Higher Speaker Impedance ;- Flatter power amp FR? - Home Theater Forum and Systems - HomeTheaterShack.com

 
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post #1 of 8 Old 04-30-16, 06:55 AM Thread Starter
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Higher Speaker Impedance ;- Flatter power amp FR?

Hello all,
I have an rmx1850hd amp and was surprised when using it to replace an old low wattage trio integrated amp that my subs did not appear to dig that low anymore but now seemed to have a stronger midbass emphasis. Although it occurred to me that this would be because I had the bass control dial all the way up on the trio and now the power amp has no such controls, I still did some reading on the FRs of power amps used in HT and I got an impression that a flatter response down to the amp's stated roll off of 5 or 10hz is obtained when the connected sub impedance is increased (although it was not categorically stated) for instance coming from a 2 or 4 ohm to an 8 or 16 ohm connection, thereby giving a better response 20hz and below. Is this really so or am I under a wrong impression?

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post #2 of 8 Old 04-30-16, 09:47 AM
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Re: Higher Speaker Impedance ;- Flatter power amp FR?

In all reality your older amp may very well have been boosting the frequency range in around 80hz as you said you had a bass control cranked on the older amp. You were possibly running into distortion levels and pushing it beyond its capabilities.
With the new amp you are running it flat giving you a much mor even response through its full operating frequency range. Likely getting much deeper into the 20hz area and little to no distortion.

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post #3 of 8 Old 05-01-16, 02:43 AM Thread Starter
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Re: Higher Speaker Impedance ;- Flatter power amp FR?

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In all reality your older amp may very well have been boosting the frequency range in around 80hz as you said you had a bass control cranked on the older amp. You were possibly running into distortion levels and pushing it beyond its capabilities..
Possibly you're right but I believe I can recognize the low rumbling frequencies around 30hz as different from 80hz and that aside, the trio was built by Kenwood in the 80s with capabilities beyond their specs. The subs also are high excursion 12" drivers. Anyway I am more intrigued by the possibility that higher impedance drivers may produce a flatter response with a power amp. If so, could it be because lower impedance speaker connection requires a lot more power to execute a flat response and power is often in short supply either from the transformer or the mains?

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post #4 of 8 Old 05-02-16, 12:06 AM
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Re: Higher Speaker Impedance ;- Flatter power amp FR?

I think you're probably experiencing two separate phenomena simultaneously. One is what Tonyvb mentioned above... the bass knob was giving you a boost and you found the resultant non-linear response pleasing. The second is that you're now using a pro-audio amplifier.

The QSC RMX series amps (which are great amps, btw) very likely do something very common in the pro audio world, which is include a subsonic filter. This is essentially a high-pass filter around 20Hz for the purpose of preventing "unusable" program material from being amplified and reproduced by the speakers. They do this for a number of reasons, mostly involving power consumption and longevity of speakers. It takes huge amounts of power and huge amounts of air displacement to get meaningful response below 20Hz in a live reinforcement setting like a concert hall or a stadium. That means amps are bigger and heavier and so are speakers and cables. All that equates to increased transportation and rigging costs, so they don't generally attempt it.

In short, aside from the flatter response from 20Hz-20KHz the RMX1850 is giving you, it is also probably chopping off everything below that.

There might be some merit to the idea that the higher impedance load is "lighter" on the amplifier, and therefore the amp responds more evenly... I'd have to look into that. My guess is that within normal parameters the differences are minute, but I could be wrong. (Happens all the time)
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post #5 of 8 Old 05-03-16, 05:02 AM Thread Starter
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Re: Higher Speaker Impedance ;- Flatter power amp FR?

No doubt there was a boost and indeed it was very pleasing to me. I guess there lies the relevance of the house curve concept, increasingly boosting through the lowest two audible octaves. It's wonderful for HT. I don't know that a perfectly flat response is better.

In my reading I found that the qsc rmx1850hd has defeatable hpf at 30 and 50hz respectively which when I disabled them left an inherent roll off at 10hz. So tonyvdb was right about it's infrasonic extension capacity. True, some power amps are intrinsically limited below 20hz for instance crown xls series because of their outdoor uses.

If higher impedance speaker connections effectively enhance infrasonic response for the reasons suggested previously, can this have practical applications especially for someone like me that does not have high spl/power needs, just profound low extension?

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post #6 of 8 Old 05-03-16, 10:29 PM
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Re: Higher Speaker Impedance ;- Flatter power amp FR?

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Anyway I am more intrigued by the possibility that higher impedance drivers may produce a flatter response with a power amp.
If the power amp has high output impedance, then the higher impedance speaker has better damping factor: A high output impedance amplifier is not a perfect voltage source anymore and its output will depend on the current delivery. This has the effect to boost the output at the speaker's impedance peak (resonant frequency) relative to the other frequencies. The RMX has very low output impedance and the aforementioned effect should be negligible. I don't know about your older amp...
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post #7 of 8 Old 05-04-16, 02:51 AM Thread Starter
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Re: Higher Speaker Impedance ;- Flatter power amp FR?

I was under the impression that modern amplifier makers all strive towards negligible amp output impedance although I must confess I don't understand the concept of amplifier output impedance. Let's assume all amplifiers under consideration have very low output impedance, in the circumstance of high power demand where the amp power supply is unable to cope at low frequencies, isn't a speaker with higher impedance a better performer here in terms of maintaining a flat FR? Is this not the reason most amplifier specs for FR are listed 20hz to 20khz at 8 ohms? In the end this may be a largely academic point but I desire to be educated on this.

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post #8 of 8 Old 05-04-16, 04:07 AM Thread Starter
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Re: Higher Speaker Impedance ;- Flatter power amp FR?

The practical application of this to me though is that since I don't play my HT at a high volume and the rmx1850hd can give 650 watts rms and my two 12" DVC subs(8 ohms per vc) are rated at 600 watts rms in parallel(4 ohms) , why don't I connect the vcs in series (16 ohms) and presumably get more profound infrasonics albeit at lower spl/wattage that is if my deduction above is correct and of practical value?

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