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Lower Driver Impedamce |
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Elliot Thompson 
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Joined: 02 April 2004 Location: United States Status: Offline Points: 5175 |
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 Posted: 10 June 2019 at 2:44am |
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A few years ago, I made a chart based on voltage drop comparing 8 ohms vs 4 ohms in addition to 4 ohms vs 2 ohms on a few amplifiers. The results were very interesting. Best Regards,
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Elliot Thompson
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Tonskulus
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Joined: 15 September 2017 Location: Finland Status: Offline Points: 425 |
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Posted: 10 June 2019 at 6:00am |
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did you mean 8 and 4ohm bridged vs. 4 and 2ohm stereo or what?
btw, there has been amps with L and R output stage modules having reversed phase. This configuration gives more stable current consumption for power supply or something.. Anyway, I have measured about dozen or so amplifiers with pure sine wave using big dummy loads. Any standard (unregulated supply) amp will suffer from mains voltage drop. As example, QSC mx3000a gave barely 2000Wrms bridged 8ohm @ 221volts mains under full load. More so, some amplifiers claims rms power here and there but its not continuous sine wave power. But its surprising that even some cheap amplifiers like T.Amp (and some behringers) gives what they claim, on sine wave! Skytecs and similar toy amplifiers.. different story. There is still one amplifier I would like to explore and measure: Peavey CS4080hz. 2U size, single toroidal transformer, over all quite light weight. 2 x 2000Wrms! How did they do it? There has to be some magic in transformer design, special quality core material and heavy windings or what.. Amplifier is standard class G/H. Claimed: 2 x 2040Watts @ 1kHz /4ohm/0.05% both channels, 2450Watts / 4ohm single channel loaded.
Edited by Tonskulus - 10 June 2019 at 6:22am |
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Elliot Thompson
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Joined: 02 April 2004 Location: United States Status: Offline Points: 5175 |
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Posted: 10 June 2019 at 11:05am |
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By voltage drop, I mean the ratio of the wattage rating under the given load. For example; Peavey CS 800X
240 watts per channel @ 8 ohms, 400 watts per channel @ 4 ohms 400 watts per channel @ 4 ohms, 600 watts per channel @ 2 ohms
There is a 3.8 volt drop comparing 8 ohms versus 4 ohms and 5.3 volt drop comparing 4 ohms versus 2 ohms.
In the case of the QSC MX 3000a...
800 watts per channel @ 8 ohms, 1200 watts per channel @ 4 ohms 1200 watts per channel @ 4 ohms, 1600 watts per channel @ 2 ohms
There is a 10.5 volt drop comparing 8 ohms versus 4 ohms and 20.3 volt drop comparing 4 ohms versus 2 ohms.
It is quite obvious that the old Peavey CS 800X has a more robust power supply than the old QSC MX 3000a based on the voltage loss moving from 8 ohms to 4 ohms and finally 2 ohms per channel. However from a wattage perspective, one would not assume such a loss is taking place.
We also need to understand that all amplifiers today are rated at 1/8 of a power. The old QSC MX 3000a was rated 1/3 of the power for 8 and 4 ohms per channel only. At 2 ohms per channel it was rated 1/8 of power. QSC also rates their amplifiers at 1% THD. So if you are concerned about output ratings under 1% THD, you will find some amplifiers will not meet their advertised wattage specifications until the THD approaches 1% whereas other amplifiers will exceed their advertised rating @ 1% THD.
The numbers game are far greater than one might imagine. This is why some just bench test the amplifier with a dummy load to know where the amplifier stands under the given scenario.
Best Regards,
Edited by Elliot Thompson - 10 June 2019 at 11:09am |
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Elliot Thompson
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toastyghost
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Joined: 09 January 2007 Location: Manchester Status: Offline Points: 10919 |
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Posted: 10 June 2019 at 11:57am |
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“1/8th of a power”?!
What are you actually on about? Sorry but yet another untrue sweeping statement. If you’re trying to refer to the crest factor and type of drive signal, then this varies depending on the standard chosen by the manufacturer - which is documented, if you care to look. As for examples to prove you wrong - Linea 44M20 and XTA APA just from the top of my head. More importantly, what kind of input signal are you driving the amps with when you use them? Is it ever actually non-stop RMS sine waves? |
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snowflake
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Joined: 29 December 2004 Location: Bristol Status: Offline Points: 3122 |
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Posted: 10 June 2019 at 12:40pm |
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I guess he means 1/8th duty cycle?
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Tonskulus
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Posted: 10 June 2019 at 1:12pm |
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Well, I do bench tests for all my amps using pure sine wave. It's just one way to measure how much stress amplifier can withstand and how much pure power it gives.
Manufacturer specs are usually less comparable than my own bench tests which are pretty much.
Yes, typical music material is not 100% sine wave but some D&B, dubstep, reggae etc has pretty long , strong sustaining bass lines which might be quite close to a sine wave "power". This kind of music indeed will be more stressful for amplifier compared to rprettyock and pop - IF pushed up to the limits. But if I had to choose amplifier beetween (thinking that I need 2x1kW of amp power) 2x1kW rms / 2x1.5kW peak and 2x1kW rms / 2 x 4kW peak - for sure I would take the latter one because it has much more head room for dynamics/peaks. anyway, for me, RMS power means continuous power amplifier can give no matter how long. Edited by Tonskulus - 10 June 2019 at 1:14pm |
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Elliot Thompson
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Joined: 02 April 2004 Location: United States Status: Offline Points: 5175 |
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Posted: 10 June 2019 at 1:22pm |
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There is a difference for those who focus on reinforcing pre recorded music and those reinforce bands. Off the top of my head Iration Steppas – Jah is my Protector offers a continuous sine wave ranging from 30 Hz – 40 Hz for nearly 5 minutes.
Hypothetically speaking. Say you have a single 8 ohm nominal woofer in an enclosure that offers an impedance dip of 5.6 ohms @ 34 Hz. You connect four boxes utilising the same 8 ohm woofer per channel and play the above track. The amplifier will see a 1.4 ohm load per channel @ 34 Hertz for nearly 5 minutes reproducing the above track. The amplifier will be heavily stressed and take necessary measures to protect itself.
I cannot answer for anyone else but, I do not want my amplifiers to premature clip, cut back their output power and/or ramp into protection mode at my events. This is why, I do not use 2 ohms per channel.
This conversation is circled around impedance of a loudspeaker and how the amplifier will react under low impedance loads. So it is not unusual to look at volts, current, ohms and duty cycles.
XTA's reputation is based on their dsp so why are you bringing up their amplifiers which is not a popular conversation on Speakerplans?
Best Regards,
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Elliot Thompson
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Elliot Thompson
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Posted: 10 June 2019 at 1:25pm |
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Absolutely. Best Regards, |
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Elliot Thompson
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snowflake
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Posted: 10 June 2019 at 3:02pm |
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just listened to it - sounds like significant peak to average ratio and duty cycle of about 50%. There is some 'music' with long sinewaves for basslines - most often amateurly produced dubstep - it sounds shit and destroys speakers.
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Timebomb
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Joined: 11 October 2004 Location: Lancaster Status: Offline Points: 2716 |
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Posted: 10 June 2019 at 4:34pm |
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Low passed at 100Hz 24dB slope, then normalised to 0dB, then hard limited at -3dB and normalised to 0dB again to simulate some clipping, and it still has about 5dB crest factor in the heavy section. Also Elliot, once you put this low crest factor signal into your drivers at full power for just a few seconds your 1.4 ohm load will increase dramatically as the coil heats up, it wont take long before RE has doubled. The ability to drive sine waves on the edge of clip into high power resistive loads is just not necessary... |
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James Secker facebook.com/soundgearuk
James@soundgear.co.uk www.soundgear.co.uk |
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gen0me
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Joined: 20 February 2016 Location: UK Status: Offline Points: 999 |
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Posted: 10 June 2019 at 7:28pm |
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4ohm bridged vs 2ohm bridged. In this case of 2 4ohm drivers vs 2 8ohm drivers. Or you can compare in specs on single channel but it wont be the real comparison as difference of power is due to the voltage drop on power supply. So with one channel off, power supply should always be well oversized as is build for both channels.
It sounds awesome
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Tonskulus
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Posted: 10 June 2019 at 7:49pm |
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Well, there are many amps having full "dual mono" configuration, so both channels has its own power supply as example QSC MX3000a etc.. But yes most todays "basic" amplifiers has very underrated power supply. They will suffer under heavy load conditions no matter if its standard transformer or switch mode psu. |
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gen0me wrote: