I scored a couple of Melcor cards off of evilbay some time ago and finally got them racked.
They were originally summing amps from a Surburban Sound board.
This presented some problems that I did not detect until I put it together.
The summing amp uses an inverted scheme, that is, the plus side of the opamp is grounded, not the minus side. So I had to reverse the phase on the output x-formers, no big deal. Otherwise, I would have had to either re-work the original board, or else make a brand new board.
I also had to put the gain control between the upper leg of the transformer and the minus input, as the plus side was already grounded via the pc board.
The summing amp cards have a 130 k feedback resistor instead of 20k or 47k, like the standard line and mic pre schematics floating around. More gain? You bet!
I find that there is a sweet spot as far as gain is concearned with these modules, and almost went as far as to fix the gain with a resistor, instead of using a pot.
You can really taylor the highend response with the cap on the feedback resistor. Some use 47 pf, others 220pf, etc.
Maybe a tone switch could be used here with different value caps?
I think I am gonna run it wide open with no cap, and limit the high end somewhere else, ie. Pultec eq. Better to have it and not need it, than need it and not have it. There is no oscillation, so what the heck.
Anyway, hewre are a few pics:
x
x
Thats a James Greenlee (remember him!) output transformer that I put in a mu can on the left.
Thats a relammed 80 percent Nickel on the right, which has one third the dcr but three times the inductance. I can hear no difference, but I have yet to see this project on the scope.
Kind of a sloppy layout (what else is new, cj!) , could have been made better with a custom PC Board, but I just wanted to get it done!
x
I am using an external pwr supply for low noise. The Jones strip is where the pwr wires are brought in. I roughed in the stock Zobel network, but noticed no difference with this particular set of input iron.
x
I took some windings off the stock bobbin to get the inductance down where it should be. This one clocked out at about 900 mH compared to abnout 350 mH for the stock x-former. Brass hardware, no varnish on the lams or coil, just some Kapton tape.
x
I kept the original lytics just to see how they sound. At this point, I see no need to change them. This circuit uses a capacitor output to the transformer. Some API circuits use direct coupled, some use a 47 ohm resistor across the cap, for what reason I do not know. Maybe a little dc in the output x-former for the funk?
x
Not a lot of bells and whistles, just an XLR in, 1/4 inch out, a phantom switch, and of course, no amplifier sounds good without the chicken heads!
x
x
Racked above a DIY Pultec and headphone amp. I figure why not wait til I get three or four projects done, then burn the Lazertran paper with as many decals as I can squeeze on there and label everything at once.
So how does it sound? Have not had time to do extesive listening tests, but my first impression is clean and quiet! A little too sterile for vocals, I would opt for my V76 for this purpose, but I bet it really kicks butt on instrument micing. I want to experiment with a pot that would go across the output cap instead of the 47 ohm resistor. This might give me a "funk" control, as it would vary the amount of dc going into the output x-former. What do you think?
Valley People micpre getting racked tomorrow!
Happy DIY! :grin:
They were originally summing amps from a Surburban Sound board.
This presented some problems that I did not detect until I put it together.
The summing amp uses an inverted scheme, that is, the plus side of the opamp is grounded, not the minus side. So I had to reverse the phase on the output x-formers, no big deal. Otherwise, I would have had to either re-work the original board, or else make a brand new board.
I also had to put the gain control between the upper leg of the transformer and the minus input, as the plus side was already grounded via the pc board.
The summing amp cards have a 130 k feedback resistor instead of 20k or 47k, like the standard line and mic pre schematics floating around. More gain? You bet!
I find that there is a sweet spot as far as gain is concearned with these modules, and almost went as far as to fix the gain with a resistor, instead of using a pot.
You can really taylor the highend response with the cap on the feedback resistor. Some use 47 pf, others 220pf, etc.
Maybe a tone switch could be used here with different value caps?
I think I am gonna run it wide open with no cap, and limit the high end somewhere else, ie. Pultec eq. Better to have it and not need it, than need it and not have it. There is no oscillation, so what the heck.
Anyway, hewre are a few pics:
x
x
Thats a James Greenlee (remember him!) output transformer that I put in a mu can on the left.
Thats a relammed 80 percent Nickel on the right, which has one third the dcr but three times the inductance. I can hear no difference, but I have yet to see this project on the scope.
Kind of a sloppy layout (what else is new, cj!) , could have been made better with a custom PC Board, but I just wanted to get it done!
x
I am using an external pwr supply for low noise. The Jones strip is where the pwr wires are brought in. I roughed in the stock Zobel network, but noticed no difference with this particular set of input iron.
x
I took some windings off the stock bobbin to get the inductance down where it should be. This one clocked out at about 900 mH compared to abnout 350 mH for the stock x-former. Brass hardware, no varnish on the lams or coil, just some Kapton tape.
x
I kept the original lytics just to see how they sound. At this point, I see no need to change them. This circuit uses a capacitor output to the transformer. Some API circuits use direct coupled, some use a 47 ohm resistor across the cap, for what reason I do not know. Maybe a little dc in the output x-former for the funk?
x
Not a lot of bells and whistles, just an XLR in, 1/4 inch out, a phantom switch, and of course, no amplifier sounds good without the chicken heads!
x
x
Racked above a DIY Pultec and headphone amp. I figure why not wait til I get three or four projects done, then burn the Lazertran paper with as many decals as I can squeeze on there and label everything at once.
So how does it sound? Have not had time to do extesive listening tests, but my first impression is clean and quiet! A little too sterile for vocals, I would opt for my V76 for this purpose, but I bet it really kicks butt on instrument micing. I want to experiment with a pot that would go across the output cap instead of the 47 ohm resistor. This might give me a "funk" control, as it would vary the amount of dc going into the output x-former. What do you think?
Valley People micpre getting racked tomorrow!
Happy DIY! :grin:
