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Thursday, January 2, 2014

RIAA, NAB & IEC Equalisation

Hi!

This is a project which got delayed a bit but I finally found the time to finish it during the holidays. It is a passive LCR EQ unit which covers RIAA for LP playback as well as NAB and IEC tape equalisation.






For IEC, both 15 ips (inch per second) and 7.5 ips curves are implemented. All networks are built as 600 Ohm LCR EQ filters. Thus they need a low impedance preamp to drive them. This particular unit will be used with a DHT preamp. But it could also be used with the multi chassis modular preamp which was intentionally split up to allow the use of different EQ modules. This way the same preamp can be used as phono preamplifier as well as tape preamplifier, which gets fed from the tape head.

The chassis style is the same as that of the modular preamp series:




Just a simple rotary selector switch at the front which switches the different EQ networks in and out of the signal path.




Nothing fancy on the backside. Just inputs and outputs and a ground post to ground the chassis.




The RIAA network follows the conventional approach. It contains two LCR filters. One for the 318/3180uS time constants (50/500Hz corners) and one for the 75uS time constant (2122Hz corner frequency).

The network for NAB equalisation only has two corner frequencies at 50Hz (same as RIAA) and 3180Hz (3180 and 50uS). So it requires only one LCR network. Since it shares the same low frequency corner with RIAA, but has a higher upper frequency corner it's attenuation is higher at 1kHz.

The two IEC curves for 7.5 and 15 inch per second tape speeds actually only have one corner frequency since they don't have a low frequency shelf. But for practical reasons a low frequency corner had to be chosen at which attenuation starts. Otherwise the inductances would have to be infinite. The low frequency corner has been chosen at around 20-25Hz. The high frequency corners are at 2274 Hz (70uS) for 7.5ips and 4547Hz (35uS) for 15ips. Due to the lower shelf frequency the attenuation at 1kHz is higher for the IEC settings.

Here a photo of the internals:




I have another variable EQ design in the planning which will have many EQ settings to cover most curves for old shellac and vinyl mono records. This design will be based on the Octal Phono Preamplifier circuit. Stay tuned!

Best regards

Thomas




6 comments:

  1. Hi Thomas,
    Very interesting project.
    I have a question: you're are saying that RIAA and NAB have the same attenuation at 1kHz. But RIAA is flat 500-2122 and NAB is not. Should not they have the same attenuation at 500Hz and NAB is -6dB at 1 kHz ?
    Also would be very interesting to see how did you implement IEC low end (i guess IEC about -10/12db compared to RIAA/NAB)
    Thank you,
    Alex
    Thank you,
    Alex

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    Replies
    1. Hi Alex,

      you are right, and that's also how it measures. I corrected the text accordingly. Thanks for pointing that out

      Thomas

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  2. Thomas,
    How did you EQ IEC ? - did you drop the R to make low corner lower then 50hz, (somewhere at 15hz neighborhood) or something else ?
    IEC drives me insane - single 35us corner asks for >50dB EQ. Whatever I tried newer sounds really good ... maybe LR is the way to go ..
    thanks,
    Alex

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    Replies
    1. Hi!

      As I wrote in the text. I did it similar to NAB, just moved the low frequency corner lower to 20-25Hz. I didn't want to go too low to not loose too much signal

      Best regards

      Thomas

      Delete
  3. Hi Thomas,

    Very nice. Will these be available to buy at some stage?

    BR

    Chris

    ReplyDelete
  4. Hi Chris,
    yes such units can be bought from me. Of course also with the matching gain stage with the low output impedance which is needed to drive these. Drop me an email if you are interested

    Thomas

    ReplyDelete