Polk Monitor 7C: which booklet is wrong?

OlivierS

I'm hooking my Monitor 7C's up to a tube amplifier with only 4 and 8 Ohm speaker out terminals.
I have the original manuals (2) and a spec sheet. Two of them say the Polks are 6 Ohms, one says the 7C's are 4 Ohms. See the attached pictures.


This matters because if they are 4 ohms, I'll have to hook them up to the 4 Ohms speaker output.
If they are 6 Ohms, it's better to hook them up to the 8 Ohm output.

I have the equipment to measure an entire impedance-frequency graph (much better than only a DC measurement), but that takes a lot of time. If there is anyone who can tell me the right impedance, that'll save me some time

Thanks!

[Deleted User]

Hello,
Welcome to Polk's forum. I'll send your question to Stu Lumsden who was the engineer for the Monitor series.
Regards, Ken

mhardy6647

Actually, whatever the nominal impedance of the 7C (and I have no idea what it is), you'll do no harm hookin' 'em to either set of taps. Indeed, I'd suggest trying one output impedance, then the other, and go with connection to the taps you like better in sonic terms.

Transformer coupling renders a vacuum tube amplifier far less picky about impedance matching (per se) than are direct coupled amplifiers -- solid state or vacuum tube (i.e., OTLs).

I say "per se" because a transformer-coupled amplifier with low damping factor (i.e., high output impedance), such as a SE DHT amp with no applied negative feedback, will "feel" the impedance characteristics of a load to a relatively great extent, and matching the load impedance with the amp's output is much more audibly important. I'm guessing that your amplifier is a push-pull amp with UL taps or some mode of NFB applied around the output stage to lower output impedance, reduce distortion, and widen & flatten frequency response, so it's well worth trying either set of taps.

The classic rule of thumb for speakers and transformer coupled (vacuum tube) amps is a 100% mismatch is readily tolerated.

And never forget that the nominal impedance of a loudspeaker (with very few exceptions, such as planars which are often purely resistive loads) is in fact a nominal quantity. The impedance varies with frequency. Depending on the impedance curve of any given loudspeaker, the nominal impedance specification may be a completely useless indicator of how the speaker is likely to behave as a load for a real amplifier in the real world.

mhardy6647

Strictly for illustrative purposes :- ) herewith, the impedance curve of another more modern Polk speaker (courtesy of a Google search and found on the AVS Forum). I in no way intend to imply that this is similar to the impedance curve of a Monitor 7C; I exhibit it merely to illustrate why I wouldn't bother to get too worked up about the nominal impedance spec of your speakers, were I you.

This looks like a typical "four ohm" nominal speaker to me, given the 4 ohm trough at a few hundred Hz. Which tap would such a speaker sound better on? Only one way to find out!

OlivierS

Hi! Thanks for the quick and in-depth responses!
The tube amp is indeed a common push-pull type with NFB and UL taps.
I've made it myself using transformers I bought on a flee market, so don't expect it to be high-end.

I know impedance matching isn't as critical as some people want me to believe.
And indeed, speaker impedance tends to vary greatly with audio frequency.
You convinced me that I shouldn't worry too much.
I've never even heard of a case where an impedance mismatch blew up transformers or output tubes (unless when the amps were playing without a load).

Today, the speakers have been running the whole day on the 4 ohm tap without problems. Tomorrow, I'm gonna try the 8 ohms tap and see if I can hear an audible improvement.

Thanks again. Not many official brand forums have such a great community and online customer service.

Greetings from Belgium,
Olivier

[Deleted User]

Hello,
I received this response from Stu:

"The 7C should be hooked to the 4ohm outputs. It really does not make that much difference if the user pays attention and keeps the gain below the level where the amp distorts regardless of the volume knob position.

Stu"

Regards, Ken

mhardy6647

^^^ ... now that's the official word from Polk folk themselves!

rikm

Thanks to you and Stu both from me as well Ken, and can we get additional comment on the variance in those spec listings?

Specifically, the jpg that shows 4 Ohms also states 91dB efficiency and 10-150wpc and has the SL2000 named as the tweeter. The other 2 show 6 Ohms and 89dB... and the one that shows the drivers lists an un-named tweeter.

Is it possible that it was the SL2000 that changed things, or is it all just old errors at the printers? I ask because I have an old EL84 amp that I have always worried was under-spec for my 7C's


thanks again,

rikm

Jimbo18

My Monitor 7's are marked as 7C on the back, although someone told me that are actually Series II because of the tweeter. They have the SL2500 tweeter, not sure if that's right or not.

In any case, I have used them with a low end Pioneer and a Yamaha AVR and now have them with the beefier HK, but still just an AVR. They have never had a problem with any of those driving them, so I think they must be at least 6 - 8 ohms.

OlivierS

Hi guys! Thanks for the great input (especially from Stu himself) !
I have been running the speakers from the 8 ohm output for a few days and I haven't noticed anything different in terms of sound quality. It just works as good as the 4 ohms output.
But I'm gonna follow Stu's advice and change them again to the 4 ohm output.

For your interest: my 7's have these numbers printed on them:

"Polk Audio
Monitor Series
7
62453"
and the other one is the same except the last number is 56147.
They have the Polk Silver Coil Dome tweeter.
So no '7C' on them.

By the way: the tube amp definitely sounds a bit different than my vintage solid state amp (a Philips 22AH386). The midrange is a lot more apparent. At first I didn't like it but now it's growing on me. I guess the Polks are a good match with this tube amp (a EL34 push-pull type, based on the Claus Byrith 4-30, which is based on the good old Mullard 5-20).

Greetings from Belgium,
Olivier

[Deleted User]

Hello Olivier,
I'm glad you are enjoying your speakers. I looked at the original Mullard applications guide for the 20W amplifier and found this comment:

"High stability cracked-carbon resistors are used in the anode, screen-grid and cathode circuits and they give an appreciable improvement in the measured level of background noise compared with ordinary carbon resistors."

This is from the middle 1950's and better sounding/measuring parts are already being considered. I've not heard of "cracked-carbon" resistors, interesting.
Cheers, Ken

OlivierS

Another chapter to this topic: it seems my Polks Monitor 7s are from different production runs.
I was replacing the bipolar electrolytics in both crossovers when I found this out.
The cabinets look identical on the outside, but their crossovers are different. This might explain the large difference in S/N.
One of the speakers is from the 'A' revision and the mid-bass driver is marked with a production stamp from June 12 1989.
The other one is a 'B' revision, with the mid-bass driver from December 18 1989.

See this topic.
The crossovers look completely different on the outside, but the schematic shows that the only electrical difference is a change from a 0.23 mH to 0.22 mH inductor. I doubt I'll ever hear a difference