Active Crossovers

[Rudy81]

Ok, here is the latest iteration of the project. The EV DC-ONE is sold. jwc is sending me his Behringer DCX-2496. I just picked up a Crown K1 to use for my woofers. I am looking for a pair of Crown D-75's to power the mids and tweets.

The hook up will be Integra outputs to a Samson S-convert to raise the consumer level signal to pro audio levels. Then into the DCX and on to the Crown amps. I sure hope this works.

[Rudy81]

Although, the passive network is no longer there

Hey Rudy

After thinking about this it dawned on me that your passive crossover (I would estimate to match the Khorn LF) is attenuating the JBL 2470 midrange by approx. -12db. That means your systems noise floor will be lowered by this much also in it's audible range and thus you might not have any noise issues when the amplifiers run through the passive crossovers but when you wire the amplifiers directly to the driver(for active operation) the nosie floor of the amplifiers could be causing a real issue.

Again the best way to test for this is to use a shorting plug(to minimize any noise leakage into the amplifier through the input and thus hear only the inherant noise floor of the amplifier's design) on the input of the amplifier and then wire the JBL 2470 directly(or through your safety capacitor) to the amplifier and then turn on the amplifier and listen for the noise level under this condition. If the noise level is objectionable under this condition then the amplifier's inherient noise by design prevents them from being a good match with very high efficiency drivers in an active system. Note: All hookups should be made prior to turning the amplifier on.

mike tn

Mike: I finally got a chance to try this out. I hooked up one of my Parasound amps directly to my mid range. Nothing else on or hooked up. I did not use a short plug for starters just because I'm lazy and the fear that I might screw something up and burn up an expensive amp.

Powered up the amp and the mid horn was perfectly silent! If I stick my head in the horn, there is a very slight hiss, but it is not audible at a foot away. I'm perplexed.

However, thank you so much for all your help.

Rudy

[Rudy81]

foiled again by the dreaded double post....

[mikebse2a3]

Mike: I finally got a chance to try this out. I hooked up one of my Parasound amps directly to my mid range. Nothing else on or hooked up. I did not use a short plug for starters just because I'm lazy and the fear that I might screw something up and burn up an expensive amp.

Powered up the amp and the mid horn was perfectly silent! If I stick my head in the horn, there is a very slight hiss, but it is not audible at a foot away. I'm perplexed.

Great..! That means the amplifier should be able to work in an active setup...IMO. It clearly shows that your noise is coming from upstream. If this is one of the amps without an input level control then a properly designed input attenuator (or Samson type device) could benefit it's use in integrating it into an active setup.

mike tn

[Rudy81]

mike: I could use those, but after learning about the signal level differences between home and pro audio gear, I wanted to try and properly match each unit's expected voltage at input. In that light, I will use the Samson S-convert to upconvert the source signal. From there on, I wanted to stay with pro gear so as to minimize the gadgets or changes I make to the signal. Thus, the search for the crown amps that may fit the bill. I have found the D-75's are pretty pricey for their age and low power. That leads me to beleive they are well thought of and must be good amps. Worst case, I know I can sell them again if they don't pan out.

When I get everything in I'll post the results....with any luck I'll get past the setup of the system and be able to get into the actual crossover and tweaking of the speaker.

[Rudy81]

Ok, here we go again. I now have the Behringer DCS2496 (thanks JC), a Crown K1 for the woofers and was able to get one D75. Problem was the D75 has a problem with the protections circuit. It pops pretty loud coming on and really loud shutting down. I only played with the new stuff for a little while since I was concerned the D75 was going to blow a driver. Good thing I have been using those capacitors to protect the drivers.

The Behringer is not nearly as feature filled as the DC-ONE was and I miss the DC-ONE. With the new pro amps, I was able to tweak the gain in order to get rid of the hiss. Yes, there was still hiss just like the DC-ONE. I guess going active is going to raise my noise floor no matter what I do. I have also found that my room lighting rheostat causes a buzzing that I had never noticed with the passive network. It is evident with the active system and I finally was able to detect it with the passive network by carefully listening with my head stuck in the horn.

So, for now I'm on hold until I can get two pro amps for the mid and tweeters....ones that don't pop coming on and off. Any other suggestions besides the D-75 for those applications?

How about a way to quiet down that lighting dimmer?

This project just keeps going and going and I'm not yet at the point of tweaking the actual crossover. Good thing this is a fun learning experience.

[Coytee]

Good thing this is a fun learning experience.

Heh...what a sport!!!

[Y]

[Rudy81]

Heh...what a sport!!!

What keeps me going is that I was able to get a glimpse of the benefits, or I would never have continued this. Yesterday, I hooked up the Crown K1 to the woofers when I was trying the other Crown as well. That K1 really produces some slam on my bass bin, unlike anything I have ever heard. The bass is very Crisp and powerfull. Another potential benefit is a much bigger amp directly controlling the woofers, that apparently makes a noticeable difference. I can only infer the same will be true of the mids and tweeters. If those D75's weren't so expensive new, I'd buy two of them.

[Rudy81]

I am currently looking into articles on how to properly set the gain structure on a system. In the active crossover, I have the source, the crossover and the amps as a minimum to balance. So, learning about gain structure would be useful. DrWho suggested one article, and I have found another that is very useful for us noobs.

http://www.rane.com/note135.html

[Pete H]

This is turning into quite the adventure Rudy! LOL, You better be taking really good notes that you can send to me when you're all done.

[Coytee]

What keeps me going is that I was able to get a glimpse of the benefits, or I would never have continued this. Yesterday, I hooked up the Crown K1 to the woofers when I was trying the other Crown as well. That K1 really produces some slam on my bass bin, unlike anything I have ever heard. The bass is very Crisp and powerfull. Another potential benefit is a much bigger amp directly controlling the woofers, that apparently makes a noticeable difference. I can only infer the same will be true of the mids and tweeters. If those D75's weren't so expensive new, I'd buy two of them

SHHHHHHHH, please do not let the secret out lest we'll be having others trying to recreate this bonafide improvement in their systems as well!

[]

[cfelliot]

Rudi,

Could I ask a question on your amps?

Why so much power? The K1 seems like putting a Hemi in a VW.

I've never had more than 100W on my Cornwalls and even loud the volume control was never above 4.

I would think a D45 on the tweeters and mids would be more than sufficient on a KHorn tri-amped.

Just curious?

[Rudy81]

This is turning into quite the adventure Rudy! LOL, You better be taking really good notes that you can send to me when you're all done.

Actually, I have been taking copious notes! It has been an adventure only becaue of my great lack of fundemental knowledge on things like crossovers, amplification, gain, audio gear limitations etc. I'm sure if I had understood these things to begin with, I'd be posting on how it sounded......but, I had to take the less traveled road. Frankly, I'm glad I did and I'm grateful for all the help and guidance I have gotten from so many forumites.

[Rudy81]

Rudi,

Why so much power? The K1 seems like putting a Hemi in a VW.

I've never had more than 100W on my Cornwalls and even loud the volume control was never above 4.

I would think a D45 on the tweeters and mids would be more than sufficient on a KHorn tri-amped.

Just curious?

A month ago, I would have agreed with you and actually there are several issue at play here. First, I have not specifically set out to put a Hemi on a VW, and don't call my Khorns a VW, I started out planning on using the consumer amps I already own. None are over 125 WPC. However, as I got into the projcet I learned that it is not wise to plug Pro product outputs into consumer level inputs. (BTW, I have confirmed this potential problem with the folks at Parasound....they said don't do it). There is a very large voltage difference. You can read the details in previous posts. That led to my search for pro amps that would fit the bill. I started looking at the D-75's and actually got a hold of one, but it had some issues and sent it back to the seller, and boy are they expensive. Since I just want to get the point of a full set up for now, I just looked around for good deals on any amps that might fit the system requirements. The K1 was a good deal and has a good reputation both here and on pro forums. So, I snagged it. It has a Damping Factor of >3000! What little I have played it on my woofers, it is friggin amazing in the way it controls the woofers. Very tight and precise bass....better than with a passive network and a 125 watt amp pushing all three drivers.

A more inportant issue is that I am going to have to reduce the amp input sensitivity in order to lower the volume level on the noise floor. This was discussed in depth by DrWho and others. The effect of chaning the gain structure is that I may end up limiting the max output of any amplifier in order to have a nice quiet system. When you do that, if you have a modest power amp, you might limit your max SPL potential even with the Khorns' efficiency. So, I really don't think the K1 is overkill. Besides, I'd rather have a Hemi on a VW, than two horses pulling a Ferrari!

I have enroute to my home a Crown CP660 multi channel amp that should work for my mid and tweeter setup. It is probably not the cleanest amp, but should work for my pursposes. We'll see how it sounds. I spent over a month trying to get at least a pair of D-75s in good condition and was not successful unless I wanted to pay over $200 each....sometimes well over $300 to get them in good shape. My secret desire was to get 4 D-75's and run them as monoblocks. The one I did get was reasonably priced, and did work once fired up. Problem was during shutdown it produced a loud pop in both channels. Not good for the drivers.

So, there are plenty of good reasons to get a large amp and none that I know of to get a smaller one. They certainly aren't that much cheaper in what I'm looking at. I also suspect that you are more likely to blow a driver with an underpowerd amp that clips...at least that is what I have read.

I hope by early next week to have the whole thing up and running if the cp660 actually works. It is supposed to be new, never used. Plan is to get the gain levels set correctly on the system and the start playing with the crossovers and driver phase......finally.

[cfelliot]

...don't call my Khorns a VW...I am going to have to reduce the amp input sensitivity in order to lower the volume level on the noise floor....

I hope you know that the VW reference was in regard to quantity not quality! [] I had a friend with KHorns and a Phase Linear 700 years ago. Even when playing loud the front meters barely moved!

How is reducing the input sensitivity any different from using a lower output amp to begin with? Your problem with the noise floor was due to the higher power amp and the high sensitivity of the KHorns?

I am following your adventures with the end result being the tri-amping of my Cornwalls.

Party on and post often!

[DrWho]

How is reducing the input sensitivity any
different from using a lower output amp to begin with? Your problem
with the noise floor was due to the higher power amp and the high
sensitivity of the KHorns?

Just because an amp is higher
power doesn't necessarily mean it is more sensitive to the input
signal. I'm wording it that way because input sensitivity works
backwards from how you're referring to it. A higher sensitivity means
you need more voltage to drive the amp to its full output, which means
the gain of the amplifier is lower and therefore less sensitive to the
input signal. I'm probably just making it more confusing, but maybe
someone else can explain it clearer.

What matters is the gain
of the amplifier, which is usually around 30dB for most consumer amps.
Some of the lower power amplifiers do have lower gain, and those
amplifiers are very well suited for the application.

It should
also be noted that the volume knob on the amplifier allows one to
adjust the input sensitivity. Lowering the volume on the amp requires a
hotter signal from the preamp, which means the noise from the preamp is amplified less.

[cfelliot]

My mistake in terminology. I was making the assumption that each amplifier achieved rated output with the same input signal.

In that case, I see no difference between using a 25W amp or a 50W that is attenuated 3db on the amp except for headroom.

This is the layout of how I plan to do a tri-amp setup with my Cornwalls.

One of the things I want to avoid is the replication of the DAC in the CD which converts the digital signal to analog to only have it converted back to digital by the DSP crossover and then again to analog. Therefore, the CD is connected to the DSP via the SPDIF input and all processing is digital at a 0db input reference. The conversion to analog is done once at the end of the DSP.

Since I can’t find a quality DSP that has a master volume control that works on all channels concurrently, the digital volume control is something I may need to design and build. I’ve been looking at digitally controlled potentiometers that have 1db steps and I think it can be done for less than $200.

With the passive crossovers the Cornwalls go as loud as I want with my 60W Bryston 2B. Since the K77 is down 3db and the K55 is down 6db from the K33 in the passive crossover, I’m looking at amps around 30W for the mids and highs. I may try tube for these.

[DrWho]

Since I can’t find a quality DSP that has a
master volume control that works on all channels concurrently, the
digital volume control is something I may need to design and build.
I’ve been looking at digitally controlled potentiometers that have 1db
steps and I think it can be done for less than $200.

I
started just such a design a couple weeks ago and was planning to share
the schematics after I verified the performance. The problem is I can't
ever make up my mind about whether to go with digital pots or VCA's.
The VCA allows for a pure analog design, whereas the digital pots you
gotta implement a microcontroller and deal with all the schnazz.
Although, once you've got a micro running, then adding remote control
functionality and independant gain adjustment becomes a piece of cake
(individual driver gain adjustment would be better done in the analog
domain after the DSP rather than inside the DSP itself).

I
was also planning to throw in switches for converting between +4dBU and
-10dBV since most DSP's run at pro levels and most amplifiers
audiophiles buy run at consumer levels.

I'd be curious what other volume adjustment approaches you've
considered. It's actually a rather difficult problem since ideally you
need better than 0.1dB accuracy. I'm not sure you can get that in a digital pot over the entire attenuation range...

[Rudy81]

How is reducing the input sensitivity any different from using a lower output amp to begin with? Your problem with the noise floor was due to the higher power amp and the high sensitivity of the KHorns?

I am following your adventures with the end result being the tri-amping of my Cornwalls.

Party on and post often!

No, the noise floor problem did not stem from using more power. First of all, my first iteration was with the same amp I use every day in passive mode, just connected directly to the drivers. If I just connect the amp to the driver and nothing else, the system is totally quiet. I also connected the K1 directly to the drivers and it is dead quiet as well. The amps are not the problem, but in fact, are part of the solution in the gain equation.

DrWho explained how one can use the amp gain to move the noise floor around on page 21 of this thread, and I quote:

The input sensitivity with the volume knob on the D75 cranked up all the way is 0dBU and your speakers with the full power of the D75 would be about 126dB SPL for the squawker. With the 106dB dynamic range of the D75, that would put the amplifier noise floor at 20dB SPL (so inaudible in even the quietest of rooms).

The DC One has 111dB of dynamic range relative to +26dBU, which puts its noise floor at -85dBU. Amplified by the 27dB of gain inside the D75 (with the volume knobs cranked) would put the system noise floor at -58dBU, or 41dB SPL...which will be audible standing near the speakers. Turning down the volume knobs of your D75 amplifers to the -30dB detent will lower the DC ONE noise floor to -28dBU, or 11dB SPL which is now masked by the 20dB SPL noise floor of the amplifier. The maximum volume would reduce from 126dB SPL to 122dB SPL (still able to drive the speakers to their max).

One thing I didn't notice earlier is that the DC One wants +26dBU on its inputs to maximize dynamic range. You would need 30dB of gain between your consumer preamp and the DC One in order to hit that spec. In other words, if you set the gain on your D75 to the -30dB indent, then driving the DC One directly with a consumer preamp will result in 92dB maximum SPL. So with your preamp turned up all the way, the loudest peak will be 92dB...which with 20dB crest factor music is only going to be a 72dB listening level (as measured by a radio shack SPL meter). If you raised the amplifier gain on the D75 from the -30dB detent to the -10dB detent, then your system noise floor will be 33dB SPL which will likely be inaudible from the listening position, but would increase your maximum volume to 92dB SPL (112dB SPL peak).

As you can see, changing the gain on your amplifiers allows you to push the noise floor around a bit, but it also affects the max SPL too. You'll be capped somewhere between 20dB SPL and 41dB SPL. Btw, all this is assuming that your speakers have a 110dB sensitivity...which I think the squawker might be close to. The Khorn LF is about 102dB depending on how you look at it (after EQ) and the tweeter is about 104dB. So the noise from the Khorn LF section will be somewhere between 12dB and 33dB SPL, and the tweeter will be somewhere between 16dB and 37dB SPL.

The noise level gets raised by the fact that I am using balanced connections, which IIRC raise the output by +6dB. Then, there is the issue of the varying noise floors of the various items between the signal generator and the amp. I had run into this issue years ago when I connected my present system using the balanced connections on my Parasound HALO pre/pro and amps. The same hiss showed up at about the same level. After much troubleshooting and finally calling Parsound, I found out that using the balanced sections raises the signal level as well as the noise floor by 6dB, which on super high efficency systems allows you to hear the noise floor from across the room. It has nothing to do with a good amp.

So, after following Dr. Who's lead, I have been studying up on gain structure in such as system, and it is very important to set it up correctly so that you end up with a quiet system that has maximum dynamic range and SPL potential to meet your speakers' capabilities. A couple of articles on the subject have been quoted in this thread. Bottom line is that having is Hemi is much less of a problem than having a small amp that barely has enough head room.

One item that these articles address is where to make attenuation or boost changes in order to avoid constricting system performance. There is a correct way and a wrong way to correct for gain issues. I will be specific with my setup when I get all the calculations done and the system set up. I will list my thoughts here so that DrWho can correct any errors in my understanding.

[cfelliot]

Since I can’t find a quality DSP that has a master volume control that works on all channels concurrently, the digital volume control is something I may need to design and build. I’ve been looking at digitally controlled potentiometers that have 1db steps and I think it can be done for less than $200.

I started just such a design a couple weeks ago and was planning to share the schematics after I verified the performance. The problem is I can't ever make up my mind about whether to go with digital pots or VCA's. The VCA allows for a pure analog design, whereas the digital pots you gotta implement a microcontroller and deal with all the schnazz. Although, once you've got a micro running, then adding remote control functionality and independant gain adjustment becomes a piece of cake (individual driver gain adjustment would be better done in the analog domain after the DSP rather than inside the DSP itself).

I was also planning to throw in switches for converting between +4dBU and -10dBV since most DSP's run at pro levels and most amplifiers audiophiles buy run at consumer levels.

I'd be curious what other volume adjustment approaches you've considered. It's actually a rather difficult problem since ideally you need better than 0.1dB accuracy. I'm not sure you can get that in a digital pot over the entire attenuation range...

At first I thought of using a ganged stepped analog volume control using discrete resistors, but the more I thought about it the more I realized that it would be a monster to build.

I think you may have hit on something using VCAs. See: http://home.comcast.net/~dreite/preamp/volume_control.htm