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Old 22nd December 2014, 08:39 PM   #1
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Question Phono Preamp Question...

Hi everybody,

I've been looking for simple schematics for a phono pre for a friend's setup and this one is about the least busy one I've found in the datasheet for the LM4562 opamp. I just don't understand the values that are circled in red. Are the resistors values supposed to be put in series with one another and that's why they have "+" signs? Are the cap values supposed to be put in parallel with one another and that's why they have "/" separating them? I've never seen this in a schematic yet.

It says that this is a passive design and seems fairly simple. Has anyone out there put this together and had success with it's curve being close to the RIAA standard?
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Old 22nd December 2014, 08:55 PM   #2
SY is offline SY  United States
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Yes, you got it, the + means series, the \\ means parallel. You can use the RIAA calculator at kabusa.com to check the values for conformance.
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Old 22nd December 2014, 09:46 PM   #3
Xoc1 is offline Xoc1  United Kingdom
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The 5% spec for the capacitors seems wide given that the 500 pf is only 2.3% of the 22nf. This makes sense if you use 1% caps.
The circuit has gain to DC which could be a problem.
A 100R resistor on the output might help if it is connected to a lead.
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Old 30th December 2014, 11:39 AM   #4
reuben is offline reuben  United States
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I built one and the equalization sounds spot on.

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Old 30th December 2014, 12:11 PM   #5
AndrewT is offline AndrewT  Scotland
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The tolerance of the resistors is just as important as the tolerance of the capacitors.

If one needs 2% tolerance, then all the RIAA components should be to this tolerance.

If one can "make do" with 5% tolerance then all the components can be 5%.
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Old 30th December 2014, 01:29 PM   #6
DF96 is online now DF96  England
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Not quite true. 5% tolerance on components may mean 10% tolerance on time constants. As high tolerance resistors are generally more easily available and cheaper than high tolerance capacitors there is a valid case for using, say, 1% resistors with 5% capacitors to get 6% accuracy. In addition, an RIAA network may use resistors to set the balance between LF and HF levels, while some capacitors may just set the transition frequency between these. Hence a cap value error may cause less change in response than a resistor error.
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Old 31st December 2014, 09:31 AM   #7
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Default interferencies

But the poles and zeros of transfer fuction are more influenced by tollerace of components if a network is une for all ?
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Old 31st December 2014, 09:42 AM   #8
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But what is the interferences from poles and zeros on the transfer function if values are affected by a large tolerance on resistors and caps ?
I think is possible to have a increase the precision of response dividing the poles one by one on its opamp . Is not best solution but is possible to use 5% tolerance components
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Old 31st December 2014, 11:36 AM   #9
Bonsai is offline Bonsai  Taiwan
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Even with 1% resistors the non-conformance can be very high.

I think you need to parallel 1% caps and select the resistors for anything like accurate equalization - seems Self and a few others also came to this conclusion before I did.
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Old 31st December 2014, 10:29 PM   #10
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That's good news Reuben. I will probably order the parts I need for this project this weekend. I always use 1% R's and I'll go with 5% caps. I probably have enough film caps laying around to hit closer to the mark if it ends up being too far from what value it should be.

Thanks SY for that kabusa site reference to double check the values. I'll post up a frequency response for this circuit when it is finished.

Last edited by shredhead; 31st December 2014 at 10:36 PM.
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