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.