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Old 24th August 2004, 06:15 PM   #1
pamaz is offline pamaz  Italy
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Default help on P1.7 (Noisy)

Dear Guys I'm asing your help for a problem with my aleph p 1.7.
the crcuit is built on Kristijan PCB with double PS.
Everything seems to be correct (voltages) , it's sound is good enough, but I've a big issue with noise.
I'm not talking of a ground loop.
In fact the noise is there on my scope and i can see it very well on my scope between Out+ and Out - (leaving the power supply not earthed so to use the scope in differential mode).
If I look at a single ended out ( let's say between Out+ and ground, while grounding in-) Istill have a noise, much more lighter, but still hearable.
The noise has a strange repetitive and simmetrical shape but is triggered/syncronised with the 50Hz ( i'm in Italy and we have 220V 50Hz freq).

Have anybody experience with such kind of problem?

Thanks and bye.

Paolo
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Old 24th August 2004, 09:05 PM   #2
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pamaz,

Did you verify the power supply ; with only a resistor to load the circuit ?
(The alpeh unplugged from it)

Second, it could be a pb with your CCS's take a look at the ZTX 450/550
(constant current sources)

Regards.

Alain.
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Old 25th August 2004, 06:30 AM   #3
pamaz is offline pamaz  Italy
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Default I'll do it today

Thanks Alain.
I'll try to give a look at both PS and CS.
One more thing is that I've settled the gain via some fixed resistor ( no POT) valued around 150 0hm.
Could this be critical in some ways?

Thanks and Bye.

Paolo
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Old 25th August 2004, 06:40 AM   #4
Klaus is offline Klaus  Germany
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Hi Paolo,

My first idea was ground loop - the input jacks, the output jacks, and the supply endangered by this. Do you have no connection from any jack to the housing and one central star point connection from PSU ground to housing, as well as channel divided signal ground? With this my P1.7 has no problem at all. Of cause the input switch have to short inv input to ground if unsymmetrical cinch jacks are connected. Another source could by the transformer. If the PSU or amy is just some cm away easily hum is induced.

Maybe check with your scope step by step the supply voltage, the CCS-current in the 22.1 and 33.1 ohms Rs. If the current is clean, the hum must come from the input.

Regards to sunny warm Italy

Klaus
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Old 25th August 2004, 01:50 PM   #5
pamaz is offline pamaz  Italy
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Default still humming

Klaus,
I've done all the things you were suggesting me, and just to avoid other problems/ ground loops, I've now the board and the 2 psu on my table, out of the box.
I'm checking just one channel and one psu to avoid further loops.
The current in the upper ccs is around 27 mA, and in the lower ccs is slightly less than 20 mA. The V at output cap is roughly 23 V, and obviously due to the partition made by the 221k resistors, 11.5 at the input of T2 ; therefore, the static working point seems to be correct.
Looking at the scope we can say that in single ended mode ( at output) I've an acceptable noise, but the situation is always the same if I look at the differential output without any reference to ground, i.e. putting the probe between OUT + and OUT- , leaving unearthed the PSU .
One thing to say is that if I check the Out+ vs Out- signal, let's say that the noise increases in the moment I connect to ground both the inputs of the preamp.
Please note once again that no gnd loops are possible, being the board alone and unconnected to anything , except the scope.
Any more ideas?

Ciao
Paolo


PS: no doubt that if one day I'll be able to fix it , I'll drink with some wine to all the guys that are helping me!
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Old 26th August 2004, 05:57 AM   #6
Klaus is offline Klaus  Germany
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Default Re: still humming

Hi again,

Well, difficulty… My last ideas about the situation.
- if I got you right you reduced the gain resistor from original mimimum value of 200Ohms even to 150ohms. This increases the gain further. Maybe the circuit doesn´t "like it", but this should not create any hum…
- is your PSU ok? Two times 1.000uF plus 3.3ohms in line on Kristijans board. On the main PSU output also no visible hum on the +62V? If so the CCS should be capable to bring any hum down to zero.
- any asymmetry between the branches of one channel.

Sorry, no more… I will enjoy another Cianti tonight and think it over!

Regards

Klaus
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Old 26th August 2004, 12:19 PM   #7
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Hi Paolo,


I have made a Aleph P1.7 clone by myself using shunt regulators instead of bipolar transistors in the current sources and current sinks. I have found that the noise performance of these current sources is very important for the overall noise performance of the preamp since the current sources are connected to the outputs just like the gain MOSFETS. So any noise generated within these current sources will show at the output. The shunt regulators I have used are not as good as single transistors so at maximum gain a slight noise (sounds like sssssssss) is hearable, but at normal listening levels you will not recognize that.
Now regarding your problem:
First I would need to know if your noise is like hum (50 or 100Hz) or more like a sssssst. Sorry I cannot describe it better.
Next comment is about your measuring method. As far as I have understood you are attaching the scope Gnd wire to the -out terminal and the scope input to the +out terminal of the preamp. I think that this will not work even though you did not earth the powersupply as there is allways some hum pickup either capacitively or inductively. So in order to measure correctly, you have to measure either -out against gnd or +out against gnd. Alternativerly you could use two channels if you have and subtract their signals in order to get the differential output signal from -out to +out.
Hope this will help you to proceed. I can confirm tha tthis preamp is an excellent choice!

regards, pozo1992
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Old 26th August 2004, 01:47 PM   #8
pamaz is offline pamaz  Italy
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Default I've done it!!!!

Pozo, Klaus and Alain.
Finally I'm stepping out of this problem!
It was a strange "ground loop". Not due to wires!
One thing that is fundamental and I was not pointing out in my previous posts, is that I've 2 PS and just one trafo with two secondaries at 60Vac.
The coupling between these two secondaries, being not them galvanically shielded each other, was causing a ground loop due to the capacitive coupling between the two windings.
So the cure is just using a single PS! ( as the original p1.7 does) and paralleling the windings
Another way to keep the two PS is to have the respective trafos completely separated (and far enough each other) so to have no capacitive coupling between them.
In any case, thanks a lot for the help that you've given me exploring this problem.
bye

paolo


PS Now the preamp is playing: still to burn in, but the first impressive thing I've noted is the rigourous control that the soun has. For sure better that the previous Aleph L and the valved Curcio Daniel I was using before.

Ciao
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Old 26th August 2004, 02:10 PM   #9
Klaus is offline Klaus  Germany
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Congratulation! Have fun - like we already had with our P1.7´s...

"rigourous control", that describes it perfect. I have never heared such a bass before... CCS´s seem to make THE difference.
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Old 26th August 2004, 02:21 PM   #10
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pamaz,

Glad you found the problem. Good listening...

Regards.

Alain.
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