DIY Mic Pre Kits, Lunchbox Mic Preamp Kits, Power Supply Kits

I'm working on a new product.... and I'm excited about this one. I think I got the parts sourcing problem figured out. I've prototyped the design on the breadboard, and computed the component values, listened to it, and tweak the values some more. It's pretty amazingly quiet, even running on a breadboard of spaghetti wires. So I can just imagine when this is on a proper PCB, it will be even quieter. Even on the breadboard, it already sounds awesome. Even my daughter noticed the difference!

All I will say for now is.... it's NOT A PREAMP.

Okay, next step is PCB design. Now that I know the design works on the breadboard, I can proceed with confidence starting the PCB design - and finalizing parts footprint and sizes. Then PCB prototype manufacturing, front panel design while we wait for the PCB, then build first PCB-prototype while we wait for the front panels.

So let's see... Probably by September I'll have the first working prototype complete with PCB and front panel. August is possible, but that's pushing it.

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Discovered a problem with the prototype... after running maybe half hour, the sound breaks up. Turning off the PSU and back on fixes the problem. Weird! It's like a Windows computer that needs a reboot.

At first, I thought it was oscillating... but it didn't sound like oscillation. So I got the oscilloscope and probed around. Fed it a clean sine wave signal and measured the output... it's clean. So it's not oscillation.

So I let it run again until the sound starts distorting and breaking up... then probed the output... yup, something goofy is going on.

Now, let me go back... this is just a prototype on a breadboard with spaghetti wires running all over the place. It was just meant to be a proof-of-concept to see if the circuit works, and maybe play around with the component values. There's no buffering, or line drivers in the circuit. The design is not even fully finished. But I've been using it for extended periods of time.... well, because it sounds good!

This distortion problem just started happening today, but during the last few days, it wasn't there.

Then I remember, I swapped IC chips. The first IC chip I used did not have this problem.... This is the 2nd chip I'm using. Maybe it's borderline defective. (I fried the 1st IC chip because I didn't notice that my PSU V+ was set to 22Volts.... soooo.... I had to throw away that IC, and replace it with a new one.... and this new one is the one giving me distortion problems.)

So I think I'll try replacing the chip tomorrow and re-test again. I'm also thinking of adding a line driver on the breadboard so the IC chip isn't driving the load directly.

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Got my big book of Mouser catalog and started browsing for parts to see which ones might work for this project.... I have very tight space constraint, and every .1" of space is precious to me. I have limited room to work with and make everything fit.

Found several very low-profile and very small switches that might work... they're calling it "sub-miniature" switches. There's some limits on how many volts I can run through it, but I'll be using it to switch line level signals, so it's all good. Of course, as the size shrinks, the price goes up.

There'll be (4) total switches on the front panel... I hope I can find a good deal on these switches.... well, if I order 100, 500 or 1000 pcs, they'll give me a good price.

I made a shopping list and will order (1) each of these switches tomorrow so I can touch them and evaluate them properly.

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Replaced the chip and added a balanced line driver to properly drive the output. I also discovered an error in my wiring. It probably happened when I started moving parts around and some of the jumper wires didn't get put back in the right location.

So far so good. Been running the prototype for a couple of hours now and the distortion problem from yesterday has not come back. I think replacing the main chip fixed that problem. (or fixing that wiring error.)

Ordered some switches from Mouser to sample them. The cost of a few switches adds up really fast. Spent more than $100 for this project/prototype.... and I'm not even on the PCB design stage yet!

I think it's time to finally write down and finalize the feature set, and start drawing the schematics in CAD.

But first, I'd have to make custom part libraries.

... a never-ending to do list.

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The switch samples are here and (2) looks promising. Very small, but high quality switches. (expensive though, costs more than the bigger switches). It looks like I ordered the same 2 switches, but one with gold plating, the other with silver. Either way, it's good to have alternative parts I can use if stock availability becomes an issue.

I have one more idea/alternative part to test, and now waiting for this new component to arrive (hopefully by Friday). If this new component works just as well (and sounds just as good), I may use it instead. The only drawback is it's larger size.... so that may be a problem fitting the component on the PCB board.

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See... this is one reason why I like to have the part on hand before I begin the PCB design (or create a part library for it).

The alternative part arrived and it's too big. The dimensions listed on the website were wrong. So no go on this alternative part. We're back to the original parts.

Found a smaller push-button switch to use, found it in my pile of "prototyping parts". So I think I'll use this one. The feel is much nicer and not as stiff as the other one.

Found a way to mount an LED so it's right close to the push-button switch. Basically, I'll mount the LED from the bottom side of the board. Problem solved.

PCB design is done, just need to do a final check on part locations, offsets and spacing. Also need to move around the part names and part values to make it look nice and intuitive (lessen chances of error during manual assembly).

I may be able to send it out for prototyping hopefully before Friday this week. Can't wait!

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PCB sent out for prototype manufacturing... should be here next week!

Now, time to design the front panel.

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I figured it out!

It's one of these...

http://www.youtube.com/watch?v=Z86V_ICU ... r_embedded

Right, right?!

Spent last night in CAD designing the front panel and making dimensions. This morning, I sent the file for manufacturing. Hopefully, I'll receive it just in time when I get the first prototype PCBs.

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Mannnnnn o mannnn.... I got the new prototype PCBs yesterday ( a week early! ) and excitedly built the first PCB prototype. All the parts fit, but I just need to nudge a few here and there to give it more room (so my fat fingers can reach into it more easily).

Power up... no smoke.

Connect XLR jacks... power up... A LOUD THUMP came on the speaker... I mean, a NASTY thump... not your normal power on thump. And the worst part is there is NO AUDIO!

Whaaaaa? Honestly, this is the first prototype PCB I did where it didn't work right of the bat. I'm pretty bummed out....

So hookup my signal gen, and oscilloscope and external PSU, and solder some XLR jacks for easy testing... NADA... No signal at the output. And the worst thing is there's 15VDC at the output of the opamp. Not good at all. I did a major screw up somewhere here.

I'm thinking maybe there's a short, that's why I'm seeing that 15VDC on the opamp's output... I opened up my Eagle CAD, reviewed my schematic, then it hit me.

I drew the schematic wrong. Well, I did it correct the first time, then I swap the + and - pins of the opamp in the symbol library, but neglect to recheck the schematic now that the pins have been swapped.

So that's the problem! Also found out on one of the other chip, I didn't ground one of the REF pins.

So got my dremel, and began cutting traces and soldering jumper wires at the bottom of the board.

Then tried it again... and this time IT WORKS! Yes... we have audio, and the circuit works just like in the prototype breadboard.

I found one other mistake though... I designed the PCB with the switches on the component side... but during assembly, the switches must be soldered on the copper side/bottom side... so the switch settings are wrong. What's supposed to be position UP, is now position DOWN.

But other than those mistakes, the rest is fine... so I think I'll have to do a second prototype PCB, just to be sure before I order hundreds of this.

The front panels will be here next week! Can't wait to test fit that to the board.

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Added some additional stuff on the board that came up during testing.

I hit this circuit with a square wave at 10Khz and 20Khz and I'm looking at how it behaves. Hmmm.... lots of ringing...

So I got some WIMA caps and tried experimenting where the ringing will disappear and we're left with a still fairly sharp looking square wave, not too rounded at both 10Khz and 20Khz... Okay.. done!

I decided to add some jumper terminal on the board so the USER can have the option of bypassing the output transformer if they want to... or if they want to save a few bucks, order the unit without the trafos (and add the trafos later on).

Did some frequency sweep testing too.... ALL GOOD! Same results from the prototype breadboard. No surprises and I'm pleased with it's performance.

The best thing is the listening test.... really brings out the best in the audio material.... you can't screw this up. No setting sounds bad. And very versatile too... you can manipulate the sound and depending on switch positions, you can do fine surgery, or broad changes.... whatever, the result is very musical.

I think I'm ready for prototype PCB #2. Will send that out tomorrow, Monday.

First prototype panel scheduled to arrive this Friday!

I think before the end of the month, if things work out well, the new PCB prototype #2 and panel will be together.

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Surprise... I got the panels today.... I wasn't expecting them until next week. I don't know what I was thinking.

It's the moment of truth... will all my measurements come out correct?

I unwrapped the panel and installed it on my board... What do you know.. it FITS PERFECTLY on the 1st try. Took some serious caliper work, and calculator button pushing and cross-referencing the PCB design with the datasheets... but everything came out exactly 100% perfect... all the switch holes aligned, all the pot holes aligned... very pleased with it...

And so now I'm ready to show it off to the world...Ladies and Gentlemen...

The FiveFish Studios Passive Equalizer, using Carnhill Inductors, for 500 Series racks. -- Model PEQ-503

Very smooth sculpting of sound, no setting is bad. Silky Low Freq and Mid Freq control.... thanks to those huge Carnhill/UK inductors (same kinds used in Neve EQs). High Freq filter also outfitted with additional inductor for better control and shaping of the high frequencies.





3-Band EQ - Inductor Based EQ
Two Frequency Selection switch per band + independent flat/bypass position
12K/AIR HF control +/- 18dB gain
1.0K/3.5K MF control +/- 12dB gain
SUB/120Hz LF control +/- 18dB gain
EQ On/Bypass switch
Relay-controlled Bypass
Electronically Balanced Inputs using inGenius Technology
Electronically Balanced Outputs using OutSmarts Technology
Output Tranformer option
Discrete OpAmp option, or PDIP chip
Using Carnhill/UK VTB inductors for LF and MF control
WIMA caps for EQ filters
1% metal resistors
Bourns Potentiometers
Solid Machined Aluminum Knobs
CNC-milled/engraved/paint-filled front panel
Double-sided, plated through PCBs
Low Noise Equalizer
Jumper settings for experimentation, changing the EQ filter frequencies

Estimated Introductory Price (assembled)... $395.00

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Here's a printout of the new panel... with the knobs laid on top of the printout.

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I'm using Carnhill multi-tap inductors on the board and I put a 2x8 headers with jumpers (similar to computer motherboard jumper settings) for both the LF and MF.

For example, on the MF band, just by moving this jumper to the next slot, you can change the Middle Frequencies from the default to

900Hz & 2.6 KHz

Move the jumper another notch, and the new Middle Frequencies will be

750Hz & 2.2 KHz

and so on....

Of course, you can also change the caps... but maybe the jumper settings will have the frequencies you want/need.

Not only that, but the Q factor also changes a bit as you move the frequencies around.... make it wider or narrower.

When I get some time, I'll write everything down.... the jumper settings and it's new Frequency selections.... if you want to change it
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