One Tube Pre

Here is a grounded grid pre which features no phase inversion, high gain and low input resistance, hence a 150:600 or 600:600 input transformer should be used. A minimal amount of feedback is injected at the grid to improve performance. Great sound and simplicity go hand in hand with this design.
It's a mic pre, for those who might be wondering.

analag

:thumb:

Any other suggestions for specs on the input trafo besides the ratio?

Cool, grounded grid! I don't think I've ever seen that before... Any suggestions on literature covering this topology?

Peace,
Al.

You might remember that grounded-grid mic amp idea I sketched not long ago... Unfortunately, neither of us are immune to the noise issues.

PDF

However, unlike me, I know that you don't have a distaste for hybrid designs, so optimization is possible here with a little silicon at the front end.

[quote author="Svart"]:thumb:

Any other suggestions for specs on the input trafo besides the ratio?[/quote]

http://www.cinemag.biz/mic_input/CMMI-2C.pdf
If you use this trannie, wire up the 6.19k as shown then go to the 47uF cap. And here is a chance to try out that wonderful XSM10K:600 from EDCOR.

[quote author="AL"]Any suggestions on literature covering this topology?
[/quote]

As they say "google it" try that first, it should bear some fruit.

http://www.transcendentsound.com/preamp%20kit.htm
not exactly the same thing, but this should give you an idea of the sound since they are in the same ball park.

analag

[quote author="NewYorkDave"]However, unlike me, I know that you don't have a distaste for hybrid designs, so optimization is possible here with a little silicon at the front end.[/quote]

If I were to use silicon in this design I would only use them as active resistors only. Believe it or not, but I have disproved much of what I hear about noise with my input transformerless tube pre. It's a thing to hear!!!
Some of these old literatures concerning tubes should be outdated, as advances though incremental have been made especially in the surrounding electronics. Hybrid designs that introduce sandstate into the plate and cathode circuit of any tube stage can be seen as a definite improvement if improvement is what we want. Unfortunately distortion is the order of the day.

i have a few xsm10k:600s laying around. would be a nice time to try them..

wow those tubes are only 3$!

truly another bargain preamp.

You should have posted this on monday when i made a huge valve order for the Fedral limiters.. :roll:

Most people use the 12xx7 and a few others, but there is a lot more tubes that gets overlooked I mean really nice cheap NOS tubes. I have about (50) 5965 sitting around in my tube box.

Ive dubbed it the "CodeCas" as it looks like a role-reversed cascode! (almost)

In addition to getting feedback into the grid of the first stage from the 68 ohm resistor above ground, there is an added benefit of injecting "anti-noise" from the PS there too for more rejection.

Excluding the xfmr gain, Analag, what is the gain of the 1st stage?

> features no phase inversion

It's got two transformers; what does it matter if the amp inverts? Swap a winding.

> high gain

1:2 input iron. 1:30 in the tube. 4:1 output iron. Gain is 15, or 23.5dB. That's assuming a zero-Z source; with typical dynamic the gain drops to 17.5dB. Take a dB or so for transformer loss, 16dB. High gain?

> low input resistance

Looks like 150 ohms.

> A minimal amount of feedback

There is no feedback in the audio band.

What you do have is some damping of the output transformer/cap resonance. A hasty calc says 68 ohms might even be "critically damped" for a transformer similar to this. And gain times 68 gives a passive output resistance that would do the same job. Better: this feedback is "exaggerating" the transformer saturation, something you could do (if you wanted) by using a smaller cheaper core.

> disproved much of what I hear about noise

I'm not interested in "noise" of a 22dB-16dB gain preamp. The input noise is around 0.5uV, the output noise near 5uV, the S/N re:+4dBV is 108dB, re:+20dBV is 124dB. In situations where ~20dB of gain is enough, noise is NOT an issue even if it is 10 times higher than theoretical.

> Great sound

I'm glad you like it, and that you can live with such a low-gain preamp. You must have hot mikes and sources.

[quote author="PRR"]I'm glad you like it, and that you can live with such a low-gain preamp. You must have hot mikes and sources.[/quote]

Active engineer here, I know what I need and how to get the sound I want. She delivers that why I posted the schemo.
By the way, did you know that a mic like the popular G7 can peak out at upwards of 1V, hmmm? Next time I'll say Condenser mic pre, my bad.

analag

> did you know that a mic like the popular G7 can peak out at upwards of 1V, hmmm?

Toys. My recital room homebrew mikes make 8Vrms every time the drummer goes nuts.

But back to normal stuff:

I've observed 400mV from AKG414s in the audience in front of an orchestra. I've inferred over 2V on hotter mikes a few feet from orchestral percussion.

But I also need 35+dB gain on some acts to bring these same 414s up to a measly -10dBV nominal (2V peak), and over 48dB gain on some small condensers for the same recording bus level. 50-55dB was common for dynamic mikes on small acts. I have not used a ribbon in years, but that's where noise gets critical. One of my mixers has over 80dB of gain on tap: I've never needed THAT much, but I've had it pretty cranked.

[quote author="PRR"]
Toys. My recital room homebrew mikes make 8Vrms every time the drummer goes nuts.
[/quote]

Where's the link to those? :grin:

> Where's the link to those?

You can't have my nutty drummers. Find your own.

just out of perverse curiosity would there be any major drawbacks to not using a transformer on the input? you could feed two differentially for a balanced input?

Many a person has tried. Usually as hissy as a box of snakes.

[quote author="Svart"]wow those tubes are only 3$! truly another bargain preamp.[/quote]

I'll send you a few for the cost of shipping. PM me if interested.

I can throw them in my One-Bottle pre to make sure they are working okay. I think I did that when I first built it, but I can't remember is I ever listened to the 5965s. Maybe I will whip up Analag's pre, all the parts are within an arm's length from me. It might be cool in front of a line amp.

I really think the idea of modular gain stages is cool. Of course it's as old as broadcast, but it would be nice to put some flexible gain staging in a single box. Switch in and out as needed, remembering to flip polarity on the output if an inverting stage is removed.

Like the gain switch in the schemo on this page. Removing the first triode when less gain is needed.

[quote author="toby"]just out of perverse curiosity would there be any major drawbacks to not using a transformer on the input? you could feed two differentially for a balanced input?[/quote]

One of the key functions of an mic input transformer is to step up the source resistance (low Z) to the optimum source resistance of the first stage of the amplifier thus achieving lowest possible noise voltage.

Transformerless transistor based pre amps uses specially selected transistors or multiple paralelled transistors to achieve decent noise results without a transformer.

With a differential input w/o transformer using tubes, there is no practical way of bringing the optimum source resistance of the tube to match the low Z of the mic.

[quote author="PRR"]> Where's the link to those?

You can't have my nutty drummers. Find your own.[/quote]

Speaking of nutty drummers, I just spent the last couple days in a session that Hamid Drake was playing on. That guy can make a stick on the rim of a snare sound like a shotgun. Frighteningly good.