Dummy load to reduce output of 300w/ch amp?

steve71

Member
2007-11-12 6:01 pm
I have a feeling this is a dumb (no pun intended) question, but here goes.

I'm using a Crown XLS-402D power amp (300w/ch into 8ohms) to power two Altec 288 drivers (105 db/w/m sensitivity and 20watts of power handling). The Altecs are actively crossed over at about 400hz.

The trouble is (as you may have guessed) that I'm getting a lot of hiss from this setup. Long term I'll probably get a more suitable amp, but in the mean time I'm wondering if I can send 2/3rd's of the power into dummy load in parallel with the Altecs.

Will this reduce hiss without compromising the sound quality at all?

BTW this setup is in a small room, home environment, so I doubt that the Altec's are seeing more than 10watts at most, so it's not like the dummy load is going to be seeing that much wattage either.
 
I'm not familiar with that model of Crown and and I didn't look it up before posting. Does the amplifier have a gain control? If it does then turn it down. If it doesn't then put a pot between the output if the preamp and the power amp to lower the level being amplified.

The 400HZ crossover point is probably a little low and you may want to consider crossing at 500hz. This is just a guess not knowing what horn you are using. Most commercial/professional drivers are meant to cross at 500hz or above. The diaphrams are prone to much damage when crossing below that.

The "hiss" is part of using a very sensitive driver/horn combination. You should be able to adjust the output of the Crown down to a point where you have a good balance between low and high frequency.

No, I don't see where a dummy load would provide you anything. Just use a pot like I mentioned and all will be fine.
 
I just went to the Crown site and looked up your amplifier and it has gain controls on the front of it so you can reduce the signal to the Crown amplifier and this will reduce the output to the horns. I doubt that you will get rid of the hiss entirely.

Do you have a pink noise generator and analyzer to use? This would be the proper way to get a good balance between the highs and lows. Also, 31 band eq's would be nice in obtaining a good flat setup. This is where a Shure DFR22 would come in handy. Don't forget the time alignment between drivers also.
 
The term for what you want is an L pad. This is made up of one resistor across the terminals of the tweeter and one resistor in series with the positive input to the tweeter.

Since it sounds like you want to knock the sensitivity back quite alot I would start with a resistor of the same nominal value (or close to it) of the tweeter. Say 8 ohms if it is an 8 ohm version (you will probably only be able to find 6.8R or 8.2R) either will be fine.

Use one 6.8R resistor accros the terminals and one in series with the positive input. this will give aproximately 1/4 the voltage accross the tweeter itself.

This will only work if the problem is hiss from the amplifer not hiss from the preamplifer. To check this simply disconnect the preamplifer or turn its volume to its lowest setting if it is intergrated. If you still have the hiss then you can use this method as you system is too sensitive for the use you are using it for. Given the type of tweeter and amplifer you have this is quite likely. So this will probably work.

The resistors should be high power types. For domestic use 7W should be fine, you could go upto high wattage parts if you plan to use it loud for long periods of time.

Regards,
Andrew
 

steve71

Member
2007-11-12 6:01 pm
Hey guys thanks for the speedy responses. Burnedfinger was right, all I needed to do was reduce the gain on the input of the crown. Most of the hiss is gone now, but I still have a slight buzz from a round loop problem. Lifting the ground on the crown didn't fix that either...

For some reason I was thinking gain was gain and it didn't matter if I used the preamp to achieve the gain, or the gain controls on the Crown. I guess I should think of the pot on the crown as input sensitivity? At any rate it stands to reason that a high gain early in the signal chain (without clipping) will yield a better signal to noise ratio.

BTW I'm using a Behringer CX2310 crossover and a second Crown for the < 400 hz signal. I'm just judging the balance by ear at the moment.

Eventually I plan on using a three way digital X-over, but for now this is just so I can evaluate the horns a bit before committing $$ to build them up into proper speakers.

I didn't know that the 288 drivers could handle 40watts, that's nice to know! Serious overkill for a bedroom HT system lol.
 
Take my advice and raise the crossover point to 500HZ and the diaphrams will last a lot longer. I install a turn signal bulb in series with the horn. It will act as a limiter and keep the voice coil from getting hot and going to hell in a handbasket. The driver was designed to perform from 500hz to about 16,000hz.
Altecs engineering notes that I have somewhere mention to cross at 500HZ on a 24db per octave.

For your setup to sound the best you will need to time align the drivers and do some serious EQ work. For this you need a Teff and a analyzer.

Serious overkill? Naa, nothing exceeds like excess.

Hey guys thanks for the speedy responses. Burnedfinger was right, all I needed to do was reduce the gain on the input of the crown. Most of the hiss is gone now, but I still have a slight buzz from a round loop problem. Lifting the ground on the crown didn't fix that either...

Are you balanced or unbalanced going to the Crown? Other amp?

Better yet draw out the signal chain and I'm sure the problem can be found.
 

steve71

Member
2007-11-12 6:01 pm
burnedfingers said:
Take my advice and raise the crossover point to 500HZ and the diaphrams will last a lot longer. I install a turn signal bulb in series with the horn. It will act as a limiter and keep the voice coil from getting hot and going to hell in a handbasket. The driver was designed to perform from 500hz to about 16,000hz.
Altecs engineering notes that I have somewhere mention to cross at 500HZ on a 24db per octave.


Never heard of using a bulb in series... is that necessary for home use... guess it couldn't hurt and better to have it and not need it that need it and not have it!

The Berhringer X-over is kind of vague as far as the exact x-over point. There's three clicks between 345hz and 700hz, so if I do the math it put's me at about 460hz. I'll try and up the x-over point another click just to be safe. I really need to get some measuring equipment.

burnedfingers said:
For your setup to sound the best you will need to time align the drivers and do some serious EQ work. For this you need a Teff and a analyzer.

Yeah if I decide to go with the horns, I'll have to sell my other speakers to fund the DIY project including power tools and measuring equipment etc.

burnedfingers said:
Are you balanced or unbalanced going to the Crown? Other amp?

Better yet draw out the signal chain and I'm sure the problem can be found.

I'm running RCA out of the AVP and XLR into the XO. From the XO into the crown's it's XLR for both the HI and LO signals.

Once again, thanks for the help!!:cheers:
 
Yeah if I decide to go with the horns, I'll have to sell my other speakers to fund the DIY project including power tools and measuring equipment etc.

Sometimes one is able to rent or borrow the testing equipment from a local business. I can't count the times I have loaded up some equipment to help out someone wanting to get that special project going. Its too bad we don't live closer because I could really make it sing for you.:D

Never heard of using a bulb in series... is that necessary for home use... guess it couldn't hurt and better to have it and not need it that need it and not have it!

Not necessary for home use unless you get too drunk and crank the snot out of it.:dead:

I use it in large systems in conjunction with a limiter as extra protection. If you push it really hard the light comes on and this limits the output to the driver thus the driver won't burn up.

I had a church that used to burn up 5-6 drivers a year and I finally got tired of the all day hassle of setting up scaffolding and removing the drivers, cleaning the gaps, and reassembling them with new drivers. I talked them into letting me put the lights in series with the drivers and I mounted a panel that held the lights in a mannor that the person running the system could see them light up. I haven't been back since to change out any drivers so I would say it works quite well.

Stormrider is correct and you should take a close look at the way you have it set up.
 
steve71 said:
I'm using a Crown XLS-402D power amp (300w/ch into 8ohms) to power two Altec 288 drivers (105 db/w/m sensitivity and 20watts of power handling).
that is capable of maximum SPL of 125db @ 8feet. but peak is 3db higher @ 128db.

Let's subtract 20db from those peaks for average listening level leaving your ears bleeding from 105db of SPL.
Isn't that a bit loud for bedroom use?

With 105db/W/m drivers you are looking for signal with a maximum of around 10W to 20W and S/N ratio better than -100db rel. maximum.
 

steve71

Member
2007-11-12 6:01 pm
Re: Re: Dummy load to reduce output of 300w/ch amp?

Stormrider said:
For the rca to xlr conversion did you follow the directions in the CX2310 manual? About shorting pins one and three on the xlr? Or are you using an active/transformer based unbalanced to balanced converter?

I'm just using an RCA to XLR adapter. I checked with a multi meter and pins 1 & 3 are shorted. The hum is pretty quite and I can only hear it when there is nothing else in the room making noise (PC, HVAC or Projector). For the moment I'll just deal with it I think rather than spend any more $ on a converter.

burnedfingers said:


Sometimes one is able to rent or borrow the testing equipment from a local business.

Not necessary for home use unless you get too drunk and crank the snot out of it.:dead:


Thanks for the tip on renting the measuring equipment.

While there is every chance that I might get drunk, I tend to avoid testing the limits of expensive stuff while intoxicated. :att'n:

AndrewT said:
that is capable of maximum SPL of 125db @ 8feet. but peak is 3db higher @ 128db.

Let's subtract 20db from those peaks for average listening level leaving your ears bleeding from 105db of SPL.
Isn't that a bit loud for bedroom use?


Overkill?? Blasphemy!!

Well before you all think I'm crazy, you should know that I got these horns for next to nothing on Craig's List on a spur of the moment decision. For now I'm just trying to get "something" up and working with what I already own or what I can buy as cheaply as possible.

I'm liking what I'm hearing so I'll most likely sell my other speakers so I have some cash to throw at the project when I get the time.

I have an unfinished basement that will house the home theater eventually but for now a spare bedroom is better than nothing


burnedfingers said:


AndrewT lighten up buddy:clown:

Its almost perfect for bedroom level as it covers up the screams of passion.:D


Let me tell ya BF, there's not too much passion from the wife since I dragged home these horns. She says they look like a PA system from a prison. :D

Here's a pic of how it's setup. Yes the horns don't match... one is a MR64 and the other is a MR94. I think the WAF will dictate selling off the MR horns and getting something a little more domesticated.

The SUB's are VMPS larger subs and are doing duties below 460hz.
 

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steve71

Member
2007-11-12 6:01 pm
burnedfingers said:
Tell her to ease off or I will fix you up with some stadium horns:D

What do you have for inputs? What are you using for a preamp?

The MR series horns are almost big enough:D

Stadium horns eh? Will they block the screen? :D

For the HT system I'm using the optical out on a Bravo D1 DVD player as a source. The Preamp is an old Denon AVP-8000.

I have a 2ch system downstairs that has a nice CDP and a hi-end turntable & phono stage. If I sell off the speakers from this system to fund the horn project I'll consolidate it all into one system.

So what does someone who works in PRO audio listen to at home? Stadium horns?
 
So what does someone who works in PRO audio listen to at home? Stadium horns?

Yes, but only in the garage. I have a wife too.:D

Had to give them up....couldn't get her SUV into the garage.

I was almost loud enough:clown:

Your going out unbalanced from the Denon to your Behringer CX2310 crossover. XLR connector or terminal strip tie pin 1 to 3 hot on two.

Do you have noise/hum when the input of the Behringer is disconnected?
 
"I'm just using an RCA to XLR adapter. I checked with a multi meter and pins 1 & 3 are shorted. The hum is pretty quite and I can only hear it when there is nothing else in the room making noise (PC, HVAC or Projector). For the moment I'll just deal with it I think rather than spend any more $ on a converter. "

I use the CX2310 in my system to integrate my stereo subs and the TM's. Both the subs and the two-ways are around 85db/1w. I use rca to xlr cables that i made, with pins 1 and 3 shorted as the manual says and i get no hum at all. I also use unbalance to balanced cables on the inputs to the amps, as they are unbalanced. Some noise is audible 2" away from the tweeter, but i think thats from the Hafler amp.

Your setup should be dead quiet or close the way you have it as the balanced cables are supposed to reject noise.
 

steve71

Member
2007-11-12 6:01 pm
burnedfingers said:


Yes, but only in the garage. I have a wife too.:D

Had to give them up....couldn't get her SUV into the garage.

I was almost loud enough:clown:

Do you have noise/hum when the input of the Behringer is disconnected?

You should have just bought her a Miata instead. :clown:

Yes I still have the buzz with the inputs disconnected from the XO.

I guess the noise has to be coming from Crown because I didn't get any noise from the last (lower powered integrated amp) I was using.

I use the CX2310 in my system to integrate my stereo subs and the TM's. Both the subs and the two-ways are around 85db/1w. I use rca to xlr cables that i made, with pins 1 and 3 shorted as the manual says and i get no hum at all. I also use unbalance to balanced cables on the inputs to the amps, as they are unbalanced. Some noise is audible 2" away from the tweeter, but i think thats from the Hafler amp.

Your setup should be dead quiet or close the way you have it as the balanced cables are supposed to reject noise. [/B]

Yeah I think it's just a combo of the 105db/w/m compression drivers and the Crown amps. The buzz would be inaudible on most normal speakers. The main thing is you guys helped me to get rid of the hiss, which was much louder than the buzz.
 

unclejed613

Member
2006-12-28 12:19 am
there is a device called a "power soak", which guitarists use to get their amp past clipping, but at a controllable level. it's usually used with tube amps. it's a dummy load with an adjustable tap for the speaker. it's great if you like lots of crunch without waking the neighbors, but for what you're looking for, the gain control is best.