BBC Grampian Head Info

[markrob]

Hi All,

Does anybody have copies of tech info on the BBC Grampian type B1/AGU cuuterhead? I'm looking for drive spec's such as impedance, max drive power, current, freq resposonse data. Also info on any maintenance aspects such as oil type. Any diagrams or US/GB patents for this head. Any info on the history of this head design would be welcomed. I'm also interested in how to apply feedback (e.g. freq and phase compensation). I've done some analysis of the RA7 amp (have that schematic) and have some ideas on what to do. Other than feedback, what were the differences of the Grampian heads as they mutated to the different models over the years? Thanks in advance for any info.

Regards,

Mark

[Simon]

Thanks to Lewis D I found out that they were made around the corner from were I live in London in Camden...

[markrob]

Hi All,

Was able to find this link, which answers most of my questions.

http://www.technicalaudio.com/pdf/Gordon_Clark_folder/Grampian-Gotham_Audio_Developments_feedback_cutter_system.pdf

I see the head used Dow Corning 200 silicone oil for damping. Looking into this, I see that its available in many viscosities ranging from 10 to 60000 cts. Does anybody know the correct type of oil? Does this need to be changed from time to time?

I've been doing some analysis of the Jamaika Grampian pre-amp provided by Flo. Looks to be a good implenetation, but I notice that it has a +2db response peak vs. the standard RIAA curve at about 5 Khz. Also uses some odd ball component values. I have some slight component tweaks the tame this. I'll build this up in the next week or so and give it a try. If anybody is interested, let me know and I'll post the changes.

Mark

[Simon]

http://www.technicalaudio.com/pdf/Gordon_Clark_folder/

Also some great other pdf's good find

[motorino]

I have some slight component tweaks the tame this. I'll build this up in the next week or so and give it a try. If anybody is interested, let me know and I'll post the changes.

Mark

Yes please

I will remove the riaa-flat switch and I will put one of three circuits

The jamaikan its a good option but I think it sounds better the Grampian cutterhead

Mark, using some filters in the feedback?

Cheers

[markrob]

Hi,

As far as I can tell, adding filtering in the feedback path is a bad idea. It will add addtional phase shift which would tend to make the loop unstable. Testing the head on the bench, I don't see any major phase shift from the head drive input to feedback output that would require any gain/phase compensation (e.g. lead - lag) to improve loop stability. You do need one dominant pole to place a limit on the closed loop bandwidth. I'll add a provision for this, but I don't think its needed. I was interested to see that the Grampian RA7 does have a small amount of lag - lead compensation in the forward path. This tends to lower the open loop gain at higher frequencies at the expense of some phase shift at lower frequencies. The Gotham amp shown in the link I posted does not appear to have this. I saw some posts here recommending using high order low pass filters on the head feedback output, but this seems just plain wrong. Any comments on this are much apreciated.

Mark

[flozki]

hmm. again
the grampian is not a feedback head. even if it says so.
at least not in the way other feedback heads work.

it has kind of a transformer. second coil. but second coil doesnt move.it is wound around the drive coil. and the moving iron is between acting as transformer core....

so compared to normal motion feedback heads. the 2nd coil is a mix of your motion and crossover through the drive coil. i never measured but i guess at high frequencies you have just the drive signal feeding into...
similar stuff they did for output transformers , for example in the early urei 1176 limiters.... to compensate low frequency roll off or distortion...

this is very important for a design.
so dont forget this. it can help.

[motorino]

Then, I understand that depending on the order of the filter to the anti RIAA correction, be mixed after the full phase of the signal feedback? We need a filter according to the specific properties of Grampian?

Excuse me but i dont understand

[markrob]

hmm. again
the grampian is not a feedback head. even if it says so.
at least not in the way other feedback heads work.

it has kind of a transformer. second coil. but second coil doesnt move.it is wound around the drive coil. and the moving iron is between acting as transformer core....

so compared to normal motion feedback heads. the 2nd coil is a mix of your motion and crossover through the drive coil. i never measured but i guess at high frequencies you have just the drive signal feeding into...
similar stuff they did for output transformers , for example in the early urei 1176 limiters.... to compensate low frequency roll off or distortion...

this is very important for a design.
so dont forget this. it can help.

Hi Flo,

That's my understanding as well. At high freq's you are really just seeing the feedthrough of the drive signal. I assume that the purpose of the feedback is to linearize the magnetic drive and resulting distortion/rolloff at low frequencies. But if you low pass filter this signal and then apply it to the feedback loop you could have stability problems if the total phase shift is too large.

Motorino,

The IRIAA is applied before the feedback, so its not in the feedback loop. I assumed you were referring to any filtering in the loop not external to it. I also assumed that you would adjust the IRIAA stage to conform to the published curve as close as possible and aply any special EQ to compensate for the cutter and/or taste elsewhere in the chain. Perhaps the designers of the Jamaika Pre tweaked the response to add a presence peak on purpose. Gave them the sound they were after.

Mark

[motorino]

hmm. again
the grampian is not a feedback head. even if it says so.
at least not in the way other feedback heads work.

it has kind of a transformer. second coil. but second coil doesnt move.it is wound around the drive coil. and the moving iron is between acting as transformer core....

so compared to normal motion feedback heads. the 2nd coil is a mix of your motion and crossover through the drive coil. i never measured but i guess at high frequencies you have just the drive signal feeding into...
similar stuff they did for output transformers , for example in the early urei 1176 limiters.... to compensate low frequency roll off or distortion...

this is very important for a design.
so dont forget this. it can help.

Motorino,

I assumed you were referring to any filtering in the loop not external to it.
Mark

No, i speak about made a "special order" iriia filter in conjuntion (phase) with the filter in the feedback loop, i believe here is the problem

[markrob]

No, i speak about made a "special order" iriia filter in conjuntion (phase) with the filter in the feedback loop, i believe here is the problem

Ok,

I understand now (sorry about being so dumb). You are looking for a for a filter to flatten the response of the Grampian head. I would leave that to software, if you are mastering using a digital system, or an external graphic or parametric EQ if you intend to stay in the analog domain. In the digital realm, I've been using Har-Bal to create a linear phase FFT based filter to do this.

http://www.har-bal.com/

I use the original mastered audio file as the reference and compare it to a recorded playback of my cut. Har-Bal will display the difference between the original file and the playback. I then use the EQ in Har-Bal to match the original and you've quickly compensated for any response problems in the cutter. Of course, you are also seeing the effect of the playback pickup, but hopefully, that is not a great source of error. You can then save this filter and re-use it on any audio file to pre-master before cutting. For $95 US, its a bargain (and I find it very useful as a mastering EQ in general).

I actually tried using Har-Bal to do the IRIAA, but it was not designed to create a filter with that much boost/cut, so it runs out of gas. So, if you use the Jamaika preamp with its analog IRIAA built in, you are already close. The Har-Bal filter needed is not so radical and will work fine.

Mark

[motorino]

You are looking for a for a filter to flatten the response of the Grampian head.
Mark

No exactly, to correct the response using an eq, I mean to calculate the filters work so well with each other without affecting the phase


The only way to do it is with a lpf in the feedback loop does not?

[Cutterwoller]

You should not really have to use eq's to flatten the response. My sytem cuts very flat and I use nothing to flatten the head response. I guess if people are using domestic amps and funny pre amps with out feedback and shit then you may need to. But, the head itself has a very good response when using the correct amplifier.

Lewis

[emorritt]

I have a copy of the original documentation for the Grampian and they recommend a silicon fluid of 500 centistokes. I bought a bottle (which as little as the head takes will last a lifetime) from Spectrum. It's about the consistency of cooking oil. Two or three drops is all that's needed.

[markrob]

I have a copy of the original documentation for the Grampian and they recommend a silicon fluid of 500 centistokes. I bought a bottle (which as little as the head takes will last a lifetime) from Spectrum. It's about the consistency of cooking oil. Two or three drops is all that's needed.

Hi,

Thanks so much for that info! Would it be possible to get a copy of your docs? I'd be more than willling to pay for that. PM me if you are interested.

Regards,

Mark