After double checking the circuit using the invaluable old highlighter technique, and putting in a couple of ground runs I had missed, I got the EQP-1A fired up without difficult.
The first thing I noticed was that the voltages from the
Hammond 270CAX power transformer I used were running rather low - almost 50V below the schematic. But Hammond transformers now have optional taps for the primary windings to reflect the fact that wall power in North America has crept up over the years from 110VAC to now typically closer to 120VAC. Since, I had originally wired it up with the 125VAC (black & white) primary combo, I just rewired it with the 117VAC (grey & white) combo to bump it up a bit. This got me a bit more than 25V back.. So the final B+ voltage is still 22V or so below the schematic, but it's not really enough to be concerned about --- approximately 7% or so below nominal, and this with my line voltage in the house running a bit low.. So it's basically within 10% and should be fine. Everything else checked out voltage wise. Popped in a nice NIB NOS GE grey plate 12AU7, and one of the new production Tung-Sol reissue 12AX7s, and we were ready for action. Everything checked out with tubes in as well.
As I pointed out earlier, one small question mark was whether there would be an issue with running the 5VAC and 6.3VAC windings of the PT in series in order to feed the DC regulator for the heaters. The numbers suggest that the 5VAC winding will have its VA rating pushed just a bit. But on the other hand, the remainder of the PT is completely loafing, as current draw for the remainder of the whole circuit other than the heaters is less than 20mA. And indeed, even with many hours of continuous operation, the PT showed absolutely no signs of overheating or any kind of stress. Hammonds are also well known for their conservative ratings to begin with. So no worries... rock solid 6.3VDC coming out of the regulator to feed the tube heaters? Check.
Alrighty then, the next issue was confirming correct operation. I used some cheapo XLR to RCA adaptors to interface the equalizer with my
M-audio Audiophile 24/96 soundcard. I've had this thing sitting in a drawer for a couple of years, since it didn't play nicely with a motherboard I had in an earlier computer. But I was recently *amazed* to discover that this sound card is still being sold, and the drivers are still supported, up to 64 bit Windows 7. The PCB on my sound card is stamped with a date in the year 2000 - practically prehistoric in computer terms! But anyway, it's still a very capable little card, which I guess is why it's still in production. It works great on my current machine.
Anyway, long story short, I was quickly able to confirm the EQ seemed basically to be working as predicted just by playing music through it as I messed with the knobs... but that's not enough! I wanted to make sure that the entire frequency response looked right - so I needed some help on the software front. Fortunately, a bit of poking around led me to the
excellent HolmImpulse freeware from
Holm Acoustics.. this software is actually intended for characterizing the frequency and impulse response of speaker setups. But it's also ideal for this application. Using this software, I was able to virtually replicate, with only extremely minor variation, the frequency responses shown in the original
EQP-1A users manual. Awesome! In some ways it never ceases to cause wonderment that it's possible to come along fifty years later and recreate something and have it behave pretty much exactly as the engineers and designers had intended.
The only minor red flag was that the noise floor of the unit didn't seem to be so great... about -70dB or so seemed to be the best it could do. However, I quickly remembered that for testing with my sound card, I was running the unit in an unbalanced configuration - and my workshop/mancave is a total swamp for EMI. I had every reason to believe it would be much better in actual balanced use in the studio....
It didn't take long to find out. I delivered the EQ to Cam DiNunzio at the Black Iris studio on Tuesday, and we quickly put it through its paces. Bottom line? The noise floor in balanced operation in the studio was somewhere
below -90dB. I don't know for sure where, as that's as low as his protools metering system would go. Anyway, as I left, Cam was already gearing up to put the EQP-1A to work on a TV spot he's working on. Here's a few pics of the finished unit and the happy new owner:
Getting it set up. Note the Wombat Amps LA-2A in the right-hand stack
Cam DiNunzio of Black Iris gives the EQP-1A a whirl
Welcome to the family!
It was fun catching up with Cam. Next blog post, maybe I'll be able to post some links to TV spots that have the used the LA-2A... there's already a fair number that have either aired or are in the pipeline. Keep your eyes and ears open right now for the Sears' Christmas commercial that airing now... it made extensive use of the Wombat Amps LA-2A. Airing soon are spots for Subaru and Electronic Arts.
