Dummy load to reduce output of 300w/ch amp? - diyAudio
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Old 28th December 2007, 09:43 PM   #1
steve71 is offline steve71  Australia
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Default Dummy load to reduce output of 300w/ch amp?

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.
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Old 28th December 2007, 10:04 PM   #2
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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.
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Old 28th December 2007, 10:34 PM   #3
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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.
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Old 28th December 2007, 10:35 PM   #4
gfiandy is offline gfiandy  United Kingdom
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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
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Old 28th December 2007, 10:46 PM   #5
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I guess I may have jumped the gun and assumed there was an amplifier for the high frequency and one for the lows. I'm too used to the bigger systems.

Lets just biamp the damn thing and be done with it.

The 288's are usually good for 40 watts with Altec drivers in them.
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Old 29th December 2007, 12:02 AM   #6
steve71 is offline steve71  Australia
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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.
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Old 29th December 2007, 12:22 AM   #7
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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.

Quote:
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.
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Old 29th December 2007, 03:48 AM   #8
steve71 is offline steve71  Australia
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Quote:
Originally posted by burnedfingers
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.

Quote:
Originally posted by burnedfingers
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.

Quote:
Originally posted by burnedfingers
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!!
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Old 29th December 2007, 04:41 AM   #9
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The CX2310 is a 24db/octave crossover, for both the high and low outputs.

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?
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Old 29th December 2007, 10:43 AM   #10
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Quote:
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.

Quote:
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.

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.
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