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freddiecas
01-03-2009, 14:35
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
01-03-2009, 15:06
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
01-03-2009, 16:13
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
01-03-2009, 16:16
Moved to D.I.Y.

StanleyB
01-03-2009, 16:48
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-LF351-Wide-Bandwidth-Single-J-FET-Op-Amp_W0QQitemZ400029380466QQcmdZViewItemQQptZBI_Ele ctronic_Components?hash=item400029380466

Stan

freddiecas
01-03-2009, 16:56
thanks again.

newbie apologies for posting in wrong forum :doh:

freddiecas
02-03-2009, 11:15
Someone suggested TLE2071 as the upgraded version of TL071, it has better specs all round. Has anyone used one?

Would the venerable old 5534AN be safe at unity gain? I have no scope to check these things now :)

Alex Nikitin
02-03-2009, 11:46
I would second OPA134, you can get these for about £1-20 each incl of VAT for 10+ quantity (http://uk.farnell.com/texas-instruments/opa134pag4/op-amp-high-perf-audio-dip8-134/dp/1178437). 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 (http://uk.farnell.com/texas-instruments/opa4134uae4/op-amp-quad-audio-fet-smd-4134/dp/1207057) - 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 (http://uk.farnell.com/texas-instruments/opa2134pa/op-amp-dual-audio-fet-i-p-2134/dp/1097574), 10+ is about £1-60 incl.

freddiecas
03-03-2009, 00:35
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
03-03-2009, 21:40
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

freddiecas
04-03-2009, 11:15
Hi,of all the mods done to the Quad 405 I built 30 years ago! (cap changes, current limiter increases, gain reduction, bridge rebalancing etc etc) the most stand out in subjective terms was the op-amp swap to eventual OPA134. This got rid of the "polite" "veiled" effect of the original Quad to real blackness in the silences and micro detail in the depth of the image.
Maybe this is not surprising changing chips originally designed and made in the late 70s. The OPA is a good audio chip and I'm sure the AD is superb too.

Thanks for the tip about the regulators, I'll be ordering them :)

johnrtd
04-03-2009, 23:32
IMHO the OPA2134 is the nicest solution. With those you might also raise the supply voltage level up to + and -18 V. That would give a bit more "headroom".
Also calculate the needed current for all those op amps. Anyhow, the regulators will get hot!

John

freddiecas
06-03-2009, 10:46
Hi there, the quiescent for OPA134 is around 4 mA, so 18 should be less than 100mA? I have a 20-0-20 torriod at 5VA so can afford to set the 317/337 at +/-16V or so and have a good voltage across the regulators, and put some of those small finned heatsinks on for safety.

At the moment I have 33k or 43k resistors in my input phono plugs (around 50% attenuation for line signals) to get the preamp volume control at 12 o'clock for a decent remote volume range on cd/dac.

Now the active crossover has output presets to balance everything up. If I put these on the front panel I can have them at ~50% nominal forcing twice as much signal through the preamp/active crossover and get rid of the attenuators at the preamp input. This should also improve S/N ratio since the attenuation is now after the main volume control.

I found a circuit for a Kef Kube which actually used 317/337 and fed the ac secondary into the main circuit box from a wall wart type plug containing transformer only - the rectifiers and regulators were in the main circuit box. The main smoothing caps were split with low value resistors in between, presumably to have some kind of L/R/C smoothing with the track inductances?

I think I will have a remote power supply box with the rectifiers and one set of smoothing caps, and run a meter or so cable of dc to the circuit box containing the rest of the smoothing and regulators etc.

This is a really helpful forum!

johnrtd
07-03-2009, 09:53
That transformer is far too small! I would prefer at least a 15 VA one or, better, a 30 VA one. Even 30 VA in a toroid is quite small.

And ......... whenever you decide to "improve" the circuit, for instance by loading the amplifier with a css, you'll need some more power.

The C-R-C idea is an old one, it's used in most "medical" electronics (and in our amplifiers). Also have a look at tips (http://www.hawkaudio.nl/tips.htm).

John

freddiecas
07-03-2009, 15:45
Also have a look at tips (http://www.hawkaudio.nl/tips.htm).

John


thanks, that's what the circuit has, 2R into the rectifier and 2R in between a couple of 1000uF, then the regulators with further smoothing.

freddiecas
08-03-2009, 13:05
Hi, I've taken your advice and found a 15VA torroid so will order that ;) The circuit is just the OPA's so I can't mod it much further. I think I'll put the torroid in an abs case remote and connected to the mains and run the secondary AC to the active crossover enclosure with triple pole power switch to the rectifiers.

Has anyone any links to any smart black enclosures please?

Nice looking ones are the extruded alu types but these are not that practical for wiring to pots on the front and phono sockets on the rear panels.

freddiecas
13-03-2009, 10:21
correction - got a 30VA torroid in the end :)

pulsestudio
16-03-2009, 23:59
Has anyone any links to any smart black enclosures please?
Nice looking ones are the extruded alu types but these are not that practical for wiring to pots on the front and phono sockets on the rear panels.

I use the Hammond cases available through Farnell, but they come at a cost, a much cheaper alternative to these can be found at Maplin, and really very good quality for the money asked, sorry I can't find the links I had but do a search on Farnell and Maplin sites, I'll keep looking in the meantime.

Paul

pulsestudio
17-03-2009, 00:03
The Hammond ones look like this:

http://i202.photobucket.com/albums/aa59/pulsestudio/Phono%20stage/STA60080.jpg

Paul

freddiecas
17-03-2009, 12:44
Hi, yes, that is very smart and would look great next to all the retail gear :)

peranders
25-04-2009, 16:06
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-LF351-Wide-Bandwidth-Single-J-FET-Op-Amp_W0QQitemZ400029380466QQcmdZViewItemQQptZBI_Ele ctronic_Components?hash=item400029380466

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.