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Realistic Optimus 21/ ADS L620 Comparo

Bodyblue

Bodyblue

AK Subscriber
Subscriber
Edit: Title changed because I compared the L620s with the 21s.

I really like many of the 70-80s Realistic speakers, although like many brands there have been a few dogs as well. The Optimus 21s are not in the dog category however.

I have been interested in a set for several years but have never found a set locally (I dont buy big speakers that have to be shipped anymore). But I found this set on Marketplace last week and picked them up yesterday. I had seen a few reviews on AK after researching them and looked at the old catalogs. The ability to mirror them I found interesting and although I like the vintage lattice look of the Nova series, I am a bit tired of it. So the more modern grill cloth was a nice change but one thing I am glad that was still there was the usual for the era real walnut veneer.

First impressions are overwhelmingly positive. I set them for left and right hooked them to my 90s Kenwood separates stack (150wpc) and let them rip. Since I have the speakers are in the corners of our big bedroom bass is never a problem but I could tell right away the bass was tight and fast and they went fairly deep. After texting back and forth with @Lavane about them yesterday it seems the left/right switch changes the volume of one of the tweeters (they both run all the time) in order to change the soundstage. And it seems to work pretty well. The soundstage is really wide, easily as good as any of the newer speakers I have had in the same spot (6 year old Polks, Baby Advent lls etc)

I have the HF compensation set to "High" and they seem to be fairly detailed but I am going to recap them (caps have been ordered) just to see if they react like the Nova 8Bs. (The 8Bs had a huge change, unlike almost every other set I have capped) They are a bit biased to the bottom end to my ears but they are very smooth and after listening to them for about 6 hours yesterday, there was zero fatigue.......


The interior was stuffed full of fiberglass that I moved out of the way of the crossover to get the pics. The veneer needs to be sanded and re oiled but I will wait for spring for that. For now some Feed N Wax made them look fine (the pics were pre Feed N Wax). I have set of Nova 7Bs that I really really like, but the 21s have a better soundstage and I think better bass.

I have them sitting on Nova 8Bs. And yes I know about getting the RCA jacks plugged.......

20240222_182040.jpg20240222_182110.jpg20240222_080657.jpg20240222_080745.jpg20240222_080754.jpg20240222_080627.jpg20240222_080843.jpg
 
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Bodyblue said:
I really like many of the 70-80s Realistic speakers, although like many brands there have been a few dogs as well. The Optimus 21s are not in the dog category however.

I have been interested in a set for several years but have never found a set locally (I dont buy big speakers that have to be shipped anymore). But I found this set on Marketplace last week and picked them up yesterday. I had seen a few reviews on AK after researching them and looked at the old catalogs. The ability to mirror them I found interesting and although I like the vintage lattice look of the Nova series, I am a bit tired of it. So the more modern grill cloth was a nice change but one thing I am glad that was still there was the usual for the era real walnut veneer.

First impressions are overwhelmingly positive. I set them for left and right hooked them to my 90s Kenwood separates stack (150wpc) and let them rip. Since I have the speakers are in the corners of our big bedroom bass is never a problem but I could tell right away the bass was tight and fast and they went fairly deep. After texting back and forth with @Lavane about them yesterday it seems the left/right switch changes the volume of one of the tweeters (they both run all the time) in order to change the soundstage. And it seems to work pretty well. The soundstage is really wide, easily as good as any of the newer speakers I have had in the same spot (6 year old Polks, Baby Advent lls etc)

I have the HF compensation set to "High" and they seem to be fairly detailed but I am going to recap them (caps have been ordered) just to see if they react like the Nova 8Bs. (The 8Bs had a huge change, unlike almost every other set I have capped) They are a bit biased to the bottom end to my ears but they are very smooth and after listening to them for about 6 hours yesterday, there was zero fatigue.......


The interior was stuffed full of fiberglass that I moved out of the way of the crossover to get the pics. The veneer needs to be sanded and re oiled but I will wait for spring for that. For now some Feed N Wax made them look fine (the pics were pre Feed N Wax). I have set of Nova 7Bs that I really really like, but the 21s have a better soundstage and I think better bass.

I have them sitting on Nova 8Bs. And yes I know about getting the RCA jacks plugged.......

View attachment 3127959View attachment 3127960View attachment 3127962View attachment 3127963View attachment 3127964View attachment 3127965View attachment 3127966
Click to expand...
I looked at that crossover closer and it does look like one tweeter is wired through a resistor connected to the cap, and the other isn't with that left/right switch. I haven't seen that kind of setup in a crossover before.
no need to make separate left/ right speakers for mirror imaging.
I've seen a speaker years ago where the tweeter was installed on its own plate that could be unscrewed and rotated for mirror imaging, but I can't remember the brand or model.
 
The three resistors provide various series attenuation levels to both mid/tweeters according to position of the HF Compensation switch. The 2Ω resistor is always in circuit and works alone in "High" setting. "Normal" adds the 4Ω resistor and "Low" adds the 6Ω resistor as well .

Optimus-21 sm.JPG

The first 15µF cap is always in series with both mid/tweeters. The second 15µF is in series with one mid/tweeter and in parallel to the other. The System Location switch just reverses which mid/tweeter is in parallel and which in series to that cap.

I had thought that whichever mid/tweeter has both 15µF caps in series would become a tweeter only, due to effectively smaller series capacitance rolling off more midrange frequencies. Which would leave the other mid/tweeter running its full range. But testing in simulation tells a different tale. Turns out that parallel capacitance across a mid/tweeter rolls off higher frequencies, relegating it to midrange duty only

Consider this example with System Location switch set to "Left"
left on baffle.JPG
Transcribed here into XSim (woofer muted to avoid distraction):
Optimus-21_L-O-B.jpg
Frequency Response graph shows HF rolled off for right hand side driver in parallel to the 15µF cap.

set_to_left.jpg

This makes more sense than my original backward thinking, which would have one driver providing only HF while the other pushes both HF and MF. Two drivers spreading short wavelength HF across the baffle invites trouble. Location switch scheme instead creates one dedicated midrange and another midrange that also pulls tweeter duty.
 
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Right?
Stuart Pedaso said:
The three resistors provide various series attenuation levels to both mid/tweeters according to position of the HF Compensation switch. The 2Ω resistor is always in circuit and works alone in "High" setting. "Normal" adds the 4Ω resistor and "Low" adds the 6Ω resistor as well .

View attachment 3128391

The first 15µF cap is always in series with both mid/tweeters. The second 15µF is in series with one mid/tweeter and in parallel to the other. The System Location switch just reverses which mid/tweeter is in parallel and which in series to that cap.

I had though that whichever mid/tweeter has both 15µF caps in series would become a tweeter only, due to effectively smaller series capacitance rolling off more midrange frequencies. Which would leave the other mid/tweeter running its full range. But testing in simulation tells a different tale. Turns out that parallel capacitance across a mid/tweeter rolls off higher frequencies, relegating it to midrange duty only

Consider this example with System Location switch set to "Left"
View attachment 3128425
Transcribed here into XSim (woofer muted to avoid distraction):
View attachment 3128433
Frequency Response graph shows HF rolled off for right hand side driver in parallel to the 15µF cap.

View attachment 3128459
This makes more sense than my original backward thinking, which would have one driver providing HF only while the other pushes both HF and MF. Two drivers spreading short wavelength HF across the baffle invites trouble. Location switch scheme instead creates one dedicated midrange and another midrange that also pulls tweeter duty.
Click to expand...
Thanks for the post. It was hard to figure out from Tony's picture.
This makes more sense. I haven't ever seen a crossover setup like this before.
 
I looked at the catalog closer about the size of the woofer. Catalog says "8inch" but they look and measure a bit bigger. They have a 46 sq inch radiating area but the 8 inch woofer in the Nova 6s have 30 sq in radiating area.

The 21 blow away the Nova 6s anything bass related and pretty much beat My Nova 7Bs in that area as well.....the "10 in " woofers have a 47in radiating area.
 
Bodyblue said:
So they are indeed 3 way speakers?
Click to expand...
sorta? One tweeter has the highs rolled off depending on where you set the switch, one or the other driver will pull more midrange duty.
So one driver does midrange AND the high frequencies, and the other JUST midrange frequencies. The catalog description states those two drivers are midrange tweeters. With the woofer running full range and rolling off naturally. Since the woofer rolled off at 58hz, I'm guessing they may have a bit of a midbass hump in the response?
 
The 21s were my first pair way back when. I used them for many years. I had the 23s , also which came with only one tweeter. Both sets go low. They are a 10 inch.
I have the Nova 8 b s here, too . They’re not recapped but they sound better than my recapped Optimus 5 b. You can’t go wrong.

Looking at that catalog, I’m nearly 70 and at the time we didn’t know how good we had it
 
Sharpstik said:
The 21s were my first pair way back when. I used them for many years. I had the 23s , also which came with only one tweeter. Both sets go low. They are a 10 inch.
I have the Nova 8 b s here, too . They’re not recapped but they sound better than my recapped Optimus 5 b. You can’t go wrong.

Looking at that catalog, I’m nearly 70 and at the time we didn’t know how good we had it
Click to expand...
If you were close by I would take those 5B's off your hands. :naughty:
I've been looking for a pair of 5B's or 8B's for some time now. All I ever see are the Optimus 1B's, Nova 7B's and Nova 6B's.
I worked part time in high school in the 70's and bought a pair of Optimus 1B's and 2B's while I worked there. Alao a SA-1000A amp, TM-1000 tuner, LAB turntable and top loading cassette deck. Oh, and I got that big 10% discount for working at RS. I still have the TM-1000 tuner and Optimus 2B speakers.
 
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Bodyblue said:
I looked at the catalog closer about the size of the woofer. Catalog says "8inch" but they look and measure a bit bigger. They have a 46 sq inch radiating area but the 8 inch woofer in the Nova 6s have 30 sq in radiating area.
Click to expand...
Optimus-21 ran for two years only. It was introduced in the 1978 catalogue as having an 8" woofer with 25 square inch cone radiating area. The 1979 catalogue changed that to a 10" woofer with 46 square inch cone area.

79-Opti-21-cat copy.JPG

78-Optimus-21_specs.JPG

79_Optimus-21-specs.JPG

All else (cabinet dimensions, frequency response etc.) is identical. Have yet to see any pics or reports of Optimus-21 with 8" woofer found in the wild.

Speculation has suggested it may have been prototyped using an 8" woofer but changed to a 10" for production, while outdated ad copy had already been submitted to meet 1978 catalogue press deadline.
 
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Stuart Pedaso said:
Optimus-21 ran for two years only. It was introduced in the 1978 catalogue as having an 8" woofer with 25 square inch cone radiating area. The 1979 catalogue changed that to a 10" woofer with 46 square inch cone area.

View attachment 3129747

All else (cabinet dimensions, frequency response etc.) is identical. Have yet to see any pics or reports of Optimus-21 with 8" woofer found in the wild.

Speculation has suggested it may have been prototyped using an 8" woofer but changed to a 10" for production, while outdated ad copy had already been submitted to meet 1978 catalogue press deadline.
Click to expand...

Mine are the 10 in.....and I bet the ad copy had went to the press and it was too late to change, just like you said.
 
My Optimus-21 aren't convenient to access just now, though reverse calculating from published "46 square inches" of cone radiating area gives the effective diameter of 7.65 inches. Tandy may have used cone diameter inside the surround for this calculation. :dunno:

Marketing does play fast and loose with speaker size. This is a 3016Aa used in Optimus-5B and Nova-8B and described as a 12 inch woofer.

3016Aa 12 inch woofer.JPG

It's exactly 11 inches across the basket, 10 inches outer surround diameter and the cone diameter is just 8.5 inches inside the surround.
 
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