HI I haven't been here for while. I've been playing the amps I've built, and sailing! Not around the world, just obsessively locally in small wooden boats.
I used to do the Rod Childers mic preamp conversion on the Akai/Roberts 770 997 units etc.
It produces usable and fun results.
My friend has asked me to build several for his lowfi studio. He has a few roberts units with bad rubber and mechanical probelms that he doesn't plan to use as reel to reels.
As my other employment is completely rebuilding the dock where I live this winter, some warm tube amp work inside which will get me back up to speed seems good as I have some other builds I'd like to try as well.
I'm making a version like the schematic I'll attach, except with shure M68 input transformers on both channels and no out level pots. THere will likely be no or just one high Z unbalanced input and I plan to add phase switches to each channel. I will try to do my own version of the schematic to help make this clearer.
My current questions are 3:
1- is about layout. The XLR ins for the units will be mounted on a panel on the back of the box they'll live in.
I'm going to add the phase switch to each channel, pre input transformer.
It seems to me sensible to mount the phase switch, the input transformer and the XLR ins all together on the back panel. (in the pictures the clips would be flipped 180 and the switches would be beside each XLR jack with the transformers mounted slightly above)
Then the wires to the switch and from the transformer can be very short and there will be shielded XLR cable running from the transformer to the preamp.
On previous versions with XLR inputs on the face of the units. I extended the transformer wires about an inch, twisted them well and mounted them to the chassis of the unit itself. In this case the phase switch would be on the faceplate too. The shielded mic cable would be the same but the twisted wires from the transformer would be longer and running through the chassis, across the board etc. (in the picture of the chassis you can see the two transformers with their extended wires poking up past the board)
Not sure if either version is better , or if it will matter that much.
2- My other question is if there's a way to tweak the EF86 Based preamp to give it more gain? (plate/cathode resistors etc. magical smoke?)
3- The schematic shows a .02 coupling cap(decoupling?) between the two halves of the 12AX7. I'm thinking a bigger value would be better for a microphone preamp to pass more bottom end. Good idea? THE EF86 has a .22 I'm thinking of at least a .1
Hope this all makes sense, Thanks in advance.
Topic: Roberts/Akai mic preamp build questions (Read 1714 times)