C12, Ela M250, AKG C28A, modern copies SP TB1, differences between circuits

Hi Ive been searching for info about these circuits have found plenty of info but wanted to ask some questions....

With the view to making from scratch a bloody good vocal mic........

I already have a AKG C28A mic with its genuine T14/1 transformer.....when I connected a large good sounding diaphram capsule to it ,it didnt sound wonderful really

I made a Ela 250 circuit with a T14/1 borowed from another C28A.......its sounded very low on bass....?

Did I read somewhere the Ela circuit has a kind of bass roll off built in ?
I did read some DIYers found the bass distorts with a T14/1..... is this the case ? is it because its small?
I notice the C12 Ela 25 & C28A all use different output capacitor values.....is this to control the bass ?
Is this how real C12 & Elas sound.....a bit bass light?
I also read that the C28A has lower noise than C12 ......why is this ? Is it just because of capsule? or the way capsule is wired ? or the way the transformer is wired not to ground in C28A.....

Modern tube mics seem to copy C12.....why ?
Studio projects TB1
Info & schematic in the interesting thread
http://www.groupdiy.com/index.php?topic=20928.0

Is this more like a Ela M250 than a C12?

Ive since made a TB1 circuit and with a good capsule & the borrowed T14/1 & it has a much fuller sound than the C28 & Ela 250 circuits.....plenty of bass however the bass does seem to go fuzzy.....
Is the T14/1 just not able to handle low frequencies ?.......

I have ADK Vienna that has had it sound modled after the C12 ....its cheap & solid state & I dont know how close it sounds to a real C12 but I like its sound ....kinda sparkly at the very top & has good bass hi fi sorta sound I guess ...its capsule Im told is a C12 copy, it looks like one, tho the back is not brass & again I dont know how good compared to real C12.....its amp Im told by the designer is flat.....I have tried the capsule on various other mic circuits Ive made &  that hi fi sound always come thru these circuits.....this capsule sounds good on the SP TB1 circuit but as I say the bass goes fuzzy.....

I waffle on thanks for reading

gary o said:

Did I read somewhere the Ela circuit has a kind of bass roll off built in ?

I notice the C12 Ela 25 & C28A all use different output capacitor values.....is this to control the bass ?
Is this how real C12 & Elas sound.....a bit bass light?

I also read that the C28A has lower noise than C12 ......why is this ? Is it just because of capsule? or the way capsule is wired ? or the way the transformer is wired not to ground in C28A.....

Click to expand...

A lot of questions here. The answers to which are, in fact, related.

It is important to note the value of the grid shunt resistor in each of the mics you mention, as this resistor forms a high pass filter with the capsule.

The C28 has twice the input impedance of the C12, 500M in the C28 as opposed to 250M in the C12.

The M251 opts for only 30Meg at the grid- there's your 'built in bass roll-off'.

These resistor values will likely account for most of the variation in LF response between the amps - which were, after all, designed to match with the particular capsule they were intended to be coupled with (SD in C28, LD in the other two) and were not designed to sound identical anyway.

The wider the response is at the grid, the greater the signal to noise ratio of the amplifier as a whole, once adjustments are made, which may account for the perceived S/N improvement in the C28 amplifier - as the increased output of the larger capsule in the C12/M251 will of course compensate for this.

The coupling cap in each case is normally tailored to roll off below the point at which the amp does. Why the 251 has such a large value is an interesting point and might have been related to parts inventory as much as design.

I did read some DIYers found the bass distorts with a T14/1..... is this the case ? is it because its small?

Click to expand...

There should be no significant distortion. Have you passed any DC through the transformer? If your circuits are distorting at low frequencies, the transformer core might be magnetised.

Dan thankyou great info .... many times you have enlightened me thanks....

SOooo the grid resistor forms a bass roll off then.....the lower the value the more roll off ?
may explain my TB1 clone....I forgot to mention I used a 1 giga ohm resistor for grid instead of 200 Meg.......sooo am I allowing too little bass roll off therefore  distorting the transformer.....or distorting the output .....its not a horrible distortion....

I havent allowed dc thru my T12/1 I dont think so dont think its magnetized ....I can test by putting metal on the core to see if sticks right.....or is it a weaker magnetism....

thanks once again

SOooo the grid resistor forms a bass roll off then.....the lower the value the more roll off ?

Click to expand...

Yes, the shunt resistance in series with the capsule capacitance form a HPF at the grid. The lower the shunt resistance the higher the cutoff point.

may explain my TB1 clone....I forgot to mention I used a 1 giga ohm resistor for grid instead of 200 Meg.......sooo am I allowing too little bass roll off therefore  distorting the transformer.....or distorting the output .....its not a horrible distortion....

Click to expand...

Depends on the source and the capsule. If the capsule has a good response to subsonics you might be picking up excessive rumble. Maybe try a 200M and see what happens.

You might want to play with the value of the bias resistor at the cathode. If the tube is biased too cold it will clip early.

Have you tried another transformer? What sort of load do you have on it?

http://imageshack.us/photo/my-images/24/tb1l.jpg/

Havent tried another transformer as yet tho I will be able to try the ADK viennas tran soon....

I only used the 1 gig coz I didnt have a 200 meg  so il try and get one then.....

I will have a play with the cathode value too i wonder if i can fit a pot here for other flavours in fact can i fit switchable grid resistors......

Not sure what you mean by what sort of load..is this the plate resistor.....

many thanks

Load = input impedance at the mic pre ...?