Q8 Reels. Any suggestions for 3 3/4 IPS NAB head preamp?

[QuadKev]

I've put all my Q8's to reel and made copies of them using a modified AKAI Quadraphonic reel-to-reel deck. Works good but I want to go further. I was able to pickup a Tascam BR-20 Stereo Reel-to-reel that runs at 3 3/34 ips. I plan on mounting a brand new quadraphonic 8 track head on a spare head stack I got off of eBay. Then I want to run the tape head output to a tape head preamp and bypass the stereo preamp inside the BR-20. Here's where it gets tricky. You can find plenty of Chinese tape preamps on eBay. The thing is if you look at the specs of the chips they use they all say cassette tape head preamp. I sure there is a difference in ohms on the head itself and then there is the tape EQ to consider. So does anyone know of a 100ohm (tape head) with 3 3/4 IPS NAB EQ preamp? Does anyone know how critical NAB EQ is to tape speeds less than 7.5 IPS? Some grey area here.

So my solution thus far is the use the preamp board from my Harmon Kardon 8+ quadraphonic 8 track player. I ordered the schematics for a Sony TC-248D quadraphonic 8 track player to see the specs, wiring and how to adjust the pots on the preamp. From what I understand in this forum it's the same as the Harmon 8+.

Any suggestions would be appreciated. I know some of you took a Fostex 8 track reel-to-reel and made it run at 3 3/4 IPS but I never read where they addressed the Tape NAB issue. 7.5 NAB EQ is different from 3 3/4 & 15 IPS.

 

[skywaveTDR]

This is the problem with people that come up with this stuff as they do not have the technicals to cover the whole project. Then I say why embark on such a journey? As most reel deck heads are around 200 ohms and 3.75 IPS is a terrible speed to use anyway due to wow and flutter specs being poor, it might be better to transfer tapes to a computer project and deal with frequency contours using EQ there. 8 Track heads as in cartridges were not made to very high standards and so will not have the best output.
Eq corrections for lack of head output at certain frequencies just make for more noise of the recording. Seems like a lot of money to sink into a item that will be not such a good result. The main thing is to set the system up to a standard so what 8 track NAB alignment tape are you going to use for that?

 

[QuadKev]

Hey Thanks for the input. I'll take the good, bad and the ugly. I've transferred Quadraphonic 8 tracks before using an old Akai 202D-SS quad reel deck. I just replaced the head with a new quadraphonic 8 track tape head. I used the Project 3 Popular Science Quadraphonic 8 track test/alignment tape (only one I know of) for alignment. Then I record the quadraphonic 8 track tapes using a 4 track Fostex DVD-RAM recorder @ 24bit 96k. After that I copy those files to the computer and mastered them on the PC. The final step is to encode them to DTS CD or put them on DVD-RAM for a 24/96 lossless version. They sound great. Of course it's not Blu-Ray audio level but it's the best I can do with 3 3/4 IPS tapes which were never intended for music in the first place. As you know not all quad's were on 7.5 IPS reel or record so if I want to get Eric Burdon's Sun Secrets on lossless disc this is the best way I know of.
With that said I'm open to suggestions. How would you transfer quad 8 track tapes to a computer project? That's a good idea about EQ corrections for the NAB EQ. Thinking I rather have that set in the circuit. When I say master on a PC I pretty much just like to normalize & dehiss the Broadcast wave file. If you get into the EQ side of things then it gets subjective. I rather leave them flat and adjust during playback if needed.
So while waiting to see if anyone has done this beside me. I found a modern day chip that I'm going to use. It's the LM381. I also found a 3 3/4 NAB EQ schematic using it. Not many parts and should be super easy and cheap to build. I should be able to make it small enough to fit on the spare head block of my Tascam BR-20. See even though the Akai works I rather have a plug an play module for my Tascam reel deck. The Tascam deck is a professional stereo deck so I don't want to screw with it. The heads are fixed so it's just a matter of swapping out the head blocks. No need to realign.

 

[skywaveTDR]

OK, The LM381 is a data sheet that say 1987 so it is a old IC and there are better ones to look into such as LM4562 or LME49720 but selecting the chip and using good parts such as Metal film resistors as well as correct lead dress and shielding may have better results. In making head preamps the logical thing to do is to keep the leads from heads to input of op amps very short. With SMD technology which I do not personally like, it should be possible to make a head preamp on a head board- in other words the head mounts on a PCB that the amp is on and then the signals amplified and sent off at higher levels are more correct.
The noise levels of the chips will be lost by a design that does not take advantage of the better chip design. Most my dislike of SMD parts come from the cheap electrolytics I keep on taking off servo capstan boards on the Tascam cassette deck and I put through hole on there after that- they work much better thereafter. Some people think these things are simple when in reality they are not. You do not need a 10 or 55 MHz bandwidth but that figure show that the amp will be able to handle with accuracy anything way below that and the noise level of 2.7nV is much better than .5uV. There is a lot of study required to select the right part but also some wireless bread board measurements. Then you will come out with a superior result. Remember the Japanese Engineers were no dummies but they often times are limited due to the old penny pinchers.

 

[QuadKev]

Thanks SkywaveTDR! Sorry for the delay. You're right the LM381 is a very old IC. The reason I choose that one is because there's a schematic for a 3 3/4 IPS NAB EQ circuit. So need to do testing as it's already been done. I assume it was used back then for slow speed decks. The newer chips you mentioned do not have that. I did find a tape NAB circuit for the LME49720 but it doesn't state the tape speed of the NAB EQ. Perhaps the same mathematical formula that is found in the LM381 schematic can be applied to that circuit. Way over my head but a good friend of mine use to work for NASA. You might have some ideas. If want to have a challenge. I can supply both schematics in pdf form.
You also mentioned a tape alignment tapes. So I did some digging. Do you know Barry's 8 track repair? I found this video where he came up with using an RCA alignment tape and a spectrum analyzer. I think it's a GREAT IDEA! I got super luckily as found a mint one on eBay with the 8k tone . The auction also contained an RCA 8 track wow/flutter test tape. I'm gonna see if I contact this guy and see if he has any ideas on what I'm trying to do. In the meantime I'm recapping my Harmon Kardon 8+ and my Technics RS-858US player/recorder. I'm might be running into the law of diminishing returns here. So maybe it's the best way to go. I still love the reels I made with the modified Akai which I'm currently recapping as well.

 

[skywaveTDR]

Yes I have heard of him but to purposely work on stuff like that is not the best idea. Yes you will have a small audience but the talents if you have them are better used on real equipment not junk like 8 tracks. Broadcast cart machines were way better but only stereo and a cue track. These worked better and were double the speed. Still people stay with those crummy 8 tracks.

 

[kfbkfb]

Maybe the needledrop model would be useful - use a flat, very low noise amplifier connected to the tape playback head and then to a PC audio input and do the NAB playback EQ in software.


Kirk Bayne

 

[MidiMagic]

Note that the tape preamp must also be matched to the characteristics of the tape head.

 

[winopener]

Yes I have heard of him but to purposely work on stuff like that is not the best idea. Yes you will have a small audience but the talents if you have them are better used on real equipment not junk like 8 tracks.

True, nostalgia effect has its toll , but for quad releases many were quad 8 track only, so it is the only real source available, and this not counting the ones that had a matrix lp counterpart, so the discreteness when decoded was something ho-hum.

 

[skywaveTDR]

Kirk might have a good solution but it has to be tried.