troubleshooting an Aleph P

I,ve run into a problem with a low level buzz from one quadrant out put, and nowhere else. It possibly could have been there from day one, as I just started using the balanced outs and it is on the neg out of one channel. It appears on the scope as an upper half of a sign wave about .5ms long and repeats every 8ms or so and is about 8mv. The only thing I used differently from the original design is BC550C and 560C for the ccs, but I have put caps across them to no avail. All my voltage readings are right on for the entire board. I have swapped supply boards, systematically replaced every component in that quadrant, reflowed every solder joint, and totally isolated the board from all input and output wiring, with no change. I'm using Kristians boards with dual regs.
Could the problem be with the higher hfe of the BCs or an inherent problem with the board layout.
The unit has always performed fine but the buzz is audible using balanced outs. I guess its actually 120hz hum.

Anybody out there seen anything like this before?

Questions or comments are definately welcomed.

Bill
 
Your $2 or 2cts is appreciated at this point, Fox. With the board totally isolated from everything but the power supply I tried grounding the board directly to the chassis, and also through a .1 uf cap with no change. I also lifted the thermistor off of ground, with no change.
I'm convinced it is in the circuitry, somehow amplifying the residual ripple that's down in the microvolt range. I don't have any previous experience with constant current sources so I don't know if this is what is happening for sure, but there aren't too many things left.
Possibly radiation from the diodes affecting only that part of the board, I guess that's possible!

Bill
 

rtirion

Member
2000-11-24 12:02 am
Did you try to test the powersupply separate from the amp board?
Specially check all power connection (solderjoints etc) between
the boards.

So with only the PS board, nothing else connected, check with
your scope (AC connected) if you find the same type of
repeating signal.

That would narrow things down a bit.
 
I didn't check the reg boards without a load, but I have a separate board for each channel. I swapped the boards channel for channel with no change. The reg board outputs are virtually identical and very clean. I haven't cascaded the scope channels as I didn't see the need to look at the residuals, but maybe I will just for kicks. I did check all the diodes on both boards, all OK.

Bill
 
rtirion said:
Did you try to test the powersupply separate from the amp board?
Specially check all power connection (solderjoints etc) between
the boards.

So with only the PS board, nothing else connected, check with
your scope (AC connected) if you find the same type of
repeating signal.

That would narrow things down a bit.

The Aleph P and BOSOZ PS-boards should never be tested without a load. If you do, they will go up in flames. Some folks at this forum can prove I am right :D

Fox
 
I don't understand what you mean for noise on one quadrant an nowhere else.
Do you run the board as single ended or balanced now?
Can you post some pics of the noise you get (in case you are referring to something that you've measured with the scope).
When you hook up the board to the amplifier-speaker, do you hear a little hiss on the tweeter as well?

As you seen i am currently having the same issue.
I have been playing around with the capacitors on the CE of BC550-450 and noticed that if i put for example 220nF instead of the 680pF suggested, i get nothing at the output and the circuit doesn't work!

I also tried to rise the gate stopper resistors without any result.

I also tried to change the type of transistors using different transistors instead of the BC550-450.
Since these last have an hfe of 350...whilst the ztxs have 110 or so....i used 2sa970 and 2sc2210 which have the same hfe as ztxs.
I had more noise at the output, so my guess or two cents are on the transistor.
As soon as i' ll get zetex parts...i' ll try them out and let you know..
 
Stefano,
I get no noise, just 120hz hum from one quadrant.

If you look at the schematic you will see each side is identical, and they are connected together at their sources. I have a single stereo board with both channels on it. My problem involves the output of one side of one channel. Being that all 4 sections are identical I call them quadrants. It is not input related and does not show up on the current sink, just the point between the gain device and the current source. If it is a ground loop, it would have to be a flaw in the board itself, embedded contamination of some sort.

You mentioned BC550 and 450, I hope you meant 550 and 560, these are low noise, I dont know what a 450 is.

Incidently, the Aleph P is a fantastic pre, do it right and you'll never regret it.

Regards, Bill
 
yes, that's what i meant.
So basically one channel of your board is running without problem while the other one doesn't.
well then i would exclude ground problems since you should have had it on the other ch as well...but you don't..so..!

Well i don't know...i get a weired thing a fuzzy noise at the output.
If i touch the mosfet's case i see oscillation at the output as they seem to act as antenna.

i was thinking that it could be because the board dos n't have a casing yet .
Would it be possible for you?
Did you have the same thing when the pre was out of the case?
 
well i think the layout is correct otherwise the circuit would failure and not be working at all.
The circuit works, it plays music without problems.

I started first with my layout where i have been working for awhile to minimize the signal paths and stuff.
Under Analog_sa' s suggestion i tried Veteran layout and i got the same exact thing.

The only thing is that by putting my ears close enough to the speakers i can hear a little 50Hz on the woofer that i can reduce lifting up the board from the floor...and hiss on the tweeter.
when i would touch the mosfet's case i would hear this noise a little louder.
By looking at the board with the scope i can see a fuzzy trace (about 2-3mV) and if i touch the mosfets i see something going on.
So i would exclude at this point the layout since i have tried to change it already.
It is strange because playing around with the capacitors across C-E junctions of the transistors npn and pnp i get very different results in term of that noise.
As i stated above, if i would put 220nf instead of the 680pf i would get a completely flat trace at the output (that is good) but i get no voltage drop across the coupling capacitors and nothing at the output when input signal is present.

As i said i' ll wait for the zetexs parts just for the sake of my curiosity.

Just one question:

When you had your board laying around without any casing: does it pick up any kind of noise, 50Hz or any disturb from the radio and stuff?
just to get an idea of what a casing can do for me.

I say so because i have read somebody else's experience in building a phono stage and until the top plate of the casing wasn't attached to the rest, he would still pick up noise at the output that would disappear only when the pre was completely cased.

Another doom question:

i'm sure this has nothing to do with the problem....but it's just a doubt i have....

Since for the evaluation and for the troubleshooting of the board i have been using tantalium capacitors (i am waiting for zen caps)....may it be possible that these capacitors are noisy and therefore can cause this noise to exist at the output?
I'm sure it is a stupid question....
 

analog_sa

Member
Paid Member
2002-08-14 1:47 pm
Cascais
Stefanoo said:

I say so because i have read somebody else's experience in building a phono stage and until the top plate of the casing wasn't attached to the rest, he would still pick up noise at the output that would disappear only when the pre was completely cased.

Another doom question:

i'm sure this has nothing to do with the problem....but it's just a doubt i have....

Since for the evaluation and for the troubleshooting of the board i have been using tantalium capacitors (i am waiting for zen caps)....may it be possible that these capacitors are noisy and therefore can cause this noise to exist at the output?
I'm sure it is a stupid question....


It is unreasonable to compare a phono and a line stage in hum sensitivity. After all the phono has several orders of magnitude higher gain, especially at low frequencies.

And the noisy capacitor theory is indeed a dud. Electrolytic caps may get noisy if reversely biased or overheated but under normal operating conditions they don't generate any significant noise.
 
well, thanks for the answer.

Do you have any idea of why if a higher capacitance on the CE junction, as i said, is used the circuit doesn't work?

that is strange and i am not able to give me an explanation.

As i said i'll wait for the zetex parts and zen caps to see if the problem will disappear.
I will also order a Plitron transformer to make sure that transformer is not inducing anything (the one i am currently using for evaluation purposes buzzes a little bit but i think that it does not have to do with it...but i might be wrong hopefully :) )
 
Stefano, noise and hum are two different things. The hum is stray fields that will be blocked by a case. The hiss is going to be your problem because the circuit is low gain and so simple that it should be dead quiet.
How much gain do you have?
I never had the urge to touch anything while it was operating, can be life threatening at times, especially with tube stuff.
Try putting .001 caps across the CB junctions.
Are all your DC voltages correct?
You might have a mistake with ground connections, make sure all the resistors to ground really are grounded, it could still function somewhat in this condition.
Tantalums are too failure prone to use except in very low voltage circuits, and still fail in them quite often.

In my case, I have new boards and ZTXs on the way, cant live without the Aleph P.

Bill