Please help me choose a chip amp for a bi-amped speaker project

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majerjack

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Joined 2010
I am planning a bi-amplified speaker which I am considering driving with chip-based amplifiers. I have no experience with such amps and hope some of you can offer suggestions.

I have read the accolades about amplifiers based on the Tripath chips and also the Texas Instruments LM3875, both of which are now out of production. I know there are others currently available, but I am ignorant of their comparative strengths and weaknesses. Some listeners report detailed sound with good bass control and others report cold one-dimensional sound. I need help sorting through all the varied and sometimes conflicting information.

One characteristic I have noticed in the specifications for amplifiers based on chips is wildly optimistic power ratings. When one looks further into the specifications one sees that those ratings are at 10% distortion. Chip amps seem to be very limited in the amount of current they can deliver. Because one of my speaker drivers is rated at 4 ohms I need to insure that any amp I choose can drive it adequately with low distortion, perhaps .1%. I need about 30 watts of clean power into 4 ohms. This will give me 3dB of headroom above my maximum volume requirement.

I have no desire to attempt to solder surface mount devices, so I need amplifiers which are complete units in chassis or already-assembled boards that I can mount in a chassis. I would also prefer to use power supplies which are already assembled. I will use these amplifiers with already-existing preamp and crossover. By the way, the Hypex products look wonderful, but they are priced too high for this project. In addition, some of the assembled boards I have seen come with a cooling fan, and I don't want that. I am otherwise open to suggestions.

Thanks to all who respond.

Note to mods: I thought this post belonged in Chip Amps, but if you think it belongs in Class D, please use your own discretion.
 
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AndrewT

R.I.P.
Joined 2004
majerjack said:
.................... Because one of my speaker drivers is rated at 4 ohms I need to insure that any amp I choose can drive it adequately with low distortion, perhaps .1%. I need about 30 watts of clean power into 4 ohms. This will give me 3dB of headroom above my maximum volume requirement......................
Click to expand...
It looks like you need a maximum output of 15W into 4ohms reactive speaker.
Let's examine that requirement.
Vmax = sqrt(2*Pmax*Rload) = 10.95Vpk
Imax = sqrt(2*Pmax/Rload) = 2.74Apk
Check: Pmax = Vpk * Ipk / 2 = 15W OK.
That 2.74Apk is into a non reactive load.
When you actively drive a reactive speaker you will need more current. Expect the transient current peaks into active speakers to be 150% to 200% of the non reactive currents.
You therefore need to select a chipamp that can output at least 2times 2.74A, i.e. 5.48A. It's in the datasheet.
Your supply rails when delivering maximum power will need to be around 4V higher than your maximum signal.
The supply rails will sag down to that maximum power value. You need to allow for that sag. Try adding on another 4Vsag.
That adds up to a quiescent supply rail of ±19Vdc to give you 15Winto a 4ohms reactive speaker.
A dual secondary of 15Vac should meet that requirement
If you have 6 amplifiers each of 15W, then your transformer should be rated for ~6*15W i.e. 90VA.
You will probably find that anywhere from 100VA to 300VA will work. 4channels would work from 80VA to 160VA
I recommend a dual rectifier each from the separate secondary. Then couple the two PSUs at the output to create your dual polarity supply for the 6 channels.
Actually I recommend two separate enclosures each having 3 amplifiers to drive each active speaker with very short cables (maybe 200mm to 300mm long !).
 
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chrisb

Member
Joined 2002
majerjack - as I've posted a couple of times, including just a few moments ago in another thread, I recently put together a nice little 4 channel amp with onboard PLLXO for bi-amping FAST projects.

https://www.parts-express.com/sure-...dio-amplifier-board-sta508-(t--320-335http://

power supply
https://www.parts-express.com/mean-...0w-regulated-switching-power-supply--320-3141

Of the "old school" chip amps, I had the opportunity to hear two LM3886 by Tom Christiansen ( Neurochrome) this August. Quite simply the best sounding chip amps of any I've heard in the last 15yrs. Modulus86 and LM3886DR. He also did a 4780 model called the Parallel86 and is working on a new model, Modulus 286 hopefully to be available by Christmas. Not inexpensive compared to the Sure boards, but AFAIC a far superior product.

He has some very strong opinions about what is often missing from the standard implementations of the 3886, and while it's all over my head, the following is good read on where he's coming from.
Taming the LM3886 Chip Amplifier
 
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AndrewT

R.I.P.
Joined 2004
chrisb said:
majerjack - as I've posted a couple of times, including just a few moments ago in another thread, I recently put together a nice little 4 channel amp with onboard PLLXO for bi-amping FAST projects. .................
Click to expand...
I read the OP's enquiry as using the passive crossover already fitted inside the speaker.
You are referring to adding a PLLXO to create an active speaker.
A quite different project.
 
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majerjack

Member
Joined 2010
Apologies if I was not clear in my original post. I have a preamplifier and crossover already built in separate units. I just need amplifiers to accept the signal from the crossover. The amplifiers should be complete in chassis or boards I can mount in chassis. If they need separate power supplies those should be already assembled as well.

Thanks for all the input.
 
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