API 2520 op amp alternatives (help the noob)

[AtticMan]

Well, after lurking around here for the the better part of a month, and reading an a$$load of old threads (about 50 pages worth), I'm starting my first thread.

Be gentle with me...

I'm planning to build an API 312 clone (already have the output xfrmrs on order with Tommytones) and was wondering what op amps were direct swappable, i.e. pin-for-pin replacements for the API 2520? I've seen the Avedis 1122 listed, but haven't seen anywhere to buy one. What about the JH990 or JLM99? Will I need to make a PCB customized to these other op amps?

FYI, I think this forum and the Group DIY folks are, quite simply, amazing. I'm floored by the communal spirit and sharing of knowledge -- especially helping out the noobies when they have problems (as I certainly will).

Many thanks, in advance.

[BjornZetterlund]

Welcome, Attic Man!

I don't know...mine are still on their way. If you've already done a comprehensive search, then you probably know more than me already. Lots of options for this one depending on what you've already got at your disposal. Many discrete op-amps have been made available through tech talk and friends of tech talk, and there are lots of options available to you. Do you have input transformers yet? They do make a difference on your selection of op-amp; anything with a FET input and you could have up to a 1:10 input transformer, whereas if you go with the John Hardy 990 I think the ideal would be more like a 1:2 (that's from memory, and probably wrong).

I've already got plenty of Sowter 1:7 input trannys and 8 output trannies on their way. I'm probably going to fit these into an 8-way rack that I've got left over from a previous project. I think I'll modify it in such a way that the power rails will carry phantom and something like +-30V or so (unregulated), allowing individual preamp cards the choice of whatever voltage they like to see, with local regulators. That's just one idea (non-purist) for this project, and I think you'll find there's lots more floating about!

Anyway, if you're really after an API clone, then you will need someone more well versed in their history than I...good luck!

Bjorn

[tkohalmi]

I was thinking about the Forssell discrete opamp as a substitute. However, while on the input it would be an OK impedance match I wonder if it could drive the 1:3 transformer on the output, being an all JFET device. You might want to give him a call and ask. http://www.forsselltech.com
An alternate solution may be using an OPA627 boosted by one or two BUF634T current buffers. I have built such a combo opamp, but it was driving a 1:1 transformer, not a 1:3. It sounds nice, but not like the API2520. :(

tamas

[Flatpicker]

Bjorn’s right. 1:2 for the John Hardy 990 and 1:4 – 1:10 for a FET type like the Forssell 992. The 990 will work with the higher ratios, but the noise will be a little higher. The 992 is the first one I’m going to try (If I can ever decide on some layout software and get this darn board laid out!). I’ve got a couple of Lundahl input transformers: a LL1538XL (1:5) and a LL1674 (1:4 – 1:8) to try. Some have said they sound great with the Forssell 992. Hopefully I’ll get to find out before too long.

Some have found Melcor opamps (the forerunner to API) on eBay for around $50-$60 each and have found them to be on par with the APIs.

[Flatpicker]

Originally posted by tkohalmi:
I was thinking about the Forssell discrete opamp as a substitute. However, while on the input it would be an OK impedance match I wonder if it could drive the 1:3 transformer on the output, being an all JFET device...

I didn't think API used it in 1:3 mode, only 1:2. The schematic I saw was only using two of the secondary windings. The thrid one was used for an output to the meter. At least that's how I plan to do mine. 1:3 is a little much.

[AtticMan]

I didn't think API used it in 1:3 mode, only 1:2. The schematic I saw was only using two of the secondary windings. The thrid one was used for an output to the meter. At least that's how I plan to do mine. 1:3 is a little much.

I think I have a copy of the same schematic. It looks like the API 2503 has three secondaries, though two of them have their taps tied together as a center tap, and each of the other legs become the HI and LO of the output. The 3rd secondary is marked "to meter". On a printed API 325 schematic I have, it shows three independent secondaries for three outputs.

Speaking of meters, does anybody know where I could find one of the slick 7-element VU meters that API is putting in their current issue 512 and 3124 units?

[jrmintz]

Just for historical interest I have a data sheet from Broadcast Tchnology, Inc on their OA400 opamp. The sheet says it is a "pin for pin replacement within its specifications" for the API 2520, Modular Audio Products 1731,5002,5004, Hardy Model 990, Aphex 2521, Fax model FA-100 and Protech A-1000. Must have been really popular products at one time.

Seth

[bauman]

I'm not sure but I guess I recall Tommytones talking about his version of a compatible opamp for 2520 position, right Tom?!?!

[img]graemlins/beerchug.gif[/img]

[Tommytones]

Yes, you are correct Fabio. I'm working on a potted opamp (a la 2520) which will be a direct plug-in replacement for 2520. With the holidays just around the corner though, things have slowed down. Nobody wants to work anymore. I should have something by mid January if all goes well.

[cjenrick]

check groupDIY for a list of opamps also, Kev's site is loaded with good stuff!