A Legendary Speaker with two Different Flavours

The ATC SCM50 a legendary HiFi and studio speaker that you can have in either a passive design where you add your own amplification or an active design where the amplifiers are built into the speaker.  It doesn’t take a long Google search to find many technical reasons why an active speaker will be better than a passive speaker, but in the crazy world of subjective sound quality is it really that simple, well that is what I was going to find out.

Back in December I reviewed the ATC SCM50 passive speakers with the ATC P2 power amplifier and to say I was impressed is an understatement. It was always the intention for ATC to convert this exact pair of SCM50 from passive speakers to active speakers by replacing the passive crossover with an active amplifier pack. Sadly COVID restrictions meant it wasn’t possible for ATC to visit to do the conversion so instead they sent me the active amplifier packs to do the conversion myself. To clarify you can upgrade your passive SCM50 to active but its not a customer do at home process it’s a back to factory upgrade as ATC need to take specific measurements of the speakers to perfectly match the amplifier packs to the speaker drivers.

There are always silver linings to every situation and getting hands on with both the passive crossover and also the active amplifier pack was very interesting for me because these are critical parts of the speaker and seeing them in flesh, feeling the weight of them and the quality gives you a different perspective of the the speaker and its manufacturing.

What are we comparing here

I think its probably sensible to start at the beginning what is going here what are we comparing and why is it important.

Firstly what is a crossover, in its most simple terms it divides the full sound signal  into smaller chunks to send to specific speaker drivers.  For example with a tweeter we only want it to see high frequencies, the crossover will divide the signal sending high frequencies only to the tweeter and lower frequencies to the other speaker drivers. In a two way speaker there is onw crossover point, in a three way speaker two crossover points and its called a crossover because there is a crossing over of the frequencies and a sound blend between the different drivers. Both passive and active crossovers do this.

A passive crossover achieves this by using certain components like inductors, resistors and capacitors and there is the challenge to try and achieve the best balance of frequency response, phase/timing and spl output and the three are intrinsically linked. I am not a speaker designer but I have asked a few about this and they confirm it’s a complex difficult balancing act, especially when your talking about high quality, high power handling crossovers and the components start to large quickly and more expensive.

At this point the sceptics among you might think ATC are bound to under engineer their passive crossovers so more customers buy active versions of their speakers, remember I mentioned silver linings. Getting hands on with the passive crossover in the SCM50 was a real eye opener to me, its big and I was going to suggest ATC trademark the name Whopper for it but  sadly it seems that has already been taken.

You can see by the size of the passive crossover ATC are not skimping here and I asked them and they confirmed the passive crossover is as good as they can make it for the SCM50. A lot of the components for the passive crossover are made in house so they have quality control over them too.

Look at the active crossover or the amplifier pack for the SCM50 it couldn’t be any more different and it doesn’t even look like is designed to do the same thing.

With a passive crossover its placed at the end of the signal chain right before the speaker drivers after the amplifier and as a result it needs to have big components in order to handle the power while also doing its job of dividing the signal with as least power loss as possible but its not an efficient process there are losses involved.

With an active crossover the division of the signal comes before the amplifier and it’s a more efficient process so the components to do this job can be a lot smaller and lower cost.

Other benefits of active crossovers are independent control of both level and phase you don’t get that in passive crossovers changes within a passive crossover will affect both

Active crossovers are not affected by temperature, passive crossovers are which is why speakers can sound different with more or less power feeding into them, we have all experienced that.

There can be lower distortion in active crossovers ATC say that in comparison to a passive system, even a bi or tri-amped passive system, an active system will show 15 – 20dB lower intermodulation distortion.

That is all extremely compelling and then when you consider what ATC are doing with the active SCM50 using 3 different amplifiers per speaker one for each speaker driver and they can set the amplifier gain to be just right for the speaker drivers impedance, this is why its not an upgrade at home process ATC have to match the amp pack to your exact speaker and I had to install the correct amplifier for the specific speaker, they were pre configured before the review.

This has been a very basic explanation, if you would like to know more Ben Lilly from ATC joined for a livestream video and we discussed this and more

The Sound and the Difference between the Passive and the Active

There are some global or macro differences you notice straight away and then over longer listening other differences stand out. Interestingly when I measured the SCM50 frequency response there was no noticeable difference to the passive SCM50 measurements at my listening position  meaning I wasn’t hearing frequency response trickery or major changes.

The first macro difference I noticed with the active speaker is what I would describe as a perceived lower noise floor, I could hear more definition to each element in the soundstage with each individual element popping out more from the background.  I asked ATC about this and they said the improved phase or timing of the active speaker improves stereo imaging creating a clearer overall picture. I think some of this might also be related to having separated “mono” amplifiers compared to a stereo amplifier like the ATC P2.

The second macro difference I noticed was the obvious increase in the efficiency of the system, firstly in volume and yes 6 amplifiers compared to 2 will do that but it was more than just volume alone. There is the audiophile cliché of an upgrade lifting a veil from the sound, well funnily enough that is what happens here with the active SCM50 but it’s a different kind of veil, I think a better word would be a filter, they sounded like a filter had been removed between me and the original source material. Music had more intensity, it had more energy and drive it’s an overall more vivid tactile presentation, which really boils down to the speaker being higher resolution.

I noticed this mostly with the mid range and treble delivery rather than the bass. The bass is more of a constant and if anything there maybe appears to be slightly less of it because the bass is tighter and more controlled. There appeared to be less of the warmer bouncy bass that I was enjoying with the passive SCM50 in favour of more precise bass and that is without a doubt more true to the original source material.

I also think this could be a perception illusion because the mid and treble, and especially the mid range driver sound like they are supercharged with the active SCM50 you can see why so many audiophiles revere that soft dome mid range driver. Vocals are presented bigger, taller and grander in scale with more detail and insight and most impressive is how natural they sound.  The term natural is used by every man and his dog, by natural I mean sounding like raw vocals going into a microphone and this is the most natural vocals have been presented in my listening room and possibly that I have heard to date.

The treble sounds the same as the Passive speaker but again supercharged to match the more vivid vocal delivery and I think the reason I like the treble so much from the SCM50 because it stands up to the rest of the presentation, its not softened in any way its always present and it stands up really nicely in my heavily acoustically treated listening room without ever sounding harsh. I do think the treble is a maybe a little less sweet with the active but I think this again could be a perception illusion because a more honest speaker shows you the good and less good in music more clearly.

Does the SCM50 ASL sound like a different speaker to the SCM50 PSL, not really but maybe a little bit, I think the balance is a little different due to the extra life and energy in the mids and treble which then wants a more substantial bass to fully balance. In saying that the active SCM50 is a technically better speaker, its also a more balanced speaker in the majority of areas but its only obviously better balanced if you enjoy the active balance better than the passive balance. For me the active SCM50 is more vivid in its overall delivery and the passive is more "cuddly", ATC speakers owners will know that ATC are not really "cuddly" sounding speakers so please consider that comment in relative terms.

Sound is not the only important factor

There are also some other factors to take into account if your going the active speaker route. The speakers get quite a bit heavier with the amplifier packs in them although the handles make it easier to move them around. You have to manually turn each speaker on and off as there is no trigger system designed into them.

You have system organisation and cabling to consider because it might be quite different to how you have things wired now in a passive system, and this could mean extra cost on top.

Another situation to consider would be for REL subwoofer users, if you don’t have an amplifier to connect your high level wiring to you would need to either use a preamplifier with an output you can use or maybe use a splitter on the balanced cable. Where there is a will there is a way but again its something to bear in mind as I think a lot of user of the SCM50 owners will end up considering adding a subwoofer or several to help support the bits the do less well and improve the bits they do really really well.

Then you have the cost factor
An SCM50 PSL with ATC P2 power amplifier costs £9820 + £3625 = £13445
The SCM50 ASL costs £13075
If I have those prices exactly correct its cheaper to buy the active SCM50 or give or take the same money for the technically better, more amplifiers based system.

There are other space saving benefits of having the amplifiers in the speaker and not on your rack but you don’t have the opportunity to upgrade or change your amplifier down the line if you want to change your sound. But you can control the sound of the active with the preamplifier, the DAC and other items so giving over control of the amplifier does not mean we are losing control of the sound balance of the system, its just a different starting point.

Final Thoughts

So from my experience should I tell everyone to discount the passive SCM50 and only buy the active and I would say of course not. If you are already heavily invested into high quality amplification you love why would you want to do that and that is why its great ATC manufacture both variants and not just the active.

I can also see some audiophiles preferring the sound of the SCM50 passive because they are a little more forgiving and I found once I set them up well I could fall very easily into their sound and be pretty much content and happy with it.

With the active version it’s a different situation, yes I am really appreciating what they are doing better than the passive and I can easily hear the benefits of the extra clarity and resolution.  But the active speaker is a different starting point so finding the right balance of sound might mean taking a slightly different approach in component choice thinking about tonality maybe more than clarity and resolution.

Spending time with the SCM50 ASL and PSL has been very enlightening to me, I now feel very strongly towards active speakers, I no longer feel as I did before starting this review.  They now make perfect sense to me as a way to achieving a better sound so well done ATC, mission accomplished.

Special Performer Award is Pursuit Perfect Systems highest accolade and is in recognition of exceptional product performance regardless of price

SCM50ASL (Active)
Drivers: HF 25mm Mid 75mm LF 234mm Super Linear
Amplitude Linearity ±2dB: 70Hz – 20kHz
Frequency Response -6dB: 38Hz & 25kHz
Max continuous SPL @1m: 112dB SPL
Crossover Frequency: 380Hz & 3.5kHz
Input Sensitivity:1V
Input Impedance: >10k ohms balanced XLR
Amplifier Output: HF 50 Watts RMS MF 100 Watts RMS LF 200 Watts RMS
Overload Protection: Active FET momentary gain reduction (amplifiers)
Power: Nominal 90 Watts, Driven 350 Watts
Cabinet Dimensions(HxWxD): 717 x 304 x 480mm including amps (stands add 250mm to height)
Weight: 48.9kg