Advice sought on building multi-channel amplification

[jwillans]

Hello,

I am in the process of building a digital church organ (for my home). I'm beginning to think about speakers and amplification which will be probably 8 independent channels in total. I've almost settled on some speaker designs which I intend to build around Common Sense Super 8 drivers along with a subwoofer to handle the lower frequency. I'm now turning my attention to amplification. I know very little about this area, but I am willing to learn.

Basically as an opening question, which chip and associated design should I be looking at if I am wanting 30W of power per channel and excellent quality sound reproduction? I have started looking the LM4765 which seems good, I would need 4 of these to get the appropriate channels. Would these be appropriate for driving subs? I'm particularly interested to hear about chips with the availability of printed PCB/Kits?

Many thanks,

James

[unclejed613]

30W/ch deems on the lightweight side if you're trying to duplicate a pipe organ, but most older Hammonds (such as the B-3) usually had a 50W amp in them, which provided more than enough volume for small-to-medium size churches.

the devices you mention should work well, and take up a lot less space than the old Hammond amp chassis did. with 4 channels and a bit of trickery in the preamp, you can even duplicate the effect of a real Leslie cabinet (4 flangers with synchronized control voltages 90 degrees apart from each other?). just an idea.....

as long as this is in your home, 30W/ch should be more tham sufficient.

i've heard the results of trying to duplicate the sound of a BIG pipe organ in a large hall using amplification, and the results were not anywhere the real thing, mainly because a pipe organ moves a LOT of air. i helped repair one when i was young and remember that the blower fan was about the same size as the blower on a commercial air conditioning unit, plus the fact that some of the pipes on this beast were 32 feet tall and a foot in diameter.

[pacificblue]

The trouble with the LM4765 will be that it is next to unheatsinkable, if you go for its maximum output power. The subwoofer will be better off with its own amplifier. Maybe one or several LM3886, if you go for chipamps. A beefy solid state amplifier would be even better. Church organs can go down to 16 Hz and there is a lot of SPL involved to make tones at that frequency heard. Link

Here is some good reading about power requirements for a subwoofer in relation to frequency and SPL. Will your speakers be up to that task?

For the other channels the 20 W per channel you can reasonably expect from the LM4765 will probably be enough. By the way, there is a Chipamp Forum here.

[AndrewT]

I was in a church that had replaced their, too expensive to repair, organ with an electronic keyboard.
They ended up with 8 off 15inch drivers in one enormous open baffle to get closer to what they were used to. I have no idea how much power or how many channels were dedicated to the speaker system.

[jwillans]

I accidentally posted in this forum - I did email the admin asking for this post to be moved but they perhaps not seen my message.

Originally posted by unclejed613
30W/ch deems on the lightweight side if you're trying to duplicate a pipe organ, but most older Hammonds (such as the B-3) usually had a 50W amp in them, which provided more than enough volume for small-to-medium size churches.

the devices you mention should work well, and take up a lot less space than the old Hammond amp chassis did. with 4 channels and a bit of trickery in the preamp, you can even duplicate the effect of a real Leslie cabinet (4 flangers with synchronized control voltages 90 degrees apart from each other?). just an idea.....

as long as this is in your home, 30W/ch should be more tham sufficient.

i've heard the results of trying to duplicate the sound of a BIG pipe organ in a large hall using amplification, and the results were not anywhere the real thing, mainly because a pipe organ moves a LOT of air. i helped repair one when i was young and remember that the blower fan was about the same size as the blower on a commercial air conditioning unit, plus the fact that some of the pipes on this beast were 32 feet tall and a foot in diameter.

Thanks unclejed613. Don't forget this is a home organ and will have 8 channels, that been said I am erring towards slightly larger drivers with a 50W (8Ohm) rating. Any thoughts on appropriate chips?

Originally posted by pacificblue
The trouble with the LM4765 will be that it is next to unheatsinkable, if you go for its maximum output power. The subwoofer will be better off with its own amplifier. Maybe one or several LM3886, if you go for chipamps. A beefy solid state amplifier would be even better. Church organs can go down to 16 Hz and there is a lot of SPL involved to make tones at that frequency heard. Link

Here is some good reading about power requirements for a subwoofer in relation to frequency and SPL. Will your speakers be up to that task?

For the other channels the 20 W per channel you can reasonably expect from the LM4765 will probably be enough. By the way, there is a Chipamp Forum here.

Thanks pacificblue. What do you mean by 'unheatsinkable'? What is SPL and what is its implication for producing low frequencies? Are you suggesting that chip amps might not be appropriate for this?

Many thanks all,

James

[pacificblue]

Originally posted by jwillans
What do you mean by 'unheatsinkable'?

That means the IC produces more heat than you can get away with any heatsink you attach. The amount of heat that can be transfered from the IC to the heatsink is limited by the junction surface area. You can stretch that limit with additional fans, if you don't mind the additional noise.

Originally posted by jwillans
What is SPL

Sound pressure level or in other words loudness

Originally posted by jwillans
and what is its implication for producing low frequencies?

If you look at the link in my former post, you will see the equal loudness curves. They indicate how sensitive the human ear is at certain frequencies. The vertical axis shows the absolute loudness in dB, and the red curves show, what humans perceive as equal loudness. E. g. conversational level is between 50 and 60 dB(A). That is not very loud. Look at the curve with the 60. At 1000 Hz it takes ~60 dB for us to perceive that, while a tone a 20 Hz needs to be nearly 110 dB loud for us to perceive it as equally loud. Musicians and their instruments make up for that. They play louder in absolute terms at those frequencies, so we perceive all notes as equally loud.

You can extend those curves to imagine the SPL that a church organ produces at 16 Hz, so that we perceive it as loud. And church organs can reproduce that note really loud, you will know that from experience.

Originally posted by jwillans
Are you suggesting that chip amps might not be appropriate for this?

That might be the case, because you need displacement volume and power to reach the necessary levels.

Displacement volume means woofers with a lot of surface area and long-stroke. In the other link I pointed you to the Linkwitz site. He uses a 12" woofer with 12,5 mm stroke. At 20 Hz he achieves 104 dB with it, which corresponds to ~40 on equal loudness, more or less a loud whisper.

He also shows the power requirements. Above the resonance frequency the power requirements grow extremely. Even a solid state amp will be hard pushed to deliver that amount of power. A chipamp is usually limited to something around 50-60 W. And before you start bridging and/or paralleling chipamps to increase the output, you should consider to either build a solid state amp for the subwoofer or to buy a plate amp for that task.

[jwillans]

pacificBlue - A fantastic explanation - many thanks for taking the time. I will look at the links you have posted in detail.

At this stage I am going to focus on thinking about amplifiers for the higher frequencies (non sub woofers) through the Super 10 speakers which can handle up to 50W, although it is unlikely that they will be driven to the max given the space they will operate in. Would the LM4780 be appropriate for this load? Given what you've said about heat sinks, I do not want to install additional fans?

James

[unclejed613]

also, look at pro-audio speaker designs. they achieve between 95 to 110 db@1watt@1meter sensitivities, which is quite a bit more efficient than standard home audio speaker designs. i regularly repair 200W home audio powered subs, and i know they're running 200W. because i can measure it, but i've heard pro bass cabinets that are louder while running 10 watts through them. i think the majority of small sub cabinets, whether ported or using passive subs just don't move enough air. a lot of these are using 8" or 10" drivers, and they just don't move enough air volume to efficiently couple the long wavelengths to the surrounding environment. i've seen cabinets, that while they are larger and use 15" and 18" drivers are so efficient that the cones barely move, but have a tendency to blow the stuffing out of the port, and they're almost deafening at 10 to 50 watts. in a home environment you might not have enough space for one of these larger cabinets, but there are efficient compromises you can make, such as a bandpass cabinet or a dipole cabinet. there is a program available for free that can give you the dimensions for a cabinet if you have a) the required rolloff frequency, and b) have some idea of what drivers you intend on using. i don't remember exactly what it is called..... speakerlab, i think.... i've got it installed on another machine.... it will even tell you if you need to change port sizes if the air velocity goes above mach 1.

[pacificblue]

Originally posted by jwillans
At this stage I am going to focus on thinking about amplifiers for the higher frequencies (non sub woofers) through the Super 10 speakers which can handle up to 50W, although it is unlikely that they will be driven to the max given the space they will operate in. Would the LM4780 be appropriate for this load?

You have not yet affirmed that you want reproduce the organ at realistic levels. So I will assume for the following that you do.

Is this the Super 10?
High efficiency and 8 Ohm speak for it. Its limited stroke against. It will need a 200-300 Hz high-pass to keep it operating within its linear stroke, if you want to play it loud. That is too high for a subwoofer, and would call for an additional woofer per box.

The relatively high mass of a 10" full-range driver requires high current in the upper frequency range. Chipamps like easy loads and dispose of a limited amount of current. It is of course a matter of trying it, to know for sure, but if you want a guess, I'd say it is not the best match.

The specs of this speaker seem to say tube amp or class A, not chipamp. And also SPL on the civilized side of loud.

[jwillans]

Originally posted by pacificblue


You have not yet affirmed that you want reproduce the organ at realistic levels. So I will assume for the following that you do.

Is this the Super 10?
High efficiency and 8 Ohm speak for it. Its limited stroke against. It will need a 200-300 Hz high-pass to keep it operating within its linear stroke, if you want to play it loud. That is too high for a subwoofer, and would call for an additional woofer per box.

The relatively high mass of a 10" full-range driver requires high current in the upper frequency range. Chipamps like easy loads and dispose of a limited amount of current. It is of course a matter of trying it, to know for sure, but if you want a guess, I'd say it is not the best match.

The specs of this speaker seem to say tube amp or class A, not chipamp. And also SPL on the civilized side of loud.

Yes those are the speakers. Another source has indicated a similar opinion to you - that chipamps are not the most appropriate for this type of application - they recommended mosfets based constructions. I think I need to do more research into amplifiers, their construction and the relative merits of different amplifier types rather than asking random questions without first doing some background. Is there a website or book that you recommend?