Designing a tube mic preamp.

Yeah, no need to reinvent the wheel, but I would like to design a mic preamp. Main goal is to learn more about tube circuits in the process.
After studying and successfully building  a dual Mila preamp, I think I can do it.

The design will be simple: 3 common cathode amp stages with negative feedback (maybe last stage will be SRPP) using 2 dual triodes. I've already done some homework, about choosing right cap/resistor values for a given tube and building a PSU.

At this point I'm choosing the transformers and tubes.

1) I would like to use Carnhill or Lundahl transformers, because they are easy to get and not as expensive as some other brands. Which turns ratio would you choose? What would be advantages/disadvantages of having higher/lower turn ratio?

2) Which tubes would a good candidates? I would prefer tubes that are still manufactured. Maybe 12AX7, 12AU7 or 12AT7?

I've read/watched youtube about technical stuff, but I would like to hear your personal opinions.

Anthon said:

Yeah, no need to reinvent the wheel, but I would like to design a mic preamp. Main goal is to learn more about tube circuits in the process.
After studying and successfully building  a dual Mila preamp, I think I can do it.

The design will be simple: 3 common cathode amp stages with negative feedback (maybe last stage will be SRPP) using 2 dual triodes.

Click to expand...

Three common-cathodes under a single overall NFB loop is attracting problems.

I've already done some homework, about choosing right cap/resistor values for a given tube and building a PSU.

At this point I'm choosing the transformers and tubes.

Click to expand...

You should choose the topology first; like a reciepe, you don't choose the spices before knowing what you cook.

1) I would like to use Carnhill or Lundahl transformers, because they are easy to get and not as expensive as some other brands. Which turns ratio would you choose? What would be advantages/disadvantages of having higher/lower turn ratio?

Click to expand...

If you had really done your homework, you wouldn't ask... This is everywhere on the web, and has been debated here a number of times. Like here http://groupdiy.com/index.php?topic=61884.0
I suggest you study the REDD47, V72 & 76, the venerable RCA preamps, such as the OP6 (which has three pentode stages, but only two are under overall NFB).

2) Which tubes would a good candidates? I would prefer tubes that are still manufactured. Maybe 12AX7, 12AU7 or 12AT7?

Click to expand...

Have you set your choice on triodes only? Some of the most successful preamps use pentodes...

About topology:
I was thinking about making stage 1 and 2 common cathode, first dual triode. Put a volume control between them.
Then for stage 3 also 1 dual triode - either in parallel (with NFB to stage 2) or as a SRPP stage. A lot of people seem to love it, for some reason - so why not trying it out.

I wanted to try couple of  different topologies, and see which one sounds better/performs best in practice. The idea is to learn a thing or two, so I would like to experiment.

I would like to avoid pentodes, because I can do more experimenting with dual triodes. For simplicity, I wanted to use 2 tubes of the same kind.
Also, I like the smaller footprint of the dual triodes. But I might consider it.

Yes, I see MILA has only 1 NFB loop, going from 3 stage to stage 2. Then you have volume control between stage 1 to stage 2. All stages are common-cathodes, with 1 NFB. I can use this topology, but then I would design something I've already built (only with different transformers and tubes)...

I already looked at REDD47  line amp, and it seems like it uses pentode in common cathode mode, and a dual triode in parallel, also as a common cathode. It seems a lot of amps use common cathode, Gates amp also had 2 pentodes, used as triodes with NFB, if I understand it correctly.


Obviously, if I had really done my homework, I wouldn't be asking  ;D The idea is to generate some food for thought.
I know higher turn ratio on the transformer means higher gain on the input and less gain on the output, and will also influence grid leak resistor value, for a needed input impedance. Which in turn would affect the tone... I guess...
I wanted to hear some subjective/practicality  thoughts.

Here is something to read for you:
http://www.recordcrate.net/diy/ClassRoom/ga699ac.pdf

Abby is right, you need to set topology first.
Oh I see you just did.
Search for your transformer question here on the forum. It has been answered in great length. Did you notice the transformer Meta? The answer is there.

A modern swiss army knife tube preamp right use two input transformers for different headroom requirements to good effect, but then people would complain about the changing sound rather than appreciating the versatility. 

12AX7 - can be noisy unless you buy old NOS Tele or 6072 (12AY7)

maye a 6DJ8 or a 6N1P for the first stage,

input transformer can be anything, best choice is Jensen 79 dollar 1:10,

output should be UTC of course,

DerEber said:

Abby is right, you need to set topology first.
Oh I see you just did.

Click to expand...

Very interesting article!
I have some ideas about the topology,  but again - it is an experiment, I want to try couple of different approaches and see which one sounds best to me.

But I'm certain it will be a class A amp, having at least 2 common cathode stages and NFB somewhere (for simplicity sake). Most likely a third stage.
Given my intentions, I think it would be logical to use 2 dual triodes.
(the article you provided seems to reinforce my belief, that it would be easier to just stick with triodes)
So 2 common cathode stages will use 1 dual triode, and the second triode will be either a 2 triode in parallel common cathode or a SRPP.
Are there other, 2 triode topologies to consider? On one hand I would like to try different topologies, but on the other hand it would be easier to just use the same setup for all stages.

emrr said:

A modern swiss army knife tube preamp right use two input transformers for different headroom requirements to good effect, but then people would complain about the changing sound rather than appreciating the versatility. 

Click to expand...

Yes, I was thinking about using 2 or even more transformers, for comparison sake.  I would like to know for myself, if it is really worth paying more for an expensive transformer.

CJ said:

12AX7 - can be noisy unless you buy old NOS Tele or 6072 (12AY7)

maye a 6DJ8 or a 6N1P for the first stage,

input transformer can be anything, best choice is Jensen 79 dollar 1:10,

output should be UTC of course,

Click to expand...

Yes, I figured 12AX7 would have higher noise.

Jensen 1:10 would good for the price, I hope they don't charge to much for shipment from US to EU.
I might also try a 1:12 Carnhill (do they produce 1:10? Haven't found one) and LL1935 (will have to remove it from Mila) or even a Hammond - these are easier to purchase in Europe.
I wonder if it will make much difference.

UTC for output would be nice, but they are so expensive  I'll probably use a Carnhill.

I would like to use parts that are easier to find in Europe. I don't want an 'exotic' preamp for this particular project.
It was pretty hard to find 4 good NOS Amperex 12AV7 for the right price (for dual Mila) - I don't want to go through that again.
I would probably make multiple preamps, when I finish designing it. (at least 4)

I have recently done something based on the UA610A.
Two dual triode, arranged in two amplifier section with NFB, so it look like the kind of topology you are looking for.
The main point here is the  the output transformer which need to be gapped.
I have used the LL1538 at the input (well only 1:5, but it is still ok) and LL2745/18mA at the output. LL2745 is oversized and not cheap but it work like a charm.
Tubes are 6N2P and 6N23P-EV but there are several possibilities ...

hey Chris_V,
how did u connect ur  LL2745/18ma : 5.6:4 or 5.6:2

5.6:1, as in the Alt R connection off the datasheet 

cool thanks, whats the B+ voltage

Only ~155V on the last triode in order to have 6-7mA of bias current without exceeding 1W of plate dissipation.
Other triodes are using a B+ of around 220V.
The cool thing is that 48V, 155V and 220V are all done from the same DC/DC converter (18 to 24V input) using a 5 stage charge pump boost converter.

Chris_V said:

I have recently done something based on the UA610A.
Two dual triode, arranged in two amplifier section with NFB, so it look like the kind of topology you are looking for.
The main point here is the  the output transformer which need to be gapped.
I have used the LL1538 at the input (well only 1:5, but it is still ok) and LL2745/18mA at the output. LL2745 is oversized and not cheap but it work like a charm.
Tubes are 6N2P and 6N23P-EV but there are several possibilities ...

Click to expand...

Very neat. The schematic will be useful for some inspiration.
I see it has 4 triodes in series (I wanted to have 3). Does such gradual amplification help with linearity and headroom or are there other advantages?

I find it hard to choose the tubes, because on one hand it all depends on the circuit, but on the other hand sellers like to label tubes with things like 'ultra low noise'  ??? But if it all depends on the circuit, then I guess it doesn't matter which one you use?  Surely this can not be true. Datasheets don't mention anything about noise either.

But if I understand it correctly, lower mu tubes will end up producing less noise, because they don't have to be loaded as much as high mu tubes with plate resistors and feedback loops? Which figures on the datasheet should give me the clue?

If I were forced to design with  12Ax7 type, it'd be 12AU7 and/or 12AY7. 

emrr said:

If I were forced to design with  12Ax7 type, it'd be 12AU7 and/or 12AY7.

Click to expand...

It seems like a lot of DIY projects use 12AU7 and it's still in production. I'll probably go with this one then.

Anthon said:

So 2 common cathode stages will use 1 dual triode, and the second triode will be either a 2 triode in parallel common cathode or a SRPP.

Click to expand...

SRPP is a poor choice for a line driver, unless you have global feedback. For open-loop designs you want a cathode follower, or better yet a White cathode follower. If you really want to use an SRPP then I would suggest:
Transformer > first triode stage > controls etc> second triode stage > SRPP --|
                                                                                                                              ^---feedback--------|

Yes, I figured 12AX7 would have higher noise.

Click to expand...

The 12AX7 is noisier than a 6DJ8, but it still makes an excellent front end for mic use. It also suffers MUCH less from microphonics than the 6DJ8. When you have ~20dB gain from an input transformer, a low-noise tube is not really a pressing requirement.

The 12AU7 is noisy and not particularly linear, but it's a good work horse for low-gain stages and line drivers. Cheap and plentiful, very 'tube sound'. It would make a cheap and cheerful SRPP, but you won't be driving 600R loads.

I have to say that  my recent measurements  re-opened my eyes (and ears!) that the ax7 can certainly be a good choice first tube in a sensitive preamp.

I'm a long time ay7 and au7 fan, BUT thesedays, my results are showing me that moderately selected common ax7s may have the edge, by a couple of dB, in terms of noise margin with a given (and decently high!)  gain.

It's a fine balance between gain, noise floor and then thd character AND microphonics!  arrrgggg 

I think one of each is needed     

That's 9 pin AND octal    !!

And it changes over the years  - I thought I left ax7 behind years ago,  but .. just when I thought I had gotten away  .. they .. pull .. you .. back in.

Couple of points.

When I first started designing tube mic pres over 10 years ago, the first thing I did was study and analyse as many existing designs as I could. This shows you the types of tube that are typically used in mic pres and also the topologies. You also notice that they nearly all start with a 1;10 input transformer.

Second point is there is nothing wrong with the 12AX7. It is one of the very few tubes actually designed for audio use and it can be ideal as a first stage. Like all tubes, it produces noise, distortion and can be microphonic. My own tests have shown it is no noisier than any other popular triode type. My own tests have shown that its distortion levels are amongst the lowest of any of the popular triodes. Lastly, microphonics. This is mainly down to manufacturer. Without exception I have found NOS types are badly microphonic. I have tested lots of current production 12AX7 tubes and the most consistently un-microphonic are the Sovtek 12AX7WA types.

Cheers

Ian

merlin said:

The 12AX7 is noisier than a 6DJ8, but it still makes an excellent front end for mic use. It also suffers MUCH less from microphonics than the 6DJ8. When you have ~20dB gain from an input transformer, a low-noise tube is not really a pressing requirement.

The 12AU7 is noisy and not particularly linear, but it's a good work horse for low-gain stages and line drivers. Cheap and plentiful, very 'tube sound'. It would make a cheap and cheerful SRPP, but you won't be driving 600R loads.

Click to expand...

ruffrecords said:

Couple of points.

When I first started designing tube mic pres over 10 years ago, the first thing I did was study and analyse as many existing designs as I could. This shows you the types of tube that are typically used in mic pres and also the topologies. You also notice that they nearly all start with a 1;10 input transformer.

Second point is there is nothing wrong with the 12AX7. It is one of the very few tubes actually designed for audio use and it can be ideal as a first stage...

Click to expand...

alexc said:

I have to say that  my recent measurements  re-opened my eyes (and ears!) that the ax7 can certainly be a good choice first tube in a sensitive preamp.

Click to expand...

Thank you - I'll certainly do some experiments with 12AX7 then! Maybe ax7 as the first stages, and au7 at the output.
1:10 transformer seems like a good start also.

I guess I'll do some shopping now  ;D

If I might make a recommendation regarding transformers....

Take a look at OEP transformers.  They're surprisingly good for the money, a lot cheaper than Lundahls.  Don't get me wrong; Lundahls are worth every penny in terms of sound quality.  But, if you're experimenting, tinker with the OEP's, and once you arrive at a design you like, you can always upgrade to Lundahls later.  This way, if you accidentally melt something, you won't be crying in your beer.