Synthi Hi-fli EMS - What Is It?!

[slacker]

The phaser is the same basic design as things like the Phase 45 and Phase 90, but instead of using JFets as voltage controlled resistors it used the string of diodes instead.
The way the diodes act as a voltage controlled resistor is something like this, each diode drops about 0.6 volts across it and this is near enough constant for different currents. Mr Ohm tells us V = IR so if V is constant then as I increases R must decrease, so the resistance of the string of diodes is inversely proportional to the current through it, so by varying the current you vary the resistance.
This is used in a number of synth designs, like the Steiner Synthacon filter, the classic Moog filter does essentially the same thing but with transistors instead of diodes. In diystompbox land Tim Escobedo's Quick and Dirty VCFs use the same trick.

EDIT: R.G. has an article on how phasers work if you need more info http://www.geofex.com/Article_Folders/phasers/phase.html

[DougH]

In the Pink Floyd Live at Pompeii film there's a short clip of Gilmour playing with this device in the studio. I think he also mentions it in the dsotm "classic album" vid but I don't remember.

[Mark Hammer]

I did a bit more reading - "bow-string" refers to a string of 20 diodes connected to each all-pass filter, the amount of phase shifting is varied by the modulation waveforms applied to this string.

Edit - anyone know exactly how this works? As in how the diodes transfer the modulation to each stage?

Look at the PAiA lowpass filter here (courtesy of Jack Orman):

The diode pair in each section of this otherwise passive LP filter forms a sort of voltage-controlled resistance to ground that gets varied by the control voltage applied to the junction of C1/D3 and C2/D1.

What's not clear to me is the relationship between the number/type of diodes in these "bowstrings" and the nature/range of the sweep.

[Earthscum]

From screwing around a little with them, basically I was finding that I had to pull bias up to keep the diodes forward biased, and even then the sweep couldn't go below a certain threshold or all the diodes would turn off at the same time on the positive signal or negaitive, or both. It seemed to me that the longer the string of diodes you have, the more they cushion each other and keep from turning completely off, as well as the range seemed to increase. I only ran up to 18V, so I was limited to about 12 diodes, if I remember right. At 9V I couldn't get 4 in and get any kind of decent sweep without distortion on either end.

But, I must point out that I had less of a clue about what I was doing and observing at the time than I do now.

[Ronan]

Here's the schem...ooops I posted and didn't see page 2. Therefore edit and crawl away

[Nasse]

http://www.diystompboxes.com/smfforum/index.php?topic=94373.msg813848#msg813848

[bwanasonic]

I know Pete Cosey used one of these with Miles Davis. You can hear bits of it here and there on Agharta and Pangaea. I don't imagine that thing was very reliable to tour with.

[Jay]

Nice thread ...enjoyed it - it took me on a journey (of the best kind, ie where I don't even get off my sofa), first to the guy reproducing these ...and in turn to the EMS website (I can recommend it), there can't be many synth manafacturers based on a road such as this...

http://maps.google.co.uk/maps?q=TR2+4NW&hl=en-GB&ll=50.334395,-4.96331&spn=0.000027,0.022638&hnear=Truro+TR2+4NW,+United+Kingdom&gl=uk&t=m&z=16&vpsrc=0&layer=c&cbll=50.334377,-4.963162&panoid=Wsb_2R8HMaZr8_yjPos3ug&cbp=12,313.59,,0,6.14

I know where that is!  About 4 miles west of my parents house!  Never knew EMS were there.

[archstanton]

hey all, sorry for performing some crazy thread necromancy here but i'm considering a project along these lines and wondered if any of you folks had ever tried to build just the filter/mod section from the synthi as a standalone device or is it too tied in with the other elements to make that possible?

[digi2t]

Later EMS MK.II models, including the Dead End FX clone, incorporate a switch that isolates the phaser/modulation only. That bypasses the top boost, fuzz, sub-octave/growl, and envelope sections. So yes, it is possible.

Personally, while the phase/modulation alone is nice, it really shines when you have all that extra harmonic content and envelope working along side. It's analog synthy chewy goodness.

[archstanton]

interesting! the service manual is for an earlier iteration then as it doesn't seem to have that?

it would definitely be cool to build the whole thing at some point. i was thinking about doing just the mod part so i could chuck some of those 'lamb' sounds into my board in a bit more of a practical size. even with all those diodes i guess that might mean losing some of the funkier settings and potentially the ramps but that crazy vibrato might just be worth it!

[anotherjim]

I can remember the Hi-fli being strongly marketed in the UK, even to the extent of giving away a demo flexi disc with at least one of the popular monthly gear magazines. I don't remember if it was International Musician or Beat (we only had those two!).
I didn't think the demo disc quality did it any favours.

[digi2t]

I can remember the Hi-fli being strongly marketed in the UK, even to the extent of giving away a demo flexi disc with at least one of the popular monthly gear magazines. I don't remember if it was International Musician or Beat (we only had those two!).
I didn't think the demo disc quality did it any favours.

But I bet someone would pay top dollar for one of those flexi discs now. Maybe not to the point of putting a child through college, but a good long supply of burgers at the very least.