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Old 21st October 2006, 12:14 AM   #1
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Default MXL 770 mics - any mods?

Yesterday I recorded a very rare record, but it has harsh highs unfortunately. It was the first time I switched off -10dB switch on MXL 770 microphones in order to gain the more as possible of dynamic range of the Yamaha MX-400 mixer.

If anybody played with mics, what is the main buttleneck? I suppose it is the transformer that overloads internal transistor amp? I searched the Net and found schematics of some MXL mics, one is original that contains an emitter follower afrer a source follower, another is a modification that uses no output transformer, but splitted load of the FET and ballanced output by pair of PNP emitter followers. I don't like thos mod since it decreases phantom power voltage needed for the capsule.

Also, I've found Neumann 2-tube schematics with a cathode follower and a 1 - stage amp, with 220K (!) in anode, and a transformer is connected to the cathode for a PFB by current. Very clever!

But is it possible to find such a transformer somewhere? I'd use a single 6AK5 since the mic is big and should have a lot of space inside... However, a filament power will be needed, since a phantom power is too weak even with a DC/DC converter for the filament, in such case more voltage may be provided for anode, anyway I will need a power source...
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Old 21st October 2006, 08:41 AM   #2
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Hi:

The thread might be more appropriate and generate more feedback under the Musical Instruments section.

If you wish to use a single 6AK5 instead of the solid state preamp you will need an external power supply that would generate +B and heater voltages. You will also need to replace the XLR connector with a multipin one. For the output transformer you may wish to consider a microphone-input transformer (ratio from 1:5 to 1:10) connected backwards.

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Milan
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Old 21st October 2006, 05:12 PM   #3
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Dear Admin;

is it possible to move the topic to the Musical Instrument's forum?

Thanks in advance!
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Old 21st October 2006, 11:17 PM   #4
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Well, the FETs will be quieter than the tube(s)...

A bit depends on what that "-10dB" switch is doing?
Is it in the Drain of a FET, in the Source?
Or is it a resistive pad on the output??

In one case we're looking for overload/clipping, in the other we're looking at possible loading due to impedance changes or perhaps overload of the mic input on the board... etc...

I boought a pair of Marshall 990 mics, which are similar I suspect, and opened them up. The preamp insidelooked like a simple two FET (I guess) single ended preamp.

You could probably improve that any number of ways...

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Old 22nd October 2006, 12:55 AM   #5
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Bear, I've opened the mic, no transformer at all...

I'll post details later.
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Old 22nd October 2006, 10:03 PM   #6
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No, I wouldn't expect one.

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Old 22nd October 2006, 10:51 PM   #7
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Quote:
Originally posted by bear
No, I wouldn't expect one.

_-_-bear
I did, so my expectations were wrong... It is strange because mics did not clip, and only highs are slightly distorted... What can cause such an effect?
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Old 27th October 2006, 10:16 PM   #8
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Ummm... distortion?
Distortion causes distortion!

I think I said much depends on where that 10dB pad is in the circuit?
The highs tend to have more peak energy, and will hit the rails faster... remember this is a 48v phantom supply, which implies Class A circuits with a swing of +/-24v max... if that much...

btw, how do you know the mics were not clipping?

The little amplifier may be clipping on peaks, or else the mic input on the board was clipping on peaks... or else something is non-linear when the pad is out. That points to a wiring error, or a design error.

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