micpre#5

maybe someone more experienced could have a look on this:



* there are two feedback loops with hopefully advanced switching possibilities - i didnt calculate the values of the resistors, i hope you get the idea of the multiple feedback gain control in the different stages.

* i hope the output stage - dc coupled 6au6 - will work that way. it´s based on what i´ve grabbed from prr´s post.

* i wonder if the filaments should be seperated for the triode and the pentode, and how i could avoid it.

if you feel like adding something to it, you are welcome :guinness:

> i didnt calculate the values of the resistors

You can't.

You have the 250K pot inside the V1 V2 feedback loop. Down to a point it does almost nothing; after that point you have no feedback.

The V2 V3 feedback loop interacts with the V1 V2 feedback loop.

The feedback loop on V2 V3 is positive feedback.

Scrap the gain pot, take one feedback loop from V3 cathode to V1 cathode.

Is there any reason to have the C-R network at the first grid?

Output drive is 2mA peak. Adequate for 10K loads.

and use a less than 10 foot long cable on its output.

thank for the answers.

reason for the r-c on first grid is adding a possible di input after the transformer.
what about adding a 4:1 output transformer?

Reason for the R-C on first grid is adding a possible DI input after the transformer.

Click to expand...

Well, then use the C only for the DI input and move the resistor in front of the cap. Wouldn't it be wise to switch the transformer out when using the first grid as DI input?

Samuel

[quote author="Samuel Groner"]Wouldn't it be wise to switch the transformer out when using the first grid as DI input?[/quote]

shure, i didnt draw it, but i think about a switched jack like in the g9. when the input transformer is switched out, the 1M becomes the grid bias resistor.

Ah, I see. :thumb:

Samuel

prr, maybe you could explain a little further
- i do understand how the 250 k pot disturbs the v1-v2 feedback loop. but what exactly happens when the two feedback loops interact with each other?
is it a matter of mixing different feedback types (negative/positive) or dont you use two feeback loops (with different sources) in general?

best regards, max

> what exactly happens when the two feedback loops interact with each other?

In this case: I don't know and life is too short to figure it out.

Anyway there is no need for two feedback connections plus a passive gain control. It can all be done in one FB loop. Or use just the local feedback you already have (cathode resistors) and the passive pot.

With a few specific exceptions, positive feedback is bad for audio. Let's get that mistake off the table before going forward.

[quote author="PRR"]With a few specific exceptions, positive feedback is bad for audio. Let's get that mistake off the table before going forward.[/quote]

so maybe there´s a way to wire the 6au6 in a different way?

thanks for taking your time :thumb:

update:

The transformer on the output is going to deliver more consistency with various loads in the real world.

Once you are commiting to a transformer, why not put it in the plate circuit of a single tube and dispense with the cathode follower and cap? One less tube. Same gain.

Oh and add some negative feedback somewhere, too=). Cathodes are bypassed so no feedback there..

You still don't need the input capacitor.

Even with your pot cranked all the way up, you've still got a 6dB attenuator. If you don't need so much gain, it's better to not produce it in the first place.

You'll have a high output impedance from the first stage, which will probably require a higher value pot. Is that why you stuck the 100k resistor in there?

The 5879 would be a good choice for input tube, since that's what it was designed for. Readily available too.

Do yo know how much gain you're going to get out of this thing?

[quote author="ulysses"]Do yo know how much gain you're going to get out of this thing?[/quote]

Some 57-60dB in the first stage, which might be appropriate for a ribbon mic, but a 1:5 transformer won't lift the signal up enough for the tube noise to be insignificant.

Peace,
Paul

[quote author="ioaudio"]update:

[/quote]
I am interested in 6AU6 based mic pres. Unfortunately your schematic pic link no longer works. Could you please repost it? Cheers

ian

i never build that thing, today i would change the second stage.
at that time i had a couple of 6au6 and wanted to use them all, and had the strange idea of using them as triodes in a srpp configuration for the output stage.