Seduction for IEC

[stellavox]

Doc,

Don't know if this post should go here or in the General category - Anyway -

I've been asked to modify an NAB seduction for IEC.  I remember you posting something on the subject in the Bottlehead forum (before this forum existed) but couldn't find them.  Could you either supply a link to them or discuss your results / parts values / etc.  I do remember you indicating that you couldn't follow the curve exactly.  Thanks

Charles

   

[xcortes]

http://www.bottlehead.com/et/adobespc/Seduction/bottleheadtapeheadpreamp.htm

go to the bottom of the page

[docb]

No one can follow the IEC curve exactly because the theoretical curve of 6dB/octave goes to infinity at 0Hz. So in actual practice you have to pick a point at which to roll the LF off. In theory one would assume the lower the better. But a preamp has a limit to the gain available. In practice the lower you start the roll off the more midband gain you a sacrifice as you try to boost the LF to higher levels relative to the midband. This is because you don't really boost the bass with passive tape playback EQ circuits, you cut the level of the frequencies above the bass.

The article mentions two LF cutoff configurations that PJ came up with. One cuts the bass at 50Hz, keeps the gain similar to the NAB EQ version, and probably meshes best with the 35Hz head bump of a stock RS1500 playback head. The other configuration is for a 25Hz rolloff that reduces the midband gain by 6dB. This might be too much of a cut for a lot of systems.

[ironbut]

I would stay with the 50Hz version. I don't think I ever had any complaints with the bass of my modded Seduction. Depending on the heads/machine you use, gain will be much more of an issue. The circuit responds well to tube rolling. If you do have preferences in the bass dept., the proper 6DJ8/6922/7308 can make quite a difference. It's really a pretty fun kit to build and the price certainly can't be beat. The wide open nature of the layout ( there are a couple of tightly packed areas as in any gear ) makes it a natural for experimenting with caps and resistors. Another thing it has going for it,.. if you upgrade to another playback amp, you can even use it as a phono stage! What'ill  they think of next!