[greg01]

Hi guys this is the first time I'm on this forum.
I own two Clarenet Line preamps (one complete board and the other I cut the boards inputs to fit it in a smaller case and hard wired it) and both have a very low hum, I've tried to find it but no luck.
All grownding checked but still the same.
Can anyone put some light on this.
As I said the hum is low but audible, the hum is the same with the volume low or high.


Greg

[poty]

Are you sure it is the Clarinet sin? If you switch off the Clarinet (or even pull out the mains power cable) - hum persists?

[greg01]

Hi and thanks for the reply.
Once I pulled out the mains from the Clarenet the hum disapeard.
Ground loop?
What can I do?

[Æ]

Hi and thanks for the reply.
Once I pulled out the mains from the Clarenet the hum disapeard.
Ground loop?
What can I do?

Try moving the whole thing to a different location or orienting it in a different direction.
Try flipping the AC cord, temporarily reverse the hot and neutral.
Hopefully you know the rules when it comes to avoiding ground loops.

[poty]

Hello, could you be more precise? What have you done?
- Even if you switch off the Clarinet you have several seconds of energy to run it - does the hum present in this period of time? After the period?
- If the hum disappears after the period - it means rawly 2 things: either there is something emanating or receiving EMI (bad input interconnects, input RCAs, some mistakes in internal wiring, risky tubes...) or something wrong with the power supply filters.
- If the hum exists after switching off the Clarinet (after the run off period), but with mains cord in place - try to disconnect the mains cord.
- If the hum disappeared - then likely it is lack or bad or polluted earth bus in the mains. The second consideration is still external EMI.
- At some point as you said - the hum disappeared - so the questions above should clarify the situation.

[greg01]

Try moving the whole thing to a different location or orienting it in a different direction.
Try flipping the AC cord, temporarily reverse the hot and neutral.
Hopefully you know the rules when it comes to avoiding ground loops.

Also moved to diff power point on the other side of the house. + reversal.
Still the same

[greg01]

Hello, could you be more precise? What have you done?
- Even if you switch off the Clarinet you have several seconds of energy to run it - does the hum present in this period of time? After the period?
- If the hum disappears after the period - it means rawly 2 things: either there is something emanating or receiving EMI (bad input interconnects, input RCAs, some mistakes in internal wiring, risky tubes...) or something wrong with the power supply filters.
- If the hum exists after switching off the Clarinet (after the run off period), but with mains cord in place - try to disconnect the mains cord.
- If the hum disappeared - then likely it is lack or bad or polluted earth bus in the mains. The second consideration is still external EMI.
- At some point as you said - the hum disappeared - so the questions above should clarify the situation.

Ok, The hum: Clarinet on or off the same hum, once I disconect the power cable from the clarinet the hum is gone. I conected an ext EMI filter still the same. Once gnd from mains connects the hum is back.
Inputs are disconected cd.
I've built other preamps but I don't have this problem.
My electronics skills are medium.

[jostber]

A short and concise article on troubleshooting hum issues from Decware:

http://www.decware.com/paper41.htm

[greg01]

Ok, going threw all the circuit and checking, then removing all the grounds and reconnecting in star config. I found that the preamp is better, the hum is still there but it is at a level that is ok. Now another thing I found was that when I conect the center tap of the 6-0-6 volt to ground as per schematics the 12au7 heaters wouldn't work  so I disconected the grounding and it all works ok.

[poty]

Ok, going threw all the circuit and checking, then removing all the grounds and reconnecting in star config. I found that the preamp is better, the hum is still there but it is at a level that is ok.

I'm curious - in the first message you mentioned PCBs, in this - "reconnecting in star config"... What have you done exactly?
The fact that the hum didn't disappear until you pull out mains cord means for me - absence of proper ground in the mains wall socket or broken cable. The usage of filters in this case is not effective at all if not aggravating. It is not clear from your message, but it can be also bad AC wiring inside the case or some leackage of the mains AC to a circuit.

Now another thing I found was that when I conect the center tap of the 6-0-6 volt to ground as per schematics the 12au7 heaters wouldn't work  so I disconected the grounding and it all works ok.

You should't do anything that is not on PCB. The Clarinet schematic for power supply have a mistake:  the center wire for 6.3V (yes! it is 3.15-0-3.15, not 6.3-0-6.3) must not be connected to ground.

[greg01]

Yes I have two clarinet preamps both pcb's, One is as I got it with all the suggested parts and the other I have cut the board (all the side with the inputs and the front with the volume, balance and selection switch) I have added my own, also I used cc resistors etc. The sound is better with the cc's also I found that the balance doesn't work so well on the original. I do like to play around with the preamp lol. But to tell you the truth I do get a hum on the full board also.
My last project was a Budgie pre amp and I can say for the cost of the build it sounds great. The 12b4a is a nice tube with good all around colour, now I’m looking to build another 12b4a, with more bells and whistles for my PMillett 6b4g. I’m also at long last finishing (after 4 years) my clone Knight KB-85; well it will be next year now. I just finished a set of 17Watt Ultra linear EL37 based on schematics from 1955, but running about 30Watts each, what a sound! too bad they don’t make those tubes anymore. I still have 3 pair’s to play with.

Regards
Greg

[jostber]

Some more good tips on this site:

http://www.geofex.com/ampdbug/hum.htm

[greg01]

Thank you

[hagtech]

Can you post a photo of what you built?  It sounds to me like some floating chassis parts.

jh