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Old 12th March 2013, 03:35 AM   #31
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The article linked seems to me (unless I skipped over some parts) not to address power supply self-noise. That being the load regulation - a headphone amp is going to be putting out peak currents in the hundreds of mA range so the power supply's output impedance is crucial.

<edit> When you examine the datasheets for the 78/79 series regulators you tend to find the output impedance plots look fairly decent, at least at LF. But then when you look closer they show the output impedance at high-ish output currents (like half an amp or so) - way beyond what's normal in opamp circuitry. At lower output currents the Zout gets considerably higher.
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Last edited by abraxalito; 12th March 2013 at 03:41 AM.
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Old 12th March 2013, 07:48 AM   #32
qusp is offline qusp  Australia
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yep and the resonant pole in the impedance will be effected and moved around by the output caps and inductance. the same can be said for the LT, just because opamps have high PSRR doesnt mean you can forget designing a decent power supply. OPCs headphone amps have good PSRR, but they also have quite hardcore bruteforce local decoupling right on the device pins and a compact layout on the board and the power supply

just assuming you will have low noise operation because you used more expensive parts seems a bit against the grain...
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Old 12th March 2013, 07:54 AM   #33
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Opamps do have high PSRR, but only for low frequencies. The PSRR always eventually falls at 20dB/decade above some corner freq which can be the open loop pole freq. Added to this, their output stages operate in classB meaning lots of harmonics on the rails. Good low-ESR HF decoupling is a must.
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Old 12th March 2013, 08:38 AM   #34
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depends on the opamp, but yep, I agree for the most part. anyway decoupling caps will almost always have the last say no matter what regulator.
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Old 12th March 2013, 11:33 PM   #35
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My 50k pot arrived today from Mouser, and I have to say the Hakko 808 is worth its weight in gold. I didn't have to go back and "unstick" even one pin. The old pot just lifted right out. Pretty amazing.

Equally amazing is no background noise/hiss at all on any gain setting or any rotation of the pot, with my old ears. I sure wouldn't have predicted that. Disclaimer: younger/better ears, better cans/IEMs, and certainly proper measuring equipment are likely to pick up noise and hiss. So YEMV (Your Ears May Vary).

I have even tried injecting noise with my finger and finally found a way. The metal shaft and faceplate on the Bourns pot is not grounded. I have a plastic knob on the pot, but by touching the shaft behind the knob or metal faceplate I can inject a small amount of 60 cycle hum with the gain on 6x and the volume pot all the way up. Even that goes away audibly on 4x gain. Touching anything else doesn't do it due to the grounding - gain switch, filter caps, bass boost switch.

So I'm going to call this arrangement of using the pot as both the input load and the op amp input bias current return resistor a big success, circuit below. I've eliminated one resistor over the more typical setup, as in the O2 headphone amp, of the pot feeding an input bias ground return resistor through a cap. The whole thing works because of the tiny 5 pico-amp input current of the FET op amp. With the 50K pot that translates into just 0.25uV of drop across the pot, worst case. Rotating the pot end to end still results in just a few micro-volts of DC offset variation at the output of the amp.

My big goal with this amp with to see if I could reduce the DC offset from that of the O2 amp. The ac parameters - distortion, crosstalk, etc - are likely a mess vs. the O2 due to the layout, buffer in the op amp loop, more op amps in parallel on the output, and other changes. This amp is just for fun to see if I could get the DC offset down. The amp sounds good to me, but that is purely subjective and the O2 was all about objectivity, so nothing here is going to give the O2 or other properly measured amps like the Wire any competition.
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File Type: jpg ODA with 50K pot - circuit.jpg (141.5 KB, 395 views)

Last edited by agdr; 12th March 2013 at 11:48 PM.
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Old 12th March 2013, 11:41 PM   #36
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qusp and abraxalito - good thoughts! I should really be more specific with the power supply stuff and separate "ripple" comments from "noise". When I think ripple I think of correlated signals like mains frequency and harmonics, or current from the signal frequency at the moment multiplied by power rail wire resistance to form microvolt signal-frequency noise on the power rails. For noise I tend to think more about uncorrelated signals like diode wideband noise.

So you guys are right, the pre-regulator setup gets the ripple down to extremely small levels, at least the mains fundamental and harmonics, but uncorrelated noise is another issue. Those LT regulators may help in that department. I agree about decoupling caps! Very important.

qusp - I'm finally messing around with surface mount stuff. So far I'm keeping it to 1206 to make them solderable with bad eyesight, lol. I see why you like those z-foil resistors. I've spent some time looking at the specs now and those are pretty amazing.

Last edited by agdr; 12th March 2013 at 11:43 PM.
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Old 14th March 2013, 01:17 PM   #37
weslito is offline weslito  United Kingdom
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agdr, using xnor's helpful link you can get an idea of how the Jonhnson noise is affected by your mods.

For your 40.2K feedback resistor:
Johnson noise = √(4k*T*R*f) = sqrt(4*(1.38e-23)*300*40200*20000) = 3.65V

And to dBV:
20 * log (1V / 3.65V) = 108.8dBV

In the absence of proper test equipment, this seems a much more solid approach than trying to measure noise with your ears
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Old 15th March 2013, 02:19 PM   #38
agdr is offline agdr  United States
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Quote:
Originally Posted by weslito View Post
In the absence of proper test equipment, this seems a much more solid approach than trying to measure noise with your ears
Lol! But ears are the whole reason for audio equipment! In my case I really am more concerned in whether I can hear the background hiss or not, just myself personally, and not the absolute (measured) value. For my purposes, if I can't hear it, whatever level that is, then it really doesn't matter (to me).

But I fully agree, the math will then come up with a good approximation on what the absolute noise voltage is at that point, which will help attach a number to the result, which would be interesting.

There is a huge problem with my design though that didn't occur to me until last night. The wiper on the pot will eventually get bad over time - they all do - and when that happens bad things will happen with the output DC offset. So *don't build this* if anyone has been following along. It could damage headphones over time when the pot goes bad. If the pot can't be directly coupled then there isn't much justification for using the pricey FET input op amp.

I'm rethinking the whole thing. Might be better to stay with RocketScientist's original design for the O2, but with the extra paralleled NJM4556 chips, and then use a DC servo just on the output stage. The servo could be done with the much less expensive bipolar dual OPA211, which also has very low input offset voltage numbers, as long as the input resistances are kept about the same to cancel input bias currents.

Last edited by agdr; 15th March 2013 at 02:35 PM.
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Old 21st March 2013, 03:46 AM   #39
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This fun little DIY project is getting a complete reboot. Again, this amp isn't going to be distortion-measured, so for the real deal build OPCs new Wire amp version! That sounds like its going to be a real winner.

* The issue of the pot wiper wearing out is solved by dropping 100K resistors from the wiper to ground to insure a DC return path for the op amp input current when the wiper eventually nuts up. Same as AMB has done on the mini^3 which also has a direct-connected pot wiper.

* OPA827 now wrapped around a LME49600 on each channel instead of a OPA627 around parallel NJM4556As. Hard to justify the lower performance output buffer chips in parallel since it is exactly the same cost now to just use a LME49600, when the cost of the paralleling resistors and IC sockets are figured in. I'm learning that lots of OPA627 + BUF634 amps are out there already (thanks qusp!) so maybe this will kick things up a notch with the OPA827, although I'm sure lots of 827+LME49600s are out there too. It has a published distortion vs.output voltage graph in the datasheet that essentially slightly beats the same LME49600 graph at (heavy) 32R load. Having a slower (BW 22mHz) chip looped around a faster (110mHz) buffer now puts the cart back in front of the horse, from a stability standpoint, vs. the faster OPA627 around the slower (BW = 8mHz) NJM4556A chips.

* B5-080 case that is 1 inch wider than the O2 amp B2-080 case and 1/4 inch taller. The extra width allows more jacks and controls, plus the power supply stuff to be heat sinked in the back.

* Power supply included on the one single 80mm x 125mm PCB, rather than two 80x100 boards in the B3-160 case with amp in front and power supply in back. Still LM317/337 prereg feeding LT1963A and LT3015 low noise final voltage regulators. Power input connector and power switch on the back.

* LME49600s mounted on the bottom of the board with a large heatsink foil area tied to Vee (their tabs), along with the LT regulators as dpak also mounted under there. Since the LM pre-regulators burn up most of the incoming power, and they are heat sinked to the case, the LT regulators dissipate fairly little with only 1.25Vdc across them. The B5-080 case has 5.8mm free under the board while the LT and LME dpacks are 4.8mm. The PCB board can go up a notch in the case too if needed.

* RCA input connectors on the back of the case, along with 3.5mm input on the front, and 3.5mm and 1/4" output on the front. 4 position gain switch remains.

* Pads for attenuation resistor in series with the pot feeds.

More surface mount to make it all fit. The OPA827, LME49600, and LT regulators are all surface mount anyway.
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Last edited by agdr; 21st March 2013 at 04:11 AM.
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Old 21st March 2013, 04:24 AM   #40
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Quote:
Originally Posted by agdr View Post
* OPA827 now wrapped around a LME49600 on each channel instead of a OPA627 around parallel NJM4556As. Hard to justify the lower performance output buffer chips in parallel since it is exactly the same cost now to just use a LME49600, when the cost of the paralleling resistors and IC sockets are figured in. I'm learning that lots of OPA627 + BUF634 amps are out there already (thanks qusp!) so maybe this will kick things up a notch with the OPA827, although I'm sure lots of 827+LME49600s are out there too. It has a published distortion vs.output voltage graph in the datasheet that essentially slightly beats the same LME49600 graph at (heavy) 32R load.
I note you're following the examples in the LME49600 DS and having the opamp and buffer running from the same power supplies. I suggest not doing this as the heavy loads that get connected to the buffer will modulate the opamp's supply and the OPA827 has very poor PSRR on its negative rail (while its +ve PSRR is fairly good). Add extra filtering or a separate shunt regulated supply for the OPA827 to reduce the HF hash introduced on to the supplies from the buffer.
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