Dahlquist DQM-9

I had a chance to move on a pair of Dahlquist DQM-9 monitors recently, so I took advantage and have them in my listening room now.

PhotoGrid_1462237735594-01They came about locally, with woofer surrounds that were shot, and cabinets and grilles needing some TLC.  The tops of the cabinets have some minor flaws.  The veneered wood sides of the cabinets are in decent condition, although one of the sides has some of the stain missing in tiny spots lower down; it really is not that noticeable unless you are close.  The one grille frame was cracked on opposite corners and I have it perched on the mounting pegs for now.  It will need angle brackets and gluing to bring it back. The aluminum trim around the drivers needs cleaning and polishing but behind the grilles, only the outlines are noticeable.

The construction is all Magnat (Germany) drivers; I’ve even read rumors that the entire systems were made by Magnat and branded Dahlquist. Nothing officially verified though, but I have seen entire systems made by Magnat from the same era (Magnat All-Ribbon series, etc.) that have similar cabinet construction, with the same “velvet” front baffle material.  The DQM-9 came in two versions–the larger one that I own, and a slightly scaled down “Compact” version with an 8″ woofer, which had a butyl rubber surround.  The woofer in the DQM-9 is a 10″ with foam surround.  Midrange, 5″ with rubber surround.  Tweeter is a treated fabric dome type.  Cabinet is a reflex design with dual ports facing rearward.

List price on the DQM-9, circa 1982, was $1200 for the pair, climbing to $1400 for the pair by the mid 80s. These were, therefore, a couple of notches above the speakers I’ve had in recent years (the Grafyx, and the Boston A-150s).

When local audio emporium Absolute Sound carried the DQM series in the 1980s, I remember them having a somewhat “chesty” resonance to the mid-bass that wasn’t too pleasant.  When I removed the woofers to redo the surrounds, I noticed that the bottom half of each cabinet had no insulation whatsoever.  The top half uses fiberglass.

I ended up putting some polyester fiberfill into the bottom halves of the cabinets before reinstalling the woofers.  I have not heard that resonance at all.  I thought maybe there was no internal bracing, but there was–a brace runs between the two sidewalls against the rear panel of the cabinet, which is the dividing line between the top and bottom halves.  I had considered using Dynamat inside as well, but it won’t be needed.  It is interesting to note that the left and right sides of the cabinet are a double thickness of MDF, so they are sturdy to begin with.

Beyond doing a full cosmetic restoration, one tweak or improvement I thought of making would be to redo the crossovers with new components.  However, the crossover uses more expensive film capacitors, so they are not prone to age like electrolytics.  I would possibly redo the cheap hookup wire with some OFC copper wiring.  An interesting design feature is that the crossover is split across three modules inside the bottom half of the cabinet.

The sound?  Quite good, and clearly a step above both the Grafyx speakers I have had for decades, and the Boston A-150s I had for several years.  Imaging is rock solid left to right, yet I do not yet hear much in the way of depth or a soundstage spread beyond the speakers.  I have not tinkered with positioning much yet.  I settled for now on having them halfway between toed in directly at the listening seat and facing straight ahead.  My stands tilt them back slightly, and I am thinking to have them tilt just a little more forward.  I also need to spike the stands to the floor (there is a concrete slab beneath the carpeting and padding), and the speakers to the stands.  I am even at the point of possibly making some stands for these.

Tonally the bass is deceptive–while it does not seem as deep the Grafyx at lower volume, they really fill out nicely when the volume is kicked up.  It takes a little volume to fully wake them up.  In fact, they may go a bit deeper; they grab the 32Hz notes easily when called upon, and orchestral crescendos are taken in stride, with no stress or strain.  The highs are not completely as smooth as I’d have thought, but that may again be due to the aged crossovers; what is there, however, is not at all bright, and that lower treble/upper midrange glare I had from the Philips tweeters in the Grafyx is all but gone now.  These are easy to listen to for hours, and while I haven’t tried them fully with rock music (which I don’t listen to much anymore), it holds up well with large-scale classical, acoustic jazz, vocals and even electronic jazz.

These will be in my system until the Martin Logan refurb project is completed.

Posted in Music | Leave a comment

Tube amplifiers

With a tube-based preamp in place, I am somewhat tentatively taking a look at some tubed power amplifiers to go with it.  I’m nowhere near a buying phase yet (unless an incredible deal comes along), but it helps to get familar with brands, models and price ranges for completed sales, to have a point of reference.

The tubes have a specific lifespan, so that is the first consideration.  How many output tubes will the amp have, how hard are they to find, and what do they cost?   Are they common tubes, or something unusual (which can be both good and bad)?  The small signal tubes have a longer lifespan but there, too, eventual replacement will be lurking in the future.

Next, what about power consumption?  At idle, tube amps can use a lot of power.  Some days if I am working from my home office, I may have the power amp running twelve hours per day.  If it is idling at 500 watts, that is quite a substantial amount of power…and heat.

Finally, to get a hefty amount of output power, will I need to find a larger stereo amp, or switch over to monoblocks?

As for brands, there are a lot of choices. Given my experience with Conrad-Johnson, I may want to stick with their products.  I passed up the MV60SE that the seller of my preamp had, but I didn’t feel 60 watts per channel was adequate for electrostatics.  It still might be a good entry point.  The Classic Sixty is another, similar model that would fit the same bill.  I really like the Premier 140, or even a Premier 11, the latter being around 70 watts per channel and the former being around 140/channel.

VTL (Vacuum Tube Logic) is another manufacturer I’ve kept an eye on.  I had to pass by a ST-150 that was several years old, yet was hardly used.  One neat feature about many of VTL’s amps is that you can use the amp in two different modes–an ultralinear tetrode mode that is faster and more accurate, or the triode mode which has the “tubey” characteristics that many like, albeit at half the output power.  A rear panel switch changes modes.

One other interesting option is a VTA M125 monoblock–it is a kit-built amp based on the old Dynaco circuitry, yet greatly enhanced.  It, too, has a dual-mode switch, and it can also run in a half-power mode with only two output tubes vs. four.  The only drawback is that these are built on a raw chassis and have no cages, something I would prefer to have with an amp.  I tend to buy most of my equipment second hand, and pricing these out, the cost is about the same as some of the other amps I have mentioned above.

Given the few drawbacks of tube amps, I sometimes wonder if sticking with a better solid state amp might be a better choice.  My current amp needs some new capacitors and a couple of small modifications which would make it into a top notch amp.  Likewise, I can get ahold of a Conrad-Johnson MF-2300 that gives me about the same amount of power I have now, but would maybe be a better match to my C-J preamp.

Posted in Music | Leave a comment

Tube rolling in the PV14L

The C-J PV14L preamp is not exactly the best candidate for tube rolling, but I do have some experience with the dfiference that tubes can make.  The PV14L only uses one pair of tubes, as the preamp only has one gain stage.

IMG_20160304_185514The second version of this preamp (which mine is) was built around the Mullard M8080/CV4058 tube.  When I received the preamp, it sounded good with the tubes provided, but I found that they were highly microphonic, exhibiting a rattling noise on certain loud notes.  I had ordered in a pair of Tung-Sol 6C4s to try.  They sounded slightly better, yet there was still some evidence of microphonics.  Just not as pronounced as the Philips ECG 6C4s I had replaced but still unacceptable.

When the Mullard M8080s came in (from a UK seller), right away I noticed that there were no microphonics whatsoever.  Sonically I did not notice a huge difference between these tubes.  If anything, the sound was slightly more mellow and balanced with the M8080 than with the others before it, the Philips ECG tubes perhaps being the brightest and maybe slightly more etched of the pair I tried. However, these latter tubes had an unknown lifespan on them–I believe the seller mentioned they were about “halfway used.”

In the meantime, I had gotten in some silicone o-rings to try as tube dampers.  A bag of 10 properly sized o-rings is only a dollar or two; some audiophile tweak companies charge ten times this amount for the same thing!  There is no difference.  In fact, the silicone o-rings that ship with the PV14L are the same material, just of a smaller thickness.  The inner diameter of these tubes is tighter, so it takes a little more effort to place on the tube.  But, that should assist in better dampening.

As one of the M8080s has gotten noisy (the seller is thankfully sending a replacement at no charge), I placed the 6C4 Tung-Sols back into the preamp.  The thicker tube dampeners have made only minimal difference in the microphonics of these tubes.  Dampeners can help with by reducing some of the vibrations but, of course, will have no effect on any of the tube components physically rattling inside the glass.

Seeing there aren’t many options in this family of tubes, I will stick with the M8080s.  The CV4058 is actually a military spec tube, made to more durable specs and tight tolerances, hence the absence of the microphonics.  There are still plenty out there, and it is not a popular audio tube.  An older McIntosh component used the 6C4, and C-J has used this in three of their preamps.  These tubes do so much right that I don’t really see a point in changing out the tubes for a different sound.

Posted in Electronics | Leave a comment

The phono stage dilemma

While searching for my ideal preamp, one of my criteria was for this new preamp to have a built-in phono stage.  That turned out to be more difficult, as the only way I could find them was in older preamps, or in new preamps that were way beyond my budget.  I finally caved in to the idea of getting a phono stage.

Yet, which one?  I really wanted tubes.  I still do, to an extent.  But my choices in tubed phono stages was limited within my budget.  And when pressed for a choice, tubes might not be my solution after all.

My first candidate was a Jolida JD9 Mk II.  While build quality seemed acceptable, a few Internet forums mentioned that there were additional upgrades that could take the JD9 up to a higher level.  Upgrades include swapping in premium op amps, better output coupling capacitors, and HEXFRED rectifiers in the power supply.  Not to mention swapping in premium tubes.  I also would have installed my own mono switch on the rear panel.

While I like getting into the innards of electronics with soldering iron and voltmeter, the idea of hacking up a brand new phono stage (and voiding the warranty) did not sit well with me.  Once I did the math, I realized the Jolida would not have been a good investment.

The other candidate in valve land was the Pro-Ject Tube Box DS, which has the unique loading adjustment on the front panel.  It is nicely built, sure.  Yet like the JD9, I could not find any mainstream reviews of this model.  I found a killer deal on it, but decided to pass.  I do not put 100% weight in reviews, of course, but they are a helpful second opinion.

My luck came about by way of a lightly used Phonomena II+ by Musical Surroundings.  Mainstream press?  Yep, Absolute Sound had picked it as an Editor’s Choice.  Other reviews thought highly of it, and especially praised the depth and control in the bass.  Perfect.  It arrived within a week of purchasing it, and looked essentially unused.  The sound is, as you’d expect, phonomenal (sorry).

Loading is quite easy on the Phonomena.  A series of DIP switches on the rear can be set in many combinations to get LOMC loading where you need it to be.  For my Dynavector, I am currently at 59 ohms, but feel that I could probably nudge it down to 50 ohms to really smooth it out.

Running a MM cartridge, though, I have the option of two different resistive loads, and capacitive loads.  The only drawback is that MM loads are limited to 47k or 100k ohms; ear bleeders like the Audio Technica MM carts are happier with different loading, maybe in the 22k range.  The next available setting is 2k ohms.

For gain, there are 13 steps available from 40 to 60dB.

I did notice a bit of “rush” (faint white noise) when turning the volume way up.  I am going to inquire whether this is normal or not, and if a better power supply (than the small wall wart) would help that.  This phono stage uses so little power that a battery power supply may be an option too.

Overall, I have not had much chance to listen to it yet, but the sound is very clean and full-bodied.  If the soundstage was improved with the C-J preamp, this really locks it in further when playing vinyl.  I do want to tweak loads a little more, but it is really sounding nice at 59 ohms.

I will have more impressions over the coming week as I evaluate some high-quality vinyl.

Posted in Audio/Video Hardware, Gadget Corner | Leave a comment

Preamp upgrade: Conrad-Johnson PV-14L

It only took me 34 years, but I finally have a new (to me) preamp in the system, and I am finally a member of the “valve audio” club.  The PV-14L arrived two weeks ago, and I have been getting familiar with it.  My unit has had the “S2” upgrade, which features improved caps, Vishay resistors, etc., and a new spec for the tubes.

Having only seen photos of the front panel, I was afraid it was going to be silver.  Instead, it is a pale gold color, which looks nice with the engraved front panel.  Operation is by pushbutton, with most features available via the remote.  Switching and volume are done internally, so worries about oxidizing potentiometers is eliminated.

The tubes were changed out at some point, as the factory spec Mullard M8080s were replaced with the Philips ECG 6C4WAs. For a quick replacement, I ordered in a pair of Tung-Sol 6C4s which were marked for FAA use.  I have the M8080s shipping from the UK, as they are reportedly less microphonic than these 6C4s are.  The Tung-Sols do sound nice in the system, slightly less bright or “etched” sounding than the Philips pair.

One thing I have noticed with this preamp is that the imaging is rock solid.  Instruments are more clearly placed left to right in the soundstage, and vocals are spookily centered between the speakers.  Clearly my speakers are the weak point (for that “last mile” of improvement), yet I can still hear the differences.  One critique of this preamp was weak bass (boooo….), but the S2 upgrade cured that.  The bass is definitely weighty in the improved version.

It also appears to be a good match to the Nelson Pass-designed Stasis amp (Nakamichi PA-7).

The only thing it does not have is a phono stage.  But, that issue has been solved as well…

I will be back with a long-term update, as well as an update once I get the Cardas interconnects built.

Posted in Music | Leave a comment