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Old 2nd July 2005, 08:52 AM   #11
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Hey Mike!..

Yep..Gary Pimm has those as parafeed caps for his output tranformers. (The page I linked to was Gary Pimm's)

Mine is still in this phase..
http://basenjes.de/tubes/gphono.htm

well slightly further....



Mine will be without tranformers for starters by the way..

Regards,
Bas
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Old 2nd July 2005, 09:32 AM   #12
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Thanks everyone; lots of good advice here. I'm going to build a simple opamp phono stage and see how this sounds. It may be good enough, using the latest and greatest opamps.
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Old 2nd July 2005, 09:39 PM   #13
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Hi Peter,

Simple's always a good place to start. The LM833 cct below as lifted from the link provided is close. only with the newer single op amps like LT1028, etc...

A couple of improvements though , if you're doing a board. A series R should be added between the RIAA network and the output feedback point. This can serve two finctions - low Z loading limiter (as the RIAA > 0 at hf) ans cutting lathe 50KHz zero provision.

When selecting the actual component values (recomment Andy C's website) or scaling the values shown, consider increasing the LF time constant considerably by increasing the 200K -> 300K for a fuller low bass consistent with a lack of 20Hz break in ther recording eq.

jcx,

The LT1028 has input stage bias cancellation so DC bias currents through the cartridge are likely acadaemic? Also the high Z source impedance source impedance you speak of , that would more suit a FET op amp, is dubious as the typical MM cartridge is usually characterised as , IK in series with 600mH and most audible nois according to Fletcher Munson is in the 500-3KHz range where source impedance is typ 2K-7Kohm. Also the RIAA eq at hf tends to further supress hf contribution. I vaguely recall NS doing an octave by octave analysis.

But I have to agree with your -
"..... 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)"

I would suggest a discrete FET diff'l pair followed by a high speed high drive op amp in one loop (all as the op amp pictured) with tailoring of the diff'l pair back to unity at hf for stability. Simple and effective AND fun!

Cheers,
Greg
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Old 3rd July 2005, 12:00 AM   #14
jcx is offline jcx  United States
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check out the lt1028 i_noise spec, 1 pA/sqrt(hz) * 2-7 KOhms is 2-7 nV/sqrt(hz) right in your peak sensitivity region, the ad743 2.5-3 nV/sqrt(hz) V_noise is a bargan, clearly beating the lt1028 with mm cart

low freq 1/f noise could be a problem with other fet input op amps with the riaa Lf boost largely canceling the A weighting noise Lf roll off - the ad743 keeps the 1/f Vnoise corner lower than any other fet input op amp I know of
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Old 3rd July 2005, 02:01 AM   #15
andy_c is offline andy_c  United States
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Hello Peter,

I've got a web page that's dedicated to showing how to get the best RIAA accuracy out of the single op-amp circuit that others have posted here. The page is located here.
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Old 3rd July 2005, 02:04 PM   #16
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Just came across a schematic using two opamps sandwiching a passive filter (attached below). Walter Jung describes the technique in his paper "Topology Considerations for RIAA Phono Preamplifiers" which you can download from his website.

At the moment, this passive filter design is favorite, any comments from the experts? [ before I waste more time... :-) ]
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File Type: jpg passively equalized phono preamp.jpg (38.6 KB, 1601 views)
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Old 3rd July 2005, 02:12 PM   #17
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Hi,

I have good expirience with that one:
http://sound.westhost.com/project06.htm

Stephan
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Old 3rd July 2005, 03:05 PM   #18
sam9 is offline sam9  United States
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I don't have any favored theory whether active or passsive will sound better. andy_c's paper is impressive and there is a certain elegance into wrapping the whole thing neatly around a asingle opamp. However, being that I'm grey-mattert-limited, I find the passive approach easier to comprehend as it lets me think about just one thing at a time.
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Old 3rd July 2005, 03:55 PM   #19
mlloyd1 is offline mlloyd1  United States
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Peter:

I've been happy using an Analog Devices AD743 driving National Semiconductor LM6321 unity gain buffer (NLA from National ). I could recommend this approach similar to the Linear Technologies data sheet for the LT1115 posted earlier. Strongly recommend to spend some time at Andy_C's site to pick the right network values, though. Great resource Andy_C!!!

Greg's suggestion about using some JFETs followed by a high drive op amp sounds intriguing, too. Oddly enough, I've got a bunch of the old Toshiba low noise high gain JFETs paired in a can (2SK146?) that I should stick in front of an AD811 or something to see what happens. I've seen this approach in a number of Japanese receivers and integrated amps from the 1980's and early 1990's but usually with nice JFETS feeding some crappy op amp.

Thanks for the flashback/reminder, Greg ...

mlloyd1
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Old 3rd July 2005, 06:03 PM   #20
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Attached is the Phono schematic that I'm settled upon (drawn in my PCB package) The design is an implementation of something I found in the TI/Burr-Brown OPA606 datasheet, and this itself draws upon ideas by Walter G. Jung (see : http://home.comcast.net/~walt-jung/w...7s-20PDFs.html)

Comments on the schematic most welcome (I'm a digital guy, so go easy on me!)

I'll post the PCB overlay later. (If it's worthwhile!)
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File Type: pdf principia phono.pdf (14.7 KB, 579 views)
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