The AM Forum

Is there a best mike to use? I saw in another post recommending an EV 638. I looked in the schematic - can't seem to find a mike gain control, although there is a clipper control. I assume the speech amp in the 500 is fixed gain. Seems to me it should be adjustable depending on the mike in use. Tnx in advance.

The quality of the schematic reprint is soooo bad - -anyway, I found R149 is the audio gain pot..... Even when blown up, the BAMA print quality is not that clear. I did a "D" size on the plotter at work, and it's not much better.

Depends a lot on what you want.

D-104 into MPF-102 is impossible to beat IMHO.

MPF-102 in place of the 12AX7 or as an external pre-amp?

The MPF-102 can be placed in the base of the mic. It is used as an impedance converter.

Alternately the first speech amp can have the grid resistor changed to a really high value like 5 megohms.

Dave: So, what ur saying is that the 1 meg input to the first 1/2 of the 12AX7 is too low for the D104 to be happy?

Yep, good ol' plain jane crystal D-104 is tough to beat. Used one for decades on the big rig until a couple years ago. Only changed it then because I was given a cool ribbon mic and wanted to play with it. I did the same thing, changed the 1 megohm grid resistor on the mic amp to 4.7 and voila - near perfect audio, or at least as good as I'll ever sound on air or in person.

It wants to work into a higher load. It'll work with 1 megohm, it'll just work/sound better at 5 or so. Make sure you have a good mic element, of course. If that's known to be good, you should be set with a simple resistor change, maybe a bypass cap or two.

I think it is on the right lower side behind something. Maybe a knob? I remember being shown the position of it but it was many years ago. Don't own one but would like to. Maybe someone will notice that you asked this question since it may have been overlooked?
OOPS, notice you found it, where was it?

Mic gain is lower left. Clipper control is on rear panel.

Lose the clipper and low pass filter if you want to sound human.

Mine is 50K impedance.  Way to low for a unpowered D104 mic.  If I plug the D104 into my stock/new J500, The audio is complete Sh*T.  Very tiny, raspy.  Just horrible.

I tried several mics and found the 638 to be great,  Smooth, clear, plenty of lows. I get great reports. 

Sure 444 also sounds good. Other high impedance dynamic mics.  The large 664 EV mic, that was common in the BC world back in the 50's sounds fantastic.  An old Turner is what Johnson sold with the Johnson name on it.

I am a big D104 fan here.  But I did not want to modify my rig. 

As others have mentioned, The grid resistor needs to be changed to 5 to 10 Meg with a heatshrink and a shield over the resistor to prevent hum.

There is no need to rip the filter or clipper out.  It wont change much of anything if you plan on using a high impedance mic.

I tested this with my spec An and an audio spec AN, part by part while reading from a book with my own voice and playing back recordings through my reference receiver.  Dont bother. Its a waste of time unless you are going to pump in external audio from an audio rack/processor and studio mic and want to be very wide and play broadcaster. 

Just match the mic,  change a few coupling caps and be done.  It will sound great!
C

Might as well just get on sideband Clark.

Rather ad-hominim, doncha' think? Have you actually heard him on the air with the 500 and his choice of mikes?

I'm fairly sure I have.