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Bi-amping/ Tri-amping

Started by exztinct01, May 02, 2016, 08:14:43 PM

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I built an LM1875 mono amp for guitar and for testing purposes, I connected it first to a 4 ohms 5 watts speaker, next to an 8 ohms 150 watts woofer with eff 93db. The signal source was my smartphone so there was no other preamp, no crossover. The only signal is a mono from the phone to LM1875 amp.

Now, I noticed that it can't give the woofer enough power to make it thump (not really great at english so I just picked the word thump  :lmao:)
What amp power, specific amp, can make a woofer with that spec sound good, assuming I have an active crossover? Of course subs will give more bassy sounds but woofers are still capable of some low Hz I think.

I am not really into building crossovers and biamped or triamped systems as of now simce I am only a beginner but there's nothing wrong with studying every aspect of amp building. Plus, my father-in-law is "obsessed" with audio systems, he owns 3 karaoke systems being rented, and he plans to buy a PA system. He's a good carpenter so I think if I gain enough knowledge to build crossovers and high powered amps, coupled with his skill at building speaker cabs, we could assemble a good PA system in the not so near future. It's very ambitious of me but nothing is wrong at aiming high  ;)
~ Stephen


An 8 ohm 150w speaker with 93db efficiency could "thump" with an LM1875 chipamp powering it depending on two primary things: size of the speaker, and power supply of the LM1875.  If the LM1875 has enough voltage going to it, it can put out over 25 watts.  That is plenty to make a 12" woofer "thump."
Life is what you make it.
Still rockin' the Dean Markley K-20X


speaker is 12" and my PS is dual 25V rails. Maybe, there are some some degrees of "thump" and I am looking for more, or just that the speaker specs is not true which is common here in the philippines for local brands. Or maybe, I will hear that thump better if the only sound I will pass to the speaker are low frequencies, not all frequencies from my phone?
~ Stephen


Sorry for double posting but is 25W enough for that woofer outdoors? say, for a car?
~ Stephen


"Thump" is a very relative term. Also it will depend on what you are sending to the speaker for signal as well as the speaker's designed frequency response. Typically a 12" hi-fi speaker should do fine for this, but if it is a 12" guitar speaker it may not.
Life is what you make it.
Still rockin' the Dean Markley K-20X


The phone might be smart but its output is only low level, you need a dedicated preamp before sending the signal to a power amplifier.
Look up crossover networks ,, there is a lot of info around to get you started.

As to Bi Amp setups you won't need them unless you are running Big PA systems.
meantime simple crossovers are all you need. :tu:


I am not yet planning to build any large PA systems in the near future, but it aroused my interest especially since my father-in-law is a bit "obsessed" with it. He doesn't have it yet but plans to buy if he acquires a good amount of money. So, I think I need to gain knowledge about those things to impress him  8|
And, I'm a musician. Not really an audiophile but most things about audio captures my interest.

Phil, you mentioned I need to research crossover networks first. Do you mean active or passive? I read that active are used for biamped or triamped systems only so which should I need to learn first?

I am a bookworm who spends about 6 hours a day reading books and articles. It's summer here so I have lots of time, but I won't be at home again for maybe half a month, so no tinkering activities for now. Just reading and researching.
~ Stephen


Crossovers are simple enough.  Usually, but naturally not always, when we say "crossover" it means the passive ones inside a speaker cabinet.  Usually when we mean active crossovers, we say active crossovers.

When a piece of equipment is called a crossover it almost always means an active one.  I am sure they exist, but I have never seen a passive crossover in a box with terminals outside.  Passives are almost always mounted inside an enclosure.

Passive crossovers work at speaker level signals, while active crossovers work at line level signals.  A passive crossover comes after an amplifier, an active one before an amplifier.

Unless you are trying to get money from him, may I suggest instead of trying to impress FIL, why not learn about what makes sense and what doesn't especially in his context.   In other words, the way PA systems and speakers are configured and used at something like a Rolling Stones or Led Zeppelin concert is totally different from a 25 watt stereo.

In my mind, biamping a little bookshelf stereo system is like cooking dinner for four, one meal at a time instead of all together.

If you read up on plain old crossovers, it will include the theory of diving audio into separate bends - highs and lows, or highs, mids, and lows, etc..  It is done with capacitors and inductors which have frequency dependent impedances.  In a speaker, we send the full range of sound to the cabinet, and the crossover splits it into highs and lows for the drivers - the woofer and tweeter.

Biamping simply means there is a separate amplifier for the highs and one for the lows.  Biamp:  two amps.  By its nature then you have to separate the highs and lows BEFORE you amplify them.  The same ideas about highs and lows and the tweeters and woofers apply, but the circuitry is different.  The actual tweeter and woofer have no idea whether their signal came from separate amplifiers or from a passive crossover.  All they know is they see the highs or the lows they expect.

I hope that helps a little.


Everything that you, good people of this forum, say are helpful  :tu:
I'll stop being too ambitious for now and settle on smaller projects. It's just that beginners have this urge of trying to know everything at once, but we know what happens next.

I'll just read a lot and take this road step by step. No more stupid questions for now, I guess.  I'll just focus on the current projects I have.

Thanks for the enlightenment  :dbtu:
~ Stephen


reurning to the original question about the punch from a speaker...

are we talking about a speaker in a cabinet or is it a "homeless" unit on the table?

speaker cabinets have a great influence on speakers low end response, simplified no cabinet no lows since the are cancelled out.
soundwave is long enough to go around the speaker and cancel the pressure difference


i removed it from its cab that time, so I think that's another factor I didn't consider. Thanks anyway  :)
~ Stephen

J M Fahey

NO speaker will thump without a cabinet.

Will not even have standard bass, go figure.

If you want exaggerated impress-the-noobs thump, build a large tuned cabinet with box tuning around 80/90Hz, no kidding, and add an EQ ( a single band from a graphic EQ will do) tuned to that frequency to increase it at will.

You'll be impressed.

Or build a car audio approved dual bandpass box, tuned to, say, 60 and 80 or 90 Hz , in all cases the trick is to have a 6/9dB peak at such frequencies.

That's the trick behind most car woofer boxes, those you hear from 2 blocks away rattling windows ... but if you listen carefully the "bass has only 1 string and only 1 fret" ... a.k.a. single note Bass.

After sometime it becomes very fake, boring and annoying but it sure sounds impressive to noobs.

Have a long hard look at this project, which is well made and has wider usable frequency response
problem is that it requires an *expensive*  woofer and 1500W RMS driving amplifier, but the basic idea can be applied to yours .

Trace Elliott once made a weird Bass amp, 10" woofer, 80W SS amp which delivered as much wall shaking bass as any 15" or 18" equipped combo ... only problem is that's all it provided, ZERO mids and highs, so much so that it was impressive in your bedroom, but absolutely disappeared (except as mushy rumble) besides any guitar amp or drummer.

Some inventive Russian player added a tweeter, but it's not enough.

The *deep*  cabinet is divided in 2 chambers, the front one tuned by the vertical slot, the rear one tuned by the wide narrow bottom one.

Notice that you do not listen to the speaker itself but to the slots.

I'm itching to build one, but adding an actively crossed over and biamplified 6" closed back midrange speaker , I bet that will be something. 8|


I walked into a Best Buy store once a long while back.  This was at the height of the "home theater" craze.  I was in front of the store somewhere, computer department I think.  And I kept hearing this rumbling noise.  It wasn;t constant, just randomly occurring rumbles.  I thought it was some problem with the air conditioning.

it wasn't until I got near the rear of the store to look at movies when I passed a hone theater setup.  The rumbling I had heard was the subwoofer from that overblown TV set.  Everything had a low rumble added.  Cars drove by on screen - rumble.  Airplane or railroad train - rumble.  But what blew me away was when the main character walked across his living room floor, and his steps were accompanied by this low rumble sound.   Seemed inappropriate to me.

J M Fahey

You bet  ;)

I easily imagine some hare brained design engineer at the factory had the bright idea of adding a "low frequency generator" , simply a divide by 2 flip flop like in an MXR (useless)  octave pedal, to turn any 60/120Hz frequency into 30/60Hz or so ... now the subwoofer *does*  something!!! :duh

Problem is that you start hearing all the time low frequencies which were never there to begin with.

It can even be dangerous: there is a rash of dead Bass speakers killed by Ashdown bass amps'"suboctave generator" which does exactly that.

Problem is that typical Bass cabinets do not reproduce flat even *normal*  bass , even mighty Ampeg 8x10" fridge has a large peak at about 80/90 Hz and then falls like a brick ... and Ashdown feeds them the 20/40Hz range  :duh  at full power.

The large cabinets fart but stand them, but now popular 2 x 10" "350/500W" cabinets burst like watermelons hit by a baseball bat.

Bill Moore

Years ago, I built the the PA cabinets for our band, and since EV supplied their drawings, I built the TL series bass bins. The bass bins were 13 cu. ft., the mids were 1/2 of the TL series 4-12 cab, and then HR90 horns. We tri-amped mono, and it worked great for our R&R band.
A few years later, my wife's sister came back to town, so we put together a country band, did away with the mids, and ran stereo bi-amped. It too worked great, but moving the large bins around in some of the smaller venues was tough. (And also the fact, that with 3 girls in the group, it was always me, and the guitar player moving them!) I decided we needed the smaller TL 7 cu. ft. bins, and built a couple. While installing insulation, I had the old PA playing music, (maybe it was a little loud). When I stuck my head into the speaker cutout, you could really hear the frequency the box was tuned to! Never thought before how the tuning thing really works!