You wish you were so lucky! To find a pair of vintage passive EQ units on an auction site and snap them up for your studio for next to nothing would be a dream. But for one Audio Masterclass visitor, this dream came true. Alexander Sundström of Helsinki, Finland, tells the story...
"I purchased two vintage passive EQ's about a year ago. I found them EXTREMELY cheap on a Finnish online auction place ( like eBay ). Turned out that the man who was selling them was the original designer of the units. He had designed them for the Finnish equivalent of BBC ( YLE ) in the late sixties.
"The units had been used in a modular mixing console mainly for vocal recording purposes. The most unbelievable thing is that the units are brand new; they've never been used. When I met the old man who was selling the units he told me that he had asked YLE if he could keep two of the produced EQ's since he had designed them. The EQ's had been collecting dust on his workshop shelf ever since.
"What was delightful about purchasing the units from the man who had originally designed the prototypes was that he could give me a load of technical specifications of the units and explain how various things worked. He also opened up one of the units to demonstrate how the unit was built inside, and to show the build quality of the EQ; which I must say is impressive. All pots, switches, you name it are gold plated; the pots are step-increment rotary pots.
"I also got to test the units with various audio sources, as well as with an oscilloscope and a spectrometer. The units just sound great. Now I'm coming to the problem I need help solving. The EQ's are passive, and the input signal is lowered by -20dB in order to be able to boost various frequencies. I have currently no use of the units because the volume I get out of them is simply too low. I've been pondering on how to fix this problem, but I'm still unsure.
Would a variable gain preamp fix this problem? I've been thinking of building a simple high quality one for this purpose, that is if a preamp will do the job. Another question is whether the preamp should should be placed at the input stage or at the output stage? I remembered the EQ's when I read your article about the $5 preamp, and started thinking whether it would do the job.
I have a rack housing for the units, with drilled holes for balanced XLR I/O's.
David Mellor responds:
Wow, what a story! And your EQ sounds great. I suspect envious Audio Masterclass visitors will be sending offers in by the dozen. Don't sell! Just use your vintage EQ units to produce great tracks.
Your question... yes passive EQ does require gain. Passive EQ loses level, therefore that level should be brought back up again.
In theory it would be better to provide that gain before the EQ unit. If you boost the level after it has lost 20 dB, then undoubtedly you would be boosting up noise as well.
However, boosting the signal before EQ is going to eat up valuable headroom. I would be inclined to compromise and provide 10 dB before EQ and 10 dB after.
The $5 preamp isn't appropriate because you would only be able to drive the EQ from a microphone, where doubtless you want to use it on line-level sources too. I would be inclined to use a couple of NE5534 integrated circuit op-amps before and after the EQ. Perhaps a noble audio engineer among Audio Masterclass's readers would help with a circuit?
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