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Old 13th April 2013, 11:31 AM   #1
nbiasin is offline nbiasin  Switzerland
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Default recommendations for 8-channel amp

Hi, this is my first post here at diyaudio forums.

I am considering to build a pair of Linkwitz' LX 521 speakers using a flat-pack. Since the LX521 are active speakers, I'll be needing eight channels of amplification. Which gives me the opportunity for a nice diy Project

I have been very interested in the Pass designs posted here, but I am unsure if there is a design that would be suitable for me: I am not a very skilled DIYer and prefer simple designs. On the other hand, even though not much raw power is needed ( I do not listen much above room level, loudspeakers do not have much of a passive xover), I am a bit worried about the heat dissipation of an 8-channel amp.

Do you know a pass design suitable for my project?
Nik
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Old 13th April 2013, 11:51 AM   #2
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by nbiasin View Post
................................. I am a bit worried about the heat dissipation of an 8-channel amp.....................
You are right to be concerned with heat in any 8channel amplifier.
But 8channel ClassA power amplifier ?????
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Old 13th April 2013, 12:00 PM   #3
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I would look at the ACA (Amp Camp Amp) and the F5 amp.

Both projects are well documented, and you can order PCB's and parts from the forum or various members.

I would think 2ch ACA , 4ch F5, and fill in the bottom end with some amp other than class A. Just keep in mind the upper mid unit used in this design has very low sensitivity 84-85db while the tweeter and lower midbass are both just above 90db.

Steve
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Old 13th April 2013, 12:00 PM   #4
nbiasin is offline nbiasin  Switzerland
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AndrewT - yea, thats just what I thought.
So it looks like I should look into class D...

I was hoping I could accomplish this with one of the pass designs, but I'd probably need new power lines for that
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Old 13th April 2013, 12:09 PM   #5
nbiasin is offline nbiasin  Switzerland
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HIPCHECK - sounds interesting - thanks for your thoughts. Do I understand right that you recomment the ACA for the (low sensitivity) upper mid, the F5s for lower mid and tweeter and maybe a class D designb for the bass?

Nik
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Old 13th April 2013, 12:11 PM   #6
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how about F5 for mid and high. and class D for low pass?
and you dont need 4ch class D amp for the low pass. so only 4ch F5 and 2ch class D

Last edited by AudioSan; 13th April 2013 at 12:18 PM.
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Old 13th April 2013, 03:48 PM   #7
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Tweeter 90db Amp Camp Amp
upper Mid 85db F5 Amp
Lower Mid 90db F5 Amp
Class D on subs
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Old 13th April 2013, 06:54 PM   #8
nbiasin is offline nbiasin  Switzerland
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AudioSan - thanks, this is what I am considering right now. I am currently researching a good class-D-design.

HIPCHECK - at this point I am not entirely sure exactly how many channels I need. I have ordered the LX521 construction plans only a few days ago and they did not find their way to Switzerland yet.

I read that there is at least an option to have a passive crossover between upper and lower mids. In that case six channels would be sufficient.
In any case - what is your reasoning for suggesting the ACA for the tweeter?
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Old 13th April 2013, 11:43 PM   #9
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Look here, and many other places on the net:

BiAmp (Bi-Amplification - Not Quite Magic, But Close) - Part 1

This section:
1.6 - High Frequency Energy Content
The basic principles described above also apply to the way high frequency signals are superimposed upon the low and middle signals. The main difference is in the energy (power) of the respective frequency bands. There is normally a relatively high amount of energy in the midrange band (see Crossover Frequency Selection in Part 2) as well as in the low frequency band. However, as the frequency increases beyond the upper fundamental frequencies of most musical instruments, the amount of energy falls off. Typically this will occur from about 800Hz and up (but will vary widely depending upon the type of program material), and the energy content will be seen to drop at a rate of about 3dB per octave (and more rapidly again above about 5kHz).

Since with a 3-way system the midrange to tweeter crossover frequency will be at perhaps 2500Hz or so, we can expect that the energy content of the high frequency band will be 9dB to 12dB down compared with the low and mid ranges. If we translate that back to our original 100 Watt amplifiers, this equates to somewhere between 7 and 12 Watts (peak) - giving an average power of around 1 Watt.
Because the high frequency energy content is such a low value (about 1/10th that of the midrange band), there is not a lot to be gained by using another electronic crossover network to separate this from the midrange signals. If the goal is to obtain the absolute maximum SPL (such as for sound reinforcement) it will be well worth the effort, but for hi-fi the law of diminishing returns indicates that it is not generally worthwhile. However, for optimum clarity, there is no comparison. An electronic crossover is not affected by driver impedance, and is (comparatively) infinitely stable.

One area where it is certainly better to use the additional electronic crossover and more amps for the tweeters, is where the sensitivities of the midrange driver and tweeter are more than 2 or 3dB different. In this case, using a separate amp will allow the levels to be matched far more easily, and will eliminate the use of resistive pads in the crossover network. There is also the potential for a useful reduction in intermodulation distortion, although with good quality modern amplifiers this is normally very low.

**If there is an option to go passive then that would eliminate the need for the smaller tweeter amp. The ACA is modest power level and consumption, the F5 is current king of DIY Pass Amps, and there are many options/documentation out there. Maybe the A-40 or A-75 but I am not sure if there is a PCB to be had off the shelf?
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Last edited by HIPCHECK; 13th April 2013 at 11:47 PM.
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Old 14th April 2013, 09:01 AM   #10
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Quote:
Originally Posted by nbiasin View Post
AudioSan - thanks, this is what I am considering right now. I am currently researching a good class-D-design.

HIPCHECK - at this point I am not entirely sure exactly how many channels I need. I have ordered the LX521 construction plans only a few days ago and they did not find their way to Switzerland yet.

I read that there is at least an option to have a passive crossover between upper and lower mids. In that case six channels would be sufficient.
In any case - what is your reasoning for suggesting the ACA for the tweeter?
you need 6 ch, as long as you can drive all 4 low pass drivers with 2 ch.
2 ch for tweeters. 2 ch for low mid and high mid(passive 6db filter) and 2 ch for low pass. are you going to use linkwitz's 3way ASP filter?

Last edited by AudioSan; 14th April 2013 at 09:04 AM.
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