3M79 - Repro Head - External Preamp

Fezzle

Member
2011-12-08 10:33 am
Hello folks,

I was hoping somebody could help me with something I'm trying to achieve with my 3M79 2 track master analog recorder. I'm not much of a technician and have very basic knowledge so please bear with me and I welcome any guidance and correction!

I have a new outboard tape preamp of high quality I would like to use with the recorder for reproduce mode. I have a dilemma with interfacing the standard repro heads with the preamp input. The current heads are SAKI ferrite and according to John French and have a medium inductance of about 250mH. According to the manual the input of my preamp accepts from "low impedance heads of between 2 - 12 uH"
I did a quick calculation and it appears in impedance (at 20Khz) this is about 30K ohms for my current repro head, and it so needs to be a very low impedance of about 1 ohm respectively.

Some questions if I may. Would it be possible to lower the impedance/inductance of the head output using a voltage follower/buffer op amp?? As far as I'm aware I need to retain maximum voltage transfer whilst lowering the impedance this much.?

Also, I would need my preamp in my studio control room and this would be about 10 metres from the machine... I may be out of luck with this one.. but to drive this length of tiny voltage without losses.. could this be possible using a Jensen load isolator?? It appears this part provides isolation to op amp outputs from capacitive loads and retains very stable low impedance way beyond 20KHz (under 1 ohm). I may be way off here with the application of this part though. Perhaps I could just use a 1:1 output transformer?

To summarize if I can get advice as to whether my goal is even possible firstly, and if so some kind of simple active/passive circuit to hep realise this. I really appreciate your time to read and welcome your response.

From the UK

Cheers :)
 
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Zero D

Member
2009-08-06 11:11 am
Hi, Lucky you with the Very nice 3M :) I'm jealous :D

I wonder if the quoted uH for your headamp is correct ? As there is a BIG difference between that & your heads !

Anyway, i think you might be better contacting some tape experts who should be able to give you the right info. Try here for eg Head relapping

All the best with it
 

Fezzle

Member
2011-12-08 10:33 am
Hi, Lucky you with the Very nice 3M :) I'm jealous :D

I wonder if the quoted uH for your headamp is correct ? As there is a BIG difference between that & your heads !

Anyway, i think you might be better contacting some tape experts who should be able to give you the right info. Try here for eg Head relapping

All the best with it

Hi mate, thanks very much yes a lovely sounding machine however a little unreliable, theres a belt driven flywheel in there its like trying to keep a vintage car running! Yes I think I will contact the preamp designer hes a clever chap, interestingly the preamp is a pretty close copy of the MCI JH16 discrete model, and the Ampex AG440.. I think John Curl worked at Ampex around the late 60's so I may see if he can shed some light. There is a chance the preamp values were meant to be in mH millihenry but the manual does state uH microhenry.. I'll try and clear this up. Cheers :)
 
Provided that the specs are correct, I think the best solution is to use an operational amplifier as impedance transformer between the head and the preamp. You need to keep the wires as short as possible, so the preamp is best placed near the machine - this is the configuration of all studio recorders. Once the signal is on the line level, you can use longer lines to the patch bay or console.
 

Fezzle

Member
2011-12-08 10:33 am
Provided that the specs are correct, I think the best solution is to use an operational amplifier as impedance transformer between the head and the preamp. You need to keep the wires as short as possible, so the preamp is best placed near the machine - this is the configuration of all studio recorders. Once the signal is on the line level, you can use longer lines to the patch bay or console.

Yeah this is what I had in mind, I would have thought that also the head output is unbalanced and there may be an op amp or a way to cleanly convert that to drive it differentially.. this and the low impedance load isolator could help big time with the cable length thing.
 
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Fezzle

Member
2011-12-08 10:33 am
Bear in mind that, when adding any element to the repro head circuit, you will want to keep the current in the microamperes range (1 micro or less) to avoid the magnetization of the head.

Also Willi can I ask.. when a buffer op amp is drawing current from the head, will it pass the same current through the outputs or will that change with a unity gain voltage follower? I've read that a low impedance load (preamp input) will want to draw more current from the source upsetting stability..