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Marantz 7T Noise and Input Bleed

TheLostBrain

TheLostBrain

Active Member
Good day all.

* This is a bit lengthy - please bear with me :) *

So my Marantz separates have been down for quite some time (waiting on me to service them) and I'm finally putting in an effort to get them going again.

I'm starting with the 7T and as of last night I'm at the following point::
  • Power Supply electrolytics replaced
  • Tone Amplfiier Board lytics and tants repalaced
  • Main Output RAEF Board lytics and tants replaced
  • Temporarily disconnected power from both the Low Level Amplifier Board and the Recording Output RAEF Board
Didn't go super fancy with the replacement caps - just some good Nichicons on the PS side and Elna Silmic IIs for the rest.

So with these done there definitely seems to be an improvement in overall clarity and most notably bass response, which is great.

However, I'm still facing some issues with Noise and Input/Source bleed. For clarification both of these issues were there before the cap replacements and still persist now.


For reference I believe this unit is the last revision of the 7T (17x serial number).

For those that want to follow along at home here's a link to a service manual for the 7T:
http://hifi-antique.com/images/Marantz_7T_Service_Manual.pdf



I'll start w/ the Noise Issue:

I should state that I'm using high(er) efficiency speakers (Klipsch Cornwall, Heresy and KG4s) so this likely compounds the issue I'm experiencing.

What I'm hearing seems to be your standard pre-amp high-frequency hiss. - a constant SHHHHHHHHHHH. It's just that it seems quite a bit louder than it should be.

It occurs equally in both channels and is completely source independent.

For testing I have:
  • Shorting plugs on ALL inputs
  • All treble and bass at the 0/bypass(ish) mark
  • Volume knob turned completely down
...and the hiss is there.

The hiss DOES go up with the volume knob (when listening to music it is quite apparent during quieter passages)

For fun I've also bypassed the internal PS completely and brought the unit up on an HP/Agilent Lab Linear Power Supply... Noise persists.

So at this point I'm thinking maybe it's just the inherent noise floor of the transistors themselves? The service manual does call them out for replacement if a channel is noisy, but the fact that both channels are experiencing the noise equally makes me doubtful that it's actually a transistor fault.

Could this possibly be Resistor/Johnson noise? It just seems awfully loud for that from what I've read on it.

Something I can do tonight to help is to bring this noise up on my scope and post some shots of that here with the measurements. Maybe it's just normal compared to what others are seeing.



Lastly there's the Input / Source bleed issue (possibly related to the above?):

So this issue is basically that you can hear the source material from other inputs bleeding into your current material. Ex: Listening to AUX1... but AUX2, Tuner, etc. can be heard in the background.

I've read this can be normal to a certain extent, but this particular case just seems extra bad.

For testing I have:
  • Shorting plugs on ALL inputs except for AUX2
  • I have iphone providing source material to AUX2
  • Volume on iphone itself is all the way up
Sanity Check:
  • Input set to AUX2
  • Eveything works as expected - volume goes up and down with knob, etc.

Testing 1...
  • Input set to AUX1
  • Volume knob all the way down
  • Bleed through...surprisingly heavy
Testing 2...
  • Input set to AUX1
  • Turning volume knob up
  • Bleed through gets louder
Testing 3...
  • Input set to AUX1
  • Continue turning volume knob up until somewhere around mid way
  • Bleed through starts to fade and then stops! (just hear the noise/hiss mentioned in first issue)
Testing 4...
  • Input set to AUX1
  • Continue turning volume knob up
  • Bleed through starts to come back in

Testing done...


So the service manual states that non-selected inputs are simply tied to ground. Does this issue sound like it's possible the contacts in the source selector are possibly oxidized, etc and aren't making a good contact to ground? Also, that behavior with the bleed-through fading out completely around the mid mark seems suspect.


Alright, well that's enough of a wall of text for one post - I would have made it two separate posts, however, I can't help but feel that these two issues are somehow intertwined.

I'll be sure to post the scope shots/results sometime tonight but if anyone has any ideas in the meantime I would love to hear them.

Thanks
Mike
 
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TheLostBrain said:
Good day all.

* This is a bit lengthy - please bear with me :) *

So my Marantz separates have been down for quite some time (waiting on me to service them) and I'm finally putting in an effort to get them going again.

I'm starting with the 7T and as of last night I'm at the following point::
  • Power Supply electrolytics replaced
  • Tone Amplfiier Board lytics and tants repalaced
  • Main Output RAEF Board lytics and tants replaced
  • Temporarily disconnected power from both the Low Level Amplifier Board and the Recording Output RAEF Board
Didn't go super fancy with the replacement caps - just some good Nichicons on the PS side and Elna Silmic IIs for the rest.

So with these done there definitely seems to be an improvement in overall clarity and most notably bass response, which is great.

However, I'm still facing some issues with Noise and Input/Source bleed. For clarification both of these issues were there before the cap replacements and still persist now.


For reference I believe this unit is the last revision of the 7T (17x serial number).

For those that want to follow along at home here's a link to a service manual for the 7T:
http://hifi-antique.com/images/Marantz_7T_Service_Manual.pdf



I'll start w/ the Noise Issue:

I should state that I'm using high(er) efficiency speakers (Klipsch Cornwall, Heresy and KG4s) so this likely compounds the issue I'm experiencing.

What I'm hearing seems to be your standard pre-amp high-frequency hiss. - a constant SHHHHHHHHHHH. It's just that it seems quite a bit louder than it should be.

It occurs equally in both channels and is completely source independent.

For testing I have:
  • Shorting plugs on ALL inputs
  • All treble and bass at the 0/bypass(ish) mark
  • Volume knob turned completely down
...and the hiss is there.

The hiss DOES go up with the volume knob (when listening to music it is quite apparent during quieter passages)

For fun I've also bypassed the internal PS completely and brought the unit up on an HP/Agilent Lab Linear Power Supply... Noise persists.

So at this point I'm thinking maybe it's just the inherent noise floor of the transistors themselves? The service manual does call them out for replacement if a channel is noisy, but the fact that both channels are experiencing the noise equally makes me doubtful that it's actually a transistor fault.

Could this possibly be Resistor/Johnson noise? It just seems awfully loud for that from what I've read on it.

Something I can do tonight to help is to bring this noise up on my scope and post some shots of that here with the measurements. Maybe it's just normal compared to what others are seeing.



Lastly there's the Input / Source bleed issue (possibly related to the above?):

So this issue is basically that you can hear the source material from other inputs bleeding into your current material. Ex: Listening to AUX1... but AUX2, Tuner, etc. can be heard in the background.

I've read this can be normal to a certain extent, but this particular case just seems extra bad.

For testing I have:
  • Shorting plugs on ALL inputs except for AUX2
  • I have iphone providing source material to AUX2
  • Volume on iphone itself is all the way up
Sanity Check:
  • Input set to AUX2
  • Eveything works as expected - volume goes up and down with knob, etc.

Testing 1...
  • Input set to AUX1
  • Volume knob all the way down
  • Bleed through...surprisingly heavy
Testing 2...
  • Input set to AUX1
  • Turning volume knob up
  • Bleed through gets louder
Testing 3...
  • Input set to AUX1
  • Continue turning volume knob up until somewhere around mid way
  • Bleed through starts to fade and then stops! (just hear the noise/hiss mentioned in first issue)
Testing 4...
  • Input set to AUX1
  • Continue turning volume knob up
  • Bleed through starts to come back in

Testing done...


So the service manual states that non-selected inputs are simply tied to ground. Does this issue sound like it's possible the contacts in the source selector are possibly oxidized, etc and aren't making a good contact to ground? Also, that behavior with the bleed-through fading out completely around the mid mark seems suspect.


Alright, well that's enough of a wall of text for one post - I would have made it two separate posts, however, I can't help but feel that these two issues are somehow intertwined.

I'll be sure to post the scope shots/results sometime tonight but if anyone has any ideas in the meantime I would love to hear them.

Thanks
Mike
Click to expand...

Wow...I look forward to reading this thread.

Correct on Selector Bleed since the selector is a "Wiper"...Was there and even with the work....Granted this is Low but if you put your ear to the speaker you can hear this. New Equipment should be using "Relays" for the inputs which will be "DEAD" silent.

My 7T Recap: http://audiokarma.org/forums/index.php?threads/marantz-7t-restoration.792097/
 
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What amplifier are you using ?
What is its gain rating?
Does the amp have gain controls?
Have you adjusted the gain controls?
What level is the 7t output level switch set at?

And what is the spl db of the speakers?

What have you done in terms of AC power filtering?

Have you done an actual signal to noise ratio measurement on 7t?

My 7t and system is completely quiet....

Don't jump to changing parts....
 
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jstang said:
What amplifier are you using ?
What is its gain rating?
What level is the 7t output level switch set at?
What have you done in terms of AC power filtering?
Have you done an actual signal to noise ratio measurement?
Click to expand...

What amplifier are you using?
Marantz Model 15

What is its gain rating?
I'm not sure honestly.

What level is the 7t output level switch set at?
I've tried both settings. Everything is lower in "low" (noise and content) which makes sense as it's just backed by a handful of resistors I believe?

What have you done in terms of AC power filtering?
Nothing at this point, I really hadn't considered it.

Have you done an actual signal to noise ratio measurement?
No, but that would make good sense... I suppose if it's within spec I could stop complaining huh? ;)


So, total disclosure here... I'm just a hack at this stuff and have never performed an SNR measurement before.

I've got a "good enough" Tektronix scope and a Keithly 2015 (I bought on a whim because it was cheap a few years back).

I'll have to take a closer look to see if the Keithly has anything native for SNR. Otherwise, a quick search is turning up this method of accomplishing it w/ the scope:

Any suggestions on this one or is that video about the best way?

Thanks
 
What the spl db of the speakers....?

Testing w/ Klipsch Heresy I...
Google says ~98 dB for 1 Watt @ 1M

Will eventually wind up back on Klipsch Cornwalls I...
Google says: ~100 dB for 1 Watt @ 1M
 
Well that's the issue... Run the 7t output level on low and call it a day.

My main system is the 7t , 500 and Ohm F.

The 500 is 26db of gain and the F are 89 db....
 
jstang said:
Well that's the issue... Run the 7t output level on low and call it a day.

My main system is the 7t , 500 and Ohm F.

The 500 is 26db of gain and the F are 89 db....
Click to expand...

Sounds like solid advice. However, wouldn't running the level on low offer roughly the same experience considering I've found the noise to track the volume setting? In other words turning up the volume seems to "turn up" the noise as well...and I'd be turning up the volume to make up for the attenuation imposed by the switch, I think.

Thanks again
 
Nope.... Because the volume pot is before the preamp's tone amp , output driver and also before the amp & speakers.. Lowering the output level and raising the volume causes the S/N ratio of the preamp & system to improve. The level switch is later in the circuit than the volume.

Raising the volume and lowering the gain ( output level) improves the S/N level of the whole system.

You have too much gain in the system....

This is called running the preamp hotter.... And is the purpose of the output level switch or gain controls on the amp ( if it has them...)

The tuning process if you have gain knobs on the amp... Is to turn the gain to zero... Then turn the preamp volume knobs to half. Then with your typical music playing.... Turn the gain knobs up so the sound level is the maximum you will ever listen too...

Then turn the volume down to more normal listening levels.... Leaving the gain knobs as set.

You have just improved/tuned the gain of the system to provide the best S/N for your system, room and listening preferences.

The Model 15 is a low gain amp if I remember just like the 500... But your speakers are high efficiency.... Very high. 92 spl db or less and you would be just fine with 7t on high output with a Model 15.

Johnk
 
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jstang said:
Nope.... Because the volume pot is before the preamp's tone amp , output driver and also before the amp & speakers.. Lowering the output level and raising the volume causes the S/N ratio of the preamp & system to improve. The level switch is later in the circuit than the volume.

Raising the volume and lowering the gain ( output level) improves the S/N level of the whole system.

You have too much gain in the system....

This is called running the preamp hotter.... And is the purpose of the output level switch or gain controls on the amp ( if it has them...)

The tuning process if you have gain knobs on the amp... Is to turn the gain to zero... Then turn the preamp volume knobs to half. Then with your typical music playing.... Turn the gain knobs up so the sound level is the maximum you will ever listen too...

Then turn the volume down to more normal listening levels.... Leaving the gain knobs as set.

You have just improved/tuned the gain of the system to provide the best S/N for your system, room and listening preferences.

The Model 15 is a low gain amp if I remember just like the 500... But your speakers are high efficiency.... Very high. 92 spl db or less and you would be just fine with 7t on high output with a Model 15.

Johnk
Click to expand...


Just had a chance to sit down at my bench and try it.

You sir are 100% correct! ... as I'm sure you might have imagined :beatnik:

Sure enough... with it set to low output, even though I had to raise the volume, the noise floor is indeed MUCH lower!

I had always I just assumed the switch was there as a means of not overpowering a particularly sensitive amp or something.

The difference is pretty drastic. I have the heresy sitting on the bench here no more than a foot or two from my ear and I can no longer hear the static at all - and that's really saying something!

Thank you, i never would have imaged the fix would have been so simple.

I'm going to see what effect, if any, this has had on the source bleed and put a little more time into exploring that.

Thanks again.
 
jstang said:
Raising the volume and lowering the gain ( output level) improves the S/N level of the whole system.

You have too much gain in the system....
Click to expand...

It actually took a bit for that to click... duh! Lol. Sometimes it's the simple concepts that escape me. :)
 
No worries, it really Is not a simple concept...and many people never come to understand how/why to adjust gain..

If you hadn't the speakers you have.... You would never had first hand experience, which would really make it tough to understand....

Lowering gain has some down side depending on the system. If you reduce the gain in the system.... Not enough gain In the electronics and low efficiency speakers.... You will likely have no noise, but will lose the dynamic headroom .....and the reproduction will be bla bla bla.....:blah:

Also hated to see a 7t get a bunch a parts replaced that did not need to ... :)

You know how many receiver people are scratching their heads right now...:confused:. Us separate folks have to stick together....:rockon:

Johnk
 
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A final point....

If you do have gain controls on the amp, it's better to use them to reduce gain than the Volume or output control on the preamp. Set the gain as I mentioned in the earlier post.

Reason being that will provide a further slight improvement in the performance of the interconnects between the preamp and amp.

It also should increase the input impedance of the amp which maybe helpful in flattening response and adding dynamics.

Cables have loses and pick up noise.... They have a S/N ratio .... By attenuating gain at the amp and driving the preamp and interconnect cables hard.... You will hear an small but further improvement in the systems S/N....


I really dislike log rhythmic volume pots, they are hard judge where the mid point of operation is. Pretty sure preamps like the 7c and 7t have linear pots.

Less noise.... Less grung between the notes.... More clarity sound....

Last last point :) .... S/N ratio is the reason I asked about the AC power filtering. Less noise coming into the system from the wall socket... Less noise that gets into the system and your ears....

If you play this game right... It will open your eyes , EARs that is... to another aspect of separates that few get to play with....and many overlook.


Johnk
 
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John ,
thank you very much for sharing those informations about gain setting , S/N and speaker efficiency , very interesting :trebon:
 
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jstang said:
Nope.... Because the volume pot is before the preamp's tone amp , output driver and also before the amp & speakers.. Lowering the output level and raising the volume causes the S/N ratio of the preamp & system to improve. The level switch is later in the circuit than the volume.

Raising the volume and lowering the gain ( output level) improves the S/N level of the whole system.

You have too much gain in the system....

This is called running the preamp hotter.... And is the purpose of the output level switch or gain controls on the amp ( if it has them...)

The tuning process if you have gain knobs on the amp... Is to turn the gain to zero... Then turn the preamp volume knobs to half. Then with your typical music playing.... Turn the gain knobs up so the sound level is the maximum you will ever listen too...

Then turn the volume down to more normal listening levels.... Leaving the gain knobs as set.

You have just improved/tuned the gain of the system to provide the best S/N for your system, room and listening preferences.

The Model 15 is a low gain amp if I remember just like the 500... But your speakers are high efficiency.... Very high. 92 spl db or less and you would be just fine with 7t on high output with a Model 15.

Johnk
Click to expand...

does this work in stereo integrated amplifier too?
 
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