PSU for precision audio opamps

Umm, what does "precision audio" mean?

PSRR is bad on most all opamps, so the ripple may shoot through. And the dc/dc guys often make spikes, which can get into the opamp front-end. If you use a converter brick, it would be prudent to add LC filtering to the input and the output.

John

The data sheet claims 120dB PSRR, but that's only if you use Monster Power Supply Cables ;-)

...Jim Thompson

"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...

Jim,

Monster power supply cables will certainly help, but the real improvement in the precisionicity of the sound will come from painting the leads of the opamp with yellow highlighter, hanging small pieces of wood in the inside of the enclosure, and plugging a Tice digital alarm clock into a nearby ac outlet.

You're welcome, in advance.

Bob

Check the graphs: -75 at 10 KHz, and getting worse. One could get potential heterodyning against power supply spikes, too, maybe.

This probably doesn't matter for audio, but might affect important stuff.

John

S/N > 120dB

Dirk

On a sunny day (Fri, 13 Jun 2008 09:13:32 -0700 (PDT)) it happened Dirk Bruere at NeoPax wrote in :

You better have stable power supply and filter to a high degree, as I do not see power supply rejection in the data sheet, but'average input offset drift vs power supply oltage is 110dB' worst case. That is referenced to input, so any supply voltage will appear 100dB attenuated at the input if I understand this right. So from that you can calculate some thing about the supply ripple / fluctuation.

Forget about monster cables, just build the whole thing in a Faraday cage ;-)

fluctuation.

Well, the only PSU I have is that of a PC. So how do audio cards like the Auzentech that boast 120dB s/n ratios handle their supplies?

Dirk

On a sunny day (Fri, 13 Jun 2008 10:20:34 -0700 (PDT)) it happened Dirk Bruere at NeoPax wrote in :

fluctuation.

Likely filter them, also there is +12 and -12 available in a PC, so you could add a LM317 in the +12 to a clean make +8, and something similar in the -12. The PC supplies are junk, especially the 5 V is full of spikes in mine.

Op amps have really amazing supply rejection around DC, and much less good supply rejection up near switching power supply frequencies.

If you use a capacitance multiplier (basically a one- or two-stage RC lowpass with a Darlington emitter follower on it) you can get upwards of

Something like this (use a MPSA64 on the negative supply)

If you make R=100K and C = 1 uF, at high frequency your filter function will be

H(f) = (1.6/f)**2 which is down by 60 dB at 1.6 kHz and 100 dB at 160 kHz.

This trick really really works. There's another approach, which is to return the first capacitor to the emitter, thus making a Sallen-Key lowpass, but that's inferior because the sneak path through the RC network limits the attenuation at high frequencies.

Cheers,

Phil Hobbs

Add a couple diodes. One from the +13.5V back to the +15V and another from the base to +15V. Just in case the +15V goes down hard and instantly for some reason, such as a short.

Good point. Hmm, what happens if you reverse a Darlington?

If it were a plain BJT, it would immediately be in normal bias as an inverted transistor...but you'd zener the BE junction, which with a big bypass on the output would turn it to lava. Check.

If the Darlington were really just two BJTs, then when it inverted the driver stage would find itself connected from the base to the 'emitter' of the inverted transistor...which should make it turn off fine, except that you'd still zener the output stage. Lava again.

Assuming the MPSA14 has a built-in pulldown resistor on the base of the output device, that would be trying very hard to turn on the inverted output stage, and there'd still be the BE breakdown. More lava.

So okay, it needs some diodes. If one were to use 1N4005s or the equivalent, though, their capacitance would be a problem--it's 15 pF, typical, even at -4V, whereas this is only -1.5V. (It's the low Cce of the transistor that makes this circuit work so well.)

To protect the CB junction, a diode from output to input would work, as long as the output bypass cap was at least 10**6 times bigger than the diode capacitance.

To protect the base, I'd probably put another smaller resistor in series with the base lead to limit the fault current. If you wanted to use diodes to protect the base, you could put one diode across each of the resistors, so the sneak path would be heavily bypassed in the middle.

Cheers,

Phil Hobbs

Yes. Also beware that the PSRR for the minus supply is often worse than for the positive one. The lower of the two numbers is the one you really need to worry about.

You should also be very careful about where the DC-DC converter's input side ripple can flow to and the capacitively driven AC current that is forced between its input side and output side.

DC-DC converters also radiate AC magnetic fields. The shield over the top of them never fully stops it. Ideally the DC-DC should be on the far side of some of the sheet metal.

fluctuation.

No, not a cage, a metal box perhaps with double walls.

Where would you get a signal that good? Or a quiet-enough place to listen to it?

Do you stop breathing when you listen to stuff like that?

John

You can get a serious amount of noise magnetically induced from fans, AC or DC brushless. One has to worry about loop areas, yet another interesting constraint in PCB layout.

Not to mention the spikey 120 Hz mag field from the ac power wiring, slamming tens of amps peak into the bridge rectifier and input cap of the power supply, assisted by some coils in the EMI filter maybe.

And all the RF that's available to rectify.

John

Try putting it through a 3kW amp and then sticking your head close to the speakers.

fluctuation.

Gosh, you don't think that they might lie? (advertise a theoretical capability based on datasheet spec's.)

Bruere

fluctuation.

Is that the way you listen to music?

John