Greetings, friends. I'm trying to understand how noise is entering my system. I have a stock Technics SL-1200mk2 and am using a DIY tube phono preamp and a DIY tube power amp. Phono pre is an EAR834p built to the LHTR specs, and the power amp is a SET with 6B4G outputs.
When the system is at idle, there's noise I've come to associate with typical urban RF. It's not noticable when the music is loud, but I can hardly listen to Joe Pass records because the noise intrudes between notes.
I believe the noise is introduced at the input jacks for the phono pre. Sometimes it's only noticable on one channel. If I move my hand to the back of the unit, the noise gets quieter, and if I squeeze the actual RCA connector coming from the TT, the noise goes away. This makes me think it's a grounding issue. Grounding scheme in the phono pre is single-point, no loops: input signal grounds are joined at the ground wire lug which is connected directly to the chassis. Output grounds are separated.
Any ideas? I'm away from home right now or I'd include pictures.
When the system is at idle, there's noise I've come to associate with typical urban RF. It's not noticable when the music is loud, but I can hardly listen to Joe Pass records because the noise intrudes between notes.
I believe the noise is introduced at the input jacks for the phono pre. Sometimes it's only noticable on one channel. If I move my hand to the back of the unit, the noise gets quieter, and if I squeeze the actual RCA connector coming from the TT, the noise goes away. This makes me think it's a grounding issue. Grounding scheme in the phono pre is single-point, no loops: input signal grounds are joined at the ground wire lug which is connected directly to the chassis. Output grounds are separated.
Any ideas? I'm away from home right now or I'd include pictures.
Switch the tt cable ends, and see if the problem follows the plug or stays with the socket.
If the noise follows the plug, then the plugs on the tt cable could have problems, if so just replace both.
If the noise stays with the socket, look for bad connections to the socket terminals inside.
The tt input common leads should be kept near the tt hot input leads, and go directly to the phono circuit audio ground,
not to the chassis. Otherwise noise pickup will be much worse due to the large loop area.
If the noise follows the plug, then the plugs on the tt cable could have problems, if so just replace both.
If the noise stays with the socket, look for bad connections to the socket terminals inside.
The tt input common leads should be kept near the tt hot input leads, and go directly to the phono circuit audio ground,
not to the chassis. Otherwise noise pickup will be much worse due to the large loop area.
Last edited:
I like that. Don't want to mess with the wires going into the TT but the jacks themselves...
E does anyone know if the stock cables on the mk2 are shielded?
E does anyone know if the stock cables on the mk2 are shielded?
Lenco Heaven? There was a recent post here with the same issue. I'll see if I can find it.
No reason you can't put a zobel across the input jacks.
jeff
Hehe that Zobel network can do anything. but what values? And when you say across, do you mean "from the input jack to the PCB" or...?
But yes, this is a noise issue I've noticed with a couple different phono pre's I've built and I would love to get to the root of it. A friend got one of those cheap Shiit phono pre's and it's dead silent in his rig, where mine was unlistenable. He lives close to a cell phone tower, it's not nearly as bad at mine. But still. I think upgrading the PSU from the stock Douk unit to the Jessica PSU actually made the noise worse. Or at least, better articulated 🙄😁
But yes, this is a noise issue I've noticed with a couple different phono pre's I've built and I would love to get to the root of it. A friend got one of those cheap Shiit phono pre's and it's dead silent in his rig, where mine was unlistenable. He lives close to a cell phone tower, it's not nearly as bad at mine. But still. I think upgrading the PSU from the stock Douk unit to the Jessica PSU actually made the noise worse. Or at least, better articulated 🙄😁
Bob Cordell recommends 75R and 220pF for amplifier inputs, the 75R being an estimate of the cables characteristic impedance and 220pF being "enough". For phono use, we'd generally want to keep the C down as much as possible. Maybe try 100pF or whatever you have a pair of, maybe less if it solves your problem.
All good fortune,
Chris
Resistor on the + or - side of things? I found a speaker wiring diagram that had the resistor on the (-) side...
Since they're in series it doesn't matter. Actually, I'd be more concerned with the wiring between turntable and preamp, but Zobels are easy to try and Mostly Harmless.
All good fortune,
Chris
All good fortune,
Chris
Its because of the hi gain design and lack of a real input circuit, causes the grid of the first tube is acting like an antenna.
Looking at the schematic, it should be changed to this standard input circuit:
Wow, thanks! Familiar with the grid leak resistor and the grid stopper, but never seen a pair of 20pF caps used like that.
Good thing I still haven't placed that order to Mouser, gonna throw some 20pF in there.
Good thing I still haven't placed that order to Mouser, gonna throw some 20pF in there.
Actually it was very common for enhancing the RFI snubbing. that normally occurs when using a grid stop. It was common in a lot of circuits, including all tube television to prevent the horizontal high voltage section from becoming an RF amp from the RFI.
So the grid stopper is more important for blocking the noise. What's the function of those 20pF caps? It's hard to implement a grid stopper in a PCB-based design.
w
w
dress the capacitor leads with the jacket of a stripped wire to insulate them and solder them on the pin side neatly.