Vintage mod of Audio Control 520b

[johnny_boy]

Hi Gurus of solid state! Any one recognizes this mod of an Audio Control 520b equalizer. I just bought this one on eBay, was expecting one with a damaged tape input (because it was missing from what we coud see on the back), but I got that surprise mod inside. Looks well made, with a board on stands (one stand was unglued) and basically one socketed TL084 per channel (dated 79, so probably period correct from when this one was issued, so an early mod) and a ton of components, plus some components added on the bottom of the original board (a big capacitor, some small caps probably to adjust the values of the original caps). Other than that, on the surface of the board the circuit looks untouched. I tested it on my oscilloscope, it looks to be doing what it should, with basically no noise and perfecly symmetrically, so looks like a job well done by somebody who knew what they were doing. I also listened to it, and it works nicely. I think they moved frequencies around also, as we can see from the new markings on the face. Anyone can comment on that, and do you guys have reocmmendations? I was disappointed initially when I saw that and decided to return it to original, but but I am no longer sure now, it does sound good.

 

[Buck Rogers]

Looks like a job for REW.

You will need to do some measurements to find out how it was modded.

Almost looks like a crossover was added?

 

[Dave1027]

That's really strange that someone would go to so much trouble and for what? Notice the frequencies are no longer symmetrical. Again, why?

I'm not a fan of EQs. I have one but I haven't had it hooked up for more than a decade

 

[johnny_boy]

Looks like a job for REW.

You will need to do some measurements to find out how it was modded.

Almost looks like a crossover was added?

I don't think it's a crossover, I have all frequencies on both sides. REW would be awesome to use; not sure I have a good enough setup though, from glancing at what I needed. I did notice the markings were not symetrical, I thought it was a typo, it's fully symmetric on the scope at that frequency. But what's not symmetrical is the 80hz (modded from 120). One side is 80, the other is 120, and the tuning caps that were added are only on one side. I think the whole retuning is on the back, and I'm just gonna remove those caps retuning it. After that, if it sounds good I'll keep it, if not I'll remove the two white wires and I should be fine, but it does sound good now.

 

[johnny_boy]

Ok, I finally got it, after reverse engineering the circuit. In fact when I said it sounded good, I was actually just listening to the original 520b, bypassing the additional board. The board ouput was hooked to the tape input - so I simply needed to switch to tape to listen to the board. When I did, it was anticlimactic - I could not hear any difference! I questioned my ears and set it up on my oscilloscope, and saw no difference with the sine waves I was looking at. I then tried with a square wave, and the card revealed itself! Now we could see a big difference between the OG and the modified eq: the OG gave me nice square waves, but the board larger amplitude waves with slanted, decaying plateaus. This meant that that board is boosting the basses. I then got back to sine waves and decreased the frequency and reconstructed the curves: it is amplifying the frequencies from 10 Hz to 25-30 Hz, and not below 10 or above 30. +6db at 15 Hz. Apparently this is a bass extension - something I did not know about before tonight (thank you Google!). What I have looks like a Linkwitz transform, who had published it in 78 (although apparently the idea was floating around for few years before that publication). So that's what it was: they switched to tape to get more bass and push their speakers!

I'm pretty happy I found out. I'll just keep it on equalizer mode (without that transform) for now, and retuned all the pots back to original bands - I now have a stock 520b with a surprise in it. Why they retuned the pots, and asymetrically, I guess we will never know for sure, maybe their setup required that and they were picky, who knows... but that's okay, and I call this investigation officially closed!

 

[loudnoises]

Interesting mod

I have a 520d that's been with me for decades. Fully redone at this point, it's a previous generation. The frequency choices are actually incredibly useful for just everyday listening, and give you much better bass control than any preamp. Plus, subsonic filter. They do best on a tape loop, get a bit funky directly in line.