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Old 21st January 2012, 08:55 PM   #11
jcx is offline jcx  United States
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oscon/polymer don't requre as much derating as normal eletrolytics but you are setting someone up for failure to dismiss derating without close reading of specs
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Old 21st January 2012, 09:10 PM   #12
Xoc1 is offline Xoc1  United Kingdom
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So with regulated voltage rails and the the power requirements of a headphone amplifier what would you concider a safe derating factor?
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Old 21st January 2012, 09:18 PM   #13
jcx is offline jcx  United States
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you need to consider reg circuit tolerance, cap manufacturers specs

3% Vreg ref tol + worst case 1% Vsetting R tol gives 15.75 V out - assuming std R values don't make the nominal setting worse

wet Al electro are often used at <80% of rated working V derating
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Old 21st January 2012, 09:19 PM   #14
gootee is offline gootee  United States
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You will have to be careful to not create any unwanted high-frequency resonances, by using caps with very low ESR, such as film, in the presence of nearby parasitic inductances (such as an electrolytic cap, and/or the supply rails).

For the small-value caps (if used), X7R ceramic is usually recommended instead of film. But if you insert some resistance, as I think GLooP suggested, then that possible-resonance issue might be alleviated somewhat.

The caps will primarily provide decoupling. i.e. Like a small point-of-load power supply, they will supply the transient current demands that might be impossible to get through the inductance of the supply rail, with accurate timing, and without causing an unacceptably-large voltage disturbance on the supply rail.

You will get better transient performance if you use multiple smaller capacitors in parallel, instead of one 47 uF capacitor, especially if their connections are also all paralleled all the way to where the decoupling is needed, i.e. not shared in common. That way, the inductances of the caps and the connections will also be reduced, in the same way that resistances reduce when placed in parallel, and you can get the inductance to be less than that of one capacitor and its connections, which only works if there is no mutual inductance. As a bonus, the total ESR will also be reduced in the same way.

You want the impedance, as seen by the supply/gnd pin pairs of the active device, to be low-enough, up to the frequency that corresponds to your fastest-possible rise time (which will most-likely be much faster than is required for a 22 kHz sine, by the way; maybe use the opamp datasheet "typical" value).

f = 1 / ((π)(trise))

If you choose a maximum allowable voltage rail disturbance value, dv, and figure out what the maximum transient current change, di, might be (probably the zero-to-rail voltage divided by the load resistance), i.e.:

di = rail voltage / load resistance (or use worst-case load impedance)

then you can calculate the target maximum impedance that should be seen by a power/gnd pin pair of the active device:

Zt = dv/di

If you use your now-known di, dv, and trise (dt), and an equation similar to the standard differential equation for a capacitor, you can calculate the capacitance needed in order to be able to supply a transient current that will slew by di amps in time dt while only disturbing the voltage rail by dv volts:

C = di (dt/dv)

I think that you can calculate the maximum allowable total inductance for that capacitance and its connections to the decoupled points by using an equation similar to the standard differential equation for an inductance:

Lmax = dv (dt/di)

You can probably estimate the actual inductance for each capacitor by summing the conductor lengths for its connections to the decoupled pins, and its lead spacing, and multiplying by around 15 nH per inch. If you get more than Lmax for the best-case layout and one cap, you might need to use parallel caps with parallel connections. But that's just the inductance. I think you'd still need to check whether or not you can meet the Zt target impedance requirement up to frequency f, too, which might be harder.

You would need to take into account the capacitances, parasitic inductances, and parasitic resistances, to see if you meet the target impedance up to frequency f. I usually just use the LTspice simulator for that, with AC Analysis with an AC=1 current source (where the active device would be) and a model of the decoupling caps and power supply rails and smoothing caps etc, including all parasitic resistances and inductances.

Cheers,

Tom

Last edited by gootee; 21st January 2012 at 09:36 PM.
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Old 21st January 2012, 10:58 PM   #15
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Quote:
Originally Posted by Sype View Post
I'm building headphoneamp using LM6171.
...
Sanyo OsCon 47F/16V are my first choise, but do these need any ceramic/plastic caps in paraller? Are there any better alternatives for less money..?
If you care about subjective sound quality I wouldn't use Oscons and similar caps.

Such caps are great for digital decoupling, though.

LM6171/2 needs rail to rail decoupling, I suggest you a 10nF film/foil cap (not metallized), like Wima FKP2, FKS2, FKC2.

On rails to ground you can use an audiograde cap 47-100uF 35V or superior, I suggest Cerafines, avoid Nichicon KZ and Silmic II (Silmic I are good).

I'm suggesting 35V voltage rating since, usually, it's the break point to lower dissipation factor and ESR.

A lot of low-esr cap are quite good (like FC/FM) but compared to audiograde ones they always have something wrong (harshness, unbalanced, etc.)

Strictly IMHO.
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Old 21st January 2012, 11:26 PM   #16
qusp is offline qusp  Australia
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Quote:
Originally Posted by Sype View Post
PCB is not yet designed, so SMD is possible. By the way, are tantalum caps obsolete, I have some 10F 35V SMD caps. How about sound, is tantalum cap good for audio circuit?
Voltage rails are 15 volts to get some headroom for high impedance headphones while playing loud.
and what do you mean by high impedance headphones? hd600 (300 ohms) will play well with about 4-5vrms with peaks of up to 7 enough for even the most staunch lover of dynamics. +/-9v is plenty unless using very old headphones higher than 600R, with an opamp that wont swing close to the rails. tantalum is not obsolete, but the worlds resources of it are running out, so its not as common and only found in fairly expensive caps these days. one of the caps i linked is a polymer tantalum
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Old 22nd January 2012, 07:42 AM   #17
GLooP is offline GLooP  Bulgaria
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Not exactly on the topic, but quite close and very useful:

Analog Devices : Analog Dialogue : PCB Layout

Also, check out the AD797 datasheet, there is very useful application data there.

AD797 | Ultralow Distortion, Ultralow Noise Op Amp | Operational Amplifiers (Op Amps) | All Operational Amplifiers | Analog Devices
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Old 22nd January 2012, 11:55 AM   #18
Sype is offline Sype  Finland
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Ok, things are going more complicated. In my first prototype I tried AD797 and OP27. AD797 was better for specs but OP27 had better sound quality. I think better power supply will improve AD797 performance.
I'll make external power supply with 200VA 2x24V toroid and rectifier + caps and 24V pre-regulators (LM7824) and for amp PCB second regulators.
Another idea for PSU was using small lead-acid batteries (like 12V 4Ah) and 13,8V float charge.
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Old 23rd January 2012, 08:32 AM   #19
qusp is offline qusp  Australia
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AD797 needs a bit of babying, you must read the datasheet thoroughly, gain must be above unity really for best performance unless compensated, the distortion cancellation pin should be used
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Old 23rd January 2012, 09:05 PM   #20
jcx is offline jcx  United States
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the impedances in Meir's circuit are way too high for best results from AD797 - its input current noise would make it worse than 10 nV/rt Hz with the reistor values shown
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