capacitor polarity in a phono stage project

[(*steve*)]

Did anything get hot? Smoke?

If this was your pre-amp than it shouldn't have loaded the power supply at all.

Did you measure the current the pre-amp was drawing, there is a small chance your power supply is at fault.

 

[giltas]

Did anything get hot? Smoke?

If this was your pre-amp than it shouldn't have loaded the power supply at all.

Did you measure the current the pre-amp was drawing, there is a small chance your power supply is at fault.

Nothing got warm or smoked.

I haven't checked the current. how do I do that?
I'm really a newbie, please forgive me.

Now that I think of it, there also might be a problem with the PSU board. the voltage across the two output ends is around 36v but when I check the voltage across the 0v and the positive/negative ends they give asymmetrical values. (like 22v between the negative and the 0 and 14v between the positive and the 0. I'm not sure about the values -will check again tomorrow- but they are asymmetric)

 

[(*steve*)]

Check the values for the voltages on your power supply. You need to get that right first.

To measure current you plug the multimeter probes into the appropriate sockets (often there are different sockets for high current ranges), set your meter for it's highest current range (probably 10A) and then connect your meter in series with one power supply lead.

In series means that you, for example, connect the negative power lead of your preamp to the power supply negative as normal, but you connect the meters red lead to the power supply's positive output and the meter's black lead to the +ve power supply input for the preamp.

If your preamp has +. -. and - voltage inputs, first measure the +ve supply current, then the -ve. There's no real point in measuring the 0V current, but you can if you wish.

For a normal preamp, you would be expecting a current of under 50mA (0.05 on your 10A range).

 

[giltas]

Check the values for the voltages on your power supply. You need to get that right first.

To measure current you plug the multimeter probes into the appropriate sockets (often there are different sockets for high current ranges), set your meter for it's highest current range (probably 10A) and then connect your meter in series with one power supply lead.

In series means that you, for example, connect the negative power lead of your preamp to the power supply negative as normal, but you connect the meters red lead to the power supply's positive output and the meter's black lead to the +ve power supply input for the preamp.

If your preamp has +. -. and - voltage inputs, first measure the +ve supply current, then the -ve. There's no real point in measuring the 0V current, but you can if you wish.

For a normal preamp, you would be expecting a current of under 50mA (0.05 on your 10A range).

Steve,
When I check the voltage of the PSU output right after connecting the mains, it shows about pretty low voltage across the negative and the positive leads and goes up in a steady pace until it reaches about 41-42 volts (takes about 10-15 minutes). However when I connect the preamp the voltage qucickly goes down until it reaches about 1.05v and stays there.
If I let the PSU charge up and then connect only the the negative lead of the PSU to the preamp, the voltage across the + and - outputs on the preamp goes down to around 12.5v, without even connecting the positive lead. Once I check the current as you described, it shows 8mA but goes down to less than 1mA in a matter of seconds.

It's about the same when I check the negative lead.

There don't seem to be shorts on the preamp circuit but there are a few points I wasn't sure I connected right and maybe they are the problem.
1. I connected all the grounds that are shown in the schematic to each other and the ground from the PSU (and the 0v) to the ground of the preamp. Is that OK?
2. wasn't sure how to connect the signal connectors to the preamp. The schematic shows two leads where it says "In". I figured that the upper lead (that connects to the + of the IC) connects to the main pin of the audio connector, and the lower negative lead connects to the ground pin of the connector. However I don't think it matters because nothing is connected to the circuit anyway.

 

[(*steve*)]

When I check the voltage of the PSU output right after connecting the mains, it shows about pretty low voltage across the negative and the positive leads and goes up in a steady pace until it reaches about 41-42 volts (takes about 10-15 minutes).

OK, you have a problem. That should happen in milliseconds. And it sounds like the voltage is going too high.

However when I connect the preamp the voltage qucickly goes down until it reaches about 1.05v and stays there.

Not surprising. your power supply probably can't supply more than microamps.

Let's get the power supply working shall we?

instead of a preamp, find a resistor (something like 1k will do) and place these across the outputs (0 to +v, and 0 to -V).

First step is to measure the voltage across C6 and then across C8 (These should be 4700uF capacitors, the schematic is not easily readable).

Now measure the voltage across D5.

And now the voltages across C11 and C12

What are the output voltages?

For all of these measurements the black probe of your multimeter should be on your 0V output.

 

[giltas]

instead of a preamp, find a resistor (something like 1k will do) and place these across the outputs (0 to +v, and 0 to -V).

do you mean I should connect two resistors?
I don't have any 1k resistors left. I have some around 2k and some around 400-500R will that do?

 

[giltas]

Iwith the 2k2 resistors the voltages are these:
C6 - 27.9v
C8 - -28.05v
D5 - 15.09v
C11 - 13.07v
C12 - -25.37v

 

[(*steve*)]

do you mean I should connect two resistors?
I don't have any 1k resistors left. I have some around 2k and some around 400-500R will that do?

I mean connect one resistor between the V+ output and 0V, and the other between V- output and 0V.

I'd go the 2K, the others might get a bit warm.

Iwith the 2k2 resistors the voltages are these:
C6 - 27.9v
C8 - -28.05v

That's good.

D5 - 15.09v

Close to 15V -- OK

C11 - 13.07v

That's a little lower than I'd expect...

C12 - -25.37v

And I'd expect that to be around -15V

You have a fault in the power supply.

Can you confirm that these readings were taken with 2K resistors placed as I suggest and also indicate the voltage at the outputs (again, with respect to 0V)

 

[Harald Kapp]

I also believe that such tiny difference as different capacitor brands doesn't make an audible difference. It just seemed to me (from recommendations in posts) that this specific project will get me a decent preamp for my turntable and that was enough for me.

The type of capacitor also plays a role. Ceramic capacitors, specially small SMD variants, can have a piezoelectric effect (http://www.edn.com/design/component...LCCs--piezoelectric-effects-and-audible-noise-). This can lead to audible noise and distortions. Leaded components are probably less prone to that effect because the leads damp the mechanical stress on the capacitors. For high end audio film capacitors may be preferred over ceramic types.

 

[giltas]

I mean connect one resistor between the V+ output and 0V, and the other between V- output and 0V.

I'd go the 2K, the others might get a bit warm.



That's good.



Close to 15V -- OK



That's a little lower than I'd expect...



And I'd expect that to be around -15V

You have a fault in the power supply.

Can you confirm that these readings were taken with 2K resistors placed as I suggest and also indicate the voltage at the outputs (again, with respect to 0V)

the resistors are 2k3. I checked again and the measures are the same.
the output voltages when resistors are connected are 0.5v on the positive lead and -1.1v on negative.

 

[giltas]

I found a mistake in the soldering of U1B. I fixed it but it still gives me similar readings. The voltages went a bit down but nothing significant. perhaps one of the components was damaged? I have spare IC's and capacitors but what should I replace?

if I plug in my turntable to check it, can it damage anything?

 

[(*steve*)]

Is it possible that you have connected up the main pass transistors incorrectly? Find the fault BEFORE you start replacing things willy nilly.

The voltage on the base of Q1 is close to correct. Can you measure (while the 2k2 resistor is in place) the voltages from the 0V rail to the base, emitter, and collector of Q1. I'm assuming the output voltage will be 0.5V at this time.

The voltage on the collector should be 28V, the base 15V and the emitter 14.5V (all approximate). We know that the base is at 13V, and that's a worry, but even that should result in the emitter being at around 12.5V.

We will ignore the other side of the power supply for now because I believe it has larger problems. I think there's a good chance the wiring around U1b is incorrect.

 

[giltas]

these are the voltages of Q1:
Emitter 11.98v
Collector 12.06v
Base 12.58v
the output of the positive end is indeed around 0.5v

 

[(*steve*)]

OK, you have some significant problems.

The collector of Q1 should be connected to C6. You have previously given the voltage here as 27.9V. You need to find out why the collector of Q1 is not the same voltage.

Secondly, if the emitter has a potential of 11.98V, and you are measuring 0.5V across a 2k2 resistor on the output it either means you have some significant miswiring, or resistor R8 is not 47 ohms but 50k (maybe it's 47k?)

Check all of these things and get back to me.

 

[giltas]

oh! the wiring was in fact correct but apparently the wire that was connecting C6 and Q1 wasn't intact. I replaced the wire and measured the resistance of both 47R resistors and guess what- they are both 47k. I am such a rookie! unfortunately the transistor voltages are still wrong: they are all around 27v (Collector 27.9v, Base 27.3v, Emitter 26.7v)
I will replace the resistors soon

 

[(*steve*)]

OK, you need to go back and measure the voltage on D5 and on both ends of R6 (the resistor between the output of the op-amp and the base of Q1

 

[giltas]

ok I replaced the resistors to the right ones (47R) and the output voltagebecame much higher- 47v across the negative and positive leads. (21v across negative and 0 and 25v across positive and 0)

the voltage of D5 is 15.17v and the voltages of R6 are around 27v on both ends. I can't give an exact number as it keeps jumping around 6.9-7.

 

[(*steve*)]

ok I replaced the resistors to the right ones (47R) and the output voltagebecame much higher- 47v across the negative and positive leads. (21v across negative and 0 and 25v across positive and 0)

OK, that's still way too high. The output voltage should be around +/- 15V, so 30V across +ve and -ve

the voltage of D5 is 15.17v and the voltages of R6 are around 27v on both ends. I can't give an exact number as it keeps jumping around 6.9-7.

I need to know ground (0V) to one end, and ground (0V) to the other end.

One end should be 15.17V if you have wired things up correctly (I assume this is where *a* fault is.

It might be useful to show us some photos of your construction so we can get some idea of what we're dealing with.

 

[giltas]

I need to know ground (0V) to one end, and ground (0V) to the other end.

the voltage is the same on both ends of the resistor

 

[(*steve*)]

OK, I need to see the other side. The wiring is pretty much the most important part

Is this supposed to be a power supply for just the preamplifier, or is it supposed to power the main amplifier as well?

I've been concerned about some aspects of the design up to now, but seeing the piddly little transistors used for Q1 and Q2, it is clear that this power supply is not going to be able to deliver any significant current without destroying both of these parts.

I'm going to bet that you have wiring errors for both the op-amps(s) and the transistor(s).

I really don't want to go to the page describing that amplifier again, so can you list the actual part numbers of these transistors and the op-amps in the power supply (I need to know this independent of what is written on that page anyway)

Damn, I went and read that page again. HAHAHAHAHA. (sorry, I actually read his "typical advantages of tubes")

Using the transistors specified, this power supply will max out at 50mA. You would be far better off getting a 7815 and a 7915 (positive and negative 15V regulators) and use them to replace pretty much everything between the filter and output capacitors.

The important thing to remember will be, if we get this design working, it will be quite a fragile power supply with (dare I say it) abysmal regulation.