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Old 18th June 2003, 04:27 AM   #1
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Unhappy Help/toruble-shooting needed for simple phono pre-amp

I've just constructed this pre-amp for a friend. When I plugged it in, all I got was a constant (earth??) hum from both channels:

http://www.reprise.com/host/circuits/riaa_preamp.asp

The only modification to the circuit I made was placing a 1000microF 35V electrolytic between the 12V rail and earth, to smooth the 12V DC from a wall-wart power supply.

I made it on perf/vero board and used only one track for all my earth connections, ie. everythging on the circuit diagram showing a "ground" symbol were all connected to the 0 V wire from the DC power pack/wall-wart.

Ive checked all the connections etc and am reasonably sure I've got the components connected as they're supposed to be according to the circuit daigram.

I'd very much appreciate any help/suggestions for whats going wrong - I'm very much an amateur on electronic matters.

Cheers,
Joseph
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Old 18th June 2003, 07:02 AM   #2
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This circuit shure looks like a horror movie - single rail opamps, cap-coupled cart... If you still want to make it work make sure that the output of the 5532 is biased at roughly half the supply voltage (same as + input) and so is the output of the TL072. As the circuit is very simple and there is nothing to possibly go wrong you have certainly a bad connection - check again.


good luck
peter
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Old 18th June 2003, 07:22 AM   #3
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Quote:
Originally posted by analog_sa
This circuit shure looks like a horror movie
Since it is a phono amp, was that a deliberate typo??
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Old 18th June 2003, 08:20 AM   #4
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Have to admit shurely not a big fan
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Old 18th June 2003, 09:03 AM   #5
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BM, one thing is obvious: You can't use those 91 + 100 k at the input without any decoupling unless the power supply is VERY silent (don't think so). Noise (or hum) is fed directly into the amp. Check also if you really have stabilized voltage! It's a must.

My suggestion is to have a voltage divider 2+2 to 10+10k and a cap of 10-100 uF down to signal ground. From the junction a 47 k resistor to the + input of the opamp.
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Old 18th June 2003, 10:29 AM   #6
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I gotta agree with the rest of those who've replied already - this circuit is distinctly lacking in merit. There is a laundry list of faults here; some, but by no means all would be:

The 5532's are so vastly superior to the TL072's in every specification that counts here that it defies reason to not use them both as the input buffer and RIAA compensation amplifier.

Using a single supply when it is not necessary - i.e., you have only a 9V battery to work with and no room for rail-splitting techniques - really lacks reason in audio applications. By running this circuit off a dual-polarity supply one could dispense with the input and output blocking capacitors and that questionable biasing network composed of the 91k and 100k resistors (these should be equal, btw...).

Also, I don't think the R and C values specified in the TL072's feedback loop will give a proper RIAA compensation curve.

Already we have tossed out pretty much every detail of the circuit as it stands, which ought to lead you to wonder whether this circuit should even be bothered with in the first place?!

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Old 19th June 2003, 12:27 AM   #7
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Thanks for your help everyone.

Haven't had enough time to recheck thoroughly, but I quickly checked the voltages at pins 3, 5 and 7 on the 5532 and they were at about 90% of the supply voltage. I suspect I've made an error translating the circuit from paper onto veroboard.

If the biasing network of 100K + 91K resistors are questionable, what values should I substitue??

I'm also wondering if the way I've grounded everything is ok/workable (if not ideal), ie. everything on the circuit diagram showing a "ground" symbol were all connected to the 0 V wire from the DC power pack/wall-wart?

I realise the cicuit is going to be LoFi, but I chose it because I want to power it from a 12VDC wall-wart power supply (fiscal prudence decision) - I'm building the pre-amp for a friend so he can plug a turntable into his all flashy disco-light bedraggled Aiwa mini system that doesn't sound too good anyway (understatement). If anyone knows of a suitable (ie. cheap) alternative that I can still power from the 12VDC plug let me know....

Cheers,
Joseph

http://www.reprise.com/host/circuits/riaa_preamp.asp
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Old 19th June 2003, 01:22 AM   #8
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Quote:
Originally posted by bm_mode
...
I quickly checked the voltages at pins 3, 5 and 7 on the 5532 and they were at about 90% of the supply voltage. I suspect I've made an error translating the circuit from paper onto veroboard.

If the biasing network of 100K + 91K resistors are questionable, what values should I substitue??
....
You sure that 91k resistor isn't actually 9.1k??? I mean, the 5532 doesn't have stellar DC specs by any stretch of the imagination, but the offset shouldn't be that high!

If lo-fi is acceptable, then save the 5532 for something better and just use a single TL074 (the quad version of the '072).

Oh, and I still don't think that RIAA curve will come out right. Course, nothin' else is gonna come out of this amp right, so what's a screwed up breakpoint or two in the RIAA curve, eh?



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Old 21st June 2003, 06:32 AM   #9
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A couple more quick questions....

Found the problem and got the preamp working now......except there's still a lot of hum in the background. I'm going to put in a 12V regulator (could this stop a lot of the hum???).

Also, the output from this preamp is far too large - I'm thinking the best way to do this would be a dual-pot on the output. What value should I use and would a log or linear pot be best???

BTW - the problem with the preamp was that the shop assistant sold me an NE5532 dual op amp and then gave me an NE5534 single op amp. I didn't check the labelling close enough when I put it into the circuit.

Cheers,
Joseph
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Old 21st June 2003, 07:52 AM   #10
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Quote:
Originally posted by bm_mode


BTW - the problem with the preamp was that the shop assistant sold me an NE5532 dual op amp and then gave me an NE5534 single op amp. I didn't check the labelling close enough when I put it into the circuit.
From which we learn, or from which you who didn't already
know it learn, always read the component labelling
before you put it on the PCB.

And, it is also a good practice to recheck all components
again after soldering, but before you power up.
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