Mic Fet booster with stepped HPF and line drive

[AnalogJoe]

Thanks for the extra detail on that ,
I know of a few old style guitar amps that use a pair of 2N3055 for 50-100W , power output depending mainly on supply and speaker load .
They really are very robust in terms of overload , they also tend to distort in a relatively musical way , compared to the usual NPN/PNP .

Im a big fan of the Neve Ba-283 output stage , Im trying to imagine a push pull version , balanced Jfet input , center tapped transformer in the 3055 collector circuit , with an additional output available from the emitters , single rail supply .

The Canford audio unit ended costing a bit more than expected ,
What started out advertised at 40 sterling (48 euros) doubled to 90 euros by the time I added shipping and import duty from the UK .
Brexit and its tarrifs has killed all the good in ordering from the UK to Ireland ,
Were getting well and truly ripped off .

The 2N3055 is one of the monsters from the ancient world. It is capable of handling lots of current but it suffers from beta droop, which means that beta drops as current increases, which creates distortion. Also, Self has shown that using a CFP instead of a PNP Darlington tends to cause more distortion. Maybe those are some of the things you are hearing that you like in the amp.

Related to brexit: I do get a benefit from our neighbors up north. Although it really bothers me when I buy on eBay that I have to pay US Taxes. There is probably a way to get a tax exemption but it is probably not easy to do. When I was young I used to get tax exemptions with my purchases when travelling to the US if I proved that I wasn't a US national. I had to go to a lawyer to give me a "manifesto", or whatever it is called, for each of my purchases and I had to pay a fee, of course, but it was worth it.

[Tubetec]

I was able to get a schematic of the Canford 20-750 ,
similar functionality to the MZA-14 ,but all passive .

TFE mic power.pdf

(44.03 KiB) Downloaded 50 times

So it seems the two 9v batteries are connected in series , then a linear regulator produces 12v , which supplies the T12 power and an oscillator for the 48v rail,
it seems a good bit more elaborate than the 555 based circuit in the MZA-14.

It also requires the use of three way 4 position rotary switches for the controls , the MZA arrangement allows more flexibillity with a single pole 12 way for instance , likewise a pot could easily replace switched attenuation and HPF resistances ,for continiously variable control of both .
The lack of repeatabillity with a pot based control might be an issue in a stereo miking situation but its easy to use REW to cal everything up precisely if the need arrises .

[AnalogJoe]

I was able to get a schematic of the Canford 20-750 ,
similar functionality to the MZA-14 ,but all passive .
TFE mic power.pdf


TFE supply.JPG
TFE audio.JPG


So it seems the two 9v batteries are connected in series , then a linear regulator produces 12v , which supplies the T12 power and an oscillator for the 48v rail,
it seems a good bit more elaborate than the 555 based circuit in the MZA-14.

It also requires the use of three way 4 position rotary switches for the controls , the MZA arrangement allows more flexibillity with a single pole 12 way for instance , likewise a pot could easily replace switched attenuation and HPF resistances ,for continiously variable control of both .
The lack of repeatabillity with a pot based control might be an issue in a stereo miking situation but its easy to use REW to cal everything up precisely if the need arrises .

It looks like the LM723 voltage regulator creates a stable voltage output which serves to power up the transistor array. The LM723 circuit is almost identical to the one in the Data Sheet. The transistor array seems to consist of a BJT multivibrator followed by a power switching stage to drive a 4-stage Cockcroft-Walton multiplier and raise the voltage up to +48 V. The IC labeled 8211 is probably the ICL8211, which is a programmable voltage detector being used to light up that LED when the battery voltage is low.

I wonder how accurate the 48 V output is, since it is not being regulated, and Cockcroft-Walton multipliers are notorious for being sensitive to load variations. When being fed with sine waves Cockcroft-Walton multipliers usually do not exhibit very low ripple, especially the multiplier being used in your schematic, which is a "half-wave" multiplier rather than a "full-wave" multiplier. However, since it is being fed a square or pulsed wave rather than a sine wave, and if the frequency is sufficiently high, it should deem much, much better results.

Overall, I would say that it is not a bad circuit but not great either.

[mediatechnology]

Nice analysis.

The 723 could easily be replaced by a three terminal regulator.
Not sure why they used one.
An LP2951 might be a good choice.

One caveat about using the 723 is not all versions of the part have an internal resistor to limit the base current of the current limit transistor when a fault occurs. (pin 10 on the TO-100; pin 2 DIP14)
Later TI parts have an internal 400Ω resistor; earlier versions (Fairchild et al) have a "naked" base connection.

I made a switching boost phantom supply with a 555 that had feedback to regulate it that worked pretty well.
It wasn't as efficient as real switcher but IIRC was more efficient than using a voltage multiplier and wasn't surface mount.

This is a Rolls MX-422 phantom supply:

[AnalogJoe]

Nice analysis.

The 723 could easily be replaced by a three terminal regulator.
Not sure why they used one.
An LP2951 might be a good choice.

One caveat about using the 723 is not all versions of the part have an internal resistor to limit the base current of the current limit transistor when a fault occurs. (pin 10 on the TO-100; pin 2 DIP14)
Later TI parts have an internal 400Ω resistor; earlier versions (Fairchild et al) have a "naked" base connection.

I made a switching boost phantom supply with a 555 that had feedback to regulate it that worked pretty well.
It wasn't as efficient as real switcher but IIRC was more efficient than using a voltage multiplier and wasn't surface mount.

This is a Rolls MX-422 phantom supply:

Very nice.
Seems like they are using the parasitic capacitances of U7A and U7B in a I-don't-care-where-it-oscillates-as-long-as-it-oscillates type of oscillator, although the 120 pF cap in parallel with R11 may also alter the frequency of oscillation..

[mediatechnology]

This is a Nixie tube supply I experimented with using a 555.
It could easily be scaled to +48V.

viewtopic.php?p=16300

This is the classic NE555/IRF740-based flyback converter needed to get the +170V needed for the anodes.
It works pretty well being about 80-85% efficient and offers OK regulation.



Note that P1 is correctly-drawn.
In many of the schematics for this circuit the wiper and top of the pot are not connected to each other.
With the base circuit connected directly to the wiper loss of wiper contact causes the supply to run open loop, lose all regulation and go to the maximum output voltage.

[AnalogJoe]

This is a Nixie tube supply I experimented with using a 555.
It could easily be scaled to +48V.

viewtopic.php?p=16300

This is the classic NE555/IRF740-based flyback converter needed to get the +170V needed for the anodes.
It works pretty well being about 80-85% efficient and offers OK regulation.



Note that P1 is correctly-drawn.
In many of the schematics for this circuit the wiper and top of the pot are not connected to each other.
With the base circuit connected directly to the wiper loss of wiper contact causes the supply to run open loop, lose all regulation and go to the maximum output voltage.

Good job with the feedback loop using the BJT or as a current source to determine the CV.

Feels weird having to read a Boost regulator from right to left.

[mediatechnology]

Here is another 555-based HV supply.
+HT looks to be about 125V as drawn.
I would provide credit but I don't recall where I saw it.

[AnalogJoe]

Here is another 555-based HV supply.
+HT looks to be about 125V as drawn.
I would provide credit but I don't recall where I saw it.

Do you know of any +HT supplies which use a more "proper" PWM IC controller rather than a 555? Wouldn't be a bad idea to create a flyback design including an optocoupler for isolation. Of course, at that point the circuit essentially becomes a SMPS and the simplicity of the design you posted disappears.

In any case, thanks for the schematic, looks like a nice and simple circuit to have around.

[mediatechnology]

In through-hole:

I clipped this circuit some time ago thinking it might be useful. It is a +12 to -48V DC/DC converter for telecom applications that could be easily modified to be a +48V supply for phantom power use.


-48V Power Supply, "DC/DC Converter for Telecom Applications," Charles Hill, Darin Kincaid, ST Microelectronics Application Note.

Full text of -48V Power Supply Using SG3524

There may be more elegant and simple solutions using modern switchers, but the SG3524 is widely available from many, many sources and the inductor requirements allow off-the-shelf components to be used.

To provide +48V for phantom, D22 and C42 could be reversed along with pins 1 and 2 which are the inverting and non-inverting error amplifier connections.

Q3, shown as a 2N4033 or MPSA56, is not drawn particularly well, having no emitter arrow. The emitter goes to +12V.

The switching frequency is a little low, 44 kHz, but it could be raised or sync'd to Fsample.

I think they mean that U19 pins 13 and 14 should float, not U19.

SG3524 Block Diagram


SG3524 Block Diagram

In TH there's also the MC34063.

I have a lot of phantom supply circuits using SMPS ICs that are SMT.

Not sure how the opto-isolator benefits unless one needs to galvanically isolate the low voltage input from the +48V output.
I see optos and TL431 combinations in a lot of line-powered switchers using transformers for isolation.

ST SG3524 Datasheet