This will be a short page.
These are Realistic brand “Minimus 7” miniature speakers with a diecast aluminium enclosure. I purchased them from a local Tandy store in about 1979 or ’80 as a teenager and have kept them all these years. They’re of very early Japanese production and are apparently quite sought after these days. The tweeter is mounted on a flat steel plate having a passivated zinc coating and painted black on the front. The first versions apparently had an aluminium tweeter plate and later production was in Malaysia with a plastic plate. Being pretty basic but ruggedly built, they found their way into the garage where they’ve been used almost daily to listen to FM radio.
A friend mentioned that he was very happy with a “Zilch crossover mod” that he’d done with his own Realistic speakers, so I thought “why not?”
The stock arrangement
Just an inductor across the tweeter and a series capacitor forming a second order high-pass. Woofers connected directly to the terminals (euch!).
The Zilch circuit
It retains the original 0.4mH inductor, but the other parts are new. The original BP electrolytic cap was discarded. I’m no passive crossover expert – that’s for sure. I don’t even like them, but for low value mini speakers like these it’s the only sensible option. The new Zobel (R1 & C1) across the woofer flattens its rising impedance as seen by the inductor (L1). But it does seem odd to have a 1st order LP filter on the woofer crossing with a 2nd order HP filter on the tweeter. I suspect that the circuit is well tweaked. Only measurements will tell! All of the specified resistor values are ridiculous! I used a standard low tolerance 3R3 for R1, a 39R for R3 and would have used a 2R2 for R2, but 2Ω was actually obtainable. I just don’t know what some people are thinking sometimes. Anyway …
The terminal board modified to that circuit
It wasn’t practical to mount the coils at right angles to one another, but they’re spaced apart a bit.
3W metal film resistors¹, low voltage polypropylene caps and a Visaton 1mH air core inductor. Parts stuck down with double-sided 3M tape and hot melt glue. Other people use an iron core inductor for lower series resistance, but these turned out to be fine (actually a 2dB loss balances the replacement woofer output very well with the tweeter anyway). C1 of the woofer Zobel is under the new inductor.
- 3W parts are more than sufficient for all three resistors. The little amp can only deliver about 6V P-P, so even 1W parts would have been OK.
The resistor (R2) at bottom right is wrong – I had increased the recommended 2Ω to 3R3 (on hand) in an effort to compensate for losses to additional series resistance of the air core inductor, but changed it to 2Ω after taking that photo.
Other (audiophile type) people change the terminals to fancy gold plated binding posts, but the Japanese style spring terminals (not shown) are absolutely fine!
I should mention that my original woofers blew and had been replaced many years ago with “Dia-ichi” general purpose 4 inch drivers, however the tweeters are stock.
This is the repalcement woofer. It’s very inexpensive, drops straight in and is still available:
Just a pic
That shows new stuffing which replaced the original glass fibre type which was pretty “toxic”.
Earthworks calibrated mic, REW and a Focusrite 2i2 sound card (power amp off camera):
Tweeter alone (after mod):
Pretty spectacular for a cheap 40 year old tweeter, but the crossover itself cannot account for that very steep 36dB/octave roll-off which looks a bit doubtful. Maybe the mic was too close to the floor. The next ones were done more carefully:
In the following traces blue is the speaker with the stock filter (modified afterwards) and red is with the Zilch xover. Microphone not moved and speakers raised up off the floor on a stool (front edge of speaker at front edge of stool), mic at about 800mm.
Just gotta say – even before the red trace was generated, I could tell by the sound of the sweep itself that it was gonna be pretty flat:
By comparison, the blue sweep was audibly very wonky. And no – a driver was not connected backwards!
These are gated quasi-anechoic measurements with even less smoothing than recommended by Toole. LF resolution is poor compared to proper anechoic measurements of course.
Horrendous destructive (5kHz) and constructive (9kHz) interference there between the woofer and tweeter in blue. That’s what you get when you don’t low-pass a woofer!
- Interesting aside: I once asked one of those bad sounding ESL panel audiophiles “how are you crossing the panel to the woofer?” His response was “mechanically”. What? “I rely on the natural roll-off”. Well that’s what you get! See above in blue!
By contrast (and as if I even need to say it) the red trace is sensational and the off-axis measurements are good up to 45° (see below under Horizontal polar traces).
Overall about 10% reduction in THD from 500Hz up.
Only a minor increase in GD at the LF end as expected from any additional filtering, but much smoother around crossover where it counts.
Needless to say the improvement is incredible for such a trivial effort.
Horizontal polar traces
These were done with the speaker on a stool, fixed mic, speaker pivoted between measurements (something of a poor man’s Toole-style “spinorama”). No audible resonances – just a slight crossover dip at 3kHz.
And these inexpensive speakers have no audiophile pretentions whatsoever! Here’s one (to the right →) with audiophile pretentions. A very well-reviewed (by idiots) American “audiophile” speaker costing over AUD20,000 not only with a deep crossover notch, but an almighty 4kHz resonance to boot! And that’s where hearing is most sensitive. What a pile of garbage. A cheap ($50) Realistic speaker with a simple crossover mod (another $50) measures better!
A video just for fun
This is silly really as it’s just a dumb video of the speakers back on the garage shelf playing whatever happened to be on the radio at the time and what you’ll hear will only be mono due to the single mic Android phone, but they do sound good in the garage:
Can somebody please tidy those shelves? 😀