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Author Topic: another mx5050 thread  (Read 277 times)

Offline drdeaner

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another mx5050 thread
« on: April 17, 2017, 01:03:50 pm »
I have read through most of the MX5050 threads about improving playback sonics via new opamps, dc coupling between stages, and power supply upgrades. Early on, I decided to abandon my Otari electronics and go with external tube playback electronics (DeHavilland)  and have been really happy with the results.I don't doubt that the high end Bottlehead preamp would be as good or better.

Recently, I revisited the Otari schematics and decided to take a stab at improving things with my B2 II machine.  I removed the first 3 opamps in the playback stages (3 total opamps for 3 stages in each channel) and installed 8 pin machined sockets in their place.  I removed the headphone opamp and installed an 8 pin machined socket.  I obtained some BrownDog single to dual SOIC to DIP adapters.  I installed 2 LT 1028 8-SOIC opamps in each adapter and plugged them into the first 3 audio playback audio stages.  I took a pair of LME49600 current buffers and kludged them into an 8 pin machined socket so they could be plugged into the headphone IC socket and provide current  but no voltage gain.  Then , I shorted (eliminated)  all of the 'lytic coupling capatitors between opamp stages because the lt1028 has extremely low dc offset and those caps seem unnecessary. And lastly, I took the audio output through the headphone jack, bypassing the final 2 opamps for each channel in the audio stage that provide balanced output.  Taking the output through the head phone jack then provides a much better cable (or headphone) driving circuit, due to the buffers.  I did not change anything in the headphone circuit, so the current buffers are driving the feedback parts that no longer set feedback.  My Otari MX5050 B2 II has the mechanical guide parts obtained from Bottlehead as well as an Anthan capstan roller.  The machine has the mic inputs disconnected and those jacks are wired for direct head output.  A pair of dpdt switches by the new tape head outputs also can direct the tape head signals back to the otari electronics. And the heads were relapped by John French with a Flux magnetics 1/2 track playback head installed.

This all came out better than hoped.  Yes, the output is lower than the balanced output but enough in my system.  The internal playback is worth listening to, which before the mod, it wasn't.  If your plan is to get into playing back tapes, this could be a path for you to get going cheap (or cheaper) and provide an easy upgrade path when your funds allow.  Let me know what you think of the idea...I have actually done several B2 IIs already. It isn't better than my tubed preamp, but it is very nice and good enough to let you enjoy those great tapes.  Of course, you could start with a working MX5050 and not upgrade the mechanical parts to reduce cost. I figured the electronic parts cost about $100.


Offline stellavox

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Re: another mx5050 thread
« Reply #1 on: May 02, 2017, 06:43:17 am »
Congratulations drdeaner on your DIY efforts - and RESULTS!

Only suggestion would be to try a few other op amps in place of the 1028 - like the LME49710, or what I feel is the best (but expensive) - OPA627/827

Charles