JBL SA600 Vintage Amplifier Review

[KEFCarver]

Speaking 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...

A sin indeed! You take it out every now and then and listen to it, hum and all. It looks great (IMHO). I have a 1950 Philco B/W TV set that I refurbished and the picture is not great by today's standards but it is cool to watch for a bit. A friend of mine's brother is converting his '58 Corvette to an electric one- that is a sin!

 

[tomtoo]

A sin indeed! You take it out every now and then and listen to it, hum and all. It looks great (IMHO). I have a 1950 Philco B/W TV set that I refurbished and the picture is not great by today's standards but it is cool to watch for a bit. A friend of mine's brother is converting his '58 Corvette to an electric one- that is a sin!

See in germany it would be called to eat sauekraut with ketchup a sin. Look at some american hot dogs.

 

[fpitas]

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

Yes. And some "audiophiles" on that other forum are trying to revive quasi-complimentary

 

[fpitas]

I bought an SA600 in the late 60s and used it for years. I never was conscious of high levels of noise and distortion from the amp. It did produce an attention-grabbing power-on thunk.

Unavailability of (exact) replacement parts was my reason for giving up on the amp. This test shows the result of such a rebuilt.

The amount of 60Hz and its harmonics this one outputs is just fishy. Something is probably still wrong.

 

[lc6]

Some internal pics and the amp schematic.
View attachment 227955

These are germanium transistors and diodes, I presume? And why would they filter supply rails for the differential input stages differently (+VCC1 with 220 Ohms / 250 μF, but -VCC1 with 160 Ohms / 160 μF)?

 

[JPA]

Definitely does not live up to the hype. I was imagining it would get closer to 60 dB SINAD based upon these measurements from Julian Hirsch.

Maybe this is what Arcam and JBL Synthesis were benchmarking against!

Thankfully as an estate find, it wasn’t anywhere close to the prices it can get in the Japanese market…

You can tell from his bow tie that Hirsch was a real engineer.

 

[fpitas]

These are germanium transistors and diodes, I presume? And why would they filter supply rails for the differential input stages differently (+VCC1 with 220 Ohms / 250 μF, but -VCC1 with 160 Ohms / 160 μF)?

Silicon, almost certainly. I'm not aware of any germanium NPN power transistors, let alone in TO-3. Probably expensive, limited SOA, and low ft.

 

[Doodski]

These are germanium transistors and diodes, I presume?

The germanium ones that I've seen where always a metal cannister package. Like the ones that I circled in this pic. I looked up the part numbers of 12536 and 12537 for the output unity gain stage transistors. I could not find a source of new parts nor a <PDF> on the pair. For pair 57 and 58 Motorola part number 12538 and 12539 the search results where barren. I stopped searching for data on the additional part numbers provided in the JBL Service manual.

And why would they filter supply rails for the differential input stages differently (+VCC1 with 220 Ohms / 250 μF, but -VCC1 with 160 Ohms / 160 μF)?

Good question. Here's a screenshot of the power supply section.

 

[Doodski]

and.... this from the service manual.

 

[pma]

I do not take the results measured on this DIY refurbished amplifier, with non-original transistors, seriously. Change of transistors would need re-calculation of resistor values at least.

According to Locanthi paper that I linked previously, the amp measured like this:

 

[fpitas]

I do not take the results measured on this DIY refurbished amplifier, with non-original transistors, seriously. Change of transistors would need re-calculation of resistor values at least.

According to Locanthi paper that I linked previously, the amp measured like this:

View attachment 228030

I'm with you. I tend to believe Locanthi, he was a good engineer.

 

[lc6]

Silicon, almost certainly. I'm not aware of any germanium NPN power transistors, let alone in TO-3. Probably expensive, limited SOA, and low ft.

You are likely right; found a schematic diagram of a similar SA660 model with transistor codes (presumably, Q5 is the same type as Q1 but, strangely, Q2 is missing):
JBL SA660 Schematic Diagram

Interesting to see the ±52 V, ±46.5 V and ±41.5 V staggered power supply rails. Here is a paper that describes the design and performance of this amp, including the "T" cascaded output stage which requires a staggered-voltage PS:
Operational Amplifier Circuit for HiFi

Also, compared to the first schematic diagram, the output inductor in parallel with a 2.7 Ohm resistor (to prevent oscillation, as described in the paper) is gone, and the 1k Ohm trim pot (for DC offset adjustment) is replaced by a fixed 91 Ohm resistor. There are also some cap and resistor value changes. The R1/R2 ratio suggests a gain of 24 times (27.6 dB). The spec load seems to be 5.5 Ohms.

 

[amirm]

Fyi peter just shared his FFT measurement with me and it has none of the power supply noise. Can't explain the large discrepancy. If I get it working tomorrow, I will play again to see if I can get different outcome.

 

[amirm]

I do not take the results measured on this DIY refurbished amplifier, with non-original transistors, seriously

Keep your insults to yourself. Peter at quirk audio performs exceptional restorations as evidenced from my previous tests if his work. There is nothing DIY here whatsoever.

 

[Eldus]

I wonder how early 70s designs would fare. Or 80s.

 

[sergeauckland]

I wonder how early 70s designs would fare. Or 80s.

My experience with my Yamaha CR1000 receiver, my son's Quad 303, my Cambridge Audio P50, my Leak Stereo 30+ all from the late 1960s or early 1970s, all measured substantially to spec. once clearly faulty components were replaced and bias current reset. There's no reason why a well restored amp shouldn't meet its original spec, or even better it, as some manufacturers were conservative in their specs.

S

 

[fpitas]

You are likely right; found a schematic diagram of a similar SA660 model with transistor codes (presumably, Q5 is the same type as Q1 but, strangely, Q2 is missing):
JBL SA660 Schematic Diagram

Interesting to see the ±52 V, ±46.5 V and ±41.5 V staggered power supply rails. Here is a paper that describes the design and performance of this amp, including the "T" cascaded output stage which requires a staggered-voltage PS:
Operational Amplifier Circuit for HiFi

Also, compared to the first schematic diagram, the output inductor in parallel with a 2.7 Ohm resistor (to prevent oscillation, as described in the paper) is gone, and the 1k Ohm trim pot (for DC offset adjustment) is replaced by a fixed 91 Ohm resistor. There as also some cap and resistor value changes. The R1/R2 ratio suggests a gain of 24 times (27.6 dB). The spec load seems to be 5.5 Ohms.

You'll want that output network. Without it, any capacitance on the output (like from a cable) will cause the followers to oscillate. That first simplified schematic was probably just to SIM the circuit (I hope). Locanthi says it probably won't oscillate without it, but it's playing with fire nonetheless. Later transistors with a decent ft (30MHz or so) certainly made the situation worse.

 

[julian_hughes]

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.

 

[Bruce Morgen]

Back in the day, nobody in the business took JBL's home stereo electronics seriously. JBL speakers had a following (which later became a stampede when the L-100 arrived), but they were usually paired with McIntosh or (U.S.-made) Marantz amplification until the better Japanese stuff (e.g. Sony's TA-3200) started to appear.

 

[DonR]

Not bad for a museum piece.