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Passive bi-amping


wgscott

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I am currently passive bi-amping, because I lost my nerve with respect to sawing out my speakers' crossovers and my lack of confidence with being able to implement digital crossovers that would be superior.

 

Currently, I feed my speakers with two DIY class D amps, which in turn are fed from the DAC and pre-amp stages of my Nova. The D amps are 120 WPC into 8 ohms. The Nova amp is currently being wasted, but it would be about 55 WPC into 8 ohms I think.

 

Would I benefit from running the mid/treble with my 55 WPC Nova (everything above the 350 Hz passive crossover) and give each woofer a bridged class D amp of its own?

 

Would Dirac DSP be capable of cleaning up mismatches in gain and phase? Would this be an improvement, especially with the treble (as class D can roll off a bit at the top)?

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I am currently passive bi-amping, because I lost my nerve with respect to sawing out my speakers' crossovers and my lack of confidence with being able to implement digital crossovers that would be superior.

 

Currently, I feed my speakers with two DIY class D amps, which in turn are fed from the DAC and pre-amp stages of my Nova. The D amps are 120 WPC into 8 ohms. The Nova amp is currently being wasted, but it would be about 55 WPC into 8 ohms I think.

 

Would I benefit from running the mid/treble with my 55 WPC Nova (everything above the 350 Hz passive crossover) and give each woofer a bridged class D amp of its own?

 

Would Dirac DSP be capable of cleaning up mismatches in gain and phase? Would this be an improvement, especially with the treble (as class D can roll off a bit at the top)?

 

I think that if you are going to bi-amp, using passive, low-level crossovers is definitely the way to go. Active crossovers just add more gain stages to your system, and generally speaking, the less electronics, the better the sound. A DSP might give one more control, but on the other hand, it's difficult to use that level of control without some serious measuring equipment - equipment that most of us don't have, and a three-way crossover such as you are asking about is difficult to do correctly without getting into level-matching problems, at least.

 

As for using the Nova amp for your tweeters, I suspect that this would depend on whether or not the top-end of the Nova sounds better than the top-end performance of your class D amps. It might be an improvement, then again, it might not. Seems like an over complication to me, but if you feel that it would benefit you, perhaps a little experimentation wouldn't hurt - just make sure you can put things back the way they were when you finish. Be like a doctor, and take the attitude "do no harm" to the music. :)

George

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My experience of passive bi-amping is that it was a waste of a good amplifier and cable. I don't necessarily agree with your statement that digital crossovers would be superior. I am a big fan of Troels Gravesen's speaker designs and the different approaches he takes to solve driver design and speaker design problems. If you haven't seen his site it is well worth a look, in particular:

ElectronicCrossovers

I do not necessarily subscribe to the notion that "less is more". Those who settle with a "coil and a cap" often pay a price on certain performance parameters as no drivers - to my knowledge - are capable of performing well from such simple measures unless we never play very loud.

Any loudspeaker crossover, being active or passive, divides the frequency band between drivers with regard to amplitude and phase and ensures a balanced presentation of bass, midrange and treble. In principle as simple as that.

If you want to use an electronic crossover (analog or digital) for any of my constructions, you need to replicate the transfer function of my passive crossover with regard to amplitude and phase, at least if you want the speaker to sound the same as I made it.

 

A good article on the benefits of active bi-amping (the ESP site has many interesting articles if you haven't visited it before):

BiAmp (Bi-Amplification - Not Quite Magic, But Close) - Part 1

 

 

 

 

 

 

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I'd say give it a go, though because your two amplifiers are different designs the results may not be so good.

 

Generally I've found B&W speakers work well bi-amped - a pair of Rotel 1070s were better than a single 1070 (with B&W CDM7NT) - though one superior amp is usually better - a Rotel 1080 was better than the 1070s.

 

As for active vs passive crossovers... I personally feel a active crossover can give superior results, but most people do not have the skills / knowledge to implement an active crossover to replace their existing passive ones so trying to replace the crossovers in your CM7s is likely to result in a worse result.

 

Eloise

Eloise

---

...in my opinion / experience...

While I agree "Everything may matter" working out what actually affects the sound is a trickier thing.

And I agree "Trust your ears" but equally don't allow them to fool you - trust them with a bit of skepticism.

keep your mind open... But mind your brain doesn't fall out.

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I would probably give it a go like Eloise says with the CM7 just to try it and heed George's advice to make sure you can replace it as is. It is also worth knowing the following from the TG link I posted which refers to his Jenzen D. Your speakers will have different problems that the designers will have tried to overcome with their passive crossover design:

As can be seen, the tweeter crossover transfer function looks pretty much like a text-book crossover. The midrange not so much with some equalisation of upper mid and a sharp notch-filter at 5 kHz. The active crossover needs to do the same thing and if not digital, not so easy to do actually.

The bass transfer function display some equalisation around 50-100 Hz as we have an elevated response around 80 Hz due to the impedance peaks created by the TL/vented box. This must be dealt with quite differently by an active crossover as the active crossover sees the linear impedance of the following amplifier and not the one created by the box tuning, thus probably no equalisation is needed. I would tune the active bass crossover section by doing near-field measurements of the bass driver.

The long and the short of it is that in order to implement the transfer function of the passive crossover we need to be able to measure what happens, and most people do not have measuring equipment good enough to do so.

 

 

 

 

 

 

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I think that if you are going to bi-amp, using passive, low-level crossovers is definitely the way to go. Active crossovers just add more gain stages to your system, and generally speaking, the less electronics, the better the sound. A DSP might give one more control, but on the other hand, it's difficult to use that level of control without some serious measuring equipment - equipment that most of us don't have, and a three-way crossover such as you are asking about is difficult to do correctly without getting into level-matching problems, at least.

 

As for using the Nova amp for your tweeters, I suspect that this would depend on whether or not the top-end of the Nova sounds better than the top-end performance of your class D amps. It might be an improvement, then again, it might not. Seems like an over complication to me, but if you feel that it would benefit you, perhaps a little experimentation wouldn't hurt - just make sure you can put things back the way they were when you finish. Be like a doctor, and take the attitude "do no harm" to the music. :)

 

If one is an expert, a well designed active crossover is a good idea, however most do-it-yourself bi-ampers don't understand the phase problems active crossovers can introduce. Gentle-slope passive crossovers will keep most people out of trouble, active crossovers? Not so much.

George

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Hi Bill,

 

I am not a big fan of bi-amping to begin with, and especially not with amps of different make/model. For example, the input-sensitivity of the WPC Nova most likely differs from your Class D amps, so you can expect unwanted results. This can be (partly?) resolved by DSP, but you will need to be able to measure it.

 

One option you might want to try is bridging your Class D amps and set them up as a dual-mono configuration.

 

While I am a big fan of digital (or active analogue) crossover solutions versus passive crossovers, it is hard to implement yourself for reasons earlier explained by George. Digital crossover solutions also require a second DAC (if supported by the software in the first place, that is) or a multi-channel DAC.

 

In all honesty I cannot say if an active analogue or a digital crossover solution will outperform an upgrade of your loudspeakers at the same cost.

 

Kind regards,

Peter

“We are the Audiodrones. Lower your skepticism and surrender your wallets. We will add your cash and savings to our own. Your mindset will adapt to service us. Resistance is futile.” - (Quote from Star Trek: The Audiophile Generation)

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Regarding active biamping the current speakers -- that is why I didn't do it.

 

I had hesitated to mix amps too, and "vertically" bi-amped to minimize the amp matching issues. i just hate having that Nova amp stage and not using it.

 

So you suggest bridging the two amp boards and running dual mono? Would it benefit from giving each amp board its own power supply (which I could do fairly cheaply but would need an external box for them, which I guess could be an advantage), and then bi-wire them (the wire etc is already in place, so I might as well?

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If one is an expert, a well designed active crossover is a good idea, however most do-it-yourself bi-ampers don't understand the phase problems active crossovers can introduce. Gentle-slope passive crossovers will keep most people out of trouble, active crossovers? Not so much.

 

That's why I stopped. I was (a) unconvinced what I would do would result in an improvement, even if I knew what I was doing, and (b) I know that I don't know what I am doing, and © I don't have the measuring equipment either.

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Regarding active biamping the current speakers -- that is why I didn't do it.

 

I had hesitated to mix amps too, and "vertically" bi-amped to minimize the amp matching issues. i just hate having that Nova amp stage and not using it.

 

So you suggest bridging the two amp boards and running dual mono? Would it benefit from giving each amp board its own power supply (which I could do fairly cheaply but would need an external box for them, which I guess could be an advantage), and then bi-wire them (the wire etc is already in place, so I might as well?

 

Hi Bill,

 

I would not say bi-wiring has special advantages, but if it is already in place, go for it :)

 

As far as adding a second PSU... Could be beneficial, but I would do a test with the bridged amps first and listen if you like it. Can you post the specs of the amp- and PSU-modules?

 

regards,

Peter

“We are the Audiodrones. Lower your skepticism and surrender your wallets. We will add your cash and savings to our own. Your mindset will adapt to service us. Resistance is futile.” - (Quote from Star Trek: The Audiophile Generation)

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I have two of these:

 

Class D Audio SDS Amplifier

 

One of these:

 

Heavy Duty DC Power Supply

 

and the accompanying toroidal transformer, all of which are in this kit:

 

SDS-254 Kit - Amplifier Kits

 

I have both amp boards going to a single power supply, so for $65 I could add in a second. It says the amps are 500W in bridged mode, so I was kind of scared off doing it that way.

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I have two of these:

 

Class D Audio SDS Amplifier

 

One of these:

 

Heavy Duty DC Power Supply

 

and the accompanying toroidal transformer, all of which are in this kit:

 

SDS-254 Kit - Amplifier Kits

 

I have both amp boards going to a single power supply, so for $65 I could add in a second. It says the amps are 500W in bridged mode, so I was kind of scared off doing it that way.

 

Hi Bill,

 

In principal the single PSU should do the trick regardless of the amp being bridged or not. However, adding another PSU-board certainly is not going to hurt as it adds 28,200 uF capacitance which could result in better dynamics. I would probably consider adding a second transformer as well. Having said that, I suggest you contact Class D Audio first to see what they recommend.

 

But I would first do a test bridging the amps with what you have. If you feel it is an improvement, you can upgrade the PSU.

 

Kind regards,

Peter

“We are the Audiodrones. Lower your skepticism and surrender your wallets. We will add your cash and savings to our own. Your mindset will adapt to service us. Resistance is futile.” - (Quote from Star Trek: The Audiophile Generation)

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Hi Bill,

 

Something I would also ask Class D Audio is if the amp in bridged mode has a problem with significantly lower than 8 Ohm loads. Normally Class D type amps are able to do that without problems, but it is worth asking...

 

Peter

“We are the Audiodrones. Lower your skepticism and surrender your wallets. We will add your cash and savings to our own. Your mindset will adapt to service us. Resistance is futile.” - (Quote from Star Trek: The Audiophile Generation)

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Did you look into Hypex Ncore amp boards? I have a Kharma MP150 amp for my woofers which is based on the preceding Hypex design and using it with a Devialet for digital XO and amp for mids/highs and I am completely convinced that a digital XO is much better than any other solution.

Passive bi-amping does not eliminate the passive XO parts in your speakers and except for power gain (which may be only of minor importance for real music) it has not much impact on sound.

Just eliminating crossover parts from an existing speaker and replacing it with a digital solution may be not very successful or at least painful. I can only recommend to think about designing speakers yourself (even if you do not like to make boxes - you could possibly find some or get them made). You will see that any speaker design decision will have more impact on sound quality than what is normally discussed in CA.

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Hi Bill,

 

I would not say bi-wiring has special advantages, but if it is already in place, go for it :)

 

As far as adding a second PSU... Could be beneficial, but I would do a test with the bridged amps first and listen if you like it. Can you post the specs of the amp- and PSU-modules?

 

regards,

Peter

 

 

I agree with that. Electrically, there should be no advantage to bi-wiring. Bi-amping, yes (if it's done right). As far as the signal is concerned there is no difference between the two wires being summed at the amp end or at the speaker end. It's still the same signal traveling down two separate lengths of cabling. The only advantage that I can see to bi-wiring is if the speaker cable being used is too small for the run and this depends on the power needs of the speaker, in which case two 8 ft runs of 14 AWG wire to the speaker would probably be sufficient to any speaker requirement or any amplifier output where-as one 14 AWG run might not be sufficient. I have frankly never heard any difference between bi-wiring and single-wiring runs, but I know people who swear its an audible improvement always.

George

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I realize bi-wiring won't do anything (I have 12 gauge wires btw), and passive bi-amping is little better than passive bi-wiring. I just never managed to take the next step to active bi-amping.

 

Given that, what I am asking now is simply whether I am better off with:

 

(a) Passive bi-amping with four 120 W channels.

 

(b) Passive bi-wiring with two bridged 500 W channels.

 

The speakers are 8 ohm nominally, rated for 50 to 150 W.

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That's too bad because the "next step" (active XO biamp) is what is really needed to lift veils and have musicians walk out of your speakers. Because steep XO sounds SOOOOO GOOOOD.

 

To answer your question, you are not "better off" if you don't hear any difference. And neither setup will make a difference unless you drive to amps so hard as to clip them. And you won't clip the amps because they have enough volts (even in non-bridged mode) to drive your speakers to very high levels.

 

But if I were in your situation, I would run the Nova to the tweeters and the class D bridged to the woofers. (assuming you can balance the gain) That maximizes the available power, which you will never use, but will appreciate because you are male.

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That's too bad because the "next step" (active XO biamp) is what is really needed to lift veils and have musicians walk out of your speakers. Because steep XO sounds SOOOOO GOOOOD.

With the caveat that you need to know what you are doing to go to the "next step"...

Eloise

---

...in my opinion / experience...

While I agree "Everything may matter" working out what actually affects the sound is a trickier thing.

And I agree "Trust your ears" but equally don't allow them to fool you - trust them with a bit of skepticism.

keep your mind open... But mind your brain doesn't fall out.

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I realize bi-wiring won't do anything (I have 12 gauge wires btw), and passive bi-amping is little better than passive bi-wiring. I just never managed to take the next step to active bi-amping.

 

Given that, what I am asking now is simply whether I am better off with:

 

(a) Passive bi-amping with four 120 W channels.

 

(b) Passive bi-wiring with two bridged 500 W channels.

 

The speakers are 8 ohm nominally, rated for 50 to 150 W.

 

If I'm recalling everything correctly, the bi-amping would involve using two different amps on the different sets of drivers. This is IMHO a Bad Idea even *if* you had really good measurement equipment to "fix" any problems with DSP. Since you don't, it's (again IMHO) a Really Bad Idea. So of the two choices you've presented above, I would pick (b). (Toroids, BTW, are more notorious for hum than, e.g., R-core, in case you were going to be doing more PSU DIY.)

 

Re crossovers, a couple of comments have pointed out a mistake that's very easy to make: ignoring phase effects in pursuit of a particular frequency response. If you're curious about trying the active crossover route one day, you need to learn about *all* of the sonic impacts of crossovers. In particular, one should not expect to plug Crossover B into speakers/drivers built with Crossover A in mind and automatically hear an improvement, regardless of whether B is active and A was passive. Playing around with crossovers is best done as part of the speaker *design* process, when you have a chance to put together all the different variables in an optimal way if you know enough.

One never knows, do one? - Fats Waller

The fairest thing we can experience is the mysterious. It is the fundamental emotion which stands at the cradle of true art and true science. - Einstein

Computer, Audirvana -> optical to EtherREGEN -> microRendu -> ISO Regen -> Pro-Ject Pre Box S2 DAC -> Spectral DMC-12 & DMA-150 -> Vandersteen 3A Signature.

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On a side-note, most of the digital cross-over solutions also include phase-correction on a per-driver basis which, in my opinion, is a second advantage of using a digital cross-over: a near phase-correct loudspeaker.

 

With manipulation in the digital domain one can do things that are very hard (if not impossible) to do without major drawbacks in the analogue domain, such as very steep filter-slopes, and seamless phase-correction. I must say that to me this all sounds the way to go... Wish I had the money for it though :)

“We are the Audiodrones. Lower your skepticism and surrender your wallets. We will add your cash and savings to our own. Your mindset will adapt to service us. Resistance is futile.” - (Quote from Star Trek: The Audiophile Generation)

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  • 6 months later...
and except for power gain (which may be only of minor importance for real music) it has not much impact on sound.

 

Actually, passive bi-amping doesn't even increase the power of the system because the weaker amp is still amplifying a full bandwidth signal, and the stronger amp's gain must be matched to the weaker amp. This is an expensive set up with no benefit, i.m.o. (unless one amp happens to excel at bass and the other excels at treble, but neither excels at both. Then you might want to take advantage of their strengths.)

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Electrically, there should be no advantage to bi-wiring.

 

Biwiring will work unless your cable has zero resistance. The resistance of the cable will cause interference between the drivers, as they interact with each other due to the 'floating voltage' of the junction at the speaker.

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