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Old 30th June 2005, 09:55 PM   #1
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Default Good Phono preamp circuit?

I need to build an RIAA phono stage to feed a Velleman Valve preamp, which in turn will feed a JLH class-A amp.

Does anyone have any favourites? - at the moment I'm leaning toward the John Linsley-Hood shunt-feedback phone stage published back in '92, but maybe I should be using a newer, Opamp based design?

All advice gratefully received!

Peter Moreton
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Old 30th June 2005, 10:17 PM   #2
Netlist is offline Netlist  Belgium
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My next project would be a Pass Aleph Ono.
http://www.passlabs.com/downloads.htm
A smaller version would be the Pearl Phono stage.
http://www.passdiy.com/preamps.htm

From what I read here, both are very nice sounding amps.

/Hugo
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Old 1st July 2005, 12:53 AM   #3
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Hi Peter,

I would consider using a single high quality OPAMP based design with full RIAA network CAREFULLY tailored around it. If you are after the best sound you would not want to be going through multiple OPAMP common mode etc.. limitations when all can be elegantly performed by just one. In fact the whole RIAA equalisation results in only +/- 6 dB NFB variation over the 20Hz - 20KHz audio band.

An LT 1028 is a good starting chip with 0.9nV/sqrt hz noise. LT's application notes will have ccts and Andy C's website derivations can help you finesse that. Take charge and learn.

Alternatively there are plenty out there who will happily sell you more complex hyped kits espousing the need for complexity to sell you more bits.

Cheers,
Greg.
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Old 1st July 2005, 11:04 AM   #4
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Thanks for the responses, the Zen Perl design looks elegant, and would save me having to create a PCB layout, as this is downloadable.

However, a quick search of the linear.com website for "RIAA" reveals a complete phono stage using the LT1115 opamp, and this looks pretty neat also.

I have attached the Linear schematic, incase anyone would like to comment. 'm leaning toward the LT1115 solution, (and if I do this, I'll publish the PCB to this group.)

Peter Moreton
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Old 1st July 2005, 02:48 PM   #5
jcx is offline jcx  United States
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a fundamental question is your phono cart type; mc (hi/lo output) or mm (again some range of source Z and output within the generic type)

mm really don't like bipolar input bias current in the coil, and Zsource is so high that "low noise" bipolars give higher noise due to input noise current - AD743/5 still the best low freq noise monolithic fet op amps I know of and will beat the LT1115/1028 on noise with high Z in

I think the single feedback loop RIAA implementation has the fewest compromizes, but many single op amps can have marginal loop gain if you need a high preamp gain (mc) - if you are adventurous I would put 2 op amps with V gain inside the RIAA feedback loop (I would probably use a high speed current feedback dsl driver as the output op amp - and remember; composite amp frequency compensation is fun)
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Old 1st July 2005, 04:02 PM   #6
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I built this http://www.geocities.com/rjm003.geo/.../diy_pho5.html
and was positively suprised by how well it performs in my setup.

I have been procrastinating on this amp for the past 2 years...
http://home.pacifier.com/~gpimm/phono.htm
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Old 1st July 2005, 04:21 PM   #7
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Quote:
the Linear schematic

I have built something similar topology-wise with AD8610 followed by a BUF634. It is very optimistic to expect compatibility with low output MCs so i use transformers. Don't see any point in such a big output resistor and certainly don't have electrolytic caps in the feedback loop. Output offset <2mV.

Is it great? No. Even with discrete regulators and resynthesised 50Hz supply. Caddock resistors and polystyrene riaa caps. And lots of BGs in the PS. I believe a passive split RIAA ala Hagerman has more sonic potential. At the expense of 4 more regulators and AC coupling.
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Old 1st July 2005, 04:40 PM   #8
sam9 is offline sam9  United States
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There is one near back of the following datasheet:

http://cache.national.com/ds/LM/LM833.pdf


It has the virtue of simplicity, i.e., you use a single opamp per channel or even use a dual for both. This is almost a generic design, both Slone and JLH show versions of it (just different RC combinations) in their books. I've seen it in other places as well. It's probably too prosaic to have audiophile status, but when I listened to one it seemed to do the job as well as the upscale designs. However, I'm one who never regreted the demise of the LP - so I'm probably not qualified to opine.

Nonetheless, since it can be constructed on a breadboard in a few minutes it can be used as a minimum performance baseline. Namely, any alternative design that does not sound at least a little better probably isn't worth pursuing. The judgement as to what sounds better I'll leave up to those more into to vinyl than I am.
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Old 1st July 2005, 04:55 PM   #9
sam9 is offline sam9  United States
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This PDF is an app note with more RIAA preamp info and ideas.

http://www.national.com/an/AN/AN-346.pdf
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Old 2nd July 2005, 02:00 AM   #10
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Hey Horneman, are those big hurky motor start caps I see in that tube preamp?

mike
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