3875 PCB: Suggestion please?

[xneakers]

Hi there,
I've been here quite long but passive as readers only. I see so many diyers here and want to ask some recommendation.

I have played a lot with gainclone (1875/3875). So far I didnt use and PCB and p2p wiring. On my last 3875 project, I used 4 trafo (around 2,5A each) around 18V AC. Quality is quite impressed for such simple design. I add 6dj8 tube buffer single stage and got better result.

Few days ago, I decided to make it better look by making the PCB. So, here's my first PCB in my life (my 2nd actually, the 1st was at my junior high school on electrical class). I need input/recommendation/critics or whatsoever to improve my PCB (or how to design a good PCB in general). I dont know a lot about good ground plane etc. So this is an hour job with Sprint Layout.

Few notes to add:
- I prepare a space for Choke. Yes I use and believe choke in power supply (though in my last 3875 I didnt use it). If you don't need it, just plug a jumper (or I will add extra route so I could use shorter jumper).
- Two filter caps use (C-L-C design). The first near the bridge, the the last near the chips. Two separated with Choke. I also add extra space for third capacitor (smaller, polypropelene or HQ non-polar is recommended). Don't know whether this will give some improvement, but if they dont, just remove them or add few jumper.
- I use 3 ground. The GND_S is for signal and negative input. The GND_C is for capacitor and speaker negative out. The last is GND_CT from transformer. All of them should be bolted together to your case. I recommend using big cables and bolt together to one of your screw case.
- This design use 4 diodes only for one channel. I will add for 8 diodes later. But that's not my main concern right now.

That's it. Any critics/suggestion/etc please post it here.

[Nordic]

I would move the two caps at the bottom, a little north, so you could change them with bigger ones if the need arises...

[xneakers]

The one after the bridge?

Suggestion accepted. Thanks.

Anyone else?

[Nordic]

Yes.
Where would yuor input caps go, good ones tend to be quite big...

[pinkmouse]

Thicker tracks. You have plenty of space, use it!

Oh, and chokes in the PSU are only a good thing for constant current supplies, such as preamps and Class A power amps. For AB they are not a good idea at all...

[Nordic]

I agree on th tracks, especialy the power and ground rail traces..

[xneakers]

@pinkmouse:

Thanks for the suggestion.

For chokes, I prepare the space so I could do A-B test easily with and without chokes...

[pinkmouse]

Originally posted by xneakers
For chokes, I prepare the space so I could do A-B test easily with and without chokes...

Fair enough, I won't argue with your attempt to check things for yourself. However, a simplified concept of what chokes do might help you understand why it is a bad idea.

A choke or inductor basically is a reactive device that attempts to keep the power flowing though it the same. This is a good thing for pre amps and class A stages that constantly draw the same current, as it smooths out any ripples or noise that the supply puts into the rails. However, the nature of a class B/AB load is that is contantly drawing the current it needs for the current load from the supply, and this changes with the music signal. The choke acts to try and keep this current constant, exactly the opposite of what is required.

[xneakers]

Originally posted by pinkmouse


Fair enough, I won't argue with your attempt to check things for yourself. However, a simplified concept of what chokes do might help you understand why it is a bad idea.

A choke or inductor basically is a reactive device that attempts to keep the power flowing though it the same. This is a good thing for pre amps and class A stages that constantly draw the same current, as it smooths out any ripples or noise that the supply puts into the rails. However, the nature of a class B/AB load is that is contantly drawing the current it needs for the current load from the supply, and this changes with the music signal. The choke acts to try and keep this current constant, exactly the opposite of what is required.

I prefer a regulator for Class A and no regulator for Class AB (so regulator somehow could kill the dynamics in class AB), but I dont know if chokes could affect the AB too. I dont play a lot with chokes, until I play with tube later this years and use big chokes (from 2 H to over 100H).

I will try on this and will discuss it later. Thanks for your input.

The bad things is imo, it's very difficult to get a good choke. Without a good choke, you better dont use it imo. Good choke will cost you a lot... so I think that's why people prefer R than L (though R has problem with it's bigger resistance and voltage drop).

Btw, about the tracks... do you think 4 mm is sufficient? You ask me to wider the tracks... but how wide? Sorry dont have enough experience designing a PCB

Thanks.

[pinkmouse]

4mm should be fine, but if you can make them wider, go for it.