Active device substitutions needed for this schematic! - diyAudio
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Old 3rd April 2002, 12:24 AM   #1
Lenin is offline Lenin  United Kingdom
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Unhappy Active device substitutions needed for this schematic!

Err...


Would someone be kind enough to take a look at this schematic and recommend some device replacements?

Before you take a look, be warned, as circuits go - it ain't pretty!
Its a big old Musical Fidelity, OK, OK, not the last word, but we've been together 15 years and the amps like a cosy old pair of slippers - you get the idea. Anyhow, I've decided it deserves a spruce up, but its with the active devices that I'm really and truly stuck.

I even tried some straight op-amp relacements (without changing any of the ancillary components - I just kinda said a little prayer and hoped for the best) but virtually everything LT1028, OPA134, OPA627 oscillate like crazy. OPA132's seem OK though...

I'm sure you guys have far better things to do but I'm really kinda stuck.

I hope the attached info is useful.


The output devices 2SK 226 and 2SJ 83 are enhancement mosfets, now discontinued.

Thanks in advance
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Old 3rd April 2002, 12:32 AM   #2
Lenin is offline Lenin  United Kingdom
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Default Whoops! Here's that schematic

URL=http://www.geocities.com/ResearchTriangle/8231/doc/ampl/musical_fidelity_a370.jpg]Here it is[/URL]

Again, thanks in advance.
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Old 3rd April 2002, 12:33 AM   #3
Lenin is offline Lenin  United Kingdom
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Default Whoops! Here's that schematic

URL=http://www.geocities.com/ResearchTriangle/8231/doc/ampl/musical_fidelity_a370.jpg]Here it is[/URL]

Again, thanks in advance.

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Old 3rd April 2002, 12:37 AM   #4
Lenin is offline Lenin  United Kingdom
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Default Will I ever get this right!!!

http://www.geocities.com/ResearchTri...elity_a370.jpg
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Old 3rd April 2002, 01:52 AM   #5
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2SK1058/2SJ162 should be fine replacements for the 2SK226/2SJ83 outputs if you can get your hands on them.
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Old 3rd April 2002, 05:39 AM   #6
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Default parts replacement

Hi Lenin,
No wonder you got oscillations with other opamps.
If you place a fast opamp in front of a buffer/outputstage you get the effect of a dog chasing its own tail. You have to slow down the opamp by placing 50 or 100pF maybe more, from the opamps output to the inverting input. The opamp originally present had compensation too, but more in a custommade manner. This has to be removed for modern opamps.
See also the Application Note #47, High Speed Amplifier Techniques at www.linear.com Appendix C The Oscillation Problem-Frequency Compensation Without Tears.
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Old 3rd April 2002, 11:46 PM   #7
Lenin is offline Lenin  United Kingdom
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Thanks guys, I'm following up your advice.

In the meantime, any suggestions for replacing MPSA43/93 and BC414/416?

Thanks again.
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Old 4th April 2002, 06:44 AM   #8
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Default replacement parts

Hi Lenin,
For what it is worth my replacement book gives
MPSA43-> BF391..393; BF420A;BFP22; BFP25
MPSA93-> BF492...493; BF423A; BFP23; BFP26
BC414-> BC184; BC550
BC416-> BC214; BC560
All kaput?
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Old 5th April 2002, 01:13 AM   #9
Lenin is offline Lenin  United Kingdom
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Thanks Elso.

Can you recommend any of those devices (or any others you feel might be suitable) on sonic grounds?

Thanks.
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