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Old 15th May 2010, 09:59 AM   #1
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Default Preamp nasty noise and hum

These signals appear on the output of a tube phono preamp.
Input connected to phono cartridge at idle.
Power supply from 50Hz mains transformer, bridge rectifier, filter caps etc, 300 v, 20 mA.
Several problems where I am stuck:
1) Hum is 50 Hz, but should be 100 Hz if it came from power supply. Removing the PS and transformer far (one meter) from the already shielded amp helps to some extent, but not enough.
2) The nasty spikes. They come and go and I guess they do not originate in the amp itself. They vary over time (1) (2), may even disappear completely for some time (3). Or pulse with a steady rhythm (4). Detail analysis shows ringing at approx. 15 kHz (5).
I tried all sorts of filters, resistor/cap, different chokes, w/o any effect. How can I get rid of these spikes ?
3) To get this far I had to connect the amplifier ground and shield to earth. Otherwise with ground floating the hum was much worse and it also had a harsh snarl. The signal plot shows why, the hum is not a sinusoidal wave but completely distorted (6). Any idea why ? It's still 50Hz and not the expected 100Hz. And doesn't grounding the amp to earth and therby the secondary of the HV transformer defeat the safety idea of mains isolation ?
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Old 15th May 2010, 11:07 AM   #2
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Your problem is best described as: weird. And my stable datum when things get weird is to suspect and look for HF/VHF oscillations.

You tell us nothing about the pre's design/topology. If it uses any tube with more gM than a 12AX7, then oscillation is a major possibility - and I've even had all 12AX7 phono pre's oscillate.

What bandwidth is your scope? Tubes like the 6DJ8 will oscillate happily at 100MHz, and WE417s even higher, so seeing the oscillation directly may be a problem.

Grid stoppers on ALL grids, especially any CFs in the circuit would be a first step,if you haven't used them already.

An oscillating amp is normally very sensitive to hand position, moving closer/further away from a tube will change the output. And unexplained hum is almost ALWAYS VHF oscillation that somehow always manifests at your basic mains frequency.

Don't expect logic to work in these situations - been there and have the scars to prove it...

Regards, Allen
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Old 15th May 2010, 11:23 AM   #3
SY is offline SY  United States
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Is your power supply in a separate box?
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Old 15th May 2010, 03:43 PM   #4
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Definitely a power supply or hum pickup problem. Does it disappear when disconnecting the phono cartridge?
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Old 15th May 2010, 09:11 PM   #5
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3-stage design, triodes have mu=20, but there is also a pentode mu(g2g1)=50.

All tubes have 1k grid-stoppers. Heating is DC. Tried lifting the filament to +50 v but to no avail. The pentode also has a 18 pF miller cap between plate and grid to fix open loop gain rolloff. Now that I talk about it - not shure what the voltage rating is, there could be 150 v across it, got to check this ...

Signal plots where actually done with PC sound card, for ease of presentation.
But sound, noise, hum, spikes do not depend on whether sound card is connected. Which is maybe also a little strange, with the PC sitting on a different wall outlet there should be a huge additional ground loop - but no effect ...

My scope is unfortunately a little out-of-date, good for AF only I suppose. You are right, I may not have seen HF oscillation. I am not shure whether I understand how HF can turn into 50Hz hum. But now that you mention it, I just had that experience with a tube power amp recently, where the hum disappeared magically once I had fixed the oscillation tendency, which manifested itself only when the input was left open, causing smoke signals from the zobel ;-)

The amp itself is inside a metal cage, and I have not noticed any hand sensitivity. However whith the turntable switched on, and before I put the stylus on a record, there is sometimes a crackling sound from the speakers when my hand is hovering over the record player. Thought this to be static ... maybe not ...

There are no female cinch sockets yet, just a pair of short shielded cable with male cinch plugs at the ends to connect to the turntable. When I leave them open, hum increases by an order of magnitude. If shorted hum is same as connected with player.

The PS is currently external as I am still experimenting. I moved it away from the amp until the hum didn't improve any more.

Last edited by payloadde; 15th May 2010 at 09:17 PM.
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Old 15th May 2010, 09:24 PM   #6
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Can you scope the house AC and compare? The spikes look typical of an ac drive. I have a well pump at home run by a drive; looks exactly like that when it turns on. Iso xfmr killed it completely.
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Old 15th May 2010, 11:53 PM   #7
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[QUOTE=payloadde;2188251]3-stage design, triodes have mu=20, but there is also a pentode mu(g2g1)=50<<

I didn't mention mu, I mentioned gM (transconductance). What tubes are they? High gM tubes tend to be VHF tubes, who LOVE oscillation.

>>All tubes have 1k grid-stoppers<<<

Right at the tube socket, or at some distance from the socket? 1mm is OK, 5mm is too far!

>>The pentode also has a 18 pF miller cap between plate and grid to fix open loop gain rolloff. <<

Does this mean it's a NFB design? Then I'm sure you have oscillation. A schematic would help a lot.

>>You are right, I may not have seen HF oscillation. I am not shure whether I understand how HF can turn into 50Hz hum<<

I don't think anybody understands that, but I assure it it does.

>> But now that you mention it, I just had that experience with a tube power amp recently, where the hum disappeared magically once I had fixed the oscillation tendency, which manifested itself only when the input was left open, causing smoke signals from the zobel ;-)<<

Exactly.

>>The amp itself is inside a metal cage, and I have not noticed any hand sensitivity. However whith the turntable switched on, and before I put the stylus on a record, there is sometimes a crackling sound from the speakers when my hand is hovering over the record player. Thought this to be static ... maybe not ...<<

That sounds like static to me.

>>There are no female cinch sockets yet, just a pair of short shielded cable with male cinch plugs at the ends to connect to the turntable. When I leave them open, hum increases by an order of magnitude. If shorted hum is same as connected with player<<

Throughly investigate oscillation. But the other poster's suggestion to check your mains supply for serious noise is also valid. Maybe it's both...

Regards, Allen
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Old 16th May 2010, 09:05 AM   #8
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Thank you for your contributions !

Transconductance is moderate, around 6 mA/V.

Regarding hum I will concentrate on oscillations.

Regarding spikes, I suspected from the beginnng that they are external and come from mains. Electric motor drive is possible, there are some in central heating and hot water supply.
But here's the catch: when I use isolation xfrmr, 50Hz hum is up, and excessive, distorted but no spikes; when I connect circuit ground to earth, i.e. protective ground from wall socket, thereby breach isolation, hum drops by an order of magnitude, takes sinusoidal shape ... but spikes appear.
Just tried to put another isolation xfrmr in front of everything, to no avail however.

Could it be that the spikes come in thru protective ground ???
If so, would there be a means to remove them by some sort of a filter ?
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Old 16th May 2010, 09:16 AM   #9
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Sometimes hum is caused by oscillation because the oscillation draws so much current from the power supply that regulators drop out of regulation. If those DC heaters are regulated, it could be heater regulator oscillation. A circuit diagram would make it much easier to guess what the problem is.
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Old 16th May 2010, 09:26 AM   #10
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What is the source of filament, AC or DC regulated? AC will make more hum/noise than DC regulated.

What about the filament and 0 - B+ connection, is filament source floating (doesn't connect to main supply)? If yes, try to give some 1/3 B+ to filament. If the topology is all AF, then it is safe to ground the filament supply.

I have the same problem and it solved by grounding/connecting the filament supply to either ground or some 1/3 B+.

Thx,

Ervin L
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