Bypassing soundcard op-amps
Aug 16, 2008 at 3:33 AM Thread Starter Post #1 of 5

b0dhi

Headphoneus Supremus
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Has anyone bypassed the JRC4580 opamps in the Juli@ (or any other soundcard)? The load to the DAC will then be an amplifier which has more than enough gain to compensate for the loss of the opamps. Any advice? Has anyone done this and found an improvement?
 
Aug 16, 2008 at 3:59 PM Post #2 of 5
what do you mean bypassed?

as in traded for another pair of opamps (like you can do on Auzen cards), or bypassed as in wiring to the D/A's output (like some SPCH-1001 mods call for, or similar transport/player mods call for)?

why not just hook up to the digital outlet with a D/A that you can switch opamps on (like the Firestone Spitfire, for example) and change opamps until you find the sound you want?

going raw from the D/A's output, assuming you hit the right pins (not at all hard to do), and assuming you can cleanly wire it (this is tricky, unless you've got surgeon stable hands, in which case, again, won't be hard at all to do), will drop the signal quite a bit, the only concern I'd have with using an amplifier with the gain up that high, is clipping (will you push the amplifier into clipping, just to handle the lower signal, is what I'm concerned about), other than that, it probably should work quite normally

the only issue I forsee is the card still having the circuitry for the opamps, and still feeding them juice and signal (you can't actually remove the entire circuit path, since some of it is inside the card), so you might have some potential problems there (and I have no idea what to suggest as far as a fix)

SQ wise, well, that'd be interesting, I know on SPCH-1001 when people go raw from the D/A they note a slightly improved SQ, but the opamps on that aren't as good as the ones on the Juli@ (probably not by a long shot, given that we're talking about a 1996-1997 era gaming console), if you can afford to replace the Juil@ board, I'd say try it out and let us know what it does to the SQ, although I'm going to venture a guess, it won't do a whole lot the SQ, because you'll loose so much gain that all of the coloration/influence the opamps had, will be replaced with coloration/influence by whatever you use to bring the signal back up to a usable level
 
Aug 16, 2008 at 5:32 PM Post #3 of 5
I'm not so worried about the loss of gain, as the DAC can output about 2Vpp itself, which the balanced M^3 can easily amplify to levels able to drive headphones. I will test this with a scope to be sure, in any case. I would also much rather the sound adopt the colouration of the OPA627 instead of the combination of el-cheapo JRC4580 and OPA627
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The other benefit will be that I can use smaller\higher quality coupling caps because of the lower signal level.

What concerns me is whether the opamp was serving some function other than just as an amplifier. I'm pretty sure the DAC chip does reconstruction filtering internally, but I'm worried that the chip's performance will be affected by driving a capacitor-decoupled load directly, without the opamp serving as a buffer. Any experts able to shed some light on this? It would be much appreciated.
 
Aug 16, 2008 at 11:00 PM Post #4 of 5
I was considering the same on a Prodigy HD2, but there are a few more things to consider.

1) The DAC must be voltage-out.
2) It must be able to drive capacitive loads (cables)
3) The opamps you're replacing must be the line-drivers and not the LPF (Low-Pass Filter).

I will be experimenting on the HD2 soon and will post back with results.
 
Aug 17, 2008 at 12:01 AM Post #5 of 5
Looking good so far. From the datasheet, the chip is voltage out (5Vpp in differential mode, which I'll be using), and it can deal with capacitive loads as long as there's a resistance too. The datasheet recommends an external LPF for SACD but I don't think it's been implemented on the board, at least not in the topology it recommends. It says the chip has an internal switched-capacitor filter?
 

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