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Old 29th April 2016, 06:13 PM   #1
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Default Noob questions

I hate to ask such noob questions, but honesty, it's difficult to find answers to the simplest questions when just starting out on a tube build.


I'm building the S-5 K 16LS. It's a very simple build. I'm gaining knowledge and realize that I can't just blindly add additional switches and the like without considering interference. And if I'm correct, the main source of interference is with the AC side of the house - hence, I'm twisting wires from the PT and trying not to make any major changes. However, I wanted to check with all of you on the following questions:


1. I'm only twisting wires that carry AC going into the PT from the board and the secondaries from the PT going back to the board. I'm also twisting the wires going from the OT back to the board where they connect to the speaker plugs. I didn't bother twisting DC side of output transformers. Is this correct?


2. I'd like to incorporate an IEC plug, fuse, switch, and power light. How do we do this while minimizing interference? Or are we only concerned with interference from the PT secondaries to the board?


3. Is it okay to relocate input signal connectors and volume control (pot) without worrying about interference since it is DC?


4. I've mounted my transformers (all three), below the main circuit board. There is a large electrolytic capacitor I'd like to relocate. Is it okay to just run jumper wires from the board to the capacitor, or are there considerations that I might not be aware of?


5. Finally, because I did mount my transformers below the main board in the chassis, I was thinking about adding a fan for cooling - will this cause interference?


I realize that this is a tall order. Any help would be appreciated!


Thanks

-mm
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Old 29th April 2016, 06:43 PM   #2
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Are you sure that signal is DC ?
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Old 29th April 2016, 07:30 PM   #3
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Well, this is embarrassing... So my general understanding is that the way the amp works is that we need AC to drive speakers, but AC is susceptible to interference, so we first convert incoming AC off PT to DC (via rectifier), then the signal is increased while it is DC, and finally we convert it back to AC at the OT, to the speakers. And when I look at schematic, both input signal, and POT is on the DC side of circuit. So I'd say yes.
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Old 29th April 2016, 07:54 PM   #4
DF96 is offline DF96  England
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No. DC is bias; the signal is AC.

I think you need to do a lot more reading and thinking before you design the physical layout of an amplifier.
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Old 29th April 2016, 09:35 PM   #5
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It might be easier to build on if you first get the idea that all audio is AC. Then think that the power to run the circuit needs to be DC otherwise you'll hear it mixed in with the music. Thus it's necessary to have DC that is pure (to a degree that is dependent on the circuit's propensity for making any unwanted AC audible).

That AC "noise" can be transmitted to the music signal carrying circuit in several ways - magnetic, electrostatic, radio frequency noise and mechanical. The degree to which these are transmitted all depend on the strength of the generated noise AC and relative proximity to the source.

Twisting wires on heaters and other AC sources has to do with putting a plus and minus voltage in such close proximity to each other that their unwanted ac fields will intermingle. You know that +1 added to -1 equals 0 . In the same way the positive field and negative field cancel out when forced together. Being nulled this way, there is far less chance of that noise having any influence on other nearby parts of the circuit.

Through experience you will get a sense of what radiates how much noise how far and what you want to keep protected. Then you develop a picture of how things need to be laid out in order to avoid unwanted interactions.

A good experiment is to take your raw, out of circuit, power transformer and put AC on it (making sure that neither the primary nor the unused secondary leads can short or come into contact with your most esteemed self.) Put your AC volt meter on the primary of a raw , out of circuit output transformer and then move it around the power transformer watching how the meter reads as you move it. You'll see how distance and position affect the reading. This tells you about radiated magnetic fields. etc etc. and why, if the way I read your post is accurate, you might decide it's better to move the power transformer farther away from your circuit board.

Gotta get back to work. Hope this helps.
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Old 29th April 2016, 09:51 PM   #6
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally Posted by mattress67 View Post
1. I'm only twisting wires that carry AC going into the PT from the board and the secondaries from the PT going back to the board. I'm also twisting the wires going from the OT back to the board where they connect to the speaker plugs. I didn't bother twisting DC side of output transformers. Is this correct?
Yes

Quote:
2. I'd like to incorporate an IEC plug, fuse, switch, and power light. How do we do this while minimizing interference?
Keep them as far away from the audio preamp circuitry as possible.

Quote:
3. Is it okay to relocate input signal connectors and volume control (pot) without worrying about interference since it is DC?
Interference tends to couple into high-impedance parts of a circuit. The volume control is typically a high impedance location. However, your quesion is rather confused and inscrutible...

Quote:
4. I've mounted my transformers (all three), below the main circuit board. There is a large electrolytic capacitor I'd like to relocate. Is it okay to just run jumper wires from the board to the capacitor, or are there considerations that I might not be aware of?
Well, you don't want long jumper wires, but you would probably be OK with three inches or something

Quote:
5. Finally, because I did mount my transformers below the main board in the chassis, I was thinking about adding a fan for cooling - will this cause interference?
Hard to predict. Don't make it permanent until you've tried it out.
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Old 29th April 2016, 09:59 PM   #7
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All excellent, and special thanks to Merlinb for answers to my specific questions!

And I think I've got a long way to go before I can really comprehend the DC bias.

Last edited by mattress67; 29th April 2016 at 10:28 PM. Reason: Additional info
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Old 29th April 2016, 11:06 PM   #8
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Merlin has a nice preamp book that I can highly recommend. Morgan Jones' books are always worth reading too.
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Old 29th April 2016, 11:32 PM   #9
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Very cool - thanks. Still accumulating data before I make the jump to "intermediate" material. Just read about the Edison effect for the first time five days ago. You'd never know I have a degree in engineering but EE was never my strong point. Barely got thru EE2. Now, later in life, my curiosities about the elusive subject have returned.
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Old 30th April 2016, 09:16 PM   #10
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The signal comes in as AC through a capacitor, then becomes DC as it is a varying value of DC voltage at the plate of the gain tube.
The signal is then fed into the output transformer in a "push - pull" config.
The transformer then converts this signal back to AC for the speakers.
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