vinyvamos
Well-known member
The purists and collectors will grit their teeth at my sacrilegious actions, but who's got an RCA OP-7 that they wish was more useful? 
Here are my simple (reversible!) mods which makes it into a 22dB per channel (measured), dual amplifier with gain and output level controls. Much more useful in the modern studio compared to it's original mono mic mixer role. Below are the original schematic and a schematic showing the mods. I have since also added the 470R & 82R resistors to O/P 1 so that it matches O/P 2 and both outputs are then loaded the same.
I am just putting each pair of channels in series and as RCA did, I am using the input trafos as output trafos straight after the attenuators. Now I can dial in a vast range of tubey harmonic overdrives with line level signals, or use it as a stereo low gain preamp. Rather than changing the old cannon sockets I will make adapter tails to give the correct gender XLRs for ins and outs.
anything more than 1Vpp on input seems to clip the first stage, so full +4 line level is too much for this box without input padding. With both attenuators fully open there is fully assymetrical clipping up until about 400mVpp input, which is giving a strong 2nd harmonic, with the 3rd following closely behind. After that the 3rd becomes dominant, giving a harsher distortion. I have yet to use it in a recording environment, but bench tests seem to be promising. I am expecting a rather quirky frequency response as these 900849 trafos gave me some surprises in my dual OP-6 build.
Let me know your thoughts, and bring on the slating of me hacking up a vintage classic from circa. 1949
EDIT: 22dB gain was measured with an unloaded channel IIRC, so I had better do more tests before confirming that. Original OP-7 has only 8dB gain from input to output so my config should give 16dB, no?
Here are my simple (reversible!) mods which makes it into a 22dB per channel (measured), dual amplifier with gain and output level controls. Much more useful in the modern studio compared to it's original mono mic mixer role. Below are the original schematic and a schematic showing the mods. I have since also added the 470R & 82R resistors to O/P 1 so that it matches O/P 2 and both outputs are then loaded the same. I am just putting each pair of channels in series and as RCA did, I am using the input trafos as output trafos straight after the attenuators
. Now I can dial in a vast range of tubey harmonic overdrives with line level signals, or use it as a stereo low gain preamp. Rather than changing the old cannon sockets I will make adapter tails to give the correct gender XLRs for ins and outs.anything more than 1Vpp on input seems to clip the first stage, so full +4 line level is too much for this box without input padding. With both attenuators fully open there is fully assymetrical clipping up until about 400mVpp input, which is giving a strong 2nd harmonic, with the 3rd following closely behind. After that the 3rd becomes dominant, giving a harsher distortion. I have yet to use it in a recording environment, but bench tests seem to be promising. I am expecting a rather quirky frequency response as these 900849 trafos gave me some surprises in my dual OP-6 build.
Let me know your thoughts, and bring on the slating of me hacking up a vintage classic from circa. 1949
EDIT: 22dB gain was measured with an unloaded channel IIRC, so I had better do more tests before confirming that. Original OP-7 has only 8dB gain from input to output so my config should give 16dB, no?
