Apex 460 Power Supply Mod: D5, D6

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

Wordsushi

Well-known member
Joined
Aug 27, 2015
Messages
435
Location
Los Angeles, CA
I'm considering modding my Apex 460 power supply by replacing Zeners D5 and D6, to lower the voltage. I'm asking if anyone has any recommendations as to which zeners would be best. I've read online about people using 62V for both or 56V.  Which would be best here?

Also, what Wattage? 1w or 3w?

Any input would be very helpful.

Thanks in advance!
 
hitchhiker said:
Ive done it with two equal zeners,  dropped to 136 volts and then dropped further inside the mic with a dropping resistor to get 120  volts. Work well and still does I am told.

Thanks for taking the time to answer. So were those two 68v Zeners? Do you recall which wattage those Zeners were?
 
You pick the zeners to get the voltage you want. If you're shooting for 120v like a C12, you'd use two 60v, if you're shooting for less, you could use two 56v. It's hard to advise without knowing what you want to supply to your mic.


The power rating of the zener needs to be greater than the power dissipated by it.

Dissipated Power=v*I
So if you are using two equal zeners for 120v, then the total power is 120*current.
The current through the zener is determined using ohm's law for the circuit. 
you need a zener with a higher power rating than the estimated power dissipation.
 
dmp said:
You pick the zeners to get the voltage you want. If you're shooting for 120v like a C12, you'd use two 60v, if you're shooting for less, you could use two 56v. It's hard to advise without knowing what you want to supply to your mic.


The power rating of the zener needs to be greater than the power dissipated by it.

Dissipated Power=v*I
So if you are using two equal zeners for 120v, then the total power is 120*current.
The current through the zener is determined using ohm's law for the circuit. 
you need a zener with a higher power rating than the estimated power dissipation.

Big thanks.  I appreciate it.

FWIW, I just heard from Brian Fox, he told me:

"The 1n4759 is a 1W diode.  They are only dissipating 0.25 Watts each in these circuits so 1W is all that is needed."
 
Maximum power dissipation in the diode is defined by the shunt current:  for example, a 1W diode @ 62V can sink maximum about 16mA.  Actual current is typically less than this (if you want the Zener to last).

In the Apex 460, we have a 200VAC secondary, which is about 280V after rectification.  If we assume two, 62V diodes, we have 124V at the output.  We have 60K of resistance (the three, 20K resistors in the PSU used for filtering) to drop 158V, which means the 20K resistors limit the shunt current to a shade under 3mA ((282.8VDC - 158.8VDC)/60K).

3mA at 62V means each Zener is dissipating about 0.186W.  Worst case is when there is no microphone attached:  dissipation will drop slightly as the microphone is plugged in, as the microphones current draw will reduce the load current running through the two diodes.
 
Maybe a dumb question: why use zeners at all? Why not just find the right shunt resistor?
Zeners are nosier right?
There where no zeners in the old german tube mics psu.
Thanks!
 
Part of the idea of regulation is to keep the 120v relatively constant despite variations in the incoming wall voltage and the load of different kinds of tubes.

However tube gain circuits can tolerate a large supply variation, so you can adjust the last filter resistor to give 120v with your mike and your wall voltage (when you measure it) and you don't need the Zener's at all.
 
Thanks Matador!
I´ve been working on my mod of a T.bone Sct-800 (same as Aurucle A5500MP and SE 5500) for a while and my problem is that the modded psu makes the mic a little nosier. I have a pair of these mics, one is 100% original and the other one that i´m working on. I use my ears and compare them all the time to get an idea what different changes achieves sonicly.
My method for noise comparing: I connect both to my sound card via a low noise stereo preamp , warm up, than i record a little soft voice talking and silence in one take. Then i Normalize both files and switch between the recording of the silence. Thats my practical amateur way of comparing SNR  :).
Anyway the mic with the modded psu i always a grain noisier. I´ve rebuilt the PSU many (many!) times testing different resistor combinations, voltage dividers, zeners, shunt resistors, added filter stages, bypass film caps. I even changed the power transformer for a lower voltage one to be able to use lower value series resistors. This have been fun and interesting in many ways but now i feel a little frustrated. Is i t the mics that gets nosier because of lower voltage? maybe the PSU is ok?
Any Ideas anyone?
 
Yes, the original mic gets even nosier with the modded psu.
I've done a lot of changes to the modded psu: higher uf, quality caps on both heater and b+, lowered heater to 5,8V, resoldederd the grounding cables...
 
Have the same problem with mine (its a tbone SCT-800 though, same mic, different name). I will try this: https://groupdiy.com/index.php?topic=62601.msg833009#msg833009
 

Attachments

  • mojpsu.jpg
    mojpsu.jpg
    176.4 KB · Views: 76
What's the input voltage to that circuit? And what's the expected current draw?

I was just thinking that if that's for cleaning up the B+ voltage, that capacitor in the transistor's base will be seeing the full 120(?) volts, so you'll want a 160V (or more) rated one there. Same goes for the BD647, though.

But since the B+ draw is usually only a handful of mA in mics, somethin like a 2N5551 would be far better suited, i think. And a 10k resistor going to the base would / will still provide plenty of drive current, as well as significantly lower the cut-off frequency of the filter.

... But that's just my opinion ;)

bernatvm said:
Have the same problem with mine (its a tbone SCT-800 though, same mic, different name). I will try this: https://groupdiy.com/index.php?topic=62601.msg833009#msg833009
 
If you wish to read up more about the concept: http://sound.whsites.net/project15.htm

You'll probably be better off with a "two-stage" version (see figure 3).

I went that route for some solid-state guitar preamps i'm designing (5-10mA load, in total).
10k resistors, 100nF caps, and a BC546 - cheap & cheerful ;D

I'd venture to say cap multipliers like these are VERY welcome for single-ended circuits like these.
I could be wrong, but i seem to recall having read somewhere that such circuits have almost no PSRR at all, and as such, benefit from all the power-supply "cleaning" as can be provided. Which fortunately is quite trivial to achieve, for single-digit mA loads :)
 
As i understand, figure 3 is the whole psu, I just want to add another stage to my psu already existing (attached). My supply has some mods, zeners are lowered to provide lower b+ voltage.
 

Attachments

  • Apex-460-PSU-Schematic.jpg
    Apex-460-PSU-Schematic.jpg
    63.5 KB · Views: 98
And is it that difficult to "separate" the part that you need to add, from the figure 3 in the link? :)

(Honestly not trying to be a "smart-a*s", just genuinely asking.)
 

Latest posts

Back
Top