RTA12C Phase, Amplification, and Bass

Crashdot

Greetings,

I've been trying to make adjustments to my sound system, and have noticed an odd lack of punchy bass or midbass from my RTA12C's. I get a some very low bass, but it almost sounds like there is phase cancellation in the higher bass frequencies, where the kick drum or certain bass lines should be, because even the little 5jr's have a kick that is conspicuously absent in the 12s.

These speakers have probably had several past owners, so I was thinking of opening them up to verify none of the connections are reversed, but I'm not sure if I would know what I am looking at since the schematics do not indicate wire colors, this model reverses some general color coding conventions, and the crossovers in this thing are quite complex to begin with. Could incorrect polarity on a driver result in what I am experiencing?

It also occurs to me that the problem may be amp related. I was looking at the specs for the 12Cs and noted that they have 500w power handling capacity and are 4 ohm, and some people have suggested they are power hungry. My current amp is a Denon AVR-4306 which is 130w/channel, but doesn't officially support 4 ohm speakers. Is it possible that I just need more power, and/or that this amp can't provide the type of current that the speakers are demanding for those strong bass beats?

Thanks for your consideration

DaveHo

Crack them open & take a closer look at the wiring diagram. It does list the wire colors on it.

http://www.polkaudio.com/forums/attachment.php?attachmentid=18713&d=1155781986

-Dave

DaveHo

Also, I think your reciever should be adequate to power them. I think the 500W power rating is a little optimistic. The two MWs will bottom long before you pump that much power into them.

-Dave

Crashdot

Whoops, you are right. I was thinking that the wire colors for the tweeter are not indicated as per the other thread on that topic. That should settle the phase issue. I just learned that my brother has a 500w amp in storage that we are going to try out this weekend to see if it makes a difference. He said that his band has experienced weak/flabby bass when insufficient power was available, and thinks it is worth testing.

I did find a few threads here where people have discussed upgrading their amp and getting much better bass response. Has anyone with RTA12's tried more than one level of amplification power?

heiney9

You need more power for them to run optimally. But it's always best to make sure all connections are correct to begin with. Perhaps there is a crossed wire somehwere.

H9

Crashdot

I had a word with Denon, who said that for these speakers I should be using an amp that is 4 ohm stable, and they recommended that I bi-amp if at all possible. I'll try to report back after I double check the wiring and test with a 4 ohm high power amplifier this weekend.

soundfreak1

I've driven 12b's on 2 denon avr's a 987 and. A 1083 with dissapointing results. But when driven by much more watts (400w@4olms) they thrived, but I believe it was more a result of high current (50 ampers) not watts! Also got an even more impressive results when combined high current and tube preamp. This combo made a huge improvment!!! I believe these newer avr's are designed for the more modern more efficient. Speakers and vice versa. Think about what was being sold as far as amps in the years that these speakers (rta's) were made. High-current/high wattage. "Just my 2 cents worth!

Dawgfish

I would say the amplification is the problem, as I do not believe your Denon reciever is giving the RTA-12s the clean power they need. They are 4 ohm speakers, so they are not the easiest speakers in the world to drive. I have driven my RTA-12Bs with anywhere from 350 to 500 wpc into 4 ohms with outstanding results. They had plenty of mid-bass punch. Get a good amp and see what these fine speakers are truly capable of. I think you will be surprised.

Crashdot

I'm trying to find a 4ohm stable amp, although I doubt it will be anything in the 300-500 watt range as there is not a lot to choose from around here, and shipping of a 500 watt amp is most likely cost prohibitive. Would a 200 watt@ 4ohms separate power amp with dedicated transformers per channel be sufficient, or would I be wasting my time?

soundfreak1

Don't get sidelined by "wattage" high-current is what makes an amp really kick! I'm getting right 400w @ 4 olms but @ about 50+ampers! I'd sacriface wattage for current anytime but both are nice!!! Don't know what kind of budget you have but you can get a very nice vintage high-current and high wattage amp for about 500 or less. Sometimes much less, just be careful and patient and do your research!

Crashdot

What is considered high current, and what specification indicates this? One that I am looking at specifies that it can supply 30 amperes of peak current. Most of the models I have looked at say "high current" in their description...

soundfreak1

If its 30 at peak its far less constant. Prob.10 to 15 amps. Look for 45+ constant or better.you didn't mention your budget?

heiney9

soundfreak1 wrote: »

Look for 45+ constant or better.you didn't mention your budget?

45 amps constant, eh? Lol, so tell me do you have 50 amp or higher breakers in your house?

45 amps constant is ridiculous and totally not necessary and not really attainable.

The amplifier typically draws about 5A (continuous RMS) from the wall during normal use,

This quote is from a pair of 1000WPC mono amps that cost in the neighborhood of $20K for a pair.

That's not to say they don't produce 20A+ peaks for milliseconds...........so the peak rating is important, but like all specs doesn't tell the whole story.

H9

Schurkey

soundfreak1 wrote: »

high current (50 ampers)

soundfreak1 wrote: »

@ about 50+ampers!

Crashdot wrote: »

One that I am looking at specifies that it can supply 30 amperes of peak current.

Amplifier specs that claim to provide some "crazy" amount of current output are essentially lies. What is consistent about such specs is that the manufacturer NEVER discusses the test procedure.

heiney9 wrote: »

45 amps constant is ridiculous and totally not necessary and not really attainable.

So true.





Nobody in High-End Audio has been able to repeal Ohm's Law. You can DAMN SURE bet that nobody in Mid-Fi Audio can repeal Ohm's Law either.

Therefore, you can be very sure that it's marketing BS; if it's true AT ALL, it's done under conditions that essentially can't happen in normal use.

Specifically, the resistance of the test load (which probably is NOT "speakers") must be nearly zero; and the duration of the high-current test burst must be so short as to be musically insignificant.

I'm not saying that "high current" amplifiers don't exist; I'm saying a lot of mid-fi, and mass-market junk claims to be "high current" but isn't; and uses misleading specifications with no details about the procedure used to generate the specs in order to "prove" the point.

Aragon refused to release a "peak" ampere spec for it's amplifiers. If you asked them, they'd say the peak amp spec was "whatever you wanted it to be", because they could design some non-musically-significant test procedure that would put the ampere "spec" at whatever you thought was impressive. They DID say that you could look at the power supply rail fuses and determine the LONG-TERM amperage rating. An Aragon 8008BB uses a pair of ten-amp fuses in each channel; it'll therefore throw 20 amps per channel continuously (until the amp overheats).

20 (continuous) amps per channel under real world conditions is H-U-G-E.

OldmanSRS

The easiest test for phasing is done by connecting a 1.5V battery momentarily to the speaker inputs and observe the cone movement. Both MWs should move the same direction and the PR should move opposite. Compare both speakers and each should be the same.

Bass can be be reduced or enhanced by speaker placement and standing waves within the room adding/cancelling certain frequencies.

Crashdot

My budget is basically as little as I can get away with spending since this is a test, and there is no guarantee the problems I am perceiving are going to be solved by more power. It may be these speakers just don't reproduce some midbass frequencies in the way that I am looking for, or even problems with speaker positioning and room acoustics. I wouldn't want to spend $1000 and find out it made no difference.

Since specifications seem to be useless, I'm not sure what to look for. I'll probably have to choose between what I can find in the area, since many people want to charge $250 to ship anything like a large amp.

The models I had been considering include a Rotel RMB 1075 (five channel) and Denon POA-8300 (three channel). I will probably just wait until something appropriate comes up.

Here is a photo of the inside of the Denon:

Schurkey

Crashdot wrote: »

there is no guarantee the problems I am perceiving are going to be solved by more power.

Excellent answer. Running 4-ohm nominal speakers with an amp NOT rated for 4-ohm loads is not the best idea. It would be wise to upgrade your amplificaton. Even so, I suspect the trouble you're describing is not due to too-little amplifier power. Entirely possible, but not likely unless you've got the volume turned up. How big a room are you filling with sound? It just seems to me that too-little amplifier power would present itself as something beyond mid-bass anomalies yet without low-bass problems.

Crashdot wrote: »

It may be these speakers just don't reproduce some midbass frequencies in the way that I am looking for

First Guess: IF (big IF) the speakers are working CORRECTLY, they'll do midbass just fine.

Crashdot wrote: »

or even problems with speaker positioning and room acoustics.

Another excellent train of thought.

"I" would be investigating the speakers/crossovers/drivers, the speaker wiring, and room interaction issues BEFORE testing with other power amps/receivers. If possible, I'd be trying to borrow a 4-ohm-rated (or better!) amp or receiver from a friend.

These speakers were sold with the understanding that they were a relatively easy load for an amp/receiver of the time; provided it was capable of 4-ohm nominal speaker loading. It should NOT take a super-amp to not have major audible problems with your speakers.

DaveHo

If you don't want to spend a ton of money, you can't go wrong with a vintage Adcom 545 or 555. I'm driving my RTA-12c's with a 555 & can't imagine needing more power. The MW's would give up long before I top out the amp. Some say this combo is a bit on the bright side, but I don't find that to be the case. You should be able to find a 545 for less than $200 or a 555 in the neighborhood of $300.

-Dave

Crashdot

Schurkey wrote: »

Excellent answer. Running 4-ohm nominal speakers with an amp NOT rated for 4-ohm loads is not the best idea. It would be wise to upgrade your amplificaton. Even so, I suspect the trouble you're describing is not due to too-little amplifier power. Entirely possible, but not likely unless you've got the volume turned up. How big a room are you filling with sound? It just seems to me that too-little amplifier power would present itself as something beyond mid-bass anomalies yet without low-bass problems.

The room is approximately 13x17.5, with the speakers and A/V equipment located at one of the long walls. I'll attach an image of the listening room. It is a terrible acoustic enviroment, so I sent my information to Auralex and got a free acoustic analysis, and have been applying studio foam and bass traps as per their recommendations. This has helped some of the problems such as the low bass buildup at the back wall and poor imaging due to reflections. The bass traps are mounted on magnets so I can pull them down for comparison and testing.

First Guess: IF (big IF) the speakers are working CORRECTLY, they'll do midbass just fine.

It is just strange that the mid bass component of kick drums or bass guitar that is punchy on the smaller speakers like the 5jr is just missing on the 12Cs. I tried cranking up individual bass frequencies on the eq, but it just doesn't seem to work.

Another excellent train of thought. "I" would be investigating the speakers/crossovers/drivers, the speaker wiring, and room interaction issues BEFORE testing with other power amps/receivers. If possible, I'd be trying to borrow a 4-ohm-rated (or better!) amp or receiver from a friend.

I was going to try one of my brother's band amps, but it only accepts XLR and TRS, so I didn't have the appropriate adapters for testing. In fact, I've seen a number of similar amps such as Crown or Carver models for sale, but am not sure these are appropriate for HiFi use as they seem to be intended for PA.

I have verified that the speaker drivers are wired correctly, but still need to trace each wire back to the crossovers and make sure than none of them were reversed when they were rebuilt or modified by one of the previous owners. They guy that I bought them from did an excellent job of upgrading these speakers, but I think I read that he got them from someone else in less than ideal condition, so it seems reasonable to double check the connections to rule this out. I suppose I also need to check to see that the cabinets/drivers are sealed properly. I wonder if it is possible to get replacement gaskets for the drivers.

These speakers were sold with the understanding that they were a relatively easy load for an amp/receiver of the time; provided it was capable of 4-ohm nominal speaker loading. It should NOT take a super-amp to not have major audible problems with your speakers.

If there is a power related problem, I suspect it is that my amp is just not designed for 4-ohm. Still, if I am going to spend money on a supplemental stereo amp just for these speakers, I would like to make sure it has sufficient power/current to drive them. For example, I have a 2000watt subwoofer amp in my car. I never use that kind of power since I have a 8inch sub in a small enclosure. It is just that I have blown speakers due to too little clean power, so I would always rather have more power than I need if possible.

Crashdot

DaveHo wrote: »

If you don't want to spend a ton of money, you can't go wrong with a vintage Adcom 545 or 555. I'm driving my RTA-12c's with a 555 & can't imagine needing more power. The MW's would give up long before I top out the amp. Some say this combo is a bit on the bright side, but I don't find that to be the case. You should be able to find a 545 for less than $200 or a 555 in the neighborhood of $300

I would be happy to keep this expense in the $300 range, particularly if I can get something that will do everything that I am looking for. I would be happy to hear of any other recommendations, although I have been watching Craigslist for Adcom, Carver, Rotel, and Denon. Ideally, I would like to get an amp that automatically powers on when a signal is detected, or one that has 12v remote turn on input so my receiver can turn it on when needed. Otherwise I will probably have to buy an additional unit to turn this on and off which would add to the expense, as some of the amps I've seen don't seem to have a power switch.

Thanks

heiney9

You're not going to have a lot of decent choices at $300.

Crashdot

What about something like a Carver M-400? (300 watts RMS per channel into 4 ohms at 1KHZ, and seems to sell for $2-300). Just throwing this out there, as I don't know much about separate power amps.

DaveHo

Auto power on you say. Parasound HCA-1000A comes to mind. Owned one in the past. I found it a little bright, but this was on some DefTech speakers which I think contributed to the brightness.

-Dave

nadams

Crashdot - Where (approx) are you located? If you're local, you can borrow my GFA-545 to test with.

DaveHo

No affiliation.

http://www.audiogon.com/cgi-bin/cls.pl?ampstran&1309817429&/Parasound-HCA-1000A-2ch-st-1-c

Good price on an HCA-1000A. $300 is more the norm.

-Dave

Crashdot

DaveHo wrote: »

Auto power on you say. Parasound HCA-1000A comes to mind. Owned one in the past. I found it a little bright, but this was on some DefTech speakers which I think contributed to the brightness.

Thanks, I'll definitely look into that. If power on capability isn't available, I can always use something like a Panamax with 12v trigger on some of the outlets, although this adds $60-100 to the expense which I would rather spend on the amp.

Crashdot

nadams wrote: »

Crashdot - Where (approx) are you located? If you're local, you can borrow my GFA-545 to test with.

I am located in Honolulu, hence the issues with buying and shipping a big heavy amp from the mainland. A lot of sellers refuse to ship to Hawaii, thinking it is a foreign country or something. Thanks for the kind offer. I am guessing it wouldn't be convenient to get your amp here and back unless you are planning a vacation

DaveHo

The one I linked includes shipping in the US. He probably didn't count on sending it to Hi or Ak, but it wouldn't hurt to ask.

-Dave

Crashdot

Sold! I was too slow, or one of you guys got a good deal Oh well, I should probably finish tracing the wires with the schematics before I buy anything.

nadams

Crashdot wrote: »

I am located in Honolulu, hence the issues with buying and shipping a big heavy amp from the mainland. A lot of sellers refuse to ship to Hawaii, thinking it is a foreign country or something. Thanks for the kind offer. I am guessing it wouldn't be convenient to get your amp here and back unless you are planning a vacation

That's about as non-local as it gets in the states, lol.. Except maybe AK.

Til it shipped back and forth, you might as well just buy one

Crashdot

I got to open up the speakers and verify the crossover and driving wiring is correct, so I think I can rule that out as a source of problems. I was told by another Club Polk member to make sure the batting was where it belonged, which it wasn't. The second layer of batting which is split at the top, had fallen down behind the passive radiator. I pushed it back up behind the drivers all the way to the top of the cabinet, and put the rolls of batting behind each driver as well.

I could just be imagining things, but now it seems that the midbass has returned and the bass is more responsive in general. Does that make sense?

Thanks!