Apex 460

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Papa Tango Charlie

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Hello, I have an Apex 460
I would like to make a mod like the one you could find online on Advanced Audio's blog
Or the Far mini Mod that could also be found online.
If someone has an idea of a C12 or 47 mod that he likes. Could you give me some schematics.
I'm not looking to make a copy I want to make an original microphone that sounds very good.
As far as the power supply is concerned I don't think I can do much better than rectifying it at 120V.

Thanks You All
PS I have a small transformer 6.5:1 from 3U
 

kingkorg

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The AA forum seems to be gone. There used to be a page with detailed instructions from Dave Thomas. However i made recently some tests of the so called "CCDA" topology both AA and Fox Audio Research rave about. That "CCDA", or cathode follower which is stock arrangement in the Apex has way higher distortion than regular plate follower topology which c12, elam, and u47 use. So i would take plate follower route.


This guy has described the mod, but i havent read the whole page, the schematic seems right. You don't have to go silver mica caps for the filter caps imho, just leave them stock. Change the diodes to get 120v b+. Leave the stock transformer for this topology. You can try the 3u transformer as well, but make sure you test it for frequency response, 6:1 is low in ratio. Use it if you leave the CCDA arrangement.


I don't like the Apex's polarization solution, but maybe once you get everything done we can discuss modding that part.
 
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soliloqueen

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The AA forum seems to be gone. There used to be a page with detailed instructions from Dave Thomas. However i made recently some tests of the so called "CCDA" topology both AA and Fox Audio Research rave about. That "CCDA", or cathode follower which is stock arrangement in the Apex has way higher distortion than regular plate follower topology which c12, elam, and u47 use. So i would take plate follower route.


This guy has described the mod, but i havent read the whole page, the schematic seems right. You don't have to go silver mica caps for the filter caps imho, just leave them stock. Change the diodes to get 120v b+. Leave the stock transformer for this topology. You can try the 3u transformer as well, but make sure you test it for frequency response, 6:1 is low in ratio. Use it if you leave the CCDA arrangement.


I don't like the Apex's polarization solution, but maybe once get everything done we can discuss modding that part.
what's your issue with it?
 

kingkorg

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Do you mean polarization? Doesn't give me good polar patterns, also i add 1G resistor after the 51G for pattern selection. I go from front diaphragm to grid without a cap, 60v is applied to the backplate, ac coupled through a cap to ground, pattern selection voltage to rear backplate. Quite common actually, but i'm trying to figure out even better solution.
 

soliloqueen

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Do you mean polarization? Doesn't give me good polar patterns, also i add 1G resistor after the 51G for pattern selection. I go from front diaphragm to grid without a cap, 60v is applied to the backplate, ac coupled through a cap to ground, pattern selection voltage to rear backplate. Quite common actually, but i'm trying to figure out even better solution.
if it's because you're worried the charge has no place to go, what about using a 5 position as a 3 position switch and grounding positions 2 and 4?
 

kingkorg

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It's actually another topology i had in mind when i mentioned that in the other thread. It involves in-mic switching. I never paid too much attention to it as i'm mostly building cardioid only mics, but there are quite a few issues with a lot of topologies.
 

kingkorg

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Hmm - false claim on their part?
The actual load on V2 is R8 in parallels with the reflected impedance at the primary of the output xfmr. For constant-currrent, the reflected load would need to be about 220k. That would mean a 10:1 xfmr loaded by 2k at the secondary; the xfmr would be very expensive and have no losses, which is just about impossible. The claim for constant-current operation is just ridiculous and plain marketing BS.
In any case the THD is substantially higher.
 

yikelu

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In any case the THD is substantially higher.
At least according to Fox, the culprit is an incorrect value for the cathode bias resistor and changing it to 1.8k should do the trick. I just did this mod with a parallel resistor but unfortunately don't have measuring apparatus to confirm the drop in THD.
 

Papa Tango Charlie

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Does any body have a schematic to make a mod for the apex 460 with single triode schematics, and U47 style polarization capsule connection (without capacitor between capsule and the first tube grid);
if i do that does it mean i have to change Microphone and PSU pcb. If any body in europe know how to do that can you contact me. Thank you. Best
 

kingkorg

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At least according to Fox, the culprit is an incorrect value for the cathode bias resistor and changing it to 1.8k should do the trick. I just did this mod with a parallel resistor but unfortunately don't have measuring apparatus to confirm the drop in THD.
I'll quote Abby explaining the issue yet again.
The actual load on V2 is R8 in parallels with the reflected impedance at the primary of the output xfmr. For constant-currrent, the reflected load would need to be about 220k. That would mean a 10:1 xfmr loaded by 2k at the secondary; the xfmr would be very expensive and have no losses, which is just about impossible. The claim for constant-current operation is just ridiculous and plain marketing BS.
@yikelu, changing the cathode resistor blindly to 1.8k doesn't guarantee anything. This resistor value should be revised for every tube to check it's bias and adjust accordingly. I've had tubes that needed values far from 1.8K.
 

kingkorg

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Does any body have a schematic to make a mod for the apex 460 with single triode schematics, and U47 style polarization capsule connection (without capacitor between capsule and the first tube grid);
if i do that does it mean i have to change Microphone and PSU pcb. If any body in europe know how to do that can you contact me. Thank you. Best
Dude, i posted the schematic in the first reply.
 

yikelu

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I'll quote Abby explaining the issue yet again.

@yikelu, changing the cathode resistor blindly to 1.8k doesn't guarantee anything. This resistor value should be revised for every tube to check it's bias and adjust accordingly. I've had tubes that needed values far from 1.8K.
Thanks for the quote and therefore the link. Very helpful thread. I just got one of these and performed the Fox mods as a start, but your measurements in that thread and my initial impressions have convinced me to go ahead and bypass the second tube stage. As far as I could tell, the mod did take care of the top end issue from the output RF filtering caps (I actually replaced with lower value caps instead of taking them out completely), but the esh-iness that Fox talks about is still there, so likely the distortion performance didn't really change with the resistor mod.
 

kingkorg

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Thanks for the quote and therefore the link. Very helpful thread. I just got one of these and performed the Fox mods as a start, but your measurements in that thread and my initial impressions have convinced me to go ahead and bypass the second tube stage. As far as I could tell, the mod did take care of the top end issue from the output RF filtering caps (I actually replaced with lower value caps instead of taking them out completely), but the esh-iness that Fox talks about is still there, so likely the distortion performance didn't really change with the resistor mod.
Here's the link to the original post with my measurements. It could be the cause of what you are experiencing.
I've been revisiting this mic just today, as i got it for 30$ without the psu.

Really the best thing to do is indeed to get rid of the second tube stage and do the plain old plate out circuit.

Here are the measurements of the circuit i made with cathode follower vs plate follower.

With -20db signal from my sound interface injected into the grid i get just 1.47% THD with plate follower (second stage removed) 100uF cathode bypass cap, 12ay7 tube. All 2nd harmonic. Which is pretty much how all the C12/elam style mics i built before measured.

Under same exact conditions, but WITHOUT CATHODE CAP i get 8.43% using stock topology (alleged "CCDA"). Also the total output of the mic is about 6db lower. So less output, and less headroom. Stock topology but WITH CATHODE CAP the THD goes through the roof!

With stock 12ax7 and stock circuit but -30db signal (so 10db lower) i get 38% of THD!!!

Stock resistor values, under all measurements. Anode voltage 65V, B+ voltage 120V.

I did couple of mods in the past listening (or reading) blindly advices found on Advanced Audio forum, and Fox Audio Research where they rave about how much better this "CCDA" is. I can't remember though if they analyzed THD anywhere.

While stock topology might get you more THD and therefore more tube "warmth" at lower levels, i'd still go with the traditional plate out variant. 8% is still too much
 

yikelu

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Here's the link to the original post with my measurements. It could be the cause of what you are experiencing.
Yup, I had read it before I replied previously. That was the post that convinced me to make the change, especially the fact that you had measurements to back it up.
 
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