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

You're well past it, Marc.

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

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

For the OP,

Are your Def 4s new or did you buy them used and they're simply new to you? There's a reason I'm asking. What are your room dimensions? Which 300B tube are you using in your SE amps?

I started with Def 1.5 back in early 2005. Then Def II, then (and still) Def 4. All three have been placed in exactly the same location. With the 4x10" back firing arrangement, I did have speakers placed close to the wall behind, but no corner loading. Under normal circumstances, the Def 4 12" sub should easily outperform the older 4x10" array, in bass depth, quality and amplitude. The old plate amp on the 4x10" wasn't as powerful as the sub amp in the Def 4. However, with 6" clearance to the back wall and 2' to the side, with the backfiring array you certainly had corner loading with the Def 1.5 that has gotten you accustomed to exaggerated bass. You might just have to wean yourself off some of that, because you are not going to get the same amplitude of corner loading from the downfiring sub in Def 4. Particularly with your 300B SET amps. You'd have corner reinforcement of rising THD bass.

On the other hand, Def 4 gives you control that laughs at the simple level knob on Def 1.5 / 2. So alot can be accomplished by getting the controls combination settings optimized. But first I need the answer to the questions whether your Def4 are new from Zu or used. Then I'll have more to suggest.

Phil

David,

I was able to speak with Sean Casey this morning. Your Def4s definitely are early enough to have to original lower-gain sub amps. You have s/n 007/008; I have s/n 009/010. You have three options, one of which requires you spend some money.

First, you can experiment with the Low Pass Filter frequency. It goes all the way to 110 Hz, so try frequencies above 60 Hz so you are well into overlap with the main drivers' bass response. I don't normally recommend this but it might work for you, and may require experimentation with the other controls to get it to sound right. You can run the level at 9, 9.5 or 10. No harm in trying.

Second, Sean says if you had Def2 and want some of the extra kick-drum slam that the 4x10" array delivered, you will get more of that out of Def4 using rubber feet instead of the stock spikes.

Third, you can simply remove your sub amp modules, return then to Zu for updating which takes about 4 days. The cost is $600/pr. They are not just making a gain change to the amp. The revision also includes Lundahl gain transformers embedded in the module. To arrange this, just call up Zu and talk to Sean or Gerritt. This will get you all the added gain you need, and you still may want to combine it with some measure of the first two options.

Phil

Definition 4 setup uses less toe-in than Def 1.5 and Def 2 in every room where I have heard both. Particularly in a relatively near-field situation.  Both the Radian supertweeter and the FRD have broader dispersion than the respective drivers in the Def 1.5/2.

You can reduce toe-in with Def 4 without tearing a hole in the middle of the soundstaging. Get some help and adjust toe-in by quarter inch increments of rotation around the inside front foot or spike as the axis of rotation until you hear the soundstage lock in. You can start with no toe-in and incrementally add some or start with the toe-in that worked for Def 1.5s and incrementally reduce it. Trying from both reference points should be revealing because you want to end up in the same place regardless where you start. It helps to use recordings from before the multi-tracks / extensive multi-mics era (pre-1964) or modern recordings known to use simple mic'ing and minimal mult-tracking, to really nail it.

Phil

Also, by the way, David: To remove your sub amps, note that the Speakon connector there is flange surrounding the connector itself. Start by removing the screws from the Speakon flange. The Speakon connector and its wiring harness stay with the Definition. After you've done that, then remove the six screws holding the amp/controls assembly to the Def cabinet. That assembly pulls away and that's what you ship back to Zu.

Phil

David,

The IsoAcoustics Gaia footers are rated for speaker weight up to 70 lbs per set. Definitions well exceed that, so the isolating compound may not as effective under greater mass.

An alternative that is effective for me is from Herbie's Audio Lab:

https://herbiesaudiolab.com/collections/loudspeaker-rack-decoupling-and-isolation/products/cone-spike-decoupling-glider?variant=12645103468599

Under Defs you can use either the regular or giant size, and the titanium spike receptor generally sounds best. This simply puts an isolating compound under the spikes.

Phil

Yes, you should solve grounding problems. You're hearing hum through the Radian compression tweet? That's strange. No, the Zu Def power cables aren't introducing ground loop hum. But you probably have induced some ground loops in your system connections, and it's likely your ground points are not all on the same plane. Solutions are always elusive. If the sub amps' power is contributing, it's not the cords, it's having them plugged into a different circuit. By chance, do you have cable TV among the sound sources into your system?

Also, is any portion of your system balanced or running balanced or XLR cables? Mixing Bal with SE can also surface ground problems. What is your analog chain (phono-pre > pre) and is there an SUT in the mix? How many (if any) of your components have ground-lift switches? Do you have any passive components that are not separately grounded? What's your phono cartridge and tonearm, and how closely are they placed to anything else with a substantial power supply?

Just some questions for which answers may narrow your suspects.

Phil

Keep in mind, the sub amp is a Class D, with relatively low consumption requirements, and the amp has limited frequency duties in this application. You have far more influential opportunities elsewhere in the system to strive for more objective sound than worrying about the power cord and fuse to each Def4.

Phil

If I can get detailed answers to the questions I raised a few posts back, I might have more suggestions. -Phil

Is this problem now resolved? -Phil

Very good. My work here is finished. -Phil