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Old 20th March 2004, 02:02 PM   #1
Vortex is offline Vortex  Hungary
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Wink MC phono preamp

Hi there,


me and my father are building a passive preamp of the best available quality for a DIY-er, it means, silver cables, very high quality RCA plugs and volume control, input switch selector both from DACT.

Now, we'd like to integrate in this passive preamp an active part, namely the phono preamp, to bring the phono's milivolts to line level.

The phono part will be for MC cartridges only, it means, very low level signal, you know how this is.


I asked my father, quite a professional electro technician, what he would recommend for the phono, to get very high quality sound with no audiophile-like magic and so on.. so just staying on the surface.

He told me, MC signal level is so small, that he wouldn't recommend a tube preamp for it because of the - in this case - relatively high dissipation noise itself - not to mention the strong microphonics at this level.


So he told me, he would build it using FET circuitry. Nearly the same characteristics as triodes but much lower noise, he argued.

The another thing was quite interesting: he told me, since this circuitry would be as small as a box of matches, the best idea would be, to build it into an existing LP, next to the point where the small level input signal comes (from the pickup). Argue was that this way, this low level signal doesn't need to run through longer cable lengths and as such collecting noise. Instead, it would at the end of the tonearm (at the tonearm-base of course) and this way the modified LP would also give the normal line-level signal just like CD players, tape decks, etc.. which signals are much more suitable for transferring them via RCA interconnects into the monoblocks, than the very low level phono (MC) signals.
And of course the 9-12V DC could also be get from the LP's drive system - no need of any separate transformer, which would of course occur in building the whole phono part outside of the LP.


And now, my questions:

1) What do you think about FET amplifying in this case ? (not overall).

2) How could the sound quality be that way. So, FET circuitry built inside the LP, not outside, or not in the "preamp". (It means, no need of extremely-low-voltage interconnect cables).

3) Can a quite audiophile level of amplification be reached this way, let's say, with an Audio Note TT1 LP and a reasonable good MC pickup ?

4) Any disadvantages, you would might consider? The simplicity of such a circuitry would also mean a good point, wouldn't it?


Thank you all!


PS: I see another phono preamp topic here, but that one was already decided to be tube-style. This isn't yet. I'm not for tubes. I'm for ultimate sound. Doesn't matter, how.
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Old 20th March 2004, 02:19 PM   #2
Vortex is offline Vortex  Hungary
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One more thing:

Check the bottom

I'd stick to that steel pipe. Alone.
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Old 20th March 2004, 03:47 PM   #3
HDTVman is offline HDTVman  United States
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Your father's plan is a good one. The reason phono systems are not setup this way all of the time is for marketing reasons, not ultimate audio reproduction.

Putting the phono preamp in the TT it by far the best way to avoid noise picked up in the cabling. Once the noise is mixed with the desired signal your stuck with it.

I have been using FET phono preamps for years, in fact I build FET cascode phono preamps with passive RIAA eq. I think they work very well and contribute no "sound of there own". The type of component used has less to do with the sound than the design of the circuit in my view. If you wanted to use a low noise opamp design I am sure very good results could be gotten that way as well.

For MM carts a tube preamp can be good also but when it comes to MC carts, the noise can be a problem and microphonics will be almost for sure. You can go through lots of tube to find the ones that are less microphonic but they all are to some degree.

Well good luck and have fun.

Later BZ
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Old 20th March 2004, 03:52 PM   #4
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Quote:
I'm for ultimate sound
but no


Quote:
audiophile-like magic
and still using silver cables, expensive RCAs, etc?!

Where do you put the border between audiophile and 'just staying on the surface'?


About


Quote:
And of course the 9-12V DC could also be get from the LP's drive system - no need of any separate transformer

you can forget. No high quality MC stage will work with voltage 'stolen' from the TT.

Otherwise your concept is good. FETs are a good way to deal with MC levels. There is the Pacifique design, often discussed on this forum, Erno Borbelly's FET phono pre to give you some ideas.
You can also use a pretty good IC, the INA103 and build the input stage around it. Remarkably low noise but possibly not as good sounding as a discrete phono.
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Old 20th March 2004, 04:14 PM   #5
sreten is offline sreten  United Kingdom
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http://www.passdiy.com/projects/pearlono1.htm

Looks ideal, won't fit it your turntable though .

The low source impedance of a MC makes it very immune
to added noise etc, even with the uV levels involved.

sreten.
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Old 20th March 2004, 04:20 PM   #6
Vortex is offline Vortex  Hungary
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High quality RCA is just because the whole passive preamp (I think, no more preamp, just a volume regulator - input selector box) .. well, this will be made in snow-white piano finish. Just like the two separate 10W Class-A monoblocks.

So optics does matter. That's the point I mean "high-quality RCA plugs".

"Silver wiring" -> "no silver wiring"


sorry for the "no", I'm just trying always to type with the least grammatic failures. My German is better, I think

So, no silver of course. I don't think it would improve sound quality much... I'm not a voodoo-fan.

Thanks for the kind answers.. we'll try the FETs.


About the LP ... you're right. I think I'll let the transformed low-voltage power cable just lead into the TT1 with some kind of a professional (and cheap!) connector, so the transformer will be outside, but the signal will stay inside. The circuit too, of course.

Nice.

I'll let my father make the circuit diagram soon, you'll see it here if interested. (Probably nothing special... but anyway..)

I hope I'll be satisfied with it.

If you were ...
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Old 20th March 2004, 04:28 PM   #7
Vortex is offline Vortex  Hungary
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sreten,


I think we'll somehow manage the shortest route of the uV-signal. Don't know how. Having a look at the pass-labs diagram, it's easy to split (just as every circuit) .. so the uV-signal WILL STAY inside, no compromises.

The rest can be placed outside.

But anyway, I think, even this Pearl could be fitted inside the TT1. Lots of space in there.

It will be quite funny looking at the LP and seeing, that it has 2 transformers and 2 power cords

(LOL)

I hope so at least.
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Old 20th March 2004, 04:42 PM   #8
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Ok, it's clear then. If you're not pursuing ultimate qualty i'd really recommend the INA103 followed by passive riaa and output opamp. It can be set to work with any MM or MC carts down to 100uV; it sounds pretty good (i think i used AD845 at the time) and is not too demanding towards the PS. Best of all it will easily fit in a cigarette pack.
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Old 20th March 2004, 04:47 PM   #9
Vortex is offline Vortex  Hungary
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what to consider just in theory, if I pursuing the ultimate for an MC ?



I don't think, I'll want to make several ones each week or month or year..

So the finished one must have to be able to do it's best for my ears for years long..

(Do I want too much at once?)

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Old 20th March 2004, 06:32 PM   #10
SY is offline SY  United States
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If you want to get ultraengineering-geeky, put the first gain stage right in the headshell, phantom-powered. This way, the voltages run down the wires are considerably higher and more noise-resistant.

I worked on a design like that with Murray Zeligman about 25 years ago, using 2N6550 FETs. We drilled two holes in the headshell to press-fit the FETs so the mass change wasn't too drastic. It was pretty cool, though truly overkill. I wonder if a tube implementation could work...
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