Bi amp pros out there I could use some help! First time Bi Amping...

My perception over the years is that many members here have achieved good results with horizontal passive biamp configurations. I recognize that in general a vertical passive biamp configuration, using identical amps, is likely to be preferable from a sonic standpoint. And that inserting an active crossover ahead of the amps can **potentially** provide added benefits, while at the same time creating opportunity for error that would compromise the results. And that removing the internal crossover of the speakers (if practical) can **potentially** provide further added benefits, while also creating greatly increased opportunity for error that would lead to poor results. And that in many and probably most cases a well-chosen single-amp approach can provide better results than all of the above, while at the same time minimizing opportunity for error.

In this case the OP **already has the amps,** and as I explained earlier the configuration he described in his original post would avoid the various technical pitfalls that can arise when passively biamping with different amps. Others have recommended a KISS approach. It seems to me that under the circumstances biamping as the OP described in his original post would in fact be a KISS approach, as George also indicated. Which as I say corresponds to an approach that has been successfully used by more than a few members here.

Regards,

-- Al

Looking at some rear panel photos of the Hsu ULS-15 it appears that it may have a two-prong IEC receptacle for its power cord. If so I don’t see any issues regarding the presence of the subs in the system.

However if they have a three-prong receptacle, and a three-prong power plug, I would suggest that you connect their speaker-level inputs to either the output terminals of the Pass amp or to the mid/hi terminals of the Dunlavys. Connecting the negative speaker-level input terminal of a powered sub that has an AC safety ground connection to the negative output terminal of a fully balanced amp (such as the BAT), which as a fully balanced amp has a full amplitude signal on that terminal, may cause problems and conceivably even damage depending on the impedance through which circuit ground and AC safety ground are interconnected within the particular amp and sub. And of course that would also apply if the connection is made to a speaker terminal to which the negative output terminal of the balanced amp is connected.

In the biamp configuration you have described the Pass amp won’t be supplying a great deal of current or power at bass frequencies, of course, but since it will be approximately gain matched with the BAT amp the subs will receive essentially the same full-range signal voltage and hence similar bass information regardless of which amp they are connected to.

Regards,

-- Al

And because there’s no way both amps will be equal in gain, you’ll have to put a passive volume control at the input of the loudest amp so you can lower it’s volume to be identical to the other amp.

Hi George,

See item 1 in my post earlier in the thread.  As well as the statement earlier in my post that I'm assuming the specs are accurate.

Best regards,

-- Al

@Haywood310 After looking at the descriptions, specs, and/or manuals for the equipment you’ve listed, if (a)you are satisfied with the sound you are presently getting with the configuration you have described, and (b)assuming the specs on the equipment are reasonably accurate, and (c)the lengths of the cables between the preamp and the amps are not unusually long (for example, longer than 15 feet or so), I’m going to offer a radical suggestion for you to consider: A good course of action may very well be to do nothing :-) Just enjoy the configuration you have described.

It appears to me that the present configuration is fine with respect to all of the major technical pitfalls that can be encountered with a passive biamp configuration:

1) Gain matching: The BAT amp has a specified gain of 25 db. By providing it with a single-ended input, while there is no change in its gain per se (i.e., the relation between its output voltage and its input voltage), overall system gain is 6 db less than if you were providing it with a balanced pair of signals. In effect that reduces its gain to 19 db. While the gain spec provided in the manual for the Pass amp appears to have a typo (it says that its gain is "20 dB balanced, 20 or 26 dB balanced") after reading the text I’m fairly certain that its balanced input provides a gain of 20 db. So you only have a 1 db gain difference between the two signal paths. Room effects most likely make a good deal more of a difference than that.

And if you were to split the XLR outputs of the preamp and drive both amps with a balanced pair of signals you would be **introducing** a significant gain imbalance, that would require insertion of a suitable attenuator between the preamp and the BAT amp.

2) Impedance compatibility: As is often the case, the signal provided by the preamp to its RCA output for each channel is the same signal that is provided to pin 2 of the XLR connector for the corresponding channel. So that signal will be loaded by the input impedances of both amps. But the numbers are such that that is not a problem in this case.

3) Upper treble rolloff that in some cases may be caused by the interaction of preamp output impedance and the combined capacitances of the cables to both amps: Again, not a problem in this case assuming the cables are not unusually long.

4) Inability to utilize most of the power capability of the higher powered amp without clipping the lower powered amp: In this case the roughly 2:1 disparity in amplifier power capability **might** result in not being able to turn the volume control high enough to utilize perhaps 2 db or so of the power capability of the BAT amp, without driving the Pass amp into clipping, but that’s probably not a major issue. If you were passively biamping say a 400 watt amp with a 50 watt amp I would not say that.

5) Degradation of the common mode noise rejection that can be provided by the amp receiving the balanced signals, due to the differing load impedances presented to the two signals in the balanced signal pair: The output and input impedance numbers in this case suggest that is unlikely to be an issue.

Others have suggested changing the equipment. That’s of course something that can be considered, now or in the future, but if you are happy with the present equipment in the present configuration it seems to me that doing nothing is also a reasonable course of action.

Regards,

-- Al