Salas hotrodded blue DCB1 build

diyAudio Chief Moderator
Joined 2002
Paid Member
Hi,

I'm back to thinking I'll run my DCB1 with a Pot on the input side of the DBC1. I was thinking of the Khozmo 25K to keep the input impedance a little higher and friendlier on the way into the preamp I'm building.... I think the 25K is a little on the higher end of the recommended range with that recommended range being more 10K - 20K.

Is the 25K ok?

Its OK.
 
Thanks dimkasta. I know you are happy with the sound you get so this is nice to hear

Personal happiness is a weird deal... It has very much to do with personal experience.

The one thing that I learned playing with DCB1 is that you can be completely happy with it (even with higher standards) and then try a different component and get your mind completely blown by what you have been missing all this time.

So I can only say that 25k is not a problem. Ideally you should try different sizes and find out what better fits your system.

What my plan is for the attenuator (after a rather failed attempt), is to make quick tests with plain resistor couples on a comfortable attenuation level, lets say -30db

For example

  • 19K6R series and 634R shunt giving 20K impedance and -30,08dB
  • 14K5R series and 475R shunt giving 15K impedance and -30,09dB
  • 9K76R series and 316R shunt giving 10K impedance and -30,07dB
And see which impedance sounds better. Of course you can try in-between impedance values as well. The nice thing is that you should get a good idea of what fits your system better, even with cheap resistors, so that way trial costs are kept to a minimum as well.

I will then try different resistor brands and or models to see which one sounds better. Again, you are saving big bugs by buying only a couple of resistors from each kind.

And after that, I will build a full 24step ladder attenuator using the results from my tests...
 
hi,

few quick questions regarding two 35VA pcb 15v transformers I have paralleled. The adjustment R is 2 ohms making it 1.77VDC and 1.92VDC on the rails. Wattage meter shows about 48VA of constant consuming. So as advised by Andrew I'm still in a safe zone :)

1. Trafos have 2x110v primaries, is it safe to use them? Checked the mains during the week and it varies from 218 till 228v.

2. I get quite audible hum when I'm close to them (about 10cm). Could that be reduced somehow? Will mu-metal be helpful here?

Try separate them as much as you have space first. See if the mechanical hum softens with distance.

didn't help. Even tried to use one of them now, but still get same mechanical hum. Does it mean that I got a crappy trafo?! Could the voltage be the reason? It's 2x110v and I just measured 225-230VAC mains V.
 
didn't help

Are you sure that you have connected primarys right?

First make sure you know which ends belongs to the same windings.

Then you need to connect the windings in series and they need to be in phase, meaning the "start" on winding number 2 connects to the "end" of winding number 1. If you connect the second winding the other way then the the first they work against each other and the trafo hums and gets warm very quick.

Staffan
 
Are you sure that you have connected primarys right?

First make sure you know which ends belongs to the same windings.

Then you need to connect the windings in series and they need to be in phase, meaning the "start" on winding number 2 connects to the "end" of winding number 1. If you connect the second winding the other way then the the first they work against each other and the trafo hums and gets warm very quick.

Staffan

the windings are in phase, just as you said and is described here.
So I tried with only one trafo and got similar hum, then I put it out of the metal box and vuala, it has disappeared. Seems the problem is in my rather small case. However if I place the trafo a bit higher (2-2.5cm) the nasty hum vanishes. I think I'll give it few tries more with a closed lid and see if the issue still persists. In case yes, need to go for some decent toroids then. Bad thing is that I'm too restricted in space.

Just to mention that it's an old pic and I know that trafos shouldn't be placed so close to each other :)

Use a bulb tester !
Andrew, I need to buy a bulb first :D
 

Attachments

  • IMG_0651.JPG
    IMG_0651.JPG
    865.5 KB · Views: 394
Last edited:
I know that trafos shouldn't be placed so close to each other

And definately not that close to the signal circuit eother, needless to say maybe;)

I think you have 2 choices: either put the trafo in a separate box, outside this, or find a small enough trafo that fits your box and your needs, place it to the right as far away from the buffer circuit as possible. Preferably with some grounded iron wall between.

Btw all inductors, spools, trafos etc are in some way affected to metal, magnetic material close by and changes their behaviour. Those should be made for mounting on metal but now they are also interacting. Or just bad trafos for the conditions.

GL
Staffan
 
Last edited:
diyAudio Chief Moderator
Joined 2002
Paid Member
If by lifting them up some interaction with their core and the metal case stops to getting them humming plus there is no hum and buzz in the audio you are done for practical purposes. If its perfect or not it takes hi bit FFT to see. Many good pc soundcards will suffice.
 
thanks, Staffan, I see what I can do. I have a 30VA toroid that does not produce any hum at all, perhaps that would be a good start then.

Anyway, I'm a bit disappointed, I thought it would be a nice compact oven including decent step attenuator and good components, but ended up eventually most likely with a need changing chassis as I do not accept separate box for PS and dreamed about >1A current :eek:

Connectors and cables to the Mosfets and the gate stoppers still on board than on gate pins and this build still looks stable for rail voltage on a scope?

haven't attached the scope yet. The output voltage is stable and equal till 0.00 on both channels.
Resistors will be relocated as you already advised earlier few month ago to some other member here :)