Seems to me that the Op Amp version would need two coupling caps at the input, one on the signal side and one on the ground side to the + input of the opamp. Then the two 470K resistors would hold the + input at 1/2 of the supply voltage. Sound right to you?
Yes it does,that is the way I have it drawn but using one input capacitor.
Using two may be a sure method stop any DC current from flowing through tapeheads.
There is supposed to be a ground symbol at the end of the two 470k resistors and it must have been cut off when I cropped the picture for posting.
jer
Using two may be a sure method stop any DC current from flowing through tapeheads.
There is supposed to be a ground symbol at the end of the two 470k resistors and it must have been cut off when I cropped the picture for posting.
jer
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I am about 1/2 the way to completing the opamp circuit. FWIW I did replace the transistors in the original circuit and there was no effect on the noise floor. So the original design is inherently noisy (the resistors have been replaced as I mentioned earlier). Its my opinion that the designer chose an operating point at the input stage which causes the transistor to be noisier than it should be.
can we see some photos? I only recently heard of the Mellotron after listening to a radio show:
RN Into The Music - 11 June 2011 - Rock and the New - Pt 2 - The Progs
and then reading the excellent wikipedia entry.
Mellotron - Wikipedia, the free encyclopedia
I think you are very lucky to own one.
col.
RN Into The Music - 11 June 2011 - Rock and the New - Pt 2 - The Progs
and then reading the excellent wikipedia entry.
Mellotron - Wikipedia, the free encyclopedia
I think you are very lucky to own one.
col.
If you want to hear this Mellotron it is used in a recording that is available as a podcast, see http://www.reviler.org/2011/05/12/reviler-podcast-2-thunderbolt-pagoda/ This recording was done live in one take.
I built up the preamp but in test it seems to boost highs excessively. I have triple checked all my connections and so far have not found any errors.
In the meantime I decided that my troubleshooting on the original tape equalizer was for the birds- and I developed a theory that maybe the 2nd gain stage was where the noise was showing up. I hand-picked an MPS2222 (npn) for the job and installed it. The result is about 8 db less noise, so it does appear that the second stage of gain was the problem.
In the meantime I am still checking the opamp circuit for bugs...
I built up the preamp but in test it seems to boost highs excessively. I have triple checked all my connections and so far have not found any errors.
In the meantime I decided that my troubleshooting on the original tape equalizer was for the birds- and I developed a theory that maybe the 2nd gain stage was where the noise was showing up. I hand-picked an MPS2222 (npn) for the job and installed it. The result is about 8 db less noise, so it does appear that the second stage of gain was the problem.
In the meantime I am still checking the opamp circuit for bugs...
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It has been a while since I have seen this thread and I am glad to see you have made some progress by swapping to a lower noise transistor.
I had excpected the opamp circuit to be a little brighter sounding according to the sims.
It is very diffucult to tune it in the simulation with out knowing the actual tape head systems charicteristics,But was a good try for a basis to start on.
I still have the files if you would like me to try any other component changes sometime.
Otherwise for it sounding to bright how did it work out in the noise department vs gain ?
And which opamp did you end up using?
jer
I had excpected the opamp circuit to be a little brighter sounding according to the sims.
It is very diffucult to tune it in the simulation with out knowing the actual tape head systems charicteristics,But was a good try for a basis to start on.
I still have the files if you would like me to try any other component changes sometime.
Otherwise for it sounding to bright how did it work out in the noise department vs gain ?
And which opamp did you end up using?
jer
I used a fairly basic opamp, the TL082. If I could work out the EQ I think the noise would be fine.
Basically how this circuit behaved was that there was a rising response rather than a falling response at high frequencies.
The Mellotron has a second amp board which is the line amplifier. It strikes me that if I allowed for a circuit board that is the same size as both boards in the Mellotron, that a single opamp could do all the EQ/amp chores.
The sound of the instrument has more to do with the tapes than it does with the electronics so this might not be a bad move...
Basically how this circuit behaved was that there was a rising response rather than a falling response at high frequencies.
The Mellotron has a second amp board which is the line amplifier. It strikes me that if I allowed for a circuit board that is the same size as both boards in the Mellotron, that a single opamp could do all the EQ/amp chores.
The sound of the instrument has more to do with the tapes than it does with the electronics so this might not be a bad move...
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