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Old 27th December 2014, 12:08 AM   #1
slapaho is offline slapaho  United States
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Default Noisy amp

Hi all...my first thread, so I hope I'm in the right place. I have a Grommes LJ4 that has 'white noise' that increases with volume. I replaced all of the coupling caps hoping to correct it, but it didn't. I'm guessing it's in the output stage being it gets louder with volume. Any help would be greatly appreciated.

P.S. I subbed out tubes one by one to no avail.
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Old 27th December 2014, 12:21 AM   #2
AJT is offline AJT  Philippines
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consider replacing ecaps instead...
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Old 27th December 2014, 12:29 AM   #3
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The noise should be coming from the preamp stage since it increases with volume. I don't have a schematic, but you might be able to remove the first tube and see if the noise goes away.
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Old 27th December 2014, 12:30 AM   #4
AJT is offline AJT  Philippines
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removing tubes may not be a good idea, B+ can rise due to decreased load and who knows?
be very carefull....

you may also want to try shorting each grid to ground using a 0.1ufd cap instead...
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Old 27th December 2014, 12:38 AM   #5
kevinkr is offline kevinkr  United States
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The source of noise will be located in a stage before the volume control or in the control itself.

If the noise increases smoothly with increasing volume settings it originates in stages prior to the control, if however it peaks and then decreases as the volume is increased further, the pot itself is the source and should be checked for DC across the pot or on the wiper. (Grid current in the driver tube may also result in this symptom)

High value pots can generate significant Johnson noise and additional excess noise if any dc current flows through the pot.

Some tube amplifiers are noisy.
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Old 27th December 2014, 01:01 AM   #6
slapaho is offline slapaho  United States
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Thanks for the input. As it turns out, the noise is associated with the phono input, but it's not the input tube ( VI). I switched them and had the same result. I switched to the tape input and it's quiet as a mouse. I did the electrolytics about a week ago, so I knew it wasn't that. I.m running a cd player...could that create noise in the phono input?
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Old 27th December 2014, 05:35 PM   #7
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CD Player should go to the "tape" or "line in" input, not the "phono" input. Phono input has too much gain for a CD player. If the CD player is NOT going to the phono input, try shorting the input connectors for "phono in" to see if this solves the noise problem. If not, it could be anything from a noisy tube, noisy resistor or cold solder joint in the phono section of the preamp. It could also be a loose tube socket pin for that tube.

What make and model amp/preamp is it?
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Old 27th December 2014, 06:20 PM   #8
slapaho is offline slapaho  United States
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Default noise

Thanks. I did figure that out. Not only is it a higher gain input, but it also has an extra gain stage. So I'm using the tape/aux input and it's dead quiet. I also took the phono preamp tube out...why burn it if I don't need to, right? Thanks for the input ...

BTW: I'm using a pair of Grommes Little Jewels...LJ4 model. One is branded Realistic... Boston Radio Shack. From what I understand, they were made by Grommes and sold as kit amps by RS.
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Old 27th December 2014, 11:06 PM   #9
seppoa is offline seppoa  Sweden
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I have a similar problem with my RIAA stage, have not get to sort it out as the music swamps out the noise. But in my case, Im almost shure the problem is heater to cathode leakage. My circuit are 3 SRPP stages in a row, according to Hiraga, and the heaters are referenced to ground. Have other projects and have not got to give the filament supply a positive bias yet.
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Old 29th December 2014, 09:57 AM   #10
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Default Schematic is here

Schematic is here :

http://www.grommesprecision.com/grom...ewelreview.PDF

The first 12AX7 is only in circuit for phono selection, so that's the place to look. One section of that 12AX7 is grid leak biased, and the associated 33M resistor seems well worth taking in for questioning by substitution I think. Otherwise, check all R values, and also check continuity of the RIAA selector switch section in case the feedback loop is open and stage gain is too high.

There's only a few Rs and Cs involved otherwise, but I'd suspect the grid leak bias circuit first - also clean the socket around the grid of that valve pin.

HTH!
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