Mic CCDA cathode follower stage - output impedance vs overall output impedance

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

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

ln76d

Well-known member
Joined
Aug 11, 2012
Messages
2,486
Location
Gallifrey
Hey Guys,

i'm confused a little, need some clarification to understand the circuit ;)
In attachment is simplified circuit of ccda tube microphone amp used in several chinese microphones (even ribbon :) ).
Filament and polarisation voltages are ommited.
Circuit was designed for 12AT7 and mostly used with 12AX7.
Based on infromations from the web, i have found that transformer ratio is 4:1 or 5:1 and some people using 10:1 (in original circuit not on fake C12 circuit).
For cathode follower output impedance should be ca. 600ohm for 12AX7,  ca. 180ohm for 12AT7.
How the hell microphone output impedance after transformer can be around 200ohm?
What am i missing here?
Any informations, explanations are highly appreciated ;)
 

Attachments

  • Alctron CCDA.jpg
    Alctron CCDA.jpg
    54.7 KB · Views: 129
> ca. 600ohm for 12AX7,  ca. 180ohm for 12AT7.

AT data-sheet boast-points of 1.5mA and 10mA.

Gm will fall-off with current, and roughly as square-root of current.

If "1" is a +100V feed, V1 with 113K load is clearly running under 1mA. I would guess 0.25mA at most. Not that we care, but we need to find the op-point for V2. I guesstimate 70V-80V across 270K, so 0.25mA-0.3mA. 12AX7 is run at about 0.25/1.5 or 1/6th of spec-point, so "600r" is nearer 1.5K. 12AT7 may be running 0.25/10 or 1/40th of spec-point, say 1.2K.

Resorting to actually looking-up the data-sheets:
12AX7 @ 100V 0.25mA Gm is around 500uMho or 2K cathode impedance.
12AT7 @ 100V 0.25mA Gm is around 400uMho or 2.5K cathode impedance.
(Both tubes use near-cathode contact potential effects to raise the zero-grid line for "normal" currents, thus show added fall-off at the very low current used here.)

> How the hell microphone output impedance after transformer can be around 200ohm?

That's the number buyers expect to find on the mike sales-sheet.

If we take 2.5K Rk, and 4:1 step-down, that works out to 156 Ohms. If we assume a real transformer shows 13% DCR on pri and sec, it comes out right-at 200 Ohms.

And really, what difference does it make? Few preamps "care" about source Z as long as it is not too high. There are a few that get ringy with a zero-Z source, but as long as there is a transformer the source won't get to ZERO. Considerations of optimum source resistance for lowest hiss are probably pointless, because the desk preamp is NOT the system low-point, the mike has considerable voltage gain and V1 is probably the key hiss source. The mike-preamp interface only has to give "reasonable" signal level, which for a large-acre condenser often means a step-down to get into a preamp scaled to work well with dynamic/ribbon sources. Those "some people" running 10:1 step-down may be working with very loud acts and must cut to avoid overloading the desk preamp.
 
Thank you very much PRR  :-*
PRR said:
> ca. 600ohm for 12AX7,  ca. 180ohm for 12AT7.

AT data-sheet boast-points of 1.5mA and 10mA.

Which datasheet did you used?


PRR said:
If "1" is a +100V feed, V1 with 113K load is clearly running under 1mA. I would guess 0.25mA at most. Not that we care, but we need to find the op-point for V2. I guesstimate 70V-80V across 270K, so 0.25mA-0.3mA. 12AX7 is run at about 0.25/1.5 or 1/6th of spec-point, so "600r" is nearer 1.5K. 12AT7 may be running 0.25/10 or 1/40th of spec-point, say 1.2K.

One thing! Why 113k load? Where from is 3k? Connected V2 plate?

"1" should be close to 150V feed.
I will try to calculate everything when i will get CCDA microphone circuit in my hands and measure it :)

So if i would like to use 3:1 transformer in exact circuit, with 12AT7, output impedance should be close to 300ohm.
Reducing V2 V1 cathode resistor to 1.8k-2k output impedance should be lower a little, am i right?
 
ln76d said:
Why 113k load? Where from is 3k?
More precisely 112.7k (don't forget the cathode resistor). Anode voltage is one thing, anode-to-cathode voltage another.
So if i would like to use 3:1 transformer in exact circuit, with 12AT7, output impedance should be close to 300ohm.
Reducing V2 cathode resistor to 1.8k-2k output impedance should be lower a little, am i right?
Yes the resulting output Z would be close to 1k, but current in V2 would be prohibitive. IMO 270k is too high, but 2k is way too low.  I would set the op point at about 1-2mA, so the cath-follower output Z would be "naturally" low, and headroom would be higher.
 
Thanks abbey!

abbey road d enfer said:
More precisely 112.7k (don't forget the cathode resistor). Anode voltage is one thing, anode-to-cathode voltage another.

Ok, i get it now!

abbey road d enfer said:
Yes the resulting output Z would be close to 1k, but current in V2 would be prohibitive. IMO 270k is too high, but 2k is way too low.  I would set the op point at about 1-2mA, so the cath-follower output Z would be "naturally" low, and headroom would be higher.

My mistake i had in mind first stage V1 not V2, but yes, it is not enough :)
 
ln76d said:
My mistake i had in mind first stage V1 not V2, but yes, it is not enough :)
Decreasing V1's cathode res would indeed result in slightly higher current, but that would also drop the anode voltage, which in turn would reduce V2's cathode current. This could easily be compensated by decreasing the value of the cath res.
V1 would be a little starving though.
 
Yes, you are right :)
I could even increase V1 cathode resistor to 3k and get 90V at V1 plate, then put 56k at V2 cathode, so the V2 current should be around 1,6mA. 
I think that real circuit and two potentiometers will do the best job to set proper values :)
 
R8 in Alctron "CCDA" circuit is way off, it is not constant current like that. The R8 should equal the V1 load, 112.7k, right? Otherwise the current drawn from PSU is not constant unless there is something I've missed (transformer load?)...
 
mhelin said:
R8 in Alctron "CCDA" circuit is way off, it is not constant current like that. The R8 should equal the V1 load, 112.7k, right? Otherwise the current drawn from PSU is not constant unless there is something I've missed (transformer load?)...
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.
 
Could someone please help explain how to calculate the optimal value for R8 in this circuit with a T14 style transformer at (11.5:1)? I'm not quite following the explanations with the shorthand style.

My guess is assuming a 2000k impedance at the secondary *11.5^2 = 264.5k

so 1/264.5k + 1/x = 1/112.7k ...then solve for x? Anywhere close here?

Thereabouts 196k ohm. The stock value is 270k. Considering the fact that the input impedances of different preamps will be different, will changing the value of R8 from 270k to 195k have any meaningful effect?
 
Last edited:
Could someone please help explain how to calculate the optimal value for R8 in this circuit with a T14 style transformer at (11.5:1)? I'm not quite following the explanations with the shorthand style.

My guess is assuming a 2000k impedance at the secondary *11.5^2 = 264.5k

so 1/264.5k + 1/x = 1/112.7k ...then solve for x? Anywhere close here?

Thereabouts 196k ohm. The stock value is 270k. Considering the fact that the input impedances of different preamps will be different, will changing the value of R8 from 270k to 195k have any meaningful effect?
The only notable effect would be little more headroom (not much, maybe 1 dB). In terms of sonic quality, there maybe some very subtle difference, but no one can predict it. You must try and judge by yourself. When you do, remember to be objective: avoid confirmation bias.
 
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
 

Attachments

  • 20220715_012410.jpg
    20220715_012410.jpg
    190.3 KB · Views: 11
  • 20220715_012422.jpg
    20220715_012422.jpg
    183.3 KB · Views: 10
Last edited:
Under same exact conditions, but WITHOUT CATHODE CAP i get 8.43% using stock topology (alleged "CCDA").
It shouldn't be so. The arrangement "common-cath + cath-follower" is a well-known circuit, used in many HiFi amps, although usually the cath-follower operates with about 4-8 times more quiescent current.
Also the total output of the mic is about 6db lower.
This is predictible.
Stock topology but WITH CATHODE CAP the THD goes through the roof!
It tends to prove that the cath-follower is not working properly.
Actually, a possible reason is that the heater voltage should be elevated. These tubes are not meant to operate with that much voltage between catode and heater.
I believe that there is something so very wrong that it's impossible to make definitive conclusions. Some more experimentation is needed.
 
It shouldn't be so. The arrangement "common-cath + cath-follower" is a well-known circuit, used in many HiFi amps, although usually the cath-follower operates with about 4-8 times more quiescent current.
I agree, but this is in this particular circuit with stock resistor values and stock transformer, something is really off.
 
Back
Top