I used to own a pair of 553's, but sold them a couple of years ago ( ). Oh well....
I think this could definitely be a nice little project for use with either the various flavors of discrete opamps as well as perhaps the option for IC's like a 5534 or OPA604, etc...
Additional frequencies would be the obvious improvement to this design. I know switching caps would change the MF selection, but how would you make the LF and HF switchable? Not too many options, mind you, but 2 or 3 each would be a wonderful upgrade to what is already a very musical EQ.
BTW, I don't think the LF and HF are shelving, though I can't tell by looking at the schematic. I just remember it responded more like a wide-Q peak, more or less, more than a shelving circuit. Very musical, as I recall.
Would any of you more knowledgable types please chime in with a little more detail on the response of these filters and how to add more frequencies?
I doubt a layout would be hard for this at all. I'd be willing to do it myself if someone can help me with the design. It wouldn't be till after Christmas, however.... too much to do at the immediate moment.
I made the hf switchable(6 frequencies) between 10kHz(stock) and @ 24kHz by using different values of capacitor in the hf. i am going to do the same with the lf. using a Neve-type inductor from Sowter, either 10H/7H/3H/1.3H or 2H/1.2/0.7H, when I have some cash! too many other projects on at the mo'. Therre are cheeper options for a 1H or 2H single value inductor. Try Cinemag, OP-amp labs, or many others. i have two of these babies permanently strapped over the output of our SSL. - AWESOME!
I didn't bother with the mf part. I remember someone suggesting that no less than Mr Woolfe himself said that you could half or double capacitor value in the mf(ie 1.4kHz - 2k8kHz(stock) - 5k6kHz) without distorting the shape of the bell with the original value of inductor-100mH.
You may have a bit of a problem finding a Profile 2503 output transformer at the moment. Sowter do a replacement, and there are others too. JLM Audio, Cinemag, etc. or you could use it without, shorting out the 27kHz resistors at the input(I think) with less Headroom(Still give you more than +20dB with +/-15dB!). There are a number of op-amp options, Original 2520 sounds absolutely fab. So does a cloned one. Also try Melcor 1731 for creamy alternative, or something like Forsell for cleanest possible. or Any of the others that will fit (Hardy. Jensen, JLM, Millenia etc. None of them sound even slightly crap, in fact, I wish I could select between them in the mix! I am currently playing around with a lash-up of the NTP m100 in a 2520 footprint. I haven't succeeded yet. i am still trying to work out how to fit it on a piece of perfboard. I do not know how to do board layout/production! - Yet . . . . you could even try a simple NE5534, if you do not use the step-up output transformer, and do not pad down the input. I tried this and was very pleasantly surprised. MUCH cheaper, and veeery very good indeed . . . .
Please do not hesitate to ask me any more questions. This is a simple thing to build, and VERY useful/musical. Mine are absolutely essential now I have them!
Definately Shelving at the HF. LF is also shelving. The capacitor(47uF) is so large as to only affect veery very low frequencies, so effectively not high passing anywhere near the inductor value. value in the ordcer of a few Hz, below audible frequencies. At least that is my interpretation . . . Please correct me if I asm wrong!
Designed to be cheap Using all parts found at Mouser, digikey, etc.
API 500 Form factor
2 switched freq. On the high and low bands
6 switched freq. using Lorlin (or what ever you want) on the mid
Hi/Lo Q on the mids
Bypass All and you can switch out the mid on the seventh position to leave just the shelfs in.
Fastron inductors (thanks NYD) on the 5 higher bands
Xicon/Mouser Trafo/Inductor (thanks again Dave) or Wilco BSL's on the Lowest mid and LF bands
Input can be ina134 or Edcor PC10k/10k (I haven't tested the Edcor'syet)
Output Edcor PC600/600 (again I haven't test the PC series yet. They might suck)
Op amp can be you Lab variety Discrete/Hybrid/whatever 990 footprint or your garden variety NE5534.
The pots will have to be Bourne 91 series, or the board will not fit into the rack right.
That's about it. It's still in the conceptual stage right now. The hardest part is the mechanical aspect. I got to make sure this puppy is going to fit right.
Here is a quick mock up of the face plate. The holes are close, but I want to get it down to .000" if possible. Don't mind the engraving. It will change. Like I said, quick mock up.
I have a pair of Apsi559's, and I have never managed to find the schemo. Whilst they are quite useful sometimes, especially on Electric Gtrs, they are not really in the same league IMHO. Lots of 4558's. Also, loadsa positions on the switches. I bet it would be bastard to try to source anything like them that might possibly fit in such a tiny space. I have often used 560's too, and they too are a tad lacklustre compared to 550's, 554's, and 553's. Just my Humble opinion etc etc. Honestly, the 553 is so simple, especially if Elco makes his boards available(Pretty pLease?), and I can honestly not tell the difference between my lash-ups and my original(when set to 10kHz at top end and mid/Lf flat). It has to be one of the simplest, most usable projects going! and, if you don't use any expensive transformers/Op-amps, definately one of the cheapest. You can always "pimp it" later with discrete Op-amp of your choice/sexy Iron of your choice. - much cooler than fat tyres/wheel skirts, and three tone Klaxon horn . . . .