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JBL SA600 Vintage Amplifier Review

restorer-john

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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.

They are doing tricks right now with other designs. Ever wonder why we don't see any high power THD tests particularly with Class Ds? 1kHz is a joke. Especially at a measly 5W.

Run full power, full bandwidth and let's see the results.

Even a full power 20-20kHz FR plot, both channels driven. When did you last (ever) see that? Power bandwidth? (-3dB) Ask an ASR member about that and see what you hear...crickets...
 

Labjr

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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.
Not long after, there were four-track heads for auto-reverse cassette players which is on the same scale.
 

fpitas

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They are doing tricks right now with other designs. Ever wonder why we don't see any high power THD tests particularly with Class Ds? 1kHz is a joke. Especially at a measly 5W.

Run full power, full bandwidth and lets see the results.
I was talking about this amp, which is class AB. Yes, you always have to do tricks to meet the Bode criterion. But today's 30MHz power transistors make life infinitely easier.
 

restorer-john

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I was talking about this amp, which is class AB. Yes, you always have to do tricks to meet the Bode criterion. But today's 30MHz power transistors make life infinitely easier.

Today's? We've had fT 20-30MHz OPTs for 30 years or more. Pick a Sanken LAPT, Fujitsu or a Toshiba....
 

mhardy6647

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Not long after, there were four-track heads for auto-reverse cassette players which is on the same scale
I don't doubt it, but (FWIW) most of the auto-reverse cassette decks I recall had heads of the ker-chunk kind. ;)
One notable exception, of course, was the truly innovative bizarre Nak RX-202E. :cool:
I am sure there was other weirdness BITD -- recreactional pharmacology was all the rage in those days. ;)

nakamichi flip.gif
 

Labjr

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I don't doubt it, but (FWIW) most of the auto-reverse cassette decks I recall had heads of the ker-chunk kind. ;)
One notable exception, of course, was the truly innovative bizarre Nak RX-202E. :cool:
I am sure there was other weirdness BITD -- recreactional pharmacology was all the rage in those days. ;)

I think because the cassette "recorder" needed the erase head to be flipped around. The cassette "player" did not have an erase head. In the 80's the Japanese decks had a tiny record/play/erase head assembly that spun around.
 
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mhardy6647

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I think because the cassette "recorder" needed the erase head to be flipped around. The cassette "player" did not have an erase head. In the 80's the Japanaese decks had a tiny record/play/erase head assembly that spun around.
Indeed, and those are what I remember. Heck, there's still at least one of them hanging around here... a rebadged Pioneer.
But we digress. :facepalm:
 

Kevinfc

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Look at those giant caps. i enjoy the reviews of these ancient relics.
 

pma

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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.
Sure, 2955 is slower than 3055. But not an order slower. It is rather about driving the output stage and switching off, taking off charge. This amp has design clever in this sense. It behaves well at 30kHz full power and that is enough.

Here is the 2955 history



I started to build complementary npn/pnp output stages about 1973 with silicon/germanium pairs and in 1975 with silicon pairs. It was difficult to get pnp silicon power transistors here earlier.



These parts were good.
 
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GXAlan

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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.
...

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.
--------------

EDIT: PSR below. It is good at middle and high audio frequencies, but deteriorates at low frequencies.

Out of curiosity, there is a post about someone taking a SMPS and getting better results into a traditional Class AB amplifier. Are there any off-the-shelf SMPS that you can simulate in this?
 

sergeauckland

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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.
I built my first SS amplifier in 1971, from a design published over 5 months in Practical Electronics starting in November 1970. It used complementary pairs of 3055/2955 and claimed 30 watts output into 8 ohms at 0.01% distortion at 1kHz. Distortion was higher at high frequencies, but still below 0.05% at 20kHz at full power, so for the time, no slouch. I used it happily for several years until I bought some IMF TLS50s where 30 watts would go nowhere. The impedance characteristics of the IMFs may not have been too kind either. The only problem with that amp was that it had a stabilised power supply and the series stabilising transistor would blow to protect the fuse...

S.
 

pma

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Out of curiosity, there is a post about someone taking a SMPS and getting better results into a traditional Class AB amplifier. Are there any off-the-shelf SMPS that you can simulate in this?
I am one of those guys who checked the traditional class AB amp with a SMPS supply:


It works, of course. You might be limited by lower instant peak current if it is requested by a low impedance load. HF pollution starts well above audio band. If the class AB is well designed, it makes not much difference what kind of power supply you would use.
Please note that the linked amp is about 30 years old and it still competes very well.
 

pma

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I built my first SS amplifier in 1971, from a design published over 5 months in Practical Electronics starting in November 1970. It used complementary pairs of 3055/2955 and claimed 30 watts output into 8 ohms at 0.01% distortion at 1kHz. Distortion was higher at high frequencies, but still below 0.05% at 20kHz at full power, so for the time, no slouch. I used it happily for several years until I bought some IMF TLS50s where 30 watts would go nowhere. The impedance characteristics of the IMFs may not have been too kind either. The only problem with that amp was that it had a stabilised power supply and the series stabilising transistor would blow to protect the fuse...

S.

Very nice. Yes that was possible to get and it is still enough today.
 

anmpr1

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Regarding the input placement: in the '50s and '60s, electronics were often installed in consoles. Only the front panel was seen, the rest hidden inside the console. The back of the console was often open. My guess is that bottom inputs were an ergonomic feature. The owner could easily plug their tape recorder, tuner, record player and whatnot into the amp from the open back of the console. With the amp installed vertically, it would be difficult to plug anything in if the connections were on the back.

But it's just a guess on my part.

I don't remember JBL too well from back then, but many of their loudspeaker designs were DIY--customers bought individual drivers/crossovers and built (or had someone build for them) a cabinet. The stuff was expensive, and in the LA hi-fi scene it wouldn't surprise me if 'installation specialists' handled everything for the well heeled music lover.

I'm surprised no one installed a complete hi-fi system into the Paragon loudspeaker console. Maybe they did.

console.jpg

jbl.jpg
 

Angsty

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The dialog on this post got me to notice the current JBL SA750 which I had not noticed before. For US$3000, it feels like a better buy than a restored SA600 (in terms of SQ) for a similar aesthetic. It's a Class G amp, like the NAD 2200 and other Power Envelope amps. However, build quality suffers a bit in the folded sheet metal chassis.

 

Jim Shaw

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I agree, but the fact is, there is a huge market for retro stuff...
I would need sales numbers to be sure, but I'd bet my next lottery ticket that yearly $sales of 1960-70 audio electronics are far less than the $revenue of one Topping or Klipsch model. Or Elac, or Polk.

A huge market? I doubt it. But a few vintage resale prices do seem huge and the value minuscule. But not without Greater Fool (or fetish) Theory.
A few audio-centric people just love collecting crap, and they are inordinately loud about it.
 

EJ3

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___________
* 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.
Unfortunately correct, as far as I know.
But the fact that it can record directly to flash drives may eliminate the tape issue. But for those of us that have cassettes of our family history, of some of our "on the air" days, and family bands, etc, having that capability to play (with an NR system as capable as the last DOLBY's that where sold in consumer cassette decks (or studio ones) and record to flash drives would be handy. And for CD's & perhaps LP's.
Many people only think of a commercial cassette of music or someone copied an a LP to cassette. They do not think of the thousands of hours on tape of personal things.
I 9.5 MM, and 8 MM movies with titles inserted from my grandfather and my mother which documented many events (family and otherwise) from prior to WWII to now.
My family was making videos in Europe and invented some things for use in making movies well before WWII. And we have historical records that are speech, community bands that my family was involved in, etc. Getting this stuff transferred into easy to use formats can be quite difficult. So TOOLS like this can help.
Yes, cassette sound quality leaves a lot to be desired but sometimes it's all you have to work with. Not all audio is about listening to music at the best possible resolution. There are stories, events, etc that make up one's history that may be worth saving.
 
OP
amirm

amirm

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Found and fixed the problem! There is an internal RCA jack that connects the power amp to pre-amp. There is a grounding screw for the shield. It had come lose. Tightened it and here are the results:

JBL SA600 Stereo Amplifier Vintage measurements.png


This is power amp only although before the fix was nearly as bad as the full amp I tested in the review. I will put it back together and re-run all the tests.
 
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