Using Aktiv Stereo cards with non-Linn amps

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Porterbrewer
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Using Aktiv Stereo cards with non-Linn amps

Post by Porterbrewer » 2019-04-09 09:59

A few weeks ago, a pair of Keltiks crossed my path, and although I hadn't planned of getting any just yet, I couldn't help myself and went and bought them anyway. I already have a pair of Keilidhs I've been planning to turn active, so I already have more amps than I can use right now. My original plan was to use a miniDSP to make a sort of diy Exakt system.

However, the speakers came with a set of Aktiv Stereo cards which I figured I could use to start with. Now these are made to fit in Linns AV5125, 2250 and LK85/140 amps, which I don't have. So I thought I could put together my own Tunebox. The first thing I found was a thread on this site where Fredrik suggested the voltage required to supply power to the cards are around 20 Volts. Well it turns out that I can get +/- 21V from the pre-amp section of my old surround amp. So now my plan is to put the cards in the amp and hotwire them there and then use four channels to run the isobarik woofers, and send the other channels to two external amps.

And now to my problem. And I have to make a n00b alert here, since I'm not formally trained and experienced in electronics. I don't want to ruin any cards or amps so I have breadboarded the cards to see if they work as they should. So I connected them up and measured the voltages. There are check points on the board marked 0V, -15V and +15V. But when I feed the cards 21V I only measure between 7,5-10V on the check points for the different cards. Why is this? It would make sense that I get the stated 15 Volts right?

When I trace the cards I can see that they first pass two parallelled 470 Ohm resistors before entering the LM317/337. So when I measure before the resistors, I do get 21 V on the boards. But between the resistors and the regulators it's down to 10-12 Volts. AFAIK the regulators would need a few volts higher supply than their regulated output, right? What am I missing here?

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Re: Using Aktiv Stereo cards with non-Linn amps

Post by sunbeamgls » 2019-04-09 23:57

Can't answer your specific questions, but I do have an observation to make about your plans.

If you use amps from different manufacturers, apart from the possible problems relating to sound quality, they may have different levels of gain, making the speakers unbalanced across the frequency spectrum. Worth considering. I know there is a member from Canada who mixes amp manufacturers with Keltiks and may be able to offer some advice.
AEDSM; AEBox/1; 3x Tundra Stereo 2.5; PMC 20.26. Blogger. Exakt Design. SO measuring.

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Re: Using Aktiv Stereo cards with non-Linn amps

Post by Porterbrewer » 2019-04-10 10:20

Thank you for your comments. Yes I'm aware of the gain problem but I'll deal with that when I get there. The plan is to set the gain for the external amps to match the AV receiver, then use the receiver as the control amp.

I'm already aware that this will not be a perfect setup. I plan to get the DSP later on and use that with measuring software to correct both for driver matching and room correction. The whole project is mainly to learn the design and implementation process. And partly because I can't afford to buy a complete system at the quality level I want to achieve. :-)

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Re: Using Aktiv Stereo cards with non-Linn amps

Post by sunbeamgls » 2019-04-24 00:45

A Tunebox has just popped up on ebay in the UK. FYI.
AEDSM; AEBox/1; 3x Tundra Stereo 2.5; PMC 20.26. Blogger. Exakt Design. SO measuring.

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Re: Using Aktiv Stereo cards with non-Linn amps

Post by Porterbrewer » 2019-04-24 09:19

Hi. Thanks for the tip! Yes, I've seen it. But I already have the cards and I'm not sure there is an aftermarket for used Keltik cards. I guess I could try and see if someone wants the extra set of cards.

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Re: Using Aktiv Stereo cards with non-Linn amps

Post by Briain » 2019-04-25 16:23

Edit: I just did the sums (see next post) and yes, I would think that +/- 21 Volts would be the absolute minimum for the cards that I am familiar with (the original stereo Ninka cards) but it was easy to lower it (down to +/- 18 Volts, or so) so if the ones you have look like the ones shown in the below photograph, there is certainly an easy option to do that. /Edit

---

Hi

A bunch of years ago, I bought a pair of stereo active cards to activate my Nimka speakers. As I was - at the time - using an LK1 and LK2 power amplifiers, there was nowhere to plug them into. As I had already built an external power supply for the LK1 (think of a Dirak circuit, but built more like Naim a Hi-Cap and using the T03 versions of the LM317 and LM337 regulators) so there was a space in the LK1 box in which the cards would fit (which was too tempting to ignore). Below shows the cards installed in the LK1.
LK1 Side.jpg
LK1 Side.jpg (158.38 KiB) Viewed 2908 times
LK1 Front.jpg
LK1 Front.jpg (160.44 KiB) Viewed 2908 times
When first looking at the cards, I noted that they were using LM317 and LM337 Voltage regulators on them, so as the the home-brew regulated PSU delivered +/- 19 Volts to the LK1 (which internally regulates that down to +/-15 Volts) I assumed that would be more than enough headroom for the LM regulators on the crossover cards to operate correctly (these regulators need 2.5 Volts minimum headroom, so on paper, they should have worked when fed by a regulated +/- 18 Volts supply) but there was an unforeseen 'gotcha' which meant that I had to so a small modification (fitting a resistor to each card).

When first tried it, there was almost no audio, so I obtained the diagram and took a dive into the circuit, discovering that that the cards had a mute line, which was triggered by a sensor on the input side of their regulators, and that at +/- 19 Volts into the card, it was mostly muting the output. I assume the idea of the mute line is likely that when the amplifier is first switched on, the crossover outputs need to be muted until the amplifier power supply rises above certain Voltage level (to eliminate any switch on 'thump' from the speakers).
XoverPSUstage.jpg
XoverPSUstage.jpg (107.94 KiB) Viewed 2908 times
The above diagram excerpt (from the original stereo crossover cards) shows the power supply stage, as well as the mute line section (in the shown area, the bass and treble cards are the same) and I have just marked it up with the teal coloured annotations. Pondering back, to make them work I had to piggy-backed another SMD resistor across the one I've outlined with the teal coloured ellipse. As you can see, R31 and R32 form a Voltage divider, which feeds into an LM393 comparator. The purpose of this is to measure the Voltage levels (on input side of the +ve regulator; an the LM317), so if the Linn supplied input was, lets say, at 22 Volts (I do not know what Linn feed into these cards, but I will later see if I can find that out) then the centre point of the two resistors would be at (4.7 / 20.7) x 22 = 5 Volts (above the 0V rail).

In my case, the power supply was only at 19 Volts, so pondering these two resistor values, the centre point would equate to (4.7 Ohms / 20.7 Ohms) x 19 Volts = 4.3 Volts (which then fees into that comparator) so clearly that was not enough to trigger the circuit to turn the mute line FET off (note the 4V7 avalanche diode on the other comparator input). Scrabbling about through the parts bin, from memory I found some 22K SMD resistors, so I used one to piggy-back R33 (the one highlighted with the cyan ellipse), thus changing it from being 16 K to 9.26 K, thus changing the equation to instead being (4.7 / 13.96) x 19 = 6.4 Volts, and that was more than enough to keep the mute circuit a bit happy about life.

Looking at it all now, if the cards really do run of something more like 22 Volts (I had assumed it to be slightly higher than that, but it was only a guess) then I could have instead used something more like 50K as in parallel with that 16K that would have equated to 12.12K, thus making the equation 4.7 / (12.12+4.7) x 19 = 5.3 Volts and that would have been enough, but I do remember looking at the LM393 data sheet and being completely happy about feeding 6.4 Volts into it.

Note that this was all done over a decade ago, so I cannot be certain that I actually used 22 K padding resistors, but from the above information, you can get the general idea of how to make crossover cards run from a lower Voltage than was originally intended (though they'd still need to be fed by more than the minimum 2.5 Volts headroom these regulators require - that plus your calculated ripple Voltage, of course - in order that the LM regulators can then successfully regulate).

Of course, if you required them to work off a slightly lower Voltage, it would be possible to swap these two regulators to suitable LDO (low drop out) equivalents, which only require about 1.5 V headroom to regulate, thus meaning that you could likely run the cards from something like a +/- 18 Volt supply (though ensure there are enough C's to keep the ripple down to well below 0.5 Volts, of course).

Bri

NB All the above sums have just been done very quickly on an on-screen calculator (not taken from old notes) so apologies if I have any glaring errors in the above sums or their resultant numbers (I'll check them tomorrow; I'm heading out, right now).
Last edited by Briain on 2019-04-26 10:44, edited 22 times in total.

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Re: Using Aktiv Stereo cards with non-Linn amps

Post by Briain » 2019-04-26 10:10

Err, in that post I said I'd done this a decade ago, but I've just noticed that the pictures show a date stamp indicating February 1999; gosh!

Anyhow, this morning I thought it all through and I remembered the values that I'd actually used to modify the cards, so the above post now has amended sums to reflect that (with that mod, they should work fine down to +/- 18 Volts).

With a bit of head scratching, a look at the LM393 data sheet (I haven't done so, but I suspect that it wont need much of a difference to 'toggle' it), it would be possible to figure out the theoretical absolute minimum supply Voltage (i.e. the lowest Voltage they'd work at without the need to piggy-back a resistor on the divider that triggers the un-mute line) but even without doing any of the above, if we just choose 21 Volts and pump that into the sums relating to the mute line...

(4.7 / 20.7) x 21 = 4.77 Volts

...that would result in just 4.77 Volts being fed into that comparator (which has a 4.7 Volt Zenner diode derived Voltage reference its other input) so I would think 21 Volts would indeed be the minimum requirement to ensure that you guarantee triggering the un-mute circuit.

If you do need them to operate at a lower Voltage, it is an easy job to add that resistor, though (from memory, they're physically quite large SMD resistors; I remember easily doing it with the standard Weller TC iron that I was using for non-SMD work).

Bri

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Re: Using Aktiv Stereo cards with non-Linn amps

Post by NinthWave » 2019-04-26 11:41

Porterbrewer wrote:
2019-04-24 09:19
Hi. Thanks for the tip! Yes, I've seen it. But I already have the cards and I'm not sure there is an aftermarket for used Keltik cards. I guess I could try and see if someone wants the extra set of cards.
Just to be sure you saw the right product. There is one with the keltik cards included but also, another one w/o cards. I would have consider it but that's far from cheap. https://www.ebay.co.uk/itm/LINN-AKTIV-C ... ctupt=true
Stream Magic 6v2 | HAKAI testing < Kairn < LK140 | LK85 < Ninka AKTIV

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