Tape playback amp

[Joshua_G]

I plan building a playback amp for reel to reel tape (Revox B77, 4 tracks). What gain do I need (from the playback head output to line level output) and what equalization?

[firechief]

Well that is a pretty general question. Is the original playback circutry still in tact? Are you trying to re invent the wheel or replace damaged circuts? Im not trying to be mean hear, we just need to know more about what you are trying to do.

[Joshua_G]

The original amp is SS, discrete bipolars. I'd like to replace it with an external tube amp. I have reasons to believe that a tube one may sound better.

[Wavebourn]

Well; you will need to replace tape heads as well since they have too low resistance to be used with tubes.

[Joshua_G]

When MC phono cartridge can be used with jfet cascoded with tube, I guess tape heads can also do it. Anyhow, when I serviced Ampex tapes, the playback amp was all tubes, though it may had stepup transformer interfacing the head, I don't remember.

[mashaffer]

Wavebourn, I don't understand why a low source resistance would be a problem. Couldn't one simply use a smaller grid leak resistor if the heads didn't like a high impedance load? I can see where a record amp might be tricky.

[Eli Duttman]

Joshua,

Allen Wright's JFET cascode will give you all the low noise gain you need in the 1st stage of your new preamp. While a newer curve is available, I suspect the tried and true NAB EQ curve is what you need. Check this Bottlehead page out.

The low impedance heads used with SS playback preamps have comparatively few turns of wire on the core. The signal generated may not be strong enough to overcome "hollow state" S/N limitations.

[firechief]

Wow, it sounds like a great project is brewing. The recorder in question is still popular, and there may be others out there who would like to try this ambitious build. Seens like the schematic of the orginal recorder would be a lot of help here. Perhaps even the service manual. I will look.

[mashaffer]

Thanks Eli. That one really had me scratching my head. (no pun intended). The step up transformer is an interesting idea but I wonder how it would react with the inductance (oops another unintended pun) of the tape head itself.

jFETs seem to be pretty non-offensive devices in general. One could try a FET input with a touch of local feedback I suppose until you could get a set of higher Z heads.

[Wavebourn]

Noises of 1/f type may be of concern with FET inputs.