Monitor 10 Mystery Crossovers

Ok Polkies, here’s a mystery for you. Today, I opened up a recently purchased pair of early Polk 10’s and found them to have rather peculiar crossovers, the likes of which I’ve never seen. Pictures below-one of each crossover.

These Polks have no serial number, but from the features, I believe them to be early 10’s (pre-10A). They have 2 fuses, brown front baffle, velcro grill attachment, stapled-on serial number tags (missing), Moretite gaskets, and a Danish-made Peerless tweeter. The mid-woofers all match and are marked with “557738”-- I assume them to be MW6500’s (although they are not marked as such).

Typically, I’ve seen the Monitor 10 crossovers to have one 33uF capacitor, TWO (2) 12uF capacitors, and resistors that vary depending on whether they have a polyswitch or fuses. However, as you can see from the photos, these very-original-looking crossovers utilize only ONE 12uF cap. Additionally, even the PCB looks different from typical Polk 10.

I sent pictures to one of our prominent CP members who has a business rebuilding crossovers. His response was that it was quite odd, and nothing he’d ever seen before.

Anybody ever seen M10 crossovers like this? Thoughts on how it well it would work, compared to the double 12uF caps?

(By the way, none of the M10 crossover schematics posted on this site show this configuration. )

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Comments

  • F1nutF1nut Posts: 44,782
    Look like VERY early 10's. Crossover looks like the typical Polk ones.
    Political Correctness'.........defined

    "A doctrine fostered by a delusional, illogical minority and rabidly promoted by an unscrupulous mainstream media, which holds forth the proposition that it is entirely possible to pick up a t-u-r-d by the clean end."


    President of Club Polk

  • mhardy6647mhardy6647 Posts: 25,740
    edited March 26
    Coated PR, though. They look like the early 10(A) morph of my early 7As (FWIW).

    11208642076_3f7f8db8b1_c.jpgPolkmonitor7Asn4900 by Mark Hardy, on Flickr
    9420465531_6f3b7ba93f_c.jpgDSC_5417 by Mark Hardy, on Flickr
    (boxes labeled "7" but speaker s/n tag clearly indicates they're 7A)

    The earliest Model 10s had uncoated paper PRs (again, FWIW).

    11208619575_674f036d9b_c.jpgP1020559 by Mark Hardy, on Flickr
  • F1nutF1nut Posts: 44,782
    Yep, you're right Mark. The very first had paper PR's.
    Political Correctness'.........defined

    "A doctrine fostered by a delusional, illogical minority and rabidly promoted by an unscrupulous mainstream media, which holds forth the proposition that it is entirely possible to pick up a t-u-r-d by the clean end."


    President of Club Polk

  • motorcityguymotorcityguy Posts: 35
    edited March 26
    F1nut wrote: »
    Look like VERY early 10's. Crossover looks like the typical Polk ones.

    So, you've seen Monitor 10s before with only ONE 12uF cap?? Westmass Dave said he'd not seen anything like this before.

    I haven't hooked them up yet, but curious as to the impact of this simpler xover.
  • F1nutF1nut Posts: 44,782
    edited March 27
    The crossover is obviously original, so it only has one 12uF, BFD! Polk changed a lot of things over many years on the fly.
    Political Correctness'.........defined

    "A doctrine fostered by a delusional, illogical minority and rabidly promoted by an unscrupulous mainstream media, which holds forth the proposition that it is entirely possible to pick up a t-u-r-d by the clean end."


    President of Club Polk

  • mhardy6647mhardy6647 Posts: 25,740
    edited March 27
    F1nut wrote: »
    The crossover is obviously original, so it only has one 12uF, BFD! Polk changed a lot of things over many years on the fly.

    Yup, we're not talking ISO 900x certified at that point in their history. Probably lots of denim and marginal personal hygiene standards on the shop floor. Maybe even some beads -- or pukka shells. ;)

    1z9z0hyud7vn.png


    Even much larger hifi concerns were pretty cavalier about versioning (as we might say nowadays), as anyone who's ever worked on, say, classic Sherwood vacuum tube hifi components can attest. :|

    EDIT: ahhh, here's the photo I was lookin' for... looks a little more Seventies :)

    6anbqlzaqj2q.png

  • F1nut wrote: »
    The crossover is obviously original, so it only has one 12uF, BFD! Polk changed a lot of things over many years on the fly.

    Yep, understood. Was curious because I've had about a dozen pairs of M10s over the past several years, including some early ones like this, and never came across this simple crossover arrangement, nor have I seen it documented on any of the forum's archived M10 schematics.

    I've got to assume the later addition of the second 12uF cap was found to be an improvement in the sound, otherwise there would be no purpose in Polk adding the extra cost. I'll have to get these speakers assembled, and give them a serious listen. If they are deficient, I suppose the crossover repairs could include an upgrade to the newer schematic. ;-)

    (And all this time, I thought "all" the early units with Peerless tweets would have the same sound!)
  • mhardy6647mhardy6647 Posts: 25,740
    If one is curious enough, I can pull an XO on one of the ancient 10s downstairs and see what it looks like.

    Just let me know... I see to have some spare time here of late :#

  • Viking64Viking64 Posts: 5,316
    mhardy6647 wrote: »
    If one is curious enough, I can pull an XO on one of the ancient 10s downstairs and see what it looks like.

    Just let me know... I see to have some spare time here of late :#

    enidudqlzd1j.jpg
  • mhardy6647mhardy6647 Posts: 25,740
    edited March 27
    I even know where they are downstairs.
    Roughly.
    And I am curious.



    107zh40pnsoy.png
    https://xkcd.com/242/
  • delkaldelkal Posts: 694
    edited March 27
    I've got to assume the later addition of the second 12uF cap was found to be an improvement in the sound, otherwise there would be no purpose in Polk adding the extra cost. I'll have to get these speakers assembled, and give them a serious listen. If they are deficient, I suppose the crossover repairs could include an upgrade to the newer schematic. ;-)

    I never understood why Polk uses two capacitors in series. This is not something you see in a crossover design book. The first cap filters out the low frequencies so there should not be anything for the second cap to filter.

    A crossover with a capacitor and an inductor is a second order crossover. It has a 12dB per octave decrease at the crossover point. My guess is that with the second capacitor It could be thought of as a "2.5" order and the slope is probably close to 15dB per octave? This was probably to filter out a little more low frequency going to the tweeter so you can crank it up more without blowing the fuse. The problem is then you are trying to match a second order slope for the woofer with a "2.5" order slope for the tweeter. That is not supposed to be done and it should not give you a smooth crossover point (but what do the books know). You might want to rebuild it as is and give it a listen. If you don't tend to crank it to 11 the original design with one Cap might give you a smoother transition at the crossover point.

    Note: the 3rd order Butterworth crossover does have 2 capacitors in series but it is not wired the same as the Polks.
  • westmassguywestmassguy Posts: 6,668
    delkal wrote: »
    I've got to assume the later addition of the second 12uF cap was found to be an improvement in the sound, otherwise there would be no purpose in Polk adding the extra cost. I'll have to get these speakers assembled, and give them a serious listen. If they are deficient, I suppose the crossover repairs could include an upgrade to the newer schematic. ;-)

    I never understood why Polk uses two capacitors in series. This is not something you see in a crossover design book. The first cap filters out the low frequencies so there should not be anything for the second cap to filter.

    A crossover with a capacitor and an inductor is a second order crossover. It has a 12dB per octave decrease at the crossover point. My guess is that with the second capacitor It could be thought of as a "2.5" order and the slope is probably close to 15dB per octave? This was probably to filter out a little more low frequency going to the tweeter so you can crank it up more without blowing the fuse. The problem is then you are trying to match a second order slope for the woofer with a "2.5" order slope for the tweeter. That is not supposed to be done and it should not give you a smooth crossover point (but what do the books know). You might want to rebuild it as is and give it a listen. If you don't tend to crank it to 11 the original design with one Cap might give you a smoother transition at the crossover point.

    Note: the 3rd order Butterworth crossover does have 2 capacitors in series but it is not wired the same as the Polks.

    In a typical Monitor 10 Crossover, the first 12uf Capacitor shunts a Resistor. This forms a contour network, which helps compensate for a tweeter whose output decreases as the frequency increases.
    The other type of contour network is a Inductor shunting a Resistor. This type would compensate for a tweeter whose output increases along with the frequency.
    Home Theater/2 Channel:
    Front: SDA-2ATL forum.polkaudio.com/discussion/143984/my-2as-finally-finished-almost/p1
    Center: Custom Built forum.polkaudio.com/discussion/150760/my-center-channel-project/p1
    Surrounds & Rears: Custom Built forum.polkaudio.com/discussion/151647/my-surround-project/p1
    Sonicaps, Mills, RDO-194s-198s, Dynamat, Hurricane Nuts, Blackhole5
    Pioneer Elite VSX-72TXV, Carver PM-600, SVS PB2-Plus Subwoofer
    Cables: Speaker: Furez 10/2 with GLS Locking Banana Plugs
    Interconnect: Furez 10/2 with SpeakONs
    ICs: Custom Furez by Douglas Connections
    Den:
    Bose 901 Series II Continentals Restored, Re-Built Equalizer with Elna Silmic IIs, Sonicaps, and Silver Mica Caps
    Carver CT-3 Pre, Carver C-500

    dhsspeakerservice.com/
    Exclusive Distributor of Gimpod's Custom SDA Circuit Boards

    "And keep her under 70, would you, huh? Betty likes a slow ride"
  • mhardy6647mhardy6647 Posts: 25,740
    edited March 27
    Also bear in mind that, absent other circuit elements (e.g., the resistor mentioned above by @westmassguy), wiring two capacitors of identical value in series halves the resultant capacitance. Generally speaking: 1/Ct = 1/(1/C1 + 1/C2)

    Practically speaking: if one had a whole mess of, e.g., 12 uF capacitors on hand, and one wanted to use a bunch of 6 uF capacitors in a product -- it's way more cost effective to use two 12s in series than to throw the 12s out in the dumpster and buy a bunch of 6s.

    :|
  • mhardy6647 wrote: »
    If one is curious enough, I can pull an XO on one of the ancient 10s downstairs and see what it looks like.

    Just let me know... I see to have some spare time here of late :#

    Hey, if it's not a PITA, that would be great! I'm really curious if I have something really weird, or just another one of many with and "early" configuration. Thanks!
  • westmassguywestmassguy Posts: 6,668
    mhardy6647 wrote: »
    Also bear in mind that, absent other circuit elements (e.g., the resistor mentioned above by @westmassguy), wiring two capacitors of identical value in series halves the resultant capacitance. Generally speaking: 1/Ct = 1/(1/C1 + 1/C2)

    Practically speaking: if one had a whole mess of, e.g., 12 uF capacitors on hand, and one wanted to use a bunch of 6 uF capacitors in a product -- it's way more cost effective to use two 12s in series than to throw the 12s out in the dumpster and buy a bunch of 6s.

    :|

    It gets a little fuzzy when you add in other elements. It appears that most of the vintage Monitors and RTAs with multiple woofers employed a similar high pass section: contour circuit, followed by a 2nd order filter.
    I would surmise that as Polk became more successful, they acquired better testing and modeling equipment, and improved their designs. Just a guess.
    Home Theater/2 Channel:
    Front: SDA-2ATL forum.polkaudio.com/discussion/143984/my-2as-finally-finished-almost/p1
    Center: Custom Built forum.polkaudio.com/discussion/150760/my-center-channel-project/p1
    Surrounds & Rears: Custom Built forum.polkaudio.com/discussion/151647/my-surround-project/p1
    Sonicaps, Mills, RDO-194s-198s, Dynamat, Hurricane Nuts, Blackhole5
    Pioneer Elite VSX-72TXV, Carver PM-600, SVS PB2-Plus Subwoofer
    Cables: Speaker: Furez 10/2 with GLS Locking Banana Plugs
    Interconnect: Furez 10/2 with SpeakONs
    ICs: Custom Furez by Douglas Connections
    Den:
    Bose 901 Series II Continentals Restored, Re-Built Equalizer with Elna Silmic IIs, Sonicaps, and Silver Mica Caps
    Carver CT-3 Pre, Carver C-500

    dhsspeakerservice.com/
    Exclusive Distributor of Gimpod's Custom SDA Circuit Boards

    "And keep her under 70, would you, huh? Betty likes a slow ride"
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