Any idea on this vintage JBL preamp?

[pyonc]

Hi friends,

Before I take it to the tech shop tomorrow for repair, I seek your opinion.
I've been using vintage JBL SG520 preamp to drive 4343.
When the volume is set at stereo mode (as seen in the pic), I hear the music coming from both speakers, pleasant to my ear.
Now, this is the issue: when I turn the volume all the way down to the left, I still keep hearing audible music from the right speaker.
When I do the vice versa, namely all the way down to the right, however, I hear nothing from the left speaker as it should.
As seen in the pic, the knob in the Balance is the volume device that slides all the way down to the left or right for volume control.
So, I think there must be something wrong with the unit. Maybe leaky capacitors or any other issue?
Given it's already more than 50 years old, I think it needs a thorough check and recap by a tech.
I look forward to your opinion and feedback. Any feedback would be welcome. Thanks.

[grumpy]

One thing that might help the tech is to determine whether the signal bleeding through is from the right channel input (or left). Just remove the right channel input cable from your source (power amps off while messing with cables) at the rear of the pre and see if the right channel bleed through remains.

If not, then the problem isn't crosstalk which may mean you could just live with it. I'm assuming the right channel output, while balance is set full left, is substantially reduced.

[Joseph Smith Jr]

Some bleed through with pre-amps or integrated amplifiers which have singular controls affecting both channels, master volume pots or sliders, tone controls or any mechanical all in one mode or function selector switch is perfectly normal operation and is not in and of itself an indication of a defect or performance problem. Especially those with any sort of phase reverse or channel reverse switch, MONO/STEREO switches etc (talking vintage gear here) Especially when unbalanced inputs are used

And it's not always going to be balanced, the bleed through, in fact it seldom is

Verify my speculation quickly with a look at the pre-amp's schematic as well as the final outputs

I'm really not understanding your problem, how much (or better, how loud) is what you are hearing when you think you shouldn't? Is it sound being projected out into the room or something you have to put your ear up close to the speaker to hear?

I have read several of your threads, even posted to a couple, most of them relating to looking for things to worry about. My favorite was the one in which you were concerned your Class A Krell, sounding better than ever after a trip to the shop wasn't running hot enough.

Hi friends,


Recently I purchased a famed Class A amp, namely Krell KSA 50 to run bass of 4343 in bi-amp mode. The other day I brought Krell KSA 50 back home after having had it checked and fixed by an experienced tech nearby. He said he diagnosed it and repaired a couple of parts including a small 'lytic cap'. He said he "adjusted bias, along with load and audio test". Currently it's running bass in a bi-amp mode for my JBL 4343 system, producing a much more dynamic and deeper rich bass and larger sound stage than before. Now this is my question: as I understand, this Class A amp is supposed to run hot and that VERY HOT. But after several hours of running, it still is running cool, not even lukewarm around the unit cover when I touch it. I checked the fan-cooler inside, and it's running all right while it's on. Is this running cool normal? If it's so, is it because of bias adjusting rather low or high done by the tech? I appreciate any comment or feedback from you who had a similar experience in advance.
After a trip to the shop, amp comes home, it's a KRELL, Class A, more dynamics and deeper richer bass than before, AND a larger sound stage than before, but after several hours it's still running cool and you're worried about it?

These threads are frustrating because after many posts and pages you never give up the final outcome or the end of the story!

Recent example, did you get your heat screen cover back into place on your amp? What did you wind up using?

Joe

[pyonc]

Thanks for your great feedback and advice.

Actually I tested the speakers with Crown IC150 preamp for double check. I got the same result with Adcom preamp.
Now, this is what I found, which I think is normal:
when I slide the volume all the way down to the left,
I don't hear any sound or noise from the right speaker.
Vice versa, I hear nothing from the left speaker.

[Joseph Smith Jr]

Thanks for your great feedback and advice.

Actually I tested the speakers with Crown IC150 preamp for double check. I got the same result with Adcom preamp.
Now, this is what I found, which I think is normal:
when I slide the volume all the way down to the left,
I don't hear any sound or noise from the right speaker.
Vice versa, I hear nothing from the left speaker.
That doesn't mean or prove anything
The ADCOM gave you the same result as what?
Exactly what piece of equipment do you think has a problem and what exactly is that problem?
How does it play and sound other than this "problem" we are contemplating?
Also, do you have a service manual for the piece of equipment that you suspect has an issue?

I will repeat myself now, you can defeat any number of volume controls, be they sliders or pots and still have a small amount of audible signal, or sound, reach the loud speakers
Depending on the circuit design, and or the type of control, this behavior can be 100% absolutely NORMAL

How loud is your bleed signal through, asked thrice now?

Joe

[grumpy]

Maybe buy one of these:

72910

[Joseph Smith Jr]

Maybe buy one of these:

72910
Already got it, many things to worry about with it, read the customer Q&A :)
https://www.amazon.com/Schylling-PANIC-PETE-SQUEEZE-TOY/dp/B001R57O88

[grumpy]

Well, you gotta get a good one! :)

[ARMED]

the left and right channel IC needs to be replaced....have it checked....im drunk

edit..let us know what your tech say...

[BMWCCA]

....im drunk

Always the best time to post! :cheers:

[pyonc]

the left and right channel IC needs to be replaced....have it checked....im drunk

edit..let us know what your tech say...

Yea, it's a vintage unit from the mid-1960s era. Now in the tech's hand for full diagnostics.

[pyonc]

That doesn't mean or prove anything
The ADCOM gave you the same result as what?
Exactly what piece of equipment do you think has a problem and what exactly is that problem?
How does it play and sound other than this "problem" we are contemplating?
Also, do you have a service manual for the piece of equipment that you suspect has an issue?

I will repeat myself now, you can defeat any number of volume controls, be they sliders or pots and still have a small amount of audible signal, or sound, reach the loud speakers
Depending on the circuit design, and or the type of control, this behavior can be 100% absolutely NORMAL

How loud is your bleed signal through, asked thrice now?

Joe

Thanks much for your kind attention and response. :)
Re signal bleed, I already mentioned "audible" in my start thread.
Let me simply this as follows:
when I slide the volume knob all the way down to the left
1) audible music still coming from the right speaker, with SG520
2) no music at all from the right speaker, with Adcom or Crown preamp.

when I slide the volume know all the way down to the right
1) no music at all from the left speaker, even with SG520
2) no music at all from the left speaker, with Adcom or Crown preamp

Now, the SG520 is in the tech's hand for full diagnostics.
Let me tell you about his explanation and fix when I get it in about 10 days.

[grumpy]

Re signal bleed, I already mentioned "audible" in my start thread.


... and there lies the lack of clarity, as both a whisper and a jet engine are audible.

barely audible would lead one direction, no real change in level would lead to a very different virtual diagnosis (i.e., one side of the balance control became disconnected). In either case, having an experienced tech review the unit was likely a good choice. Glad to hear you're getting it sorted out :)

[pyonc]

... and there lies the lack of clarity, as both a whisper and a jet engine are audible.

barely audible would lead one direction, no real change in level would lead to a very different virtual diagnosis (i.e., one side of the balance control became disconnected). In either case, having an experienced tech review the unit was likely a good choice. Glad to hear you're getting it sorted out :)

Let me report back as soon as I hear from the tech. Thanks, grumpy!

[Joseph Smith Jr]

... and there lies the lack of clarity, as both a whisper and a jet engine are audible.
As for the magnitude or SPL of the music signal emanating from his right channel with the control defeated on the old JBL pre in play I don't know, but the question of how much is bleeding through is clearly not audible at all ;)

Joe

[pyonc]

Some bleed through with pre-amps or integrated amplifiers which have singular controls affecting both channels, master volume pots or sliders, tone controls or any mechanical all in one mode or function selector switch is perfectly normal operation and is not in and of itself an indication of a defect or performance problem. Especially those with any sort of phase reverse or channel reverse switch, MONO/STEREO switches etc (talking vintage gear here) Especially when unbalanced inputs are used

And it's not always going to be balanced, the bleed through, in fact it seldom is

Verify my speculation quickly with a look at the pre-amp's schematic as well as the final outputs

I'm really not understanding your problem, how much (or better, how loud) is what you are hearing when you think you shouldn't? Is it sound being projected out into the room or something you have to put your ear up close to the speaker to hear?

......

Joe

Disappointing news from the tech who diagnosed this unit. He verified the signal bleed through the right channel, but couldn't find the so-called fade control that fits this specific SG520. It's obsolete, non-obtainable even from JBL. Returned to me not repaired, accordingly.
Now, I'm back to this thread, and appreciate very much your detailed and kind reply. :applaud:
You asked: "Is it sound being projected out into the room or something you have to put your ear up close to the speaker to hear?"
My answer is 'audible bleed' (about 2 out of 10 in volume level) when I put my ear up close to the speaker on the right when I slide the volume knob all the way to the left.
When in stereo mode, however, I don't hear any audible imbalance between the left and right speakers.
I think I have to focus on listening to music from now on rather than worrying ... :)

[grumpy]

I think I have to focus on listening to music from now on rather than worrying ... :)

a healthy attitude!