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JBL SA600 Vintage Amplifier Review

D

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I was really curious how this amp should perform and how it could have performed when it was new. As the circuit is simple, I have put it into MicroCap, trying to keep parts as close as they were.

This is the schematics
View attachment 228185
and it was designed by a competent circuit designer. R4+C2, R27+C9 and C6 make very interesting and perfectly working frequency compensation. Small signal frequency response looks like this
View attachment 228186
It is wide and flat. Remember I speak about the power amp section only, without bass/treble eq.

The idle current is quite low, due to simple bias circuit with 5 diodes in series. This is the point that might have been grossly improved. Because of the low idle current, distortion is rising at low power. However, the value of distortion above 100mW is fully acceptable.
View attachment 228187
In this distortion/power simulation, please take into account that ideal voltage sources are used to supply the amp.

I will investigate the PSRR as well, and I expect it will not be as good as these basic parameters.
These are things that your average person doesn’t realize, or even think about it myself included.

I have a feeling that even though this amplifier was lovingly put back together, it’s not quite performing like it originally did.
 

beagleman

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These are things that your average person doesn’t realize, or even think about it myself included.

I have a feeling that even though this amplifier was lovingly put back together, it’s not quite performing like it originally did.

I have a similar era older Kenwood receiver.
NEVER has it been measured obviously, but going from just common sense and sound quality comparisons I have done in relation to much newer and frankly "More HiFi" type of components, it is FAR better than this unit appears to be.

Yes it has noise, but not "End of the world" horrible noise,

It does not roll off extremely in the bass at all, but simply has a somewhat "Fatter" sounding bass and a bit of a rolled off "top end" type of sound.

Honestly not great sounding, but EASILY listenable, considering the age, and NOTHING have ever been done to it.
 

mhardy6647

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I like it! Would be interesting on Amirm's bench. I also like the

AD-850-SE (It needs something like Dolby-S), though.​

No flavor of Dolby NR hardware* is being produced any more, to the best of my knowledge.
Looks like the cassette deck part of that beast has no NR at all (not even generic noise reduction or "DNR").
So, yeah, that'll be great. ;)

ad-850-se-b_main.jpg


That warm analog hiss -- like embers in a dying fire. :cool:
Pair that TEAC with a vintage Certron cassette tape and relive 1974 all over again. Now, where did I put the keys to that Ford Pinto?

s-l400.jpg


___________
* EDIT: I should qualify this and say "no flavor of consumer Dolby hardware" (i.e., B, C, S, or HXPro) -- and, again, to the best of my knowledge.
 

restorer-john

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I am pretty sure I would be able to re-create this power amp section of the Locanthi amplifier, with new PCB design and parameters close to the original.

I just knew you would do this Pavel! Let's rebuild a Locanthi design in 2022. :)
 
D

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No flavor of Dolby NR hardware* is being produced any more, to the best of my knowledge.
Looks like the cassette deck part of that beast has no NR at all (not even generic noise reduction or "DNR").
So, yeah, that'll be great. ;)

ad-850-se-b_main.jpg


That warm analog hiss -- like embers in a dying fire. :cool:
Pair that TEAC with a vintage Certron cassette tape and relive 1974 all over again. Now, where did I put the keys to that Ford Pinto?

s-l400.jpg


___________
* EDIT: I should qualify this and say "no flavor of consumer Dolby hardware" (i.e., B, C, S, or HXPro) -- and, again, to the best of my knowledge.
The cassette tapes… Too funny!

I remember possibly buying cassettes at the drugstore as we called them here or in the US many moons ago. They were available everywhere, and to suit every budget, just wonderful memories for me.

I started out at around 12 years old putting the cassette recorder up to my parents radio or record player. I’ll bet the SINAD was at least 3.5 dB on those recordings.

Is it live or is it… 10 for $1.75 cassettes?
 
D

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I also enjoyed 8-tracks, as long as I had a matchbook handy, so that track 1 didn’t bleed onto track 2 hahahaha

Head alignment? Nah matchbook cover!
 

mhardy6647

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You should look into that. Sandwich heads etc.
The head wouldve cost more thaan the whole player did, ca. 1970! ;)

I can't say I ever really enjoyed 8-tracks, but I used 'em. I do miss my Weltron "space helmet" portable AM-FM-8 track, though. I am not sure what actually happened to it. :(
It actually sounded pretty good for what it was, and it sure had panache.
 

fpitas

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I just knew you would do this Pavel! Let's rebuild a Locanthi design in 2022. :)
Sounds like fun, but the tough part will be finding transistors bad enough ;) especially the power transistors. They really were terrible back then.
 

Jim Shaw

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Measuring a 1966 "transistorized" hifi amp is much like measuring horsepower compared to a real horse. Which horse and how old is that horse? And, what does the answer tell you?

In 1966 audio, replacing vacuum tubes with transistors was a very difficult design chore... and except for uptime in use, very unrewarding. We just didn't have the solid-state devices and design methods that have come since. ICE hadn't been invented. Engineers still relied on graphs. Capacitor material science was many decades behind today. Comparatively, transistorized devices were bronze-age.

[It] "is like a dog's walking on his hind legs. It is not done well; but you are surprised to find it done at all." ― Samuel Johnson

Why any reasonable human would pay >$8K for a relic is a question better left to museum curators. And value best associated with the "greater fool" theory.
iu
 

fpitas

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Measuring a 1966 "transistorized" hifi amp is much like measuring horsepower compared to a real horse. Which horse and how old is that horse? And, what does the answer tell you?

In 1966 audio, replacing vacuum tubes with transistors was a very difficult design chore... and except for uptime in use, very unrewarding. We just didn't have the solid-state devices and design methods that have come since. ICE hadn't been invented. Engineers still relied on graphs. Capacitor material science was many decades behind today. Comparatively, transistorized devices were bronze-age.

[It] "is like a dog's walking on his hind legs. It is not done well; but you are surprised to find it done at all." ― Samuel Johnson

Why any reasonable human would pay >$8K for a relic is a question better left to museum curators. And value best associated with the "greater fool" theory.
iu
I agree, but the fact is, there is a huge market for retro stuff. Most of which is at least half-broken, and/or wrong parts were used for repairs, and doesn't perform even vaguely like the original.
 

restorer-john

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Sounds like fun, but the tough part will be finding transistors bad enough ;) especially the power transistors. They really were terrible back then.

Actually, they weren't.

You want to go back to germanium, sure, they were flaky at best, but once we transistioned into the early 1970s, silicon was getting really good. It only got better and better as time went on.

There's a ton of misinformation and revisionist BS out there in relation to 1970s silicon. Consider RETs came about in the mid 70s, LAPTs in the late 70s and basically all diffused emitter devices after that.

Sanken, Fujitsu, Hitachi, Sanyo, Sony, Toshiba etc. Without those guys, silicon would stilll be in the stone age.
 

fpitas

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Actually, they weren't.

You want to go back to germanium, sure, they were flaky at best, but once we transistioned into the early 1970s, silicon was getting really good. It only got better and better as time went on.

There's a ton of misinformation and revisionist BS out there in relation to 1970s silicon. Consider RETs came about in the mid 70s, LAPTs in the late 70s and basically all diffused emitter devices after that.

Sanken, Fujitsu, Hitachi, Sanyo, Sony, Toshiba etc. Without those guys, silicon would stilll be in the stone age.
Well, this is from 1966. A lot of important things happened in the following years.
 

pma

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I agree with @restorer-john . As soon as we had 2n3055/mj2955 pair (late 60’s), it was possible to make a competent solid state amp design.
 

fpitas

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I agree with @restorer-john . As soon as we had 2n3055/mj2955 pair (late 60’s), it was possible to make a competent solid state amp design.
Did those exist in 1965, when Locanthi was doing his design?
 

fpitas

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They had parts especially produced for JBL. See their special marking.
Right. And I bet they had low ft and limited SOA. Trying to design an effective feedback amp with those old transistors was a pain. Just look at the distortion figures Locanthi shows at 20kHz. You had to do tricks with zeros etc.
 

restorer-john

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I agree with @restorer-john . As soon as we had 2n3055/mj955 pair (late 60’s), it was possible to make a competent solid state amp design.

The 2955 was slower than the 3055. All the amps I built in the 70s/80s were quasi-comp 3055 designs. I think it was the MJ802 and its complement pair (4502?) where PNP got partialy decent for us hobbyists back then. The the MJ-15003/4 for 'big' amps.
 
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