rmaier
Well-known member
Dropbox link to an audio sample of the buzzing:
https://www.dropbox.com/s/xz6a9qk784gwc03/Preamp%20Buzz.m4a?dl=0
https://www.dropbox.com/s/xz6a9qk784gwc03/Preamp%20Buzz.m4a?dl=0
Feeler: EZ Tube Lunch Box
- Thread starter ruffrecords
- Start date
Help Support GroupDIY Audio Forum:
Hi Rmaier,
I'll look at it when i get home but did you try feeding the output trafo from out 1? Just to see if its working....
You might want to use screened cable for out 1 to in 2.
Do you plan to use the relay for the direct in? Is the relay working? Maybe try jumping the relay to see if thats the problem...
Regards,
Pierre
I'll look at it when i get home but did you try feeding the output trafo from out 1? Just to see if its working....
You might want to use screened cable for out 1 to in 2.
Do you plan to use the relay for the direct in? Is the relay working? Maybe try jumping the relay to see if thats the problem...
Regards,
Pierre
ruffrecords
Well-known member
I cannot see how you have the level pot wired up. I cannot see the ground connection. Does the buzz vary with the level control pot setting?
Cheers
Ian
Cheers
Ian
rmaier
Well-known member
Hi Ian, hi Pierre,
As you can see, I used unshielded wire between stages...
As for the buzz, it is the most extreme when the pot is fully open. Having said that, although the unit passes a signal (I ran one of my cheap dynamic mics through the input and went out into a powered monitor), I'm hearing more buzz than signal. What does make it through is very distorted.
Thanks!
Ralph
As you can see, I used unshielded wire between stages...
As for the buzz, it is the most extreme when the pot is fully open. Having said that, although the unit passes a signal (I ran one of my cheap dynamic mics through the input and went out into a powered monitor), I'm hearing more buzz than signal. What does make it through is very distorted.
Thanks!
Ralph
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ruffrecords
Well-known member
You seem to have only two wires connected to the pot. There does not appear to be a ground connection to the pot.
It is also worth checking out your grounding scheme. You have a remote power supply. In that power supply the HT 0V needs to be connected to the safety earth point which should be a bolt on the chassis near the main input connector. A short lead from the mains inlet earth pin should also go directly to this bolt. Not making these connections often leads to hum. Also you need to make sure that the chassis of the amps box is connected to the chassis of the power supply box. If it is not you can get hum. So you need a lead in your power supply cable that makes this connection. The idea is to make the amp chassis an extension of the power supply chassis and the purpose of both is to screen the sensitive electronics from things like hum. So, it is a good idea to make this connection a screen as well. I normally use a screened power cable for connecting power supplies to amps boxes and use the screen of this cable to connect the two chassis together.
Cheers
Ian
It is also worth checking out your grounding scheme. You have a remote power supply. In that power supply the HT 0V needs to be connected to the safety earth point which should be a bolt on the chassis near the main input connector. A short lead from the mains inlet earth pin should also go directly to this bolt. Not making these connections often leads to hum. Also you need to make sure that the chassis of the amps box is connected to the chassis of the power supply box. If it is not you can get hum. So you need a lead in your power supply cable that makes this connection. The idea is to make the amp chassis an extension of the power supply chassis and the purpose of both is to screen the sensitive electronics from things like hum. So, it is a good idea to make this connection a screen as well. I normally use a screened power cable for connecting power supplies to amps boxes and use the screen of this cable to connect the two chassis together.
Cheers
Ian
rmaier
Well-known member
Thanks Ian and Pierre. I knew about the connection of the units via the screen of the multi-cable, but had no idea that HT needed to be earthed (even though I had already read it around post #269 - amazing how it doesn't really sink in until you encounter difficulties yourself!). Missing the ground connection on the pots was a pretty silly mistake - I need to pay better attention to the schematic! I used shielded cable from Out1 to the HPF assembly and through the pot to In2 (ground connected at one end only). In any case, I now have four working channels of yummy tube pre goodness! While the unit still needs more extensive testing, my first impression is extremely positive - made my cheapest dynamics sound lush and refined. Thanks once again, Ian, for your generosity in making this kind of project available and for your patience in guiding newbies like myself through it.
Ralph
A few pictures below:
Ralph
A few pictures below:
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rmaier
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Rocinante
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Awesome job man. Looks tight. Im very curious about your feelings on it in various applications. Please let us know. Also your thoughts on the edcors would be welcome.
rmaier said:
Hi Ralph,
I had been waiting for your reactions after the first moments with the preamps....brings a smile to your face right. I'm happy you found the solution!
I have not tried the hpf mod yet, if you have time to comment on it eventually i would be interested in your opinion!
Regards!
Pierre
ruffrecords
Well-known member
Well done Ralph! I am really pleased you got it going and that you enjoy the sound. Don't ever be afraid to ask a question. When I was in my teens I joined the local amateur radio club - there was no internet in the 60s - and the old guys there we really helpful and and answered all my silly teenage technical questions. One of them even ran a radio amateurs exam course at the local technical college which I passed when I was 15. I have never forgotten the help I got from those old boys and I am more than happy to continue the tradition.
Cheers
ian
Cheers
ian
rmaier
Well-known member
Thanks, gents, I appreciate the encouragement and the opportunity to continue to learn. I'll post more comments on the unit once its seen a bit of use.
Ralph
Ralph
ruffrecords
Well-known member
rmaier said:
Would you like to email me some photos so I can pass them on to Pierre to put in his gallery of builds from my designs?
Cheers
ian
rmaier
Well-known member
Hi Ian,
Just sent you a Dropbox link.
Ralph
Just sent you a Dropbox link.
Ralph
ruffrecords
Well-known member
rmaier said:
Received OK. I will pass the zip file on to Pierre.
Cheers
Ian
ruffrecords
Well-known member
It has been a couple of months since I posted anything about the Lunch Box project but I have not been idle. I have been building a lunch box for a German customer whilst still experimenting with the 500 series module width version using Frank's enclosure. As part of this I have been checking out AC heaters and I also ran into some hum problems (which turned out not to be related to the hum problem)
First the hum problem. The plan in Franks enclosure is to use an internal power supply and AC heaters to keep cost down. The plan for the German lunch box was to use an SRS sub-rack and an internal power supply with dc heaters. I did not expect any problems with the German lunch box because it is basically the same as the prototype rack except the power supply is not inside the 28HP plug in module. As it turned out, I got a significant hum in the output which got worse as the modules approached the toroidal mains transformer. To cut a long story short, this turns out to be stray field from the toroid reaching the mic input wiring and/or the mic input transformer. The level of hum is very low but it it not zero. Using a classic module in the slot nearest the transformer with 70dB gain, the hum only worsens the noise floor by about 2dB. I did some experiments with magnetic screening materials in various positions and this did reduce the hum by about one dB. So it appears the only way to eliminate the hum entirely is to place the toroid in a magnetic screening box. I am presently sourcing a mins transformer in a mu-metal enclosure for this project.
Mindful of the above hum problems, I tried AC heaters in Frank's enclosure and again had a hum problem. This time it worsened the noise floor of a Classic module at 70dB gain by 2 or 3dB in the module nearest the transformer and lesser amounts if the module was fitted further away. I was careful to ensure all five slots were powering tubes so any hum from the AC heaters would be maximised. There was no way I could tell if the hum I was hearing was due to the transformer of the AC heaters. So, I bought a cheap steel box off eBay for £5. It is 100mm x 100mm x 50mm. I fitted the mains transformer into this box, fitted it back into Frank's enclosure and brought the transformer leads out through a hole in the lid of the box. I am very pleased to say that this completely eliminated the hum. The noise levels with AC heaters and the transformer in a cheap steel box are the same as with an external power supply with DC heaters.
The bottom line is that AC heaters and internal power supplies in Frank's box are perfectly viable without the need to resort to expensive mu-metal screens but some screening of the toroid is required for best performance.
Cheers
Ian
First the hum problem. The plan in Franks enclosure is to use an internal power supply and AC heaters to keep cost down. The plan for the German lunch box was to use an SRS sub-rack and an internal power supply with dc heaters. I did not expect any problems with the German lunch box because it is basically the same as the prototype rack except the power supply is not inside the 28HP plug in module. As it turned out, I got a significant hum in the output which got worse as the modules approached the toroidal mains transformer. To cut a long story short, this turns out to be stray field from the toroid reaching the mic input wiring and/or the mic input transformer. The level of hum is very low but it it not zero. Using a classic module in the slot nearest the transformer with 70dB gain, the hum only worsens the noise floor by about 2dB. I did some experiments with magnetic screening materials in various positions and this did reduce the hum by about one dB. So it appears the only way to eliminate the hum entirely is to place the toroid in a magnetic screening box. I am presently sourcing a mins transformer in a mu-metal enclosure for this project.
Mindful of the above hum problems, I tried AC heaters in Frank's enclosure and again had a hum problem. This time it worsened the noise floor of a Classic module at 70dB gain by 2 or 3dB in the module nearest the transformer and lesser amounts if the module was fitted further away. I was careful to ensure all five slots were powering tubes so any hum from the AC heaters would be maximised. There was no way I could tell if the hum I was hearing was due to the transformer of the AC heaters. So, I bought a cheap steel box off eBay for £5. It is 100mm x 100mm x 50mm. I fitted the mains transformer into this box, fitted it back into Frank's enclosure and brought the transformer leads out through a hole in the lid of the box. I am very pleased to say that this completely eliminated the hum. The noise levels with AC heaters and the transformer in a cheap steel box are the same as with an external power supply with DC heaters.
The bottom line is that AC heaters and internal power supplies in Frank's box are perfectly viable without the need to resort to expensive mu-metal screens but some screening of the toroid is required for best performance.
Cheers
Ian
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