Reel Tape Head Stage vs Phono Stage

well.. not to taking in account quality of the Technics RS1500/1506 playback amplifier, it is very easy to add proper IEC EQ (t list for 15IPS), not like RS1520 that changes only high frequency EQ, leaving 3180 uS as is. For the EQ switch, I found that mic sensitivity is a good device to use for EQ.
NAB / IEC EQ amplifier vs. RIAA amplifier.
Let's say we want to have output level similar to CD/DAC. With modern low inductivity playback heads we need to get about 50dB amplification at 1kH. In a way similar to many phono stages (EAR 834P for example).

EQ is very different. NAB (7.5 /15 IPS) has 2 pols 3180 and 50uS
RIAA has 3. 3180 / 318 /75
So, for RIAA additional amplification for EQ is +20 dB. 3180uS is 50Hz and 318 uS is 500Hz. With 6 dB/Octave, or 20 dB/decade we have -20 dB at 500 Hz compared to max elevation at 50 Hz, then flat plato and then another -6 db/octave from 2.2 kH
So, to get 50dB amplification @1kH for RIAA we need 70dB amplifier and EQ will eat 20 dB
NAB has no plato in the middle of EQ. -6dB/ octave goes from 50 Hz to 3k2 (50 uS). Compare d to RIAA we are -6dB at 1kH . So to get 50dB
@1kH amplification we need 76 dB amplifier.
CCIR /IEC at 15 IPS has no low frequency pole and only 35 uS high one. In theory CCIR calls for an infinity amplification at 0 Hz (DC). Well.. we can limit our EQ somewhere around 15-16 Hz. Lets say we select a pole at 3 x 3180 uS or 50 Hz / 3 = approximately 17 Hz.
So, to get low end of CCIR reasonably flat, we need another 10dB EQ, compared to NAB (and 16 dB compared to RIAA) so CCIR playback amplifier should have 86 dB amplification to get 50 dB @1kH.
Another word, to convert phono preamp to CCIR tape playback, we need phono with ~ 66 dB amplification @1kH and NOT the MC transformer based, due to completely different requirements for the input impedance. MC cartridges are happy with something like 100 ohm load, playback head needs somewhat like 100K.
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Tape playback EQ is actually defined based on the flux on the tape. The eq must have adjustment to compensate for the loss in the head and gap. And its different for eack speed. Without a reference tape you will be in the dark. Also a good tape playback system has a phase equalizer that corrects for the large phase shift through the system. Its not a simple tweak on a phono equalizer unfortunately. The early preamps from Dyna and MacIntosh had "tape eq" but not a very good implementation.
Postings like this reappear every few years. You CAN use a preamplifier initially designed for phono use for tape playback.

Both Levinson and Cello did this back in the day. Levinson used the same plug-in preamp modules in their (later) ML1 and LNP1 preamps and in the later plug-in Repro cards used in their ML5 (Studer A80). Cello took their P100/101 phono stage (used in their Audio Suite) and made the P603 tape module (for the AS) that they sold a to a number of mastering/recording studios.

A tape head looks electrically pretty much like a MM cartridge as regards impedance and output. Most repro heads in transistor machines were/are around 100-200mH - some as low as 30-50 - higher (up to 600) in the older tube machines. Cello repackaged the P100 module in a separate box as the "Remote Phono" stage that included a 40/50/60dB gain switch to accommodate MM/MC cartridges - along with appropriate loading.

Having made dozens of tape/phono preamps based on the P100/603 design mainly for use in "outboard" tape preamps, I've found that a little over 50 db gain is all that's necessary to get the output of most repro heads up to around 0dB - plenty for most prerecorded tapes on the market today. For tape, The multi-pole RIAA network can be replaced (switched-out?) by a series R/C network of around 15nf and a 5K HF adjust potentiometer - 5K allows adjustment for either NAB or IEC high frequency EQ. An additional R of 150 to 200K needs to be switched in across the cap to compensate for the low frequency NAB pole. The parallel R can also be adjusted to reduce head bumps on "inferior" playback heads. DO have to select an input loading parallel R/C to best match the PB head being used - 50 to 100K in parallel with around 100p seems to do the job.
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Tape playback EQ is actually defined based on the flux on the tape.
If playback EQ follows the standard, with modern heads (including stock RS-1500 /1506) it will have the accurate result. It is practically tested with number of heads and MRL 21J203.
BTW, RS-1500 /1520 does EQ the most primitive way. It has 3180/100uS in the NF loop (3 3/4 IPS) and for 7.5 and 15 IPS adds another passive RC to compensate HF and get close to 50uS. RS-1520 for CCIR EQ @15 IPS adds another passive RC to get close to 35uS and did not change 3180. So CCIR has -3db @50hz and -6dB @25Hz. The only EQ adjustment is the 100uS setting in NF.
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What I've ended up doing is building the NAB -> IEC equalization network described here:
and added a B1 buffer in front of it, to deal with the impedence mismatch. (The RS-1500 has output impedence "less than 3k ohms", so above the 600 ohms limit for that circuit.) I haven't yet been able to connect it all up for lack of time, but it'll at least allow me to listen to IEC tapes, and I would guess it should be fairly neutral otherwise. I'll report back when I've had a chance to try it out.
The complex internal heads/preamp connections of that RS-1500US include complicated switching along with precise preamplification, and as I remember, are highly involved and difficult to even get to.
The wiring from the selector switch (for 2 track/ 4 track playback) to the decks's main amp board terminates with a 6 pin single-row connector. This is just one of multiple connectors for the various circuit boards in the deck, which are labelled with letters or numbers. In this case, it is connector V, and no particular problem to access.

So bypassing the amp board by disconnecting the wiring at the input end should not be difficult. It can just be unplugged. To then re-reroute the signal directly to RCA output would take some extra adaptation. It should be possible to mate connector V (female) to wiring terminated with a male connector so that the main board can be plugged and unplugged as desired. From the schematic, it looks as if the six wires for V are the left and right channel with the third pin of each shorted to the respective second pin. Maybe someone can explain it better:

RS1500 heads out.jpg

Using connectors (some would have to be added), it should be possible, without major surgery to the deck, to combine direct wiring to an outboard amp with the capability to revert everything when needed. With the number of in/out connections on the back panel, another option is to leave the original wiring to the output RCA connected, but rewire another of the RCA terminals so that it connects directly to the INPUT of the deck's main amp board. Then an RCA interconnect, connected when needed between the two sets of RCA terminals, would return the "outboard" signal to the main amp board. The purpose of doing this is to retain the capability to use the internal amp, which can be done by plugging/unplugging an interconnect on the outside of the the deck rather than having to open everything up.

But in any case, it seems that the use of connectors in the Technics makes it possible to revert things fairly quickly rather than having to make changes that are more permanent by soldering everything in.

By the way, in the discussion of this deck, it has been mentioned that they can go for upwards of $8000. That's chiefly applicable to J-corder decks, which are customized for look and charged accordingly. The internal work sounds fairly minimal: by some accounts at least, recapping is generally not done, while the prevailing opinion these days seems to be that the Technics is now prone to leaky caps and does need recapping. If for nothing else, to make it run properly in future.

I would say that those dolled-up RS1500s are carrying at least a $5000 premium, as $1000 to 2000 is a more realistic price to pay for you and me (and J-Corder, which has been actively buying up old decks for 10 or 15 years now, would have paid nothing like that for much of their own inventory). Most of the models that I see tend to the garish, and the exterior look is where almost all of the extra pricing goes. While it may be craftsmanship, it doesn't indicate the actual value of a Technics RS1500, which is far lower, and then might have a few hundred dollars added if fully serviced by a technician. Without heavy customization, there is no way to squeeze $8000, or even half that, out of a customer for one of these.
I'm just a senior professional serive tech with 40+ years in a shop.
I don't mess with modifications or butching of equipment like a Technics RS1500, I bring them back to original spec and reliability for the customer, and made good money doing it.
As far as I can tell, Technics went to a lot of trouble to perfect them, and the specs show that.
I'm just a senior professional serive tech with 40+ years in a shop.
I don't mess with modifications or butching of equipment like a Technics RS1500, I bring them back to original spec and reliability for the customer, and made good money doing it.

All very appropriate, and understandable from a tech's point of view. Though there'll never be full agreement among techs, of course.

The OP has already made a different choice, by the look of it, which is much in line with what can be expected on this site. That too is appropriate, given the name of this forum. There is plenty of modification of circuits discussed here.

While Technics may have perfected their design, the $8500 deck prices that you mentioned are the preserve of custom restorers who choose to turn an RS1500 into an exhibition piece. J-Corder is the best known and probably most prolific of them. They're much too bling for my eyes, a bit of a desecration, actually, and the price clearly reflects the customization work rather than the underlying price of the deck, the cost of adding a full servicing (which, other techs have suggested, those decks don't get), or the perfection of the original design, which we can actually acquire for about $6500 less. That customization is also a form of butchery, and once done, it will never be practical or affordable to return those units to their original understated aesthetic.
The rear photo I posted doesn't show all of the inside, but trust me, in order to access and service this deck, some major dissasembly and removal of components are required.
And that takes hours of work.... hours!
The massive capstan direct-drive motor and it's huge flywheel (pictured) needs removal in order to access wiring of the head block.
Boards must be moved too, to get to mechanical adjustments and microswitches.
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