jfets are relatively immune to static, and can be handled like regular bipolars but MOSFETs need all the care you can use.
Using a wriststrap is not overkill, better safe than sorry.
regards, Allen
Your hum could be caused by the details of your grounding arrangements. There are lots of ways to connect grounds, and only a few of them will eliminate hum.
Thank you; I thought about this endlessly yesterday. I had star earthed everything with all power supply caps and bleed resistors going to a point,all grid leaks on a bus as another point, all g2 connections as a 3rd and finally the reference for the 6sn7 heaters. These 4 points were then connected to the safety ground point on the chassis. I would have thought this would be ok but please tell me if I am mistaken. The troublesome hum is 50Hz however and when I noticed that a negative supply output wire was passing rather too close to a power transformer and improved that situation this reduced it to a much more acceptable level although I have only tried it through headphones as yet. I have not yet installed grid stoppers on the sinks but shall do so tomorrow. Without pretending to understand why,I have read that UHF oscillation shows up as a mains frequency hum.At least now it is working fine and I am pleased with the sound or rather the lack of it. I am still curious to know the reason for the apparent inexplicably high current draw reading 30V across the cathode resistor of the 6p15p. I have not reused the offending tubes and perhaps Wavebourne's suggestion is true despite the high pedigree of these tubes. It is unlikely to suppose that since the break-up of the mighty Soviet Union every 6p15p-er now offered for sale has been stored in ideal conditions. Many thanks to all who have taken the trouble to reply to my posts.
Grounding within the unit seems OK. Could it be an external ground loop? You could check this by temporarily disconnecting the link to safety ground - but be careful, don't touch anything while you do this! The idea is that the signal ground gets grounded via other units upstream or downstream. If this solves the hum, then there are ways of maintaining a safety ground while still separating it from the signal ground.
RF oscillation can cause hum by the RF getting into the mains rectifier and so being modulated at 50Hz or 100Hz. The modulated RF then finds its way back into a signal circuit. Small grid stoppers won't do any harm and may do some good so best to add them.
RF oscillation can cause hum by the RF getting into the mains rectifier and so being modulated at 50Hz or 100Hz. The modulated RF then finds its way back into a signal circuit. Small grid stoppers won't do any harm and may do some good so best to add them.
Hi Piano,
I have had a few problems with DC. heater arrangements and how they induced 50hz hum.
If I understand correctly, you have raised the potential of your heaters?
Could this not be a source of noise?
Just my two peneth worth.
Regards
Bernie
I have had a few problems with DC. heater arrangements and how they induced 50hz hum.
If I understand correctly, you have raised the potential of your heaters?
Could this not be a source of noise?
Just my two peneth worth.
Regards
Bernie
Hello Bernie,
The cathodes of the pentode constant current sinks are at about -220V so there was absolutely no choice but to elevate the heater voltage as goodness knows what the result would have been otherwise. The elevation was done from the output of the supply which is very well filtered. The hum is also definitely 50 rather than 100 Hz-one advantage of having spent my entire life incessantly practising the piano is that I can easily hear the difference between them or in combination without needing to turn the oscilloscope on. I am quite sure that DF96 is correct when he suggests that it is an external loop and my 250mV(!) sensitivity commercial SE amp(which has served me very well for a dozen years) does very little to help matters.However the fact that you have had problems with elevated supplies is an interesting topic although one which I am not qualified to comment on. I am sure that any of the heavyweights above who have replied to my thread may have something apposite to say.
Regards,Nicholas
The cathodes of the pentode constant current sinks are at about -220V so there was absolutely no choice but to elevate the heater voltage as goodness knows what the result would have been otherwise. The elevation was done from the output of the supply which is very well filtered. The hum is also definitely 50 rather than 100 Hz-one advantage of having spent my entire life incessantly practising the piano is that I can easily hear the difference between them or in combination without needing to turn the oscilloscope on. I am quite sure that DF96 is correct when he suggests that it is an external loop and my 250mV(!) sensitivity commercial SE amp(which has served me very well for a dozen years) does very little to help matters.However the fact that you have had problems with elevated supplies is an interesting topic although one which I am not qualified to comment on. I am sure that any of the heavyweights above who have replied to my thread may have something apposite to say.
Regards,Nicholas
DC heaters are supposed to avoid hum! However, if done carelessly the fast rectifier spikes can induce interference (probably at 100Hz) which would not occur with much slower 50Hz AC.
I have now taken every possible precaution; screened HT+ and HT- wires, 1k grid stoppers on pentode sinks, no AC or electromagnetic components anywhere near the signal circuitry, everything possible to twist twisted and when monitored on headphones on my Panasonic 3700 DAT recorder,XLR to XLR with the recording level to maximum all is acceptable. As soon as I connect to my 300B SE then the hum is there and the level is independent of the attenuation at line stage. I broke the mains earth connections to chassis on phono and line(with some trepidation,I might add!) which made no difference until I remembered I had not broken the pin 1 to chassis connection at the linestage output and then the noise became alarming. What else can I try? Would resistors beween 0V and chassis on phono and line as some recommend be likely to help? The resistance from pin1(or 3) to the chassis of the power amp is about 100 ohms.2 gain stages of 6SN7 before a 300B is really far too much gain for a power amp! My previous preamp which matches the power amp had the 0V to chassis connection completely cut!
Of course,much as I dislike modifying someone else's work,particularly in a commercial amp I wonder has anyone ever modified this 6sn7 cascade-I think it is known as a Reichert cascade- to get lower gain? It is a single valve so no topologies which might exceed cathode to heater limits are possible. Perhaps just paralleling the 2 sections in a simple voltage driver might give a reasonable output impedance for driving the 300B. This lower gain might cause my seemingly intractable hum problem to go away.
Just to clarify the ground issue: each chassis should remain connected to mains (safety) ground. The signal ground should be maintained separately throughout the system, and connected to one and only one of the chassis somewhere. In addition, you may add resistors etc. from signal ground to safety ground in other units - the resistor (or diode bridge etc. - there are several options) is supposed to provide a backup safety ground and blow a fuse when necessary yet have high enough impedance for small signals that it does not create a hum loop.
If the problem only occurs with your 300B amp, could that be injecting hum into the ground circuit? Maybe your new preamp is not the culprit but the victim?
If the problem only occurs with your 300B amp, could that be injecting hum into the ground circuit? Maybe your new preamp is not the culprit but the victim?
Thank you for your clarity, DF96. I found that I had downloaded the schematic of my 300B amps some time ago which confirms that the signal ground is connected to safety ground via a 100 ohm resistor(I hope it is a very high power one!). It is hard to say whether there is anything suspicious although the power supply as drawn,which is not necessarily how it is wired, has one slightly concerning detail. It would be totally inappropriate to mention manufacturers on this forum. Would it be possible to send you the schematic by private email ?
Also, the thought just occurred to me, if the poweramps were injecting hum,would the fact that I have 9 Mohm resistors to ground at the preamp output make things worse?
Yes, send me the circuit. However, hum problems are often caused by things not shown on the circuit, such as exact grounding arrangements and physical location and orientation of components.
The 100 ohm resistor just needs to survive long enough for the fuse or circuit breaker to blow in the event of a short. However, a resistor here means that the main ground connection is upstream somewhere, such as in the preamp or source equipment.
The 100 ohm resistor just needs to survive long enough for the fuse or circuit breaker to blow in the event of a short. However, a resistor here means that the main ground connection is upstream somewhere, such as in the preamp or source equipment.
On adding a ground lift resistor of 10 ohms I measured a 1 volt difference between signal 0V and mains earth. Surely this means that I have a problem with my electricity supply. Maybe I should really lower the tone of my nice London suburb and bury a car in the garden...
Not necessarily a supply problem. It doesn't matter, within reason, what voltage your 'earth' is provided that everything is referred back to the same voltage.
You seem to have 100mA of leakage from somewhere! Are you sure? This ought to trip a breaker, or does your house only have old-fashioned fuses? Mains filter capacitors can inject current into the earth, but only about 100uA per item. Where did you insert the ground lift?
You seem to have 100mA of leakage from somewhere! Are you sure? This ought to trip a breaker, or does your house only have old-fashioned fuses? Mains filter capacitors can inject current into the earth, but only about 100uA per item. Where did you insert the ground lift?
I am quite sure and when I did a listening test with this configuration not only was the hum enormous but there was all kinds of interference as well. The ground lift was simply connected between the star point on the signal earth and the safety bond on the chassis. The only listenable configuration is with the signal earth star point connected to the chassis of the (differential) phono stage. I have been highly suspicious of my supply for quite some time as my AC voltage tracks exactly expected local current demand,dropping to about 210 V(and sometimes below 200!) in the early evening and rising to about 240 at bedtime. This,as far as I know,is illegal and it means I must run my hifi off a variac. Perhaps these two problems are connected? My house does have circuit breakers( not RCD's) and indeed this puzzles me.
Very strange! You would expect some hum if the signal ground is slightly floating, but 10ohms is more or less a short circuit for line-level impedances. Was there an input source, with its own ground or just an open/short circuit at the preamp input?
Your mains supply seems to have a poor connection somewhere. Perhaps you should get an electrician to check it. Do any of your neighbours have similar problems? About every third house will share the same phase, so your immediate neighbours may be on a different phase from you. It needs checking. Poor connections in houses can cause fires, and poor connections in the street have been known to go bang in the middle of the rush hour! You could talk to your local electricity distributor (EdF in London?).
Your mains supply seems to have a poor connection somewhere. Perhaps you should get an electrician to check it. Do any of your neighbours have similar problems? About every third house will share the same phase, so your immediate neighbours may be on a different phase from you. It needs checking. Poor connections in houses can cause fires, and poor connections in the street have been known to go bang in the middle of the rush hour! You could talk to your local electricity distributor (EdF in London?).
The Garrard turntable at the input has its body safety grounded, of course, but the arm is totally electrically isolated from the motor unit and is connected to the phono preamp with a phono to XLR adaptor with the little ground lead connecting to the preamp chassis as usual.
I have had one act in the circus with the suppliers already; on the night this year when it snowed heavily in London my voltage dropped so low that my lights were very noticeably dimmer. I phoned the supplier and they told me it was probably not a big problem. However,a guy turned up at 8.30 the next morning hardly able to get up the path in the snow. He told me all was within normal parameters and left. I measured it a few minutes after he left with several different meters-it was 217V. My sister, who lives next door, does not have a problem and always measures 230-40 so she is clearly on a different phase.
I have had one act in the circus with the suppliers already; on the night this year when it snowed heavily in London my voltage dropped so low that my lights were very noticeably dimmer. I phoned the supplier and they told me it was probably not a big problem. However,a guy turned up at 8.30 the next morning hardly able to get up the path in the snow. He told me all was within normal parameters and left. I measured it a few minutes after he left with several different meters-it was 217V. My sister, who lives next door, does not have a problem and always measures 230-40 so she is clearly on a different phase.
217V is legal, but only just; I believe the lower limit is 216.2V in the UK. Perhaps you should complain about the huge variation, rather than the low value, although you might have difficulty explaining the difference to Darren or Tracy in the call centre!
Is the phono preamp the point at which the safety and signal grounds meet? This makes sense, provided that the input cable distinguishes between screen and signal return - most arm wiring should be OK. It might be worth checking at this end, preamp, XLR plugs etc. Does the ground lead connect to the screen or the signal return? At the preamp, does the ground terminal connect to safety or signal ground? As you probably know, cartridge connections are treated as balanced even when the phono preamp has an unbalanced input, so the screen just does screening and is not the signal return conductor. Does the phono to XLR adaptor maintain the right connections?
Is the phono preamp the point at which the safety and signal grounds meet? This makes sense, provided that the input cable distinguishes between screen and signal return - most arm wiring should be OK. It might be worth checking at this end, preamp, XLR plugs etc. Does the ground lead connect to the screen or the signal return? At the preamp, does the ground terminal connect to safety or signal ground? As you probably know, cartridge connections are treated as balanced even when the phono preamp has an unbalanced input, so the screen just does screening and is not the signal return conductor. Does the phono to XLR adaptor maintain the right connections?
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