Calling all Polkies:Official Polk thread - Page 475

So I've been reading a searching for suitable 750-1000 watts Class AB amps with low THD and high SNR. I've been looking at a lot of the Class D amps on the market and have realized that there are literally hundreds of low cost Class D pro-amps out there. The one thing that leads me to doubt, is the relatively high THD and "noise". I looked at the specs for a RMX 4050HD from QSC and have noticed that the higher fidelity and higher quality Class Ds are fairly expensive as well ($1,600.00 and up). Some are over $2,200.00.

My Onkyo NR3009 has a rated RCA pre-out of 1V/470 ohms with a maximum output of 5.5V/470 ohms. Most of the Pro amps have an input sensitivity of 1.2 - 1.4 V at around 10K ohms unbalanced. This is problematic to say the least. I know that the XPA-1 is a beast and is rated for 500W RMS and will power my speakers to no end., however, two of these bad boys will cost me ~$2,000. I have also been considering a second XPA-2 and running them in bridged mode. That should give me over 500W RMS per speaker.

Are there any high quality, high power amps out there (preferably Class AB) that won't break my budget. I can't find any. Just curious.

Advice/suggestions are welcome folks.

Stand pat. Not that you might not prefer another amp to the Emo, but the 2-3 db gained in headroom you won't notice due to the damage you are doing to your hearing .

Climber, I see 0.01 THD for that RMX amp which is pretty low to my understanding. That should give a good sound.

Also, the preout volts seem high enough for the amp. The pro amp needs 1.4V input to produce the max amount of watts to the speakers. If the 3009 can put out 5.5V, then that should be more than enough (even with resistance taken into consideration(Ohms)). I think it puts out 5.5V when the volume knob is turned all the way up.

I too am looking at buying a pro amp this week. Another forum member said that the 3009 should not have a problem with the QSC GX5 amp that I'm looking at buying.

What did you say... I can't hear you for the ringing in my ears. LOL I have Tinnitus anyway. All those years around aircraft and life fire weapons systems. The 240G MG (7.62mm) will get rid of your hearing for hours if you are next to the muzzle when they rip off a burst.

Thanks for the input guys.

Let us know how the Pro amp goes when you get it hooked up freeyayo50.

I just contacted Emo and their tech support said I can Bridge the XPA-2 with nominal 8 ohm speakers. I know amps see less impedance when bridged and I am just cautious because all "nominal" ratings are just that. My speakers can probably dip as low as 4 - 5- ohms at certain freqs. It's just a thought anyway. I won't be upgrading for quite some time. The XPA-2 pushes my speakers hard enough that it hurts anyway. LOL Besides, The neighbors might just be calling the MP's if it gets too loud.

Just trying to understand but if you give a 300 watt 8ohm speaker 300 watts at 4 ohms from the amplifier, is that not safe for the speaker? Question to anyone.

The only way to "put" a 4 ohm load on an 8 ohm speaker is to wire two of them in parallel to the same amplifier terminal. Although 8 ohm speakers may dip down to 4 ohms or even less temporarily, the speaker is rated at a "nominal" load and either presents a load that is "typically" 8 ohm or typically 4 ohms. Or did I misunderstand the question?

Rob, your saying an amp designed for 4ohm will provide a 8ohm current to a speaker designed for 8ohm use. Only in parallel will a speaker be brought up to 4ohm?

The amps output does not have a resistance. It is designed to match an output impedance of 4 - 8 ohms to provide the voltage amplification. 2 ohm stable amps are somewhat more rare as they are harder to drive.

The speakers won't suffer from a drop in impedance. Speakers are not resistive type ohms. They are reactive resistance as their impedance will change (rise and fall) with the input frequency. The real problem is when you drop the load on a transistor amplifier. The output is where you develop the voltage. E=IR (voltage = current x resistance).. If you drop the resistance, the current increases but not the voltage. Lower resistance speakers offer very little opposition to amps and they can over-current very easily trying to produce the expected voltage. Transistors are very susceptible to over current. This is why most amplifiers are protected against a short (super low resistance) and an open. Audio amps will sometime cook if the resistance is too low.

The goal of an amp is to produce a voltage (obviously with enough current to flow through the voice coils of the speaker without overheating it) that draws the voice coil back and forth at an adequate distance and speed to produce sound waves. Not enough voltage and too much current cause the voice coils to heat up as they are not moving far enough, but are passing a high amount of current (just like clipping).

Tomorrow I will be doing some comparisons between my XPA-3 and my QSC CX902 powering my RTi A9s.

Just an advice to all people looking into buy pro amps: I don't see any problem using pro amps to drive HT speakers, but just be careful with the amp you chose. RMX and GX series are really noisy. I only recommend RMX and GX series if you have a closet far away from your HT room. Just my 2 cents!!!

BTW, I'm not kipping the QSC CX902 driving my A9s, this only an experiment. Even the CX902 (the most quiet amp from QSC) is noisy for my.

Just as reference


Edited by enricoclaudio - 7/10/12 at 9:31pm

Tuned in for the results.......

Can't wait to hear your thoughts.

64db doesnt seem that loud from an inch away. Sitting 8-9 feet away should drop it down to about 20-30 db correct.

I'm thinking that's the fan noise from 1" away from the back of the amp .

People looking at pro amps should also consider a quality used amp. My Adcom (100w per at 8 ohms) has been going strong for 20 years now. I bought it used. The 200w model sells for around $300 on ebay. Great amps. I'm sure there are others.

Yes, it's the fan noise and it's very annoying even at 10 ft.

I employ two pro amps in my HT with great success. I have a QSC GX-3 (class B) >.02% THD powering my CSI a6 (350WPC one channel used) and a QSC GX-5 (class H) >.02% THD powering RTIA9's (500WPC X 2). I sit roughly 16ft from the amps/speakers and during low levels found the fans quite annoying. I added a 10watt 100ohm resistor to the fan circuit and that slowed them down enough to the point they are not audible unless I am within 1ft. or so Took roughly 35mins and cost $3.

I have not noticed any tonal difference between the two amps as I have switched them around a bit for comparison. I initially had the amps running on unbalanced RCA interconnects using a Yammy RX667 (mid/lower mid level AVR rated a ~1v) as a pre/pro & amps at full gain. I found that I was at times clipping the AVR pre-out section. Further research lead me to the difference between a balanced and unbalanced audio signal, the importance of providing the amp with a powerful signal, and gain staging. I then purchased consumer to pro converter and RCA to XLR cables (AVR to converter) XLR to XLR cables (to the amps) then re-ran YPAO. I turned the amps to half gain from full and the output values on the Yammy went from F L/R +4.5db to -5db & Center +5db to -3.5db. No clip since. In my instance the AVR pre-out amp section did not seem to provide a strong enough signal to the pro amps and the converter remedied this issue. Higher end AVRs may not need such an addition. As far another device in the signal chain..... I guess I will live with it as no discernible difference has been detected. Quite the opposite in fact.

Most of the Pro amps have an input sensitivity of 1.2 - 1.4 V at around 10K ohms unbalanced. This is problematic to say the least. ...



The reason amps have a high impedance on the input is to avoid the draw of energy from the source.

Just noticed the XPA-2 has a 23.5 KOhm unballanced input impedance. Twice that of the Pro amp from QSC. the 1.2V RCA input sensitivity on the pro amp shouldn't be a problem except at very low volume levels (probably below 10 or so of 100 max). A converter would probably solve this as it did your you chief.

Thanks for the guidance brother.

BTW, my wife got a little pissed last night anyway with my 300W XPA-2 running at 75 on the volume. She said it was too loud! LMAO. She's not been in the house when I ran it up to 90...

Hey Chief - Yammies seem to have low output voltage. There have been posters in the RXV-565/665 thread having problems setting their subwoofer levels.

Is that 23.5 KOhm unbalanced input impedence the same on each channel (right and left)?

I'm asking because my XPA-2 is making a loud thump on the left channel each time a there is new material sent to the amp from the receiver. Heck, even when I'm changing inputs, I can hear the thump. So, I'm not sure if there is a problem with that particular channel. How can I check that?

Oh, and I already used different wires, removed the XPA-2 from my setup, connected the front speakers directly to the receiver and "no" thump. So, I know something is going on with the amp.

Chief, how high you have to put the volume on your AVR with the GX5? Is it more towards the top end or low end

That's hilarious. She should get together with my wife. I am certain they would have a nice "conversation".

You have to take note of a few things: Is the rating full power? Also what is the distortion #'s of you sub woofer alone. Usually way beyond that of the amp. Plenty of people with awesome sounding subs using the Behringer EP2500 and now the Crown XLS Drivecores. You can always go to Guitar Center and get ears on some pro-audio amps. I don't think it is made out to be what you are making it out to be.

If you think $1600 is expensive try a Crown MacroTech HD at $6K (and that is the STARTING price )

ok all I need some serious help. So when comparing AV Receiver amplifiers to external amplifiers it is not apples to apples, I know that. So does a, say parasound 125 w/channel amp compare to a 130w/channel denon amp. Are they about apples to apples? What I am trying to get at is that I know that say an emotiva 200 w/c amp will make my fronts (say in a two channel amp) sound much much better. But how does that make the system overall balance. Would it just make those fronts pop better at lower volumes? Believe me I understand that it is going to be better, but really want to understand how much. Too bad I wasnt an electrical engineer instead of a civil! Argh...

The RX-667 is a great unit but mid/mid-low range nonetheless. Every time I consider replacing it I always find myself asking the same question. Why?? I payed ~$265. It does a fine job as a pre/pro. When it craps out I will move to a higher end unit. YPAO does not properly set my subs either. I had to use an SPL meter.



I ran YPAO then balanced each channel with a meter. YPAO was surprisingly accurate when compared to the SPL meter minus the subs of course Just to give you an idea, I typically listen to music at ~ -35 movies @ ~ -30. Keeps the wolves at bay. These values are typically not a good frame of reference as it is quite relative(). Anything near -15 in my room is unbearable. Hope that helps.

I have a Yammie in the bedroom and one in the family room for TV and background music and they have been great for those purposes, but now that I have two Denons, having picked up a 3312CI on end of year closeout, I probably will be retiring one of the Yammies. The difference in SQ is noticeable even at low volumes.

Civil... you're funny. I wish I were an engineer! I wouldn't be looking to save 100 bucks on an amp. LOL

An amp will only make it sound better when the AV receiver isn't able to give the speakers what they want. SO, at lower volumes, there isn't much difference. The speakers are already getting what they need at low volume. Don't get me wrong, an external amp may have some benefit even at lower volume, but at 1:00 am when you are listening quietly because the family is asleep, the difference will be unnoticeable.

BUT, when you crank the volume up to 1/2 - 3/4 you will start to see some amazing benefits. Over 3/4 is where you will get a stupid smile.

If you go from a 130 WPC AVR to a 120 WPC external amp, you won't have an amazing experience when only listening to stereo or "pure audio" mode, but when you are in a home theater surround environment, the AVR will not have to power the fronts and they will be able to sing with the best of them.

The 200 WPC (or 300 WPC) Emos will make your speakers seem like new ones! That's a fact.

I'm not sure how the Denons distribute power. The Onkyos are rated with 2ch. driven. So my 807 is rated at 135 with 2ch driven. Once I add a 5.1, my AVR is now rated at 105 at 0.1% distortion. With 7 ch driven, it now drops to 29.9 watts per ch. @ 0.1% distortion. If I tried running my system with just the AVR, I'd be risking clip city.

With my emo amps, my 3ch is rated at 200w per ch. and that's what you get. No drop off in power. My 2ch amp is rated at 300w. per ch. So my mains are getting juiced with 300w, wides and center; 200, and my AVR feeds the surrounds with 135 2ch driven.

So, the morale of the story is; with power amps, you are able to get more power to your speakers than just using your AVR.

I was actually looking at high power pro amps to power my fronts. I've always used Class D amps for subs, but never for stereo full range speakers. I'm a little concerned about he modulation frequency of the Class D's MOSFET and how it will handle fast and high freqs. I need to go down to the music store and listen to a couple on some stereo speakers I guess...

$6K is too rich for my blood. Maybe when my kids are graduated and on their own (in 15 years)...

Very eloquently put Geoff4RFC. I love this forum.
Edited by climber07 - 7/11/12 at 8:32pm

Geoffrey,

Me neither. Here are the power specifications for my amp:



Take a look and tell me what you think. This might not mean anything, but worth a try.

Chip