Apex 460 mod- keep or remove cathode follower?

[jordan s]

So I got a couple of these mics to mod, now it's time to order capsules and transformers. It seems like most people have removed the cathode follower and replaced the transformer with Cinemag CM-2480, this is supposedly to keep it with the same design as C12/ C24. Dave Thomas strongly suggests keeping the cathode follower and using BV11 transformer. He says this is an improvement over the C12 design and allows for a flatter sound, he also uses it for his CM-12 mics which seem to get good reviews.

This site seems to suggest that removing the CF and using the cinemag creates a high frequency roll off. Perhaps this is desired for a c12 type capsule?
http://www.foxaudioresearch.ca/transformers1.htm

And about the dreaded Cathode Follower (CF) circuit ...

When you are driving a transformer with a little tube that has high Plate Resistance (Rp) it is more difficult to get really flat response across the entire audio range even when we wired both 6072 triodes in parallel to lower Rp in half!

The cathode follower contrary to many does not fix a bad transformer. As we saw above each transformer showed some roll off; even the very fine quality units from Peluso and Cinemag. The CF circuit fixes an inherent problem with the plate follower circuit; high output impedance. By using a CF output circuit we do the first "transformation" in the tube and amplify the current we can deliver. The transformer then does the final step down to mic levels and balances (floats) the output signal.

 

[Matador]

I'm in the process of doing this very upgrade (BV11 and keeping the CF)....I'll post some results soon.

One advantage:  low output impedance means that you can use a lower ratio transformer to drive a load of a given impedance.  This should provide less voltage gain loss overall from the output circuit....enough that you can get rid of the cathode bypass capacitors in the front end to linearize the first gain stage and keep the overall gain constant.

 

[pasarski]

Perhaps this is desired for a c12 type capsule?

Apex 460 has a K67/K87 type capsule which can benefit from top roll off.

 

[strangeandbouncy]

  . . . I am not sure who is in a position to judge whether cathode follower is BETTER than C12 or 251 . . . .


  I have not come across anyone ever that would want to change their C12 or 251 to cathode follower, and that should speak volumes. C12vr that is cathode follower is a great disappointment. Why try to reinvent the wheel. . . . .

 

[mista min]

The Sony C800G is a cathode follower.  And so is the Sony C37a.

 

[strangeandbouncy]

. . . true, but we are talking about C12 here . . . . if it aint broke . . . .

 

[Gus]

first the C800 is a CF.  C800G is plate out

Now to the 460 type circuit

It all depends on what you want and want to spend.

What tube will you use?  What capsule? stock or different transformer?

You can adjust the stock 460,1050,1150 etc circuit with a few changes if you know what you are doing, keeping the stock capsule and transformer.

IMO question what you read on the web even what I post.

That said I will not post what I would do because there is the possibility someone will take it and start a new microphone mod service.

Most microphone circuits are simple little things that a good tech or engineer should understand without much trouble.

 

[jordan s]

I'm just trying to mod these for my own studio use. I understand you not wanting to divulge too much though, if you're running a business. I plan on putting c12 type capsules (Peluso?) in these. I already have the tubes, they're a EH 12AY7/6072, not the best but enough to get started. I'm trying to decide to remove the CF or not because if I do, I will use the cinemag 2480, if I don't, I will use the BV11. Seems most people prefer to remove the CF, if not only because that's how the C12 is.

 

[strangeandbouncy]

Hi,



  I would strongly suggest that you look at Tim Campbells capsule. Nothing against Peluso, especially as I have not even tried it. Its just that Tims is a fantastic version of the original CK12.


    Kindest regards


      ANdyP

 

[jordan s]

So I suppose the consensus is to remove the CF?

 

[abechap024]

I think what they are saying is it just depends on the circuit as a whole. Many very respectable mics use CF. I modded my 460 to still use the CF. I've also made c37A clones that sound great to me, and they use CF. I think in 2 properly designed circuits you will find that the CF has hardly any sound at all.

 

[0dbfs]

If you are using an original-CK12, clone-CK12, or other replacement-grade-CK12, why not just go with the C12 transformer and circuit?

I am planning an M49 style in those bodies with a 5840 tube instead of an ac701. I will use the stock capsules and may swap in some m7's. I would like to get a set of Tim's capsules and put together a C12 clone though. May get a diymic.com kit and use the body for that project.

Cheers,
jb

 

[pucho812]

many a well respected mic has a cf circuit

 

[Rossi]

I think we should differentiate between

a) mic circuits that are cathode follower circuits (single stage)

b) mic circuits that employ a cathode follower as a second stage.

And of course there's always the question whether the circuit is properly designed.
Are the bias voltages sensible?
How many coupling caps and what quality (lytic, polyester, polyprop etc).?
Does the capsule need frequency compensation, and if so, how is it achieved?
etc.

 

[mitsos]

As Rossi hints, the 460 is not a real CF. I'm no tubey mic guru, (in fact I'm no guru at all) but to do a real CF you'd have to eliminate one of the the tube stages. Look at the AKG C60 schematic.

In fact, the same site the OP links to calls the Apex circuit a CCDA, constant current draw amplifier:
http://www.foxaudioresearch.ca/CCDA.htm

So C60 falls into #1,

APEX 460 falls into #2.


Rossi, I'd love to hear your thoughts on the two.

 

[Rossi]

I don't think my personal views are of much importance. But all the tube classics are single stage, and even some FET classics such as the U87(a) or the KM80 series are essentially single stage. That's not to say single stage is the gospel, but it may be something to consider.

That said, depending on your goals and your limitations a good mic circuit can be single stage or multiple stage, transformer coupled or transformerless. What's really essential is to have a design goal in the first place. Then think about how much money you are willing to spend and what other limitations you have to reach that design goal. The circuit will follow from that.

Also think about the mic as a whole. It's no use talking about circuits in isolation. The capsule comes first, and the circuit must accomodate its output and frequency response.