I re-simulated my playback phase correction circuit since I realized a better way to select values.
One thing that is cool about this circuit is that you can create an RC network *second-order* all pass phase correction with a single triode section that only passes the HF content to be corrected. You can even throw in a couple db HF eq for low tape speeds, if desired. Everything else at lower frequencies is summed through a separate resistor - this is about as close as one can get to non-sonic signature signal processing and this topology is suitable for the input to the following stage in the same tube bulb which will have the peak unlimiting. Component sensitivity ought to be low enough with this topology to allow it to be used without special gain trimming.
When you don't want to use a shipload of ICs, you need to make fewer tubes do more. Btw, I never saw any reference to such a circuit which is reasonably understandable because most people don't consider phase correcting LP or tape response to the HF limit - I worked it out myself and proved its efficacy by prototyping.
One thing that is cool about this circuit is that you can create an RC network *second-order* all pass phase correction with a single triode section that only passes the HF content to be corrected. You can even throw in a couple db HF eq for low tape speeds, if desired. Everything else at lower frequencies is summed through a separate resistor - this is about as close as one can get to non-sonic signature signal processing and this topology is suitable for the input to the following stage in the same tube bulb which will have the peak unlimiting. Component sensitivity ought to be low enough with this topology to allow it to be used without special gain trimming.
When you don't want to use a shipload of ICs, you need to make fewer tubes do more. Btw, I never saw any reference to such a circuit which is reasonably understandable because most people don't consider phase correcting LP or tape response to the HF limit - I worked it out myself and proved its efficacy by prototyping.
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Hi -
I actually had that done for my Ampex 351 transports, particularly with the right guide, borrowing from the roller guide design of an old computer tape transport. Before that, when I was using EE tape (by this time, I had Saki glass ferrite heads in and the record bias pushed way up at 450 khz) , I was having some HF dropout issues apparently due to the oxide wearing at the fixed guides due to friction and possibly the tight radii. After I replaced the fixed guides with the rolling guides, this problem essentially disappeared. This was never a problem with standard tapes though in the Ampex 351, and the dropouts due to oxide shedding were mostly evident at 3 3/4 ips even with the EE tape. Putting roller guides on the Ampex 351 transport did not have any adverse effect that I could see.
Actually before I got the Ampex tube machines, I had bought a Revox A700 brand new, and while I really liked the way it handled tape, I wasn't very excited about the sound of its electronics - it used some fairly primitive op amps and modest quality caps, and being a consumer type deck, had many more controls - the quality of the front panel audio switches didn't impress me that much, either - they tended to drop out the sound if not perfectly centered. So I never seriously considered modifying it, and eventually traded it for an Ampex 300-4 channel. When I bought my first Ampex 351, it killed the A700 sound quality wise, even before any mods.
One thing I like about the B67 transports is that they come with rolling tape guides, so this is not an issue. There is one thing I did to both of mine in the head block that may help performance slightly. From what I could determine, the B67 headblock from the factory came with two ruby guides - one near the middle and one just as the tape exited, but I noticed that there was a mounting hole for a third ruby guide at the head block tape input, and so installed a ruby tape guide there - it may have brought audible stability up a little. I haven't been able to determine why Studer put it there, and have never seen where it was used in their standard product. The stock B67 PB electronics, tapped before the output amp, don't sound that bad for solid state, but I definitely want to replace them with tube circuitry.
I actually had that done for my Ampex 351 transports, particularly with the right guide, borrowing from the roller guide design of an old computer tape transport. Before that, when I was using EE tape (by this time, I had Saki glass ferrite heads in and the record bias pushed way up at 450 khz) , I was having some HF dropout issues apparently due to the oxide wearing at the fixed guides due to friction and possibly the tight radii. After I replaced the fixed guides with the rolling guides, this problem essentially disappeared. This was never a problem with standard tapes though in the Ampex 351, and the dropouts due to oxide shedding were mostly evident at 3 3/4 ips even with the EE tape. Putting roller guides on the Ampex 351 transport did not have any adverse effect that I could see.
Actually before I got the Ampex tube machines, I had bought a Revox A700 brand new, and while I really liked the way it handled tape, I wasn't very excited about the sound of its electronics - it used some fairly primitive op amps and modest quality caps, and being a consumer type deck, had many more controls - the quality of the front panel audio switches didn't impress me that much, either - they tended to drop out the sound if not perfectly centered. So I never seriously considered modifying it, and eventually traded it for an Ampex 300-4 channel. When I bought my first Ampex 351, it killed the A700 sound quality wise, even before any mods.
One thing I like about the B67 transports is that they come with rolling tape guides, so this is not an issue. There is one thing I did to both of mine in the head block that may help performance slightly. From what I could determine, the B67 headblock from the factory came with two ruby guides - one near the middle and one just as the tape exited, but I noticed that there was a mounting hole for a third ruby guide at the head block tape input, and so installed a ruby tape guide there - it may have brought audible stability up a little. I haven't been able to determine why Studer put it there, and have never seen where it was used in their standard product. The stock B67 PB electronics, tapped before the output amp, don't sound that bad for solid state, but I definitely want to replace them with tube circuitry.
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record/playback square wave photos
I found a couple scope photos I took back when I prototyped the phase correction circuit for a recorded and played back signal. At the time, I always had it set for one compensation which I dialed in at 7.5 ips, so it didn't do quite as good a job at 15 ips, but I plan to have individual compensations for each speed in the new version.
For comparison, I show the 15 ips record/playback square wave response of the Studer B67 as reviewed by Studio Sound when it was a new product. It can be seen how much less leading edge overshoot there is and much less horizontal section tilt to the square wave with the phase compensation in (to be fair, I did something at the low end also that helped the latter).
I found a couple scope photos I took back when I prototyped the phase correction circuit for a recorded and played back signal. At the time, I always had it set for one compensation which I dialed in at 7.5 ips, so it didn't do quite as good a job at 15 ips, but I plan to have individual compensations for each speed in the new version.
For comparison, I show the 15 ips record/playback square wave response of the Studer B67 as reviewed by Studio Sound when it was a new product. It can be seen how much less leading edge overshoot there is and much less horizontal section tilt to the square wave with the phase compensation in (to be fair, I did something at the low end also that helped the latter).
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Unfortunately, I've been letting B67 things slide a bit due to other priorities, but recently picked up a used B67 2-track headstack at a fairly low cost based on its photographs since the heads looked like Saki amorphous ferrite heads.
Guess what? When I got them, I could see that they were - the 'Saki' sticker on each one was the tip off
This head technology is still used in the duplicator business due to its long lifetime, and it was the technology that Studer evolved to in its latest hi-end tape recorders (I don't believe Studer ever developed an amorphous head option for the B67).
I'm pretty happy with this head stack. On top of the Saki heads, it has a flutter idler. The combination results in *much* better inner detail and more detailed imaging. And while the amount of actual usage on the Saki heads is hard to determine, there is almost no visible evidence of any head wear, which is a great improvement over the original Studer heads that have worn noticeably in the time I've used the recorder in playback, even though I haven't done much more than play my old tape library so far on this machine.
Guess what? When I got them, I could see that they were - the 'Saki' sticker on each one was the tip off
This head technology is still used in the duplicator business due to its long lifetime, and it was the technology that Studer evolved to in its latest hi-end tape recorders (I don't believe Studer ever developed an amorphous head option for the B67).
I'm pretty happy with this head stack. On top of the Saki heads, it has a flutter idler. The combination results in *much* better inner detail and more detailed imaging. And while the amount of actual usage on the Saki heads is hard to determine, there is almost no visible evidence of any head wear, which is a great improvement over the original Studer heads that have worn noticeably in the time I've used the recorder in playback, even though I haven't done much more than play my old tape library so far on this machine.
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Latest studer update - I added the third headguide to the left of the Saki equipped headstack and found that a spring that holds back one of the tape lift arms had slipped off its footing some time in the past (before I owned it) so that the lifter was always dragging on the tape in play or record. When I restored the spring mounting and listened to it, a great deal of the original subleties of my recordings were restored through the B67. However, I also noticed that the record head for this stack was vertically misaligned, so once I get the electronics rebuilt in tubes, re-aligning the heads will be my next priority.
Moving on from Studer, I am getting well along in putting together my own Ampex 440 based tubed 8 track machine using 354 electronics. My plan is to add silent fan cooling to allow all 4 354's to be adjacently rack mounted. I plan to keep the whole machine on the original 440 rollaround cart but extend the electronics side panels to accommodate all 4 sets of electronics.
To keep this project manageable time wise, I will limit the electronic mods somewhat, but they will include replacing all the original HV filter 'lytics with polypropylene caps in the 354 electronics. The PB head is being fabricated by Flux Magnetics as one of their super bass extension types good to 20 hz at 30 ips. I'm thinking of putting simul-sync functions in the 354 electronics themselves.
Moving on from Studer, I am getting well along in putting together my own Ampex 440 based tubed 8 track machine using 354 electronics. My plan is to add silent fan cooling to allow all 4 354's to be adjacently rack mounted. I plan to keep the whole machine on the original 440 rollaround cart but extend the electronics side panels to accommodate all 4 sets of electronics.
To keep this project manageable time wise, I will limit the electronic mods somewhat, but they will include replacing all the original HV filter 'lytics with polypropylene caps in the 354 electronics. The PB head is being fabricated by Flux Magnetics as one of their super bass extension types good to 20 hz at 30 ips. I'm thinking of putting simul-sync functions in the 354 electronics themselves.
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B67 Protection TDA1000?
Hi,
A very interesting thread!!
I own an A700 with a similar capstan speed control as your B67 Mk1.
Could you please upload a (handdrawn) schematic how you actually have protected the TDA1000 by 47Ohm series resistor in the I/O lines and TSV's ? (can you please quote a type number for the TSV's)??
Thanks very much!
Hi,
A very interesting thread!!
I own an A700 with a similar capstan speed control as your B67 Mk1.
Could you please upload a (handdrawn) schematic how you actually have protected the TDA1000 by 47Ohm series resistor in the I/O lines and TSV's ? (can you please quote a type number for the TSV's)??
Thanks very much!
A few things you probably know. The torque might change with lifter position, they may act as tape tension sensors ( so no tape = no tension= max torque). You will need alignment tapes from "Mr Weber" to align your heads and playback electronics. Your noise level will change with bias level which should be adjusted for different kinds of tape. Popular way to do this is record 30hz and adjust bias till minimum noise.
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