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BmWr75 12th September 2021 04:30 PM

ACA amp and speaker matching
I've built a couple of these ACA amps. But am having some trouble finding speakers they match well with. Thought a pair of Tyler Acoustics PD15s would be a good match since they are high sensitivity, but there must be an impedance issue in the bass or mid-bass region with these speakers because they distort easily in that range on bass heavy music.

What speakers have you successfully mated with an ACA amp?

waxx 12th September 2021 07:18 PM

Like all low power speakers they don't like a ragged impendance and huge impendance peaks. That is what causes the distortion i think.

I use mine now in an own design MLTL with a single Mark Audio CHN110 (88dB sensitive) driver and no crossover or filters per cabinet. But all speakers of reasonable sensivity and a flat impendance should work very good. I know the Tannoy Arden works well with this kind of amps. I coupled mine also once to old Goodman Mezzo SL's and those did it also good on it.

ad010685 12th September 2021 07:45 PM

I mate the 2 aca in parallel config with nomex 164 with great success.i found significant control of mid bass changing speakers wire. Best energy transfer is with large gauge and very short wire. If you need more specific info on my case you can pm.
No need for fancy cables, just short and big.

res07njc 12th September 2021 09:58 PM

If volume is an issue, perhaps there is a reason for more amp than just ACA. That said, the speakers can be singles, multi's... big or small... whichever speaker is your fancy... regardless, it should be easy to drive and responsive in a 'home audio' environment. I'm really not the best person to chime in to be honest, I like lower volume experiences. In my case I need/like that low/high frequency 'loudness' with modest power requirements. Which is pbly why my easy-to-drive classic 8 ohm 3-ways are my speakers of choice. Very single ended friendly. There is no scientific data from my end, but this relationship between electric work and mechanical work, in iso terms, seems best to me. Neither should work too hard or too little. ymmv.

BmWr75 13th September 2021 12:55 PM

Volume is not the issue, distortion in the bass/mid-bass region is the problem with the specific set of speakers mentioned.

waxx 13th September 2021 01:08 PM

That is because an impendance peak in that region causes that the amp must send a lot of watt to conquer that peak, even on low volume, so it distorts in that region. This aca amp has only a bit less than 8 watt so it can't deliver the juice to do that and distort. This is typical for low power amps and speaker with huge impendance peaks, and more powerfull speakers can handle that so you don't hear it there. This means the amp is not fit for that speaker or any other speaker with high impendance peaks.

ItsAllInMyHead 14th September 2021 12:33 PM

waxx has excellent points, but one conclusion I disagree with. Low volume is a relative term... and so is "a lot" of watt. I'd expand their excellent thoughts further using data from your own speakers or those you may consider.

I'd recommend calculating the available current and voltage into a load at an expected power level. You know the max available current, and you know the max available voltage swing of the ACAs you built in your configuration. So, you can know whether it's voltage or current limited into the load at the power delivery you expect. You also know what % distortion at 1W is. I think others have measured across a number of loads and power ranges. However, if it's 0.7% at 1W @ 8 ohms, what is the distortion at "full power" at your low frequencies?

Armed with that, you can calculate whether it will reach the desired SPL you want using the lowest impedance (likely current limited) and highest impedance (likely voltage limited) of the speakers at a distortion level that's desirable to you.

Summary - given the sensitivity and nominal impedance of the TA speakers (I am not going to look up any other info), I doubt that power delivery is an issue unless you sit quite a ways away in a big room and listen at hearing damaging levels.

Where I differ - IMO, the amp is perfectly fit for speakers with impedance dips. If, you listen at low levels, and/or the speakers are efficient. Even if your speakers have low impedance at low frequencies... if they are 98dB @ 1W / 1m and 8 ohm, then you likely have enough power (current and voltage across the impedance range of your speakers) to drive them to very loud levels within the expected distortion profile of the amp.

Back to your question and adding a thought - My WAG is that your speakers (I'm not familiar with them) may like an amplifier with a bit lower output impedance. Looking at it from another angle, speakers that can get away with less damping factor might be a nice choice for your ears. Some people enjoy the slightly "bloomed" bass from less damping / control. It sounds like you may not. I'm speculating on both accounts. I don't have any solid speaker recommendations for you, but it may help. Another option could be to try two ACAs run in parallel if you have another available to test the theories.

Either way, have fun!

Edited for clarity. I shouldn't take a break mid-response for a phone call. :D

MEPER 14th September 2021 01:49 PM

Put a scope on speaker output terminals and look at voltages when playing. A DMM can also be used set to AC and if is can measure peaks it is also useful.

If there are distortion and you never exceed ....say....3 V....then I would say amp has a problem or speaker impedance goes very low (< 3 ohm).

I can't see why a high impedance peak could make ACA distort?

To test ACA using sine input into a 8 ohm resistor and look at sine at a scope (output) is a good initial test after build to check that amp seems to function correct.

When I pushed my ACA well beyond my listening level I could see peak voltages above 10V and sound was still OK.

ItsAllInMyHead 14th September 2021 04:21 PM

I never like to speculate on others abilities. If you'd like to follow MEPER's excellent advice to check things out with a scope, be sure you're familiar with the grounding scheme of the ACA. If you're not familiar, do a quick search. Many have posted about their issues.

tl;dr - If the amps were built to spec per the guide, the red speaker output is GND. You may know that, but juuuuuuuuuuuuuust in case.

With that said, another fun exercise for seeing if your bass and/or imaging changes to your liking is to simply reverse the speaker leads to the amp (reverse the phase). Again, this has nothing to do with choosing "suitable" speakers for your amps, but it may indicate a few things.

Edited b/c I misread. DOH!

planet10 14th September 2021 05:28 PM

As has been mentioned the Rout of the ACA reaches in to the realm of what is considered highish.

This means that one has to take into consideration the (real) impedance response of the loudspeaker. One want sit to be flattish, as anywhere the impedance is high the FR could be boosted.

With an ACA with channels paralleled into monoBloks the Rout (halved) is low enuff that it is not much of a consideration. One just has to ensure there is sufficient mechanical damping in the loudspeaker that you do not get “boomy” or unevem bass.

With an ACA with channels bridged into monoBloks the Rout is now high enuff that it is one has to worry. Rout is similar to the Rout of a “typical” SET.

In stereo mode you are halfway in-between.

The ACA also has low power (but clips so well it seems MUCH bigger), so reasonably efficient is typically required., but this is not as important as havinf a suitable impedance load.

Waxx mentions his ML-TLs with a single FR driver. Drivers like the CHN110 with copper polepieces have quite flat impedance excepst for at the bass resonance, and in his case, mechanical damping can be adde din the form of volume damping in the line. So the boxes bass can be adjusted to suit the amplifier.

Most speakers with crossovers all to often care little about th eimpedance assumming, like most, you have an amplifir with very low Rout, ie not the ACA.

I can’t find a measured impedance for these, but given the size of the horn and the woofer, this speaker will have issues with lower mids, upper bass (ie near the XO) and a woofer that big is going to struggle with the midrange, and the ACA will lay that bare.

Can you measure the impedance?


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