Active Avantgardes?

[Eric van Spelde]

I must have had the tales and pictures of Scalford's communal mega-horn system in the back of my mind when I asked Armin Krauss of Avantgarde Acoustic whether my trusty old Duos would be improved with fully active operation, halfway expecting him to laugh the idea away. Unfortunately, he didn't, stating that getting rid of passive crossover components in front of driver units was always a good idea, as particularly capacitors tend to do nasty things no matter how you implement them - in fact, they had planned to bring an advanced active crossover for their horns to Munich, but ultimately other projects such as the Zero One took priority (rightly so, I'd say).

Anyway, the only passive crossover components in my Duos sit in the pathway to the high frequency unit, a 12 dB/octave with the crossover point @ 2,000 Hz. The mids are directly driven by the amplifier and of course, the bass is active already, with both level and crossover frequency variable.

So in fact, fully active operation of the Duos in its simplest for would mean a separate amp for the highs with a low pass in front of it.

Ideally, the amp for the midrange horn would also have a low pass crossover in front of it, but that would mean driving the internal amp for the subs directly from another separate preamp output instead of from the amp speaker terminals - which as we've seen is rather tricky because of the huge impedance the line input sees (i.e. the same as the speaker level input, which is designed to minimise current demand from the main amp). So to start with, I'd just bi-wire the midrange horns and subwoofers from the speaker outputs of my 2A3 SET, and add a true flea power with reduced bandwidth for the HF horns.

An Elektro Mekano S6 has been suggested as a power triode for the HF amp, giving about 2W with relative ease.

So - what about the output transformer? What sort of bandwidth would be needed for this specific task, given the 2 KHz crossover point? What sort of overall layout and components when tailoring the performance for this application? Am I right in saying this could be done, and done properly, relatively cheaply? Any 'gotchas'?

 

[AmDismal]

I don't understand - you have an amp that outputs to mid and tweeter, this must have a passive crossover - at the very least, a capacitor on each driver. The sub bass can be done actively.

Unless the midrange is not a compression driver - but even then, it must have something to stop the 20Hz stuff going to it, surely?

 

[Eric van Spelde]

There's no crossover components whatsoever in the mid horn with Avantgardes - the roll-off at both ends is entirely a function of its acoustic design... How they manage it I don't know but having taken the thing apart, I can assure you there is nothing whatsoever between the input terminals and the driver but the cable...

 

[AmDismal]

Interesting. Anyway, for the tweeter, what about something low power, 45 or 10Y maybe, with an air core transformer?

 

[i_should_coco]

Interesting. Anyway, for the tweeter, what about something low power, 45 or 10Y maybe, with an air core transformer?

Gets my vote.

For the mid? Something VFET/SIT-based.

 

[Eric van Spelde]

Mids would continue to use my Welter 2A3s - they're awesome and have properly good output trannies so they'd be a waste to use on the highs...

 

[AmDismal]

Sounds like a plan

 

[paulf-2007]

I believe the mid drivers on duo's are 4" but would they stand 20Hz up 'em?

 

[Eric van Spelde]

They're 7" in total, and they do. It's how they've always been from the factory - effectively a single speaker system with an additional tweeter and a sub, if that makes sense...

 

[Eric van Spelde]

Interesting. Anyway, for the tweeter, what about something low power, 45 or 10Y maybe, with an air core transformer?

Low power was always the plan - the S6 has a 10W plate dissipation so would be good for just over 2W. Interestingly, like the 10Y its main use in amps these days is as a driver tube for 300B.

Did a quick Google on air core output trannies but have found lots of ponderings and no actual implementation - is their anything available off the shelf that would fit the bill for this particular application?

 

[Parkandbike]

I realise your main concern is to bi-amp the horns (and I look forward very much to hearing your findings), but the comments below confused me a bit.

....but that would mean driving the internal amp for the subs directly from another separate preamp output instead of from the amp speaker terminals - which as we've seen is rather tricky because of the huge impedance the line input sees (i.e. the same as the speaker level input, which is designed to minimise current demand from the main amp).

I can't see why it would be difficult to drive the sub amp from a pre-out as there is a switch to select either speaker level input or line level input. I'm assuming the line-level input has a much higher input impedance than the nominal 8ohms of the speaker-level input, but I can't find the spec for it anywhere. Just to reassure you this is effectively what I am doing as I drive the subs from a second line-level output on my CDP- with no problems that I can hear.

More recently the line-level signal has also been going through an Antimode Dual Core which allows some taming of room modes without any concerns about impact on the mid range. It also allows you to implement low-pass and high-pass filters in place of those on the AGs.

Apologies if I have misunderstood your post.

 

[AmDismal]

They're 7" in total, and they do. It's how they've always been from the factory - effectively a single speaker system with an additional tweeter and a sub, if that makes sense...

Interesting - presumably they are a cone driver then, rather than a compression driver? I was surprised, on peeking into JVS's Acapellas, they they are the same. I guess that it would have enough strength to resist the lower frequencies, since its sensitivity there would be much lower, and the horn will choke a lot off as well.

I think that I would consider getting some kind of digital active crossover to see what it does - even a Behringer would work just as an experiment - if you add low pass to the bass, band pass to the mids and high pass to the tweeter and see how it sounds. It might sound good...

Low power was always the plan - the S6 has a 10W plate dissipation so would be good for just over 2W. Interestingly, like the 10Y its main use in amps these days is as a driver tube for 300B. Did a quick Google on air core output trannies but have found lots of ponderings and no actual implementation - is their anything available off the shelf that would fit the bill for this particular application?

I think you'd have to get them custom wound. Theoretically it should be much better than using a core, but there's no way you'll get the bandwidth. But if you don't need or indeed want the bandwidth...

 

[Hornucopia]

I'd imagine little improvement. After all, they are already Bi-amped, given the Bass amps?

And, as previously mentioned, the natural frequency limits of the horns means a simple pass for tweeter.

 

[AmDismal]

I'd imagine little improvement. After all, they are already Bi-amped, given the Bass amps?And, as previously mentioned, the natural frequency limits of the horns means a simple pass for tweeter.

Not really - using the natural choke of the horn cut-off introduces a lot of distortion, most drive units have break-up modes higher up and horns will beam higher up as well. I can understand that there is a balance between this and adding a complex passive crossover, but if you're going active anyway, I think it might sound better.

I have always found AGs (especially older ones) coloured but amazingly compelling and dynamic. If you could reduce the colour, they could be properly up there - worth a blast for a few hundred euros. (Although a Najda rather than a Behringer would be my preference.)

 

[Eric van Spelde]

I realise your main concern is to bi-amp the horns (and I look forward very much to hearing your findings), but the comments below confused me a bit.

Well, I bought a preamp with two outputs specifically for trying this - and found that even with the bass level turned up fully on the AGs the bass output was so low I had to put my ear on the bass bins just to check that they were working...

I then took the bass amps out of the AG's bass bins and took them to Triple M Audioshop to see if they could reduce some resistor values or summat on the line input.

Their conclusion was that the impedance was the same between speaker and line level inputs and there was only a stupidly low amount of difference in resistor values - hardly worth talking about and no one knows quite why the bothered - between the two. Anyway, to up the input sensitivity on the line level inputs by any significant amount you'd need to remove the input transformer from the signal path, which didn't seem too clever to us.

There's an article on the AG website from some Italian magazine that stated you need to use the line level input on the bass to get good sound quality with the Duos - but after having opened it up no one can quite figure out how to do it except by adding a FET buffer stage to a dedicated preamp output...

Since, I had the input sensitivity on the Welters reduced by 8 dB or so, so I can use more than 10% of the preamp's volume pot travel... but I doubt it'd be enough, from feel the difference between bass and horns output was more like 20 dB at least...

 

[Eric van Spelde]

Interesting - presumably they are a cone driver then, rather than a compression driver? I was surprised, on peeking into JVS's Acapellas, they they are the same. I guess that it would have enough strength to resist the lower frequencies, since its sensitivity there would be much lower, and the horn will choke a lot off as well.

No compression drivers on AGs - mid is sort of a dome/cone hybrid made out of some modified paper pulp, tweet is an aluminium/titanium/whatever dome of sorts.

I think that I would consider getting some kind of digital active crossover to see what it does - even a Behringer would work just as an experiment - if you add low pass to the bass, band pass to the mids and high pass to the tweeter and see how it sounds. It might sound good...

No doubt this is what Avantgarde themselves have in the works as their Zero One already uses the tech, but that takes the digital signal from your iPhone or DAC or whatever and splits/modifies it in the digital domain where it makes sense...

OTOH I'll be damned if I'm using an anlogue record player and single ended, non-feedback triode stages throughout just to have some cheap box of chips convert it into digital and back into analogue before it going into the speakers... Just doesn't make sense to me.

Should I win the lottery or something I might call Kevin Scott to hear whether that Kondo-san 5-way crossover he's got kicking around for God knows how long can also work as a 3-way.... ;-)

 

[Parkandbike]

I'm really mystified- especially as I think we are running similar vintage Duos and I have plenty of bass and headroom to spare. Do yours have the 225 sub?

I would have though that a pre-amp would be able to provide in excess of the 2V standard output from a CDP. However, if you do not have any means of adjusting the volume of your Welters other than the pre, I suppose it might be that they are so sensitive that there is insufficient volume adjustment on the subs to balance the system. I'm using AN P4s which only need 0.5V for full output, but they do have input pots to attenuate the input signal, so that makes them seem less sensitive. You might try some attenuators on the input to the Welters as a cheap quick experiment.

However an 8ohm input impedance clearly would not be sensible for a line level signal. Might be worth a quick email to AG.

By the way I also though the Duos sounded much better with the subs being fed through the line-level input, but thought it would be heresy to say it!

Well, I bought a preamp with two outputs specifically for trying this - and found that even with the bass level turned up fully on the AGs the bass output was so low I had to put my ear on the bass bins just to check that they were working...I then took the bass amps out of the AG's bass bins and took them to Triple M Audioshop to see if they could reduce some resistor values or summat on the line input.

Their conclusion was that the impedance was the same between speaker and line level inputs and there was only a stupidly low amount of difference in resistor values - hardly worth talking about and no one knows quite why the bothered - between the two. Anyway, to up the input sensitivity on the line level inputs by any significant amount you'd need to remove the input transformer from the signal path, which didn't seem too clever to us.

There's an article on the AG website from some Italian magazine that stated you need to use the line level input on the bass to get good sound quality with the Duos - but after having opened it up no one can quite figure out how to do it except by adding a FET buffer stage to a dedicated preamp output...

Since, I had the input sensitivity on the Welters reduced by 8 dB or so, so I can use more than 10% of the preamp's volume pot travel... but I doubt it'd be enough, from feel the difference between bass and horns output was more like 20 dB at least...

 

[anubisgrau]

Low power was always the plan - the S6 has a 10W plate dissipation so would be good for just over 2W. Interestingly, like the 10Y its main use in amps these days is as a driver tube for 300B.

actually on contrary, the tube has a serious revival in state of the art DHT preamps and extremely good SE amps, like the one i use at home:

 

[Eric van Spelde]

I'm really mystified- especially as I think we are running similar vintage Duos and I have plenty of bass and headroom to spare. Do yours have the 225 sub?

Yes, later version 225s with the rubber woofer surrounds and PA102(?) amps...

it was also quoted as the reason the Dutch AG importers don't use the line level inputs either - serious mismatch between bass and the rest with any and all pre/power combos they have tried...

However an 8ohm input impedance clearly would not be sensible for a line level signal. Might be worth a quick email to AG.

IIRC it was more like 10,000 ohms on both line and speaker level, designed so the main amp wouldn't have to waste any significant current on the bass when using the speaker level inputs...