Bi-amping question for Gene

["Rich B"]

Hi Gene,

I have a technical question regarding passive bi-amping that I was hoping you might help me to understand.

Here is my example :

I have a 100W amp that I'm using for bass, mids/highs. I find a 50W amp lying around, and decide to connect it up. So now I'm using 100W just for the bass, and 50W just for the mids/highs.

In this scenario, do I have additional headroom in the 100W amp handling only the bass as I offloaded the mids/highs?

Would there be any benefit at all in this scenario?

Your technical expertise would be most appreciated.

 

[gene]

Well your speakers will typically suck more power in the bass than the mids/highs so it could benefit you. However, you need to make sure the voltage gain of both amps is similar or adjust accordingly, or you won't properly match the levels.

 

["Rich B"]

Hi Gene. Thanks for the reply.

If you had to estimate how much additional headroom I could expect in such a scenario, would you suspect 1 dB extra, less or more?

Truth be told I've asked this question on the Hydrogen Audio forum and they gave me questionable advice. They told me that I could possibly gain less headroom in this scenario.

They also basically told me that passive bi-amping is utterly worthless and that no person could, under any circumstances, hear the difference. No audibility studies... etc.

 

[TLS Guy]

Hi Gene. Thanks for the reply.

If you had to estimate how much additional headroom I could expect in such a scenario, would you suspect 1 dB extra, less or more?

Truth be told I've asked this question on the Hydrogen Audio forum and they gave me questionable advice. They told me that I could possibly gain less headroom in this scenario.

They also basically told me that passive bi-amping is utterly worthless and that no person could, under any circumstances, hear the difference. No audibility studies... etc.

The folks at hydrogen Audio, are basically correct, except for one thing. You won't have less headroom, but what you will gain is insignificant.

However a 150 watt amp would give you more head room than what you propose, as in your case you are very unlikely to use the full power of the 50 watt amp. If you had one 150 watt amp, then you would have extra headroom that you would not have if your 100 watt amp ran out of gas.

 

[Speedskater]

Sometimes on some forums, every discussion quickly turns into an argument.
But over on the DIY forum, they are actually having a discussion on this topic.
I'll look for the link.

 

[Speedskater]

Found the thread, it's rather short.

Dynamic Range of Active Crossovers

http://www.diyaudio.com/forums/multi-way/276102-dynamic-range-active-crossovers.html

 

[Big Jake]

Hmmm... this has gotten my attention.

1). Exactly what is being meant with the term "head room?"

2). A person has a 150W per channel amp. They biamp their front L/R, so instead of 150W per speaker, they are now using 300W per speaker. There is no benefit to that?

 

["Rich B"]

The folks at hydrogen Audio, are basically correct, except for one thing. You won't have less headroom, but what you will gain is insignificant.

However a 150 watt amp would give you more head room than what you propose, as in your case you are very unlikely to use the full power of the 50 watt amp. If you had one 150 watt amp, then you would have extra headroom that you would not have if your 100 watt amp ran out of gas.

So in my example you are saying that the amp handling the bass won't have less headroom due to offloading the mids/highs to the secondary amp.

How much headroom would I gain from doing this? Gene thinks it could be of some benefit, but I'm still waiting to hear an explanation.

 

[Swerd]

1). Exactly what is being meant with the term "head room?"

Head room is a vague term for added dynamic range. It can apply to speakers, amplifiers, or their combined performance.

All speakers have a range of loudness, from the quietest to the loudest, over which they can produce sound without deviating (too much) from the amplified electronic signal. All amps also have a dynamic power range. When a speaker is underpowered by an amp, the amp runs out of ability to deliver power before the speaker runs out of ability to get louder. The amp clips while the speaker can still get louder without distorting or running into thermal compression.

In that sense, a more powerful amp would generate more "head room".

All this is a vague generalization. Limits to dynamic range or head room depends on both the specifics of the amp and the speaker.

It pays to remember that power is a an estimate, not a direct measurement. It is calculated based on direct measurements of voltage and current. Of course, in speakers, current varies with the impedance, which varies with the frequency of the audio signal. It isn't a constant. Power also varies significantly with time. Some sounds, which demand high power, are very short, while others last longer.

How much headroom would I gain from doing this? Gene thinks it could be of some benefit, but I'm still waiting to hear an explanation.

You only gain head room if the original amp lacked enough juice to drive the speakers over their full dynamic range. If the original amp was powerful enough, then you gain nothing by adding more. Again, this is hard to accurately predict, so as a result, most people add more than enough power, just to be sure.

Edit:
Again, this is hard to accurately predict, so as a result, most people (who fret over head room) add more than enough power, just to be sure.

In reality, MOST people don't care.

 

[TLS Guy]

Hmmm... this has gotten my attention.

1). Exactly what is being meant with the term "head room?"

2). A person has a 150W per channel amp. They biamp their front L/R, so instead of 150W per speaker, they are now using 300W per speaker. There is no benefit to that?

1). Is how much power you have in reserve before the clip point on the loudest musical peaks.

2). There is insignificant increase in power, because the HF amp is for most speakers only supplying power above 2.5 KHz or so, where there is minimal demand for power.

The acoustic power split is around 400 Hz. However below 400 Hz is in the region of the transition from half space to full space radiator, for monopole speakers. The precise point is dependent on the width of the cabinet. This is called baffle step compensation. Below that transition power has to be increase 6db. Every 3db requires a doubling of the amp power. So you can see that the power demands below 400 Hz have to be doubled twice for the same acoustic output above 400 Hz or so.

The bottom line is that passive biamping with two 150 watt amps is no way equivalent to powering with one 300 watt amp. The 300 watt amp will gain you 3 db the biamp arrangement will barely register and not be audible.

Biamping is to achieve what can not be done with passive crossovers, and the benefits increase the lower the crossover and offer marked improvement below 500 Hz crossover points.

Bottom line, is that passive biamping will never get you a 3db gain doubling the power available, but doubling the power of single amp will.

 

[TLS Guy]

There is another issue to biamping when using different amps even form the same manufacturer.

You all thought that the red terminal on your amp or receiver always meant positive, didn't you? Wrong! It may or may not be. All you can generally be sure of is the the black is ground and is connected to the ground plane. Nothing says that the output has to be in phase with the input and is often out of phase with it. Don't look in the specs it seldom tells you.

So in addition to setting the gains of the amps identically you have to know the phase response. Is it phase inverting or not? If one amp is inverting and the other is not, then your high pass and low pass sections are now out of phase!

Before I use different amps in ANY bi amp situation, I always check the phase response between the two amps.

You do this by connecting the output of one amp to the X plates and the other to the Y plates of an O scope.

If the amps are on phase you see a nice round circle and if one is phase inverting a straight line.

Bi-amping with different amps is fraught with problems if you don't understand the issues.

 

[gene]

There is another issue to biamping when using different amps even form the same manufacturer.

You all thought that the red terminal on your amp or receiver always meant positive, didn't you? Wrong! It may or may not be. All you can generally be sure of is the the black is ground and is connected to the ground plane. Nothing says that the output has to be in phase with the input and is often out of phase with it. Don't look in the specs it seldom tells you.

So in addition to setting the gains of the amps identically you have to know the phase response. Is it phase inverting or not? If one amp is inverting and the other is not, then your high pass and low pass sections are now out of phase!

Before I use different amps in ANY bi amp situation, I always check the phase response between the two amps.

You do this by connecting the output of one amp to the X plates and the other to the Y plates of an O scope.

If the amps are on phase you see a nice round circle and if one is phase inverting a straight line.

Bi-amping with different amps is fraught with problems if you don't understand the issues.

I don't know of any modern power amplifiers that flip phase from input to output. But I suppose anything is possible, especially in high end audio

 

["Rich B"]

Gene, so just to be clear, in my example, if I used the 100W amp just for the bass, and the 50W amp just for the mids/highs, you DON'T think there would be any additional headroom in the 100W amp handling just bass on account of not handling the mids/highs?

I've been getting conflicting information on this one. Some people tell me - yes, you'll get more headroom as the bass amp is not handling the mids/highs anyway, and others say that no, you won't gain any more headroom at all, and possibly may have worse headroom compared to using a 100W amp non bi-amp.

Your advice would be appreciated.

 

[ski2xblack]

Um, Rich, read all the posts and think about it, slowly.

In the case of using a single 100 watt amp, and assuming a two way speaker, you're powering both the woofers and tweets. Assuming a fairly typical crossover frequency, lets say 2 khz, the tweet may be getting 10% or so of the power. 10% of 100 is 10 watts.

Now you off load the tweeter to the 50 watt amp. The woofs get full use of the 100 watt amp. So you went from ~90 available watts (100 watts minus the tweeter load, whatever it happens to be) to 100 watts. That equates to a small fraction of a decibel, an insignificant difference. You're not magically achieving more power/headroom from the 100 watt amp, just reallocating what is available strictly to one driver rather than both.

It's still a piss poor argument in favor of bi-amping.

 

[slipperybidness]

There is another issue to biamping when using different amps even form the same manufacturer.

You all thought that the red terminal on your amp or receiver always meant positive, didn't you? Wrong! It may or may not be. All you can generally be sure of is the the black is ground and is connected to the ground plane. Nothing says that the output has to be in phase with the input and is often out of phase with it. Don't look in the specs it seldom tells you.

So in addition to setting the gains of the amps identically you have to know the phase response. Is it phase inverting or not? If one amp is inverting and the other is not, then your high pass and low pass sections are now out of phase!

Before I use different amps in ANY bi amp situation, I always check the phase response between the two amps.

You do this by connecting the output of one amp to the X plates and the other to the Y plates of an O scope.

If the amps are on phase you see a nice round circle and if one is phase inverting a straight line.

Bi-amping with different amps is fraught with problems if you don't understand the issues.

X Plate and Y Plate? I'm kind of thinking that modern scopes tend to not have these anymore, I don't think I've ever seen that. I'm assuming these would be on CRT scopes?

You could do the same thing by simply monitoring input/output on dual trace and confirming that there is no phase shift?

 

["Rich B"]

Um, Rich, read all the posts and think about it, slowly.

In the case of using a single 100 watt amp, and assuming a two way speaker, you're powering both the woofers and tweets. Assuming a fairly typical crossover frequency, lets say 2 khz, the tweet may be getting 10% or so of the power. 10% of 100 is 10 watts.

Now you off load the tweeter to the 50 watt amp. The woofs get full use of the 100 watt amp. So you went from ~90 available watts (100 watts minus the tweeter load, whatever it happens to be) to 100 watts. That equates to a small fraction of a decibel, an insignificant difference. You're not magically achieving more power/headroom from the 100 watt amp, just reallocating what is available strictly to one driver rather than both.

It's still a piss poor argument in favor of bi-amping.

Sorry if I've upset you.

What I've been told by Hydrogen Audio is that if the passive crossovers were 100% efficient, I would gain several watts in the bass amp, because the mids/highs, assuming a typical 3-5 kHz crossover would account for a 10-15% of the total power at most.

So that power I would gain, but it would be negligible, less than 0.8 dB. BUT, they say that because the crossovers are not 100% efficient I won't even gain the 10-15%.

They tell me that I'll gain nothing. Is that correct? I came here because I wanted to ask for unbiased advice - the people at Hydrogen are trying to beat me over the head that it won't make any difference in any conditions.

 

[ski2xblack]

Rich, I'm not upset, aside from having to leave this wonderful interaction with you to go to the office.

For all practical purposes (in the real world, when kicking back and enjoying some tunes) the gain is so inconsequential to be considered nothing. Believe me, if you're running your 100 watt amp up to it's limits you have bigger fish to fry than worrying about passive bi-amping. In the outlier situation you're describing, more sensitive speakers, more powerful amps, or both would be in order before trying to get the fraction of a decibel, most likely audibly indistinguishable, difference you may get from passive bi-amping.

Now I'm not unbiased, but I am a practical person. This hobby is about the music, not parsing through all the marketing bullshit and audiophool mysticism that is presented along the way. Passive bi-amping kind of falls into that second realm. Life's too short to worry about this, Rich.

 

[Speedskater]

I don't know of any modern power amplifiers that flip phase from input to output. But I suppose anything is possible, especially in high end audio

I don't know of any either.
But Stereophile does test for absolute polarity of all amps.

 

[gene]

Gene, so just to be clear, in my example, if I used the 100W amp just for the bass, and the 50W amp just for the mids/highs, you DON'T think there would be any additional headroom in the 100W amp handling just bass on account of not handling the mids/highs?

I've been getting conflicting information on this one. Some people tell me - yes, you'll get more headroom as the bass amp is not handling the mids/highs anyway, and others say that no, you won't gain any more headroom at all, and possibly may have worse headroom compared to using a 100W amp non bi-amp.

Your advice would be appreciated.

As others have pointed out, not much.

You can try it if you want but make sure you match the gains. Polarity really shouldn't be an issue. I've never measured an amp that inverts the outputs.

 

[markw]

They tell me that I'll gain nothing. Is that correct? I came here because I wanted to ask for unbiased advice - the people at Hydrogen are trying to beat me over the head that it won't make any difference in any conditions.

You can believe who you want to believe. If you see a significant difference between "nothing" and "inaudible" then, yes, you do gain something, even though you can't hear it. But, like other things in this hobby, people often hear what they what to hear, in spite of the facts.

[edit] I just peeked into the thread at Hydrogen Audio you mentioned. http://www.hydrogenaud.io/forums/index.php?showtopic=109331
You really turned that into a cluster f**k. Is that your intent here? What's interesting is that the first response you get references your exact same thread, with the exact same results at AVS.

You don't seem to believe what anyone tells you.

I smell a troll.