Reading posts and magazines while figuring out purchases has raised something that I would appreciate everyone's input on. When shopping for amplifiers, how much THD can actually make a difference? I was reading a comparison on high power amps for SPL purposes in one of the mags this month and the comment was made that during SPL contests, most of the amps are at roughly 4% distortion when they are racking up those big sound pressure levels. Obviously, this has little to do with sound reproduction and fidelity but it is a heck of a physics experiment. So, at one end you have the Macs with their .005% THD ( I think, Percy is this correct?), and then you get to Helix with a claimed 0.009%, Alpine up to 0.08%, and ads at around 0.1%. Not trying to slight the SPL specific amps but these are the amps I have been looking at and they seem to be in a different range than most other amps. Are these just lies, damn lies, and specifications or is there something here that justifies the price tags, especially the Macs, with an older set of ears? I notice Mean Gene mentioned that anything under 1% is not really noticeable. Is that what everyone's experience is? If that is the case, what are the best performance/price tradeoffs acknowledging that the Macs have the performance end pegged but also are fairly well up there on the price end as well. Not trying to get heresay and product defense going on but more from the actual sound standpoint and admitting that neither our ears nor our cars can be considered perfect audio tools/environments. So what's the opinion, how much THD is perceptible and where do we establish the lower limit to preclude those components from purchase into a Lexus? Thanks in advance for comments.

 

Ron,

You already know my take on this! Some companies will measure at full audio spectrum, from 20 to 20khz at below rated power, which is actually a HIGHER number than if you were to measure AT the rated power. Others will simply dump a "0.04%" and not specify at what frequency they made a measurement at. If they do this then it's at 1khz, the area of lowest distortion, number wise. If we were to look at a typical distortion chart, you'll notice that the low end (20hz) and towards the high end (17khz and up) show the highest amounts of distortion. This is also true with speakers and their operational range.

Mc will use the 20 to 20khz at below rated power for their distortion numbers. Ken C. Pohlman tested the MC4000M and it is STILL the amp to beat in terms of signal to noise ratio (at below rated power and AT rated power), channel seperation (critical to imaging and soundstage) and distortion (measured at 0.005 or a few thousandths above that number). Brax managed to pull off a 73db signal to noise ratio in a recent review. Mc was in the low 90's.

Speakers have the most distortion, usually in several percentages. MBQ will have anywhere from 5 to 7 percent distortion in the midbass while Dynaudio is anywhere from 1 to 3 percent. But the Dyns get VERY good at 100hz and above where distortion is LESS than 1 percent for 2nd and 3rd harmonics throughout the spectrum!

Always look at how they're claiming their measurements for amps. The more information, the better. A simple "slapped on" number doesn't tell much.

As for speakers, distortion charts are VERY hard to come by. Dyn is the only one that I know that will publish their results on the web. Pretty much everyone else has something to hide...INCLUDING the $7,000 Rainbow Reference speakers. No charts or REAL specs (distortion, 2nd and 3rd harmonics, frequency response graphs, etc) there, but I'm working on finding the numbers for these guys.

Percy

 

Ron - I'll stand by my 1% statement as being the upper limit of human perception. SPL amps can have more distortion; in fact, some of that distortion can register on the pressure meter. However, when U run pink noise thru your system ( as in competitions & viewed by B&K quality meters ), the additional THD can cost ya points. Same deal goes with interconnects - the $5 Radio Shack RCA's will be just fine for everyday listening, especially in an SPL setup. Run the system thru the testing for a flat response with the same interconnects & U'll quickly see peaks & dips in the response curve as well as noise - all things that top of the line connections like those from Stinger & StreetWires can help with.
Ron - on a different note: my wife gave me the Griot's Detailer's Handbook for Christmas & it provided us an answer about their polishes. Remember when I asked if U felt that their Machine Polish was the same as their Hand Polish? Turns out that the Hand Polish is still water based like the Machine stuff but it's formulated to be somewhere between the Machine Polish 1 & 2 in terms of abrasiveness. Griot's says that since it's difficult to maintain a steady pressure when polishing by hand, he feels that the Hand polish needs to be slightly more abrasive to achieve acceptable results. Just thought I'd throw that in there!!

 

Gene,

Opening up a new ball of wax here...

Interconnects will make a difference. Same thing with op amps. I've tried different interconnects and op amps with pink noise and an RTA meter. They both measure the same, frequency response wise. Now if you play actual music through them then it turns out to be a vastly different story. Strange how the measurements just can't pick up what the human ear can!

Little known fact. That Radio Shack cable is actually made by none other than Monster Cable. Depends on which wire you're looking at though as some are produced by different manufacturers.

Percy

 

This is going to take a while. If you guys don't mind, could we start with the basics. How are the amp makers measuring the distortion? Sending a sine wave and driving the amp and then looking at the variation from the theoretical response seems logical. I notice in the Mac amp specs state that they rate their distortion over the entire range that the amp is recommended to be used over. I can't say that I have seen that from the other manufacturers. When THD is reported is that an average or the maximum distortion at any point in the spectrum?

If the speakers are the real culprit, how much better than the speakers should the amp be? It seems that the Mac amp is much better than you can actually hear but then I assume that the brands that you guys never talk about are the ones where the distortion is very noticeable. I am trying to not make this a simple "I don't want to spend the bucks for the Mac" but rather if money is relevant (isn't it always) what kind of amp performance (and I assume we are talking about THD for the time being but please throw in the other specs, what they mean, and what you look for) would you look for to insure getting suitable response if you are using, say, the dyns or diamond hex or any other quality speakers?

Mean Gene - Well isn't that interesting. I had a feeling that the hand polish was a bit more agressive than the fine machine polish. One more question, just out of curiosity. I think you posted some time back you might look at getting a car and building it up for SPL competition. Being a physicist and seeing the impressive response of the SPL competitors, albeit at higher distortion, I am very interested in what you would look for in a car to build it up for SPL? I assume if a sponsor were willing to put bucks up for a certain model that would over ride but lacking that what makes a good platform for SPL competition? Will anything do or is it most important to just have enough room for componentry? It would seem that there will be limits to combinations of power and drivers and that other vehicle dynamics will eventually make a difference. Do people design the interior of the car the way they design a sub box to try and get resonances to put a lot more zippity in their doo dah at a certain frequency and location in the car? Obviously this has very little to do with listening to music but from the physics standpoint it is kind of fascinating. As I recall from the days on aircraft carrier decks, 80db is the break where you start having hearing loss so these numbers over 160 db are scary but, once again, it is actually a quite impressive display of physics. As long as you guys are starting me with the basics, how do these SPL contests work. I assume a pickup is located within the vehicle and then some type of signal (pink noise) is fed to the system and the results monitored - correct? Percy - no one is going to listen to music at these levels but is the only hindrance to producing high power clean signals the cubic dollars required? Hope this isn't too dull for everyone else but I figure the only way to realistically be an informed consumer is to understand the specs and their impact on performance.

Subwoofer side note. For those of you who have occasionally joined in on the sub selection threads, the subwoofers in Percy's beloved Dynaudio range are recommended for either enclosure or free air application. With the unavailability of the Vels, probably ol' Osama is sitting on the last ones in his cave, the Dyn subs might be a good choice and permit both infinite baffle and then installation in an enclosure when the spirit moves you.

As always, thanks for the responses.

 

Percy - one more favor. I figure this is covered in the book but just so I understand what we are talking about. Power cables are the substantial wires carrying all that current from the battery (ies) to the components. Speaker wire is the, generally, non-connectorized wire to go from the amp to the speakers. Interconnect is the RCA cables that go from the head unit to the amp or LLC to amp. Correct?

Obviously all of these wires have an impact on performance and especially noise or distortion. So are there rules of thumb of what to look for? I am still trying to figure out the cleanest and most durable way to grab the 12 volts from the battery to run back to the amp. Just looking at prices, there is a big variation in price in terminals for this application. Any input, or just wait for the book? Not to harp on Mac but once again, can you realistically take advantage of that .005 distortion with everything esle involved? By the way, Happy New Year to you two and thanks again for your work on this board.

 

Ron - First of all, U've got the wiring figured out ( in your last question to Master P ). IMO, look for oxygen free copper wires for the speakers & possibly with the wire strands twisted ( to cancel out noise ). Look at the Fosgate Gamma series for a closer look. The power cables should be based on the total amperage draw of your system & the distance from the battery to the components as to the correct gauge to use. On RCA's is about the only place that Percy & I disagree. I've used pink noise & an RTA before in setting up systems & have seen a small difference in response curves when changing from basic RCA's to the higher end models. Percy says that his ears can tell the difference & I have no reason to not believe him - I'm sure I've lost some hearing over all these years of working around machinery ( even with hearing protection ) but my ears could not tell a difference. As far as SPL testing goes, USAC & IASCA rules lay out the exact microphone placement to be used ( so many inches from the dash, window, etc - don't have my rulebooks handy at the moment ). Some events allow U to use your own music - "Flashdance" is the unofficial crank-it-up national anthem while others ( like USAC & IASCA ) have a competition CD with a 10-80 Hz bass wave sweep. As far as my ideas on SPL, look at most of the World Record holders - they're either vans or extended cab pickups. Just cram in as many woofers as U can & fire them into the smallest cabin U can get away with - ever see some of the pics in the mags where the driver's ( yeah, right!! ) seat consists of a wooden box? Hardly a daily driver!!:eek: When I was thinking about playing with the SPL gang, I was looking at the new IS3 Sportcross. The body shape is very similar to that of a ( gasp! ) Civic hatchback which even Percy will admit is well known for generating bass. Kind of an inside joke between he & myself also!! These cars give the impression of big bass but in actuality don't really make the numbers U'd think they would. The rounded hatchback shape helps the bass wave smoothly form ( "mature" ) plus the cargo area gives ya plenty of room to fabricate a multi-subwoofer enclosure. As U mentioned, they also yield some space for the amps & spare batteries. In fact, another friend opened up an audio shop & we're ( yes, I'm in the middle of it!! :eek: ) planning on using his Civic hatchback to house (8) 15" RF Power HX2's in a chambered sealed box ( might try six 15's in a multi-ported enclosure ). Gonna be his shop "rolling business card"!! We've already applied lots of Dynamat & tack-welded ribs on the roof for structural integrity & plan on cutting the floorboard to make room for some Interstate diesel batteries. As far as redesigning the interior for certain frequencies, that works out better for SQ cars - look at Steve Brown's Alpine sponsored Bimmer. He gutted the whole car ( only stock piece is the headliner!!:eek: ) & completely redid the interior. He ordered European ( right hand drive ) steering components to make room for some of his front stage speakers & the back of the car's pure fiberglass - quite beautiful, IMO!! Still, he only placed third at this past year's Nationals. The only perfect SQ score in USAC history was in some guy's Pontiac Grand Am or Grand Prix where he moved the steering wheel to the center of the dash & extended the pedals to reach almost the back seat - talk about equal pathlengths!!:eek: Just some food for thought.

 

Gene - Never, and I mean never too old to learn. I think that is fascinating. My tiny mind began thinking about modelling a specific vehicle to see if you get resonances at certain frequencies at certain locations. Probably science fiction but it you work it out right you could theoretically increase the pressure pulse substantially at that specific location. As long as you keep dumping power into the standing wave it will keep increasing probably until you ignite the air (just kidding for you non techies). It's also amazing that in my younger days you would walk around the pits and find some guy bent over a big block with open pipes twenty inches away from his ears goosing the throttle and saying "Man, that is sweet." But you try to talk to the guy later and he keeps saying "What?." I can understnad Percy's approach, after all look at the much larger bucks we spend on a Lexus for that minor improvement over other cars. Hope that doesn't get me flamed but in life it seems that getting that last 5 or 10% costs double or triple the cost of getting the first ninety per cent. So I ordered a Mac 446. But after waiting and still not being able to confirm delivery, I cancelled. Starting looking again and I added the a/d/s PH30.2, Alpine MRD-F752, and Helix HXA-500Q. The Mac is around $1200 while the other three are in the 7 to 800 range. Hey, $500 is $500. Undisputed the Mac has the best specs but I am not sure my old ears ( I wasn't the guy with the big block but I did put my time in around open pipes years ago) will be able to hear the difference. So I figured I would find out what everybody feels is really "hearable".

You know, when the IS came out I wasn't that thrilled with it but I have really grown to respect it. And I think the Vehicross really hit the design target in the middle. Just briefly looked at a review that included the IS in one of the auto mags and they make the point that the biggest failing from the factory is the tires and with better skins it would be a much different car. Can't argue with that, it is amazing the difference the bilsteins alone made on my GS and I have to tell you the more I drive Yok AVS dbs on my wifes Maxima, the more impressed I am with them. They are not only head and shoulders quieter than anything else I have seen but they are a very decent handling tire in the dry and the wet. Very impressive. I realize that if you compete there are things you would not want to reveal but I am interested that if you do build that car up for competition posting some of the decisions you make, the tradeoffs, and results.

 

Gene,

That was Kirk Perry's car. Pontiac LeMans. Completely gutted interior! Very unusual and he literally has to drive in the rear seat. IASCA wanted to make some rules outlawing that car (did they?) since it was such a drastic change from a "daily driver".

Steve Brown's new M3 should do fairly well if it was decked out right. But, not having Dyns will lower the score a bit.

Ron,

Most manufacturers will measure distortion and pick it's lowest point to make the numbers look good. Mc is the only one that I know of that will measure at the HIGHEST POSSIBLE point. So, that 0.005% is the maximum that you'll get out of any Mc amp.

Best way to run the power wire is to keep the power wire on the passenger side of the car. Speaker wires (high level signals) can go on the right hand side but NO INTERCONNECT should be placed near the power wire. Otherwise you'll get lots of hissing and whining from the alternator. Noise is picked up from induction. Any wire that is of the right guage (4 guage recommended for the long 13 to 15 foot run).

 

Percy,

That Mac spec is truly impressive. That is obviously an inaudible amount of distortion from the amp. It isn't even that easy a number to pick up with instrumentation. Just moving cables around on the test setup would probably make that much distortion. I guess you do at least keep a passing interest in the SPL area. While it is amazing to gut a car just for competition purposes, it ain't unusual. Anybody remember Trans Am cars in the 60s versus what they eventually became? Let me put you on the spot. If there was one area you would think that potentially could make for an SPL winner, what would it be? Still can't get over the physics, generating those pressure levels is impressive.

OK, I think I have it. Obviously your concern is that an interconnect has a lower level signal and even though it might be shielded, you want to keep it away from the higher level signals. The a/d/s and Helix amps take speaker level inputs but the Mac and Alpine do not so the closest I would get to what you call an interconnect would be after the LLC, right? Does this mean it is better, if you are using an LLC to put it as near the amp as you can? I can't imagine anything worse than ripping everything apart and getting it all back together and having alternator whine. Don't forget, I am just a weekend warrior on this stuff, going to do the install more for therapy (and I want to be sure how everything is done and goes back together) than anything.