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Old 19th January 2006, 04:08 PM   #1
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Default having issues with my 'El Cheapo' assembly

This one has got the best of me. I'm assembling an 'El Cheapo' phono pre, and I'm testing the first channel and getting some unusual results.

When I ground out the input, I get a fixed 0.140V output on pin 6 of the OPA637. However, after the output capacitors, I get a very weird DC offset and low frequency oscillation.

When I turn on the power, the output will spike to 3-5V, then drop to -3 or -2V, then start to climb again. This will change over the course of minutes, it's very slow, but I have no clue what's causing it since the output of the OPA is solid

Ideas?

I should note that I've eliminated the MC input, it's only MM and all my polystyrene caps came from parts connexion. What's weirding me out is that I thought all polystyrene caps were non-polar, but mine do have markings. You can see examples of them at:

http://www.partsconnexion.com/catalo...itorsFilm.html
(click on the polystyrene cap tab on the bottom of the page)

I'm hoping I didn't blow money on polars when I need non-polars....

Anyway, if anyone has ideas or needs further info from me please let me know.

Thanks,

Rob
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Old 19th January 2006, 05:38 PM   #2
Limhes is offline Limhes  Orkney Islands
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Looks like a typical transient response as a result of the power supply's 'step input'. Does it damp out completely or is the LF oscillation still there after these few minutes?

I think (but I could well be wrong) that you can eliminate this with a proper LPF after the output. Resistor + capacitor that would be; is this called a zobel or am I mixing up things?
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Old 19th January 2006, 06:37 PM   #3
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it is decreasing in amplitude. It's settled at -24mV offset after about 5-10 minutes.

Most preamps have DC blocking on the inputs right? Is this something I can ignore?

I'm not too worried about -0.24mV, but when it's swinging around +/- 5V at startup I'm not sure if that's an issue...
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Old 19th January 2006, 06:47 PM   #4
Limhes is offline Limhes  Orkney Islands
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It could do harm to your speakers ofcourse, having 5VDC before your main amp and most probable a lot more volts at the speaker outputs!

I just found out a zobel is supposed to do something entirely different, but some load might damp the oscillation out a lot faster, though. What is the load after the op amp? Have you connected a resistor to ground? What value is it? You might want to decrease it...
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Old 19th January 2006, 06:49 PM   #5
lineup is offline lineup  Sweden
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Quote:
However, after the output capacitors, I get a very weird DC offset and low frequency oscillation.
If you put a load 'after the output capacitors', say 10 kOhm resistor
from output to 0V,
Does it get any better?

There should be a 100 kOhm already from output to 0V.
See attachment.

This is the only I can think of, if you get strange values AFTER output caps.
In fact you should get strange values, if no load from output to 0V
besides a multimeter.
Because takes very long time for caps to get loaded,
using very very low current that goes into a multimeter.
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Old 19th January 2006, 06:58 PM   #6
Limhes is offline Limhes  Orkney Islands
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Aha, he is measuring on a floating capacitor? Then it makes sense indeed.

The capacitor in series with the scope (or whatsoever) impedance might give the slow oscillation (ringing?), then. Say, 1M + 1uF in series is about 0.2Hz or something...
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Old 19th January 2006, 08:33 PM   #7
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The 100k is in there from the cap to GND, just as shown.

When an additional 10k is put in parrallell with it, the initial values did improve, it now starts off at about -0.5V and slowly moves toward 0V, but it's not getting there any quicker.....

I should mention I swapped the chip from a 637 to a 627 before these last few tests. I have five of the later and only two of the 37s so I'm trying to keep them 'safe'. With the 627, the output on pin six is 0.403V to 0.397V, which is also curious since both -In and +In are at 0.000V.....
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Old 19th January 2006, 08:41 PM   #8
lineup is offline lineup  Sweden
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Quote:
Originally posted by robselina
The 100k is in there from the cap to GND, just as shown.

When an additional 10k is put in parrallell with it, the initial values did improve, it now starts off at about -0.5V and slowly moves toward 0V, but it's not getting there any quicker.....
I can only say, you should check your wiring.
Probably you have put some component wrong.

It is easy for example to confuse -V supply with 0V (GND)
and also to put an electrolytic cap +pin to negative.
There are two large caps 330uF in output.
You should check that you have done this part of circuit correctly.

I think when you find this bad thing, all will be alright.
Because probably rest of circuit is correct.

Quote:
I should mention I swapped the chip from a 637 to a 627 before these last few tests. I have five of the later and only two of the 37s so I'm trying to keep them 'safe'. With the 627, the output on pin six is 0.403V to 0.397V, which is also curious since both -In and +In are at 0.000V.....
The DC offset at output, I think is normal in this circuit.
This is why you need these output caps!

Even a very small offset at input, or input bias current
can result in such an offset at output, pin6.

The DC-gain is 1 + (681k + 56k2 + 2k21 )/475 = 1557 ........
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Old 19th January 2006, 09:22 PM   #9
lineup is offline lineup  Sweden
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I am beginning to think it is quite normal
this circuit is taking several minutes to make output at 0.00 V.

Because 100k resistor from between 330uF is connected to -12V.
The time before these BOTH caps has been loaded
is very long, using 100 kOhm.

When they have been loaded, the junction in between caps ( -pins )
is at -12V, while positive (+pins) are at 0V.
So both caps has to get 12 volt across them.
And the little current in 100k Ohm has to do it.

If the output has got near to 0.0X V after 5-15 minutes, it is okay.

The time constant of 330uF+330uF x 100k Ohm is
0.000660 x100.000 = 66 seconds

And to get caps near to full load (12V) will take even longer than this.

Now dont go make that 100 kOhm smaller, because this will increase the load of output.
It is meant to be this way.
And this time it takes to get 0 V at output is probably also normal.

I hope Kuei Yang Wang will come and help us and explain if there is something wrong.
He is the designer of El Cheapo!
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Old 19th January 2006, 09:32 PM   #10
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Sure enough after a few minutes it's hitting 0.0V dead on.

Also now that the caps have been charged, when I flick the power off for 30 minutes and then back on, I'm starting so much closer to the gnd reference it seems to be correcting within a few minutes.

I'll take it down to the lab and hook it up to the frequency generator and oscilloscope and see what it does tomorrow

thanks for the help,

Rob
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