bi-amping with 'A' networks

[Erik Mandaville]

I recently finished building a pair of the 'A' networks to use with my 70's Klipschorns. Because of the simplicity of the A network, it is very simple to separate the branches of the crossover, and I have one 2A3 pair of monoblock amps for dedicated woofer use, and another recently completed pair of DIY parallel-feed 2A3 amps for mid/tweeter use.

If the nominal impedance of the Klipschorn is 8 ohms, am I correct in assuming that the woofer is a 16 ohm driver? What will be the impedance of the mid and tweeter sections if they are connected in parallel and separated from the low pass section of the network? I am not certain about their respective impedances; however, I'm assuming that their parallel combination results in an impedance of 16 ohms which, when connected in parallel with the woofer would result in an overall system impedance of 8 ohms.

Any thoughts or suggestions are most appreciated -- thanks in advance!

[JohnA]

While the K-horn is nominally 8 ohms, the woofer runs about 6 ohms, the mids at 30 and the tweeter at 8 ohms. See:

http://forums.klipsch.com/idealbb/files/Fred%205c.jpg

Biamping with two 2A3 amps is likely a very good idea. You should recover a HUGE amount of headroom that way. May I suggest you not drive both woofers with the same amp. There will be more power available for the woofers if the second channel of each amp is driving the highs. It might even reduce crosstalk between channels.

[sfogg]

Erik,

If you really want to bi-amp and since you are using first order crossover at the moment anyway you might want to consider moving the woofer to mid/upper crossover to passive line level and remove the speaker level components of that part of the crossover. You will need to know the input impedance of your amps to build a line level crossover for them. This would increase the effective power of your amps more since they won't be sent full bandwidth anymore and you wouldn't have any looses in the speaker level crossover. You will need volume controls on at least one of your amps so you can balance the system.

Basically at speaker level you would just need the autoformer to balance the squawker and a cap to roll off the tweeter, the line level components would split the signal at 400hz to go to the different amps. If you took this approach I'd suggest maybe playing around with a few extra things.

First would be to add a coil to the squawker to roll it off above 6k to try to tame some of its nastyiness up there. Or on the flip side trying the P-trap mod.

Second would be a little more contriversal. Instead of using the autoformer to scale back the squaker to the tweeter use it (or the Universal Transformer model) in reverse to increase the voltage to the tweeter to balance it to the squawker run without the autoformer. Then use the volume control of the amp running that pair to balance the two to the woofer section. This would let you use the extra sensitivity of the squawker to try to get more out of the power that amp has.

Or for a totally odd approach use one amp (fed full range) to feed the woofer and tweeter keeping their speaker level crossovers and the second amp to feed the squaker using a line level crossover (maybe just high pass or possibly band pass) and no autoformer to take full advantage of the K55s extra sensitivity to maximize the power from whatever amp is feeding the K55.

Shawn

[Erik Mandaville]

Thanks Shawn and John:

I really appreciate your thoughts and extra information on this idea.

John: You said exactly what I wanted to do. Both woofer sections would be on their own amps, with just the low-pass coil between the amps and speakers.

and then this from Shawn: Or for a totally odd approach use one amp (fed full range) to feed the woofer and tweeter keeping their speaker level crossovers and the second amp to feed the squaker using a line level crossover (maybe just high pass or possibly band pass) and no autoformer to take full advantage of the K55s extra sensitivity to maximize the power from whatever amp is feeding the K55.

This is interesting as well, and I was just now actually looking at some possibilities for both active and passive 'crossover-ing.' The one thing I see with this that may or may not be an issue, is the parallel combination of woofer (6-8 ohms horn loaded) and an 8 ohm tweeter. This would result in a parallel connection of around 4 ohms. This is because right now, the amps I would use for these two drivers (Moondogs)are configured for the 15 ohm Lowthers. I would just need to rewire the OPTs for a nominal 4 ohm load. Not too big of a deal, but I'm running out of wire length on those things.

So, the remaining midrange horns would be driven with the former Horus, now parallel-feed DRD monoblocks. The OPTs on those are 8ohms, but the 16ohm load from the mid-horn shouldn't be a problem, and in fact might work even better.

But. On the AA an A networks, there is a 13uf capacitor from the input to the autoformer. What I could do is 1)use a passive line level crossover, like you mentioned above, in combination with an variable L-pad in series with the driver (after the 13uf capacitor) instead of the autoformer. It might be easier to blend the three drivers that way.

Another thought was to make a series speaker level crossover for the tweeter and woofer, which would provide both an easier (about 16 ohm)load, plus provide some of the benefits of a series network (which gets kind of involved in a technical sense). And then use the other amp as mentioned above, with some kind of attenuation (either on the amp itself or in-line)available to match the output to the other two drivers. I'm not thrilled about drilling another hole in the amp chassis for a volume control, though. It could just be mounted on a metal bracket on the crossover board.

BTW: for the sake of a line level crossover, the input impedance on the four monoblock amps is 100K -- that's 100k on each amplifier.

This gives me lots to think about -- all really great help! Thanks again.

Erik

[sfogg]

Erik,

" The one thing I see with this that may or may not be an issue, is the parallel combination of woofer (6-8 ohms horn loaded) and an 8 ohm tweeter. This would result in a parallel connection of around 4 ohms. "

That won't really tell you enough since in this setup you will need to have a passive speaker level crossover as well which will alter the impedance seen by the amp.

" there is a 13uf capacitor from the input to the autoformer. What I could do is 1)use a passive line level crossover, like you mentioned above, in combination with an variable L-pad in series with the driver (after the 13uf capacitor) instead of the autoformer."

I'm not sure of which configuration you mean here... if the squawker has a dedicated amp just for it there is no reason to use the autoformer. Just use a line level volume control to that amp (or the amp running woofer/tweeter) to balance the two. This way you won't be wasting power through the autoformer to maximize the available power from the amp running the squawker.

" Another thought was to make a series speaker level crossover for the tweeter and woofer, which would provide both an easier (about 16 ohm)load, plus provide some of the benefits of a series network (which gets kind of involved in a technical sense). "

I've never looked to much into series crossovers but is it possible to build one with a large difference between the high pass and low pass sections? I know one of the advantages of series is when the impedance changes the crossover point change together relative to each other to try to avoid holes or overlaps. I don't know how they would react with a large gap built right into a series crossover.

Realistically with the K'Horns and one amp for woofer/tweeter you might not even need any sort of low pass. Just feed the woofer full range and let the horn/driver roll it off and then just have a crossover (just a cap if you stay first order) for the tweeter. Basically for the amp feeding the woofer/tweeter treat the speaker as a very simply two way speaker with a minimal crossover.. then with the other amp and line level crossovers deal with the squawker.

" I'm not thrilled about drilling another hole in the amp chassis for a volume control, though. It could just be mounted on a metal bracket on the crossover board."

You could just build a passive line level volume control into the interconnects feeding the amp. Or build a simple one input passive volume control that you could put between your regular pre-amp and one of the amps. Or on the flip side do the above till you find the proper balance point then just measure the two resistances of the volume control and build that right into the input of the amp with a couple of resistors. This would change the input impedance of the amp though which could throw off any passive crossovers ahead of it.

" BTW: for the sake of a line level crossover, the input impedance on the four monoblock amps is 100K -- that's 100k on each amplifier."

Good, you need to know that if you were to work with passive line level crossovers. If you use an active line level crossover this isn't important.

"This gives me lots to think about -- all really great help! "

When going to bi-amping (and altering crossover points and/or moving to line level crossovers) there are literally dozens of ways you can try running the speakers... which of course gives you lots of ways to screw up the sound. But with work you might end up with something you love. I've only bi-amped one speaker and converted it from a passive first order speaker level crossover to an active 8th order line level crossover with a different crossover point. It took a lot of work (and a fair amount of measuring) but the end result was very good. Based on that and my recent experience with Al K's ES600s I've become a big fan of high order crossovers.

BTW, many of these ideas might turn out to stink... I am just kind of brainstorming and throwing them out there for you to consider.

Also, if you do consider active line level crossovers the units from:

http://marchandelec.com/

are very good. Be careful with some others that have nasty turn on/off transients... those are *much* more devestating when you bi/tri-amp and have no speaker level crossovers.

Shawn

[Erik Mandaville]

Shawn:

Fantastic information -- and yes, some of which might really mess things up. I've done that before lots of times!

I have never bought or built anything from Marchand, but I have known about them for many years. I almost their passive preamp until I figured I would build one for about 1/16th the cost -- except their attenuator was/is lots nicer than what I came up with!

I will ponder these possiblibities. I have built a couple of series networks in the past, and the worked well; but as you say, there was not such a gap or hole between the two drivers in question. They sound very good though, I think. And sure, I could use an inline pot, find the right balance, then just replace that with fixed resistance. That would be better.

As far as on and off transients: I learned a really neat little trick from an old Dynaco preamp schematic: Something like a .01uf/Kvolt ceramic cap across the switch. It's totally effective, and I've used this on all of the preamps I have built. In fact, I'm using an SS Dynaco PAT 4 right now that has the same thing.

Your assistance with this is appreciated, Shawn -- you've obviously got lots of experience with this sort of thing behind you! Thanks again!

[sfogg]

Erik,

"I've done that before lots of times!"

Best way to learn IMO....

" I learned a really neat little trick from an old Dynaco preamp schematic: Something like a .01uf/Kvolt ceramic cap across the switch. It's totally effective, and I've used this on all of the preamps I have built"

I'm not sure that would help for all transients. Some crossover (Audio Controls and Behringers for example) have a massive DC and above turn on/off thump. With no speaker level crossover that thump goes directly to a tweeter which can fry it very very quickly. It is so bad I just leave that box on all the time to avoid any chance of that noise. The Marchand crossovers short their outputs to ground on turn on/off so they make no noise at all. I used one of these in my bi-amping awhile back. I actually left a cap on the tweeter far below the line level crossover point just for added protection from DC for it.

Shawn