op-amp suggestions welcome

[freddiecas]

Hi, I've got hold of an Kef active crossover design from the 1980's specific for my Kef Reference speakers and fancy a go at building it since I now have a power amp available for each drive unit in a passive tri-amp arrangement anyway.

I replaced the passive crossover elcaps with polyester and polypropylenes many years ago, matching the values to better than 0.5% on a bridge and adding power resistors for DF compensation, the crossovers are external near the amp terminals so speaker leads are hard tri-wired from the passive crossover outputs.

The active circuit from the 1980's used LF351's on +/-9V. It's more than a few years since my building days so am not up to date with latest devices (but I did manage to put a sm LM 4562 in my Beresford 7510 6/4 after lurking here a while ).

However the active has 9 op-amps per channel, one input buffer and 3, 3, 2 path for tri-amp outs with sallen key filters and shaping circuits, so 18 of those would be quite expensive.

I would be thankful for any suggestions for unconditionally stable unity gain op-amps suitable for this job... dual or quads if the quality won't suffer on veroboard (I have some layout experience) and easily available since I have no trade accounts anymore (retired).

Are there any reasonably priced kits for dual rail power? I have some smallish toroids left over and understand that simple 78 and 79 type regulators might need extra circuitry nowadays to satisfy audio applications.

any comments appreciated, thanks to you all

[StanleyB]

The only one I can think of is the OPA134. See the datasheets:
http://www.national.com/ds/LF/LF351.pdf
http://focus.ti.com/lit/ds/symlink/opa2134.pdf

[freddiecas]

Hi, honoured to have a reply from you Stan, thank you

If I get the active crossover done I won't need a preamp anymore since can use the 7510 6/4+ as a preamp.

I built a Quad 405 many years ago when they first came out, and used OPA134 as the op-amp swap as it got upgraded to the later versions and mods etc a la Bernt Ludwig and Keith Snook. The 405 is not used in the main system anymore but still works fine with the original transistors after all these years

Anyway 18 of those 134's would be quite expensive for a retired person and I was wondering if a cheaper alternative would still give acceptable performance...?

op-amp recommendations, suppliers and regulator suggestions welcome, thanks all.

Fred

[snapper]

Moved to D.I.Y.

[StanleyB]

The LF351 is not a bad chip at all. You can get quite a few for little money from http://cgi.ebay.co.uk/10-pcs-LF351N-...em400029380466

Stan

[freddiecas]

thanks again.

newbie apologies for posting in wrong forum

[peranders]

The LF351 is not a bad chip at all. You can get quite a few for little money from http://cgi.ebay.co.uk/10-pcs-LF351N-...em400029380466

Stan

LF351 is not particularly exciting. The design is from the stone age and most opamp are better. A nice one is AD8610 and ADA4898.

[Alex Nikitin]

I would second OPA134, you can get these for about £1-20 each incl of VAT for 10+ quantity. One other cheaper possibility would be plain TL071 - it could be a bit better than LF351. OPA134 has however much lower noise and distortion.

Alex

P.S. NE5534 are not always stable at a unity gain - it depends on a make somewhat but I would not recommend these to replace a FET input opamp anyway. If you are not following an exact layout and can use an SMD part it would be sensible to use OPA4134 - with 4 opamps in one package it would half the price.

P.P.S. - another cheaper option is a dual OPA2134 in a DIP package, 10+ is about £1-60 incl.

[freddiecas]

thank you for the brilliant link, OPA134 it is then, I thought they were much more expensive than that! brilliant!

I already sketched a veroboard for single 8 pins, it comes out pretty neatly with 0.1u around each op-amp supply to ground and a smallish electrolytic every three op-amps.

Supply will be a toroid and 78/79 regulators with plenty of smoothing at the moment on a seperate board.

I'm not sure about the earthing. The circuit is effectively 3 parallel circuits from a common input phono ground to 3 phonos out to power amps. I envisage star earthing the parallel ground paths at the input phono ground for signal, but where do I then connect the centre supply? Or should I star earth at one output ground with the supply ground? Is the star location important in circuits with only mAs supply?

thanks again to all for the suggestions and links, it's got me enthusiastic to get on with it now

[pulsestudio]

Can I add my 2 penneth here, I have not tried the OPA134's can not comment on their performance, however I do use AD825's, (a very fast high slew rate J-FET)on the outputs of my Arcam Alpha 5 which has been highly modified to Avondale spec, I tried countless op-amps in this position and found that the AD825's offerd by far the best up-grade. Sadly they are only available as SMD devices so a Brown Dog? adaptor is required, or if you are good with a soldering iron then you can make them fit onto a 8 pin DIL socket of the turned pin variety ( Harwin ), you will need to straighten out the legs and bend them into position, tricky but it does work.
In place of the 78/79 series voltage regulators LM317/337T's are a preferred choice, they are variable regs but easy to work out the resistor value for the correct set voltage, much lower impedance and quieter, just a thought

Paul