PMC Twenty.21 Bookshelf Speaker Review

[m8o]

$2000 for what seemed like would be $50-100 performance. Then was barely that...

A fool and his money are soon parted.

 

[tuga]

I am hesitating between the PCM 25.22i

After seeing measurements of several different PMC models posted in this thread you are still hesitating?

 

[CDMC]

About 15 years ago my former boss was upgrading his system and one of the speakers he was looking at were the PMCs (don't remember the model, they were about $10,000 a pair). They were touted as accurate. To me they were harsh, bright, and in your face. He seriously considered them, but ultimately chose the Revel Salons, B15 sub and a Mark Levinson to power them. I would say that was a much better choice.

I have to wonder if the studios that like these have excessive absorption sucking up the high frequencies and in turn leading to them being more balanced.

 

[BikeSmith60]

After seeing measurements of several different PMC models posted in this thread you are still hesitating?

Yes, I am. I'll explain why. I demo'd them at a dealer with my own amp and cables. Frankly, they were very good. The unknown variable is my own listening room. The next step is to demo them at home along with my other shortlister, the ATC SCM19. I hate to see those measurements just like the rest of you. However, that cannot account for my ears, my room and my amplification. I am 56 years old and have been "sick" with audiophilia nervosa since age 13 or 14. I have owned and parted ways with Magnepan, Martin Logan, Sonus faber to name but a few, powered by tubes and transistors. Now as life progresses and priorities shift and change I am indeed downsizing this particular system (1 of 4 at home). I will know what sounds right to me when I hear it, irrespective of good or bad lab measurements. So yes, facing a certain dilemma that is very time-consuming!
Brgds.

 

[Kal Rubinson]

So yes, facing a certain dilemma that is very time-consuming!

Tread carefully. Can you get a home demo?

 

[sergeauckland]

I think we're on to something here - if the problem with TL design is creating sufficient length due to the difficulty in forming long enough 'tunnels' using rigid wall construction, why not use a soft material that folds and can pack tightly into the cabinet?

The problem with doing that is that sound won't do as it's told and follow the foam. It'll go just as happily sideways, and fill the entire box. What you're suggesting is pretty much what's done with sealed bass boxes, which are filled with foam/wool or whatever.

The point of a TL is that is is a long tapering absorbent tunnel and the sound has no option but to go down the line, getting weaker as it goes, until there's nothing of any consequence left.

S.

 

[Pluto]

…a higher end audio store in 1974…
…we had master tape copies to play…
…we sold Revox and other large-spool tape recorders including the fancy Technics model with oval head-block

REW in Charing Cross Road?

 

[BikeSmith60]

Tread carefully. Can you get a home demo?

Yes, currently arranging this for the beasts in question. My current rear-ported Sonus fabers are very upset with my 4m x 8m room. Magnificent speakers but only sing when 120cm away from rear wall. So, WAF is in my disfavor. My room won't change but bass reflex SBIR at 45cm are causing problems at 42, 64, and 78 hz. Above that I'm delighted. Only surviving now through Roon convolution filters I made with REW. Pseudo-TL and infinite baffle technologies are my best hope now for attenuating the problem and ATC and PCM are known to be somewhat room-friendly and very compatible with my amplification. I wish myself lots of luck!
Brgds.

 

[QMuse]

We're experiencing some confusion here. The nautilus has a long line to almost completely absorb the rear radiation of the drivers. This is a good thing, and it is an unusual design. Linkwitz's PLUTO or whatever he called the last iteration of the sewer pipe speakers work in a similar way. The principle is simple - if the rear radiation has to go through 3 feet of polyfill, it will eliminate that energy from bouncing back through the cone. Of course if you want to absorb bass, you need a long line.

The term 'transmission line' also refers to a certain class of woofer loading where the bass output of the speaker is augmented by using the back-wave of the speaker. The idea is very simple - you create a labyrinthine path which delays the rear wave of the speaker, and when it comes out the box, it reinforces the front radiation at a certain frequency set by the length of the path. This is called 'quarter wave' loading sometimes, because the rear radiation is delayed by 1/4 wavelength compared to the direct radiation. As I'm sure most know, if you delay something by 1/2 the wavelength, it will be out out phase; 1/4 wavelength and you get a bit of a boost.

There are other ways of using the rear radiation of a speaker to boost bass. The most common is a bass reflex, where you have a resonating mass of air which really likes vibrating at a certain frequency. A passive radiator does the same thing. This principle can also be combined with a transmission line, where you have a short transmission line which 'loads' a port. This is called a mass loaded transmission line, also called an MLTL. It allows you to get more bass in a smaller box than a transmission line. You will also notice that sometimes the line starts big and gets narrow, or starts narrow and gets big. The former is called a tapered quarter wave tube (TQWT). The latter is sometimes called a back loaded horn.

Quarter wavelength augmentation is not without problems. First of all, the claims that it has less group delay are somewhat misleading. Group delay is a function of the roll-off of the bass; it doesn't really matter what means you use to achieve it. A QL speaker will have a roll-off somewhere between a sealed and ported enclosure, and group delay will be somewhere in between. The idea that group delay is signficiant in bass indoors is sort of ridiculous anyway, we're talking about frequencies which bounce around the room a dozen times before they even reach one period.

The biggest problem with QL augmentation is that the line needs to be big, but for DIYers and expensive speakers this can be overcome.

There are some issues with resonances in the line; these manifest as a series of sharp peaks in the response in the midbass. This is where stuffing comes in. By adding acoustic absorption to the line, you can let the bass pass but kill all the other stuff. This is necessary for good performance.

In my opinion, the best thing about QL augmentation is not the bass but the handling of the back wave of the speaker.

PMC speakers aspire to use quarter wavelength augmentation. I have no idea what they think they are doing, but with their smaller speakers, the lines aren't long enough, and to make them as long as they are, the sectional area of the line is quite small. Both of these factors compromise the performance of the line.

Most DIY designers trying to make reasonably sized transmission lines will use the MLTL topology, which offers reasonable size, good back wave absorption, and good bass efficiency. See this link for an audioxpress article about a DIY speaker by Paul Kittinger which uses MLTL loading.

My speakers, Castle Harlech S2, are of twin pipe quarter-wave design meaning each woofer is laoding it's own pipe. They are 2 way design, app 55L of internal volume, 2*130mm woofers and 28mm soft-dome tweeter. Crossover is at 1800Hz, LR24. Pipes are tappered and stuffed with port at the base of the plyinth.

Here is the uncorrected in-room response, measured from LP which is 4 meters from the speakers. As you can see it is relatively smooth for a speaker of such age and no spikes there:

Another thing why I like these speakers is this:

As you can see they are able to achieve THD of 1.75% while playing 40Hz tone at SPL of 100dB measured at LP, so 4 meters from the speakers. At 95dB measured at 1m they give 0.45% THD. Not often you can find such clean bass and my guess is this is due to twin pipe quarter wave TL design.

 

[DSJR]

REW in Charing Cross Road?

No, the more rural KJ in Watford

There were only three 'domestic' Crown dealers in the UK back then, the third being The Audio Centre in Sheffield from memory..

 

[DSJR]

Castle Harlech 2's were very nice as I remember, but the larger Howard's from the same era didn't work at all in a smaller room (15' x 11' in old English money).

Just to repeat, the PMC 20 series (now two generations on I think from the one reviewed just now), Facts and so on are NOT used in studios to the best of my knowledge..

 

[AudioSceptic]

When I started in a higher end audio store in 1974 (as a Saturday boy originally), Crown driven IMF speakers were the dogs danglies (excluding the Crown/Amcron electrostatic hybrids, JBL L200's and AR LST's)and we had master tape copies to play into and through them, as 'vinyl' wasn't judged any good (well, most direct drives fed back due to bad siting and springy belt drives suffered wow quite often back before the Linn LP12 took the UK over ). An IC150- D-150 driven pair of IMF pro Monitor III's could sound awesome with a good source and they did an excellent job with a tape from Angus McKenzie (a notorious reviewer of the era) of Hendon Brass band playing Stars & Stripes Forever

We bought the final batch of III 'Improved' speakers (half a dozen pairs in around 1981 I think) and driven by 'bolt up' Naim and an LP12 source, they were horrible (the amp had a higher output impedance and Lord knows what an LP12 was doing to the bass back then, but we didn't care in our ignorance with the bass light boxes we started to sell and almost brain washing at the time - the master tapes had been 'mislaid' over the intervening years, although we sold Revox and other large-spool tape recorders including the fancy Technics model with oval head-block.

In a smaller room, the very low bass of the IMF could give uncomfortable 'pressure waves' on vinyl sources of the times and the RSPM mk4 model from 1976 offered considerably tautened low bass with lf filter options too. I'd love to hear a pair today driven from a decent modern amp and digital source (to be fair, the ancient Crowns I still use were measured by ken Rockwell and still offer a balanced solid result even today I think).

PMC make several ranges of products all aimed at a slightly different market. The 'hot' ehf is common to most I've heard I think and a definite design intention going back to the LB1 original, which sounded great on a Quad 606 amp but horrible on the then market leading Naim 250 (why my then employer turned the brand down as 'we' were so Naim centric back then). The top domestic model Fact Fenestria are awesome sounding boxes though, but totally overpowering in a smaller space and a chat with Pete Thomas, whom I've known a little bit since the late 70's when he was at the BBC, he indicated that prospective purchasers should look to DSP to blend the speaker to the room better.. I think in fairness the domestic PMC's have been improved over the nearly? ten years since the speaker reviewed here was introduced. The market for these doesn't look so much at price and Naim is routinely used with them locally...


As for an impending ATC review. I'm dreading it myself as I've held a brightly lit torch for the brand for thirty years now and owned and loved a few of their products, only giving up some much cherished 100A's for love and marriage (not saying more on that one ).. Hopefully the 19's are current issue with the latest tweeter design. Listening to so many bling boxes in recent times, I return to ATC's (11's are fine in smaller UK rooms) with a huge sigh of relief! I also fell in love with the Kii Three's as well and going against pre-conceived ideas of Class D amp modules, they sounded so clear and sweet in the dem I sat in on.

Forgive the lengthy post and hope some of the above experiences from the mid 70's bring back memories for some of you.

When I lived in London in the 70s, I visited an audio dealer called Studio 99 (I think), at Centre Point, several times. They had all that gear and I remember being very impressed with the Amcron hybrid speakers driven by, of course, the Amcron amps. I think the source was TEAC reel-to-reel. This was before the whole Linn-Naim thing took off, so probably 1974-5. Thanks for reminding me.

Edit: Studio 99 was somewhere else. REW now rings a bell.

 

[Sonny1]

Thanks Amir! Wow, that was much worse than I expected. I’ve always liked the clean look of PMC speakers. This was a myth buster review. These speakers are held in high regard by many audiofools. Measurements don’t lie. Hope the company sees this review. Poorly engineered speakers with low performance at a HIGH price. Not good.

 

[AudioSceptic]

No, the more rural KJ in Watford

There were only three 'domestic' Crown dealers in the UK back then, the third being The Audio Centre in Sheffield from memory..

When I bought my BC1s from Audio T in West Hampstead in 1976, they were using Crown/Amcron as their reference amp.

 

[CDMC]

Had these in the 70’s driven by GAS Thaedra & Ampzilla, great combo.

View attachment 71342

My brother in law had a pair. Great sounding speakers. I told him if he ever wanted to sell them, I would buy them. Of course they were sold and I never got a chance to purchase them.

 

[Thomas savage]

My speakers, Castle Harlech S2, are of twin pipe quarter-wave design meaning each woofer is laoding it's own pipe. They are 2 way design, app 55L of internal volume, 2*130mm woofers and 28mm soft-dome tweeter. Crossover is at 1800Hz, LR24. Pipes are tappered and stuffed with port at the base of the plyinth.

Here is the uncorrected in-room response, measured from LP which is 4 meters from the speakers. As you can see it is relatively smooth for a speaker of such age and no spikes there:

View attachment 71398

Another thing why I like these speakers is this:

View attachment 71399

As you can see they are able to achieve THD of 1.75% while playing 40Hz tone at SPL of 100dB measured at LP, so 4 meters from the speakers. At 95dB measured at 1m they give 0.45% THD. Not often you can find such clean bass and my guess is this is due to twin pipe quarter wave TL design.

Are you still using the Richmond 3 as surrounds ?

 

[CDMC]

That is not high fidelity!

But it is high frequency.

 

[AudioSceptic]

By eye usually anchored at 200 hz.

Thanks. I'm sure you know that this can be tricky to get right. ;-)

 

[amirm]

Thanks. I'm sure you know that this can be tricky to get right. ;-)

I do indeed. I am not sure a mathematical solution is ideal though as weighting of the frequency bands is not equal audibly speaking.

 

[BikeSmith60]

Thanks Amir! Wow, that was much worse than I expected. I’ve always liked the clean look of PMC speakers. This was a myth buster review. These speakers are held in high regard by many audiofools. Measurements don’t lie. Hope the company sees this review. Poorly engineered speakers with low performance at a HIGH price. Not good.

Gee whiz. You sure do come to definitive conclusions is a hurry. Was that with or without actually hearing them make sound, which is what transducers do? Secondly, if every happy PMC owner learned you consider them to be "fools" they may have some choice adjectives for your good self. I agree that the measurements presented are indeed dismal. However, in today's Western capitalist marketplace any company regularly proposing poorly-designed products would neither survive nor gain legions of customers. I have personally heard the Twenty5 22 on my amplification and cables last week. I detected no major flaws or niggles that would dissuade me from acquiring them. I am neither for or against PMC just brutally subjective in determining what sounds good to me or not. I am not pleased with the measurements presented here of a 2-generation old PMC speaker but if in the end it outperforms, in my room, the ATC SCM19 then I will not have a problem with acquiring them.
Brgds.