Aikido microphone preamp

Hey guys,

Having thrown some ideas out there for a simple tubed mic preamp, I have changed my mind a bit. In my concurrent seach for a phono stage to build, I ran across the Aikido amplifier circuit at tubecad.com. It looks ideal for a mic preamp.

Basically the Aikido does 3 cool things - it injects the inverse of the power supply noise into the cathode follower to cancel power supply noise, and it loads both the gain stage and the cathode follower with the same tube, which has a neat distortion cancelling effect. As a bonus, you can use many different tubes in it without any changes to the circuit.

Here is the Aikido info:
http://tubecad.com/2004/blog0011.htm
http://tubecad.com/2004/blog0013.htm
http://tubecad.com/2004/blog0016.htm#MoreAikido
http://tubecad.com/2005/January/blog0030.htm
http://tubecad.com/2005/January/blog0032.htm
http://tubecad.com/2005/July/blog0051.htm

Of course you can just go to tubecad.com and read all about it.

I would like to adapt this to a mic preamp using an input and output tranformer for balanced operation, and around 40dB of gain with only 1 gain stage and a cathode follower. Of course using the Aikido topology you need 2 tubes for this so the whole one-bottle thing is out.

Here is the schematic I have in mind:


I figure with Altec 15095 input and output transformers, and a 6sl7 for gain and a 6sn7 got the cathode follower, I should get about 36dB gain, minus various losses. To get less gain, you could use a 6sn7 for the gain tube. For more gain, you could use an Altec 4722 as the input transformer , or an Altec 15335 10k:10k as the output transformer, or both.

And if you used 9 pin tubes, you could play with *tons* of different tubes for various levels of gain and to play with the sound of all the available 9 pin tubes.

What do you guys think about the volume pot right off the input transformer? Is there a better way to do it?

I also plan on adding a -20dB pad, a phase switch, and phantom power.

I have this thing started, and will post more info as things progress. I have been trying to perfect an eyelet circuit board and ground bus construction technique, that I hope will be neat and quiet.

jsn

I´m not an expert on mic preamps but 36dB seems to be a little low, compared with 60 or 70dB on others preamps.

it run on class AB?

Yeah 36dB may be low for some environments. I am recording using a usb soundcard with it's own gain controls, so the mic preamp doesn't have to do all the work, and I don't have to drive long cables or 600 ohm inputs.

This design with a 6sl7 gives you 30dB of electronic gain, not counting the transformers. You only get half of the tube's inherent gain.

If you wanted the most gain possiblw out of this thing, you could use an Altec 4722 (1:18 = 25dB) on the input and an Altec 15335 (1:1) on the output, and get 55dB of gain out of it, at the expense of a little bit more output impedance.

And if you need less, you can just plug in different tubes and different transformers.

jsn

Here is the finished preamp:
http://boozhoundlabs.com/microphone-preamp-aikido/

It's CRAZY quiet, like with the mic unpluged it doesn't even register on the meters in my recording software. I am using an Alesis io2 for the ADC. It appears that the playback noise is more than the noise from this thing because I hear hiss on top of hum, but nothing registers on the meters. I don;t have my setup dialed in, and my laptop is definitely contributing noise, but still, that is pretty quiet.

The sound - very neutral. I wouldn't even call it "warm" necessarily.

With 1509 input and output, it needs a bit more gian, although it does the job. I want to try a 1:1 output since the output impedance is only like 350 ohms.

I also want to try something like this with Edcor transformers, Allied power transformer (<$20) and Sovtek tubes. Cheap.

jsn

Try a 2k4:600 2:1 output.

Nice stuff there jsn! I like the Aikido circuits too. :thumb:
Edcor will wind you the XSM2.4k/600 for the same price as the standard XSM line. I have a couple them here.

Cool. I need to call Edcor. I am only a couple hundred miles north of them so it is only fair to try to use thier stuff whenever possible.

jsn

That looks really cool. Great work!

Full kits are now offered for this amp. :grin:

http://glass-ware.stores.yahoo.net/newhardware.html

are they your kits volta?

From the site linked to:

"COMPANY PROFILE

We at GlassWare are audiophiles like you. We want superior musical sound; that's why we build with vacuum tubes. But we know that doing all the math involved in circuit design may not be your idea of a good time...and that's why they invented computers, wasn't it? Our software does the hard math for you. So you can spend your time doing what you love: building and enjoying tube high-end audio components. For more information, visit our Web sites: www.glass-ware.com and www.tubecad.com (The Tube CAD Journal). Links are on the home page of this store."

[quote author="ipagel"]are they your kits volta?[/quote]

No,these are offered at tubecad.com
Not as DIY as jsn's amp but still DIY in the sense you have to assemble and test.

[quote author="volta"][quote author="ipagel"]are they your kits volta?[/quote]

No,these are offered at tubecad.com
Not as DIY as jsn's amp but still DIY in the sense you have to assemble and test.[/quote]

thanks i was just trying to work out what you need to build the kit.
what power supply ect..

i can never work out what you need to get :?
im the same when i look at the JLM kits so many options i just get lost.

"All that is needed to finish the entire line stage is a chassis, power supply, volume and input selector controls, and a handful of RCA jacks and hookup wire. Although the Aikido PCB kit isn't all that is needed, it is a great start."


Simple power supply needs. 300V B+ and heaters, in the following image you can see how simple Jsn's is:

http://boozhoundlabs.com/microphone-preamp-aikido/images/aikido-mic-pre-2.jpg

Looks like a full wave rectifier followed by 3 RC filters.

thanks i was just trying to work out what you need to build the kit.
what power supply ect..

Click to expand...

If you do not want to build a power supply from scratch you can give these guys a shot.

http://www.welbornelabs.com/ps8.htm

http://www.welbornelabs.com/ps10.htm

jsn, what kind of cart?
must be moving coil, with the xfmr up there...

heard the shure v15 mixes the channels because of the magnet.
is this true?
better seperation with a moving coil on the cymbals?

is that amp to bump up a signal for a phono in, or does it do all of the amplification-ready for the pwr amp input?
in other words, line out level, is that what you are trying for?

[quote author="CJ"]is that amp to bump up a signal for a phono in, or does it do all of the amplification-ready for the pwr amp input?
in other words, line out level, is that what you are trying for?[/quote]

It's kinda in the same spirit as the Neve 1272......line amp turned into a mic pre.
The site where the kits are sold suggest it can be used for line or pre,etc....

some have returned to the purity of mono hi-fi (half the cost, half the headaches), on the other hand some are running five-channel systems and they require a single Aikido board for the center channel, some need a single microphone amplifier

Click to expand...

forgot to mention...... nice work jsn :grin:

Bump.

I was just wondering, while putting my own Aikido-ish thing together with a 6SN7 as the output tube - I suppose you need to bias the heaters up by DC, or we would be violating the heater-to-cathode limits of the top 6SN7?

The heater-to-cathode DC limit of a 6SN7 seems to be 100v, and the cathode voltage of the top tube will be 150vdc with a 300vdc B+.

EDIT: The more I think, the more I know I'm on the right track here. :grin: I assume biasing up by +75vdc would work well?

Now that I think of it.. My Analag opto comp has a B+ of 250v, and a 12AU7 at output in a similar configuration. That'd be 125vdc cathode voltage, and the limit is 100v. I think I need to do some little biasing here, too.

Now that I think of it.. My Analag opto comp has a B+ of 250v, and a 12AU7 at output in a similar configuration.

Click to expand...

Isn't the same true of the G9?

Those Akido PCBs look nice. I might knock something together.

[quote author="zebra50"]Isn't the same true of the G9?[/quote]

G9 seems to be an SRPP output, but as I understand it, the cathode voltage will still be 125v at the top tube.

I actually began to modify my Analag compressor, I just wanted to see what it does. Will report back on monday..