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agdr 21st June 2013 06:10 AM

Phono amp top slot board for O2 amplfier in B3 case
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Here is a revised version of a phono amp board I posted yesterday in the main O2 amp thread that would fit in the top slot of an O2 headphone amp in a B3 case. This version uses stacked film caps in the feedback path instead of 1206 COG/NPO MLCC ceramics. This is just the simple single stage phono amp from the LT1028 data sheet.

If RCA jacks are mounted on the O2 front panel they should not be grounded to the B3 chassis, even through it is grounded to the O2 PCB. Instead the jacks should be the type that are insulated from the chasis. The two signal wires from the RCA jacks (left and right) should be twisted together with the RCA jack's common ground and all 3 soldered onto this board's inputs. I've added a ground PCB hole to attach the RCA jack ground wire to.

Then the 3 wire PCB holes for power and ground go to the O2. There is a hole in the O2 PCB that allows V+, V-, and ground wires to go under the board and solder to the output of the O2 mosfets.

I haven't fabricated or built this one and I'm not going to since I don't have a turntable. :) So builder beware, there may be an error somewhere I've missed, so you might want to do your own layout and just use this as a guide. The board goes upside down, with parts down and solder joints up. The circuitry area is placed over the (low noise) signal input section of the O2. The BOM has both Mouser and Digikey part numbers. Mouser doesn't have the LT chips.

qusp 21st June 2013 08:06 PM

I would be surprised if pps are lower noise/distortion than high quality np0

you REALLY hate ground planes dont you mate?

agdr 22nd June 2013 03:10 AM

qusp - I was wondering about that (COG/NPO)! On the initial version I seemed to recall that COG/NPO solved the ceramic capacitance change vs. voltage issue along with temperature stability, but when I went looking to verify that I couldn't find anything. So hence version 2 where I made the change to film. Lol - you are exactly the one I figured would barbeque me (and rightly so) for putting ceramics in the feedback path on the first version. :D

What are some better phono amp circuits that are in the public domain? I see the one on the LT1115 data sheet which looks like it may be a bit more precise. Seems like the OPA1641 / OPA1642 would be a good modern chip for this kind of thing with the JFET inputs.

Yep, I'm not feeling the love with ground planes for "lumped" elements where the wavelengths involved are vastly longer than the size of the parts. I did some microwave stripline / transmission line research work a long time ago with GaAs MMIC's (microwave integrated circuits we designed) where ground planes were part of the transmission line. At high frequencies the wavelengths involved are comparable to the size of the parts and everything is treated as a distributed transmission line. But at audio frequencies I don't get it. I can see some value for electrostatic shielding but I am more concerned about magnetic field cancellation. That is why I have the power supply rail traces sitting right on top of each other on the Amplifier Previously Known As An ODAC Version. :) The few milliohms that may be saved by the large ground plane vs. a properly sized ground trace seem pointless to me for an audio amp. As for trace inductance, at lower audio frequencies it won't matter and at higher (audio) frequencies and oscillation frequencies of the op amps, that is what bypassing and decoupling capacitors are for at the end of the various power line segments.

I would however be concerned about ground return currents flying all over the place on a solid ground plane that is carrying currents, running by and through parts I don't want such currents near. For example, on the ODAC thing, follow the current loops from the input. The high frequencies trace a short route from the input jack through the EMI X2Y cap back to the ground on the input jack. The lower RF current trace a slightly longer route through the 220pF and 274R filter, but all still very near the input jack. Those RF ground return currents never leave the premises, so to speak. Then with the RF removed the audio currents trace a slightly larger loop through the 10K ground return resistors in front of the gain stage op amps, etc, etc. Finally the output of the gain stage op amp traces the full loop back through the ground star point. The heavy output stage return currents never get anywhere near the gain stage parts.

But... I'm probably just out of date. Everyone seems to be using ground planes with the audio. I've bought all of Doug Self's and Cordell's amp books recently and I see reference to the Henry Ott EMI PCB stuff. As time permits I'll do some reading and maybe I'll have an epiphany. :p

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