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Author Topic: Inverse tape playback circuit  (Read 5980 times)

Offline tapepath

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Inverse tape playback circuit
« on: January 15, 2008, 04:39:48 PM »
Hello,
Is anyone familiar with a test instrument that would convert a sine wave, for example, into various tape playback curves? There are inverse RIAA circuits that allow an engineer to examine a potential RIAA circuit and see if it will output a flat response. I was wondering if there is such a device that would do a wide variety of playback EQ curves?
Thanks, Ken

Offline Tubes n tapes

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Re: Inverse tape playback circuit
« Reply #1 on: February 08, 2008, 12:34:12 PM »
Ken,

I've seen inverse RIAA filters that you can put in between your generator and the Device Under Test and I'm sure that there is test equipment around that has the inverted RIAA curve built in, but for tape EQ's it is less obvious because the characteristics of the heads play a significant role in the total EQ. Therefore the EQ curves you'll find in most record and playback amplifiers standalone appear to be slightly off-spec, especially for lower tape speeds.

So, a perfect inverse EQ filter to check your record and playback amplifiers wouldn't necessarily give you good results.
Arian Jansen.

SonoruS Audio.
VP of technology of the Los Angeles and Orange County Audio Society (LAOCAS).
ESL/OTL builder and modest Studer/ReVox collector.

Offline tapepath

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Re: Inverse tape playback circuit
« Reply #2 on: February 08, 2008, 03:18:24 PM »
Hello Arian,
Thanks for the response, like you I'm trying to build a good sounding tube playback amplifier. I've built the Audio Amateur inverse RIAA network and use it to examine phono circuits. I was hoping someone was familiar with a piece of test gear that would do the same for tape playback. It's hard to imagine the original design engineers, at Ampex or Studer, would use a test tape to evaluate each circuit change they thought up. If so, they probably went through lots of tapes in the days before computer modeling came on the scene. It would seem to me that they would have wanted a steady source to look at on a 'scope and distortion analyzer. Then test it with actual heads and tape.
Are you basing your design on any classic playback circuits? If you need any schematics, let me know, I might have it available.
Take care, Ken

Offline High and Outside

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Re: Inverse tape playback circuit
« Reply #3 on: February 08, 2008, 03:27:19 PM »
What Arian said. The curves are not so easily defined. They are actually defined sort of: "The curve that gives this result, in these circumstances."

However, there is a better answer to your question, and that is (which is also what all the Ampex and Studer guys did) to use a flux loop. This can be imagined as a record head that you clip onto the front face of your playback head, then you feed signals into it from a generator of your choice.

I'm not sure where you find a ready-made flux loop these days, but that's what you're looking for.
Paul Stubblebine
Managing Director, The TapeProject

Offline tapepath

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Re: Inverse tape playback circuit
« Reply #4 on: February 08, 2008, 03:56:33 PM »

Thanks! Now I understand much more!
Take care, Ken

Offline Tubes n tapes

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Re: Inverse tape playback circuit
« Reply #5 on: February 08, 2008, 06:41:38 PM »
I'm sure those Ampex and Studer flux-loops ended up in the garages of the engineers after they eventually all got sacked when Studer was sold and went digital and Ampex management decided that they could sit out the rest of their days only managing a rapidly aging IP portfolio.

In any case those guys had their tape heads pretty well characterized. If you are developing electronics for those heads, the best method is to just measure the EQ characteristics of the original machines and use that as a starting point for the EQ curves of your own design.

I currently have an unconventional tube playback design that works extremely well, but I cannot publish that very design for a few practical reasons. Over the coming several weeks I will be publishing some stuff that can help people to get a Tape Project compatible machine running fairly easily as long as they have some soldering skills. 
Arian Jansen.

SonoruS Audio.
VP of technology of the Los Angeles and Orange County Audio Society (LAOCAS).
ESL/OTL builder and modest Studer/ReVox collector.

Offline mstcraig

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Re: Inverse tape playback circuit
« Reply #6 on: February 08, 2008, 06:49:47 PM »
Arian, are you referring to a home brew NAB/IEC preamp by any chance?
Craig Sypnier
(Magnepan, VPI, Sumiko, Shure,
Belles Research, ReVox (A-77 and B-77), Scully 280-B, Teac A-2300SD, Nordost)

Offline Tubes n tapes

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Re: Inverse tape playback circuit
« Reply #7 on: February 10, 2008, 03:40:10 PM »
It will be several months before I can publish a derivative of my tube preamp that is sufficiently easy to build. At this moment the EQ is is switchable between 15ips IEC, 15ips NAB (which is the same as 7.5ips NAB) and 3.75ips NAB.

Eventhough it is easy to add, 7.5ips IEC and 3.75ips IEC seem fairly useless to me. Anyone who has a different opinion on that?
Arian Jansen.

SonoruS Audio.
VP of technology of the Los Angeles and Orange County Audio Society (LAOCAS).
ESL/OTL builder and modest Studer/ReVox collector.