I'm looking for a power amp that can reproduce infra-sound from near DC to 30Hz or so. This amp needs to power a subwoofer that is rated at 150 watts RMS into 8 ohms. My research has determined that a direct coupled amp design is the way to go but so far I cannot find a suitable amp. The closet that I have come is a vintage Technics SU-V3 Stereo Integrated DC Amplifier with a FR of 2Hz to 100kHz. Unfortunately it's power output is only 40 watts per channel into 8Ω (stereo). I then thought that I could build an amp on a board but I am having trouble finding a suitable design. Can anyone point me to a design or a consumer amp that can meet my needs?
Thanks,
Scott

 

Many audio power amp designs can be modified to suit your application by removing (or designing around) the input and output coupling capacitors. Many amps with dual power supplies do not have an output coupling capacitor, so that leaves dealing with the input. Also, many amps have a capacitor in series with the shunt resistor in the feedback network. This reduces the gain of the amp to 1 at DC, preventing the amp's input offset voltage error from appearing across the speaker as a DC component. One way around this is to add a "DC servo" circuit (basically, an opamp integrator) as part of the feedback loop.

Do you want to design and build from scratch, start with a kit and modify it, buy a commercial audio amp module and modify it, or something else?

ak

 

I am not certain there is a suitable design. What designs have you considered and rejected?
The Mythbusters did an episode in 2005 where they were looking for the brown note in the range 20-100 Hz. I couldn't find any information on the amplifier they used. It sounded cool in the film clip.

 

At the concept level, the block diagrams and most of the schematics of a traditional bipolar opamp (741, etc.) and a 1970's audio power amp are identical: NPN input differential pair, voltage amplifier, complimentary output stage. Here is a quickie grab from the innergoogle that uses multiple parallel output transistors for 1 1500 W output.

https://circuitszone.com/high-power-audio-amplifier-circuit-1500-watt/



The only difference between this and a "DC amplifier" are C1 and C3. Remove them and you have a DC amp just like a 741. And, just like with a 741, you now have to deal with the offset voltage. Like this:

https://electronics.stackexchange.com/questions/354983/integrator-as-dc-servo

ak

 

Given the need for a lot of output power, and the very modest requirement for upper-end frequency response, it seems to me this could be a good place to use a Class-D (i.e., a PWM-based) amplifier. It would certainly eliminate the need for a massive heatsink for the output transistors.

Hard to say whether the technique is applicable here without knowing more about the requirements, but it's worth a thought.

 

Thought about that, but finding one without a low-frequency high-pass corner in its internal feedback loop could be hard.

ak

 

Hello,

Have a look at this project:
https://sound-au.com/project68.htm

Bertus

 

Where did you find a subwoofer that produces infra-sounds from 5Hz to 20Hz? Most simply boom a resonance at only 30Hz.
150W into 8 ohms is not much, is it a little 8" woofer?
Subwoofer speakers at Parts Express are usually 15", rated at 600W, cost $190US today and resonate at 19.5Hz. A speaker performs poorly below resonance unless you drive it very hard.

 

Hello,

Have a look at this project:
https://sound-au.com/project68.htm

Bertus

I've admired some of the stuff from ESP before, but the note about Toshiba Transistors being obsolete and most likely counterfeit would mean a detailed search for an alternate manufacturer.

 

reproduce infra-sound from near DC

How near to DC?

 

How near to DC?

Until we can find a speaker that will work well below 20hz, maybe near DC is not the problem. Many amps will do 10hz. (or with a change in caps will do 10 or 5hz)

 

How near to DC?

Post #1 says 2 Hz.

 

I asked Bing to play 2Hz and I got songs from the band called 2Hz and also got a "binaural beat" which was created using a combination of a 57Hz sine wave and a 55Hz sine wave split. It sounds like a 2Hz sinewave causes ramping a 55Hz sinewave up and down in one stereo channel while a 57Hz sinewave was ramping down and up in the other stereo channel and the levels alternate between the channels. Try it :

 

A suitable number of LM3886, LM1875 or similar chip amps in parallel and/or bridged?

Regarding the Mythbusters "brown note" episode, they did it wrong. Off-the-truck pro audio gear, set up outdoors. It should have been a no-brainer to build a chamber large enough for a human guinea pig with one or more subs mounted in the side and a hole for the victim's neck to stick out. Or just use a car with a sealed-box subwoofer in the back; I think there's a couple of those in California.

 

So what an infrasound weapon will be built?? Fear panic-attack weapon, or kidney-collapse weapon? Or less innocent kill-all-around weapon? Infrasound is rather tricky thing, especially when larger wattage.

 

It's probably just for home theater use; some movies have "sound" down into the single digits. See the "Way Down Deep" article. The spectrogram of "Black Hawk Down" shows high levels at 7 Hz:
https://www.soundandvision.com/images/archivesart/604way.waterfalls.jpg
https://www.soundandvision.com/content/way-down-deep-i
It'll be impractical to achieve flat response to 7 Hz in a house, but sealed-box subs in a car should come close (thanks to "cabin gain"), if the electronics can pass those frequencies.

 

Today I saw the trailer for the new "Top Gun" movie coming in June. I listened to it playing on my 4W amplifier driving my ported speaker that has a 6.5" woofer (down to about 50Hz) and a dome tweeter. I was amazed that the sound had very low frequencies in it.
Oh, I forgot. My amplifier is fed with my bass enhancement circuit that boosts the very low frequencies +8dB.