content: You might want to take a look at the circuit around C8,R12 and C7.

author: microbug
author_rank: Frequent Contributor
timestamp: February 07, 2016, 09:30:39 pm
content: Well spotted. C7 and R12 should be connected to Vout.EDIT: Fixed schematic attached.

author: dannyf
author_rank: Super Contributor
timestamp: February 07, 2016, 11:38:20 pm
content: You probably want to think about how this amp is going to be used.For most headphones, you wouldn't need to swing more than 1-2v to produce sound that's too loud for most people. With that in mind, I would suggest that going to 9v isn't worth it, particularly at the cost of having a dc/dc converter this close to the audio amplifiers.For most of those designs, it is the current drive capabilities that matter -> for low-impedance phones. 20ma max is OK but you may want to buffer it, with an IC, a diamond buffer, or even an emitter follower.If I were you, I would put one dual-opamp per channel. One acting as a rail splitter and another as a follower. Simple and no cross-talk.

author: Dave
author_rank: Super Contributor
timestamp: February 07, 2016, 11:57:33 pm
content: I see a potential problem in your circuit. If you were to simultaneously plug in charging and audio cables from the same device (a computer, for example), you would be shorting out the virtual ground to your actual circuit ground.

author: microbug
author_rank: Frequent Contributor
timestamp: February 08, 2016, 08:07:38 am
content: I added a negative charge pump to the boost converter, giving me a negative rail so that the charger and op-amps are running from the same ground. Schematic attached.Dannyf- I'm only going to be using this to drive high-impedance headphones. My phone (which will be feeding this amp anyway) will do fine for 32 ohm things. I can bring the voltage of the boost converter down; I agree that 9V is too high - it is left from when I was going to use an LDO with it. The op-amp's common-mode voltage range is Vdd-2 to Vss+2, so it wants at least ±3V for the max ±1V signal it will get from my phone, unless I am mistaken. I could attenuate the signal before it is amplified, of course. Is there a viable alternative to having a switcher this close when only using one battery?Dave - The new negative rail should solve this.EDIT: Added a second charge pump (so the rails give symmetrical voltages), and reannotated schematic. Rails are now ±5V. Updated schematic attached.

author: dannyf
author_rank: Super Contributor
timestamp: February 08, 2016, 09:58:26 pm
content: You probably want to experiment a little on the need for gains to drive your phones to sufficient loudness. High impedance phones generally are more sensitive than low impedance phones. So it is unclear to me if you really need that much drive.If a 2Vpp is all it takes, a 3.7v battery will work fine for most opamps without any DC/DC conversion.

author: dannyf
author_rank: Super Contributor
timestamp: February 08, 2016, 09:59:48 pm
content: As to the use of a charge pump, you should look into their dynamic performance - as you are using them to power class b amps.

author: microbug
author_rank: Frequent Contributor
timestamp: February 08, 2016, 10:00:58 pm
content: Would an inverter be better? In the other thread, someone suggested the MAX660 which would appear to be more foolproof than a charge pump.

author: dannyf
author_rank: Super Contributor
timestamp: February 08, 2016, 10:52:23 pm
content: I guess I am trying to say that the best solution to any problem is not having that problem in the first place.I don't think you have established that you needed sufficient gain to drive your phones. Unless that's established, it makes no sense, in my view, to expand the supply rail.

author: microbug
author_rank: Frequent Contributor
timestamp: February 08, 2016, 11:25:21 pm
content: Ok. The problem here is that my headphones haven't arrived yet, but from reviews I gather that around 1.5V RMS max is needed to drive them. My phone will do max 1V RMS into a high impedance source. To account for weaker sources, I will have this amp at 2x gain. The OPA1612's common mode voltage range is 2V from either rail: the solution then is to have no boost and an inverter for the negative rail, plus a small voltage divider on the input (accounted for in gain) to stop the input going past 1V peak when the battery is low (3V).I'll post a schematic tomorrow. Thanks for helping me realise the above.

author: Audioguru
author_rank: Super Contributor
timestamp: February 09, 2016, 12:25:56 am
content: I saw the tiny low value output capacitors then the same as the virtual supply filters in the 25 previous schematics and I knew that there will be no bass sounds. I am glad you removed them so now the bass sounds will be awesome. Your gain switch will change the volume only a little and is not worth having.

author: dannyf
author_rank: Super Contributor
timestamp: February 09, 2016, 12:41:40 am
content: 1.5v rms on a 32 ohm load means 70mw. Most phones are at least 90db/mw and putting 70mw into such a phone will make deaf, at least temporarily.Either your phone is indeed extremely insensitive or you seriously over engineered this thing.

author: rickey1990
author_rank: Regular Contributor
timestamp: February 09, 2016, 12:59:38 am
content: MY DESIGN CRITIQUE.1)We live in a consumer society, most stuff is made cheap and isnt really made to last. so its great that you want to make something and that your trying to make it as good as can be.2) The best thing to do as you said is to test it. id recommend using a cheap pair on head phones and do some destructive testing on it, get some cheap boards printed, plug the battery in the wrong way round, overload the circuit. But id also say breadboarding first, even though computers can simulate something very well. hands on prototyping will let you see how well your circuit really works.3) After you've finished your project you may want to sell it, so see if you can get it to work well with other brands of headphones.4) Advice is great and always recommended. But real innovation comes from new idea's & experimenting, okay you may make mistakes, we all do, but you will learn from them.5) Keep up the good work.

author: dannyf
author_rank: Super Contributor
timestamp: February 09, 2016, 01:17:17 am
content: also, 1.5vrms into 32ohm means peak current delivery of close to 70ma.Most opamps wouldn't be able to deliver that kind of current. and I would be surprised if yours can - check its datasheet.If that's really the kind of performance you are going after, look for 150ma / 200ma current capable opamps. Particularly xDSL drivers - they tend to be CFB opamps and delivering 200ma is a piece of cake for those guys. AD815 or THS6012 - it has a headphone amp counterpart too.If you aren't looking for super-duper specs, I would suggest that you look into tda1308 - it is my opamp of choice right now,

author: dadler
author_rank: Unknown
timestamp: February 09, 2016, 03:54:05 am
content: Consider using BUF634 as higher current output buffer.

author: microbug
author_rank: Frequent Contributor
timestamp: February 09, 2016, 05:57:58 pm
content: Audioguru:QuoteI get the pointQuoteI have come to the same conclusion myself and will remove it.dannyf:QuoteMy headphones are 80 ohms, so the max power I can get from 1.5V RMS is about 28mW. While 1.5V RMS is overkill, the max voltage you could realistically want according tothisreview is 1.2V RMS, and I'd like a little room for audio sources potentially weaker than my phone (PC sound cards, for instance).Quote18.75mA into 80 ohms. Totally doable by the OPA1612, but see end of post.QuoteIt is, at 80 ohms.QuoteThanks! I think I will -- see end of post.rickey1990:QuoteThe included protection IC will stop any current from flowing past a few hundred mA.QuotePossibly. Depends on demand. If I do I might add an output buffer so it can work with lower impedance headphones.QuoteThanks! Much of the work here has been done by those helping me thoughQuoteI agree.blueskull:QuoteChanged to 1M and 47nF, for a 3.4Hz roll off.QuoteQuoteQuoteSee end of post.QuoteWill do.If I use the TDA1308:This chip presents yet another power supply problem, in that it won't be able to do both a positive and negative rail from my lithium ion battery with just an inverter (max voltage +7V or ±3.5V).After Audioguru's comment I looked up the value of a coupling capacitor with a -3db frequency of 15Hz, and it was 130µF (round to 150µF). I googled whether a tantalum cap would do in audio coupling and came up withthis post in another forum, answered by (presumably the same) Audioguru who said, no. Electrolytics are apparently bad too. This leaves me with the following options:- Fake ground solution (can't charge and use simoultaneously)- Buck/boost to 3.5V w/ inverter (noisy)- Magical capacitors I haven't heard about that are not physically too large at 150µF- LDO to 2.8V w/ inverter (UVP for the battery kicks in at 2.8V)If I use the OPA1612, I need a buffer and it needs to have a 600R+ input impedance for the op-amp to work properly. For the buffer I would choose a unity-gain chip (or high-current op-amp), rather than do it with discretes, due to board limitations and complexity.I'm leaning towards using the TDA1308, with the last option.Also, the TDA1308 datasheet lists common-mode voltage range as 0 to 3.5 pA. I'm sure that can't be right; does anyone have some idea what it will be?No point in doing another schematic until I have chosen an amplifier, but the other changes are- Film caps on input- Lower input roll off- Remove gain switch and set gain at 1.5×Thanks,microbug

author: microbug
author_rank: Frequent Contributor
timestamp: February 10, 2016, 07:16:20 pm
content: OK, schematic attached. Most changes are above (I went for the TDA1308). I also added a battery low LED, as the LDO might drop out before the battery protection circuit (at this point there is very little charge left in the battery anyway). Since the datasheet doesn't give any info on input range, I added a voltage divider to each audio channel to prevent non-linear activity.EDIT: Re-annotated schematic.EDIT 2: Corrected name of comparator from TLV3001 to TLV3011.

author: rob77
author_rank: Super Contributor
timestamp: February 10, 2016, 08:37:14 pm
content: charge pump driven negative rail for an amplifier ? i would rather stick to single suply + output caps.from the max660 datasheet:? 6.5? Typ Output Impedance? Guaranteed ROUT < 15? for C1 = C2 = 10µFactually 15ohm output impedance is not what you need from a supply providing the juice to an amplifier driving 32ohm headphones.

author: microbug
author_rank: Frequent Contributor
timestamp: February 10, 2016, 08:41:46 pm
content: OK, but what type of cap? For a 15Hz -3dB roll off on 32 ohms, you need a 330µF cap!EDIT: Also, looking at the graph from the datasheet it seems more likely to be 7-8 ohms. Max voltage drop of 400mV with a min output of -1.5V... the datasheet says it should work.EDIT 2:More details: at 32 ohms the TDA1308 can do 0.75V above its negative supply voltage. 0.75+0.4=1.15V drop. -2.8V+1.15V = -1.65V worst-case minimum output voltage.Calculated it wrong. Lowest possible output voltage is more like -2.1V, so the inverter would not work when at max volume even when the output impedance is high.

author: rob77
author_rank: Super Contributor
timestamp: February 10, 2016, 08:55:42 pm
content: Quote from: microbug on February 10, 2016, 08:41:46 pmwhat's the problem with the 330uF cap ? you can have a SMD electrolytic in a relatively small package... for example "SMD; 330uF; 6,3V; Ø6,3x7,7mm; ±20%" is priced 11 cents if you buy 10pcs (TME).max voltage drop 400mV means the absolute value of your negative rail will be max 400mV less than the positive rail (unloaded negative rail)even 7-8 ohms is 25% of the load , it means 25% less swing on negative part of the waveform => lot's of distortion.

author: microbug
author_rank: Frequent Contributor
timestamp: February 10, 2016, 08:57:25 pm
content: Quote from: rob77 on February 10, 2016, 08:55:42 pmThis is the problem.OPA1612 + buffer is starting to look more appealing...

author: rob77
author_rank: Super Contributor
timestamp: February 10, 2016, 09:02:36 pm
content: Quote from: microbug on February 10, 2016, 08:57:25 pmyou provided a link to some article about TANTALUM caps not ELECTROLYTIC - those two are different things.

author: microbug
author_rank: Frequent Contributor
timestamp: February 10, 2016, 09:05:50 pm
content: From link:QuoteEDIT: second quote from link:QuoteI wonder if @Audioguru would like to comment?

author: rob77
author_rank: Super Contributor
timestamp: February 10, 2016, 09:17:20 pm
content: ok so you're afraid of inaudible distortion coming from the electrolytic caps but your'e ok with a butload of distortion coming from a weak negative rail... makes sense

author: microbug
author_rank: Frequent Contributor
timestamp: February 10, 2016, 09:19:31 pm
content: I just want a second opinion on the electrolytic caps. Seems reasonable enough since they have been criticised elsewhere. And if you weren't following, I did realise that the inverter is not an option at -2.8V.

author: Audioguru
author_rank: Super Contributor
timestamp: February 10, 2016, 09:23:58 pm
content: An electrolytic or tantalum capacitor changes its value when the voltage across it changes. So how do you prevent the voltage across it from changing? By simply increasing its value then the lowest frequency it passes will produce only a small voltage across it.

author: microbug
author_rank: Frequent Contributor
timestamp: February 10, 2016, 10:43:12 pm
content: Going from 330µF to 470µF sounds like it should do then. Schematic attached, with a boost converter to 6V, virtual ground for amplification and normal ground for output (AC coupled). Hopefully this should be good enough now. It should do 32 ohms quite easily.

author: rob77
author_rank: Super Contributor
timestamp: February 10, 2016, 11:23:30 pm
content: you still have errors there. check your GNDA vs VGND in the schematic. and i'm afraid your 100k/200k feedback resistors are too high. and also the 100K dividers will make a weak artificial ground.btw... what's wrong with the typical application schematic ? - datasheet page 6 fig. 4http://www.nxp.com/documents/data_sheet/TDA1308.pdf

author: Audioguru
author_rank: Super Contributor
timestamp: February 11, 2016, 04:05:57 am
content: Quote from: rob77 on February 10, 2016, 11:23:30 pmI think it is designed for a tinny-sounding toy radio:1) Electrolytic input capacitor produces even harmonics distortion and a cutoff at 41Hz.2) Output capacitor as above but with a cutoff at 50Hz.Then both cutoffs affect 100Hz and lower.

author: nano
author_rank: Contributor
timestamp: February 11, 2016, 06:56:40 am
content: Regarding the discussion of electrolytics in the signal path. Self found no measurable distortion introduced by electrolytics if you don’t drop more than 80mV across them (Book is Small Signal Design).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 11, 2016, 09:03:06 am
content: Audioguru: Tantalum and electrolytic capacitances are independent of applied voltage. Tantalum more so. You're thinking of ceramic capacitors.Ceramic capacitors also exhibit piezoelectricity. They were recommended against very early in the thread.Tim

author: rob77
author_rank: Super Contributor
timestamp: February 11, 2016, 09:06:16 am
content: Quote from: Audioguru on February 11, 2016, 04:05:57 ami meant the circuit itself - you can adjust the capacitor values to bring the frequencies down.OP is using 100k+ feedback resistors, while the recommended circuit uses feedback resistors in a kohm range. furthermore OP is using a nonivenrting setup while the recomended is a inverting. OP is using 100k dividers for virtual ground (weak) while the recomended has a harder virtual ground with 10k...

author: microbug
author_rank: Frequent Contributor
timestamp: February 18, 2016, 11:28:29 pm
content: I haven't abandoned this project/thread; I've been away for a week.Quote from: nano on February 11, 2016, 06:56:40 amQuote from: T3sl4co1l on February 11, 2016, 09:03:06 amGood to know. I've replaced all the 10µF ceramics with electrolytic equivalents. Correct me if I'm wrong, but it seems unlikely to be a problem with the SMPS as the caps will be low ESR and the current will be low. If need be I could use tantalum in the boost converter.Quote from: rob77 on February 10, 2016, 11:23:30 pmI've switched to the inverting config as per the datasheet now, thanks. The datasheet lists most specifications for inverting only so I assume that it provides lower THD+N.Quote from: rob77 on February 11, 2016, 09:06:16 amFixed. Feedback resistors are now 22k and 33k, with the input film caps at 2.2µF. I have set the virtual ground to use 10k resistors now.I looked up an older datasheet for the TPA1308 and found that the common mode voltage range is V- to V+-1.5V (the current datasheet has a typo; the units for this field are pA when they should be volts). According to theWikipedia page on line levelthe peak input voltage will be around 1.7V with pro gear and 1.4V with consumer gear, so I upped the boost regulator to 7V (±3.5V with virtual ground) for a little more margin: the max supported input voltage will now be 2V peak, 1.4V RMS which is plenty.Re-annotated and updated schematic attached. The organisation is now battery-related things on the top, power supply in the centre and amp on the bottom.

author: dannyf
author_rank: Super Contributor
timestamp: February 18, 2016, 11:30:48 pm
content: "I assume that it provides lower THD+N."Probably not a good assumption.

author: microbug
author_rank: Frequent Contributor
timestamp: February 18, 2016, 11:47:46 pm
content: Quote from: dannyf on February 18, 2016, 11:30:48 pmOops, you're right. Just looked at the datasheet: the only specs which specify inverting operation are slew rate and bandwidth. Nevertheless it seems safer to go with inverting,

author: dannyf
author_rank: Super Contributor
timestamp: February 19, 2016, 12:00:23 am
content: That makes sense as an opamp doesn't know if it is in a non inverting or inverting configuration. All it does is to take the difference between the input signals and amplify it.BTW, most audio amps are mom inverting I think for historical reasons.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 19, 2016, 06:02:38 am
content: Well, you probably still want ceramics around the SMPS, to keep ripple low. A little piezoelectricity in the supply rail won't make much difference to the amp (on the order of ~10mV due to microphonics, at audio frequencies where the amp is fine with it -- versus ~1MHz switching ripple that'll go right through the amp!).Tim

author: Warhawk
author_rank: Frequent Contributor
timestamp: February 19, 2016, 07:43:20 am
content: Inverting topology for the opamp may reduce common mode distortion. Some time ago I built this:http://www.redcircuits.com/Page119.htm