Musical Fidelity V-DAC2 - opamp comaptibility

[wiland1]

I'm wondering if anyone can help with selecting a compatible op-amp, or give guidance on re-working the loading around the op-amps, in the V-DAC2? I've done some Op-amp rolling, but i'm no depth expert on compatibility - i'm an enthusiastic DIYer. Here's what i've done:

Original Musical Fidelity spec in the circuit is a 4-channel 33079 (ST Electronics) followed by a 2-channel 33078 (ST Electronics).

First I replaced the DC Blocking caps by the RCA outputs: 47uF SMD with Nichicon ES 470uF Electrolytics (there is just room inside the case!).

Then to the opamps: I replaced the 2-channel 33078 ouput opamp with 2 x OPA827 on a Browndog. Lovely improvement in detail, whilst keeping the 'musicality'.

Then I got excited and replaced the 4-channel 33079 with an LME49740. But the bass just disappeared - almost like a low frequency filter. Really not sure why (could it be the larger current requirement of the 49740 at 18.5mA, versus the 8mA of the 33079?). So I switched the 49740 to an OPA1644. Bass came back with good detail and spaciousness. However, over time (i.e. a few hours) the sound became relatively fatiguing. Seemed tight and clinical - it lacked the musical warmth, despite representing instruments accurately.

So I decided to switch the 2xOPA827 on Browndog with an OPA1642. Now this should be a happy result, and mostly it is the best sounding so far (i.e. the combination of OPA1644 and OPA1642 in the circuit), but the mid bass has dropped away again - significantly reduced - coming back with deeper bass notes being present, although softer.

Can anyone give me some guidance of whether this is just audio character of this op-amp series, or is there likely to be an incompatibility with the audio circuit leading to the reduction in mid-bass? It was interesting that i saw this effect with both the 49740 and then the 1642, which are both in different locations. I have an OPA1612 ready to test, and i can go back to the ST chips, although they are not my first choice - they are musical but after hearing the depth of detail produced by the others i've been testing, i'm loath to go back! Any ideas anyone?

Will

[wiland1]

I think i've answered my own question: i went back to the OPA1642, but used 2 x OPA1641 chips instead on a Browndog. Then the solution to the bass: 0.1uF decoupling caps across the supply pins - really sorted the bass and balances the presentation generally.

So, if you're interested in upgrading the V-DAC2 opamps, i recommend the 4-channel OPA1644 and the 2-channel OPA1642, with 0.1uF decoupling caps on the supply pins of each one - sound is more transparent than stock, but maintains that musicality.

[percy]

Hi wiland1,
how easy (smd?) do you think it is to completely bypass the analog stage and use custom I/V ? Can you post some pictures so I can see how it looks in that area ?
Also, did you change anything about the power supply ? Or are you using the stock wall wart ?
I am seriously considering this DAC and would like to get some feedback on it.

[wiland1]

It's a great sound out of the box - i just like tweeking I'm using the stock wall wart supply - i'm sure the V-Dac supply would clean it up further.

Only trouble is that i think those new OPA's are not totally happy - still slightly bass shy and harsh treble now (oscillating). As you can tell, i'm not a pro so cannot answer your questions. Here's a couple of photos:



And one of the underside with the dead-bugged PP Panasonic caps on the supply pins

:

I'll try and take a photo of the whole board from above so you can check it out.

I've got some 4.7uF PP Claritycaps on order now - the VDac Mod thread on the forum suggested this would be a better option than a big Elec Cap. I wonder if that is the reason my bass is not right? I had heard that a big EC is a good solution for signal path DC blocking - it's the small value ones which veil the sound...

[wiland1]

Here are 3 pics of the board for folks to look at - the three photos effectively stitch together. I've just replaced the 2 x 470uF green Nichicon ES Electrolytics with 2 x 4.7uF Claritycap PP caps from Falconacoustics - they're big and only just fit - and it sounds fabulous. Really clean, natural, balanced sound now, with lots of space around insruments. And it's still running in! So the advice about using PP caps in the signal path appears to be very true - if you can make them fit .