High end Missing In Action...

[Phil_in_CA]

Hi Everyone,

Finally got new heads for the Technics deck and find that the lack of high end response through the Seduction Tape Head Preamp isn't totally due to the worn heads!  I got an "as new" head block and the sound (following head alignment tweaking*) is as if the highs are rolled off through the Seduction...

Suggestions...  I wonder if anyone else has had this problem.  It is in both channels.

* I have a new MRL tape, signal generator, AC-VTVM, oscilloscope, and manuals... and requisite skills to use them properly.

I undid the head direct-out wiring and re-established the internal electronics on the Technics to find that it sounds quite nice.  Before you ask me what kind of tapes I am listening to, know that I am listening to TP-02 Blackjack David for critical evaluation.  With the Seduction, it is missing it's open sound, as if the top is being clipped off, like a bad "flat top" haircut my dad used to give me!

I would appreciate anything you'd care to suggest.

Phil

[Grainger49]

 Are you confident in the tubes you have in the Tape Seduction?  It sounds like you have a new rig, but thought it a good idea to ask.

[Paul Joppa]

Man, there's one thing I hate about this forum software. Grainger posted while I was composing my response, so my post did not go through - and the warning is in small print, orange on black and I never see it. I'll try again:

Sounds like you have an equalization error, or a tape head loading error. There were a few versions of the tape head modification to the Seduction, and I don't recall if you said anything in the other thread about what version you have.

(They are documented at http://www.bottlehead.com/et/adobespc/Seduction/bottleheadtapeheadpreamp.htm in case anyone needs the link.)

The earliest versiuon had trimpots for the HF equalization (adjusts the 5-10kHz level)) and for the tape head loading resistor (adjusts the 15kHz level - i.e. the damping of the tape head resonance). If you have something different, we can make further guesses but we'd need to know what your Seduction circuit is and the equalization error values in the range 1kHz to 15kHz. To get the loading resistor matched to the tape head, you might look at the circuit values in the original built-in preamp. Heads vary widely in their optimum loading!

Doc B should be back online on Monday; he has the most experience with these issues and may have more to offer.

[Doc B.]

There are a few different reasons the high frequency loss could be caused. The thing we have seen most often is that the cable between the head and the preamp causes problems. In fact we have one set of cables here that we have relieved from any tape head duty because they really kill the top end. The place to start is run that MRL tape and look at the output of the Seduction on a scope. Figure how many dB the high frequency response is at 10kHz and 15kHz relative to 1kHz. If a different cable doesn't help it might be worthwhile to put a 250K ohm trimpot in place of the input grid resistor, which is 47K in a stock Seduction, and might be 47K or maybe 100K in the tape head version. Try adjusting the value of the trimmer up and see if that brings up the high frequencies.