Biamping with very different amps


I'm considering biamping the HF/MF units of KEF Reference 107/2s with a pair of Cary CAD 572SE monoblocks and the LF with a pair of Parasound JC 1 monoblocks.  Does that seem a crazy idea?

db
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No not at all. That's something I have wanted to try for a long time. That's the idea a bass specific high current amp and a tube amp for the magic imagery and tone. 
Not crazy. But you will typically need a means of adjusting gain as between the separate amps to make them equal. Usually done at the active crossover stage.

If passive biamping, the JC 1 will provide about equal to the maximum output power of the CAD 572SE.
Active biamping is the way to go!

Biamping with very different amps

I’m considering biamping the HF/MF units of KEF Reference 107/2s with a pair of Cary CAD 572SE monoblocks and the LF with a pair of Parasound JC 1 monoblocks. Does that seem a crazy idea?

No not crazy, sweetness of the tubes for mids and highs, and control for the bass with the jc1’s.
If your passive biamping which it sound as what you are doing, both amps would have to be exactly the same gain, which is near impossible unless the same brand of amps.

But this is an easy fix, the louder of the two amps just needs a passive volume control on it’s input to bring it down to the same level as the other amp, once that is set, the main volume is then done by your main preamp or source volume

Easiest way is to get a Schiit Sys $49 passive volume control.
http://www.schiit.com/products/sys

Cheers George
You really need an active crossover to split the signal into high and low pass outputs to drive the bass and mid/high amps! An active crossover will allow you to set the roll-off slopes (not doeable with passive) and levels for the best sound! Been there, done that!
roberjermanYou really need an active crossover to split the signal into high and low pass outputs to drive the bass and mid/high amps! An active crossover will allow you to set the roll-off slopes (not doeable with passive) and levels for the best sound! Been there, done that!

To do that successfully with an active xover, he would also have to completely by-pass the internal xovers inside his Kefs, way too much work.

No, passive by-amping will be fine, as those Kefs have a very well thought out quality passive xover network, with for memory conjugate matching circuitry as well.

Cheers George
Post removed 
Thank you George! You are correct in this matter!
While the KUBE equalizer I believe you are using with the speakers would probably allow you to avoid the gain matching issue some of the others have mentioned, that is the least of the issues that would come into play.

I suspect that many of those who have responded do not realize that the Cary is a 20 watt tube amp (and rated at only 15 watts for 1% distortion), while for the 4 ohm nominal impedance of the speakers the Parasound is rated at 800 watts!!

As Imhififan correctly indicated, if you were to passively biamp using those amplifiers you would in effect be converting the 800 watt amp into not a great deal more than a 20 watt amp. That is because while a passive biamp configuration reduces the amount of current and power each amp must supply, both amps must still output voltages corresponding to the full frequency range of the signal. And therefore if the volume control is turned up high enough to utilize a substantial fraction of the power capability of the 800 watt amp the 20 watt amp would be driven into clipping.

But that’s not all. The speaker’s crossover between the two sections that are accessible via its input terminals (i.e., the crossover between the low frequency driver and the mid/hi drivers) occurs at 150 Hz. Most music has most of its energy above that frequency, most of the time. Which means that a passive biamp configuration would require the 20 watt amp to supply most of the power most of the time, while the 800 watt amp would be doing very little most of the time.

And if you were to use an external active crossover with its crossover point set high enough to avoid asking too much of the 20 watt amp, I suspect you would have to set that frequency somewhere in the treble region. The result would be little or no sound between that frequency and 150 Hz!!!

And all of that is not to mention the potential for loss of coherency due to differences between the intrinsic sonic characters of the amps.

Yes, to use your words it’s a crazy idea IMO. In audio, IMO, simpler is generally likely to be better. And in a case like this especially.

Regards,
-- Al

Thanks Al, I value your opinion.  The current drive train for the 107/2s is Parasound (JC 3), JC 2 BP, 107/2 KUBE, and JC 1.  When I replace the Esoteric SA-60 with an Ayre DX-5 DSD, a Bryston SP3 may replace the JC 2 BP so HDMI can be used for SACD and BD surround.

I've used a CAD 572 SE to drive LS50s with surprisingly good results, so that stirred my imagination without thinking it through.  The sensitivity of the LS50s is 5 dB less than that of the 107/2s, so I may try setting up a switch between the amps, but the 107/2s do sound magnificent driven by the JC 1s -- a JC 1 also drives the Reference 204/2C center.

db
george, the suggestion of using a volume control device such as Schiit Sys will not work in this particular situation since that will reduce the overall volume going into the preamp and not change the gain of the amplifier. Al has accurately and completely covered all the pitfalls of this proposal arrangement. If the amps were closely matched in their power/voltage outputs, some of the other concerns could have been overcome or tolerated at best.
Thank you, Kalali. Note though that George suggested placing the passive volume control device at the input of the amp having higher gain, not at the input of the preamp.

In any event, though, the KUBE would probably make that a moot point, while leaving the other issues I cited unresolved.

Best regards,
-- Al
george, the suggestion of using a volume control device such as Schiit Sys will not work in this particular situation since that will reduce the overall volume going into the preamp and not change the gain of the amplifier.

Read again, the Shiit is to go between the active preamp (or kube if being used) and only the power amp with the highest gain, so to just reduce it to the same gain as the other power amp. Then the active preamp will be master volume.

Cheers George


n any event, though, the KUBE would probably make that a moot point, while leaving the other issues I cited unresolved.
If a Kube is used then it would drive the (lowest gain) poweramp and also the Schiit, which drives the (highest gain) poweramp, then the main volume control duties are up to the active preamp.

Cheers George
Hi George,

The KUBE would allow the gains of the two amps to be matched without using a separate passive volume control. See section 3.6 of the following document:

http://www.kef.com/uploads/files/en/museum_pdf/90s/Reference_Series_Model_107_2_Installation_Instruc...

Best regards,
-- Al
Hi Al, not on the Kube I once had used, looks like the op’s one is different and has high and low gain controls, if so and he has the range to match the amps, then yes no need to use a Schiit passive then. If not get the Schiit is only $49. That way he can also do it without the Kube in, as it’s full of opamps TL071/2 from memory, and they sound very soso in the mids and highs.
http://www.canuckaudiomart.com/details/649052620-kef_kube_1042/

Cheers George

I stand corrected. I misread george's post.
The 107/2 KUBE has both HF and LF level controls.  I've tried the 107/2s with and without the KUBE, and prefer their sound with the KUBE.  As I understand it, the 107 and 107/2 KUBEs differ substantially.  The 107/2s remain among the best sounding speakers I've heard, and driven by the JC 1s do a credible simulation of the sound I hear at pipe organ recitals and jazz clubs.  I've abandoned the crazy idea.

db 
I’ve abandoned the crazy idea.

As I said It’s not crazy, you said you have Kube level controls over both highs and lows .

The Kube drives all your amps JC1’s on the bass and Cary’s on the mid/highs.

You use your JC 2 BP as your main preamp to drive the Kube.

Cheers George