This looks like a very interesting project. If you're into audio measurements, this project might also interest you.

From the Pmillett website...

Most of us DIY audio types have been using PC sound cards to make measurements.  There are excellent, inexpensive programs available to do test and measurement of audio equipment available.  Personally I use Audiotester.  If you've been around my web site you've seen FFT plots generated that way.

What has always been lacking is a decent interface between the sound card and the device under test (DUT for you geeks).  There has been much discussion in forums like DIYaudio about this, and many proposals and circuits shown, but so far I've not seen anything implemented.

The problem with sound cards is that they are designed for fixed line-level inputs and outputs.  Depending on the card this may be between 1V and 5V RMS maximum.  They are also not tolerant of overloads - accidentally deliver 20V into your sound card input and you will most likely be looking for a new sounds card.

http://www.pmillett.com/ATEST.htm

A DIY Tube Microphone.... and yes, it's a KIT!  What's even more awesome about this is the microphone body, grille and other stuff is included in the KIT.  Anybody that's into building DIY microphones know this is the most hardest thing to DIY... the microphone case. Not the electronics, not the power supply but the mechanical enclosure! So this kit packages everything nicely.

From the writeup:

Beautifully machined (unpainted) brass microphone body and head-grill assembly
Internal mounting cage assembly to mount circuit board, capsule, transformer and connector housings.

Components included

1 - 32mm/1in large diaphragm pressure gradient capsule (assembled)
1 - Plastic capsule mount
1 - Pre-amp circuit PCB
1 - Tube socket PCB
1 - Vacuum tube, 12AX7
1 - 9-pin tube socket
1 - 7-pin XLR male connector
2 - Ceramic insulators for both ends of C4

1 - Film Resistor,  1/2 W 10KΩ
1 - Film Resistor,  2W 100KΩ
2 - Film Resistor,  1/2 W 270KΩ
2 - Resistor, 51M
1 - Resistor, 200M
1 - Film Resistor, RJ 1/2 W 3KΩ
1 - Resistor, 1 KM
3 - Capacitor,  1μ/400V
2 - Capacitor,  .022μ/630V
1 - Capacitor,  1000 pF/630V
1 - Capacitor,  100μ/25V
3 - Capacitor,  0.1μ/63V
1 - Capacitor,  1000μ/10V
1 - Ceramic Capacitor, 2pF
1 - Transformer, turns ratio 10:1

1 - Fully assembled power supply
1 - Power cable for power supply
1 - 7-pin microphone cable

Printed documents
Schematic

I bought this kit on eBay and sure hope to find the time to build it.  I'll post pictures of my build.

Last October, I held a sale of my X-12mk500 preamps.
 http://fivefishstudios.com/index.php/X-12mk500-Mic-Preamp-Kit.html

These are some of the units that went out to customers... <drool>

Thanks to all my customers that support FiveFish Studios DIY Mic Preamp kits.
Some photos:


from another angle

Lunchbox pres racked in a 1u case

Just a test.... checking clearances, and how the whole thing will look.

The real deal will be a setup for (2) X-12 pres.

update:



Finished X-12 preamp, 2 channel in a 1u rack.




check it out... We now have audio samples of the X-12 Mic Preamp.

http://fivefishstudios.com/index.php/Audio-Samples.html

Special thanks to Farview Recording (Jay Walsh) and Jason Mallow. You guys ROCK! Also thanks to Madison Rhoades for the vocal clips.

Check out the Servant song... all tracks recorded through an X-12 Mic Preamp.

It's done :)  My switching DC-to-DC PSU.

I've been working on this design on and off since October 2007. Several prototypes later, and many $$$, I'm happy with it.  All output voltages are regulated and adjustable via multiturn trimmers, +18, -18 and it's a true +48V phantom power... all derived from a single 12Volt input.  Outputs are short circuit protected. Just a big spark, but keeps on ticking. Nothing gets blown. 




Powering my preamp and my I/O module (input and output trafo, pad, polarity) with Active DI, via 12V wall wart. No hashing sound, no RF leak, no noise. (Well, okay... when you're at 66-72dB gain... but could be caused by everything not in a metal enclosure.)





Output on all 3 rails are typical like below:



Switching spikes on the DC output of about 0.06Vpeak-to-peak, and interval of 33.333Khz. I can set switching freq. to about 60Khz but the spikes get larger.

So I compromised for smaller spikes at a lower switching frequency... i.e. smaller spike/transients, less RFI energy emitted, less chance that it will interfere with sensitive mic pre circuitry.

Did the noise floor at max gain drop at all?

By -1dB! But it's probably awash. It could have been better.

Using the external gain switch with about almost 6" of wires (3" going to, 3" coming back) made RF noise worse. See that clump of white wires there going to the switch?

But "grounding" the case made the unit quieter. In fact, it's a different kind of quiet hissing sound. And again, you only hear this hiss sound at max volume and at +66 and +72dB gain. Anything below that gain and it's super quiet/silent.

Maybe that's why a certain commercial preamp's max gain level setting is only 59dB! I won't mention names. :)

My recommendation: On the final kit, don't save a few bucks to buy a $6 plastic rotary switch instead of the $16 Bourns PCB-mounted, metal selector switch originally spec'd. The wiring job is tedious.

See that big clump of white wires in the photo? Touch it and you'd pick up all sorts of AM radio. (+72dB gain = 4000x amplification!)

But I do know some people are interested in using hookup wires instead of the onboard selector switch so I had to try it. It could work, it will just take more time assembling it, and there could be more potential noise/RF problems. Neat wiring and soldering job is a must if going this route.

Right now, I measured noise floor at -63dB and -64dB @ +72dB gain.
And -70 to -69dB at +66dB gain.
-76 to -75dB at +60dB gain.

photos at +72dB gain.

FINISHED!!!!!!!!!

Worked on this project this Friday, Saturday and Sunday.... whew!

Since I used an external 12-pos switch, I basically had to solder 13 wires, so that equals 26 solder points per channel, or 52 solder points for 2 channels!!! I wouldn't recommend this approach. The wires are also prone to RFI pickup. I made the wires as short as I can but still, it's prone to pickup interference.

I would recommend you guys use the onboard PCB selector switch instead.

To fix the RF problem at 66dB and 72dB Gain, I "discovered" a new (new for me at least) of fixing ground loop/rf problems.

I wired the ground of the 1st channel to the XLR output gnd of the 2nd channel and vice versa. That lowered RF problems, but there was still some slight noise but I noticed that if I parallel the 2 gnd wires and stick them flat against the case, it fixed the RF problem. So that's what I did. You can see the "black tape" holding the 2 gnd wires flat against the case.

I don't know what to call this method... It is definitely NOT a star ground. It's more like an X ground.

Now, the only thing you hear is normal "hiss/white noise" at 100% volume @ 72dB gain! There also seem to be a spike at 15.7Khz. But at normal listening volume (unity gain on mixer), the hiss/white noise is only slight even at 72dB.

Some photos!!!

From this....

To this....

Some close-up photos.

 

 

 

 

 

 

 

Rev3.00 PCB will be coming in middle of this week from the PCB manufacturer, and then I'll test those samples by building prototype #003 and #004.

April 20, 2007

I'm racking prototype #001 and #002. Getting ready to use it for sample recordings, so I need to be able to take it anywhere easily. I'd also like to see if racking it in a metal case will improve noise floor further... hope so!!!

Also going to test how the 7818/7918 PSU type regulators stack up vs. 317/337 type regulators. I want to see if 317/337 are really better than plain stock 7818/7918, or if it's just a myth.

This is still the PCB without the onboard pots and selectors. So I guess, we'll also see noise performance when using external pots and switches with hook-up wires.

Here are some photos.... sorry for the harsh shadows, I just used a single flash on the right side and didn't use any reflector on the left.

If you use on-board pots and switches, you can easily fit 4 preamp channels on a 1u rack... with plenty of room to spare in the back.

http://www.fivefish.net/diy/sc1/images/protocase2.jpg

The front view of the prototype rack. (On the background, you'll see my new "electronics assembly" factory. I'm getting ready to package some PSU kits to sell.)

Closeup of prototype #001.

The switch you see in the picture is just a jury-rigged DPDT switch using component leads soldered to the switch and PCB.

Closeup of prototype #002.

Knobs and switches and pots on front panel.

Back view showing IEC AC line filter connector, fuse, and Neutrik XLR male and female jacks.

Getting ready to build a PSU for this. I already have an 18-0-18 designed PSU, but I think I can still improve upon it. I'll try my Rev1 PSU design, and compare it with a 317/337 type psu. See which one is better.

The PSU kit will be sold separately from the SC-1 kit.

Current consumption for (2) channels.

+/- 26.4 mA (2 channels, with phantom power LED OFF)

If phantom power is ON, the LEDs consume a few milliamps.

if using a diffuse RED LED, it's consumption is 3.1mA x 2 = 6.2mA
total = 32.6mA

if using a clear RED LED, it's consumption is 4.9mA x 2 = 9.8mA
total = 36.2mA

Add another LED for the power on switch indicator... 4.9mA

And estimate about 10mA to 14mA for each phantom powered mic attached = 28mA

GRAND TOTAL ESTIMATE: 69.1 mAmps for 2 channels

To be on the safe side (with 100% safety factor), let's round it up to 140mA.

So the VA needed will be 18V x .140 Amp = 2.52VA

Looking at available toroidal transformers, we can buy a 3.2VA transformer (18V @ .178 Amps) - Good enough!

But if you'll be adding additional modules in the future, maybe the 7VA transformer (18V @ .388 amps) is a much better buy considering it's only $1 more expensive compared to the 3.2VA. - Recommended!

The SC-1 has one feature that I have not talked about much. It's got provisions for an INSERT.

Just like the Inserts in your mixer, the concept is I want to be able to add future signal processing to the SC-1 easily.

I envision in the future creating add-on modules to the SC-1. For example: Passive EQ, Active EQ, optical compressor, VCA compressor, etc... so all you'll need to do is solder 3 wires, remove the jumper JP2 and the add-on module is now part of the signal path.

If you're a purist, and you don't want anything else changing the sound... just don't add any modules.

If you want a preamp with an EQ, you can have it.
If you want a preamp with built-in compression, you can have it.
If you want THE WORKS, preamp with built-in compression, EQ, etc...

Just add the modules you want. You can see the 3 solder pads in the PCB in the photo below, near the lower left corner.

As far as the insert modules go, would it be possible to wire this as a 1/4" insert jack?

I looked into the datasheets, and it could work.

The SC-1 will have a 5K input impedance for the insert return. And the send can have a max +/- 13V swing (on a 15-0-15 supply) output voltageon a 2K load.

Note: the SC-1 will be powered by 18-0-18 so the output voltage swing could be higher.

(The Mackie 1402VLZ has 2.5k input impedance for the insert return. )

I'll try it out one of these days.

The saga continues... Time to get picky! I want to stamp out every possible problem that could happen.

Hooked up the preamp via 20ft xlr to trs cable. From the test preamp it went into a patch bay, then a 3ft cord, then to MOTU inputs.

Used Digital Performer to record the tracks and measure the "noise" DP registered in it's VU meters.

Gain position - Gain dB - VU meter reading
============================
1 - 0dB -90dB
2 - 6dB -90dB
3 - 12dB -90dB
4 - 18dB -90dB
5 - 24dB -90dB
6 - 30dB -90dB
7 - 36dB -84dB
8 - 42dB -74dB
9 - 48dB -66dB
10- 54dB -59dB
11- 60dB -52dB
12- 66dB -44dB

As you can see, noise started to increase at the 8th gain position -74dB reading for a 42dB of gain.

During my testing the other day, I did notice the oscillation on the 5th, 6th and 7th position which was reduced/eliminated by the bypass capacitors.

But the 0.1uf bypass caps were ineffective for gain position 8th and higher. ... and it shows in the readings above.

-74dB is still not too noticeable, but -59, -52 and -44dB are very high.

So I need to find a solution for this and get rid of this problem. I'm thinking a different value of bypass cap in parallel with the 0.1uf may work.

So for now, Rev3.00 PCB is on hold for manufacturing.

UPDATE
Incorporated some fixes. Added more bypass capacitors to the volume potentiometer. I think considering the volume pot is connected up via hookup wires instead of onboard the PCB is a contributing factor to the noise problem.

Picked an AM station when the preamp is hooked up directly to the RP8 monitor. Didn't happen when the pre was hooked up to the MOTU. Added another bypass cap to the volume knob and that solved the problem. Bye bye country radio!

Gain position - Gain dB - VU meter reading - New reading
============================
1 - 0dB -90dB -90dB
2 - 6dB -90dB -90dB
3 - 12dB -90dB -90dB
4 - 18dB -90dB -90dB
5 - 24dB -90dB -90dB
6 - 30dB -90dB -86dB
7 - 36dB -84dB -84dB
8 - 42dB -74dB -78dB
9 - 48dB -66dB -78dB
10- 54dB -59dB -65dB
11- 60dB -52dB -60dB
12- 66dB -44dB -55dB

DP's VU meter readings are better... especially at the 11th and 12th position. Oscillation at the higher gain settings fixed too.

Just for some layman terms...

60dB is 1000x amplification of the original signal.
66dB is 2000x amplification of the original signal.

BTW, without the preamp connected the best my system can do is -90dB. So the above -90dB readings probably should be lower.

I still want to be able to push noise lower for gain settings 54, 60 and 66dB maybe down to the -78dB mark.

The thing is since I'm using about 2" jumper to simulate my selector switch, most of the noise is coming in via this jumper. I can tell because depending on how the jumper wire is positioned, or if I'm touching it, the amount of noise varies. If I had a real selector switch soldered in, I know the readings will be even lower than above.

Another possible area where noise is coming in is via the hookup wires that connects to the external potentiometer. This piece of wire then goes directly to the balanced driver stage.

On Rev3.00 PCB, the gain selector switch and volume potentiometer will be onboard the PCB so we're talking about very minimum distances, no hookup wires and very close to the ground plane of the PCB. I don't think we'll have this problem on the Rev3 design.

So maybe I might go ahead and push to manufacturing...

more update
I decided to plugin again the SM57, crank the gain selector all the way to 66dB, and lowered the volume knob since the SM57 doesn't really need that much of a gain... hooked the preamp directly to the RP8 monitor and fired it up.

Better than yesterday and earlier today! It is quiet. The volume knob pot is like 1/5th up and it is already loud, of course considering it's at 66dB gain (amplifying 2000x) and no RF noise, no hum noise... YES, at 66dB setting!

Noise measurements:

I'm a little bummed that at the 60dB and 66dB gain settings, the SC-1 noise floor isn't lower than I expected. (@60dB, -52dB improved to -60dB; @66dB, -44dB improved to -55dB)

So I looked around the net at other equipment specs...

There's a lot of mumbo jumbo out there... Some measuring their noise with the inputs shorted to ground (which personally I don't agree) and some not specifying what's the volume level at (i.e. you can set gain to 66dB, but if you leave the volume only at 50% can get a low noise floor reading... deceiving don't you think?)

I found a Rane note (http://www.rane.com/note145.html) that says GAIN should be set to maximum (I did that) and the input TERMINATED with the expected source impedance. (Oh oh.... I did NOT do that.)

So, I goofed up. When I did the earlier noise measurements, I left the inputs hanging.

It should have been terminated with a 150-ohm resistor to simulate a mic connected to it. So I got a spare XLR jack, soldered a 150ohm resistor on pins 2 and 3, plugged it into the mic preamp and measured performance again.

WOW! Big difference! Much better.

(This also explains why the other day when I hooked up an SM57, it seems quiet and great, but when I checked the noise floor in DP (with inputs hanging), the readings were high.)

So here are the NEW and improved numbers!

Gain dB = Noise floor (Rs=150 ohms, Max Gain settings, volume at max)
================================
0dB = -90dB*
6 dB = -90dB*
12 dB = -90dB*
18 dB = -90dB*
24 dB = -90dB*
30 dB = -90dB*
36 dB = -88dB
42 dB = -85dB
48 dB = -80dB
54 dB = -74dB
60 dB = -69dB
66 dB = -64dB

* Note: The best my setup can do is -90dB, so I cannot measure anything below this.

** These are not EIN numbers.

And yes, the above values are at Max Gain, Volume knob all the way to the right - settings. This is real-world results, not theoretical calculated noise figure. This is what you'll see on your VU meters in your DAW software.

So this is the current score... we're at -64dB noise floor at 66dB gain.... much better! Now, the challenge is to reduce this further. But I'm just nit-picking here.

Noise Obsession

I've been pondering this noise issue for a couple of days now.

...... so last night, I'm trying to review my design and reviewing the data sheets....

I *KNOW* I have the resistor values for the mic pre stage I have were computed for 0 to 66dB gain. Then it goes to a servo, then to an unbalanced to balanced line driver.... which then goes to the MOTU inputs.

So I'm thinking... maybe it's the line driver stage giving me the noise problem??? The 1510 chip.

So I looked at the datasheet and something caught my eye.... the line driver stage ADDS ANOTHER 6dB GAIN!!!! DUHHHHH!!!! Of course! Going from unbalanced to balanced, gives you an additional +6dB gain!

So really, my preamp does not go from 0-66dB in 12 steps, BUT RATHER it goes from +6 to +72dB gain!!!! (i.e. 66dB mic pre gain + 6dB gain of balanced driver = 72dB total gain!)

So the correct noise figures compared to dB gain are:

Noise Floor at max gain settings, max volume, 150 ohm terminated inputs

6dB = -90dB* 12 dB = -90dB* 18 dB = -90dB* 24 dB = -90dB* 30 dB = -90dB* 36 dB = -90dB* 42 dB = -88dB 48 dB = -85dB 54 dB = -80dB 60 dB = -74dB 66 dB = -69dB 72 dB = -64dB

So there you go!

The FINAL SCORE!!!....

-69dB @ 66dB Gain with Rs=150 ohm terminated inputs, volume at max 100%. Which is about comparable to the RNP's noise floor and other non-transformer based IC mic pre design.

And for the icing on the cake, mine can go up to 72dB of gain, not just 66dB!!! -64dB @ +72dB gain (which is 4000x amplification!)

Not bad!

Found an oscilloscope plugin...
http://mdsp.smartelectronix.com/ffttools/2005/03/freakoscope-08-public-beta.php

continuing with this noise obsession... using the above plugin.

Since a picture is worth a thousand words.

Update:

Well, I spent some time working with the prototype.

Noticed some slight oscillation at the 5th, 6th, and 7th gain settings of the selector switch. So finally fixed that with additional bypass capacitors on the supply rails. We're talking about less than 0.2Volts oscillation on top of a 30Vpp output. So it's insignificant. But still, it's gone now. :)

As I said, this is Rev1.00 PCB I'm working on and it looks like on my Rev3.00 PCB, I've already incorporated those caps in the design.

Then connected the mic pre circuit output to the balanced line driver. And what do you know... the less than 0.2Volt oscillation fixed itself. The addtl caps weren't needed.

I can't wait any longer, and it's time to hear this with my own ears. So I connected my RP8 monitor to the preamp output jacks. And run the signal gen from 20Hz to 20Khz. (I didn't run it higher than that since I might burn out my tweeter!) The RP8 cannot reproduce 20Hz, and I only start to hear sound around 30+Hz... makes sense since the RP8 freq response is stated at 45Hz to 20Khz.

It passes audio, sounds great! Gain selector switch working. Even at max setting of 66dB!!!

Connected a dynamic mic. Very clean even at high gain settings. I sometimes thought the unit was off because I can't hear any residual noise when I have my ears near the RP8. Then I tap the mic and yup... it's on and working.

Next, passed some music material. Very clean, no hum, no noise .

Played "Misssing (Remix version)" by EBTG. Woohoo... the bass is so deep and clean and distinct. Not muddy. Played some synth heavy music. The highs and mids are very clear too. And quiet! No hum or noise.

I think I'll probably build a 2nd prototype of the Rev1. PCB so I can listen to it on stereo! :)

I still need to test this using a condenser mic. But from all signs, I think it will work just fine and just be as good.

More update: Condenser Mic Testing/SM57 Mic Testing:

Tried testing with some condenser mics today. Niiice! I just used +18V for the 48V phantom power since my bench psu doesn't have 48V. You don't necessarily need to have +48V for your condenser mics to work. The mics all worked just fine even at +18V, no problem!

As for the sound... it really sounds very full and very nice.

And I was blown away when I plugged in an SM57 dynamic. It *really* sounds great. Not boomy or thin, not harsh, but you can hear everything clearly.

I don't have any high-end condenser mics, but I can tell you... even a cheap MXL 990 shines on this thing. Very clear. Lots of response.

And yes, also tried a $20 Nady dynamic mic. It's not as great as the SM57, but it's still clean and quiet. A little lacking on the high end compared to the SM57.

Rev3.00 PCB Prototype

So what's next?

Rev3.00 PCB Design

I'm going to send PCB Rev3.00 (shown above) for a prototype run, build a couple of units using the Rev3.00 PCB, check the final parts list and bill of materials... and if those units work great (which I'm sure), I'm going to send the PCB out for mass manufacturing... mass as in maybe 100-250 PCB units. I'm just a small-time potato running this operation in the garage.

What's new in Rev3.00?

1. Added J.W Miller inductors at the output for RFI protection
2. Added coupling caps for balanced line driver in the output stage
3. Added jumpers for easy setup
4. Added coupling capacitor in case you don't want a Servo design... or you want a Servo + AC coupling setup. Added jumper for those who don't want AC coupling.
5. Change value or Resistor for LED phantom power indicator. Made the LED brighter. (it was a bit dim on Rev1.00)

Ordering Info

I'm shooting for mid May to end of May when you can start ordering them.

So start saving!!!! For sure, it will be less than $100 per channel (for the kit version, with all the PCB and parts included. Just supply your own soldering iron.)

Discounts will be given for those purchasing 2 channels, or preamp and psu combo. Bigger discounts for those wanting to purchase 8 channels.

The SC-1 preamp worked and it sounds great, very quiet, very clear, with a great freq response.

A lot has happened to the SC-1 mic preamp that I'm developing... that I didn't have time to update this site. Sorry about that.

Here's some new materials/information regarding the SC-1 mic preamp kit.

Basically, I'm getting close to when I can start selling the kits. I made 2 channels of prototype using the Revision 1 PCB design and I can tell you, I'm more than pleased with how it turned out. 

I also updated the design, added more enhancements and some small fixes (mainly, additional bypass capacitors, RF protection) and came up with a Revision 3 PCB. This Revision 3 design was sent out for another batch of prototype manufacturing, and I'll start building prototype #003 and #004 next week.

However!...  I made the decision to make a Revision 4 PCB design with some more enhancements. I think this is the last PCB revision before I start mass producing the PCB and then sell the kits to you guys.

Revision 4 PCB adds the following features:

1. Slow-start phantom power - ramp up the 48V phantom power slowly so mics will be happier, instead of subjecting them from 0Volts to 48Volts in an instant.

2. Added protection for INSERT out in case the output is short-circuited. The preamp will have INSERTs or has the capability for INSERT jacks and interfacing with the outside world. So you can use your compressor, or limiter using standard INSERT cables with the SC-1 preamp.

3. Added some changes to the gain selector stage to prevent popping at the higher gain level settings when switching gain. Again, this is not a *must have* fix, but it makes for a better and refined user experience.

So that's the latest on the SC-1 preamp.

Some photos of my SC-1 prototype #001 and #002.

Insert jacks not shown on this photo. This was taken before I added the INSERT jacks.

Rev1.00 PCB Prototype #001 Testing

Stuffing parts on the prototype #1 PCB, i.e. the PCB with the backwards selector switch. I want to get this working first, before I order another prototype for the REV 3.00 design.

Here's a photo I took of a partially stuffed PCB. I'm waiting for my 8-pin IC sockets to come in on Monday, then I'll be able to stuff the IC chips and get it working!

The switch shown on the photo is not the actual switch I spec'd in the parts list. But I don't want to use a $6-$8 switch for testing, so I jury rigged a $1 switch. It's not bad... and very stable and secure.

Did some changes on the Rev3.00 board... fixed the width of capacitors, aligned some components and some routing changes.

Signal Gen/Oscilloscope Testing

Soldered the IC sockets, applied 18-0-18 voltages, measure if the correct voltages are present in the correct pins... so far so good....

Insert all 3 IC chips....

Turn on power.... wait for any popping sound... None... Sniff for burnt smell.... None.... touch IC parts for hotness.... None. Everything is cool!

Proceeded to adjust knobs on my oscilloscope for a steady waveform, set my signal generator, and first I test the balanced driver circuit.

Here's a photo of my test rig/prototype.

I soldered some XLR jacks to make testing easier.

Checked the balanced driver stage....

Looks good!!!! I set the signal generator to sweep to a higher frequency... higher... higher... higher.... IN CASE YOU DIDNT NOTICE... That's 60Khz coming in and out of the balanced driver stage!!!! I'm pretty pleased.

Waveform looks pretty clean even at 60khz! WOW!

Since I don't have a selector switch, I just used a jumper to simulate a selector switch adjusting the dB gain selector. I then tapped from the output of the servo... which is after the mic pre. So in this waveform, the balanced driver isn't connected. I want to test each section of the preamp individually first.

So it works... I'm very pleased.

And of course, I know you'll want pictures so here it is.

I want you to take note of the waveform on the oscilloscope you see on the photo above, okay?

A separate photo of that same waveform is posted below... enlarged, with the camera about 5 inches away from the scope.

The top waveform is the input, and the bottom waveform is the output... just before clippinng. The oscilloscope was set to 5Volts per division. So it's roughly 30Volts peak to peak on +18/-18 power. The input is set to 2V per division.

Like it?

Well, you should. Because that waveform shot is at the max gain before clipping which is already an impressive 30Vp-p !!!!

And what's really impressive is it's doing this amplification to 30Vp-p of a 100Khz waveform AND the waveform still looks clean!!!! YES! 100 KILOHERTZ!!!! Yeay!

Here's the signal generator showing the 100 set on the dial, and the 1Khz multiplier.... as proof :)

This is going to be good!!!!!

It's getting late, so tomorrow, I'll connect the output of the pre to a potentiometer, then to the balanced driver stage.

And No... I haven't heard the preamp yet. I'm just doing signal gen testing, looking at the waveforms and looking for signs of trouble like oscillation and things like that.

But so far, so good. I'm liking what I'm seeing.

To do:
1. Add volume knob (because this prototype pcb didn't have the volume knob)
2. connect output of mic pre to balanced driver stage.
3. Test using a square wave
4. Connect RP8 monitor and do some listening tests using signal gen.
5. Play some music through it, via line level signals.
6. Test 48V phantom power.
7. Testing using condenser microphone
8. Optimize gain staging and dB steps of the preamp.
9. Test using long cables the mic pre output.
10. Test using long cables for the microphone input.
11. Really, really look for any oscillation.
12. Test at 15Hz or lower frequencies.
13. Find out highest possible frequency it can reproduce without distorting.

It's been a long wait, but I think it's going to be worth it. I am extremely pleased with the results I'm getting. And PCB rev3.00 will be even better.

Okay, I received the PCBs from the fab.  It's pretty.  Measuring a tiny 2-1/2" by 2-1/8".

I'm selling a few extra PCBs as I ordered a few more than what I needed. Cost is $20 per board.

I test-built one of the PCBs to test that it's working properly. Here's my completed prototype PSU (+44V) and (+48V) for phantom power, suited for those racking their Yamaha PM1000 channel strips.

And here's a shot of the bare PCB.




I have a few extra PSU boards so if you're interested, the cost of the PCB is $20 each, +cost of shipping fees. I'll include a Bill of Materials with Digikey part numbers along with your PCB so you can order the parts direct from Digikey or Mouser or wherever.

Email me if you're interested to buy. Thanks!

I just gutted the guts of a Yamaha PM1000 channel strip. I will try to rack them in a standard 19" rack case. I'll be adding a custom power supply, and the usual XLR/TRS input and output jacks, phase, phantom switch, filter cutoff low-pass, hi-pass, etc...

So far, I'm successful in my testing after ripping and soldering everything together and adding a few modifications (i.e. using simple DPDT switches instead of 3Pole selector switches).

The Yamaha PM1000 are usually called Japanese "Neves" because of a similar approach in design. Just like the Neves, this is all discrete transistor and uses an input transformer and output transformer.

The EQ section is especially sweet because there is not a single IC on this preamp. The EQ section uses inductors and capacitors... totally analog manipulation and shaping of your frequency. (circa 1974)

I've tested them on drums and vocals and they really make the sound FAT. The EQ section shapes your sound anywhere from slight barely noticeable to extreme gain/reduction. Bass frequencies are solid. Of course, being an analog EQ, it is not as precise in shaping the sound as IC chip Equalizers. But the curve is smoother I would think.

This is what a PM1000 mixer looks like (circa 1975). Note, I did not use my fully operational 16 channel vintage mixer for this project. I have a few (8) spare PM1000 modules I got from eBay. I also have 2 master strips.

The SC-1 Mic Preamp is a THAT 1510/1512/SSM2017/SSM2019/INA217 based Mic preamp with phantom power, DC servo and Balanced line output driver.

Rev.1.00 PCB Design for the SC-1 Preamp

Introduction

The THAT 1510 is a high performance audio preamplifier suitable for use in microphone preamp application. It is pin compatible with the now discontinued, Analog Devices SSM2019 and SSM2017 IC chips, and the Texas Instruments INA217 and INA163 mic preamp chips.

Designed from the ground up in THAT’s complementary dielectric isolation process and including laser-trimmed Si-Chrome thin film resistors, the THAT 1510 improve on existing integrated microphone preamps by offering lower noise at low gains, wider bandwidth, higher slew rate, lower distortion, and lower supply current. The parts feature internal ESD overload protection on all critical pins.

In short, the THAT 1510 provide superior performance in a popular format at an affordable price.

Features

* Low Noise:
1 nV/ ÖHz input noise (60dB gain)
34 nV/ ÖHz input noise (0dB gain) (1512)
* Low THD+N (full audio bandwidth):
0.0005% < 40dB gain
0.005% @ 60dB gain
* Low Current: 6 mA
* Wide Bandwidth: 7MHz @ G=100
* High Slew Rate: 19 V/µs
* Wide Output Swing: ±13.3V on ±15V supplies
* Gain adjustable from 0 to >60dB with
one external resistor
* Industry-standard pinouts

You can buy this kit from PAIA, including the cool-retro case, complete with electronic parts, power supply, knobs, manuals. They even throw in the wires.  You have the option of using a desktop case with wooden sides, or mount it on a 2u rack panel.

The price I think is also pretty reasonable considering that some of these things sell in the several hundreds.

The kit includes a burned EPROM (containing the programming). The code is also available on their website.  And note, this is not just some analog synth that doesn't play with others... it has MIDI. It responds to note-on/off data, including pitch, velocy and gate. So you can hook it up and make it part of your MIDI studio.

Writeup from their website.

The FatMan has all of the features that give analog it's big bottom
and punch in a MIDI controlled package. The classic Voltage Controlled
normalization is brought up to date with a Velocity CV not available on
pre-MIDI synths. Pitch wheel modulation is supported and Pitch and
Velocity CVs and gate signal are provided on the front panel for
driving PAiA or other linear response synthesizer modules.

And all those knobs. They invite experimentation and invention in
ways that a single control with functions buried under layers of
obscure menus never can. Just a few hours with the FatMan will teach
you more about the nature of sound than you might ever learn otherwise.
It's as at home in the physics lab as on-stage.

 

Isn't this a thing of beauty? And for a few dollars, you can build your own analog synth, a'la Bob Moog! Link to Project

Write up from the website:

I've seen the new Zoom H4 Handy Recorder. I think it's a neat little thing! And best of all, it's "cheap" at $299. Of course, "cheap" is a relative word, but compared to other gear out there, this seems to be one of the low priced new entry. It looks good too!

While reading the H4 specs, this caught my eye... "The H4 features 2 studio-quality electret condenser microphones configured in an X/Y pattern for true stereo recording."  The keyword here is "electret microphone." And the picture seems to confirm that it's really just an electret mic.

So I'm thinking, maybe I can DIY my own stereo mic.

I've used an electret microphone on a non-audio project. I made a sound-triggered flash sync for my Canon EOS30D camera. In this case, I just used the audio output from the mic to trigger an SCR, which then triggered the camera flash.

Can we used the same electret mic to create a good enough sounding stereo mic? I think we can.

So I begin adapting my sound-triggered flash sync and while doing some Googling around, I came across this product.
Stereo Super Ear Amplifier Kit

It's a kit made by Velleman and it uses 2 electret mics, and a headphone output jack. Hmmmm.... interesting. So I ordered a kit (hey, it's less than $10 so if it didn't work, it's not too much of a loss). You can use the link above to order your own kit.

The circuit is simple... the Left and Right channel is the same, so the operation of the circuit is identical. Basically, an electret condenser microphone is biased for operation using a single resistor and capacitor. The output of the microphone goes to a potentiometer which acts as the volume control. This is then amplified by the NE5532 opamp, which is then routed to a headphone jack. Since the whole circuit operates from a single supply voltage (4.5Volts), the opamp uses a virtual ground by the use of 2 resistors in it's input pin. This of course, means that each leg of the split power supply will be only Vcc/2, where Vcc=4.5Volts.

I built this kit in less than an hour... while watching TV. And construction isn't hard and it's very simple. So if you're ready to jump into this Electronics hobby and looking for your first audio project, why not try this stereo mic kit? It's only less than $10.

Here are some pics during construction... enjoy.



There are assembly instructions and schematics on the box. Very easy to follow.

Moogheads, Tubeheads, VCOVandals, VCFFreaks, DCOOverlords... let's build some modular synths! Get your soldering irons hot, open up your browsers to Digikey and Mouser and your favorite surplus store because you'll be building some wicked synths, old  school style. Yup.. old school style, without PIC programming or embedded firmware that a bunch of codewarriors just programmed. No siree!... we're going to be pushing, bending, forcing electrons to do the work... not some silicon nerd that's just faking it.

You can buy ready-made units from here, pick and match what you need, wait 4 weeks, and then wait for Mr.Brown to deliver your packages. Then you'll be able to create sounds like this.

If you can't wait that long, or want the excitement of DIY, you can buy the ready-made PCBs, add a few parts and there you have it.  Look at all the stuff you can build.

This site is run by Ken Stone.

CV processors

• Analog Logic voltage processor
• Analog Shift Register
• Cascade Mixer
• C.V. Cluster
• C.V. Mega Mixer
• D.C. Mixer
• Joystick controller
• Matrix Mixer
• Modulo Magic
• Voltage controlled amplifier

  CV Generators

• Diatonic Converter
• Gate Sequencer CV Adapter
• Infinite Melody
• Pedal board/Mini Keyboard
• Psycho LFO
• Programmer/Sequencer
• Super Psycho Modulation source.
• Utility LFO.

  Clock/Gate/Trigger processors

• Burst Generator
• Gated Comparator
• Gate to Trigger Converter
• Gate Sequencer
• Master Divider
• Pulse Divider and Boolean Logic
• Quad Logic Gate
• Slope Detector
• Sub-oscillator/harmonic sequencer

  Signal Processors

• Bandpass Filter
• Bi-N-Tic Voltage controlled Filter/oscillator
• Cascade Mixer
• Delay Development Board
• Dome Filter
• Modulo Magic
• Real Ring Modulator
• Saw Pitch Shifter/Wave Multiplier
• Simple Wave Folder
• Steiner Voltage Controlled HP, LP & BP Filter
• Steiner Voltage Controlled HP, LP & BP Filter, Eurorack format.
• Sequential Switch
• Tube VCA and wave folder
• Wave Multiplier
• Voltage controlled divider
• Voltage controlled amplifier

  Signal Generators

• Chime Simulator
• Cynare (drum simulator)
• Digital Noise
• Drum Simulator
• V8 Engine Simulator
• VCO
• Wavetable for VCOs

  General and Accessories

• +/- 15 Volt Power Supply
• Display Adapter
• LED Driver
• Power Supply Delay
• Stomp Box Adapter (Use CGS modules as effects pedals)
• Stomp Box Adapter (Use effects pedals as modules)

  Digital Synth subsections

• D/A Converter
• Digitally Controlled Oscillator
• Keyboard Scanner sub-module
• Parallel port adapter

Schematics, parts list, and description and operation of the circuit are pain-stakingly written on the site. This is an excellent, excellent website! Check out Ken Stone's Modular Synth website.

Some pics to get your blood going...

I now have a mic preamp kit inspired by the API preamp. Check it out! You can buy the complete kit on my website. http://www.fivefish.net/diy/ http://www.fivefishstudios.com/ffimages/X12-Preamp-Fivefish-large.jpg This is my own version of a classic preamp, popular in the 70s and still popular today. It's not a clone since I'm using a totally different signal path and design, and different input and output transformers. But the sound nevertheless is awesome. Big and beefy with lots of transformer mojo goodness. Note: This kit is my 1u rack version with on-board local voltage regulators. Using Input and Output Transformers 12-step Gain from 22dB to 68dB, at 4dB/step -20dB Input Pad High-quality parts Option to use Discrete OpAmps with the 990/2520 format Using high-performance chipsets, currently the best chip around Using High-speed 2000V/us, video buffer, high-current transformer line driver Using Grayhill switch - high quality mil-spec with output trim Bourns potentiometer - so you can overdrive the input trafo and be able to trim the output of the pre. Null offset trimmer adjustment LED-lighted push buttons for 48V phantom, pads, and polarity reverse Relay Controlled Polarity Reverse Relay Controlled -20dB Pad Soft-start/Ramp-up 48V Phantom Power Includes on-board voltage regulators Professional PCB, Rohs PCB with double-sided, plated-through holes, 2oz. copper with soldermask, silkscreen layout, Includes everything... 5-LED VU Meter, spacers, screws, knobs