Had some fun over the last week or two. This is what I've been up to:
At a glance it looks like an old EV velocity ribbon mic, but hold on! What's that glinting away under the hood? Looks like gold....
Undress it a little - there's a tube in there - and a lundahl!
Very simple circuit with our favourite small tube. Black electro caps are for local B+ and heater supply filtering. NOS Mullard 'Mustards' metal&film for the capacitors in the audio chain. There is no cap between the capsule and grid. I pulled inspiration from royer, gyraf G7 and a few other places. Note the star ground in the middle.
Power supply is slightly unusual. There are switches to vary the capsule polarisation voltage (30-60V in 10 V steps) and (regulated) heater voltage (5 - 6V) for experiments. Looks a bit crazy with huge ugly power supply trannies and caps - one of the rules for this build was that I could only use things that I had in the cupboard or shed (can't afford to buy bits at the moment) - this is all recycled or leftover stuff. The two transformers step the AC down to 12 V for the heater circuit and then back up to 120V for the B+ etc - just like the gyraf method.
The polarisation switch was based on an idea from Gus a while back. It is a pretty neat way of changing the sensitivity of the mic. It works well.
Mic and supply back together. The mic sounds pretty good to my ears actually - I'm pretty sure I can here the sound of the grill/body, but it seems 'musical' to my ears. I think the body and the mustard caps give it quite an unusual sonic character. And it is pretty quiet in terms of noise too.
I'll draw up some bits of the circuit and scan it soon.
NB: No microphones were hurt in this project - some twat had ripped out the inside of the EV mic a long time ago. I got it as a shell for some fun - this is the fun :grin:
OK!
Stewart
At a glance it looks like an old EV velocity ribbon mic, but hold on! What's that glinting away under the hood? Looks like gold....
Undress it a little - there's a tube in there - and a lundahl!
Very simple circuit with our favourite small tube. Black electro caps are for local B+ and heater supply filtering. NOS Mullard 'Mustards' metal&film for the capacitors in the audio chain. There is no cap between the capsule and grid. I pulled inspiration from royer, gyraf G7 and a few other places. Note the star ground in the middle.
Power supply is slightly unusual. There are switches to vary the capsule polarisation voltage (30-60V in 10 V steps) and (regulated) heater voltage (5 - 6V) for experiments. Looks a bit crazy with huge ugly power supply trannies and caps - one of the rules for this build was that I could only use things that I had in the cupboard or shed (can't afford to buy bits at the moment) - this is all recycled or leftover stuff. The two transformers step the AC down to 12 V for the heater circuit and then back up to 120V for the B+ etc - just like the gyraf method.
The polarisation switch was based on an idea from Gus a while back. It is a pretty neat way of changing the sensitivity of the mic. It works well.
Mic and supply back together. The mic sounds pretty good to my ears actually - I'm pretty sure I can here the sound of the grill/body, but it seems 'musical' to my ears. I think the body and the mustard caps give it quite an unusual sonic character. And it is pretty quiet in terms of noise too.
I'll draw up some bits of the circuit and scan it soon.
NB: No microphones were hurt in this project - some twat had ripped out the inside of the EV mic a long time ago. I got it as a shell for some fun - this is the fun :grin:
OK!
Stewart
