Noob questions

[mattress67]

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

 

[picowallspeaker]

Are you sure that signal is DC ?

 

[mattress67]

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.

 

[DF96]

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.

 

[Hearinspace]

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.

 

[Merlinb]

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

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.

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...

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

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.

 

[mattress67]

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.

 

[soulmerchant]

Merlin has a nice preamp book that I can highly recommend. Morgan Jones' books are always worth reading too.

 

[mattress67]

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.

 

[stocktrader200]

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.

 

[HollowState]

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.

Sorry but that is not true. The signal enters the amplifier's first stage and is amplified as an AC signal that is super imposed on a high level DC potential needed by the tube. The signal is not converted to DC at any time in a conventional tube amplifier. It remains as AC but rides on a DC level throughout. The output stage may further amplify the AC signal and the output transformer converts that AC signal to an appropriate match for the speaker. It does not convert DC to AC.

 

[Parafeed813]

Maybe this helps to understand:
Voltages can be added to each other. Like putting two 1.5V batteries in series to get 3V (all DC in this case), signal voltage (AC) and the voltage that powers an amp (DC) can add as well. Looking on a o-scope you see the same signal, only with a DC offset (elevated by the DC).

Looking at it mathematically:
Complex waveforms are made up from a combination of sinewaves of different frequencies. A Fourier transform would show you these frequencies and their individual magnitude.
DC is just one of these possible frequencies: 0 Hz.

 

[Malcolm Irving]

DC stands for ‘direct current’. In other words current that travels in one direction only. The (conventional) current flowing through a tube from anode to cathode is always flowing in the same direction, so perhaps should be described as ‘fluctuating DC’.
Strictly speaking ‘direct current’ cannot be a description of a voltage! But we do it all the time! There is some confusion because some folk take DC voltage to mean ‘constant voltage’ and some take it to mean a voltage which is always positive (or always negative) but can fluctuate.
Sorry, if this has just added to anybody’s confusion!

 

[mattress67]

Hollow state, that was an excellent explanation.

I finished the K-16LS last night and it sounds and works great despite my slightly modified layout. Although I did incorporate all suggestions. Still, when I powered on with no signal, I noticed just the slightest hum at max vol, (had to put my ear right next to speaker) and no hum with the iPhone connected. If anything I get a slight crackle when I turn volume knob (it's just dangling via wires for now) but that is all. Doesn't seem like my transformers being mounted below the board has any negligible effect.

Certainly, I am no audiophile. And after testing the amp through a few music tracks, I then proceeded to test the same tracks on all my other stereos equipment, old and new. I'm not sure I noticed much of a difference. Most of my equipment is decent and If anything, the sound quality seem to be more affected by the speakers then the amp.

Still, it's a pretty cool thing to see a piece of history operate with such great results. Now I've got to start hitting the DIY speaker forum!