Matching my mismatched Monitor 7's

so_ein_pech

I picked up two early Polk monitor 7's. They are early versions with the 8" PR's. I got them knowing one tweeter was busted and would need to be replaced. I figured I could just grab a Peerless tweet on the 'bay and call it quits. Looks like it might not be that simple. The serial numbers are only 1000 apart, but it amazing how much they changed.

Speaker A (S/N 00495):

Speaker B (S/N 01555):

Here's what they have in common:

-Cabinet geometry
-Woofers (P/N 6KU132)
-PR's (P/N 7450)

That's about it.

Here are the differences:

-Tweeters:

-Crossovers:

-Binding posts:

-A isn't fused
-Acoustic damping material (A uses fiberglass, B uses something more like polyfill)
-Badges
-Wiring convention (A: White is positive. B: Black is positive.)
-No veneer on the edges of A

OK so the reason I am posting all of this is I am hoping to get some advice on getting the right tweeters and modifying the crossovers. The working tweeter (tweeter looks like an early Peerless, except that there is no hole punched in it (possibly a Danish Peerless, I gather). But the back is perfectly round, which I haven't seen anywhere else. And there is no number or label on it, which is odd. Maybe it is a drop-in replacement?

What concerns me is the presence of an extra 6.7Ω resistor in series with tweeter B. There is no corresponding resistor in speaker A. Other than that the crossovers look identical. I'll probably replace the tweeters with a matched pair of hole-y Peerless or SL-1000's or possibly RD0194's. But that leaves the question of how to mod the x-overs. Obviously one or both needs to be changed. Does anyone have any information about different crossover values for different generations and different tweeters?

Here's a couple of threads I found about IDing these tweeters:
http://www.polkaudio.com/forums/showthread.php?25377-Identifying-your-vintage-tweeters
http://www.polkaudio.com/forums/showthread.php?99168-Vintage-Peerless-clarification

Ben

westmassguy

Your best bet is to buy two matching vintage Peerless Tweeters, or two Midwest Speaker, Peerless clones. Forget the SL1000, not a good tweeter, and the RDO-194 is not compatible. I'd make a schematic of each, and compare them. Speaker A's crossover might have been modified to accommodate the different tweeter. I had an early pair of 5s with Peerless, which had the 2.7 and 6.2 ohm resistors in them. I'd also check that the Inductors are the same.

TNHNDYMAN

The serial # 495 is the earliest model I have seen. It may have some historical significance just to show the model's progression. Personally I would not modify it and keep it as a show piece only. It would be easier to buy a matching set of 7's that would give you less problems to sound coherent together.

MHardy or other's- has anyone else seen or have pics of an earlier model 7?

so_ein_pech

westmassguy wrote: »

Your best bet is to buy two matching vintage Peerless Tweeters, or two Midwest Speaker, Peerless clones. Forget the SL1000, not a good tweeter, and the RDO-194 is not compatible. I'd make a schematic of each, and compare them. Speaker A's crossover might have been modified to accommodate the different tweeter. I had an early pair of 5s with Peerless, which had the 2.7 and 6.2 ohm resistors in them. I'd also check that the Inductors are the same.

You're right it's a 6.2Ω resistor, I typed that wrong in my first post. I thought that the SL1000 and the Peerless were similar and the SL2000 was the one that is generally disliked. Why wouldn't the RDO-194 be compatible?

I don't have any way to test the inductors short of unwinding them and measuring the length of the wire.

My sister lives in Western Mass, by the way. Its a beautiful area.

so_ein_pech

TNHNDYMAN wrote: »

The serial # 495 is the earliest model I have seen. It may have some historical significance just to show the model's progression. Personally I would not modify it and keep it as a show piece only. It would be easier to buy a matching set of 7's that would give you less problems to sound coherent together.

MHardy or other's- has anyone else seen or have pics of an earlier model 7?

It is an interesting speaker, but I need a working set more than I need a broken museum piece. Besides, the only modification I plan to do is change out the tweeter and possibly add a resistor in series with it--not exactly an irreversible mod. I'll keep the stock tweeter lying around. It may even be possible to use the stock faceplate on a new tweeter.

I am tempted to change out those awful binding posts, but truth be told, I will be too lazy to bother with it once I have it hooked up.

westmassguy

so_ein_pech wrote: »

You're right it's a 6.2Ω resistor, I typed that wrong in my first post. I thought that the SL1000 and the Peerless were similar and the SL2000 was the one that is generally disliked. Why wouldn't the RDO-194 be compatible?

I don't have any way to test the inductors short of unwinding them and measuring the length of the wire.

My sister lives in Western Mass, by the way. Its a beautiful area.

Speaker A, looks like it was modified. That doesn't look like a Peerless, could be wrong though. The Peerless generally require different Resistor, Inductor, and Capacitor values compared with the later tweeters. The SL1000 was Polks replacement for the Peerless, but it's more closely related to the SL2000. That's why the RDO-194-1 is an approved to replacement for the SL1000 and SL2000, but NOT the Peerless. If you look at the later schematics for the 10s, you'll see different values for the Hi-Pass sections that used the SL2000. You would need an LCR Meter to check the Inductors.
Western MA is beautiful, especially in the fall.

so_ein_pech

westmassguy wrote: »

Speaker A, looks like it was modified. That doesn't look like a Peerless, could be wrong though. The Peerless generally require different Resistor, Inductor, and Capacitor values compared with the later tweeters. The SL1000 was Polks replacement for the Peerless, but it's more closely related to the SL2000. That's why the RDO-194-1 is an approved to replacement for the SL1000 and SL2000, but NOT the Peerless. If you look at the later schematics for the 10s, you'll see different values for the Hi-Pass sections that used the SL2000. You would need an LCR Meter to check the Inductors.
Western MA is beautiful, especially in the fall.

I suspect that Speaker A (S/N 455) is stock. This thread has a brochure of the first gen monitor 7's and 10's: <http://www.polkaudio.com/forums/showthread.php?14441-Original-Model-7-and-Model-10-Brochure-Scans>. The tweeters and crossovers look the same, except that the corners of the face plate are not cut off on my tweeter.

Its funny to note how Polk still isn't coherent about the tweeters from one page of the brochure to the next. The photos show tweeters that look similar to mine, with the wiring hidden behind the faceplate, whereas the drawings clearly show the familiar Peerless tweeters.

LCR meters aren't exactly cheap enough for a one-off use, although maybe I should get one since I'm studying EE. I could hypothetically calculate the inductance with a function generator, a capacitor, and an RMS voltmeter. But that sounds like a PITA. I supposed I could just walk into a lab at school and use one of their LCR meters. I can at least test the winding resistance to cross compare the inductors. I would guess that they are the same, since the woofers are the same and both inductors part of the woofer's xover, (I think). But you never know.

westmassguy

Like I said, I could be wrong
See if you can borrow one from school, even if you have to sign for it or leave a deposit. Best to be sure. The crossovers are perf-board, so it's all point to point underneath. Should be easy to add or remove any component you want.

so_ein_pech

Do you have any experience with the MW audio Peerless clones? I can't find much about them except that they are made in China (of course so is everything). Do they sound like the originals?

westmassguy

I've heard nothing but good things about them. Member Faustin has used them, and says they sound great next to the Peerless.

mhardy6647

Why not clone the XO values and wiring of the Peerless-equipped Monitor 7 into the other one?
The only risk (which is minimal, I'd opine) is that the choke values (DCR and/or inductance) are very different.
If you have a DMM and/or an LCR meter, that's easy enough to check.

I'd bring the non-Peerless speaker into Peerless condition :-), normalize the XOs as much as possible given your skill and interest in spending $ on the parts, and listen to the results.

It would be a good idea to add a 1A fuse in series with one leg of the tweeter on the fuseless one.

PS In early days, I am pretty darned sure that Polk themselves used un-holey :-) tweeters. My early pair of Monitor Series Model 10 speakers is my empirical evidence of that.

P1020559 by mhardy6647, on Flickr

I think Polk went to adding the hole to the tweeter dome (presumably/ostensibly to reduce certain resonances, I think) later in the history of their "Monitor Series" speakers. Interestingly, it is said that Polk modified the tweeters using the tip of a soldering iron!

so_ein_pech

mhardy6647 wrote: »

I'd bring the non-Peerless speaker into Peerless condition :-), normalize the XOs as much as possible given your skill and interest in spending $ on the parts, and listen to the results.

Yeah this looks like the best course of action. I'll replace the electrolytic caps while im at it.

Anyone have a line on some non-wirewound 6.2Ω 5W resistors? Mouser stocks some metal film type ones (http://www.mouser.com/ds/2/351/XC-600044-204222.pdf). I don't know if these are best for xover applications. Usually you see those square ceramic resistors in xovers, but I can't find any of the right value that aren't wirewound.

Edit: Some quick googling has led me to believe that so-called non-inductive audio-grade resistors (like the ones at parts-express.com) are standard wirewound power resistors. They are inductive, but their inductance isn't big enough to have any real effect on audio frequencies.

westmassguy

Mills MRA-12s, or Mundorf MOX are popular around here

mhardy6647

You can use Ohm's law to make resistors of the value you need :-)

The Mills noninductive resistors are classics - and good.

That s/n 495 Model 7 is really, really early! Quite a find, I'd opine.

so_ein_pech

mhardy6647 wrote: »

You can use Ohm's law to make resistors of the value you need :-)

The Mills noninductive resistors are classics - and good.

That s/n 495 Model 7 is really, really early! Quite a find, I'd opine.

I know, it must be one of the first 500 Monitor 7's ever made. Too bad it's not a working matched set. Makes you wonder what its history is; what happened to its mate?

It looks like I can get mills 6.2Ω 5W Mills resistors at soniccraft.com for $4 each. Expensive for a resistor, but not bad.

Faustin

MW Peerless clones are a good substitute for original Peerless. You can find original Peerless on Ebay etc if you are patient, that are in good condition and at the right price.

so_ein_pech

OK update time.

First, let me just say that I hate this new forum format. All the links are broken in the first post and I can't edit it to fix them. Here are the links to the tweeter ID threads that I referenced.

http://forum.polkaudio.com/discussion/25377/Identifying-your-vintage-tweeters
http://forum.polkaudio.com/discussion/99168/Vintage-Peerless-clarification

And here's the link to the thread with the old brochures in it:
http://forum.polkaudio.com/discussion/14441/Original-Model-7-and-Model-10-Brochure-Scans

You would think they could put in some redirect mechanism to handle the old links, but I digress.

Anyhoo I was able to find a Peerless tweeter that visually matched on Ebay (no hole, perfectly round magnet) and decided to take the plunge. I modified the older x-over with a generic 6.2Ω power resistor. I went ahead and replaced the electrolytics for good measure.

I pulled the original fiberglass stuffing out of the old speaker and shared the polyfill stuffing between the two. Then I dumped a bag of acousti-stuff or whatever its called from PE into the cabs to fill them back up.

Here is the x-over schematic that I was able to deduce. In red is the stuff that's only in the newer speaker (resistor and fuse). I have no idea what the values of the inductors are.



This xover circuit is different from the oldest official schematic that I can find. (From this thread.) Namely, the resistor in series with the tweeter's inductor is not in the later iterations. Also, the values are slightly tweaked as you can see.

So after testing these speakers, they sound balanced to me. I can't say for certain that the inductors are identical in each speaker, but I can't hear a difference.

My initial observations are thus:

• The bass goes quite low, but is not very tight. I guess that's to be expected from a PR design.
• The mids seem a bit shouty and sometimes harsh. Maybe I'm just not used to speakers that put out confident mid range frequencies.
• The highs are detailed enough but seem to get lost sometimes.

Overall, I'm pleased with the sound but not blown away. Chalk it up to overinflated expectations maybe.

Future modifications:

• Reseal the cabinets to hopefully tighten up the bass
• Put in non-inductive resistors. Probably 7Ω(5W)||56Ω(1W) to get 6.2Ω and replace the tweeter capacitors, to get more air out of the tweeters.
• Dynamat on the baskets.
• A little more acousti-stuff

I'm thinking (hoping) maybe the harshness in the mids is due to basket resonance. Does anyone have any experience with the Dynamat mod?

westmassguy

The Cabinets should NOT be filled completely. The Dacron Batting should be rolled, and installed in the upper section behind the tweeter and woofers. It should extend no lower than the bottom of the woofer openings, or the top of the PR opening. The bottom of the cabinet should be completely empty.

TNHNDYMAN

Yes many here have experience with the Dynamat mod. Helps stop unwanted resonance from the driver baskets themselves. Should give more clarity to midrange and vocals with less smearing of the sound. Pretty inexpensive and effective.

so_ein_pech

westmassguy wrote: »

The Cabinets should NOT be filled completely. The Dacron Batting should be rolled, and installed in the upper section behind the tweeter and woofers. It should extend no lower than the bottom of the woofer openings, or the top of the PR opening. The bottom of the cabinet should be completely empty.

Interesting. In their stock states they were both definitely packed quite full of batting. I guess I can experiment with different amounts. What would be the effect of under- or over-filling them?

so_ein_pech

TNHNDYMAN wrote: »

Yes many here have experience with the Dynamat mod. Helps stop unwanted resonance from the driver baskets themselves. Should give more clarity to midrange and vocals with less smearing of the sound. Pretty inexpensive and effective.

Smearing is a good way to describe how the mids sound.

Inexpensive? I can't seem to find the stuff for less than a bill!

TNHNDYMAN

^ check amazon

so_ein_pech

TNHNDYMAN wrote: »

^ check amazon

Oh ok, I guess I was looking at the huge packs of the stuff. Should I go with the 0.067" thick stuff or the 1/4" thick sheets? Also, about how many square inches will I need for two woofers and two PRs?

westmassguy

so_ein_pech wrote: »

westmassguy wrote: »

The Cabinets should NOT be filled completely. The Dacron Batting should be rolled, and installed in the upper section behind the tweeter and woofers. It should extend no lower than the bottom of the woofer openings, or the top of the PR opening. The bottom of the cabinet should be completely empty.

Interesting. In their stock states they were both definitely packed quite full of batting. I guess I can experiment with different amounts. What would be the effect of under- or over-filling them?

You're not the original owner. They could have been fiddled with in the last 30 years, by previous owners.
Over-filling mimics a larger enclosure, so the bass will be sloppy because the PR tuning is now incorrect. Under-filling will smear the image, because the mid and high frequencies will be bouncing around inside the cabinet, and eventually make their way out through the woofers and PR. If you want tight bass and clean mids, remove all the stuffing. Get some Blackhole5 or Sonic Barrier. For BH5, cut strips about 3-4 inches wide, and apply them to the back wall behind the woofers, extending up as high as the tweeter. The bottom should be no lower than the bottom of the woofer opening. Sonic Barrier is not as efficient as the BH5, so cut strips about 5-6 inches wide, and apply them to the back wall behind the woofers, extending up to the tweeter, and again, no lower than the bottom of the woofer opening.
Re-install the Dacron as follows: two tight rolls behind the woofer and tweeter. Tack the Dacron to the sides, and again, no lower than the bottom of the woofer opening, or the top of the PR opening. The bottom should be completely empty.

so_ein_pech

Yeah I can't prove that the dacron and fiberglass are stock, but I suspect that they are. Why would they be filled the same way but with two different materials if someone modified them down the line?

Bear in mind that these are the original 8" PRs so they may have been tuned differently than later models out of the factory. Also note that the woofers in these must be somewhat somewhat less efficient than later woofers because the tweeters need to be padded down so much. My guess is that the woofers were designed to match the original mystery tweeter which must have been less efficient that the peerless.

Another possibility is that may not have been as refined as the later models and Polk didn't decide to start filling only the top part until later.

Still I will try what you suggest and see if it improves the sound.

westmassguy

I'd toss the fiberglass, vacuum out the cabinet, and divide the Dacron between the two. I'm not allergic to it like some poor souls are, but it's not good to have around, even though the cabinets are sealed.

so_ein_pech

Getting ready to order some parts...

Does anyone know the rating of the fuse for the tweeter?

As I mentioned before, only one speaker came with a fused tweeter, and I plan to install a fuse in the other. The value of the fuse in the speaker that has one is 1A, but I have no way of knowing if that is original size. People tend to toss whatever fuses they have lying around in when one blows (I know I am guilty). This is a standard AGC fast-blow fuse, not a polyswitch.

@westmassguy when you say install strips of Sonic Barrier, do you mean one per speaker or multiple strips? Why not cover the whole width of the back wall?

boston1450

1amp fast

westmassguy

so_ein_pech wrote: »

@westmassguy when you say install strips of Sonic Barrier, do you mean one per speaker or multiple strips? Why not cover the whole width of the back wall?

One strip per speaker. You can overdo it with BH5 and Sonic Barrier, and suck all the life out of the music.

so_ein_pech

Progress is slow, but I finally got most of the parts together. Also, I picked up some Paradigms 7se's that I'm really enjoying so the Polks ended up on the back burner. I also took some measurements of how these perform. I am planning on replacing the 6.2Ω resistor with a potentiometer after looking at the frequency response curves. I posted about it on audiokarma since I spend more time there but feel free to respond here if you don't have an account.

http://www.audiokarma.org/forums/showthread.php?t=654106

Frequency response measurements. These are not particularly accurate since I took them with an iphone mic, but they give a picture of what's going on in the mids and upper frequencies.

Speaker A is stock, speaker B is the modified older one. They are pretty closely matched up to about 10kHz. I'm not super bothered by that since there's not much sound that high anyway. Hopefully new high-quality caps in the tweeters will help match them a little better. But in the end, if I want them to match perfectly, I might need new ones. Maybe some holey-peerless tweets.



The red dotted line in the graph below is the hypothetical response if the tweeter was 4dB louder.




For comparison here's how a 7se measures with my iphone:

kachilda

I bought a pair of Polk audio monitor 7's that had one peerless tweeter and one silver tweeter. Are the brand new peerless tweeters manufactured today a good replacement for my Polk audio monitor 7's and if so would I need to replace just one are both tweeters? If they are a good replacement could someone post a link to a trusted source and part number please. Thank you.