XLR to RCA to XLR

Thanks everyone for all the helpful info!

Here is more info on my setup and my latest experiment...

My preamp has both RCA and XLR outputs, which can be used simultaneously.

I only want the KUBE on the LF in a biamp'd config. I have read that even if you do not adjust the HF contour on the KUBE, it can still muddy up the wonderful midrange on these speakers just by being in the signal path, which is probably another debate altogether...

Anyway, I am feeding the XLR outputs of the preamp straight into the the XLR inputs of my amp for the HF.

I then figured I'd use the RCA outputs of the preamp to feed the KUBE, and then the KUBE output would go to a second set of amp inputs (it's a 7 channel amp).

Last night, I tried this with just RCA to XLR cables on the output of the KUBE to my amp using cable #17 on this page: http://www.rane.com/note110.html

It "worked", but the output from the LF side was significantly lower, even when using the variable output of the KUBE and cranking the pot on the back.

I sent a 60Hz tone through the system at low volume and with a volt meter, measured .40v on the HF speaker terminals and .30v on the LF speaker terminals. I'm sure this is not very scientific, but I wanted to somehow confirm what my ears were hearing.

I guess I have voltage mismatches by using the unbalanced outputs from my pre to the KUBE compared to the straight balanced connections for the HF.

Now I am thinking I should NOT use the unbalanced preamp output, but rather split the balanced preamp outputs with XLR Y-adapters (this is sounding less and less high-end as I write), feed one directly to the amp for the HF, then send the other to a Jensen PI-2XR to feed the KUBE.

The unbalanced output of the KUBE would then go through another PI-2XR to the second pair of balanced amp inputs.

I figure in this setup I should (hopefully) have similar gain/voltage at both pairs of amp inputs when the KUBE is inline (and bypassed).

Here are some quick specs on the gear:

KUBE 200:

Input Impedance 51 k ohms.
Maximum Input Voltage >1.9 V rms above 20 Hz.
>8 V rms above 50 Hz.
Maximum Output Voltage >5 V rms.
Output Impedance 100 ohms.
Output Noise Level <6 uV A-weighted.
Signal-to-Noise Ratio >104 dBA ref 1 V rms.
Distortion <0.005 % for 1 V rms input @ 1 kHz.

Preamp:

Unbalanced line-level outputs:
Maximum Output Level: 4 Vrms
Output Impedance: 100 ohms

Balanced line-level outputs:
Maximum Output Level: 8 Vrms
Output Impedance: 200 ohms

Amp:

Balanced line-level audio inputs:
Input Impedance: 24k Ohms balanced
Maximum Input Level: 4 Vrms balanced

Also, since it just says balanced and unbalanced "line-level" I don't think I need to worry about +4 to -10 conversion, etc.

Apologies for the long post. Any advice or suggestions at this point (or a slap in the face if I'm doing something stupid) would be most appreciated!

Rdeckard, I took a look at the manual for the Kube 200, and I see that its level control provides a maximum boost of 5 db for its variable output, relative to the level of the fixed output. Your findings seem consistent with that.

The approach you are considering would work, except that it may be that a PI-2RX should be used between the Kube and the power amp, rather a PI-2XR connected "backwards".

HOWEVER, in the light of Grant's (Tvad's) comment above that ...

Jensen makes 1:1, 1:2, 1:4, 2:1, 4:1 transformers, and I suspect any of these could be built into their XLR>RCA or RCA>XLR converter boxes at a customer's request.

... a call to Jensen would seem to be in order, to explore additional possibilities.

Those possibilities would include stepping up the signal level by 1:2 (6 db) between the Kube and the power amp; or stepping down the mid/hi frequencies by inserting into the XLR connection between preamp and power amp a PI-2XX (XLR ins and outs) specially configured with a 2:1 stepdown transformer (with no transformers being used in the low frequency part of the signal path, unless one is needed to eliminate groundloop hum).

Looking at all of the numbers that are involved I don't see anything that would necessarily rule out any of those alternatives. But my instinct, assuming that you are not presently using your volume control near the upper end of its range, would be that it is generally better to step down than to step up, given the cautions I expressed yesterday in my post about the Rane unit. And also given the very high degree of transparency Tvad and others have reported the Jensen transformers as providing.

Hope that helps. Regards,
-- Al

Rdeckard wrote, "I only want the KUBE on the LF in a biamp'd config. I have read that even if you do not adjust the HF contour on the KUBE, it can still muddy up the wonderful midrange on these speakers just by being in the signal path, which is probably another debate altogether..."

According to the KEF museum, the KUBE is just an option for the 105/3, not integral to the design as it is for the 107/2. Why not just try it between the preamp and amp, and judge for yourself whether it muddies the midrange?

The 107/2 KUBE doesn't muddy the sound of my spectacular 107/2s that are bi-amped between the amps and the speakers.

db

Since my last post, I was lucky enough to speak with Bill Whitlock of Jensen, who is both very knowledgeable and willing to give thoughts and suggestions about my situation. He observed that the gain mismatch I am getting would be best remedied by somehow getting more output from the KUBE. I sent him schematics of the KUBE and he replied back very quickly, instructing to change out 2 resistors in the variable output circuit for lower values, which would extend the gain by a few dB. He said that I could stick with the RCA to XLR cables the way I had them wired, but suggested that if I got any noise, I should try combining the shield with the black conductor at the RCA end. If no luck there, then I should look into getting a PI-2RX, etc.

I have yet to do this because I haven't gotten the parts, but in the meantime, I figured I'd had a go with db's suggestion above and just try it for myself. I hooked it up, non-biamped, and gave it a go.

I have to say I really don't notice any discernible difference in the HF with the KUBE inline. It may be that my gear is just not good enough to expose anything, but I went back and forth a few times and I'll be damned if I heard any detrimental effects on the mids or highs.

The first difference I did notice is, I have less gain from using the RCA to XLR outputs from the KUBE to my amp. I have to crank the preamp up a bit more, but I thankfully don't have any hiss, buzz or hum. I'm going to live with this setup until I get the parts I need to mod the KUBE for biamping, or I may just make another set of RCA to XLR cables and forget the balanced preamp output altogether.

Now, as far as using the KUBE...and forgive me if I am taking this thread off into the weeds... The whole point of getting one (for me) was to see just how well it could extend the low end. As db pointed out, the KUBE is not "required" for the 105/3, but it's an option if you feel you're missing out on the lower octaves. I can say that it does indeed work as advertised, but I think it may be at the expense of overall SPL and some very obvious strain on the woofers.

A few observations:

- With the LF and HF controls set at the center notch (which I would think would be flat), there was a noticeable bass boost when bypassing and un-bypassing the KUBE, so it seems those knob positions are not flat, or the same as bypass.

- When engaging the KUBE at a decent volume with the LF contour slightly increased and some moderately bass-heavy material, the bottom woofer's movement started to scare me...

- The KUBE seems to not only extend the bass, but also change the tone of the bass, if that makes any sense. I noticed this regardless of whether the LF knob was centered or not. It almost seems like I lose a tiny bit of upper mid-bass as soon as the KUBE is un-bypassed.

Overall, I think the KUBE on the 105/3 is a nice option if you want to get a little more oomph, but only at low to moderate volumes. For "critical" listening or filling the house with music, I would be wary of using it for fear of damage. IMO, the KUBE tries to make the 105/3 woofers do something for which they were not designed.

Unlike the KUBEs that are integral to the design of the speaker, I think the only purpose of the 200 KUBE is bass boost. I had a 200 KUBE at one time and I don't recall if it had a button you can push to remove the boost circuit, but I suspect it may not. The 102 and 107/2 KUBEs I use both have such a button. I use those KUBEs without the added boost.

db