Hi!
This reared its head at the 'other place' so stop me if you've heard this one before.
Once upon a time I had a couple of little battery powered mic preamps that came as kits from Maplins back in the early-mid eighties. They had a crappy looking unshielded input transformer (that was prone to hum), aOP27 op amp, and unbalanced output. A trimpot set the gain. I lost one about ten years ago, but the other lived in my toolbox/bench for quick testing & troubleshooting jobs.
Earlier this year it stopped working. I opened it up and replaced the electrolytic caps with some decent ones (which fixed it), and whilst I was inside noticed that there was space for a decent transformer. I slipped in a Sowter 1:7 mic input transformer. I also changed the 'loading' resistor at the xformer secondary to 100K.
Here's the circuit (curtesy of someone - sorry but I lost details of who posted it when TT went down - thank you whoever you are):
After these changes I thought it actually sounded rather good and thought I'd try to make a stereo pair as a portable pre for doing minidisk recordings etc. I made some changes to the circuit based mostly on what I had in my parts box, and what I remembered from the suggestions at TT.
Also added a phantom supply powered by a stack of 9V batteries.
The changes were:
T1 Sowter 1:7
C1 omitted completely
C2 changed to 220pf
C3 Ployester not disc type
R1 100K not 18 K (n.b. I also tried 330K)
C5 output cap changed to non-polarised MKT poly type.
Gain trimmer now an external pot.
Phantom power added
Run off 2 x 9V battery for +/- 9V operation (the origineal was a single battery)
The unit is quiet and has more than enough gain, even for ribbon mics. The phantom supply works well. Unfortunately, I'm now a bit disppointed by the sound and it is duller & less detailed than I remember the 'prototype' being.
Really I'm asking you all if you any of the changes I've made may have caused unforeseen problems. Any suggestions?
Here's some pictures of the mostly-built stereo unit.
Thanks!
Stewart
This reared its head at the 'other place' so stop me if you've heard this one before.
Once upon a time I had a couple of little battery powered mic preamps that came as kits from Maplins back in the early-mid eighties. They had a crappy looking unshielded input transformer (that was prone to hum), aOP27 op amp, and unbalanced output. A trimpot set the gain. I lost one about ten years ago, but the other lived in my toolbox/bench for quick testing & troubleshooting jobs.
Earlier this year it stopped working. I opened it up and replaced the electrolytic caps with some decent ones (which fixed it), and whilst I was inside noticed that there was space for a decent transformer. I slipped in a Sowter 1:7 mic input transformer. I also changed the 'loading' resistor at the xformer secondary to 100K.
Here's the circuit (curtesy of someone - sorry but I lost details of who posted it when TT went down - thank you whoever you are):
After these changes I thought it actually sounded rather good and thought I'd try to make a stereo pair as a portable pre for doing minidisk recordings etc. I made some changes to the circuit based mostly on what I had in my parts box, and what I remembered from the suggestions at TT.
Also added a phantom supply powered by a stack of 9V batteries.
The changes were:
T1 Sowter 1:7
C1 omitted completely
C2 changed to 220pf
C3 Ployester not disc type
R1 100K not 18 K (n.b. I also tried 330K)
C5 output cap changed to non-polarised MKT poly type.
Gain trimmer now an external pot.
Phantom power added
Run off 2 x 9V battery for +/- 9V operation (the origineal was a single battery)
The unit is quiet and has more than enough gain, even for ribbon mics. The phantom supply works well. Unfortunately, I'm now a bit disppointed by the sound and it is duller & less detailed than I remember the 'prototype' being.
Really I'm asking you all if you any of the changes I've made may have caused unforeseen problems. Any suggestions?
Here's some pictures of the mostly-built stereo unit.
Thanks!
Stewart