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Old 14th March 2004, 05:03 PM   #1
JTT is offline JTT  United Kingdom
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Default Phono PreAmp

I have recently had need of a free-standing Mag PU preamp, and resurrected a design I have often used in the past. It is based on the NE5532 dual low noise IC, includes a 25Hz 12db/oct HP filter, and may be powered by a couple of PP3s. It performs extremely well, despite the design being rather long in the tooth, and does not cost an arm and a leg to produce.

I thought it might be of interest to some of yous, and I have zipped up a set of files (324Kb) including the schematic and PCB layout. I'll be happy to send it on receipt of an email request. I can also provide a (higher) res image of the PCB track layout, or Gerber files (if you don't know what those are, you don't need them!) on request.

JohnT
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File Type: jpg phono.jpg (13.9 KB, 1831 views)
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Old 15th March 2004, 01:10 AM   #2
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Default The circuit.

Hi John,
There are many phono preamp circuits right here on this forum.
The NE5532 is a pretty good opamp despite its age.
However in a phono preamp that I built recently , I found the Ad826 or even the OPA2134 sounded better at LF and in percussion sounds-than a NE5532. Have you made any such comparisons?
It would be interesting to see what the circuit is like.

You could post it on the forum or maybe you could email it to me.
amadhavan(at)yahoo.com
Thanks.
Cheers,
ashok.
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Old 15th March 2004, 01:20 AM   #3
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the best to do by phono preamps is to use low voltage opamps and power the preamp from 2 9volt batteries. Thats very quiet...
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Old 15th March 2004, 12:27 PM   #4
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Default Phono PreAmp

I know there are several other pin compatible ICs which are considered more desirable than the 5532, and in my earlier days I tried (literally) dozens of different preamp designs - discrete, balanced, FET, passively equalised, single IC, multi IC ............ I was always paranoic about noise level, which kept me away from MC cartridges for some years, and although I've still got a couple lying quietly in their boxes, I'm happy to use a MM for what little exercise I give my vinyls. And I'm equally happy with the 5532 - it does exactly what it says on the box!

JohnT
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Old 20th March 2004, 05:07 AM   #5
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Default Thanks.

Hi John,
Thanks for the zipped file. I simulated it and found that the rumble filter cuts off too early for my liking. I would say that the RIAA error with filter should not be more than -3db at 20 Hz. That way the bass will be intact and the LF from the turntable will be minimised. Additionally the HF error shows a rising trend after 20KHz . That might make surface noise more irritable.

I will try to work on this and send you a modified circuit. Since you have already got a circuit board , I will stick to change of component values. I did some quick mods and there was an improvement . I will also build the circuit and test it with various OP amps. Will let you know results soon.
I like the idea of running the whole thing off a battery. Great circuit for beginners. Maybe you should sell the bare pcb's on the net.

Another thing I was concerned about. I never like to apply ac signals or pass ac signals through a non biased Elec cap. In your case it might be better to run the whole amp from 18 volts (9+9) with ground on the -ve supply. That way all Elec Caps will get polarised at 9 volts or so. In this circuit input and output is dc decoupled and so there should be no problem. Check it out.
Cheers.
Ashok.
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Old 20th March 2004, 03:03 PM   #6
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Default RIAA Eq

Hi Ashok

I can't get too worked up about the LF turnover point. If you're using a pair of loudspeakers in the 20K bracket ( or $ - I doubt the exact amount matters that much at that level) then what happens around 20Hz may be of interest. But for those of us who have to manage with more frugal offerings, which can be fitted into the space allowed between the wife's set of vases and, maybe, the kid's Scalectrix, then 20Hz only exists in dreams!

My loudspeakers are 6db down at 40Hz (20W75 in 1 cub ft, nearfield), and will be somewhat better than that when room gain is taken into account. But, 20Hz .... well .................

So far as the rising HF error is concerned, this is a well known drawback with series feedback equalisation. Parallel feedback, or passive equalisation topologies, can better approach the ideal, but series feedback designs give lowest noise floors. You pays yer money and yer takes yer choice! None the less, I do not recall trying to minimise out-of-band error when I put this design together many years ago (using standard values for R and C), and no doubt the measured error could be improved. But ..... while my loudspeakers limit what I can get at the bottom end, my ears no longer respond to anything above about 9KHz at the top. So it may be that some wealthy youngsters will have to be the beneficiaries of any improvements you come up with. But don't let that stop you trying!

JohnT
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Old 20th March 2004, 03:22 PM   #7
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Default RIAA Eq

Hi again Ashok

Forgot to address the Electrolytics question.

Yes, it's true that use of a single ended supply, and a shift of the DC reference point, would ensure absolute polarisation of the electrolytics. However, recent research suggests that polar electrolytics are at their most linear when polarising voltage is low, and I seem to remember that this is maintained with low levels of reverse poarisation. Better still, and best, is apparently to use non-polar electrolytics, which can approach the performance linearity of solid dialectrics, but with high values and low bulk. (See series of articles by Cyril Bateman in Electronics World over the past year or so).

JohnT
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Old 21st March 2004, 02:04 AM   #8
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Default To tweak or not to ?

Hi John,
You are right about the unity gain limit causing the error at HF. I had overlooked that till I tried to tweak some parts and looked at the actual stage gain.

That information about the electrolytic caps is interesting and reassuring. I used to get Wirless World regularly till a few years ago. I must search out the past issues .

About tweaking the parts, I am sure the improvements will probably not make any audible difference as you so rightly pointed out. Additionally deviating from standard component values is of no help as it will mean using more than one component value at one point.
However I have been working with some RIAA circuits recently and hence the "urge" to tweak spilled over to this circuit !
The RIAA error on this circuit is pretty low and as I said earlier the improvements will probably make no audible improvement.
I simulated the circuit with a cartridge modeled as a 500 ohm resistor and 0.5H inductor in series.
The response error ( deviation from RIAA curve) is attached.
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File Type: gif riaa-1 copy.gif (5.1 KB, 1367 views)
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Old 21st March 2004, 02:16 AM   #9
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Default Second graph.

The vertical scale is not in db's away from RIAA. The deviation away from 1Khz is what I looked at.
I changed the rumble filter values from 100n/150n to 150n/150n and the RIAA feedback cap 3.3nF to 3.4nF and got the following result.
All this just for tweaks !!

About the rumble filter. It does introduce phase shift .I do not find much low frequency flutter or intolerable rumble in my playback system without a rumble filter. So if you have a similar situation maybe you should try to defeat the rumble filter and see if it sounds OK in your system.
I am sure that many people do not have systems that can reproduce audible sounds down to 20 Hz . Mine surely doesn't.
It would be interesting to know how this preamp sounds without the filter in your system.

The error graph with the modified parts is attached.
Cheers.
Ashok.
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File Type: gif riaa-2 copy.gif (5.0 KB, 1302 views)
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Old 21st March 2004, 04:31 PM   #10
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Default RIAA Error

Hi Ashok

I hope we're not boring the rest of the community with this ping pong play.

I said earlier that I could not remember being concerned about the out-of-band response, and I now remember why. The design was put together in the days when simulators were the provence of large companies running IBM 360s (or even its predecessor) and with the oodles of money needed to develop the s/w. I would have put it together by trial and error, and when it looked halfway like it worked, I would have stopped!

The change from 3.3n to 3.4n leaves me totally unmoved .. !

However, the filter change to 150n/150n I would regard as a retrograde step, as it does away with an (unintended) advantage to my mind. On my last set of loudpeakers (17w75XL in16L - now mothballed as standbys) I used a resistor bypassed inductor in the xover LF section - 1mH/2R2 - to produce a gentle bit of lift at the LF extreme, bolstering the baffle step compensation introduced by the design of the main LF xover section. It operates pretty much over the range of LF lift introduced by the filter Q highlighted in your error graph. Neat, eh .. !

But, joking aside, it is well known that most LPs were cut with increasingly steep filtering below 30, or even 40Hz, so the error-induced lift from the 100n/150n combo might not be such a bad thing. I think that integrating an LF filter into the RIAA preamp was probably something I just wanted to try out, and since it worked well I was happy to leave it there. The actual turnover frequency is a really just a matter of personal choice. My Thorens TD160 does not produce much in the way of rumble, but some records do, either because of poor pressing onto poor quality vinyl, causing warping or rippling, or because of rumble induced by the original cutter platform.

I don't know why I have got caught up in this, since I am not a member of the Vinyl-is French-Baguette-but-CDs-are-worse-than-sliced-bread brigade. I have to say that the sound from many of my LPs sounds significantly compressed when compared to the best of my CDs (but I do also have to say that some of my CDs are grim!), so I'm staying put!

Over to you!

Ouch!

Regards

JohnT
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