Mic Preamp PCB kit offering (measure FR etc)

[Vikash]

Brian and I have decided to put together a PCB offering of a mic preamp perfect for measuring loudspeakers. Jason Neil has offered a PCB in the past, but we (actually Brian ) has redone and improved the layout/design (circuit is Wallin's preamp v2) incorporating the connctors and battery onto the PCB, and we also have a case in mind.

We may be offering made up mic wands using the WM-61A electrets as well if there is interest:

I think it would be excellent to have a high quality and cheap kit available to measure your loudspeaker designs. Our initial thought is just to offer PCB's. Let us have your input.

Here's the PDF of the design so far.
One connector will be RCA, the other BNC.

[Bill Fitzpatrick]

I'd be interested in the whole package.

[wintermute]

Sounds like a good plan to me I made preamp II and one thing I'm a little concerned about is the size of the caps on your pcb layout, quite a few look too small to me.

Attached is my layout (on experimenter board) with the actual cap sizes (these were the smallest ones I was able to source in panasonic FC series, and rubicon ZA. Note especially C9 (don't remember what brand that was, but there is quite limited choice in NP's here in Aus).

One other thing that I'd think was possibly worthwhile, is a modifiction to the circuit to have a gain of more than 20 (if that is possible)..... after testing on the weekend, it would have been nice to have a bit more gain when doing the 1M on axis tests, rather than cranking the volume on the amp up so high (was wearing earplugs, but not sure what the neighbors thought)

Tony.

[BrianGT]

Originally posted by wintermute
Sounds like a good plan to me I made preamp II and one thing I'm a little concerned about is the size of the caps on your pcb layout, quite a few look too small to me.

Attached is my layout (on experimenter board) with the actual cap sizes (these were the smallest ones I was able to source in panasonic FC series, and rubicon ZA. Note especially C9 (don't remember what brand that was, but there is quite limited choice in NP's here in Aus).

One other thing that I'd think was possibly worthwhile, is a modifiction to the circuit to have a gain of more than 20 (if that is possible)..... after testing on the weekend, it would have been nice to have a bit more gain when doing the 1M on axis tests, rather than cranking the volume on the amp up so high (was wearing earplugs, but not sure what the neighbors thought)

Tony.

The electrolytic capacitors were spec'd as Panasonic FC caps, and C9 is to be a Panasonic SU 16v 10uF cap. The datasheet says 5mm diameter, 11mm height, as with the majority of the other caps on the board. (only larger caps are the 100uF 16v FC caps with a 6.3mm diameter).

For the 5pf cap, Mica seems to be the only cap that I can find of that value from Digikey, which is not surface mount. They are quite expensive in that value, so I am looking for an alternate part, any suggestions?

I am still finalizing the selection of parts, and will do a digikey order of the parts to verify the sizes on the layout before ordering the prototype pcb.

For this project, all parts will be available from Digikey, except for the RCAs, which I sourced from Vampire Wire (sound connections) from a previous project.

--
Brian

[audionut]

I'd be interested in the whole thing...when do we start?

[AAXenoStrange]

I'd beinterested too ! Been wanting to do this on my own but this makes it so much easier!

[planet10]

Originally posted by Vikash
Brian and I have decided to put together a PCB offering of a mic preamp perfect for measuring loudspeakers. Jason Neil has offered a PCB in the past, but we (actually Brian ) has redone and improved the layout/design (circuit is Wallin's preamp v2) incorporating the connctors and battery onto the PCB, and we also have a case in mind.

Just before you posted this i was chatting with brian, suggesting a group buy on boards. I have 2 of Jason's 1st gen boards/kits, and the boards are VERY primitive compared to what we have come to expect these days (that was some 5 years or so ago).

I'm (acually JasonL) just getting around to building up the old boards... if this is going to come together i'll hold off on the 2nd one and i'll have parts already.

dave

[wigginjs]

Heh, I was just looking into buying a ART MicroMix and Behringer ECM8000 for measuring my speakers. I'd be very interested in the preamp and microphone together as a kit, depending on price of course.

[BrianGT]

Here is a preliminary digikey parts list for this project:

This assumes that Vikash and I would only be selling the PCBs and RCAs, and the rest of the ordering would be done by the builder through Digikey (or similar vendor).

I am going to order all the parts and do a test fit on paper, and then if it looks good, I will get some prototype pcbs done, and if it works, then I will order some production pcbs (probably red ). I only started this project on Saturday (2 days ago), and I don't expect to have any production boards made for at last a couple of weeks.

One thing not mentioned earlier is that the Vampire Wire (Sound Connections RCAs have the same footprint as the standard BNC board mount connector, so they can be interchanged easily. Here are pictures of the connectors on the website:
http://vampirewire.com/mcart/index.c...3&page=2&cat=5


--
Brian

[pinkmouse]

From a European point of view, more vital than the RCA would be the capsule, everything else is relatively easily obtainable over here. As however you are making the boards to fit a particular connector pinout, the kit should be the two connectors, the board, and the capsule. What do you guys think?

[Vikash]

makes sense. however, you tend to go through several capsules before getting it right unless you're experienced which is why i thought offering a completed wand might be better.

[wigginjs]

Completed wand is a cool idea, and I would actually prefer to buy a completed wand, but this makes the entire project a giant logistical pain. I would be ALMOST as happy with a kit that contains all the parts for the wand and instructions. I guess the feasibility of that would depend on how hard the assembly process is.

[Vikash]

Making the wand is mostly straight forward - but it can very fiddly soldering the tiny and delicate capsules. I went through at least 3 the first time I tried it (and soon found the importance of a good soldering iron and tip!)

We might offer various options with the electret, but primarily this is about getting preamp PCB's done at reasonable cost as this is generally the hold up for people wanting to make their own measuring equipment.

The more I think about it, the more I think adding a capsule to the mix is the best way.

[pinkmouse]

Is that because of the mod to the capsule, or just because they are awkward to solder connections to?

Maybe just price the kit with a pair, then one could be made normally, with a spare to mod if needed.

[sbolin]

Raises hand for the full package!
I would love to get a full kit (mic, wand, and chassis included) if possible - home assembly no problem. If not that, at least the PCB with all parts included. Maybe make two options, one for the PCB alone, one with the PCB+all the fixins.

Brian, you have been incredibly busy lately, with this and the gainclones, too. Much appreciated!

[planet10]

I've got lots of capsules -- how to get them calibrated?

dave

[jleaman]

Ill get them built for you then we can order NEW parts no something that is 15 years old and have de-colored..Then we can try them on my new speaker's..


Arrived tonight an setup at 11 pm : O )

[wintermute]

Originally posted by BrianGT


The electrolytic capacitors were spec'd as Panasonic FC caps, and C9 is to be a Panasonic SU 16v 10uF cap. The datasheet says 5mm diameter, 11mm height, as with the majority of the other caps on the board. (only larger caps are the 100uF 16v FC caps with a 6.3mm diameter).

I think my problem was that Farnell only stock certain voltages and not the whole range, and the ones I could get were higher voltage and therefore bigger. An order from digikey from Aus incurs a $17 US postal fee, a $6 handling fee, and if the order is less than a certain amount and additional $6 US handling fee, making a total of $29 US overhead on top of what might be $10US worth of parts so local sourcing, or being able to get a complete kit of parts, I would think would be desirable for some people outside of the US

For the 5pf cap, Mica seems to be the only cap that I can find of that value from Digikey, which is not surface mount. They are quite expensive in that value, so I am looking for an alternate part, any suggestions?

I had a lot of trouble finding them, but eventually found that Dick Smith electronics (last place I would have expected) had NPO ceramics. so that's what I used..... just had a look at digikey, and I see what you mean. Since I built mine, farnel have upgraded their search engine, and now I find that they have this
http://au.farnell.com/jsp/endecaSear...U=896354&N=401

Went to see the US site, but it appears that they don't have one!

I am still finalizing the selection of parts, and will do a digikey order of the parts to verify the sizes on the layout before ordering the prototype pcb.

For this project, all parts will be available from Digikey, except for the RCAs, which I sourced from Vampire Wire (sound connections) from a previous project.

--
Brian

I think it get's dificult when you try to get everything from one place, I usually end up sourcing stuff from two or three, because no one has everything I want. (in this case I bought from Farnell, Dick Smith, and Jaycar) .

Tony.

[Vikash]

Originally posted by planet10
I've got lots of capsules -- how to get them calibrated?
dave

I'm not too sure. But depending on how much accuracy you require, you might get away with a generic calibration file. From what I've seen this will reduce the common hump in these electrets.

Calibration file (6th one down)

10 measured casules with similar curves

[Vikash]

Some good measurement info about these electrets here: http://orca.st.usm.edu/~jmneal/pre_meas.htm

[Puggie]

could you design some form of variable filter to correct the peak and have a simple one pot 'tweeker' to calibrate it a particular capsule on the pre amp. the variation in response looks like pretty much the same shape peak just varying in amplitude. then calibrate them against a known accurate mic?

[Vikash]

would be easier to just supply dedicated calibration files if we had a reference mic and equipment to calibrate against.

[Puggie]

Hmmmm, B&K 4133 and mic calibrator kit any use to you? I could sort you an accurate calibration print out for a few capsules and you could calibrate the remainder from that if your mounting them in 'wands'. Is that any help?

[Hornlover]

There is already a kit available. You can get the bare board, complete kit including mic capsule, or complete kit with plastic enclosure. Works very well, and has very low battery consumption.

http://www.highefficiencyloudspeakers.com/News.html

[jleaman]

Jason L finished this for dave !!