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JBL SA600 Vintage Amplifier Review

GXAlan

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It’s so sad, I am speechless. The SDR-35 has a lot of features, of course, but it’s an $8550 piece of equipment.
 

pma

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Using internal power amp connectors, we get good performance for both channels so the issue above is the pre-amp:

This is very usual for the integrated amplifiers, especially those vintage units. When we check the service manual, we can see that though the power amp circuit is a competent design, preamp and bass/treble EQ circuits are much, much worse. Together with mediocre wiring it results in much worse overall performance of the complete amplifier.
I think many of us will remember that these amps sounded much better when the power amp only was used or at least pure direct to bypass EQ. It may be a similar issue even in nowadays AVR systems.
 

pma

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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.

This is the biggest issue of the power amp section. The fixed diodes do not allow for adjustment of the idle current, the idle current is low and it is the crossover distortion that degrades the parameters.
By simply changing the diodes for a Vbe multiplier, we can adjust the idle current and get much much lower distortion especially at lower power. Unfortunately, Vbe multiplier circuits were not used back in 1966. Below we can see how it works with crossover distortion reduced by a properly set idle current.

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This would be a competitive amplifier, even nowadays. Removal of crossover distortion would remove the "transistor sound" of the sixties.
 

Neddy

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Review updated with the latest measurements.
Thank you Amir.
These results (and causes!) match my (10yrs of use) recollection 'more better' now...!
Makes me feel pretty good about how much I liked that amp.
I even got used to having the balance knob always set a 'tad off' too!
The JBL dealer I bought it (SA660) from in the 90s had run power and distortion tests after repairs, and said it met spec.
Now...what to do what to do? with it.
Might be interesting to have it rebuilt and drive the (ahem, slightly newer) JBL L-25s Primas I use for TV watching - would make for a pretty distinctive pairing!
Many thanks again!!
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fricc

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This is the biggest issue of the power amp section. The fixed diodes do not allow for adjustment of the idle current, the idle current is low and it is the crossover distortion that degrades the parameters.
By simply changing the diodes for a Vbe multiplier, we can adjust the idle current and get much much lower distortion especially at lower power. Unfortunately, Vbe multiplier circuits were not used back in 1966. Below we can see how it works with crossover distortion reduced by a properly set idle current.

View attachment 228360

View attachment 228361

View attachment 228362

This would be a competitive amplifier, even nowadays. Removal of crossover distortion would remove the "transistor sound" of the sixties.

Oh awesome, is this a p-spice simulation? Could you show results with both configurations (diodes and vbe) for the sake of comparison?
Thanks!
- Fabio
 

pma

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Could you show results with both configurations (diodes and vbe) for the sake of comparison?

Hi, it was already shown :)



 

GXAlan

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Just to add a final coda to this.

In Amir's test, he ran the amplifier as originally intended. Feed it 0.16V in and then adjust preamp/amp to get to 5W.

I tried feeding it 2.1V straight from the Panasonic UB9000 and attenuating the volume knob until I got 5W out at the amplifier. With this, I was able to get numbers going through the pre-amp that come close to "amplifier only" and would be reflective of how it sounds when paired with a modern DAC.

There is a glitch with the Multitone that stereo recording won't show the spectrum beyond 20-20.

Conclusion:
The SFS-80 will beat the SA600. The SA600 gives you more H2 harmonics for a "tube like" sound.


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