[dude_500]

I just built a phono amp and it had a 60 hz square wave ripple that was very loud on the output. If I disconnected the heater wire with the rest powered up the sound immediately went away so I believe it has to do with the heater supply. It is a 10V-AC trans through a 4A bridge rectifier, into a 10,000uF capacitor, then an LM317 voltage regulator. As far as I can think, this shouldn't produce any ripple.

However, since it seems this circuit is causing it (perhaps RF interference), I realized that the entire heater supply is ground-isolated. I figured I'd just ground the circuit to the chassis (which also then grounds to the PE of the 120 plug).

I plugged this in and I didn't notice the ripple (though it may not have warmed up enough - perhaps 5 seconds into testing) I noticed the rectifier was smoking slightly. I immediately cut the power. The heaters were on during this test.

I disconnected the ground wire I added and the rectifier still works.

In my other amp I have the rectifier output chassis grounded, why would it be making this one smoke? It is a 4A rectifier and I do not draw more than perhaps 1.5A for the heaters.

 

[dsavitsk]

Good to hear you are making some progress. Do you have shielded tube sockets? If not, that is likely your noise. If you don't have them, wrap the tubes in aluminum foil -- make a little tube of it for each tube, and be sure it is grounded. Probably the reason that you get no noise with the heaters disconnected is because without the heaters connected, the tubes are not doing anything. If you measure, there should not be any current flowing.

For the burning rectifier, any chance you grounded the wrong side? Be sure this transformer is fused with an appropriate fuse. Your heaters draw about 1.2A. At 10V, this means ~15W rounding up. So, a 125mA slow blow fuse on the primary of the 10V transformer would be appropriate.

 

[dude_500]

I got some shields today. I originally had 120hz as well as 60hz. The shields eliminated the 120hz but the 60hz was unaffected.

My fuse is a 1A on the primary on the 120V before it goes to the two transformers in parallel. I know it's a bit more than it needs however it definitely protects against shorts on the secondary and serves as a sufficient safety valve. There are no local stores that sell under 1A fuses so I'm just using that value.

I grounded the correct side, I put the ground right onto the negative side of the filter capacitor so it's definitely correct. I read one site online that said you should never PE-ground the output of a rectifier, however I don't understand why you wouldn't (and I have in my other amp and that one is fine).

When I cut the power to the heaters the sound stops IMMEDIATELY. There is still a 120hz hum for a few seconds if the shields are off implying it's still operating. Wouldn't there be enough residual heat to keep it going for at least a half a second?

 

[dsavitsk]

Quote:

Originally Posted by dude_500 /img/forum/go_quote.gif Wouldn't there be enough residual heat to keep it going for at least a half a second?

Probably. Maybe post some pics?

 

[dude_500]

Here are some picts.

Notes: In the third photo it looks like the rectifier is shorting to chassis. It is not. It's out right now just for testing purposes.

These shields pull the 120hz interference to not audible on normal volumes but the 60hz sound is not affected by the shields at all.


I believe it's 60hz, I'm not positive however it sounds like about 60 to me. It also is not a sine wave as it's not a tone, it's a collection of clicks. It sounds to me like a 60hz square wave

 

[dsavitsk]

Are your RCA jacks connected to the chassis? If so, you likely have a ground loop. Make sure they are insulated.

 

[dude_500]

Yes they are connected. I will try isolating them, but they still just connect to the ground wire anyways which is on the chassis. Do I insulate them and connect the input/output grounds but not even wire them to the ground wire of the whole amp?

 

[dsavitsk]

What you want is for the wall earth to connect to the chassis. Everything in the circuit should connect to a single ground point (that "point" can be a bus wire like I have). Then, the circuit ground should connect to the chassis ground at one point, preferably close to the IEC. You might also make this connection through a small value high wattage resistor (1R 10W), though it isn't necessary in mine. But, don't use the chassis as a signal ground at any point. Grounding is always tricky, and with tubes it is extra tricky, and in a high gain circuit like a phono stage it is extra extra tricky.

 

[dude_500]

I just tried it with the rca plug barely in so that the ground isn't contacting and the sound is identical, so I'm thinking that means grounding isn't an issue?

It seems to me that grounding the heaters to the chassis should fix it but it starts smoking hah.

UPDATE: I found a 2-3 voltage between isolated heater ground and chassis ground. Why on earth is there a variation between that? When the tubes are removed, there is no voltage between them, however with the tubes in it grows from zero volts to 2-3 volts then VERY slowly drops down. It gets down to 1 volt after about 2 minutes. Is there something in the tubes that would cause this induction?

I noticed I had a center tap on my tube transformer on ground previously (including during the previous smoking situation) and that yielded a 4.5 volt difference, perhaps I should just ground this one out.

 

[nikongod]

if you grounded the CT of the transformer, DO NOT try to ground the regulator.

grounding the CT creates a virtual ground, grounding the regulator creates a "conflicting ground" and this will let the smoke out.

I agree, all of the RCA's should be isolated from the chassis. Star ground is your friend.

also, where is the ground lug for the tonearm ground? Have you tried to run the amp with shorting plugs on the inputs: this will eliminate the possibility of hum coming from the TT.

edited:
It looks like you have 2 "ground busses" going across the front and rear of the chassis. In a phono amp, a ground buss is probably better than a "true" star ground for reasons of practicality, but a x-mas tree configuration returning to a central star ground is still the way. multiple ground points to the chassis is BEGGING to pick up hum.

 

[dude_500]

I cut the CT on the transformer and grounded the circuit and the noise went away. My RCA jacks are still directly grounded, but it doesn't seem to matter in this case. I'll remember to buy isolated jacks in the future.

Thanks to everyone for the help, especially to dsavitsk for the schematic and tons of help on building this. This thing sounds amazing compared to my old solid state amp.

 

[dsavitsk]

Glad to hear you got it working

I have a few modifications I am going to be trying out in the next few weeks/months, and I'll post them when I get a chance.

In case anyone else is curious, the schematic is available at ecp phono stage.

 

[nikongod]

could this be built with 12AV7& 10K:600 OPT

naturally adjusting all of the RIAA values to suit.

I just cant figure out what the gain is!
My guess is is that as it stands now its around 56dB (voltage gain=600ish) which is more than I need.

With 12AV7 it would be just about 43dB (voltage gain=160, before the output transformers knocked it down)

am I way off base with my estimates?

 

[dsavitsk]

Embarrassingly enough, I don't know the exact gain. But, it is pretty high.

The design itself is not that unique -- just a two stage RIAA filter with a cathode follower on the output. I guess the unique part is the use of the CCSes throughout which lower noise and reduce the current loops to keep the PS out of the circuit.

To lower overall gain, you could use an OPT on the circuit as is. You could also use a lower gain tube for the second stage. I might use a triode strapped 6AK5 to keep with the 7 pin concept. But, your favorite 12A*7 is fine, too. The 6GK5 on the input is probably best left as-is as it is pretty good for the task (6ER5 works, too), and the things that compete with it (D3a, 5842, etc.) are all much harder to work with. There is nothing magical about the 6T4 for the follower, other than I had a box of them and nothing else to use them for. Anything will work there, too with a 6922 being a pretty standard choice.