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4S preamp project
http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=6392
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Author:  Gab [ 20 Nov 2018, 12:38 ]
Post subject:  Re: 4S preamp project

So, if I understand, with a B+ of 250V and lets say 20mA (10mA for each LEDs), I would need a 12.5k resistor wich can dissipate 5W of power ?

Author:  Suncalc [ 20 Nov 2018, 17:29 ]
Post subject:  Re: 4S preamp project

Gab wrote:
So, if I understand, with a B+ of 250V and lets say 20mA (10mA for each LEDs), I would need a 12.5k resistor wich can dissipate 5W of power ?
A couple of points.

1. Put your LEDs in series. This way you're not asking the B+ supply for lots of current. Remember with vacuum tube B+ supplies voltage is cheap, current costs.

2. Unless you're looking for supplemental room lighting think more like 5ma for the LED chain.

3. So, for a 250V B+ supply; R = 250/.005 = 50kΩ. Power rating is: P = (0.005^2)*50000 = 1.25W. I'd use a 2W resistor.

Does this make sense?

Author:  Gab [ 20 Nov 2018, 19:04 ]
Post subject:  Re: 4S preamp project

Totally. Less current means less power to dissipate, less space and less costs. Is there any minimum current specs or recommendations for an led ? I didn't find any minimum spec anywhere !

Again, your answers are appreciated. It helps me a lot, especially when I look around the web and can't find exactly what I need.

Author:  Suncalc [ 20 Nov 2018, 19:32 ]
Post subject:  Re: 4S preamp project

Gab wrote:
Is there any minimum current specs or recommendations for an led ?
Not really.

If you are designing for mass production you need to worry about minimum current because of variability in luminous flux at low currents. My son (EE PhD specializing in MicroElectronics) rants at me about feeding LEDs with too little current and causing item to item variability.

My experience (30 years plus) tells me that a few mA is more than adequate for the vast majority of cheap red/yellow/green LEDs on the market today. If one is a little brighter or dimmer than another, I chalk it up to the unpredictability of the universe and take another sip from my beer. :beerchug:

Author:  Gab [ 21 Nov 2018, 10:14 ]
Post subject:  Re: 4S preamp project

As long as they light up bright enough to be seen, i’ll have no problem living with a couple lumens difference ! (And won’t wait for any unpredictability of the universe to have a sip from my beer ahah !)

Author:  Gab [ 21 Nov 2018, 10:47 ]
Post subject:  Re: 4S preamp project

The power supply components are installed. I ran the B+ to the signal side, but I am still thinking about the layout. The holes in the divider might be too small for all the wires I need to get through.

I also wan’t to buy terminal strips locally so I won’t have to wait for the shipping, but can’t find anything up to this point. Totally forgot to order them at the same time as my first order from Digi-Key. We are getting a lot of snow these days so going out on the road to Montreal to buy parts is out of question.

Author:  Chris65 [ 21 Nov 2018, 10:49 ]
Post subject:  Re: 4S preamp project

Always thought LED's on tube amps are superfluous, the tube heater glow being a good indicator that the amp is 'On'.
(unless the tubes are enclosed inside the chassis).
Anyway, it is a good looking build.

Author:  Suncalc [ 21 Nov 2018, 11:44 ]
Post subject:  Re: 4S preamp project

Gab wrote:
I ran the B+ to the signal side
One quick note. Make sure you run the B+ power return / signal ground back through the same hole as the B+. If you don't, then the divider metal forms a closed loop conduction path link with your power supply loop. This arrangement can couple field noise into your B+ circuit.

See this shot from the Lacewood V2.0. All the B+ feeds and the ground buss penetrate the metal divider at a single place.
Attachment:
PowerFeed.jpg
This is the way to do it. :up:

Author:  Gab [ 21 Nov 2018, 13:52 ]
Post subject:  Re: 4S preamp project

Suncalc wrote:
Gab wrote:
I ran the B+ to the signal side
One quick note. Make sure you run the B+ power return / signal ground back through the same hole as the B+. If you don't, then the divider metal forms a closed loop conduction path link with your power supply loop. This arrangement can couple field noise into your B+ circuit.

See this shot from the Lacewood V2.0. All the B+ feeds and the ground buss penetrate the metal divider at a single place.
Attachment:
PowerFeed.jpg
This is the way to do it. :up:


Good to know ! Should I consider that having longer wires that run across the enclosure is better than creating this kind of loop ?


Chris65 wrote:
Always thought LED's on tube amps are superfluous, the tube heater glow being a good indicator that the amp is 'On'.
(unless the tubes are enclosed inside the chassis).
Anyway, it is a good looking build.


Thank you for your comment. The LEDs shows wich input is selected. For the "power status" LED, I consider the heater glow to be nothing more than the confirmation that the heaters are on. The heaters and power supply/B+ are two completely different circuits so it might be useful if something goes wrong with the amp. The "if" is very important in this sentence !

Author:  mwhouston [ 21 Nov 2018, 16:28 ]
Post subject:  Re: 4S preamp project

When building all my battery powered devices which require a led (always 3mm) to indicate on firstly; I like to use retro yellow (most go for blue these days but being retro, I go the old amber/yellow). Secondly I buy the highest output LEDs I can. These you can run on extremely low current and they will still glow. I like a barely "on" extremely low glow - because I don't want distraction when listening to music, always in a darkened room and in this mode they consume very little battery power.

Even my mains powered devices I have the yellow LEDs barely on. Nothing worse than a bright LED cast a searing beam across your listening area.

To sum up, high powered 3mm LEDs on extremely low current.

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