Topic: Making battery box and pre-amp for SP-CMC-8 (AT943?)

[graemecogger]

Hi,

I am about to buy the SP-CMC-8 with omni capsules and (as an ex-electronics engineer) intending to make a pre-amp that also supplies power in the manner of the SP battery boxes.  I'm fine with the electronics, but not sure what I need to design for.  If anyone can help, I'd appreciate it.

For the battery power I believe that all I need is to connect 9V via a resistor to each mic input line, the use a capacitor to the output.  Is that correct and, if so, does anyone know what resistance value works best?  I e-mailed the guys at SP, but they were a bit cagey about answering - I think they wanted me to buy their battery box

For the pre-amp, SP told me that it needed an input impedance of 10k.  I suspect that is just so the bass rolloff values on their battery box is correct, but I'll run with it.  Does anyone know what sort of gain I'll need to bring these mics up to line level?  I'll be using them for recording classical acoustic performances, either with the mics right in front of the stage or some distance back in the first few rows of seating.

Most of the pre's I see have gains of 20/40 and sometimes up to 60dB, but I'm not sure whether it's necessary to go that high and whether I'd sometimes need gains down to near 0.

Thanks

Graeme

[cybermansrev]

I'm not convinced you want to use the sound pros battery box as a design, these mics do overload at high spl and by all accounts work better with a 3 wire battery box (see http://taperssection.com/index.php?topic=49909.0). I'm currently playing around with a 3 wire battery box and pre amp design (based on poorlyconditioned drawings in the linked thread) but as someone with no experience of electonics would love to see what you come up with

[Weazel]

i like to hear some samples made with a 3 wire box.

[lordbelial]

Hi,

I am about to buy the SP-CMC-8 with omni capsules and (as an ex-electronics engineer) intending to make a pre-amp that also supplies power in the manner of the SP battery boxes.  I'm fine with the electronics, but not sure what I need to design for.  If anyone can help, I'd appreciate it.

For the battery power I believe that all I need is to connect 9V via a resistor to each mic input line, the use a capacitor to the output.  Is that correct and, if so, does anyone know what resistance value works best?  I e-mailed the guys at SP, but they were a bit cagey about answering - I think they wanted me to buy their battery box

For the pre-amp, SP told me that it needed an input impedance of 10k.  I suspect that is just so the bass rolloff values on their battery box is correct, but I'll run with it.  Does anyone know what sort of gain I'll need to bring these mics up to line level?  I'll be using them for recording classical acoustic performances, either with the mics right in front of the stage or some distance back in the first few rows of seating.

Most of the pre's I see have gains of 20/40 and sometimes up to 60dB, but I'm not sure whether it's necessary to go that high and whether I'd sometimes need gains down to near 0.

Thanks

Graeme

Welcome!

I'll keep noticed on this thread, I'm also interested in the design of a transformerless preamp for my CMC-4.

What recorder do you have? The line input impedace refers to this detail...

[cybermansrev]

What recorder do you have? The line input impedace refers to this detail...

I'm not so sure about this if he's running a battery box with variable bass roll off won't it be the impedance of the following pre-amp which determines the actual roll off not the recorders line in impedance (or a combination of both)?

[graemecogger]

cybermansrev - thanks for the thread link.  I did a search, but missed that one.  It seems a shame to chop the plugs off the SP mics, but if necessary...  I'll certainly post what I've done (it may be a long while before finish it!), although it might be a case of using components I already have rather than the easiest to buy.

lordbelial - I've just bought a Sony MZ-NH1 (Hi-MD recorder) and I'm waiting for it to arrive.  I was trying to determine the input impedance of the pre-amp I'll be designing, not the recorder I'll be driving with it.

How about noise levels - is it worth going for ultra low noise devices, or are the mics themselves noisy?.

[graemecogger]

What recorder do you have? The line input impedace refers to this detail...

I'm not so sure about this if he's running a battery box with variable bass roll off won't it be the impedance of the following pre-amp which determines the actual roll off not the recorders line in impedance (or a combination of both)?

Yup - the rolloff is determined by the capacitor in the battery box and the input impedance of the next stage (i.e. the pre-amp).  OK, the source impedance also matters, but should be fairly negligible.

[cybermansrev]

It seems a shame to chop the plugs off the SP mics, but if necessary...

It's only a 1/8" jack with the basic version and you can always reuse it (if you manage to scape the epoxy off sp uses). I would also shop around a bit and see what prices you can get for the mics elsewhere. I'm not really convinced that there's a whole lot of "matching" done for these mics certainly in hindsight my AT853's bought 4 yrs ago from Sp have always been 1.5 db different.
You may also want to look at getting other mic elements, as in some venues far from the performers you may want cardoids.

[graemecogger]

It seems a shame to chop the plugs off the SP mics, but if necessary...

It's only a 1/8" jack with the basic version and you can always reuse it (if you manage to scape the epoxy off sp uses). I would also shop around a bit and see what prices you can get for the mics elsewhere. I'm not really convinced that there's a whole lot of "matching" done for these mics certainly in hindsight my AT853's bought 4 yrs ago from Sp have always been 1.5 db different.
You may also want to look at getting other mic elements, as in some venues far from the performers you may want cardoids.

I'm hoping to be able to be able to record close up most of the time and, from the samples I've heard, these omnis are ideal for classical vocal work.  I'll look around for the capsules...

[Sanjay]

i like to hear some samples made with a 3 wire box.

I promise after this weekend i'll put one out for you.

[Weazel]

i like to hear some samples made with a 3 wire box.

I promise after this weekend i'll put one out for you.

thanks