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
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
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
good luck
peter
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.
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.
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?!
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?!
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
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
bm_mode said:
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?
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
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
bm_mode said:
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.
bm_mode said:
In this case, probably. This is because to use a single sided supply you have to directly connect the power supply to the input of the amplifier (the voltage divider). In fact, you throw away one of the really good qualities of op-amps by doing this: power supply rejection ratio.
Changing the gain of the input stage would be a much better proposition, but if you want to put a volume control in, put it between the output of the first stage and the input of the second stage (where the RIAA equalization is supposed to be done). A log ("audio taper") pot is theoretically necessary, but a better tracking, more accurate "log" pot can be made by putting a resistor in parallel with the wiper to ground. The pot should be 6.67x the value of the resistor, so a 10k pot gets a 1.5k resistor. The resistor serves a number of other useful functions besides modifying the law of the linear pot, but that may be more than you really care to hear about at this moment.
Hi Peranders
Thanks for your suggestion; I've added a (7812) 12V regulator into the circuit which has got rid of a lot of the hum - but there's still a bit too much there (even for the LoFi aspirations of this circuit).
I'm not entirely sure how to implement your suggestion: do you mean to use 2 equal resistors in the 2K - 10K range in place of the 100K and 91K resistors, and from the junctions of these two resistors have the 47K resistor in series between the 1uF input caps and pin 3 of the first opamp???
Also, should the 10-100uF cap be an electrolytic and should it be connected from the junction of the voltage divider to signal ground??
Cheers
Joseph
http://www.reprise.com/host/circuits/riaa_preamp.asp
Hi Peranders,
Just been sifting a bit more on the net and found this circuit, is this what you were suggesting???
http://www.maxim-ic.com/appnotes.cfm/appnote_number/1931/ln/en
Just been sifting a bit more on the net and found this circuit, is this what you were suggesting???
http://www.maxim-ic.com/appnotes.cfm/appnote_number/1931/ln/en
bm_mode said:
The Maxim circuit is A LOT better than the one you were suggesting.
To begin with it filters the resistor divider with 100uF, which should have two equal resistors. As you can see there's also a 47K load which should work with almost any MM cartridge or modified accordingly. That's very important.
Then also the main RIAA filter is on the first stage instead of the second.
It is not a coincidence that Erno Borbely used a similar topology (though discrete) on his RIAA preamp published in The Audio Amateur years ago.
Go for it and ditch the other circuit. If possible use 5532s on both stages, or put them on the first stage.
Use regulators, if possible 3X7 types.
Carlos
Well, I've just got the maxim phono pre-amp going (it's the same as one Rod Elliot has on his site). The only modification I made was to use TL 074s (in place of 5532s) and I included a 7812 12V regulator on the power supply. It's working ok apart from that old chestnut of a little too much hum, which I think is from the power supply.
So I'm after some more helpful advice on reducing this hum - I was thinking of either:
A) placing another 12V regulator in series with the other one (would this do much at all???), or
B) taking the already regulated 12V and reducing it to 9V with a 9V (7809) regulator.
Thoughts, facts, opinions and suggestions please.....
Cheers,
Joseph
Link to the phono pre-amp circuit:
http://www.maxim-ic.com/appnotes.cf...mber/1931/ln/en
So I'm after some more helpful advice on reducing this hum - I was thinking of either:
A) placing another 12V regulator in series with the other one (would this do much at all???), or
B) taking the already regulated 12V and reducing it to 9V with a 9V (7809) regulator.
Thoughts, facts, opinions and suggestions please.....
Cheers,
Joseph
Link to the phono pre-amp circuit:
http://www.maxim-ic.com/appnotes.cf...mber/1931/ln/en
Back to basics.
Loose contacts in the tonearm or turntable will cause hum.
It is usually necessary to connect the turntable frame to the amplifier frame.
Phono preamps need to be in closed steel cases; breadboards will suck hum from every light and wall-wart in the room. (Yes, sometimes I can get away with an open-case phono preamp, but other places it just won't be clean.)
Don't put the preamp power supply anywhere near the preamp. I have seen some with transformers inside the steel case that didn't hum too bad, but it takes suplemental shielding and layout experimentation to get that to happen.
Heck.... if it will run on 12V, it will probably run on a 9V battery. If it hums on a battery, you KNOW it isn't power supply hum.
Loose contacts in the tonearm or turntable will cause hum.
It is usually necessary to connect the turntable frame to the amplifier frame.
Phono preamps need to be in closed steel cases; breadboards will suck hum from every light and wall-wart in the room. (Yes, sometimes I can get away with an open-case phono preamp, but other places it just won't be clean.)
Don't put the preamp power supply anywhere near the preamp. I have seen some with transformers inside the steel case that didn't hum too bad, but it takes suplemental shielding and layout experimentation to get that to happen.
Heck.... if it will run on 12V, it will probably run on a 9V battery. If it hums on a battery, you KNOW it isn't power supply hum.
Thanks or your help PRR.
Yep, gave the basics a go and....... the hum's not due to loose contacts on the tonearm (no hum there when I plug in my other phono pre-amp). I'm reasonably sure it isn't picking up hum through the ether as when i moved the pre-amp around a bit and put it in a biscuit tin (tin box), the level of hum stayed the same, suggesting the hum's coming in through the power-supply.
I haven't tried a 9V battery yet - but I'll have a dig round at home to see if I can unearth one.
Cheers,
Joseph.
Yep, gave the basics a go and....... the hum's not due to loose contacts on the tonearm (no hum there when I plug in my other phono pre-amp). I'm reasonably sure it isn't picking up hum through the ether as when i moved the pre-amp around a bit and put it in a biscuit tin (tin box), the level of hum stayed the same, suggesting the hum's coming in through the power-supply.
I haven't tried a 9V battery yet - but I'll have a dig round at home to see if I can unearth one.
Cheers,
Joseph.
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