New B-format preamp build

So I had a couple of issues with my original B-format preamp. One gang of the 4-gang pot also got a little crackly, and I have a large classical gig this weekend (60-piece orchestra and 80-voice choir... and only one chance to get it!) so I wanted to solve the problem once and for all.

I found some Extron video sync converters some of which worked, and others of which were dead... but they had nice cases, and they were $5.00 each, so I tossed the guts out of a dead one.

Here's the old Extron unit with the lid off:




Made a new back panel and built in a B-format preamp...

Here's the new back panel:




and the temporary front panel (I'll make a better one later!)




Here's the back once it's all closed up:




...and from the front all tucked away:




Here it is with the ST250 control unit which it is going to be used for:



...and from the front:




/They work well together: about the same height, and will both fot side-by-side in a 1RU space:



-I feel MUCH better about the gig this weekend!

Keef

Ooooh

I too am into ambisonics.
I DIY'ed my own ambisonic microphone.
I used Henry's B-Format ciruits to go from the mic
to wxyz outputs. My mic uses 4 mxl 600 capsules.
(They were available and not that expensive). The
head amps are 4 Dorsey circuits, All crammed into a
Marshall Nuemann clone case.

Is the schematic for what's on the perfboard available?

OT: Are you happy with the Meeeeters?

Your work looks perty.

:grin:

:LSB

Oh, the front panel controls:

The four interlocking pushbuttons (kind of like an LA-1176 switch bank) are as follows:

0dB
+10dB
+20dB
+30dB

and the rotary control is ±12dB.

The reason that the front panel is so temporary is that I'll be adding 4-channel LED metering for the WXYZ axes later on... so more holes being cut and filed.

Keith

Bitman,

The MEEEEETursss are excellent, thanks! :thumb: :thumb:

Into ambisonics huh? fabulous! -I just finished an album last weekend which I did in B+ format: The producer was a pain in the butt at times, not getting the whole thing, and wondering what those 4 tracks were for, but when I finally decoded everything he stopped grumbling and enjoyed it. He didn't understand, but he did like it, which is what matters.

The schematic: well it's pretty simple: the ST250 puts out unbalanced signals when switched to B-format, and -as a consequence of how the unit is designed to run off of intternal battery, AC wall power OR phantom power, unfortunately the outputs also carry about 25Volts of DC superimposed on the signal.

The first thing on all 4 channels is to block the DC with a 100µF cap, then refer the negative plate of the cap to audio ground.

Then there's the 0/+10/+20/+30dB stage, which is one half of a philips 5532 per channel. The signal input (referred to audio ground via 47K resistor) goes to the noninverting input. The inverting input is coupled to the output through a 22k resistor. The gain is switched by tying the inverting input to ground (attenuating the feedback) by a 6.8K resistor(+10dB) a 2.2K resistor (+20dB) or 680ohms(+30dB).

The second stage looks like the second stage in my original version: -See this thread:
http://www.groupdiy.com/index.php?topic=7528

but with 3.9K instead of 39k, and with a 10k pot instead of a 100k pot... oh, and a 5532 instead of an 072 (though they're socketed.. so that can be switched easily.)

I like the gain switching by just changing the feedback shunt leg (as I tried to describe) much better than the original version shown in the schematic in the link: MUCH better. -I think Paul Stamler may have suggested that one.

Keith

Oh, just went to the gig for a quick location/setup scout...

Looks pretty nice... very pleasant room sound. Here's a picture:


...A choir of eighty-something voices


...Should be quite big sounding!

Keith

Excellent!

Let us know ow it goes. Are you using a soundfield mic?

Excuse my ignorance is that a preamp?? :roll: Who's design it is anyways....

Top work, Keef! :thumb:
Good luck on the gig.

alright did some research...

My oh my...

:shock:

That was a big gig last night!

Seems that I was misinformed about the size of the choir. It ended up being 196 voices... plus four soloists. Then there was the 50-piece orchestra.

The performance was reasonably good, the preamp worked flawlessly.

Note to self however: -Build an XLR snake with boxes at both ends: the stage end to plug the soloist & spot mics in, the recordist end with ground lift switches available for each channel.

I had a tube mic (I have a reasonably nice U67) as a soloist mic, and there was a ground loop when I plugged in the power at the stage end. Lifting the audio ground worked, as would lifting the power ground, but I had to consider safety, so audio ground lift it was. -I have some short ground-lift XLR male-to-female cables, but it'd be MUCH quicker just to throw a switch and compare...

I need to make a nice snake anyhow, so this is a feature to add.

the second concert is this afternoon, so I'll try and take a few pictures of the setup if I can...

The stereo spread was fantastic. The orchestra is set up in a very "wide-and-shallow" arrangement, to accommodate the choir at the back. That put the brass off in a little 'transept' to the right, and some of the trombone/trumpet line-of-sight was a little lost, but other than that and a couple of other comparatively small balance familiarity issues, everything was acceptable, but the stereo width was quite spectacular.

It ended up being mainly B-format, with an XY pair on the choir, used just to support the diction (it's quite a live stone room chapel, so the diction could definately get a little mushy in there at a distance... adding just a small amount of the closer XY pair helped clarify the choir a great deal)... and a single soloist spot mic.

But yes, the preamp worked marvellously. Convenient, quiet, reliable and no crackles. Scaling the resistance values down by a factor of ten around the variable-gain stage seems to have cleared up a lot of the stability issues that the previous preamb had.

Next I'll be adding output balancing and 4-channel WXYZ bargraph metering... then it'll be time to make a professional-looking front panel.

:thumb:

Keef