Feedback heads

[emorritt]

Emoritt,

Could you post pics of your cutter head you built?

The only pic I took of the 'guts' was before I finished wiring everything, but the main board is complete. I did a "quick and dirty" version - didn't bother making a PCB. PS is at the top of the board and Flo's jamiaca circuit is at the bottom.

[JayDC]

off the top of your head, about how much did it cost?

[markrob]

Hi All,

Here is a link to a zip file with all of the info you should need to build up the summing amp. If there are any questions, feel free to ask.

Here is a link to some picts of the completed board.

http://home.comcast.net/~markrob1066/pwpimages/Grampian%20FB%20Summing%20Amp%20Top.JPG

http://home.comcast.net/~markrob1066/pwpimages/Grampian%20FB%20Summing%20Amp%20Bot.JPG

A link to the archive.

http://home.comcast.net/~markrob1066/docs/Preamp.zip

Mark

[emorritt]

Jay: I had the parts for the power supply on hand as well as most of the resistors and small caps; just had to buy the amp chips which I think was about $20 worth. If you had to buy everything (transformer, components, pilot lamp, switches, pots, enclosure, etc.) you would probably end up spending around $80. I buy practically all components that I use from Mouser. For all HV caps for vacuum tube work I use justradios.com - he's in Canada but I've never received bad components from him like from some vintage guitar amp suppliers in the states.

[flozki]

great.thanks.
put another double opamp for the riaa encoder in front or use the first opamp for that. and there is a nice standalone version...

f.

[petermontg]

Hi All,

Here is a link to a zip file with all of the info you should need to build up the summing amp. If there are any questions, feel free to ask.

Here is a link to some picts of the completed board.

http://home.comcast.net/~markrob1066/pwpimages/Grampian%20FB%20Summing%20Amp%20Top.JPG

http://home.comcast.net/~markrob1066/pwpimages/Grampian%20FB%20Summing%20Amp%20Bot.JPG

A link to the archive.

http://home.comcast.net/~markrob1066/docs/Preamp.zip

Mark

Thanks Mark,I shall try knock them out in the next few weeks.

[RVonse]

Hi,

I did do some experiments adding feedback to my Presto 1D head. In that attempt, I basically did what the Grampian does. I added a second winding that allowd me to close the loop around the magnetic flux generated in the head. So, the feedback tended to correct only for low frequency effects (mangetic saturation, and the L/R rollof of the head below 500 hz). I started to play with adding true motional feedback to the 1D, but never got very far. Instead, I've been gearing up to re-visit my DIY stereo head.

I've also been doing experiments with my Grampian head. I had been using it open loop and decided to see what feedback would do for it. I've discovered that you need to add some special circuitry to properly achieve the 20db of feedback that is specified by the manufacturer. Flo's simple Jamaika pre is missing some key elements that prevent this from happening. The original RA-7 or Gotham amplifiers had specific compensation built in to provide this. It was sold as a complete system. If you try to close the loop on Grampian head with a simple summing stage as the Jamaika pre does, you will not take full advantage of the head. You'll be lucky to achieve 5db of feedback and stability will be marginal. I plan to post my results soon.

One thing to keep in mind is that you have to have a very good understanding of the open loop gain and phase characteristics of your head before you attempt to close the loop. You want the open loop response to be free of secondary resonances and high frequency poles that cause exess roll-off and/or phase shift. Otherwise, you will have a very hard time closing the loop. The other major problem is finding a way to add sensing that is a true representation of the head's motion. It took some clever engineering by Neumann and others to add a sensing coil that does not pickup up crosstalk from the large magnetic fields present in the head. I've been looking at using the Fairchild RF method to avoid this. Finally, you need a firm grasp of the math and access to some measuring tools to get it right.

Mark

How does the Grampian head physically sense feedback? Do they actually measure physical movement of the stylus or is it just another winding fed off the primary signal path?

Seems to me that if heads were being made with today's technology they would probably incorporate some kind of optical feedback system thereby illiminate any sort of resonant issue.

Since I own a Presto 1-d I would be very interested in any more details you might share with the experiment you did on yours. Other than the feedback mechanicals, does a 1-d head look similar to the Grampian?

I think all of us would love to convert a cheap head to feedback if we could pull it off.

[markrob]

Hi,

The Grampian head does not use motional feedback. The feedback loop is around the magnetics. There is a feedback coil embeded within the drive winding. So the system looks very much like a transformer. When you close the loop around the head, the feedback corrects for non-linearities in the magnetic circuit only. One result of this is that the L/R time constant is eliminated (within the range that the feedback active). This has the effect of making the head response flat vs. velocity. Additionally, the feedback can correct for low frequency saturation effects to some exent. The back EMF that is generated by the motion of the stylus does have some second order affect on the amgnetics, but its small in this type of design. Its probably not a fair compairson to a true motional feedback head like the Neumanns.

When I did my experiments with my Presto 1C, I removed one of the drive coils and wound a few turns on the stylus armature. I hoping to get a bit more motional feedback than the Grampian design. While it did work, it was not a slam dunk winner. Removing the coil increased even order distortion (due to loss of push-pull action) and sensitivity.