weird feedback coil?

[sameal]

So i got this RCA head with a weird coil and what looks to be a reed switch behind the armature dampening. This looks to be some kind of homebrew.

Anybody seen anything like this? Know what it is?

[markrob]

Hi,

I'm wondering if it is a heater and thermostat combination to keep the damping material at a constant durometer so that is better controls the resonance of the head. Just a guess.

Mark

[sameal]

Hi,

I'm wondering if it is a heater and thermostat combination to keep the damping material at a constant durometer so that is better controls the resonance of the head. Just a guess.

Mark

That makes more sense then a feedback coil.

I wish they built it a little better. I went to solder new wires to it and it basically crumbled. The wires were soldered between a pad on the phenolic wall and the device in the middle.

So the reed switch would be the thermostat. I wonder what voltage to feed it? I'm guessing ac.

[markrob]

Hi,

AC or DC should not matter for a heater. As far as voltage goes, I'd measure the coil resistance and bring it up on a Variac slowly to see if that is really a heater an if the switch is a thermostat. You could also try heating the "thermostat" with a heat gun or hair dryer and monitor the contacts to see if they open at a reasonable temperate before trying to run current through the coil. TBH, don't think I would use it.

Mark

[sameal]

I was thinking the same thing. Not only are the other RCA heads pretty good and this one probably the same, i think itd be a better idea to swap dampening materials for a desired effect rather then heat the dampening material.

I still play around though see if this is a heater

[mrd]

No, but it did remind me I saw someone make some kind of feedback using a tape head on an RCA:

[YouTube2]https://www.youtube.com/watch?v=jrndjNVyGmU[/YouTube2]

[markrob]

Hi,

Very cool idea. Eddie Celetti is a legendary audio tech guy. I like they way he is testing to make sure he is picking up the motion of the stylus rather than the drive current into the coil.

Mark

[sameal]

I like the idea, but id have to find a REALLY small tape head. Maybe one of those old dictation pocket recorders

[sameal]

Well, I got one on the way.

Now, is it as simple as summing the signal back?

[markrob]

Hi,

If you get it to null out the drive coil over the entire frequency range and are able to prove that it is really picking up the armature motion like in the video, you will need to run an open loop frequency and phase plot to determine if you can just sum it or if you will need some form of loop compensation. Moving coil heads if well designed, can get by with simple summing or perhaps some small amount of loop compensation. Not sure about a moving iron head as it's velocity response should be flat up to resonance. I'm thinking you will need to do some compensation.

One check I would do would be to compare the feedback coil response to the head cutting response to see if they match. I would run a white or pink noise cut (depending on your analyzer type) with a full IRIAA curve in place. Capture the feedback coil to an audio file at the same time as you cut. Playback and capture the cut and compare the two files on a FFT or RTA analyzer. They should match if the feedback is tracking the stylus motion.

Mark

[sameal]

Hi,

If you get it to null out the drive coil over the entire frequency range and are able to prove that it is really picking up the armature motion like in the video, you will need to run an open loop frequency and phase plot to determine if you can just sum it or if you will need some form of loop compensation. Moving coil heads if well designed, can get by with simple summing or perhaps some small amount of loop compensation. Not sure about a moving iron head as it's velocity response should be flat up to resonance. I'm thinking you will need to do some compensation.

One check I would do would be to compare the feedback coil response to the head cutting response to see if they match. I would run a white or pink noise cut (depending on your analyzer type) with a full IRIAA curve in place. Capture the feedback coil to an audio file at the same time as you cut. Playback and capture the cut and compare the two files on a FFT or RTA analyzer. They should match if the feedback is tracking the stylus motion.

Mark

Is there a way I could plot it without cutting a blank? Like monitor it with an O-scope?

Cuz I am burning through blanks! I also found the worlds smallest tape head

[markrob]

Hi,

I'm thinking that you could remove the magnet from the head so that the stylus no longer moves when a signal is fed to the drive coil. If you monitor the feedback signal with an analyzer while feeding the head, you will only see magnetic leakage pickup from the the drive coil. If you adjust the pickup so that that this is minimized, then you should be able to use this as a baseline to compare to the feedback signal spectrum when the magnet is in place under the same drive conditions. As long as you have about 10 db higher signal with the magnet installed at all frequencies of interest, then you should be able to close the loop and know you are correcting motion rather than leakage. As an additional check, maybe try to physically clamp the stylus from moving with the magnet in place and see if you get a similar response.

Hope this makes some sense.

Mark