Minimus 7 PZ-2.2 Mod DIY Circuit Board by Pete Basel

[Pete B]

Edit: Attached an actual sized layout drawing in .pdf format, see instructions for use in the last
paragraph of this post.

I looked back in my notes and noticed that while I owned 2 pairs of Minimus 7s since
the 1990s and kept them stock, I first began measuring them for mods around August
of 2008, that was a long time ago! lol. I worked out the PB-1 mod through measurements
around that time and offered it to Zilch in December of 2008. A few adjustments became the
PZ-2.1 mod. Link to the original thread, PZ-2.1 is finalized around page 7 on that thread:
http://audiokarma.org/forums/index.php?threads/minimus-7-loudspeakers-measurements.199790/

Several people have asked if there is a PC board available for the PZ-2.1 mod for the Minimus7
and I have done a layout as shown below. I used 5W resistors and there was no 40 ohm so I substituted
a 47 ohm resistor and thus renamed it PZ-2.2 to avoid confusion. I also added the option for a tweeter
level control. I do not expect there to be any sonic change due to the resistor difference. I am of the
opinion that the design is so simple that it can be done on proto board, thin masonite, or even terminal
strips. If you aren't able to make a circuit board, then go ahead and build it with terminal strips.

Here is the layout on a 4"X3" board, done in Express PCB but not for production, just to have a
clean layout. Note that the components are placed closer to the top to provide room behind the woofer
so that a deeper woofer can be used as a possible future upgrade. I have added a tweeter level control,
since I see many different opinions on the best level for the tweeter.
Notes:
1. R2 is increased from 2 ohms to 15 ohms and the 25 ohm tweeter level pot is wired across R2.
Those who do not want to include the level pot should simply use a 2 ohm resistor leaving out the holes
for the wires to the level pot.
2. This view of the board is the copper side so we are looking through the board at the components. The
level pot is as it would be seen looking into the speaker enclosure to wire it up.
3. Two locations are provided for L1 the larger one to the left is the 18 gauge iron core from Parts Express,
the smaller steel rod type is mounted on board that I bought surplus. Only one location is populated depending
on which coil is used. The PE inductor should be hot glued down.
4. Note that the connection between R1 and C1 is in the air as can be seen in the picture with a twist.
The layout tool has no way to show this. The other lead of R1 reaches over to the IN- strip down the
middle also as shown in the picture. These are the only leads that do not go through holes close to the
component.


This board is not designed for a printed circuit board shop, rather I cut the copper away on a table saw
as described in this thread at DIYaudio. I also mention there mounting components to the copper side
so that through holes do not have to be drilled, these components are heavy and therefore through holes
are used in this design. This board could easily be fixed up for a PC shop to fabricate:
http://www.diyaudio.com/forums/cons...ype-pc-boards-etch-table-saw.html#post4910039

Here is the board after cutting on the table saw:


Here it is after tinning with solder, this step is not really needed but the solder does not tarnish as much
as copper and it just keeps it looking a bit better:


This board also completely replaces the old plastic cover for the input terminals and provides nice binding
posts on standard .75" centers. Note that one of the posts on the back of the box sticks up about .1" higher
than the mounting point for the XO board. There are two ways around this, double up on the gasket between
the XO opening and the PC board or cut the post down with a Dremel cutoff disk. I used the thicker gasket.
I used plastic type 5-way binding posts left over from another project with the mounting screws working
perfectly with the board. The posts in the parts list must be removed from the mounting board which is
not needed.

Component side view of the completed board:


Notes:
1. important THE METAL NEAR THE SCREW HOLES MUST BE REMOVED WITH A LARGE
DRILL BIT TO BEVEL THE EDGE, to avoid shorts to the screws.
2. All resistors should be mounted elevated as shown in the lower picture, forgot to do it at first,
but resoldered all the resistors after bending the leads as shown.
3. The above picture was made prior to the final layout above, it is 2.5" wide whereas the final layout is
3" wide. The wider board provides room for a new and larger tweeter inductor if you must use one, in
the event one doesn't have the original inductor.
4. I used parts that I had on hand, the Dayton 2 uF cap is longer than the one shown, also I used a 10W
for the 15 R only because I had it on hand, 5W is specified.
5. Hot glue the heavy components down, both inductors and the large cap. E-6000 craft glue, or even Shoe
Goo can be used instead of hot glue.
6. Double stick, foam, mounting tape is used to make a gasket around the binding posts, and it should be 2
layers deep because of the tall post on the back of the enclosure as shown below:


Two layers of foam tape is needed around the binding posts, then one layer up the center
and a second layer to avoid vibration against the top two posts:


Rear view showing tweeter level pot, and new binding posts:
View attachment 845270

Here is the PZ-2.2 schematic, note a few minor component value change of R3 = 47 ohms:


2020 EDIT: Stuart's L-pad version is better than how I did the tweeter level control so this is
what people should build for the version with a tweeter level control:
Minimus 7 PZ-2.2 Mod DIY Circuit Board by Pete Basel

Use the long shaft version of the 15W L-pad as I show here to provide clearance for the XO
board so that you don't have to cut the board:
https://audiokarma.org/forums/index...ard-by-pete-basel.750334/page-4#post-12561307


Parts list for the version with a tweeter level control, R2 is 15 ohms, note the use of a surplus woofer inductor.
The level pot is 25 ohm 5W from Radio Shack #271-0625:


Parts list for the version without a level control, the only difference is R2 being 2 ohms, note the use of a
surplus woofer inductor:


Copper PC Board 4" X 6" Single Sided board from Parts Express #055-134 can be cut in half to
make two 4" X 3" boards.

Edit: Attached an actual sized layout drawing in .pdf format. Print it out, be sure to check actual size
and tape it to the PC board to punch points indicating where to drill. Site along the edge and mark with a
wide Sharpie where to cut copper on the table saw if you plan to use this method. Align the sharpie mark
on the table saw for cuts by eye - this is how I do it.

Merry Christmas, and Happy Holidays, all the best for the new year!

 

[Pete B]

Only 10 pictures are allowed per post,

2020 EDIT: The LPAD level control is better but the long shaft version should be used and
the hole must be drilled in a different position:
https://audiokarma.org/forums/index...ard-by-pete-basel.750334/page-4#post-12561307

LPAD version by Stuart, nice work!
https://audiokarma.org/forums/index...ard-by-pete-basel.750334/page-3#post-11937263

Potentiometer version only:
Drill a 3/8" hole .75" down from the top and centered on the back, punch and first drill a 3/16" guide hole:


Resistance check, before installing the crossover and before hooking up the drivers,
the level control should be wired up. Look for gross errors, +/- 10% should be fine:
From ........ Condition ........ To ........ Reading #1 ........ Reading #2
IN+ ........... Level down ...... T+ ....... 57.5 ohms .......... 57.7 ohms
IN+ ........... Level up ........... T+ ....... 48.0 ohms .......... 48.3 ohms
IN+ ...................................... W+ ...... .3 ohms .............. .3 ohms
IN+ ........... Level down ....... IN- ...... 56.1 ohms .......... 56.6 ohms
IN+ ........... Level up ............ IN- ...... 46.7 ohms .......... 47.2 ohms
T+ ........................................ T- ........ 1.1 ohms ............ 1.1 ohms
W+ ........... Level down ....... W- ....... 56.6 ohms .......... 56.8 ohms
W+ ........... Level up ............ W- ....... 47.2 ohms .......... 47.4 ohms

On test fitting the crossover into the box I found that it was difficult to get to the openings for
the wire in the binding posts due to the deep recess in the back. The posts I used had two
nuts so I used one nut as a spacer between the post and the PC board, that was still not enough
and three #6 brass washers were added on top of the nut. If doing this again I'd probably use
six brass washers and use the nut to lock against the one that holds the post in place.
The #6 brass washers are 12 for $1 at Home Depot in the $1 plastic bags. Two bags are needed
to use six washers on each post.

Plastic or fiber washers are needed between the box and the PC board to act as a spacer and
insulator for the mounting screw to tighten up against. These are 3 to a bag for less than a buck
each, 3 bags are needed. I held these in place with plastic tape as shown below:

People have asked how much fiberglass was used as built from Radio Shack, this is what was in mine,
it is about 1" thick:


This is how I stuffed it with the larger piece folded behind the tweeter:

 

[ghazzer]

Thanks Pete! This has revived my interest in LTC for my Minimus7s. Now I need to search through the other thread and figure out how to open the case, correctly.

 

[Pete B]

You're welcome!
To open the systems, take a very small screw driver, or an opened paper clip into
a hole along the edge of the grille - NOT TO FAR IN to avoid damaging the woofer,
and pry up against the enclosure. Use an old credit card or the top of a match book
cover to protect the paint. There is a soft goop, like poster stick goop that holds it in,
it is reusable. Then you remove the screws holding in the drivers and work through
the driver holes.

 

[Zonker92]

Wonderful, Pete; thanks for the information!

 

[Robie]

Still loving my PZ-2.1 modded M7s that Pete and Zilch designed (forgive me if I have left anyone out or have that wrong).

They now sit on my wife's desk as her computer speakers and sound very nice indeed even though only powered by a lowly modded Accurian.

I do remember how hard it was to do the mod so Pete's boards would be welcome. I'm also intrigued by the tweeter level control. Nice job, Pete!!

 

[Pete B]

Thanks everyone, I'm kind of impressed by how neat this turned out.

 

[Pete B]

Looked back in my notes and I first began measuring them for mods around August
of 2008, lol. The PC board was a push to finish them. They sound great, by the way:



Should put knobs on the level controls:

 

[Stuart Pedaso]

Way cool, Pete.

I was just working up a PCB layout for PZ-2.1 to take outside and etch when the weather clears up. Glad I caught this thread. Your solution is so elegantly simple from a construction standpoint, yet has all the subtle fit and placement details worked out as well. Makes my path going forward obvious -- why reinvent the wheel when I can just stea ..., er,... borrow what already works?

The tweeter level control option looks potentially useful. I have some 22Ω pots that should work. But I also have some 15W 8Ω L-Pads. Would there be any performance advantages between either L-Pad after the standard HF filter vs pot with R2 upped to 15Ω?

 

[Pete B]

Hi Stuart,
I posted the layout for people to use so please go ahead, the grid in the layout is .2" per dot.
Did you notice the small buy out inductors at PE that I called out in the parts list?
Also wondering are you keeping the stock woofers?
An L-Pad is the better way to do this since it keeps the impedance/resistance much
closer to 8 ohms loading the crossover. The crossover point moves when the load
resistance changes as with the pot that I used.
If you use the L-Pad, I'd make R2 zero ohms so that you can get more level when
you want.

I've been listening to them in the kitchen, first on a shelf, not in the corners, and I found
the best balance around 2-4 ohms on the pot. Next I tried them on either side of a
receiver (only about 2 ft apart) in a corner and the balance is about right all the way
up. The bass boost in the corner is impressive.

 

[Stuart Pedaso]

Good deal, Pete. I'll build up a set with L-Pads to play with. The practical advantage looks appealing -- especially given the portable nature of these little speakers.

Good to know the buyout inductor is a valid substitution. Already have a pair that I haven't tried yet. Ordering a few more this morning.

Using 5W 47Ω at R3 is another welcomed tip. 10W 40Ω items seem to get back ordered at both PE and ERSE as often as the standard I-cores at both sources. I've had to substitute in 20W and even 25W at times, which are ridiculously humongous. (I blame the popularity of PZ-2.1 for these stock depletions). I do recall you originally suggested 50Ω here to correct an impedance dip. Zilch settled on the 40Ω compromise as it was the highest value available in PE's audio-grade line. Light bulb should've lit, but I haven't crested that learning curve yet.

I'm using stock woofers currently. Might fab a temporary adapter plate to experiment with the MCM woofers. But they're ultimately destined for custom box builds -- once this deluge that's soaking the west coast passes and I can pull out woodworking tools. Maybe I'll build an ark instead. Anyone know how long a cubit is?

 

[Pete B]

That all makes perfect sense Stuart and you have a great memory about the resistors!

Thoughts on resistor power ratings:
If we turn the tweeter level all the way up, then the 47 ohm is directly across the input
terminals. When driven by a voltage source (solid state amps are very close to one)
it does nothing but draw slightly more current and burn it up as heat. But for an amp
that has some source impedance, such as a low damping factor tube amp, the source
impedance forms a voltage divider with the speaker input impedance. The 47 ohm
knocks down the peaks while only slightly lowering the dips in the impedance. If the
impedance peaks to 47 ohms, then adding the resistor lowers that down to half or
23.5 ohms. But at a minimum of say 6 ohms, 47 doesn't change it much, the resistor
is not the perfect solution but it works pretty well. It could be left out for those who
don't care about the system input impedance.
As far as the power rating goes, 47 ohms is roughly 6 times the rated 8 ohm impedance
and therefore the speaker gets 6 times more power than the resistor. The resistors 5W
rating is therefore reached when 30W continuous is driven into the speaker and given
the crest factor in music, I'd say that 60 to 100W peaks should be fine and probably
more than the drivers will take anyway.

The 2 ohm in series with the tweeter sees 2/10 ths of the system power, or 6W at 30W
in if we ignore the effect of the tweeter crossover cap. Factoring in the crossover cap
the resistor probably sees roughly 1/3 of that given the lower power seen at high
frequencies in most music. Taking the crossover cap into consideration and the crest
factor of music the 5W resistor should be loafing along.

The 3 ohm on the woofer shunt sees very little power because the series cap makes
it mainly take high frequencies but the inductor acts to roll off high frequencies. Not
going to do any math but it should be fine.

Resistors by their nature get hot and air flow is needed to remove the heat so it is very
important to elevate them. I should say that they get very hot when run at their full
rated power.

 

[Mofi]

Hey Pete, I just stumbled across this new thread and it's just what I was looking for. I've had the parts for the crossover upgrade for a few years and just hadn't gotten to it. I always thought there needed to be a circuit board involved. Taa Daa ! ! !
I doubt there's much difference but I'll be installing mine into the wooden cabinet model to begin with. I can't think of any mods other than making the binding posts fit the opening on the speaker.
Am I missing something about the connection between R1 and C1? It looks like R1 has a thru-hole on the left but the photo shows its left lead over to the negative input. Same between R1 & C1. Are those thru-hole connections or just the leads soldered together. (Sorry for the dumb questions.)
I am curious how you attached the crossover to the speaker. I see big (mounting?) holes on each side of the binding posts. Are these for attachment screws and did you make any solid attachment at the top of the crossover, behind the tweeter? Looks like the back on the wooden models is around 3/8" thick so some small bolts may be in order.
I like your table saw technique! I figure I can do about the same thing with a Sharpie, straight edge and a Dremel tool with the mini-router attachment. Just route out the Sharpie marks.
Thanks for the board layout!!!
Randy

 

[djnagle]

If you want them to really shine, strip the cabs and polish them.

 

[Robie]

djnagle, those are econowaved? I've not been to active on AK and therefore probably missed an earlier thread.

 

[elitopus]

If you want them to really shine, strip the cabs and polish them.

View attachment 858686

I have a pair of Optimus Pro7AV that are stripped down like that. They look pretty cool.

What drivers are you running in yours?

 

[djnagle]

Here is a link to the build thread.

http://www.diyaudio.com/forums/mult...ever-think-rs-minimus-11-could-look-good.html

 

[Pete B]

Am I missing something about the connection between R1 and C1? It looks like R1 has a thru-hole on the left but the photo shows its left lead over to the negative input. Same between R1 & C1. Are those thru-hole connections or just the leads soldered together. (Sorry for the dumb questions.)
I am curious how you attached the crossover to the speaker. I see big (mounting?) holes on each side of the binding posts. Are these for attachment screws and did you make any solid attachment at the top of the crossover, behind the tweeter? Looks like the back on the wooden models is around 3/8" thick so some small bolts may be in order.
I like your table saw technique! I figure I can do about the same thing with a Sharpie, straight edge and a Dremel tool with the mini-router attachment. Just route out the Sharpie marks.
Thanks for the board layout!!!
Randy

Hi Randy,
Very good questions actually. You are right that the connection between R1 and C1 is in the air.
When you place a part in the CAD tool there is no option to undo the dot that looks like a through
hole. You are also correct that the other lead of R1 reaches over to the center IN-, again just the
tool. The side that you mount the parts on has no copper so you can bend R1's long lead tight
against the board then up near the component to elevate it. C1 should be hot glued or Gooped
down so it provides the mechanical strength on the other side.
The board mounts in place of the input terminal cup and uses the two screws on either side to
hold it down. Double stick thick foam tape is used to shock mount it against the other posts in
the metal cabinet. You might have to improvise in the wood cabinet if it is different, I've never
seen one inside so let us know.

Yes sure a router should work fine, let us know how it works for you and any tips you might have.
You can find a lot on the net about using CNC routing to make PC boards. But this is so simple
that you can do it just as you said. We should get an actual size of the drawing to make it easier
to copy. The grid of dots in the layout are on .2" centers.

The two screws holding down the board are probably not good enough for shipping these speakers
I'd probably hot glue the other end down if they had to survive shipping.

 

[Blacksmith59]

Hi Pete, I'm clear on where the leads of R1 go, that's good... but what I'm not clear on is the location of the terminal of L1 in relation to R1 and C1...the PCB layout shows that lead as soldered to a through hole but the photo appears to show that lead (L1)as soldered to the 'in the air connection' along with R1 and C1 as a "T, must be an illusion?The schematic seems to show something else as the L1 is shown as connected to r1directly .I have all the parts but this last detail has me stymied.

.

 

[Blacksmith59]

I think I figured it out....or at least made the schematic consistent with the PCB layout at any rate.....
I inverted R1 and C1 in the schematic so C1 is on top and hooks up with L1.....