Mic Pre
- Thread starter analag
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Hey, I got this device breadboarded and its quite interesting. On the scope it bahavs very well. Even beyond 100 kHz. No traces of oscillations. I put a 10 ohm in series with the +-V rail and a elyt cap to ground .With a rail supply of +-18V I got -7V offset on the output! We really need to go up to +-24V for a good symm. swing. I tried lifting up R1/R7 to raise the offset but it became unlinear at the top freq. above about 50kHz.With R1/R7 at 10kohm the offset was nearly at 0V but the problem started even by mooving it from 1 kohm. Q1 is a 550, Q3 is 546 and Q2/Q4 is BD140. No listening tests so far but,
is there a sonic way to raise the offset a few volts?
I had a 2x50k revlog. pot (measured 40kohm) and with that the gain change was about 30 dB from CCV to CV. The C2/C4 had about -1,3V at the top so it doesnt need to be high DC Voltage type.
PS. as PRR pointed out the current is 1+10 mA per stage. DS
Cheers Bo
is there a sonic way to raise the offset a few volts?
I had a 2x50k revlog. pot (measured 40kohm) and with that the gain change was about 30 dB from CCV to CV. The C2/C4 had about -1,3V at the top so it doesnt need to be high DC Voltage type.
PS. as PRR pointed out the current is 1+10 mA per stage. DS
Cheers Bo
Like I posted I did not do the math I had to go somewere that was why the fast post with an error. The DC offset does not bother me. As abbey road d enfer posted the dual gain control might work "better" as a rotary switch. And maybe the gain could be set different at each stage at the same rotary setting.
As posted by others why a low input impedance? Might work well with say a sm57 but some transformer out microphones will have reduced gain due to the loading and the reflected reduced drain or plate loading. IIRC Paul S. has posted about loading tests with sm57s. I see no phantom supply so maybe you are designing for dynamics?
As posted by others why a low input impedance? Might work well with say a sm57 but some transformer out microphones will have reduced gain due to the loading and the reflected reduced drain or plate loading. IIRC Paul S. has posted about loading tests with sm57s. I see no phantom supply so maybe you are designing for dynamics?
Use a FET like the PN4393/2N4393 on the input and you can fix both input impedance and offset problems. Actually, you will still have a volt or so offset, but not seven...
clintrubber
Well-known member
With all respect and in all ignorance, what does this topology achieve what the pre-amp of the 'recent' BBC-thread doesn't ? It likely won't sound exactly the same, but that other one seems to have solved a few things already that are still a potential issue here.
Let's simply merge these threads, not ?
(Note that I'm deliberately provoking a bit here)
http://www.groupdiy.com/index.php?topic=4134.0
Bye,
Peter
Let's simply merge these threads, not ?
(Note that I'm deliberately provoking a bit here)
http://www.groupdiy.com/index.php?topic=4134.0
Bye,
Peter
> is there a sonic way to raise the offset a few volts?
Basic circuit-diddles. Add 27K from first emitter to -24V. That's crude, but this is NOT a "zero offset" circuit.
> Use a FET ... fix both input impedance and offset problems.
No.
Input impedance is set by the 3K32. Which IS clearly "user choice".... Q3 Q4 all works the same (pretty near) with 221K in the same place. Try R3=50K-100K for a nice 2K input.
The offset is "fixed". We need 0.7V across R1=1K. As Bovox found, omitting R1 gives poor HF performance and possibly higher THD... you want R1 to swamp Q2 input current nonlinearity. That 0.7V in 1K must all come up through Q1. It can only (as shown) come through R2=10K. So there is a semi-fixed 7V drop in R2. Changing to an FET won't help (well, 1% due to no Base current).
R1 (and R6) could be smaller. But if "much" smaller, it becomes a significant load on Q2+R4.
27K to -24V flows ~~0.86mA to Q1, to make our 0.7V across R1, without sucking through R2, and without much upset of the audio NFB network. 24V/27K slightly over-corrects, the output now sits a bit positive. You could go further, make the output clipping symmetrical, or less-far, for whatever reason Analag left it sitting negative. Don't get fussy because it will drift 20mV/deg C, and worse with rail drift or Q2 replacement.
Basic circuit-diddles. Add 27K from first emitter to -24V. That's crude, but this is NOT a "zero offset" circuit.
> Use a FET ... fix both input impedance and offset problems.
No.
Input impedance is set by the 3K32. Which IS clearly "user choice".... Q3 Q4 all works the same (pretty near) with 221K in the same place. Try R3=50K-100K for a nice 2K input.
The offset is "fixed". We need 0.7V across R1=1K. As Bovox found, omitting R1 gives poor HF performance and possibly higher THD... you want R1 to swamp Q2 input current nonlinearity. That 0.7V in 1K must all come up through Q1. It can only (as shown) come through R2=10K. So there is a semi-fixed 7V drop in R2. Changing to an FET won't help (well, 1% due to no Base current).
R1 (and R6) could be smaller. But if "much" smaller, it becomes a significant load on Q2+R4.
27K to -24V flows ~~0.86mA to Q1, to make our 0.7V across R1, without sucking through R2, and without much upset of the audio NFB network. 24V/27K slightly over-corrects, the output now sits a bit positive. You could go further, make the output clipping symmetrical, or less-far, for whatever reason Analag left it sitting negative. Don't get fussy because it will drift 20mV/deg C, and worse with rail drift or Q2 replacement.
I like the simplicity; should be easy enough to build on a piece of perfboard. I wonder if it wouldn't be worthwhile to adapt it to +48V instead of +/-24 to eliminate the need need for a separate phantom power supply.
I would agree that a higher input impedance would be preferable. 150 ohms will make some condensers distort at relatively low levels, and will result in treble and bass losses (as well as overall gain loss) with most dynamics. But as PRR said, that's "user choice".
I would agree that a higher input impedance would be preferable. 150 ohms will make some condensers distort at relatively low levels, and will result in treble and bass losses (as well as overall gain loss) with most dynamics. But as PRR said, that's "user choice".
3nity
Well-known member
You're right....Rotarys for exact recalling....but hey..Audio maintenance offers Dual 47K rev log....i think we have a winner part...
Jorge Aristondo.
mikep
Well-known member
Rossi said:
good idea. and if presented that way initially I betcha there wouldn't have even been an offset discussion. I think the existance of symetrical supples is what leads one to wonder why the audio is not centered around zero volts. nice circuit.
mike p
Thanks PRR, it solved the offset but...gain whent from unity til about 3dB per stage and current consumtion whent up about 50%. Of course ,offset 0V at the output gives the 1k, from the emitter of Q2/Q4, to -V. A loss of 18V on my testplate.
All things where stable. I even tried with 4 different output Tx. -No good, in my opinion.
I believe the output Z is to high.
-Otherwise is sounds good and there is no noise problems.I keep up with more tests.
Cheers Bo
All things where stable. I even tried with 4 different output Tx. -No good, in my opinion.
I believe the output Z is to high.
-Otherwise is sounds good and there is no noise problems.I keep up with more tests.
Cheers Bo
skipwave
Well-known member
Can I sub BD140 for the MJE253? Either one appears to be overkill in this app, which is a good thing in my mind.
