Would this work (simple OPA1632 phono preamp)? - diyAudio
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Old 1st February 2006, 05:55 PM   #1
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Default Would this work (simple OPA1632 phono preamp)?

My quest is for a simple inexpensive balanced phono preamp. Not too many examples out there. So I drew up my own. It is basically just Rod Elliott's RIAA opamp eq from Project 25 http://sound.westhost.com/project25.htm thrown into the feedback loops of an OPA1632, as shown.

My questions are pretty basic: Would it even work? What enhancements would you make?
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Old 2nd February 2006, 08:42 AM   #2
sreten is offline sreten  United Kingdom
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Hi,

at a glance it appears it would work.

However for certain noise performance will be
poor due to the shunt feedback series resistor.

Many turntables, arms and sometimes cartridges will
need rewiring carefully to provide balanced operation.

/sreten.
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Old 2nd February 2006, 09:08 AM   #3
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The big problem is to get the RIAA network 100.000% identical

Notice also the input impedance's. 2 *(18//100) kohms, not what you have in mind?

If you want low noise you must keep the resistor values low and 18 k is pretty much.
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Old 2nd February 2006, 03:21 PM   #4
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Thanks for your comments. I did a bit more searching on phono stages and I think that I will probably just use the RIAA eq used in the VSPS http://www.geocities.com/rjm003.geo/.../diy_pho5.html
The input impedance improves since the input resistor drops to 220R and the shunt resistor drops to 24K.
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Old 2nd February 2006, 09:05 PM   #5
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As the OPA1632 behaves more like an inverting topology, you might better have a look at RJM's "VSPS Ultra": http://www.geocities.com/rjm003.geo/.../diy_pho7.html
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Old 2nd February 2006, 09:17 PM   #6
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Quote:
Originally posted by HBarske
As the OPA1632 behaves more like an inverting topology, you might better have a look at RJM's "VSPS Ultra": http://www.geocities.com/rjm003.geo/.../diy_pho7.html
Actually, the VSPS is closer to what I need. The VSPS Ultra omits the input resistor to get extra gain for an MC cartridge. I am building for an MM cartridge and need that input resistor. I was not going to mistakenly short the input resistor to ground if that's what you mean
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Old 10th February 2006, 03:18 PM   #7
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Quote:
The big problem is to get the RIAA network 100.000% identical
Take a look at A. Wrights RTP Riaa.
http://www.vacuumstate.com/schematics.htm
http://www.vacuumstate.com/schematics/rtp3c_s.gif

Carsten
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Old 10th February 2006, 04:25 PM   #8
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Wow, didn't know anybody was still reading this.

Quote:
Originally posted by carawu
Take a look at A. Wrights RTP Riaa.
True, doing that type of passive eq would avoid matching problems, but I have read good things about doing active eq in the feedback loop so I would like to try it.

This is what my current thinking is. The addition of iron to convert to unbalanced may look silly at first, but that is because I am going to use cheap Edcor 10k:10k transformers I have on hand for the unbalanced output, which will likely be a rarely used connection, and I expect the OPA1632 to do a better job.
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