Phantom Power from 18-0-18 ac Transformer

Slouching Raymond said:
Transformers are specified in RMS voltages. The peak rectified voltage would be much higher, as ecc83 says.
Your kit may work adequately on a voltage lower than 48V.
You could try connecting up a couple of little 12V batteries. They exist - I bought a pack of various battery sizes, and in there was a little 12V battery, I think for hearing aids.
I'm assuming this is just a one-off exercise.
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We shall be in the dark Ray until and unless our OP gives us much more detail about the transformer. For instance, very, very small transformers, under 15VA say, have very poor regulation and so their off load voltage is much higher than their rms full load rating. I recall a 12.5VA traff rated at 16V/0.8A that had an O/C voltage of 22V and would thus produce an off load rectified voltage of some 30V.

Dave.
 
Slouching Raymond said:
Your kit may work adequately on a voltage lower than 48V.
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You do have a point with this as most are 5-12V internally set by a zener inside.
But as far as building mic preamps, I think people should ditch the phantom circuit all together as its a point of loss. But this user I guess is trying to power a mixing board channel strip. Which could be easily done just getting a power supply. But buying one vs. copying and building some one else's' circuit is going to be a wash.
But a power supply circuit from a budget portable mixer would work.

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"I think people should ditch the phantom circuit all together as its a point of loss" How then do you consider people are going to power their microphones?
And yes, the phantom power resistors constitute a loss but taking the worse case of a 200 Ohm mic and 6k8 the the loss is a mere 0.25dB. In practice however the input impedance of the actual pre amp will be a lot less than 6k8 so the effect is swamped.

Dave.
 
ecc83 said:
And yes, the phantom power resistors constitute a loss but taking the worse case of a 200 Ohm mic and 6k8 the the loss is a mere 0.25dB. In practice however the input impedance of the actual pre amp will be a lot less than 6k8 so the effect is swamped.
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The extra termination circuit formed by the phantom power feed resistors is negligible compared to the measured non linear losses of the coupling capacitors.
 
ecc83 said:
Oh! FFS Read some Self.

Dave.
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Mic preamp design is a complicated subject that should be carried on in a different thread. Mainly because the end user frame of reference is way different than the though behind designs. But Ideally, the source impedance should be 0 ohms while the input impedance of the preamp circuit be infinite.
 
drtechno said:
Mic preamp design is a complicated subject that should be carried on in a different thread. Mainly because the end user frame of reference is way different than the though behind designs. But Ideally, the source impedance should be 0 ohms while the input impedance of the preamp circuit be infinite.
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No it isn't, two transistors with low rb and a couple of op amps and you have a circuit that performs close to thermal noise limits. You have to be careful to stop RFI getting in but very good quality designs have been around for decades. REAL engineers in the REAL world where finite and non-zero resistances exist manage very well.
Easier to get low noise and RF immunity with a transformer front end but good transformers are costly and prone to hum pickup.

You can make the basic circuit more operationally useful, for instance, it is hard to get a gain range better that 50-55dB without "gain bunching" of the gain control in the last few degrees of pot rotation. A good solution is a multiway gain switch in say 5dB steps. Esoteric designs use digital control of analogue parameters and you just "dial in" the gain you want. There has been a "fashion" for some time now to have various different input impedances. Ribbons especially are said to benefit from a load of up to 15k but there is scant A/B evidence of this idea?

But all these complications and refinements are secondary, a basic very low noise, very low distortion mic pre amp is but a handful of cheap components. Do not believe the advertising BS trying to part you from your hard earned!

Dave.
 
ecc83 said:
No it isn't, two transistors with low rb and a couple of op amps and you have a circuit that performs close to thermal noise limits. You have to be careful to stop RFI getting in but very good quality designs have been around for decades. REAL engineers in the REAL world where finite and non-zero resistances exist manage very well.
Easier to get low noise and RF immunity with a transformer front end but good transformers are costly and prone to hum pickup.
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In reality, the amplifying devices and the circuit chosen dictate the limitations from the ideal model.
I guess I will start a new thread because this is off topic.
But please keep it professional. Just because you learned something one way doesn't mean it always going to be taught the same way (which there is good and bad examples of that in electronics theory).
 
drtechno said:
In reality, the amplifying devices and the circuit chosen dictate the limitations from the ideal model.
I guess I will start a new thread because this is off topic.
But please keep it professional. Just because you learned something one way doesn't mean it always going to be taught the same way (which there is good and bad examples of that in electronics theory).
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Ok "Mr Professional" give us a circuit of a mic pre amp that meets your criteria and the reasons why?

Dave.
 
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