An attempt making singlesided Opa Alice and Opic Pcb. Need Your Help.

[zawad_tahmid]

Greetings GroupDIY Members,

At first, I'd like to convey my sincere appreciation to Mr. Jules and Mr. Rogs for their remarkable contributions in creating Opa Alice and Opic circuits.

When I first find out about Opa Alice, I was really excided to try it. Mr. Jules already provided detailed, step-by-step instructions on Instructables. However, I got stuck at the premade PCB, realizing that its cost, including shipping, surpassed what I could manage for a DIY alternative. With limited knowledge in circuit or PCB design, I began searching for a comparable preamp solution.

My searching led me to the Opic circuit, a simplified veroboard version by Mr. Rogs, with a comprehensive PDF guide that could help any newcomers like myself. This helped me to successfully assemble my very first microphone preamp circuit (with a hex inverter).

After a year or so I found a video explaining easyeda. I didn't know how easy it was creating PCBs from diagrams. I couldn't resist making these pcbs. I already had some singlesided PCB boards (Doublesided seemed bit harder for toner transfer and etching) so I went with singlesided design.

Witnessing the community's supportive environment and the willingness of members to share their designs, I find myself reaching out for assistance. As a relative newcomer, I'm a bit scared etching these boards. I seek your guidance so that I can fix mistakes that I made.

Kindly inform me if this post does not go with the forum's guidelines.

update: Hex inverter and opa alice

Warm regards,

Zawad Tahmid

 

[rock soderstrom]

Hi Zawad, I don't really know either project, maybe it would be helpful to post the schematics as well.

A few basic comments from my side. I am not a PCB designer expert. Like you, I taught myself EasyEDA not so long ago.

I would test all the component symbols or footprints to see if they match your avaible parts (or the ones you can source) and if the sizes are correct. Usually you then make minor changes (I prefer physically larger solder pads for the larger components such as capacitors) and while you're at it, you can also integrate visual 3D data, it's not that difficult. The result will be that after a short time you will build up a small (your own, tested) component library.

A few thoughts on the actual PCB design:

You should take the greatest care with the high-impedance area where the signal from the capsule meets the 1G resistor and the OPamp input. Keep this trace as short as possible and maybe a signal routing detached from the PCB would be better. Air insulates better than a PCB. Another idea would be to use ceramic insulated solder "turrets" for this sensitive area.

I would definitely rotate the OPamp chip so that the input is at the top.

I would also try to have all polarised capacitors pointing in the same direction. This makes the actual mounting easier and safer.

Try to avoid right-angled traces, if possible.

Try to keep the PCB traces the same width (especially the grounds), use other resistor footsprints that give you more space to run the traces.

Keep self etching!

 

[zawad_tahmid]

@rock soderstrom,

My apologies. here are the schematics for opa alice and opic circuit.

Thanks for the tips. Seems like i already made several mistakes (i saw this coming).

As for resistors I was going for vertical mount, so made footprint by myself. Guess i have to modify those and align the caps in one direction and will keep all the traces at 0.5mm.

Mr. Rogs, in his pdf showed how not to connect 1G to the board so, i will stick with that.

Cheers

 

[rock soderstrom]

Seems like i already made several mistakes (i saw this coming).

Pretty normal, only those who do nothing make no mistakes! There is always something to improve

 

[rogs]

I see you have chosen to use an early (2021) draft of my OPIC circuit, which included a differential audio output, similar to Jules orignal OPA Alice configuration.
You also seem to have changed to using a OPA2134 dual op-amp?
That dual opamp will draw around 10 mA from the phantom power supply, which is around double that of the current required by an OPA1642.
It's also noisier than the OPA1642 op-amp.

May I recommend you take a look at the current versions of either the 'FETless' or 'LDC' OPIC project notes, the details of which you can select from here: www.opic.jp137.com

This latest version uses an OPA1641 op-amp for a single sided audio output (The line is still impedance balanced).
There are a couple of advantages over the earlier version:

• The OPA1641 only draws about 1.8mA of supply current, which will allow an unregulated (but decoupled) DC supply voltage of around 24v to be applied to the op-amp. This will help optimise the available headroom.

• The use of a single sided audio output will also result in a lower noise figure. A second differential audio output amp will add extra noise, as well as extra signal.
As I mentioned above the single sided version still provides an impedance balanced output.

 

[zawad_tahmid]

@rogs,
Hi Mr. Rogs,

I extend my gratitude for your presence in this thread. Your insights are always appreciated.

The OPA2134 is just an excuse to use OPA1642 with an adapter (like you did on stripboard version) as I am not that good with smd components and do not want to mess up on the PCB. also i have some tl072 laying around i might use those to test the PCB

I am aware of your latest preamp with the OPA1641. But the thing is I have already purchased the genuine OPA1642 from http://lcsc.com/, anticipated to arrive by month-end.

Your work on the hex inverter PCB is amazing. The stripboard version of hex inverter works as intended.

Once again, I'm a fan of your work. Keep up the good work.

Cheers

 

[rogs]

Glad you've found some of my notes useful !

I've used TL071 and TL072 DIL opa-amps for testing, and found them pretty good. Current wise they only draw about the same - or slightly less - than the OPA164* series. They are a bit noisier, but still quite useful for testing...

I find stripboard layouts quite useful for prototype testing .. and if you're only going to build one or two mics of a particular type, they often can be cheaper than PCBs.
Not so easy for projects like my RF mic though ... a PCB was pretty essential for that , so I was very grateful to Khron for his work on that layout...