sbeach
Active member
Hi,
Here's a schematic for a mic preamp design I've been working on. It's not been built yet but I will try to put one "test" channel together on a breadboard to experiment & troubleshoot.
http://twin-x.com/groupdiy/displayimage.php?pid=200&fullsize=1
It has the Burr-Brown INA103 instrumentation opamp as the 1st gain stage (I have several free samples). This opamp is used as a sort of input transformer substitute which has gain adjustable from 0 to +30 db in 5 db steps. This approach will save $$ compared to an expensive input transformer & hopefully will be quite transparent sound-wise. I've heard that the Grace 801 preamp uses this same instrumentation amp so I expect it to be somewhat like the Grace's "straight wire with gain". Also, please assume that before the input there will be suitable 48v phantom power in place with 47 uF blocking caps. I purposely left that out to save space on this schematic.
The second stage is a simple class-A JFET amplifier with a JFET follower. This amp's drain voltage is set at 2/3 the supply voltage & is mean't to mimick a triode tube preamp stage - not unlike that found in a Fender guitar amp. The "boost / color" switch engages a source bypass cap which optionally should reduce all but 2nd order harmonic distortion and boost the stage's gain by 6 db. Hopefully, when driven hard, it should provide a smooth & sonorous tonal color. The simple source follower JFET gets it's DC blocked on the output to go to a 10k audio pot (front panel) before the final third amplification stage.
This third gain stage is structured so that it maximizes the available supply voltage & places the audio signal at near 1/2 the DC supply - much like the input to Scott Hampton's Hamptone design from his now-famous TapeOp article. This stage too has a source bypass cap that can be switched in or out. This gain stage meets the outside world through a buffer that can put out some serious current if need be. The output is left unbalanced without a transformer as this has worked very well for me in the past.
Yes, I know a lot of folks don't like coupling caps in the audio chain but my experience is that, if correctly biased & bypassed with polypropylene caps, coupling caps are nearly inaudible in most cases. However, I do think the fewer the better.
I expect this preamp to be able to suitably provide clear, clean gain for a classical recording approach. Optionally, by increasing the 1st stage gain & trimming back the output of the second gain stage with the 10k pot, it should be able to add a smooth to dirty & colorful tonal component to the signal. I could be wrong :sad: . It's happened before.
I have tried to choose the individual JFETS in each stage for appropriate input headroom & desired gain. My plan is to adjust the drain trimpots for each JFET to the chosen DC voltage & then replace the trimmers with regular resistors.
OK, now please tell me where I'm going wrong! Plus a few questions for you all ...
On the final stage would it be better to use a simpler, single-supply output buffer (maybe a single NPN transistor ie BD139) and lose the coupling caps between the gain JFET & buffer? Is the single 24 v supply feeding the two final stages capable of enough headroom for normal use? Are there resistor or capacitor values that should be re-thought? Is the whole aproach to gain structure balanced & flexible enough?
Please peruse the schematic & tell me your thoughts about this project. I truly welcome all of your opinions and pointers. Thanks,
Skip Beach
Here's a schematic for a mic preamp design I've been working on. It's not been built yet but I will try to put one "test" channel together on a breadboard to experiment & troubleshoot.
http://twin-x.com/groupdiy/displayimage.php?pid=200&fullsize=1
It has the Burr-Brown INA103 instrumentation opamp as the 1st gain stage (I have several free samples). This opamp is used as a sort of input transformer substitute which has gain adjustable from 0 to +30 db in 5 db steps. This approach will save $$ compared to an expensive input transformer & hopefully will be quite transparent sound-wise. I've heard that the Grace 801 preamp uses this same instrumentation amp so I expect it to be somewhat like the Grace's "straight wire with gain". Also, please assume that before the input there will be suitable 48v phantom power in place with 47 uF blocking caps. I purposely left that out to save space on this schematic.
The second stage is a simple class-A JFET amplifier with a JFET follower. This amp's drain voltage is set at 2/3 the supply voltage & is mean't to mimick a triode tube preamp stage - not unlike that found in a Fender guitar amp. The "boost / color" switch engages a source bypass cap which optionally should reduce all but 2nd order harmonic distortion and boost the stage's gain by 6 db. Hopefully, when driven hard, it should provide a smooth & sonorous tonal color. The simple source follower JFET gets it's DC blocked on the output to go to a 10k audio pot (front panel) before the final third amplification stage.
This third gain stage is structured so that it maximizes the available supply voltage & places the audio signal at near 1/2 the DC supply - much like the input to Scott Hampton's Hamptone design from his now-famous TapeOp article. This stage too has a source bypass cap that can be switched in or out. This gain stage meets the outside world through a buffer that can put out some serious current if need be. The output is left unbalanced without a transformer as this has worked very well for me in the past.
Yes, I know a lot of folks don't like coupling caps in the audio chain but my experience is that, if correctly biased & bypassed with polypropylene caps, coupling caps are nearly inaudible in most cases. However, I do think the fewer the better.
I expect this preamp to be able to suitably provide clear, clean gain for a classical recording approach. Optionally, by increasing the 1st stage gain & trimming back the output of the second gain stage with the 10k pot, it should be able to add a smooth to dirty & colorful tonal component to the signal. I could be wrong :sad: . It's happened before.
I have tried to choose the individual JFETS in each stage for appropriate input headroom & desired gain. My plan is to adjust the drain trimpots for each JFET to the chosen DC voltage & then replace the trimmers with regular resistors.
OK, now please tell me where I'm going wrong! Plus a few questions for you all ...
On the final stage would it be better to use a simpler, single-supply output buffer (maybe a single NPN transistor ie BD139) and lose the coupling caps between the gain JFET & buffer? Is the single 24 v supply feeding the two final stages capable of enough headroom for normal use? Are there resistor or capacitor values that should be re-thought? Is the whole aproach to gain structure balanced & flexible enough?
Please peruse the schematic & tell me your thoughts about this project. I truly welcome all of your opinions and pointers. Thanks,
Skip Beach