Bardamu

Trafo ratio in tube mics
« on: July 26, 2019, 11:01:39 AM »
Can someone explain to me the importance of xformer ratio in relation to tube choice/total equivalent plate (load) resistance when building a mic? I know nearly nothing about electronics, but with the help of someone who does I've built a d-ef47 mic with an ef800 tube and a bv8 is, as far as I can tell, a popular approach that Oliver Archut came up with.

However, I've read somewhere that the output impedance of the ef800 is high and should therefore be combined with a 12:1 xformer. If this were the case, I don't understand why many u47 clone manufacturers are using 6:1 xformers with tubes that have a high output impedance. I'm aware of the fact that the original design used this bv8, but in absence of the vf14 or tubes with a similarly low output impedance, isn't it very illogical from a designer's viewpoint?

I discussed this with my friend, and he says a high mic output impedance can be compensated for by using shorter cables or cables with lower capacitance/meter, but he admitted that his knowledge on tube circuitry is limited.

I found this experiment done by forum member OPR that seems to address this question:

https://groupdiy.com/index.php?topic=58091.0

To my ears, the single ef800 paired with the 12:1 ratio xformer sounds more extended and clear than with the 6:1, but who am I, when all these people that are more knowledgeable than me use 6:1 ratios?

Hopefully someone can chime in. Don't be hesitant to get technical,  I'll let my friend read along.
« Last Edit: July 27, 2019, 07:58:26 AM by Bardamu »


Gus

Re: Trafo ratio in tube mics
« Reply #1 on: July 26, 2019, 05:03:29 PM »
Don't assume people know what they are doing when you read stuff on the web.

squarewave

Re: Trafo ratio in tube mics
« Reply #2 on: July 26, 2019, 05:27:38 PM »
It also depends on the load. Tube mics are frequently specific to the supply / amp they're connected to. So you have the ability to tailor the circuit it's connected to. So if it's 6.5:1 and the load is 200 ohms then the load reflected on the tube would be 6.5^2*200 = 8.5K which is too low. But if the load on the transformer is 2K then it's 6.5^2*2K=85K which is probably pretty good. Or if you use 12:1 you could make the load on the transformer more like 600 ohms and also get 85K (12^2*600=86K). Why use one way over another? Lower impedance is less susceptible to interference over the cable but requires more amplification. Not sure which is better. I'm not really a tube guy but I did make a FET mic once.

Re: Trafo ratio in tube mics
« Reply #3 on: July 27, 2019, 05:14:37 AM »
A mic may have been expecting 200-600 ohms load in the olden  days,  now typically balanced mic inputs are 2-3 kohms ,bigger step down  ratio might impact freq response and your never really going to see 200 ohm inputs anymore .
I sometimes run tube condenser mics into line level 10 or 20 kohms balanced , especially if theres a loud sound source in front of it . The higher load combined with smaller step down means you can put out more level with less distortion , you might experience a little more droop in the high end because the effect of cable capacitance is magnified , but with short cable runs its not an issue .

Bardamu

Re: Trafo ratio in tube mics
« Reply #4 on: July 29, 2019, 06:02:49 PM »
Thanks guys, for clearing things up for me, makes sense. Think i'll just buy buy another trafo in the neighbourhood of 12:1 ratio and let my ears decide.

However, I'm still curious what others have to say, especially those who advocate for a higher ratio trafo with the ef800 in this circuit. OPR, granger.frederic, to name a few.

Gus

Re: Trafo ratio in tube mics
« Reply #5 on: July 29, 2019, 06:48:47 PM »
It is not just the turn ratio you need to think about

Bardamu

Re: Trafo ratio in tube mics
« Reply #6 on: July 29, 2019, 07:34:28 PM »
Care to explain what else?

Gus

Re: Trafo ratio in tube mics
« Reply #7 on: July 30, 2019, 07:42:53 AM »
There is a lot of information on the web.
Some are scans of books with information about transformers.
There are pictures
This forum and other forums with information and clues.
sometimes you can find transformer measurements.

No simple post on a forum will give you this knowledge.

You are on to a good start with your question.

abbey road d enfer

Re: Trafo ratio in tube mics
« Reply #8 on: August 06, 2019, 04:37:42 AM »
Can someone explain to me the importance of xformer ratio in relation to tube choice/total equivalent plate (load) resistance when building a mic?
It is a quite common situation where the final stage of a tube (pre)amp has a low-ish load. Particularly pentodes, which have a very high plate resistance; transformers cannot be built with a nominal impedance higher than a few tens kiloohms. That means that the power transfer is not optimum, but who cares? There's plenty available (several dozen mW) compared to what's needed (about 1mW). OTOH the tube works in an area of lesser distortion, which we care about.

Quote
  I don't understand why many u47 clone manufacturers are using 6:1 xformers with tubes that have a high output impedance.
Transformers with lesser nominal impedance are easier and cheaper to build and offer potentially better frequency response. Considering the fact that most of the recordings are done close-miced, the sensitivity loss is not an issue compared to the gains in response and distortion.

Quote
To my ears, the single ef800 paired with the 12:1 ratio xformer sounds more extended and clear than with the 6:1, but who am I, when all these people that are more knowledgeable than me use 6:1 ratios?
I can only guess that the 12:1 xfmr has received more attention in its design and manufacturing than the 6:1.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

 

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