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Old 17th February 2004, 11:13 AM   #11
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Default NFB of input stage for phono stage.

The input stage of a phono pre amp has to have voltage series feedback ( non inverting input ). Shunt feedback could be used in the following stages if done carefully.
At the input the amp will see a minimum of 47K ohms source resistance in the shunt feedback case ( like your circuit).
In series feedback the 47K will be parallel with the dc coil resistance of a few hundred ohms and the coil inductance. This will result in less source resistance and result in a lower noise floor at the input.
Since the output of the cartridge is already low , the source resistance's noise voltage becomes significant. In your case I think the S/N ratio will worsen by possibly about 20db or more as compared to a non inverting input stage.
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Old 17th February 2004, 11:43 AM   #12
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Quote:
The input stage of a phono pre amp has to have voltage series feedback
WHY?

Quote:
In series feedback the 47K will be parallel with the dc coil resistance of a few hundred ohms and the coil inductance. This will result in less source resistance and result in a lower noise floor at the input.
Just like in the case of an inverting circuit. Don't forget the summing node is a virtual ground. There is indeed a noise penalty with a the chosen topology, but it can be done.

- put both inputs of the unused opamp to ref
- get rid of the last gain stage, nothing worse than 2 gain stages when it can be done with a single one (the second amplifies all the noise of the first and adds it own noise).
- use lower resistors for your last gain stage. A 5532 has no problem driving 600 Ohms.
- use a regulator to get your 15V if you don't have one already (or drop to 12V) and plenty of capacitors, something like 8x 4700uF split as 2 groups of four with a resistor of 100 Ohm in the positive will do.
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Old 17th February 2004, 12:33 PM   #13
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Havoc,

Just out of interest, what is the tool that you're using to test your stage's noise?

At the moment, I'm particularly interested in collecting such pieces of software to help me out. I'm considering buying a copy of Fluke View for my Fluke scope! Yay!

If anyone has tried the full version of Fluke View, I'd be interested in your opinions of it. I only have the demo at the moment, and no interface cable...
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Old 17th February 2004, 04:44 PM   #14
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Havoc.............
quote:
The input stage of a phono pre amp has to have voltage series feedback


WHY?.........................


With a NE55xx opamp this would be the only way to get least noise when loaded with a cartridge that needs a 47K load and to keep output impedance down..

While the inverting circuit input is a "virtual" ground , the signals do not pass to ground ! It's not a signal ground ..... its just zero signal volts for simplicity.
The 47k and the cartridge will come in series with the input ! Any text book will clarify that . Check it out.

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Old 17th February 2004, 06:31 PM   #15
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Eeka: I wrote it in the first post with the results. For compactness: load -> preamp -> RME ADI-8 AE -> RME Hammerfall -> Cool Edit Pro. Then I use the tools inside CEP. It is not a real measuring instrument, but it gives a good idea where you are. To be complete, I should have worked out the gains and reference levels of every stage to get a "real" measurement. This gives only the noise of the complete setup. But I know that the noise of the RME is at least 10dB lower than the cartridge/preamp, so can be discarded. As said, the gain could be 6dB higher without problems. This would just shift everything 6dB higher.

A better -more correct- result would be to connect my HP3400A, but then I would have to make a filter to exclude the MHz band and calculate the influence etc etc. Far to complicated. And it tells nothing about the frequency response of the noise. And this is far more important IMHO! I don't care about a couple of dB with a LP, you get 50dB at most once it is playing. But having 50Hz coupled in is to be avoided. It also shows nicely when the rectifiers of the psu are coming through (or the TV or the fridge).


I could put the RME in 96kHz sampling, but this would only raise the noise floor of the convertor, while the contribution of the preamp goes down. And the frequency response of a LP/cartridge is not to write home about once you are at 20kHz, so it doesn't matter after all. One of the basics in low noise design is not using more bandwidth than absolutly needed.

I don't trust scopes for this sort of measurement, certainly not digital ones. Their input sensitivity is not good enough, and they start from only 8bit if you are lucky. Not that I wouldn't like one around.....

Ashok: if you do the calculations, then you will see that the difference can be neglected. I don't know how to get a formula into this, but the only difference between the 2 is the placement of the 100 Ohm impedance of the cartridge.

In the non-inverting, it sits at the + input, and contributes to the current noise. But 100 Ohm is so small that even with the 0.7pA/sqrt(Hz) it can be neglected.

In the inverting case, it sits in series with the 47 kOhm. But that is so large that the difference is just as neglectable.

Agreed, this is just "back of envlope" stuff as you would need to take into acount the riaa correction, Z(f) of the cartridge etc etc.

Best result would be to use a NE5534A and just drop the compensation capacitor. He would gain a bit in noise figure and a lot in slew rate.
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Old 17th February 2004, 07:42 PM   #16
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I pretty much have to limit the gain of one of the stages to 10 so the RIAA step filter can be combined with it. That means that if I only used one other gain stage, its gain would be 40dB, with resistor values like 10k and 1M. Sure this would save the noise generated by an extra stage, but at the cost of extra noise picked up by the high resistance value. I think there could be more noise overall.

Havoc mentioned slew rates. This could only an issue if the gain per opamp stage is too great, and in fact I think a low slew-rate would be better because the opamp then inherently filters out RF and reduces the need for external capacitors.

I'm still undecided whether inverting or non-inverting is better in this case. It's not just the inherent noise of a given topology that matters but it also depends on the op-amp. Incidentally I have quite a few NE5532ANs but NE5534s so they're not an option.

CM
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Old 17th February 2004, 07:46 PM   #17
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BTW I don't see anyone else posting in the noise performance of their phono filters. Maybe I'm onto something?

CM
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Old 17th February 2004, 11:22 PM   #18
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The trouble is that there are lots of ways of measuring noise, and they all give different results. For instance, Havoc's method of measuring noise is completely non-standard but extremely revealing of problems on his set-up. The bandwidth of the measurement system affects the results because white noise is proportional to the square root of bandwidth. As an extreme example, applying "A" weighting allows super figures because it not only restricts the bandwidth, but removes hum and 1/f noise.

You really do want to change you input stage to non-inverting configuration. The NE5534 is a quiet op-amp in terms of voltage noise, but (like all bipolar op-amps) generates a lot of current noise, making it sensitive to the resistance to ground. That 47k resistor not only adds Johnson noise of its own, but converts the op-amp's current noise into a voltage noise source in series with the cartridge...
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Old 18th February 2004, 10:45 AM   #19
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Quote:
For instance, Havoc's method of measuring noise is completely non-standard but extremely revealing of problems on his set-up.
Agreed, like I said in last post. I only tought about the problem of translating it to other setups later. The issue is that noise is in most setups not the problem, but hum and grounding is the main problem. And this can best be seen by something that gives you the frequency response of the noise.

Quote:
its gain would be 40dB, with resistor values like 10k and 1M.
You can just as well use 100 Ohm and 10 kOhm. Or even 10 Ohm and 1 kOhm!

Quote:
I think a low slew-rate would be better because the opamp then inherently filters out RF and reduces the need for external capacitors
While this may seem so, it means that you will introduce TIM distortion. Do not rely on the inherent bandwidth to limit the final bandwidth. Use external roll-off to define the BW as you need it.

I still think you need to check how your noise floor looks in terms of frequency content. You may have other problems besides the inverting input.
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Old 19th February 2004, 09:53 AM   #20
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Quote:
Originally posted by Havoc
...You can just as well use 100 Ohm and 10 kOhm. Or even 10 Ohm and 1 kOhm!...
Good idea! Now, where did I put those 1500uF caps?

Quote:
...it means that you will introduce TIM distortion.
Didn't think of that. Wouldn't be surprised if some of the audible noise will probably be a result of IM distortion of ultrasonic noise.
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...Do not rely on the inherent bandwidth to limit the final bandwidth. Use external roll-off to define the BW as you need it...
Op-amps tend to have a much wider bandwidth at a lower gain anyway. At a gain of 10 the NE5532 has a bandwidth of around 2MHz anyway, as opposed to 200kHz at a gain of 100.

Does anyone have a nice and easy way of accurately calculating the theoretical noise performance of an inverting vs non-inverting phono input stage (as opposed to rules of thumb where "this is better than that")? Here is some useful data:
50V/mV open-loop gain
300k ohm input resistance
5nV/sqrt Hz noise voltage (@1kHz)
0.7pA/sqrt Hz noise current (@1kHz)

In any case, I still need to put in a regulated voltage reference as opposed to that RC network. That might make the biggest difference of all.

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