OPA Alice Failure

[Khron]

These were OPA1642AID - couldn’t find just 1642 without the suffixes.

There's no such thing as "without the suffixes".
-A seems to apply to all(?)
-I stands for the "industrial" temperature range (-40C to 85C; "Consumer" is 0-70C)
-D is the SOIC package designator (the only other one is "DGK" for the even smaller VSSOP package)

 

[Toobme]

Thanks, @Khron - that confused me - so thanks for the teach!
Still, i did order a new batch, so we’ll see - I’m still betting it’s a fault on my end, but few hairs remain to pull, so gotta try.

 

[graysam]

Hey guys.. I’m 2 chips deep also in a batch of 5 from Digi-Key I purchased early April…

Both boards are doing nothing but sending mains hum into my preamp at massive level..

My voltages all measure right, only thing left is that they’re naked, ie not in any enclosure. Sure, I’d expect some noise unshielded like this, but this much is overloading my pre’s.

No change when connecting/disconnecting a capsule.

Swapping in a known good condenser mic confirms the pre and phantom are in working order (still!). Surely it’s not a grounding issue that only affects this mic? The interface I am using is USB Bus powered, but as I said every other mic I have is dead silent.

At work at the moment so can’t check but do these things have date or batch codes on them?

 

[Khron]

Photos of your build?

 

[graysam]

Photos of your build?

I’ll upload some when I’m home from work >=6 hours from now.
Thanks, Sam

 

[graysam]

I’ll upload some when I’m home from work >=6 hours from now.
Thanks, Sam

Here we go, just home from work, here’s the most recent one.
Sorry I haven’t cleaned off the finger prints!!
All parts are tested and other than the 100n WIMA film caps from the BOM supplied, ordered from Digikey.

It would be good to know if I’m just wasting my money ordering another lot of OPA1642s from Digikey as postage to Australia is brutal!

 

[Khron]

Well, trying these out in a shielded body / enclosure would indeed show whether the hum pickup is indeed "environmental" or not.

To be fair, even in his own instructable, Jules mentions:

Inspect everything and if it looks good, put the basket over the capsule and secure it with yet two more m2.5 screws. This provides a faraday cage for the capsule and if you don’t have that, there will be an amazing amount of 50/60 hz hum, buzz and noise.

 

[graysam]

Well, trying these out in a shielded body / enclosure would indeed show whether the hum pickup is indeed "environmental" or not.

To be fair, even in his own instructable, Jules mentions:

Thanks for your reply,
I suppose I’ll have to grab the sieve from the kitchen and try shielding the whole shebang. I’m not confident that it will prove to be the issue at hand, but it’s worth a shot!

The trouble is that I am building the housings for this project and I haven’t finalised the design yet, much less have one available to try.
I had planned to finish the design once I was set on what I’d be putting inside it, as my general plan requires some machining to be done by someone who isn’t me.
Perhaps I’ll consider making a prototyping enclosure, some sort of faraday cage.

 

[Khron]

... Or just hunt down a $10-20 BM700 or BM800 or Neewer NW700, if even only for testing

 

[pucho812]

Here we go, just home from work, here’s the most recent one.
Sorry I haven’t cleaned off the finger prints!!
All parts are tested and other than the 100n WIMA film caps from the BOM supplied, ordered from Digikey.

It would be good to know if I’m just wasting my money ordering another lot of OPA1642s from Digikey as postage to Australia is brutal!

may I ask why you are missing D1 and D2? it's on the schematic but I do not see it populated on your PCB? is this intentional?

 

[Khron]

may I ask why you are missing D1 and D2? it's on the schematic but I do not see it populated on your PCB? is this intentional?

That seems to be an option, D1 & D2 being 6.8v, or D3 being a 12v. D1+D2 are in parallel with D3.

But since apparently the DC voltages check out (and D3 is connected correctly - despite the diagonal positioning, cathode is where it's supposed to be, anode goes to ground), that should not be an issue.

 

[graysam]

That seems to be an option, D1 & D2 being 6.8v, or D3 being a 12v. D1+D2 are in parallel with D3.

But since apparently the DC voltages check out (and D3 is connected correctly - despite the diagonal positioning, cathode is where it's supposed to be, anode goes to ground), that should not be an issue.

Yep, I wanted to install the diode laying flat, for a few reasons that aren’t all that important… and since the pads were there and technically the same point in the circuit, I thought why not.

Well, trying these out in a shielded body / enclosure would indeed show whether the hum pickup is indeed "environmental" or not.

To be fair, even in his own instructable, Jules mentions:

So I stand humbled, grateful and relieved.

I shielded the fly leads from PCB to electret, and 99% of the noise all but vanished. I put the whole lot in between two metal sieves, and it was silent. Colour me surprised! I suppose this is a far higher input impedance than I’ve worked with previously.

I guess I’ll be doing an enclosure V.0.1 sooner than later.

Thanks all for your input!

 

[Khron]

I wanted to install the diode laying flat

Pretty sure it wasn't meant to lay flat. Even though there's more than enough space for all the passives to lay flat on that board, but that wasn't chosen because reasons...

 

[pucho812]

That seems to be an option, D1 & D2 being 6.8v, or D3 being a 12v. D1+D2 are in parallel with D3.

But since apparently the DC voltages check out (and D3 is connected correctly - despite the diagonal positioning, cathode is where it's supposed to be, anode goes to ground), that should not be an issue.

thanks for the clarification

 

[JeffKall]

... Or just hunt down a $10-20 BM700 or BM800 or Neewer NW700, if even only for testing

My first DIY build were a pair of Jules’ Pimped Alice in Neewer BM800 bodies that exhibited the same hum issue anytime they were powered up “sleeveless” !

It was a great learning experience for a first time DIY Newb !!!

Glad you got it sorted out !!!

Best Regards !