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Old 8th November 2013, 12:50 PM   #421
BigE is offline BigE  Canada
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I tried using the OP27EZ in place of the OPA627 in one channel last night. Nulling the DC offset in this chip seems to be a 'good idea'.
Thankfully, there is a set of three pads that can have their traces cut that can be used to mount the 100K trim pot I had (10K optimal). The OP27 is all N-channel BJT and specs out quieter than the OPA627 FET based op amp.

I prefer this chip to the OPA627; it has a quieter background and it seems to reproduce longer reverb/decay than the 627. Generally sounds cleaner and less congested than the stock circuit. I will be mounting the other trimmer tonight and report my findings.

Beware: the offset adjust pins are different on the OPA627 than on the OP27. Once this circuit is added, the parts are no longer compatible.

Last edited by BigE; 8th November 2013 at 12:52 PM.
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Old 8th November 2013, 10:22 PM   #422
BigE is offline BigE  Canada
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Did the second one. The results are stellar. Far better performance with nulling dc at first stage and using op27 devices. Quieter and more open better all around than the opa627bp.

Wondering about the 637 section now....
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Old 12th November 2013, 12:31 AM   #423
BigE is offline BigE  Canada
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Replacement of the opa637 with an op27 was again excellent. This stage is finally there.

Now using opa627bp in the power supply and op27ez in the phono circuit. It sounds more natural with way better bass depth and staging.

Anyone that has this stage should try op27 chips. Marvellous!
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Old 13th November 2013, 06:05 PM   #424
morinix is offline morinix  United States
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The slew rate on the op27 is 2.8v/us verses the slew rate of the op637 being a minimum of 100v/us. It is easier to fabricate a low noise component when sacrificing slew rate speed. However the transient information of the music will also be sacrificed. Nothing inherently wrong with these trade-offs. Just understand, what might be, unintended consiquences.
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Old 22nd November 2013, 02:23 PM   #425
BigE is offline BigE  Canada
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Most real music has a slew rate under that of 2.8 V/uS. The only excepetion I am aware of is the rim shot. It is because of the low slew rates on the OP27 that putting them in the shunt regulator without swapping out the op637 is a "bad idea". That actually sounds awful.

I've found no sonic issue with using them in the first stage with no other changes.

I am sure that it would be better to use a faster and quieter opamp, but there is so little room on that board to attach the bypass caps necessary to run something like an LME499990. but truthfully, I've not tried attaching the bypass caps to the pins under the boards....

Also, these were just opamps that I had lying around. Thankfully, I like them a lot.
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Old 27th November 2013, 04:03 PM   #426
BigE is offline BigE  Canada
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Can this be made balanced by simply adding a second phono stage circuit and sending +'ve to one circuit and -'ve to the other?
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Old 15th December 2013, 10:24 PM   #427
BigE is offline BigE  Canada
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For those of you that want more bass, try changing C2 to 0.0047uF - 0.0056uF. That's 4700pF - 5600 pf.

I will be doing that once I find a good cap and report back -- 5600 gives a lower sum of squares when calculating deviation from RIAA.

Also, replace 10uF C6 with 47uF is good too. 22-47 should be fine.

These figures yeild best numbers in my spreadsheet.

Last edited by BigE; 15th December 2013 at 10:44 PM.
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Old 15th December 2013, 11:06 PM   #428
BigE is offline BigE  Canada
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Finally, 330K resistors can be 300K with theoretically improved performance.
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Old 15th December 2013, 11:45 PM   #429
ahaja is offline ahaja  Poland
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Are you sure about this C2 ?
Look: riaa

Here - what happen when C2=56nF : riaa

If you change 330k's then you change riaa also.

Only way to get more deep bas - add another 10uF parallel to existing 10uF.
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Old 16th December 2013, 12:45 AM   #430
BigE is offline BigE  Canada
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In my spreadsheet, I've changed C2=56nF, C6=47uF, and both 330K resistors to 300K.

This decreased the sum of squares of the error from RIAA using 78 frequency samples from 220 to 17.

My spreadsheet is not really ready for posting, but I would be happy to email it to you.

What software is that you used?

Last edited by BigE; 16th December 2013 at 12:49 AM.
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