JBL SA600 Vintage Amplifier Review

[Joe Smith]

I truly love the face of this amp but the jacks on the underside would just be a deal breaker for me, no matter how it performed. The jacks on a Beomaster receiver drive me kinda crazy; this is worse.

Shame about the testing, hope she's not blown by the sweep test. Well, maybe ultimately OK for quieter listening? Hope so as it sounds like it was a fairly long rebuild with some $$ involved for the owner...

 

[testp]

great, where is CD-in?

 

[Joe Smith]

great, where is CD-in?

Back in 1966, that would be covered by "AUX," or as this amp elegantly puts it "Auxiliary"...

 

[Keith Conroy]

This is a review and detailed measurements of the JBL SA600 vintage (1966) stereo amplifier. It is on kind loan from a member.
View attachment 227934

The classic look endures the test of time. It is more usable and stylish than many amps of today. Inputs though, are oddly placed underneath the unit:
View attachment 227935

Prices for these can go way up with one currently listed at US $5,600!

Here is the background from the owner:

"“This was an estate find that came with an original owner’s manual (not a photocopy!) that looked and sounded great albeit with a large channel imbalance. I reached out to Peter to do a “classic car” restoration in that I wanted to figure out how to keep this amp running another 60 years from now but to also keep things as close to original as possible instead of doing a restoration-modification like transforming a vintage car with a Tesla powertrain. I’d like to send it to Amir after restoration so we have an rough estimate of how the very first solid state integrated amps from the 1960s likely performed. This amp is interesting because JBL didn’t know how the amp performed because it was at the limit of the test equipment at the time before negative feedback was applied, so no one really knows how it performs once negative feedback was applied. I sent Peter a full set of new-old-stock JBL branded, Motorola manufactured transistors from a different estate sale from someone’s whose dad worked closely with Ed May. Per Peter, several of these transistors had already deteriorated on their own, but he was able to use a few of them in the restoration.”

Peter is humble, and I would add that he spent about a year researching the idea and reviewing the technical and original designer’s writings before actually embarking on the restoration. This is almost a 2 year project! Ironically with the goal of not making it “too good” that it would defeat the purpose of having a benchmark to compare against."

And notes from Peter from Quirkaudio:

"The SA 600 has four PCB.

The power supply new electrolytic capacitors throughout with, in the main supply, the original 4500Uf 40v replaced with low ESR 10k 63v. The diodes were also replaced with 5A ultrafast types. The secondary voltages of which there are three, also had the original 1000uf 6v replaced with 2200uf 25v and the diodes again replaced with 1A ultra fast types. The film caps were also replaced as were the dropping resistors.

The preamp board had all of the electrolytic caps replaced, with a combination of low impedance and audio types, including the Nichicon Muse UES BiPolar types, depending on function. Some of the values were increased where appropriate. The film caps were mostly replaced with polypropylene or polyester depending on function. A number of ceramic capacitors were also replaced, generally with WIMA polypropylene.

Each main amp channel has a single PCB. The input pair were replaced with low noise modern transistors, and all the electrolytics were also replaced, again with a combination of low ESR and audio types as applicable. The input filter cap was replaced with a polypropylene film type as were the ceramic caps. The power rail caps were increased in capacity slightly. The main output transistors were all replaced with modern OnSemi types as they all measured poorly. Some of the resistors on the main amp boards were also replaced, mainly to reduce noise at input and feedback."

Here are the distortion and power specs:
View attachment 227943

JBL SA600 Measurements
While there is a set of pre-outs, they seem to be just pass through so I measured the amp as a whole unit. Here is our dashboard:
View attachment 227936

Not a pretty picture although much of the fault falls on the shoulders of the power supply. The 120 Hz harmonic dominates SINAD at just 40 dB. If we replace that power supply with an ideal one, SINAD would be around 60 dB given the second harmonic at -65 dB.

As noted in the intro and in my measurements, channel balance is quite off although it depends partially on the volume control position. In the review image you can see where I had to set it for the measurements.

SNR suffers of course due to that power supply noise:
View attachment 227937

Then again at full power it is dominated by distortion which is close to what I guessed.

There is no tone defeat. With the controls not be precise at center position, response was quite variable:
View attachment 227938

The solid lines are the closest I could get to a flat response without too many trials. The peaking is above audible band fortunately. The low frequency cut off is not. The input is capacitor coupled so some of that roll off is due to that. Here is the effect of the loudness control:
View attachment 227940

Multitone shows the level of distortion we saw in the dashboard:
View attachment 227941

Crosstalk was quite poor:
View attachment 227942

Here is our power response:

View attachment 227945

View attachment 227946

Even allow for more distortion I could not reach the spec:
View attachment 227947

I then attempted to run my frequency vs power sweep but the amp shut down with the first test (20 kHz). The fuse blew and unfortunately I don't have a slo blow 2 amp one to put in there. I have one on order. Hopefully that is all it is and not a blown output stage.

Conclusions
1966 is a long, long time ago. We are talking early decade in life of transistor amplifiers. Was this a good attempt or not? Based on current measurements it is not but there is no way to know how it originally performed. Company specs are certainly a lot better than what I am seeing. Maybe the power supply can be further improved. Not sure.

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As always, questions, comments, recommendations, etc. are welcome.

Any donations are much appreciated using: https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/

To some degree I feel bad for the current owner. I can only imagine the total money invested here?? I do also think some of the parts replacements might have a negative affect on the circuit. However to be fair, I'm not really in a position to judge this. One thing about the historic part of the JBL design & its current performance. I'll use classic cars as an example. Lets take a stock 1966 Corvette Sting Ray. This is a car I have drive. I can tell you when its performance is compared to a 2018 corvette it comes out terrible. As to abilities & performance between the 2 cars the 1966 Sting Ray scores badly! However these cars are very sought after. People love them. They are worth a lot of money. Or we could still use cars and go back further to a model T. Not even close to any modern car of today in performance. However people love them and drive them. In audio take old record players & 78 records. At some level its about perspective ???

 

[mhardy6647]

The P/S should give no audible hum -- it's measuring at least 20 dB higher than it should.
Heck, even my P/S abominations don't hum... and as a DHT (ahem, that's direct heated triode) user, I am pretty oblivious to a bit of mmmmmmmm in the background on 104 dB per watt @ 1 meter loudspeakers.

 

[Neddy]

I truly love the face of this amp but the jacks on the underside would just be a deal breaker for me, no matter how it performed. The jacks on a Beomaster receiver drive me kinda crazy; this is worse.

Shame about the testing, hope she's not blown by the sweep test. Well, maybe ultimately OK for quieter listening? Hope so as it sounds like it was a fairly long rebuild with some $$ involved for the owner...

I agree about the jacks, but I just put the stack on a Nelson Bench. Worked great.

 

[PeteL]

I'll use classic cars as an example. Lets take a stock 1966 Corvette Sting Ray. This is a car I have drive. I can tell you when its performance is compared to a 2018 corvette it comes out terrible. As to abilities & performance between the 2 cars the 1966 Sting Ray scores badly! However these cars are very sought after. People love them. They are worth a lot of money. Or we could still use cars and go back further to a model T. Not even close to any modern car of today in performance. However people love them and drive them. In audio take old record players & 78 records. At some level its about perspective ???

I could be argued that compared to a 2020 Hiundai Accent it's "performance" comes out terrible, depending on the performance metric.

 

[Keith Conroy]

I could be argued that compared to a 2020 Hiundai Accent it's "performance" comes out terrible, depending on the performance metric.

Hmmmm...........................Maybe the model T is better??? Turn that hand crank to start can be rough though!

 

[Joe Smith]

To some degree I feel bad for the current owner. I can only imagine the total money invested here?? I do also think some of the parts replacements might have a negative affect on the circuit. However to be fair, I'm not really in a position to judge this. One thing about the historic part of the JBL design & its current performance. I'll use classic cars as an example. Lets take a stock 1966 Corvette Sting Ray. This is a car I have drive. I can tell you when its performance is compared to a 2018 corvette it comes out terrible. As to abilities & performance between the 2 cars the 1966 Sting Ray scores badly! However these cars are very sought after. People love them. They are worth a lot of money. Or we could still use cars and go back further to a model T. Not even close to any modern car of today in performance. However people love them and drive them. In audio take old record players & 78 records. At some level its about perspective ???

Yeah, while admire the amp for its classic design, I think it's unrealistic to expect top level performance from it, even restored...I like my restored Marantz and Kenwood receivers, but I don't expect them to be able to match the best of today's amp technology...I like using them though, and restored, they are solid performers...

 

[Joe Smith]

I agree about the jacks, but I just put the stack on a Nelson Bench. Worked great.
View attachment 228081

Eureka! You have solved it!

 

[fpitas]

Back in 1966, that would be covered by "AUX," or as this amp elegantly puts it "Auxiliary"...

Some also had Tape, and sometimes even Line. As I recall, they were all just line-level inputs.

 

[Hipster Doofus]

Thanks, I still like to see the old stuff reviewed from time to time. My ears are from 1954.

 

[restorer-john]

peaking of restoration: One might be tempted to build a contemporary amp in the cabinet of such a "dinosaur".
Yes, the retro freaks would consider it a sin, but still...

It's not a sin. When an amp is so far gone, all bets are off- people can do what they like.

Kind of fish-tank in an old TV is even fair game.

 

[restorer-john]

Interesting and welcome review of a lovely old JBL! I would have liked to have seen and tested the amp before any restoration. So many 'restorations' wreck the originality to the point you are essentially measuring a completely different amplifier.

I'm dealing with a similar thing here: https://www.audiosciencereview.com/...orkbench-right-now.11155/page-20#post-1301615

 

[Steven Holt]

I've seen worse with single rail quasi complimentary pair design. This JBL amp is pretty good for the era.

What I can't get over are two things : Where are the heat sinks? Why are they hiding the inputs on the bottom? Also, it's JBL, why no torodial transformer? To give credit, the capacitors look nice.

 

[restorer-john]

What I can't get over are two things : Where are the heat sinks? Why are they hiding the inputs on the bottom? Also, it's JBL, why no torodial transformer? To give credit, the capacitors look nice.

The entire rear is a die-cast block with massive heat disipation capabilities for an amplifier of that capability.

The inputs in the middle underneath is a very good design. Sony did the same, as did others like Sansui etc. The preamp is at the front, the power stage is at the back. By sending signals in/out right at the point of the preamp stage, you are not running shielded cable across the amplifier and high current stages to rear panel connectors. Sure, it's unconventional, but it is very sensible.

A toroid would have taken up more space, even if they were making them in numbers in the 1960s, which they really weren't.

Basically, the JBL is built like test equipment.

 

[Labjr]

Hmmm. The amplfier circuit looks pretty textbook. Not sure about the preamp circuit.

Maybe use a lawn mower to cut the high grass in that multitone test.

 

[Steven Holt]

The entire rear is a die-cast block with massive heat disipation capabilities for an amplifier of that capability.

The inputs in the middle underneath is a very good design. Sony did the same, as did others like Sansui etc. The preamp is at the front, the power stage is at the back. By sending signals in/out right at the point of the preamp stage, you are not running shielded cable across the amplifier and high current stages to rear panel connectors. Sure, it's unconventional, but it is very sensible.

A toroid would have taken up more space, even if they were making them in numbers in the 1960s, which they really weren't.

Basically, the JBL is built like test equipment.

John, thank you for this, I learned something. If you and Doodski like it, this settles the issue for me.

 

[Jamie Alexander]

What I can't get over are two things : Where are the heat sinks? Why are they hiding the inputs on the bottom? Also, it's JBL, why no torodial transformer? To give credit, the capacitors look nice.

Toroidal Transformers didn't show up in Hi-Fi until 2 years later. At least according to Cambridge Audio, in there model P-40.

 

[audioholic63]

Something I like a lot about this old amplifier: the visual simplicity and convenience. I am so bored with lousy modern design such as unhighlighted text /moulded/pressed/stamped on casings such that it can only be read in bright light or at an angle. Or how about grey text on dark grey or black backgrounds? Usually tiny!
This amp has been designed by people who appreciate that you might want to glance at it from across a decent sized room and see easily and unambiguously its state and settings. Of course, the substantial size matters and makes it easier, but it is a thoughtful design and nice interface.

It's missing retina frying blue led's.