• WANTED: Happy members who like to discuss audio and other topics related to our interest. Desire to learn and share knowledge of science required as is 20 years of participation in forums (not all true). There are daily reviews of audio hardware and expert members to help answer your questions. Click here to have your audio equipment measured for free!

Kenwood L-08M Monoblock Amplifier (Vintage Review)

restorer-john

Grand Contributor
Joined
Mar 1, 2018
Messages
10,016
Likes
28,648
Location
Gold Coast, Queensland, Australia
On the topic of vintage audio gear from Japan, I am about to finish restoring a Sansui AU-999 and can drop it off with Amir if there is interest.

Classic Sansui! :)

When you say 'restoring', what is the scope of your restoration and what have you changed/modified/repaired?
 

itsikhefez

Member
Joined
Mar 20, 2019
Messages
27
Likes
26
Location
Greater Seattle Area
I know there are differing approaches and I think I have read some of your comments in this regard.
Given that this is a 52 year old unit, I preferred to replace anything that has failed or could fail in the future.

I replaced all electrolytics, green caps and small signal transistors with parts of equivalent value.
I replaced the output transistors to MJE types because one was not original (seemed to have been replaced sometime in the past)
The only mod I did was the kevzep preamp mod to increase headroom.
My unit did not have the subsonic filter and I left the speaker protection in tact.
 

Eldus

Member
Joined
Jun 8, 2021
Messages
74
Likes
67
What’s weird is that I didn’t touch anything between the three different measurements. In the first round where the high frequency noise was introduced, all I was doing was turning the knob on the preamp to increase volume.

Then later, when everyone was asleep, I just retested without changing anything and the noise was gone!

Just wasn’t sure if ~30 kHz was a classic tell-tale ________.

Since the THD+N measurements are just 20-20kHz, it doesn’t affect the numbers much other than a fraction of a dB lower noise

It’s impressive that Kenwood achieved this performance which is transparent to CD quality five years before the advent of CDs.

I am not sure what transistors are in the L-08M. The service manual lists the transistor type but not the manufacturer.

View attachment 233063
I would love to see measurments of the first commercial CD player. The Sony CDP-101.
 
OP
G

GXAlan

Major Contributor
Forum Donor
Joined
Jan 15, 2020
Messages
1,370
Likes
2,170
I would love to see measurments of the first commercial CD player. The Sony CDP-101.

Not quite the CDP-101 but still interesting!
 

charleski

Addicted to Fun and Learning
Joined
Dec 15, 2019
Messages
836
Likes
1,660
Location
Manchester UK
I would love to see measurments of the first commercial CD player. The Sony CDP-101.
There's a test report of the first Philips player (CD100) on the Miller Audio site (scroll down to HFN Oct 2011). He wants people to register (free, you get the user/pw by automatic email reply), so I won't post the direct link, but here's a taster (there's a lot more in the full report):
Screenshot 2022-10-01 105850.jpg

Quite a few FAILs, unfortunately.
 

poxymoron

Active Member
Joined
May 7, 2021
Messages
271
Likes
296
Thanks for the review @GXAlan.
This is really impressive and very interesting. My love for vintage Hifi diminished a bit some years ago when I thought we had all been fed lies and marketing billsh!t for so long, but it's great to see that there were some real gems and real efforts made to produce something worthy of the Hifi hyperbole. This ASR forum and everybody that contributes to it the best thing that's happened to the industry. Please keep up the great work, well done and thank you.
 

Vear

Member
Joined
Dec 28, 2019
Messages
28
Likes
36
Kenwood L-08M Monoblock Amplifier Review
(Vintage Gear; QuirkAudio restored)
View attachment 232966

Where were you in 1980? In Tokyo, Japan, a small team of engineers released the most sophisticated amplifier of its time...

A bit of history
In the 1970's, an elite team of veteran Trio-Kenwood engineers left the company to form what is today known Accuphase. Half-rivals and half-colleagues, Kenwood would actually maintain a sizeable ownership of Accuphase all the way through the mid-1990's. Accuphase had the goal of building flagship audio equipment using the very best in craftsmanship and engineering. With a focus on quality not quantity, Accuphase continues today as a leader in high-end Class A and Class AB amplifiers and has a deep inventory and vault of spare components and parts for their electronics going all the way back to its founding.

Meanwhile, the young and talented engineers that remained at Kenwood wouldn't let their mentors take all of the glory to Accuphase-Kensonic Laboratory. At the start of what would we today recognize as Japan's "bubble economy" of the 1980's, an elite team at Kenwood set out to build the very best amplifier its engineers could achieve. With Accuphase-Kensonic already giving the Trio-Kenwood executives access to traditional high-end audio, the engineering team at Kenwood were not encumbered by the requirements of tradition or heritage. They were truly free to try out different concepts and ideas.

The L-08M represents the pinnacle of Kenwood's efforts in the audiophile arena never again to be matched on the specification chart. Let's look at the advertised specifications:

View attachment 232913

Want a high damping factor?

"How about 15,000!" Benchmark Media has a great blog explaining why that number should seem unbelievable -- speaker cables set a practical limit on damping factor. Kenwood engineers knew that too. To solve this issue, they had a second pair of wires going from the amplifier to the speaker terminals so that the negative feedback could be measured in such a way that included the effects of the speaker cable. This technology was called Sigma Drive.

Amplifiers need to run cool?
"We're going to make the whole chassis a sand casted heatsink." See that 27.5 lb weight? All heatsink.

Need a wide bandwidth?
"DC to 600 kHz!"

Want a fast slew rate to avoid TIM?
"How about 200V/microsecond!"

Want low distortion?
"We're going to apply negative feedback all the way to 50 kHz!"

Worried about crossover distortion?
"We'll avoid this with an ultra-high speed design!"

Worried about electromagnetic issues?
"We'll use as much non-magnetic material as possible! State-of-the-art material plastics!"

Repairability?
Why are you asking so many questions? Just look at how beautiful this is!

View attachment 232969

Maintaining a stock of old parts?
Our colleagues at Accuphase are stockpiling transistors for no good reason. We'll just order replacement parts from our suppliers whenever we need them, have them delivered to us via Pan-Am Cargo. With the way the semiconductor world is growing, I'm sure in 2 or 3 years, we'll be able to order anything we need to repair these from a Sears Mail Order Catalog.

(That's what they call foreshadowing.)

Very few specimens of the L-08 line still exist. Even rarer is my120/240V version with a German Type F plug. The plastic portions for the power switch and rectangular component of the amp and top-heavy beefy heat sink design makes it easily to topple or get damaged in transit. If the Sigma Drive cables are incorrectly wired, the whole unit is destroyed. If the remote trigger is wired to a 12V trigger, it probably destroys the unit too. This remote trigger was designed specifically for the L-08C and just wants the open circuit to be closed like a simple switch, as the entire trigger mechanism is self-powered by the amplifier itself. I'm sure user error adapting a 12V trigger to this also destroyed a lot of these amplifiers.

This particular unit was damaged during transit as it was shipped from Europe to the United States at the very beginning of the pandemic. It looked like the box was dropped, and it was clearly a stressful time for all of the world. It was repaired and recapped by QuirkAudio and in full disclosure, Peter from QuirkAudio has made additional recommendations to further improve its performance which haven't been applied yet. There's a real chance we can improve the performance even more.

Right now, Sigma Drive is operational on the L-08M, but I was cautioned by Peter that if the unit fails from a bad transistor while Sigma Drive is enabled, his opinion is that there is a real chance it will destroy components that just aren't available anymore and I'll end up with a beautiful heatsink. If I leave Sigma Drive off, it probably won't make a big difference in sound and the chances of catastrophic failure decrease substantially. Even the switch is apparently particularly complex. So... this unit is being tested with Sigma Drive disabled. We probably get even better performance with the feature enabled. And again, Peter from QuirkAudio still wants to make some tweaks some of the capacitors, but he and I have both been too busy. These amps see regular use driving my primary speakers in my home theater.

Oh, the marketing photos don't give you a true sense of the scale for the monoblock amplifiers. A pair of L-08M's takes more volume than the Marantz PM-10 and this photo comparing it to the svelte JBL SA600 gives you a good sense of scale.

View attachment 232961


Test Conditions
The signal chain is Panasonic UB9000 -- balanced-2V --> Marantz PM-11s2 as pre-amp --single ended--> Kenwood L-08m --Vishay Dale 1% NH-250 4.48 ohm resistor -- AUX in -> E1DA Cosmos
I have tested these in the stereo mode of the E1DA and ultimately used the AUX input for my final tests.

Initial testing idiosyncrasies...

I started with XLR input for 5W. I've blocked out the numbers so you can pay closer attention to curve and the ultrasonics in particular...
View attachment 232935

and then switched to the AUX to test higher power output
View attachment 232936
View attachment 232937

Whoa! Did you see that? What's going on at ~30 kHz and ~70 kHz? I went back to the original 5W tested power and back to using the XLR inputs again. No change -- I still had those spikes.
I took a break for dinner and then went back to testing again. This is where it gets really interesting. I just stuck with the AUX input on the E1DA Cosmos ADC.

View attachment 232939

View attachment 232940

View attachment 232941

See those humps at ~12 and 20kHz?

View attachment 232942

Did you see that? The humps at 12 kHz and 20 kHz dropped! The graph explains what I did. I turned off the lights in the room. LED lights really can affect signal quality. I did not have a dimmer on the lights so they were running full brightness as well. With the LED lighting out of the way, I continued increasing power.

View attachment 232943

View attachment 232944


It's pretty interesting to know that LED lights can introduce issues in the audible spectrum. Although the actual contribution is low, I can certainly imagine this playing a role. Now that I know that LED lights create this noise pattern, it gets even more interesting that different interconnect cables are more or less sensitive to the interference of the LEDs.

Oh, by the way, did you notice those numbers?

93.5 dB ~5W SINAD achieved 42 years ago in 1980 with high gain (1V input sensitivity). From Kenwood.
(And that's on my not-as-good-as-Amir's test setup. My PM-90 is 8-9 dB worse than his PM-90 amplifier measurements.)

As with the case with solid state, as you increase power, the noise goes down, allowing us to nearly reach 100 dB SINAD score at higher power. I am pretty sure I can improve the numbers if I fed the unit a better DAC with digital volume control. Right now, it's UB9000 then a PM-11s2 as pre-amp and then the amplifiers. It'll also be interesting to see the numbers when I can use the mono summed version of the E1DA Cosmos ADC. The broken one has a weird hump in the 7-8 kHz range which takes SINAD down about 3 dB

View attachment 232968


Subjective Impressions
Truth. Transparency. Makes bad recordings worse. Makes great recordings incredible. If I only had one amplifier, it'd be the Marantz PM-10 just for its versatility. If I am allowed to keep two amplifiers, my #2 would easily be the Kenwood L-08C/L-08M combo.

When I first brought the L-08M's home and fired it up, I was actually very disappointed. Vocals sounded thin and I just didn't enjoy it as much as my tube amp or warmer sounding Marantz PM-90 which measures worse. I was listening to the soundtrack from La La Land where you have actors/actresses singing (not necessarily professional singers). As I switched back and forth between the Kenwood L-08M and the Marantz PM-90 fed directly by a Topping D50s, I quickly was able to identify the difference. On the Kenwood, voices from the La La Land Soundtrack sounded like they were coming from something a little over an inch wide: a human mouth. With tube amps or the PM-90, the voices were larger than life as if I was watching something at a movie theater rather than having someone actually in the room.

Switching back and forth, it was pretty clear to me that the Kenwood was a more accurate amplifier that could bring Emma Stone into the room. But no matter how much my brain told me that I should want and prefer the absolute truth and transparency in my gear, I still personal have the preference for a larger-than-life sound and euphonic colorations that I get with the tubes or my Marantz gear -- at least for that specific soundtrack.

But classical music? Wow.
LP vinyl? Wow x 2.

Classical music is brilliant on this and it makes grand pianos sound authentic. I cannot describe exactly what is special and a null test might be a great way to assess this. I'm very familiar with piano music, and back when I was actively taking lessons, I could walk through a hotel or mall, hear an actual piano in the background and tell you with complete accuracy whether it was a Steinway, Kawai, Yamaha, or other. What really stands out with the L-08M is what I call micro-imaging for lack of a better term. Even with LP vinyls, you not only get a clear phantom center, a clear left and right image, but what separates this L-08M from basically every other amplifier I've listed to, other than the PM-10, is an uncanny ability for this amplifier to place instruments in sharp focus in a way unlike any other amplifier in the space IN BETWEEN phantom center and left or right. I have had other monoblocks such as those from Adcom and PS Audio 200c, but none of them achieved what the Kenwood could. I actually think the Kenwood L-08M beats the Marantz PM-10 for delivering the most narrow phantom center image (specifically for the La La Land Soundtrack) but the PM-10 gives me the chameleon of clean vocals with the tiniest amount of tube warmth with all of the crystalline clarity of classical music where direct comparisons can be made to reality.

The L-08M does great in my home theater setup. Harmonious Live on Disney+ sounds truly breathtaking with the L-08M. Even though the show is streamed in lossy Dolby Digital Plus, it indicates to me that the L-08M doesn't need some sort of fancy audiophile master recording. The L-08M is just a window of truth. It cannot make very good singers (who are better at acting) like Emma Stone and Ryan Gossling sound like spectacular singers the way that my Marantz gear can. The L-08M instead makes me feel like Simon Cowell -- suddenly I can easily hear the difference between world-class singers and very good singers.

Conclusion
More than 40 years ago, state of the art amplifiers like the Kenwood L-08M were driven by engineering and science. The designers behind this amplifier at Kenwood never gained the status of icons like Nelson Pass, Sidney Corderman, Frank McIntosh, Gordon Gow, Ken Ishiwata, John Siau or even the Accuphase founders, Nakaichi and Jiro Kasuga. As far as I know, they remain anonymous. But it is clear with my measurements that they deserve recognition in the history of hi-fi audio. They were not afraid to try new concepts or do things. Service manuals are readily available, and though many of the parts are now unobtainium, any patents are long expired.

This amp easily beats the average amplifier tested here and likely would have been the best measured amplifier of its time... two years before the introduction of the Compact Disc (in the USA). With Sigma Drive active, the negative feedback in real world loads would have been even better and this unit would have been fully transparent to even the 16-bit/50 kHz Soundstream recording technology of the era.

In testing this, I did discover the nugget of truth behind a lot of audiophile myths. Clearly something introduced those high frequency spikes during my testing, and this could be in the area where "AC line conditioners" could have made a difference. None of my other amps tested so far showed this kind of sensitivity to household electricity, so it's possible that older amplifier designs were more sensitive to "dirty power" at least in the realm of measurements.

I've attached PDFs of the technical explanation behind Sigma Drive and the original Kenwood L-08C/L-08M brochure. I don't remember where I sourced these, so apologize for not giving credit where credit is due.

Oh, and one more thing...

I tested the L-08M this evening yet again... when most of the neighborhood is asleep and most of the lights were off in my home.

View attachment 232980

View attachment 232981

Sorry if I missed it but were these original unrestored units?
 
OP
G

GXAlan

Major Contributor
Forum Donor
Joined
Jan 15, 2020
Messages
1,370
Likes
2,170
Sorry if I missed it but were these original unrestored units?
Restored by Quirkaudio. The capacitors can go bad on these (after 20+ years).
 

Vear

Member
Joined
Dec 28, 2019
Messages
28
Likes
36
Restored by Quirkaudio. The capacitors can go bad on these (after 20+ years).

Yes, restoration is critical for any unit from that era. I have a pair of restored L-05M that I've been thinking about to sending to @amirm .
 
Last edited:
OP
G

GXAlan

Major Contributor
Forum Donor
Joined
Jan 15, 2020
Messages
1,370
Likes
2,170
Yes, restoration is critical for any unit from that era. I have a pair of restored L-05M that I've been thinking about to sending to Amir.

That's a great idea! The L-05 and L-06 are physically easier to ship safely!
 

Vear

Member
Joined
Dec 28, 2019
Messages
28
Likes
36
That's a great idea! The L-05 and L-06 are physically easier to ship safely!

Plus I have the original boxes and packaging but I'm on the east cost and not sure I want to spend the $ on shipping...
The L-05M have the same published specs as the L-07MII only with 100W per monoblock vs 150W. It would be really interesting to see how they measure on a state of the art AP analyzer.
 

ConnorG

Member
Joined
Jan 7, 2020
Messages
9
Likes
34
Location
MA
Speaking of the L-07MII, I just finished a comprehensive restoration and repair on a set yesterday. The video can be found here:

 

restorer-john

Grand Contributor
Joined
Mar 1, 2018
Messages
10,016
Likes
28,648
Location
Gold Coast, Queensland, Australia
Speaking of the L-07MII, I just finished a comprehensive restoration and repair on a set yesterday. The video can be found here:


Nice looking pair of mk2s. The mk2s usually end up a lot more battered due to the soft black surface on the aluminium.

Your measured power output, however, is unusually low. All my multivoltage L07ms and L07mk2s clipped at [email protected]/[email protected] because the assymetric clipping you show is also not typical- they clip evenly. Check your ASO (current limiters).
 

Vear

Member
Joined
Dec 28, 2019
Messages
28
Likes
36
Speaking of the L-07MII, I just finished a comprehensive restoration and repair on a set yesterday. The video can be found here:


Nice to see you used the Sanken 2SA1186 & 2SC2837 I recommended to you in the other forum a while back. Those are exactly the same ones I had chosen to put in my L-05M.

For comparison, I believe my restored L05M benched at ~120W @8Ω & ~185W @4Ω (spec's rated them at 100W @8Ω & 150W @4Ω).
 

anander

Member
Joined
Oct 13, 2021
Messages
35
Likes
26
Wow... that's a sexy amplifier if I've ever seen one. Cool story, thanks.
 
Top Bottom