Marantz 7 clone tape head preamp

[JohnP]

I agree! The RCA commercial tapes from the late fifties are fantastic I’ve compared some of. them to some of the 15ips tapes I bought from Acoustic Sounds. I was hard pressed to hear any difference between them. I also like the Everest commercial tapes from that period. Definitely a bargain.
John

 

[Atmasphere]

Mercury bought Everest's tape machine after Everest realized doing independent recordings could be kind of expensive. Mercury discovered an EQ error in that machine (the one that used 35mm tractor recording tape) which resulted in roll off below 100Hz; 6dB/octave IIRC. This was a problem on their LPs so I am assuming their tapes had this issue too. Mercury fixed that problem which is why they had 35mm recordings that had plenty of bass. A bass control could be really handy...

 

[JohnP]

Mercury bought Everest's tape machine after Everest realized doing independent recordings could be kind of expensive. Mercury discovered an EQ error in that machine (the one that used 35mm tractor recording tape) which resulted in roll off below 100Hz; 6dB/octave IIRC. This was a problem on their LPs so I am assuming their tapes had this issue too. Mercury fixed that problem which is why they had 35mm recordings that had plenty of bass. A bass control could be really handy...

That’s interesting. I use an EQ for all my recordings so I can correct problems like this when I hear them.

 

[U47]

God of Tape(Foxbat) inspired me to hook my my 1959 Marantz 7(re-capped with Arizona caps) to an old 5050 B2 with stock heads. Have a bit of hum, when the 5050 going into my Wright phono stage was quiet as a mouse.
Playing an old tape I picked up last month. Unknown group from 1964, recorded in Hartford.
King/Cello will hopefully check this out later. Have not thrown a test tape on yet. Marantz 7 has high frequency trim pots on rear. Bass is quite heavy at present.

Test video about 3 minutes after hooking it up-

 

[hannemanpl]

Work on my version has been completed. The casing is temporary because I didn't want the preamp to pick up interference. Once I've played around with it a bit and decided that I like the project and the sound, I'll put something better together and I also plan to add a VU meter for a nice look.

I have a question about the gain. In my opinion, it is a bit too low. I used a 330 ohm resistor instead of a 510 ohm one. Can I use, for example, a 270 or 220 ohm resistor? Will this result in higher gain?



Additionally, I wanted to ask @Foxbat about the measurements, how did you do them? I tried to use RMAA, but the signal is too distorted and the results are nonsense.

 

[Foxbat]

I mentioned earlier that the gain is on the low side, still usable with the majority of tape heads, but don't expect miracles.

Reducing that resistor will push the gain closer to the open loop number, and will compress the LF response as result. You can fight this to some degree by putting in selected higher gain tubes, but not by much, and it is not really necessary.

I calibrate my preamps first on the Audio Precision system where I set the curves based on typical calibration values. After that it usually does not need to be calibrated in the real system with the tape head, as the result is usually very close - I typically see the less than 1dB residual error, which is already fine, but you can further tune it out with the test tape.

Here is the test plot - you can clearly see that the closed loop response is getting too close to the open loop one at LF, so I would not go any further.

The gain at 400Hz is 50dB, which is not too bad, actually.

PS. I love your caliper! I have one like that... actually, more than one... don't use them but keep them as memory. Very few people know how to use these.