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Old 5th May 2004, 06:37 PM   #1
rjm is offline rjm  Japan
Richard Murdey
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Default DIY phono preamp - cheap and simple!

Very Simple Phono Stage

Only 27 parts for the whole stereo unit.

User configurable gain 30-50dB.

Standard value components, even for the RIAA eq.

Allen Wright modded RIAA response. Accurate to +/- 0.25 dB.

Use any standard dual opamp.

Layout diagrams and photos included. Easy to build. (For those who previously visited, I have today updated the page to make it clearer and simpler to understand for the neophyte.)

-rjm
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Old 5th May 2004, 09:33 PM   #2
Ropie is offline Ropie  United Kingdom
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Excellent page, RJM, with great scematic drawings.

How about including a battery psu as an alternative to mains? From what I have read/been told recently, a battery psu will give the best performance for a sensitive device like a phono stage.
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Old 5th May 2004, 10:34 PM   #3
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Default Batteries and other PSU options.

Hi Ropie,

As noted, you could use batteries. I've never much cared for the results and that article at TNT does lend some credibility to that little prejudice of mine.

Be that as it may, as for a schematic since anything from 2 9-volt batteries on up through lead-acid cells up to caseloads of magnesium C-cells will work, I felt those who like the sound of battery PSUs are best left to implement their individual favorites.

I expect that while most people will be quite comfortable with the amp circuit, the regulation part might raise eyebrows. Pretty dinky hey? Sure, but it is "sufficient" and sounds surprisingly good - natural & musical. Please try it before experimenting with more conventional solutions.

All roads lead to Japan or some such nonsense. And I'm quite serious about those carbon resistors, too.

-rjm
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Old 5th May 2004, 11:23 PM   #4
matjans is offline matjans  Netherlands
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did you do any measurements? noise floor level etc. would be interesting to know.

And, most important, how does it sound ?

I have recently bought a project phono box and after replacing a number of components it sounds pretty good.
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Old 6th May 2004, 01:51 AM   #5
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Quote:
Originally posted by matjans
And, most important, how does it sound ?
I'm avoiding plunging into a description of the sonics for two reasons: the practical one is that my current system is a bit of a knock-up affair and the speakers especially not really of sufficient bandwidth to make a full evaluation. And then the more acadamic one that the designer isn't really the right person to make an objective review now is he?

For all their faults though the speakers are very coherant and clear in the midrange, with a true to life instrumental timbre even if dynamics are rather compressed.

OK? With that, and remembering that the Black Gates haven't yet been fully broken in, my "golden ears" tell me the following:

RIAA is spot on, though I swear I can hear the Allen Wright mod as a slight increase in clarity over the normal eq curve. Hum is absent, noise is low but I can't get a good feel on this. Distortion is also low. High freq oscillation - assorted opamp nastiness - is mercifully absent. That at least I can tell with certainty.

Top to bottom (remember though I don't get the true extremes with my speakers) it all seems remarkably natural. No part of the music is over-accentuated, there's no overhang or smearing. No grain, no flatness. For any LP I play - and with the VSPS hooked up I'm playing everything I can get my hands on - I run into the limitations of the speakers well before I can detect any problem with the phono stage. Note the limitations of my QED Discsaver are readily apparent under the same circumstances.

Take that as you will - I think its pretty nifty. What's musically important to me it nails authoritatively - yet its dirt cheap and a quick and easy build. Cool!

-rjm
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Old 6th May 2004, 11:44 AM   #6
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Default Re: DIY phono preamp - cheap and simple!

Konnichiwa,

Quote:
Originally posted by rjm
Very Simple Phono Stage
I have a few comments that may or MAY NOT interest you. They are meant constructive, in the interest of improving the design further....

1) For a MM sensitivity Phonostage to be used with a MM Pickup the Op-Amp's current noise dominates the noise unless the Op-Amp is very noisy. This due to the very high inductance of MM Cartidges, up to around 1H which causes material levels of impedance to appear on the input.

With FET input Op-Amp's input current noise is virtually nil, MOST BJT Op-Amp's have a rather high input current noise which will make the stage noisy with MM Pickups. For MC picups (even medium/high output types) a BJT input Op-Amp tends to be apropriate as the source impedance is low and the input noise current matters much less.

Also, MM Cartidges require a correct capacitive load (usually quite high) which forms a resonance circuit with pickups inductance to boost the treble. Usually the best course of action is to shoot for a capacitive load that places the peak at 18-20KHz and to adjust the load resistor untill a pink noise spectrum in the middle of the record shows no HF lift or droop. Then there will usually only be a small amount of HF loss in the inner groves.

2) This is a trick I learned from Allen Wright and have used many times by now. return the AC coupled Feedback loop components (R3/C1 & R4/C2) not directly to the Op-Amp output but to the output after the output coupling cap C3, follow this by the "build out" resistor R6. By insluding the output coupling into the feedback loop it's sonic influence is lowered somewhat and LF RIAA accuracy improves. Sounds notably better wherever I tried.

3) The RIAA capacitors are chose quite high in value. Remember that output stage must supply the current to drive the RIAA network and any external capacitances too. The load at high frequencies from the current values is equal to 1k in series with 3n3, this is quite a notable load which may make many an Op-Amp distort more than neccesary, especially in terms of the various non-harmonic distrotions.

I found scaling the impedances in the NFB Network up by a factor 10 (meaning all resistors * 10 and all capacitors /10), all else being equal sounded better with all the common Op-Amp's I tried. Also smaller value capacitors can often be had in better quality. For fun, try a air dielectric AM Receiver variable tuning capacitor adjusted to 330pF in the HF section of the EQ (they often have two identical sections with excellent tracking making one good for stereo). Very earopening....

The noise is usally dominated by the source and Op-Amp together, so the increase in noise from the higher feedback impedances tends to be minimal.

Combining 2 & 3 can mae a surprisingly large sonic change, which I at least consider a bona fide improvement.

Lastly, if you can find a 4-pin connector for the powersupply consider keeping the "grounds" of both supply lines seperate and only connect them together at the "power star", which would be the point where the output current loop and PSU decoupling capacitor current loop are "starred". I would recommend also seperate ground lines from there to the Input ground with R2 returned to the respective channels input "ground" point.

Anyway, just a few notes.

Sayonara
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Old 7th May 2004, 12:57 PM   #7
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Your suggestions here are all very reasonable ones. In the spirit of discussion Iíd like to respond to a couple of points.

Impedance of MM cartridges and op-amp noise.

If, indeed, the inductance of the cartridge is in the order of 1 Henry, then your suggestion that a FET input op-amp would likely have lower noise than a BJT input is at higher frequencies generally true and I urge anyone with such a cartridge to consider the OPA2134 etc. over the NE5532 in this circuit.

With a FET input op-amp it might also prove useful to scale the feedback components by 10. With larger impedance it is easier for the op-amp to drive, and with the high source impedance + low current noise op-amp there is no noise advantage to keeping it low anyway.

Since scaling the loop still keeps all the parts as standard values, its easily done.

Not all cartridges have such high impedances however. My daily-use Grado 8MZ has a resistance of 470 ohms and inductance of 45 millihenries. The more expensive models of that series have a resistance of 70 ohms and inductance of only 8 mH. Over most of the audio band the source impedance is below 1k, and best noise performance is obtained with bipolars (low voltage noise) in combination with low values for R2. This configuration will however favor op-amps with low distortion drive into 1k ohm loads - the NE5534 being one of these.

C3 in the FB loop.

Thatís also an easy mod and worth trying. The only curiosity I see with doing that is the feedback at DC falls to zero. I actually prefer the sound of those nonpolar 4.7uF Black Gates to a piece of wire (I find it adds a hint of warmth with no loss of transparency) so I'm personally quite comfortable with the conventional position. The low frequency response is 3dB at 4 Hz for 10k load, or -0.25dB at 20 Hz, so likewise the RIAA error it induces isnít something Iím going to lose sleep over.

Grounding.

Absolutely keeping the input grounds separate until the star point is the correct method. Call me unprincipled but considering the channel separation of the cartridge itself I just canít get excited over 4 cm of shared input ground.

Separate power grounds to the transformer windings are likewise the best implementation Iíve so far seen. I would have hooked up both my Gainclone and the VSPS that way but for my lazyness: itís a lot easier for me to come by 3 wire 16-18 gauge power cable than 4 wire.

-rjm
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Old 8th May 2004, 07:27 PM   #8
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Konnichiwa,

Quote:
Originally posted by rjm
Not all cartridges have such high impedances however. My daily-use Grado 8MZ has a resistance of 470 ohms and inductance of 45 millihenries.
Yes, Grado's use a different technology to Moving Magnet Cartridges (they are a "third kind" of cartridge outside the MM/MC division with Decca's being the "fourth kind"). The much more typhical Shure V-15 clocks in at 680mH or 0.68H....

Quote:
Originally posted by rjm
I actually prefer the sound of those nonpolar 4.7uF Black Gates to a piece of wire
Well, that is a personal choice.

Quote:
Originally posted by rjm
The low frequency response is 3dB at 4 Hz for 10k load, or -0.25dB at 20 Hz, so likewise the RIAA error it induces isnít something Iím going to lose sleep over.
True. Another Mod I had not mentioned but actually highly usefull and somewhat original would be to make the EQ curves adjustble to match also the DECCA & COLUMBIA Stereo EQ curves and possibly CCIR (Europe mainly eastern)....

Quote:
Originally posted by rjm
Call me unprincipled but considering the channel separation of the cartridge itself I just canít get excited over 4 cm of shared input ground.
I understand that, channel separation was never the consideration, rather current loops per se are.

Sayonara
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Old 9th May 2004, 02:25 PM   #9
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Default Schematic = Drawing / NOT!

In the RJM circuit schematic, R3, 4 & 5 share the negative IC lead through R2 to ground.
In the "beginners" drawing they are depicted as a closed loop on the output leg.
What gives?
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Old 9th May 2004, 07:24 PM   #10
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Checked it out and noticed the trace from the top of R5 to the blue trace connecting R2 to the IC isn't visible. It's "under" the resistor - there are only a couple of pixels peeking out! I'll fix that shortly.

Thinking about T's comments I'll branch the component list early on into a Version A (low impedance, bipolar-op-amp, for Grado-type carts) and a Version B (high impedance, FET op-amp, "typical" MM carts) with the difference being the 10x scaling of the feedback loop.

Since I have to fix the layout for the invisible trace I migth as well modify the grounding too.

-rjm
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