Help me understand how to optimize bass on the Zu Definition Mk IV


I am a longtime owner of Zu Definition Mk1.5 speakers and recently also bought a pair of Definition Mk4's. I based this decision partly on the reviews / feedback posted here on Audiogon in various threads by members such as 213cobra, gsm18439, spiritofmusic and others. I'm really hoping some of you Mk4 owners can chime in and help me out here!

Basically, my issue is that I found the Mk1.5's put out incredibly deep and impactful bass, especially with some of the music I love, like R&B and reggae. And this was with the bass amplitude knob at 12 or 1 o'clock. However, after setting up my new Mk4's, I find the bass underwhelming and anemic, even after fiddling around with various settings and with the volume and PEQ cranked. I have them on spikes into bases on a hardwood floor, with about a 1.5" gap on the bottom. What am I missing here?? Based on the reviews I read, I was expected the bass to equal or exceed the lowly 1.5's. 

Thanks so much in advance for any helpful advice! 


temicxoch
The reason I asked about new vs. used is that in the early Def4, Sean set the internal amp's gain fairly low, such that in many rooms, including mine, the bass level had to be cranked to 8+, often 9, just to get in the realm. I pointed out to Sean that in a bass-shy room a user would not have enough sub amp gain headroom to properly load some rooms relative to the FRD efficiency. So he subsequently changed the internal gain setting on the internal amp so that the external pot yielded a far greater range of useful gain. I know all the Rev.B speakers were corrected, but some original series were too. I just don't know the serial # cut-off.

Call Sean with the serial numbers of your speakers, and he should be able to tell you if that's the case with your speakers, and he can send you a fix. If you have your sub gain cranked almost to 10, I believe that's your first order problem. In most environments, floor-to-plinth gap on Def 4 is practically immaterial. There are some situations where it is conceivable to matter but I have yet to actually encounter it. It doesn't hurt to experiment but I think you won't have to.

The TJ / Full Music Mesh Plate 300B (really a perforated plate) has three distinctive characteristics, compared to a more objective tube: a rich and euphonic top-end spray, classic coveted 300B lush midrange, and comparatively bloated bass that easily infects midrange transparency and articulation. It's a rich, creamy, decidedly not-objective sound. Now, all the Definitions have internal sub amps that derive their input signal from the bass character of the full range power amps. But the Def4 12" downfire is capable of much better bass articulation and is more dynamically disciplined, so you will have more to work with if you switch to one of the better solid plate 300B tubes, chief of which I suggest the KR 300B balloon glass. But if you need to spend less, one of the Sophia or TJ solid carbon plate tubes, or even the Shuguang Natural -T will be better.

IIRC Sean suggests new owners of Def4 start with the LPF set to 60 hz, which is right for almost nobody, but working down from there gives you an audible ramp into what works in your room. Then you can dial in amplitude and go after the PEQ settings to refine. But it seems highly-likely to me that you have an early pair of Def4 that made it into the field before Sean altered the internal amp gain to a more useful level.

Check and report back, or send my your serial #s and I'll call him tomorrow.

Phil


Phil... wow, amazing feedback! Yes, the mkIV's seem bass shy at reasonable volumes against the FRD efficiency (almost as if they were tuned brighter than my mk1.5s), but can clearly crank out deep frequencies when turned up. Just checked and the serial numbers here are 007 and 008, so I'd guess these are very early models. I'm working tomorrow and don't really know the Zu guys, so if you'd have time to call and ask Sean, I'd be very appreciative! 

I suppose I've liked the 300b mesh plate because I listen to so much music that isn't that well recorded and the rich and creamy character of these tubes hides some of that... 70s reggae made in the ghettos of Kingston just isn't audiophile grade. But I also have a strong collection of jazz, blues, R&B, and rock on vinyl as well, some of which are superb recordings. So for me it has been about finding a balance over the years of a system that's capable of being highly resolving, but also forgiving of poor recordings. It's a delicate dance. I'm certainly willing to try out some new tubes though ;)

Thanks again for all your help!
David
OK, David. I'll call Sean tomorrow and circle back. At 007/8, I am now certain this is your problem. I didn't get mine corrected because "9" gets me to the right spot, but if I moved while owning these I'd surely have to get the change myself. Standby.

Phil
Interesting and useful info from Phil. My Def4s are also a very early pair. Sean probably should update the downloadable owner's manual from the Zu website.

Gary
My Def 4s are nos.179 and 180. Where does this fit in the production run re sum amps' gain?