setting gains in a bi-amplified system

[KyleScha 10+ year member]

Hey everyone.

I have some boston Pro60's on the way and am planning to bi-amp them. I'm going to use a bridged JL300/4 (~150x2, but ive heard they really do about 190x2) for the woofers and then use my Alpine MRP-T220 (which should be 60w x 2) for the tweeters.

How should I go about setting the gains on the tweeter amplifier so it matches with the woofer? Should I just set it to 1000hz test tone and set it to the same voltage as the JL? (of course the tweets will be disconnected)

Thanks,

Kyle

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[FoxPro5 5,000+ posts]

I don't klnow the answer to you question for sure, but I'm subscribing to your thread to find out the correct answer for myself. My instinct tells me to match the votages.

Also not sure of the tweet's impedance, but I'll guess it's 6 ohms. If you do the math:

Tweets: 60x6 = sqrt 360 = 18.9v

Mids: 150x3 = sqrt 450 = 21.2v

They come out to be pretty close if I guessed right on the impedance.

 

[DBfan187 5,000+ posts]

you should use a tone that in the middle of your tweets playing frequency.

 

[helotaxi 5,000+ posts]

Actually you should use an RTA and use that to match the levels and crossover point so they match in output. You can't just guess and hope.

As an aside, the Alpine amp isn't going to put out 60W @ 6 ohms so the math above is off assuming a 6 ohm tweet. You would be looking for 15.5V, same as if you were using a 4 ohm driver and the power output would be right at 40W. The only reason that the JL amps have different voltages for different loads is they produce the same power at any load into which they are rated. The Alpine doesn't do this so the voltage is theoretically the same regardless of the load. You really wouldn't know for sure what you could get out of it without an o-scope.

Me personally, I would save myself the hassle and use the supplied crossover, which will do all the matching for you and just run the whole set off the JL amp bridged.

 

[KyleScha 10+ year member]

I didnt know tweeters were 6 ohm, i figured they were 3ohm. So yeah that brings my amp down to about 40w (its 50x2 @ 4ohm).

I feel cheated though with the JL Amp. I originally bought it to power 4 speakers (at 4 ohms each) and wasnt planning on running 3ohm speakers bridged. So my amp is capable of 150w at an 8 ohm load, if it was a normal amp it would be well over 200w at a 3 ohm load. Oh well at least the SQ is good. And the JL's have a pretty good resale value. I'm just worried about not having enough power for the Pro60's.

But I'll just approximate the voltage and adjust it slightly for what sounds best.

btw I still will use the crossovers that come with it

 

[squeak9798 5,000+ posts]

I would use the DMM method only to determine your probable point of clipping.

Then simply level match with an RTA or by ear, and just be sure you are not exceeding the point of clipping for the amplifier.

But to be totally honest, I'd just run that JL bridged to each side and not bi-amp.

 

[KyleScha 10+ year member]

Alright I'll do that first. I guess if I find I really need more power I'll try the bi-amping.

anyway thanks for your replies.

Kyle

 

[helotaxi 5,000+ posts]

You won't get more power by bi-amping. You will actually get less to the tweets and the same power to the mids.

 

[squeak9798 5,000+ posts]

You won't get more power by bi-amping. You will actually get less to the tweets and the same power to the mids.

Yup.

Bi-amping is usually only really helpful for increased processing abilities...such as being able to time delay the mid and tweeter separately, etc.

 

[KyleScha 10+ year member]

Strange, I'd think that the tweeter would use up power that could otherwise be used by the mids.

 

[helotaxi 5,000+ posts]

The amp provides power across its entire frequency response range (adjusted by any active filtering). The passive crossover divides the power between the drivers attached it. Power above the crossover point goes to the high freq driver and power below to the low freq driver. Except in the Besel filter's resistor there is almost no loss in a passive network.

 

[KyleScha 10+ year member]

Thanks.

I'm really hoping it will be enough power for them. I did buy the amplifier (well it was a christmas present...albiet an extremely overpaid one) at Tweeter. I dont know what the return policy is, but hopefully if it really isnt enough I can return it.

SQ wise ive been very happy with the JL, would switching to something such as an orion (which could give me 300x2 @ 3ohm) be taking a dive as far as SQ is concerned?

Thanks for all your replies.

 

[squeak9798 5,000+ posts]

SQ wise ive been very happy with the JL, would switching to something such as an orion (which could give me 300x2 @ 3ohm) be taking a dive as far as SQ is concerned?

Only if your current amplifier won't drive the speakers to an output level that is satisfactory for you.

The main thing a larger than needed amplifier does for you, other than allow for higher output levels, is allow you to have some "headroom"....which basically means the amplifier has enough excess power that on highly dynamic transients at loud volumes you won't clip the amplifier (whereas you might with the smaller amplifier at a high volume).

 

[alphakenny1 10+ year member]

you should use a tone that in the middle of your tweets playing frequency.

sorry for the hijack but is this is what recommended when setting gains for the tweets? i have my system bi-amped too and i set the gains using the tutorial for both mids and tweets using a 1khz test tone. so if you are correct should i use something like 10-12khz for my tweets?

 

[helotaxi 5,000+ posts]

I have more than enough power and headroom with 100W to make my ears bleed. 150 should be more than enough unless you have hearing problems...seriously. IIRC Bostons tend to be fairly efficient anyway. You should be more than good to go.