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Old 27th February 2014, 05:58 PM   #11
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This topic has a long history and there are numerous DIY audio threads on using LT opamps in RIAA preamps.
See here. LT1115 - Ultra-Low Noise, Low Distortion, Audio Op Amp - Linear Technology
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Old 27th February 2014, 06:11 PM   #12
jcx is offline jcx  United States
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still doesn't make low noise bjt Inoise any better with mm coil's high Z

~1 nV heavy bias bjt inputs LT1028/AD797 are really only OK with low Z source less than few 100 Ohms

Scott's AD743/745 would still be the winner - if AD still wanted to make them (seems they do but under protest Product Status:Not Recommended for New Designs)
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Old 18th March 2014, 09:51 PM   #13
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OK, I did some changes to the schematic (se post #1). By simulation, it looks quite good and accurate in theory (+-0.0035dB) but, ... of course, component tolerances and wrong load capacitance kills that accuracy.

Anyway, I have couple questions:
1. Is variable capacitor an option for to control the load capacitance?
2. Does computer power (ATX) as a voltage source for LT1028/LT1010 need some extra filtering?
3. Is there any advantage in building the final R/C values by using multiple components instead of lowest count (tolerances repeals?)?

Last edited by jiiteepee; 18th March 2014 at 09:57 PM. Reason: spelling
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Old 18th March 2014, 11:31 PM   #14
mlloyd1 is offline mlloyd1  United States
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I agree about AD743/AD745.
OPA627 makes a pretty good MM front end also.
It's just outrageously expensive to due how it's made.

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Old 18th March 2014, 11:41 PM   #15
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I have no knowledge of variable caps so to jump over that to your 2nd question. I'd definitely avoid an ATX power supply for this application. Its certainly possible to filter the output enough to remove the differential mode HF hash but the common-mode is going to be a serious problem. There's also the question of how much LF noise is going to be present as ATX power supplies are designed for computers not audio and tend to be highly cost-engineered.

While on the subject of power supplies - you've lost much of the advantage of using the buffer in connecting the power supplies together. The front end opamp should get some extra filtering (LC) so its not exposed to the nasty switching noise produced by the buffer.
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Old 23rd March 2014, 02:44 PM   #16
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Quote:
Originally Posted by abraxalito View Post
I have no knowledge of variable caps so to jump over that to your 2nd question. I'd definitely avoid an ATX power supply for this application. Its certainly possible to filter the output enough to remove the differential mode HF hash but the common-mode is going to be a serious problem. There's also the question of how much LF noise is going to be present as ATX power supplies are designed for computers not audio and tend to be highly cost-engineered.
Hmm... does it make any difference if the pre-amp is going to be used only with PC setup (-->pre-amp --> USB sound card --> PC)?

Quote:
While on the subject of power supplies - you've lost much of the advantage of using the buffer in connecting the power supplies together. The front end opamp should get some extra filtering (LC) so its not exposed to the nasty switching noise produced by the buffer.
Any suggestions ... I did add the JFET system as suggested for LT1115 based design (link given in rayfutrell's post) but does that help anything in this?
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Old 23rd March 2014, 02:46 PM   #17
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I made couple measures (see post #1). Is there something wrong because of spikes seen there in plots? Also is the level at 1kHz around +10dB ... shouldn't that be less than 0dB?

Last edited by jiiteepee; 23rd March 2014 at 02:51 PM.
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Old 23rd March 2014, 03:57 PM   #18
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Quote:
Originally Posted by jiiteepee View Post
Hmm... does it make any difference if the pre-amp is going to be used only with PC setup (-->pre-amp --> USB sound card --> PC)?
If everything runs from the same PSU then yes, it makes a difference. You won't need to worry about common-mode noise, only differential mode.

Quote:
Any suggestions ... I did add the JFET system as suggested for LT1115 based design (link given in rayfutrell's post) but does that help anything in this?
You mean suggestions for actual values and part numbers for inductors? I would only be able to do that if I was designing this for real with knowledge of the desired performance relative to the PSRR spec. In general to get the best SQ its necessary to keep HF hash off the rails as much as possible - LC filters are the best way to do this. You could start with an online LC filter designer to design a filter which gets HF rejection to match the opamp's PSRR curve.

The JFET current source is a good idea to reduce HF hash generated by the opamp itself, the LC filtering is to keep the buffer's hash out.
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Old 24th March 2014, 07:02 PM   #19
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Originally Posted by jiiteepee View Post
I made couple measures (see post #1). Is there something wrong because of spikes seen there in plots? Also is the level at 1kHz around +10dB ... shouldn't that be less than 0dB?
LOL ... both questions got solved (I'm still learning the LTSpice software ... as well as electronics). Spikes was caused by PULSE -command for another small circuit which I had left active while running the .tran simulation. Level was just too high because of V was set to 0.1V.
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Old 24th March 2014, 07:21 PM   #20
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Originally Posted by abraxalito View Post
If everything runs from the same PSU then yes, it makes a difference. You won't need to worry about common-mode noise, only differential mode.
This is just what I thought.

Quote:
Originally Posted by abraxalito View Post
You mean suggestions for actual values and part numbers for inductors? I would only be able to do that if I was designing this for real with knowledge of the desired performance relative to the PSRR spec. In general to get the best SQ its necessary to keep HF hash off the rails as much as possible - LC filters are the best way to do this. You could start with an online LC filter designer to design a filter which gets HF rejection to match the opamp's PSRR curve.

The JFET current source is a good idea to reduce HF hash generated by the opamp itself, the LC filtering is to keep the buffer's hash out.
I made new transient analysis and now the result looks much better AFAIK.

As I'm in learning mode, can you or someone else point me some examples showing the usage of LC filters (schematics, LTSpice project files).
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