The "Freebird" ultra clean ultra simple preamp design

[Russ White]

kmj,

I would love to help, but I don't have enough information. I would have to know what kind of trafo you are using and how you have it connected, what your PS circuit looks like, etc....

Pictures would help a lot.

Cheers!
Russ

 

[kmj]

trafo is 2x12V and 2,25VA per secondary winding.
capacitors are 150uF-150uF-100nF before the regulator and 100nF-220uF after.

I tested the supply with a 4,7K and later a 1K load and it worked fine, then I connected it to the preamp and one of the regulators stopped working.
they are (+ - )12V and rated to 1,5A

 

[Mick_F]

Maybe, if you draw some current, the secondary voltage of the transformator drops too far, so that it becomes lower than the sum of output voltage and dropout voltage.


Mick

Edit: I just see that you have a 12V trafo. That should be fine....

 

[kmj]

Well, I had at bigger transformer earlier. 17,5V and 85mA per winding.

I went over to the local supermarket just now and bought a 9V battery. The Preamp works, thats for sure. I did however blow a pair of speakers earlier trying it with the powersupply. Cheap pieces of crap but the were useful when testing amps.

 

[kmj]

Hi again, I built a powersupply using the datasheet for the 7812-regulators and etched a new pair of Mono-yardbird PCBs. I have 47pF on the cap designated CRF and on the RG I have 0,85R since I didn't have the exact value.

My casing is made of MDF with aluminumfoil on the inside connected to ground. Internat wire is unshielded.

The problem is a buzz combined with a sparking sound that is more or less constant. What should I look after to solve this?

 

[zei]

Bump

Anyone have any thought on what can cause this preamp to buzz....?



/Z

 

[kmj]

An update and some changes. I added 470uF between each fuse and the first capacitor.
So on the positive rail it's fuse->470uF->0,33uF->reg->0,1uF
and on the negative it's Fuse->470uF->2,2uF->reg->1,0uF

The positive, negative and common rail ends with a dual pinhead which connects to the two boards.

So the powerstarground is at the PSU-board and the signalground is as specified on the board. The signalground of each channel is separated from the common with a 0,82ohm resistor.

When the CD is connected to the poweramp directly there's no buzzing sound so it should be the pre that is the source.

Attaching a schematic of the PSU (one psu to two channels)

 

[BrianDonegan]

Is your mains ground connected to the star ground?

 

[kmj]

No, Swedish apartmentbuildings doesn't have i mainsground in the wall-socket. It is stated in some law that when renovating a threepronged socket containing mainsearth must be installed but that is when and if thers a renovation. So no.

And I can't seem to think of a way to seperate the ground of the two channels.

Oh, one more thing. If I disconnect the ampboards from the PSU and drive one channel only with a 9V battery, when there's still buzzing.

 

[Russ White]

Oh, one more thing. If I disconnect the ampboards from the PSU and drive one channel only with a 9V battery, when there's still buzzing.

You could not power it from a single 9Volt.

You would need two, connect in series, with the mid point (where you connect the two) being your ground.

Cheers!
Russ

 

[kmj]

Ok, I just tested but Batterypower is not an option i'm sorry to say. It's for a friend not fond of the idéa.

 

[Russ White]

Ok, I just tested but Batterypower is not an option i'm sorry to say. It's for a friend not fond of the id�a.

And did it work?

If it did then the problem is with the PS.

Do you have a dual scondary trafo?

If so I would use two bridges.

 

[BrianDonegan]

That's fine. I was going to recommend disconnecting it if it was. A picture could be helpful.

 

[kmj]

Oh, you missunderstood me. I just tested with the one battery. I only have one but I could buy another tomorrow. And I have more bridges to add if it might help. And the trafo has dual secondaries

 

[Russ White]

Oh, you missunderstood me. I just tested with the one battery. I only have one but I could buy another tomorrow. And I have more bridges to add if it might help. And the trafo has dual secondaries

It would make better use of the trafo.

 

[Russ White]

The key thing will be to test that you have a good clean +12 to GND and -12V to GND from the respective outputs of the the PS.

I would test the preamp itself with two batteries first though, just to be sure there is not a problem there.

 

[kmj]

I measured the voltage to approx 11,9V positive and negative. The layout is from the datasheet exept for the added 470uF caps, shouldn't it be good enough?

Attatched a pic of present setup, one channel disconnected.

 

[Russ White]

I measured the voltage to approx 11,9V positive and negative. The layout is from the datasheet exept for the added 470uF caps, shouldn't it be good enough?

Attatched a pic of present setup, one channel disconnected.

Is it possible you have the opamps on the solder side?

If so they are upside down.

 

[kmj]

Is it possible you have the opamps on the solder side?

Not if you turn the paper upside-down when putting it in the UV-box
I managed to etch it on the wrong side and that's why the OPs are on the copperside, they are however correct. Forgot to mention it

I went for a run to the nearest gasstation and I'm now in possesion of two 9V batteries. Stay tuned!

 

[kmj]

damn, that basicly blew my eardrums. All I got was a high pitch screeching when connecting them in series and using the common point between them as ground.