capacitor polarity in a phono stage project

[giltas]

About the transistors- I couldn't get the once specified in that page (you don't have to go there again) so I used other low noise transistors.
Maybe I should just replace them? The power supply is intended to be used only for the pre-amp.

I will go over the wiring a few more times to verify that it's correct (or not).

I guess this project was too much for me, maybe I should have bought a ready phono stage for the price of the parts :/

What do you think I should do to make the most of this project? i don't want all tge efforts and money I've put in it to go in vain.

Also I appreciate your help a lot!! Honestly I feel uncomfortable with all the time you've put into finding my stupid mistakes.

 

[(*steve*)]

About the transistors- I couldn't get the once specified in that page (you don't have to go there again) so I used other low noise transistors.
Maybe I should just replace them? The power supply is intended to be used only for the pre-amp.

It's massive overkill in some respects, and a series of missed opportunities in others. I don't think the designer really had a good grasp of power supply design.

I will go over the wiring a few more times to verify that it's correct (or not).

I suspect that you have the wrong transistors (e.g. PNP instead of NPN) or you have wired up the base, emitter, and collector the wrong way. But even that doesn't explain what's going on with the op-amp output.

I guess this project was too much for me, maybe I should have bought a ready phono stage for the price of the parts :/

Heh. But at least you're learning stuff.

What do you think I should do to make the most of this project? i don't want all tge efforts and money I've put in it to go in vain.

As I said before, I would replace most of the stuff between the capacitors with a three terminal regulator. Whilst they have a little noise, the massive capacitance on the output will pretty much eliminate it.

Also I appreciate your help a lot!! Honestly I feel uncomfortable with all the time you've put into finding my stupid mistakes.

Don't feel uncomfortable. I have selfish reasons... (If I can help you find errors maybe I can get better at finding my own )

 

[giltas]

Haha it's good that you have your own interest in helping me, I just hope you won't get tired of it too soon.
Do you think I would get better sound quality if I used those terminals? If so what are they called and how should I use them?
You said that if I do finish the project the outcome would be bad. Is that because my work is bad or because of the design? The guy who designed it claims that it's based on the black cube haha

About the transistors, I did take notice of the PNP/NPN but maybe I accidentaly switched them. Or maybe I just damaged them? I have some spare transistors I could use.

BTW what would happen if I connected the power supply to the pre amp now? Would it fry anything on the preamp circuit or inside the turntable?

 

[(*steve*)]

Do you think I would get better sound quality if I used those terminals?

Three terminal regulators (they look like a big transistor). Three terminals = three connections.

I don't think it would make any difference to the sound.

If so what are they called and how should I use them?

You need an LM7815 and an LM7915. I'm not sure (or I can't remember) where you are, so I can't advise you where to shop. A couple of dollars each.

You said that if I do finish the project the outcome would be bad. Is that because my work is bad or because of the design?

I don't think the design of the power supply is very good. I've not seen your work, but assuming it's not completely abysmal, it really won't change the outcome much.

The guy who designed it claims that it's based on the black cube haha

I don't even know what that is.

About the transistors, I did take notice of the PNP/NPN but maybe I accidentaly switched them. Or maybe I just damaged them? I have some spare transistors I could use.

We probably need to get a few other things figure out first. Photos of the other side of your board would be great. I may be able to spot some problems.

BTW what would happen if I connected the power supply to the pre amp now? Would it fry anything on the preamp circuit or inside the turntable?

Previously the 47k resistors in place of the 47 ohm resistors made the power supply pretty safe. That's not true now. I wouldn't connect anything to it until the output is under control.

 

[giltas]

Ok I will upload a photo of the back side soon but I don't think it will be easy to figure out what's going on.
It's also pretty damn ugly. I've never really studied electronics so no one (other than the little experience I have) taught me how to solder or arrange a circuit right.
I will check if I can get those regulators in a local store and if not I'll order them from ebay.

 

[Harald Kapp]

The "black cube" is a supposedly very good phono preamp.

 

[giltas]

where do the V- on the upper op285 and the V+ on the bottom one connect?
I did not connect them to anything because it wasn't shown on the schem. was I supposed to?
Now that I read a bit about transistors I think that maybe mine went into saturation because the voltage was to high for them, is it possible?

 

[giltas]

can anyone just tell me where to connect the V+ and V- on the OP285's?

 

[(*steve*)]

can anyone just tell me where to connect the V+ and V- on the OP285's?

Pin 4 is -ve, pin 8 is +ve.

See here.

 

[giltas]

Pin 4 is -ve, pin 8 is +ve.

See here.

I know this. I meant that V- on the upper one doesnt connect anywhere on the schematic. Should I leave it like that?

 

[(*steve*)]

I know this. I meant that V- on the upper one doesnt connect anywhere on the schematic. Should I leave it like that?

No, all the power pins must be connected. I assumed the diagram was suggesting a dual op-amp, but you're using single op-amps?

 

[giltas]

actually I used two double op-amps, and used only the pins that were needed for each side. should I resolder everything to one op-amp? or is it possible to fix it without completely resoldering them?

 

[(*steve*)]

There's no real problem using just one op-amp out of a dual package, but you must ensure that all the chips have power supply connections.

 

[giltas]

I connected the missing voltage pins and suddenly the voltage of the Q1 changed to 15.16 at the base and 14.53 at the emmiter.
Q2 changed to 15.39 at the base and 14.75 at the emmiter. output voltage with the 2k3 resistors is 28.72 (14.27 positive lead and -14.46 negative)

I've connected the preamp to the power supply. the output of the power supply is about 24v (arround 12v for each lead) when the preamp is connected. is that good or is it too low?

Ok so I connected the turntable to the preamp and the preamp to an amp but the sound was very weak and distorted. Both channels did work though. after a few minutes I noticed that some components on the power supply got pretty hot (almost too hot to touch)- these were especially Q2 and the op-amps. some of the resistors and capacitors around them warmed up too.

 

[(*steve*)]

OK, that sounds a lot better.

The task now is to determine if the pre-amp is drawing too much power, or if you are coming up against limitations of the power supply.

I need you to hook the PSU up to the amplifier and measure the voltage across the following resistors:

Side	Measuring	Resistor	Value	Voltage	Current
V+	Q1 IB	R6	100	?	?
V+	Q1 IC	R8	47	?	?
V-	Q2 IB	R7	100	?	?
V-	Q2 IC	R9	47	?	?

Set your meter to the voltage range then measure across each resistor listed (place this in the Voltage column. If you get a negative value, just put the absolute value in.

Then divide the voltage by the resistor value and multiply by 1000. So, if you get -1.7 volts across R6, put 1.7 in the voltage column, then calculate (1.7/100)*1000 = 17 and place that in the current column.

Looking at the design of the preamp again -- jeez, he loves 4700uF capacitors doesn't he? -- you'll need to monitor the voltage across R8 until it stabilises. The poor power supply has to charge up all those caps before the voltage can rise to a reasonable value.

It might also be a good time to read the voltage across all of your 4700uF capacitors just to make sure that the -ve lead is always -ve. Place the black probe on the -ve lead of the cap, the red lead on the other. If you read a negative voltage you have the cap around the wrong way. This can cause excessive current consumption.

Then turn everything off before it gets too hot.

The distorted output may be caused by one of several things:

1) Poor design
2) Wiring error
3) Component fault
4) incorrect input level
5) Incorrect input voltage

We can't really eliminate any of these yet.

Is the distortion the same on both channels? (Have you built 2 channels yet?)

 

[giltas]

Side	Measuring	Resistor	Value	Voltage	Current
V+	Q1 IB	R6	100	6mV	0.06
V+	Q1 IC	R8	47	1.6v	34
V-	Q2 IB	R7	100	11.4mV	0.114
V-	Q2 IC	R9	47	2.6v	55

the measurements I took are by placing the probes of the multimeter on each side of the resistor. Wasn't sure if I was supposed to do that or place on on the component described on the left of the table (transistor legs)
I noticed that Q2 gets hot almost instantly.

I have built the two channels. Since I wasn't sure how to connect them to the power supply as there weren't any directions, I connected them in parallel by connecting the V+ lead of each channel to one combined lead (and the same with the V- leads) and to those I connected the power supply leads.

 

[(*steve*)]

Did you also connect the 0V terminal?

And are both channels distorted?

 

[giltas]

ok I listened to it again and for some reason this time the sound was pretty clean (other than a minor noise from one of the channels but it might be from the record).
and yeah the 0V is connected

 

[(*steve*)]

It is a little unusual that the current being drawn from the negative supply is significantly larger than from the positive supply.

And yes, you took the measurements correctly

Can you now do the same thing with resistors R1, R3, R8, and R9 on the preamps (8 measurements). What we're looking for in general is that the current through R1 is very similar to R3, and similar on both preamps. The same pattern should apply to R8 and R9.

The SSM2017 should draw between 10 and 15mA, and the OP275 between 4 and 5 mA. That should result in a total supply current of between 28 and 40 mA. V+ seems right in the middle of that, but V- is too high. What I want to find out is if we can attribute that to one of the ICs.

Incidentally, how warm does Q1 get? And are the op amps in the power supply still getting hot? (And you may as well indicate how hot Q2 is. )

How hot is hot? -- see my finger calibration post here (Yes it is a bit tongue in cheek, but it gives me a good indication)

 

[giltas]

ok I will start with the hotness.
Q2 is the first to get warm. after about 30 seconds it becomes to hot to touch. If I put my finger on it when it's already warm I can keep touching it for maximum 1 second. so it would number 6 from your calibration. It seemed like it didn't get any hotter than that but kinda stabilized on number 6, but I didn't want the transistor to get burnt so I didn't leave it connected for more than 7-8 minutes. Q1 takes more time but after a couple minutes it reaches number 5- you can touch it for a few seconds but after a while it gets too hot. You wait a few more mintues and it reaches 6 as well. the IC's take longer to warm up, but after like 7 minutes they get around 4.


now the measurments-
R channel
R1 - 0.926v - 12.3mA
R3 - 0.933v - 12.44mA
R8 - 0.366 - 4.88mA
R9 - 1.232v - 16.42mA
L channel
R1 - 0.857v - 11.4mA
R3 - 0.876v - 11.68mA
R8 - 0.371v - 4.94mA
R9 - 1.237v - 16.49mA
as you can see there is a difference between the channels and R8 and R9 are not similar at all. I will check everything for miswiring until your reply

EDIT: seems like I found the miswire. Apparently I didn't understand the schematic correctly. I thought there was a connection between R9 R7 and R6. Resoldering right now.

EDIT: I fixed the mistake. the voltage of R8 and R9 changed to 0.409 and 0.410 respectively (0.404 and 0.405 on the left channel). Plugged it in and... it works! I almost gave up hope!
I am not much of an audiophile but sound is clear enough, there don't seem to be any noises or distortions. the transistors keep getting hot however, I wonder if that could damage them or if that's a normal temperature for transistors to work on.