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Author Topic: otari 5050: impedance and output of the playback heads?  (Read 5167 times)

Offline monotube

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otari 5050: impedance and output of the playback heads?
« on: December 30, 2012, 03:08:28 AM »
Hello,

I'm a newbie about reel to reel machines and I have bought an Otari 5050 first version, now perfectly calibrated, to play old 4 track prerecorded tapes ( and obviously 2 tracks... ). Actually it's only one curiosity, but I would like to know the characteristics of the two playback heads ( output tension and impedance ) to evaluate if I can modify one tube phono I have build time ago. I have read about Nab and IEC eq. and it's not a big problem  to substitute my old RIAA, I don't know if a step up transformer is a good choice for these playback heads. Many thanks to all

Offline stellavox

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Re: otari 5050: impedance and output of the playback heads?
« Reply #1 on: December 30, 2012, 11:13:15 AM »
Welcome monotube,

I'd guess that the impedance of the PB head is in the range of 100mH which will work fine into a solid state preamps.  Typical values for tubed PB heads were in the 400mh range.  The higher impedance means that you get a few more dB out of the head which reduces the preamplifier gain requirements.  You could just try hooking the head up to your phono preamp and (recognizing that the EQ network is "wrong") just see if there appears to be enough gain out of it.   it  If so then yes, change the RIAA network to tape (I'd suggest lifting one end and putting in a switch so you can switch back to RIAA if you'd like).  For what it's worth, tape pre's seem to need about 50dB of gain (depending on the "sensitivity" of the rest of your system)  - push a tubed unit more than that and you will start getting into noise issues - perhaps requiring a step-up transformer somewhere in the circuit.  In any case - please experiment - and share your results!

 Good Luck

Charles
« Last Edit: December 30, 2012, 11:17:39 AM by stellavox »

Offline monotube

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Re: otari 5050: impedance and output of the playback heads?
« Reply #2 on: January 09, 2013, 03:31:30 PM »
Hi Stellavox,

many thanks for your reply. Do you know the playback head values in ohms and in mvolts? In effect for me the value in mh is not useful to interface a step up transformer

Offline docb

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Re: otari 5050: impedance and output of the playback heads?
« Reply #3 on: January 09, 2013, 08:16:57 PM »
Output is roughly a couple of millivolts with a 250nW/M tape or maybe a bit higher. Don't worry about the head's ohms. It likes a load of around 100K ohms. I would try to avoid a step up transformer and stick with active devices if I found my preamp didn't have enough gain, as I have not had much luck with typical MC step up transformers and repro heads.
Dan "Doc B." Schmalle
President for Life, Bottlehead Corp.
Managing Director - retired, The Tape Project

Offline ironbut

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Re: otari 5050: impedance and output of the playback heads?
« Reply #4 on: January 09, 2013, 10:10:39 PM »
Hey Monotube (btw, we use our real names on this forum. Please read the Forum Rules),

I've found that it pays to experiment before worrying about the gain needed.

My first outboard tape head preamp had pretty low gain. For some reason, I decided to "supplement" the gain by adding an old preamp I had sitting around (I put it between the heads and the tape head preamp). I did that for a while because I had to turn the volume knob up on my preamp or headphone amp I was using it with.
Turns out that I didn't really need that extra gain at the listening levels I was using (in other words, the noise floor was already low enough). I just felt uncomfortable turning up the volume so much higher than I was used to.
Where the noise floor is depends on the rest of the system as well as the tape preamp (and sometimes the tapes).
So, my advice is to do real world tests before getting out the soldering iron.
You may be perfectly happy with the way it is.
steve koto
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