Best input Op Amp (TL071CP replacement for SPL Volume 2). - diyAudio
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Old 28th October 2012, 09:05 PM   #1
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Default Best input Op Amp (TL071CP replacement for SPL Volume 2).

I'm in the process to upgrade the SPL Volume 2 balanced preamp with the best input Op Amps avaiable.
The stock version uses two Texas Instrument TL071CP (JFET input) and PSU is rated at 1,6A and 15V each channel.
Inside pic taken from 6moons review (strangely in that sample there is an ST part in one channel and a TI part in the other) http://www.6moons.com/audioreviews/spl/open_big.jpg .

I've selected on paper a few Op Amps with JFET input (recommended) and same pin configuration and DIP8 socket (SMT or other mounting type are discarded).
The list and general specs:

Brand * Code * Icc min/typ/max * IN bias current Iib * IN offset curr. Iio
TI * TL071CP * -/1,4/2,5 mA * -/65/200 pA * -/5/100 pA
BB * OPA604AP * -/5,3/7 mA * -/50/- pA * -/3/- pA
BB * OPA627AP * -/7/7,5 mA * -/2/10 pA * -/1/10 pA
BB * OPA627BP * -/7/7,5 mA * -/1/5 pA * -/0,5/5 pA

IN Volt. Noise typ (Vn) * Sl. Rate unity gain (V/nS) * THD % @1K * Unity gain B1 Voltage R.R. Ksvr
18 nV @1K * 8/13/- * 0,00300 * 3 Mhz * 70/100/- dB
11 nV @1K * 15/25/- * 0,00030 * 20 Mhz * 80/100/- dB
5,6 nV @1K * 40/55/- * 0,00003 * 16 Mhz * 100/116/- dB
5,2 nV @1K * 40/55/- * 0,00003 * 16 Mhz * 106/120/- dB

Other candidates:
OPA134PA
AD797
LME49710

I'm not sure which to select since some Op Amps like the OPA134PA have pin 5 and 8 inverted, others like AD797A comes with two NC instead of Offset Null 1 and 2.

1st question: if pins do not correspond do I must avoid those chips?

2nd question: do the internal power supply and circuit could works with any Op Amp listed?

3rd question: do the unity gain bandwidth (B1) of higher Mhz could damage the following circuits?

Any other suggestion which is the absolute best Op Amp for audio?

I've already ordered the OPA604AP and OPA627BP, but I'm not sure if I can use them safely in the SPL Volume 2 preamp.

If someone is so kind to help me choose which will works safely, I'll post reviews of each Op Amp tested so to find the best.


Thanks
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Old 29th October 2012, 12:49 AM   #2
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An opamp can be no better than its power supply. To make a worthwhile SQ improvement you'd need to improve the power supply design here. If you do that then probably even the TL071s won't sound at all bad but of course other better performaning opamps will show improvements. Upgrading the opamps without attention to PSU quality in my experience is daft.

A quick look at the pic you linked shows the PSU regs are too distant from the opamps they power. This means a big low impedance loop of wire which act as an antenna. Result - lots of HF noise on the supplies. So my first suggestion would be to turn the 3 pin regs into current sources and install TL431 shunts directly at the opamp power pins.
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Old 29th October 2012, 09:26 AM   #3
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and improve the local decoupling.
Then adjust the local decoupling to suit each opamp and only then measure/listen.
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Old 29th October 2012, 01:52 PM   #4
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Thanks for the suggestions, but I'll not do any hardware mod, I'm not expert even with soldering, I just want to try changing the Op Amp for fun and a simple plug-n-play approach.
If no sound differences will be apparent I will report.


Could you please suggest me which replacement to use for the TL071CP input JFET Op Amp?

Thanks

Last edited by Celadus; 29th October 2012 at 02:03 PM.
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Old 29th October 2012, 02:22 PM   #5
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Stick with JFETs which means OPA604, OPA627 or OPA134. The others you've mentioned might not be suitable. Do check the power supply voltages are within the recommended range before swapping.
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Old 29th October 2012, 02:24 PM   #6
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I'm thinking to start an hardware mod, but I'm not an expert so I can't understand what you suggest.
Can you guide me to turn the 3 pin regs into current sources and install TL431 shunts directly at the opamp power pins?
How to improve local decoupling and adjust to suite each opamp?
Which measuring instruments and kits do I need to start the mod?

I hope it's not that difficult to do those mods.

Thanks
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Old 29th October 2012, 02:28 PM   #7
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Quote:
Originally Posted by abraxalito View Post
Stick with JFETs which means OPA604, OPA627 or OPA134. The others you've mentioned might not be suitable. Do check the power supply voltages are within the recommended range before swapping.
Do you have a clue watching the inside about power supply voltages?
I read 1.6A 15V each channel.
The opamp list above are all specified working at a range which include 15V but I'm not sure.
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