Preamp for reel-to-reel

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#21
Those big repro reel2reel heads maybe have different characteristics to cassette heads, but nice info compiled there. I just did a sim run from 5mH to 200mH watching the last octave, plays for a dB or so and the variable input load compensates.

P.S. Gain is ~35dB
 
#23
That manual was about studio multitrack. It mentions that tube heads are high at 500mH but the modern replace heads for solid state or transformer input are low. For 270mH and 35k load the response is not affected in the JFET examples. For 100k load it picks up high range treble. They can be easily tweaked for 10-20kHz in high load. Do you have any info on measured source resistance also?
 
#25
"Do you have any info on measured source resistance also?"
Sorry Salas, no such info. I just picked up on the 5mH. I don't have any experience with transformer coupled PB heads. The output of such head must be very low.
Regards.
 
#27
That resistive part of the head's coil, yes. Guys, I had inserted 300 Ohm and 250mH in series to a voltage source in the sim with the pot and resistor load combination circa 35K Ohm. It does not lose it in the highs. Studeb, lets see where stands yours for mH too. Those circuits were an older exercise just based on curve data I could see floating on the web. I had used old Ampex papers for NAB. I thought I better post them ccts, this thread rang a bell, so not to remain as dusty files only. In any case I don't have the hardware opportunity to practice them. If anybody will ever be curious enough to try them in a reel 2 reel deck, only then we will know if and what they might need in tweaking. Studio units have enough EQ pots on them, proof its a tweak thing to get it right with each machine anyway.
 
#28
magnetoresistive PB-heads in R2R machines

Sorry for being off-topic, but do you know about the use of magnetoresistive PB-heads in reel-to-reel tape recorders? These have a higher output signal than induction heads and do not need IEC/NAB EQ. They were used e.g. in the RS-AZ 6 and 7 cassette tape recorders from Technics. The later has a direct head output.
 
#29
franzm said:
Sorry for being off-topic, but do you know about the use of magnetoresistive PB-heads in reel-to-reel tape recorders? These have a higher output signal than induction heads and do not need IEC/NAB EQ. They were used e.g. in the RS-AZ 6 and 7 cassette tape recorders from Technics. The later has a direct head output.
Click to expand...

Technics created these myths about MR heads in tape recorders. In reality: 1) The Signal-to-Noise ratio on these heads is not better than that of good conventional heads - amorphous alloy for instance. 2) MR heads need quite complicated EQ to provide a flat PB response in a tape deck. 3) The "direct head output" in the RS-AZ7 is just a marketing gimmick.

Even in cassette decks MR heads don't give any real advantage, as for R2R decks - the tape noise there is so high that the heads output and noise is not a problem whatsoever is any properly designed recorder. On the other hand MR heads won't last as long as conventional heads in a R2R.

Cheers

Alex
 
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#30
As on the thread topic. There are quite a few good circuits for PB amps. Even my Kora 3T phono stage circuit (MC version) can be easily adapted into an excellent PB EQ amplifier with gain about 70-72dB at 315 Hz which is enough for most PB heads. For R2R decks a resistive load to control the head/load capacitance resonance is usually enough so the design can be fairly simple. For a good cassette deck PB head (ferrite of amorphous alloy) a "virtually cooled" head load can give several dB improvement in the SNR. If there is enough interest I can show how to make a top quality PB EQ amp out of my phono stage. Otherwise a PB EQ amp from, for example, the Techncs RS-B965 is a good circuit to copy - the service manual is available on the Internet.

Cheers

Alex
 
#31
It would be good to find out just how good something like a Nak can sound with a modern PB amp - the heads are known to be top-notch, I wonder how much of an improvement an amp without Dolby IC's would sound - I understand Nak's used the reference IC's but I still think there is un-tapped potential there.

In a perfect world we could look at the record circuit as well but I think that would be a whole other kettle of fish...
 
#33
studeb said:
Head measurements:

GX-4000D:
387 Ohms 1.98H
376 Ohms 1.99H

GX-77:
235 Ohms .513H
238 Ohms .513H
230 Ohms .492H
235 Ohms .491H

All measured at the end of the pigtail.
Click to expand...

The measured inductances are far too high. I have a set of heads here from the Akai GX-4000D and the PB head (P4-241-Σ) measures about 270mH on both channels at 1kHz with DC resistance of the coils about 230 Ohm. What did you use to measure the heads?

Cheers

Alex
 
#40
studeb said:
Hi Alex,

B&K 875B

I can check again tonight, they seemed high, and the resistance is off from what i was expecting. Maybe the meter is out.
Click to expand...

You mean BK 875B? A hand-held one? It could be just not good enough to measure the heads. I am using the Racal-Dana 9343M and that is barely OK for that purpose. The best meter I know of to measure heads (and I had a chance to use it) is the HP4192A as it allows to measure coils at a very low AC voltage (down to 10mV) , avoiding the saturation, and at any frequency from 5Hz to 13MHz, both avoiding and finding resonances.

studeb said:
just the pigtail, i am measuring them at the disconnected connector.
all heads are installed
Click to expand...

That, at least with a good meter, should not be a problem, I do regularly measure heads together with connecting cables and it usually gives me only a small (within few percent) error.


Cheers

Alex
 
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