[Gus]

An idea for a low gain high input level preamp

   A one transistor EF pre

   +24vdc supply

   Bias string 10K one end to 24 V other end to a node A  one end of a  22K. the other end of the 22k to ground.  A 100uf cap connected to the node A and other end to ground.

  The EF stage a 2n4401 collector to 24V.  Base to node B emitter to a 2.2K emitter R and the other end of the 22k to ground.

 the secondary of an input transfromer  one end to node A and the other to node B  Maybe a transformer like the LL7905 setup as 1:5.6 or 1:11.2.  

  With something this simple the secondary  of the transformer might need a resistor  from node A to node B.  The input R of the transistor will be about hfe( at about 8ma) X the emitter R so a resistor in parallel with the secondary might help with termination.

  The unbalence output will be taken off the emitter of the transistor.  Yes we want the emitter to sit at about 18V

feedback please.

[[email protected]]

Why not? I like simple things like that. Just put it on a perfboard then throw it in a cookie tin.
I like to use a constant current source on emitter/source followers for less distortion and lower output impedance.
What are your goals? Simplicity, coloration, something else?

Tamas

[adrianh]

Gus;

I drew the circuit so everyone here could discuss.
i.e. for PRR to bleed all over it.
That is not a problem for me because I will learn something!

Gus's circuit;


My mod is to change the bias feed point and AC couple the Xformer;


Tamas's mod is to place a current source in place of R5.

[bcarso]

If you are going to drive a long cable with the preamp output I recommend 100 ohms or so in series with the 3904 emitter to make rf oscillations less likely. If instead it is close to your console you probably don't need to worry.

The noise peformance is not going to be too good at that bias and that transformed source impedance (150 ohm going to 3.75k---15k, depending on T1 turns ratio, for example).  You will have reasonable power supply noise rejection with the e-follower configuration at least.

Brad

[Gus]

I meant something like the first picture R5 should be 2.2K and R4 was not stated because R4 will be picked by the transformer and by the loading of the transistor base (should be about hfe X 2.2K)

I did not think alot about noise I was more interested in handling the signal input from high output level condenser microphones.  

The LL7905 can handle very high levels  

This will not be a general use preamp.

I will build one soon I have the parts.

[bcarso]

Note that Gus's power rail is 24V not 48V.  Then the operating point of the 3904 is about 7mA if R5 is 2.2k.

I don't know how noisy the mic output is already, or what its output impedance is, but that is going to be a way noisy unity-gain buffer.  The noise will be dominated by current noise which for a transformed Z of 5kohm will be about 40nV/root Hz at 1kHz and rising at lower frequencies.  Again, maybe the mic is already noisier than this and it is a non-issue.

Adding a PNP with emitter to +24, collector to Q1 emitter, and with a base-emitter R of about 10k, and collector of Q1 driving that base, (a so-called Sziklai follower) would help a lot and reduce distortion as well.

[PRR]

> noise will be dominated by current noise which for a transformed Z of 5kohm will be about 40nV/root Hz at 1kHz and rising at lower frequencies.

Or say 6 microVolts across the audio band.

Note that the proposed operating level, ~1V, is about 100dB higher. When you really have 100mV-200mV at the mike, 1V after a step-up, your raw dynamic range is large compared to the recording/playback channel. When I'm set for orchestral peaks, direct to CD, I don't really care if mike-noise is 14dB SPL or 25dB SPL. True, there are better recording media (few playback rooms are even 96dB). But the loud sources that leap to mind are close-mike Fender and drums. S/N on Fender is just moot. Many (too many) drummers never stop, so noise is a non-issue.

Yes, a Darlington or Sziklai would give lower noise (and the Sziklai lower distortion).

But a nothing-special JFET would also give lower noise, would not need 100Ω series resistor (for short-protection if not for oscillation), and might be more euphonious, or look good on the product brochure. And no input DC current to sock the iron, and allow much higher bias resistors for smaller bias bypass cap.

In general I'd have to say most mike transformers are not meant to whack a volt or more output. I see Gus has pondered this.

If your mike levels are really this hot, many of the frills in mike transformers are pointless. You can get away with 20dB less shielding, quite a lot more parasitic resistance, etc.

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