Routing signal for Grampian D, better "A" or "

[motorino]

i have "The question"

as it is the best form to routing the signal? A or B? i believe its better A, but...

Please, help Misters

A



B

[cuttercollector]

I do not claim much expertise in setting up systems with feedback, so my opinion is just that. Take it for the little it is worth.

Choice "B" seems to make the most sense to me, though I philisophically question the need for a further eq in the feedback loop. The whole idea of feedback is a correction signal. It is trying to make the head produce an exact replica of the input signal, so other than inverting the phase and mixing it back in at the proper level there should be no other need to correct the eq on it. By inverting, you are subtracting the cutter error from the origional input signal. Any eq used there will have a reverse effect on the total system response. You could (theoretically) put an riaa playback eq and gainstage (AKA a phono preamp) in the feedback loop and have that be your record curve as it's effect would be "backwards" before the phase inverter and mixer.

Another way of saying this is that if you went with your diagram "A" I don't think you would need the RIAA record curve stage you have just before the cuttter power amp. And still no eq on the feedback loop. What you do to pre-process the signal before the entire cutter "system" is up to you.

[cuttercollector]

Some further thoughts.
Not to muddy the water too much on this, but some boxes are not phase coherant input to output - that is their phase is inverted between their input and ouput. Obviously this is an issue when ever you are dealing with something in the feedback loop. You need to not have an unintended phase inversion! Also any network that does not have flat frequency response tends to shift phase at some frequency. EQs obviously do this.
This can also be a problem in that your negative feedback can shift to positve at some (typically high) frequency due to phase shift and cause oscillations even at ultrasonic frequencies and blow up amps and cutters etc. So, I would say the less stuff you can have in the signal path with the least phase shift and flattest response, all the better. BTW, The feedback coil is just a coil. By that I mean it's output theoretically is balanced with respect to ground. This should allow the phase inversion to be done simply by connecting the coil in the right polarity and not having to run through another phase inverter "stage".

[JayDC]

so what your saying in a nutshell, well at least to me is:

Phase invertion is not to be handled by software, and should be accomplished by switching the + and - from the wiring of the output to the feedback coils. Just like you would do with a speaker if you wanted to invert the phase.

cool!..

[flozki]

hello

once again. the grampian type D is not a real Feedback cutterhead !!!

this is very important. it is more a transformer. the feedbackcoil is inside, or outside the drivecoil. and both coils are fix place. and the moving iron is the moving part.

therefore you will have a lot of crosstalk. almost only crosstalk from the drivecoil. 2 windings with an iron core build a perfect transformer.

so for grampian just get the low frequency singnals to feed back. all the rest brings nothing but trouble.
----------------------------------------------

for a real feedback (moving coil ..ortofon,westrex,neumann or for a capacitve feedback...fairchild) the schematics is more the b type.

preamp with riaa-deemphasis , ----> some phase correction network and then ------> inverse phase and ------> mix with input signal.

the good thing if you do riaa deemphasis at first stage is also that you can directly monitor the feedback signal...

flo

[JayDC]

ahh... The feedback is an input to the system, not and output. I thought that the main drive coils were in phase, and the feedback coil were out of phase to the output of the mixer. Kinda push/pull, to give more steady movement.

Now I see the feedback coil as a mic, and it give you a source of what the head is doing.

(edited )

[JayDC]

(edited)

[motorino]

Thanks friends

I am going to directly connect feedback to a pre one of the mackie, connected in xlr inverted., will use a direct out by db25 to take it pre mc phono and monitoring..

Thanks

[motorino]

ahh .. transformer, yes, but feedback no?

[vinnymeyer]

Drawing B is correct.

V.