PRR varimu experimentations

[lolo-m]

I've build a PRR varimu today... The result is really interresting and I thought :
OK, it's sounding well and it's limited to 12db of gain reduction ...

The max gain reduction is a result of the 12AU7 characteristics...

I may want more gain reduction without too much distortion ( i'm not sure about this but I experiment )...

Let's had another gain reduction stage !!!

Because I have a regulated PSU, I did it and it's working ( I had a 12AU7 stage with a 2:1 transfo between the 2 GR stages... )

My questions are :
What's the ratio of the tranfo between the 2 Gain reduction stages to keep the original overall gain or isn't it necessary ? Or another way to ask: what's the gain of the GR tube stage with CV = 0V (only tube stage ) ?

 

[PRR]

> Let's had another gain reduction stage !!!

{sigh} There is always some way to complicate any simple thing.

> what's the gain of the GR tube stage with CV = 0V (only tube stage ) ?

This is very obvious from the table on my page. 10mV in gives 100mV out, so total gain is 10. You know by inspection that the 5532 stage gives gain of 11. So gain of two transformers and the tube is nearly unity.

Stacking gain cells does NOT change the input noise or overload level. Output dynamic range is essentially the input range (noise to overload) minus the maximum gain reduction. If the output should be "good", then stacked gain cells is probably "bad". If instead your goal is a "bad" output, no theory will guide you as quick as try-it-and-see.

 

[lolo-m]

Thanks PRR, you're the one who made me understand quickly the principe of that magic and mysterious word : VARIMU .

[quote author="PRR"]{sigh} There is always some way to complicate any simple thing.[/quote]
Yes, but it's funny !
My goal isn't to make "THE COMP"... My goal is to play with a "simple" tube project, trying to understand how it is working, and to learn about tubes. I'm a complete newbie in tube technology. So, of course I say and think completely stupid things. I certainly do not find the easiest way to do what I want. But I don't care, this project is for me really interesting to learn.

I can (and I did) clone things without understanding what I do. DIY is close to provide clone kits. Just put R1 (10K) on the board at the place marked R1 and it's working !!! I always try to understand how it's working.

Stacking gain cells does NOT change the input noise or overload level. Output dynamic range is essentially the input range (noise to overload) minus the maximum gain reduction. If the output should be "good", then stacked gain cells is probably "bad". If instead your goal is a "bad" output, no theory will guide you as quick as try-it-and-see.

Your VARIMU has got a simple path. Of course if I change something, I know there's a lot of chances that it will work bader than before. But changing things help me to understand how things are interfering together.
The better way to have more compression is to choose a more appropriate tube to do the task of course !
My goal in stacking GR cells is just to see if what I understand is right.

Sorry, the title "evolution" isn't what I meant, I changed it to "experimentations". :grin:

 

[lolo-m]

no theory will guide you as quick as try-it-and-see.

So I did...
My goal was :
- to increase the maximum input level without distorting
The 6,25:1 input transfo does it ... :grin:

- to have a possibility to compress more without distorting too much...
This is working really well. The comp with extreme settings and input levels, reacts closely to a brick-wall limiter without a lot of distortion :grin:



With this stage added I certainly raised the noise floor... But I knew that before doing the mod... For the ones who want to try, here is the schematic of what I plugged :
http://pagesperso-orange.fr/NBNS-Studio/PRR-VARIMU/PRR-varimu-variation.bmp
Schematic for ExpressSCH :
http://pagesperso-orange.fr/NBNS-Studio/PRR-VARIMU/PRR-varimu-variation.sch

I will certainly do more precise tests later (one week or two) and I'll post a report... But test it if you own already everything, it has a really interesting result IMO

 

[b2]

Hi,:grin:

That's interesting to play with this comp,Have you try to put an ECC189 (instead of the 12au7 in front end,and for the output let's try an Srpp output stage with the 12au7or a 6922(it's the same output that's is used in the G9 Gyraf,Ok).

And let's go. :thumb:

A+,Bruno.

 

[lolo-m]

[quote author="b2"]Have you try to put an ECC189[/quote]

Bonjour b2, :wink:
That's another step. At the moment I only have 12AU7... So I try and check all I understood...

My last test was made with the basic PRR schematic but:
- 150V PSU
- R1 and R2 at 47K
- 6:1 input transfo
- 1:1 high impedance output transfo.

My goal was :
- Increase the input level range >> 6:1 transfo
- Increase the peak reduction >> higher power supply means longer Ia=f (Vg) means bigger GR range.

I just checked very very quickly but it seams that it's working... I will tell you more later...

My next step is to work on the GR control amp...