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So you think you want to play with tape: An Otari Story
So you think you want to play with tape: An Otari Story
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Old 14th August 2019, 12:17 AM   #231
kevinkr is offline kevinkr  United States
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So you think you want to play with tape: An Otari Story
Interestingly there are 11 pairs of 0.1uF bypass caps with a pair right near these op-amps. (I may add some more)

Scope shows no evidence of oscillation - I may need to dig out the 100MHz analog scope.

I did not look at the turn off behavior so I will need to revisit it. There may be a relatively simple fix or I may simply not care.

The biggest benefit comes from installing in the first stage which is muted. There was an incremental improvement in playback with the additional op-amps.
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Old 14th August 2019, 09:05 AM   #232
dotneck335 is offline dotneck335  United States
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Hmmmm.... Rod Elliott also found this strange phenomena with TL072 chips in his crossover project, and put in a mute circuit to avoid it.......He said 4558s did NOT do this, but I'd hate to use those for anything!
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Old 16th August 2019, 02:52 AM   #233
kevinkr is offline kevinkr  United States
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So you think you want to play with tape: An Otari Story
These replaced 4560s. The other machine with stock op-amps does not do this and I am 99.99% sure this one didn't before I changed the op-amps.

The day I forget to mute the pre-amp could be the day that I fry my bass amps and woofers so I will either need to fix this eventually or undo this mod - a shame because it's definitely an audible improvement.

I will not be changing out any more in this machine since modern high voltage op-amps appear to be unsuitable for audio use unless you use a mute circuit.

I will verify that the supplies collapse in a symmetrical manner, other than that there isn't much I can do unless I add a muting circuit.
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Old 17th August 2019, 03:02 AM   #234
anatech is offline anatech  Canada
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So you think you want to play with tape: An Otari Story
Hi Kevin,
Use a shorting relay to mute. That won't affect the sound quality.

-Chris
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Old 17th August 2019, 07:24 AM   #235
dotneck335 is offline dotneck335  United States
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Quote:
Originally Posted by kevinkr View Post
I will not be changing out any more in this machine since modern high voltage op-amps appear to be unsuitable for audio use unless you use a mute circuit.
Man, that's a shame! Surely one of the geniuses here on the forum can come up with a solution. Bleeder resistors across the power supply caps? Lower the +/- V and use 4562s? Someone MUST have come across this before.
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Old 17th August 2019, 09:30 PM   #236
kevinkr is offline kevinkr  United States
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So you think you want to play with tape: An Otari Story
I can think of several solutions and may try a few when I have a bit more time.

The LME49860 is reputedly nothing more than the high voltage version of the LM4562 so I suspect swapping it in with a lower PSU voltage probably isn't going to fix it.
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Old 18th August 2019, 10:25 AM   #237
dotneck335 is offline dotneck335  United States
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Originally Posted by kevinkr View Post
The LME49860 is reputedly nothing more than the high voltage version of the LM4562 so I suspect swapping it in with a lower PSU voltage probably isn't going to fix it.
Good point; but I have dozens of 4562s in my gear and have never encountered the problem you are describing. Hmmmm.... wonder if it is somehow power-supply related? Maybe you could shoe-horn in one of LV's NoNoiser circuits? D-Noizator: a magic active noise canceller to retrofit & upgrade any 317-based V.Reg.
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Old 18th August 2019, 02:45 PM   #238
kevinkr is offline kevinkr  United States
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So you think you want to play with tape: An Otari Story
It's definitely something I need to measure, the schematics indicate that the supplies are symmetrical or should be depending on the loading.
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Old Yesterday, 02:01 AM   #239
kevinkr is offline kevinkr  United States
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So you think you want to play with tape: An Otari Story
It's pretty much the op-amps.. LOL I installed some more tonight, and it got even worse. I will likely add muting relays at the output at some point.

Apparently the better op-amps are audible, although given the whole record and playback process I sort of wonder why. I figured the process was so flawed that replacing a device orders of magnitude closer to mathematical perfection would be inaudible due to the gross errors elsewhere, this doesn't seem to be the case. I will admit I have made a lot of changes to this machine that contribute to the improved performance so at some point I need to measure a few things and see how good this machine is against the original performance claims.

I should probably take some more pictures of this machine, it looks a whole lot less shabby than when I got it. (Still some obvious signs of wear and tear but a lot of things have been replaced..
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