Internal placing of components

[mosfetmaster]

Hello everyone,

I'm starting making my first tube amp; I have no experience with tubes, so I need some advices about internal placing of the components in order to get a noise-free result.
Here is an image to better understand what I'm talking about:
Imageshack - img008611.jpg

- There are inputs (on the right top of the image) so near the power transformer; moreover, in that corner would "lodge" one of the two output transformers (the second one would be on the opposite corner.

- on the forepart would lodge the two tube amps boards (left and right channel), on the sides, and the unique stereo pre-amp in the middle: the amps inputs would be on the front, so very far from transformers, while the pre-amps inputs would be exactly close the power transformer; alternately, if I "180° rotate" the board I would get the inputs on the front but the pre-outputs close the transformer...

- The power supply board would be under the pre-amp board.

I need your opinions and suggestions: is it possible to get a good, noise-free work in so little space, or should I think about placing power supply unit apart?

Thanks very much to everyone.

[DF96]

The inputs and outputs (including OPT) must not be close together. The outputs can be near the power supply, but you have to be careful to avoid inductive coupling. Keep current loops as small as possible. The ideal layout is inputs-driver-outputs-power supply. The OPT and power transformer can couple together to give hum, but this is less likely with toroids. You don't want the power supply anywhere near the inputs.

[Wavebourn]

It is not enough for the amp to be quiet; tubes and other components generate heat that have to be properly dissipated. Resistors, diodes, tubes generate heat that increases speed of aging of themselves and other components, especially capacitors that use organic materials. That means, they have to be placed accordingly. For example, tubes can heat up capacitors, but they also can create air flow that cools down resistors and capacitors.
Also, it have to be mechanically stable.
You can use shielded cables to protect inputs from AC power and output fields. You can use shields on transformers. You can rotate transformers such a way they are less coupled. You can use (and you do) toroidal transformers that generate less of magnetic fields. However, the longer is the distance, the less is the field: fields created by even toroidal transformers cancel each other on the infinite distance that is infinitely large than size of the transformer.
There are many variables in equation, how to optimize design and layout. Even rotating a transformer properly you can shorten paths of high peak currents that go from transformer to rectifier and filter caps. It can save a lot of headache people have when as the result of suboptimal layout some rectifiers "sound" worse than other rectifiers!

I would suggest you for the beginning to place all components on the chassis, then think how to rearrange them optimally.
Also, some good and bad examples help: if you look at recently posted by AirPower photos you can learn a lot about layout design. His layout had been done very professionally.

[mosfetmaster]

Thanks for your replies.
Preamp and final stages would take place on PCB, so the passive components placing is not an issue.
Hum would be the problem.
I have so little space, and some limitations. I cannot dispose components in any other way. Maybe I could not rotate transformers to minimize couplings as there is no space to do it.
In that case, I could only place them outside, on the top panel ( i prefer to avoid this but...).
I only can keep input cables as far as possible from power transformer. I'm a bit skeptical about shielded cables or shielding transformer as I already tried this previously on another (solid state) amplifier with no success...

[DF96]

By "not an issue" do you mean you can design the PCB layout around your other placement decisions, or that it is out of your hands because you already have the PCBs?

This physical design issue is just as important as the electrical design, but even commercial equipment can get it wrong. I once bought a metal box to build a PSU. I spent a long long time trying different positions (a dry run, not for real) and finally bought a bigger box. When I came to build it, I only had just enough room in the bigger box.

[ChrisA]

Originally Posted by mosfetmaster

Thanks for your replies.
Preamp and final stages would take place on PCB, so the passive components placing is not an issue.
Hum would be the problem.
I have so little space, and some limitations. I cannot dispose components in any other way. Maybe I could not rotate transformers to minimize couplings as there is no space to do it.
In that case, I could only place them outside, on the top panel ( i prefer to avoid this but...).
I only can keep input cables as far as possible from power transformer. I'm a bit skeptical about shielded cables or shielding transformer as I already tried this previously on another (solid state) amplifier with no success...

Shielding works. Perhaps you did it wrong and the shields were not properly grounded to the single point signal ground. or you shielded the wrong wires

If this is your first project why not make it easier. Get a bigger chassis and spread things out more. Why are you using a printed circuit board on a new design. First get it to work and tweak the design using conventional layout, then once you have a good amp and you like the sound design a PCB if you like. If you are building this using a PCB you bought like a kit then follow the designer's instructions to the letter

The advice you got was good, Input, driver, power tubes then power suply in that order. But I'd add one more rule: If the power supply is a CRCRC type multisection filter do NOT bunch up all the filter caps. Far better to distribute them so each cap is physically near the load it server. So the first plate load resister in the input would be almost directly connected to the last filter cap. At the other end of the amp the filter cap that feeds B+ to the output transformer should be close o the output transformer.

Always bigger is better, don't try and cram a big amp into a small box. If you need physically tiny amp, build a chip amp. Space helps cooling and many coupling, noise and feedback problems are solved simply by the inverse square law, if you give the law room to work.

[M Gregg]

Hi,

You can screen input cables, however if you have input capacitors they will pick up hum if not screened. Sometimes a twisted pair can work well for inputs.

Remember your heater cables will produce hum so they have to be twisted and kept away from any input cables.

Star earthing works well.

Input loops can be a problem if you connect all inputs to earth at different points!

Rule of thumb is keep power away from any inputs and any circuit with gain. Star earth. Don't take individual earths from inputs to circuit. keep Ouput Tx away from inputs and gain stages. Heater circuits are a pain!

Best of luck!

Regards
M. Gregg

[Wavebourn]

Originally Posted by ChrisA

Shielding works. Perhaps you did it wrong and the shields were not properly grounded to the single point signal ground. or you shielded the wrong wires

Or the main source of hum and noise was ground loop.

[mosfetmaster]

Originally Posted by ChrisA

If this is your first project why not make it easier. Get a bigger chassis and spread things out more. Why are you using a printed circuit board on a new design. First get it to work and tweak the design using conventional layout, then once you have a good amp and you like the sound design a PCB if you like. If you are building this using a PCB you bought like a kit then follow the designer's instructions to the letter

Well, I taken different schematics from some different reviews; so I have a preamp circuit that comes apart from final stage circuit, that comes apart from power supply circuit, that comes apart from input selection circuit.
Each single circuit has been tested from the designer and should work at best. So my problem is only to make a correct coupling and avoid hum and ground loops.
Obviously I'm not obliged to mount components on PCBs but it seems to be the best way for a beginner; mounting components on chassis and wiring them manually appears to be a much more troublesome way to proceed.

Always bigger is better, don't try and cram a big amp into a small box. If you need physically tiny amp, build a chip amp. Space helps cooling and many coupling, noise and feedback problems are solved simply by the inverse square law, if you give the law room to work.

The case has the maximum dimensions the manufacturer was able to build with his equipment; internal, in millimeters:440 x 280 x 80h.
Tubes would jut out from the holed top panel.
Alternately I could lodge the output transformers or the power transformer outside as already said, or, even lodge them in a case apart, reserving this case only for strictly audio circuits...

[mosfetmaster]

Originally Posted by M Gregg

Hi,

You can screen input cables, however if you have input capacitors they will pick up hum if not screened. Sometimes a twisted pair can work well for inputs.

Remember your heater cables will produce hum so they have to be twisted and kept away from any input cables.

M. Gregg

I would supply cc heating voltage.

...Input cables even if shielded could "catch" hum from the solded ends (on the RCA connectors)...