[Design in progress] Varimu preamp

Hello everyone!

First of all, I want to say that I am sorry because I am not really good at speaking English, so, sorry for the mistakes :'(

I'm opening this subject because I would like to build some stuff, and I do not know wich solution I should choose.

The goal is to build a microphone preamp first, then I will make a compressor, all for the singer of my band.
I first thought to build it all in one rack, but I think that would be easier in two separates boxes.
The only limitation I have is that I would like to build the two devices with valves. I am not newbie with those components, as I made some stuffs and repaired other's using those components, but I have a lack of knowledge about the different input/output of those devices.

My preference would be to design from scratch, but as I do not know a lot about mic input, line levels with exact datas, I am not sure I will be able to make it. So I thought I should build some stuff already designed, and this is how I found this forum

So, about the first rack, I will build a mic preamp. All valves, and one channel. Simple thing.

I searched a lot on the internet and I found too designs that seems interesting:

The Gyraf G9 (I studied the schematic, and it is really interesting), modified in a one channel preamp
The Redd47, that is simple to me, the only difficulty is to have some 0A2, but I have about 30 6X4 in stock

Their is not limitation about money, I'll take my time to do it, and no specification about the sound, as it will be a surprise

So, what do you think?
- Should I build a G9 by modifying the schematic?
- Or A Redd47?
- Or starting from scratch?

In the last case, I think I'll have many questions


François


PS: Sorry for the big post and stupid questions

Check out the EZTubeMixer LunchBox project:

http://groupdiy.com/index.php?topic=60988.msg772483#msg772483

and

http://groupdiy.com/index.php?topic=53765.0

Cheers

Ian

Hello Ian,

Thank you for your answer! I'll check this when I will be back home. I cannot acces to any drive from work

Goodbye!


François

EDIT: I looked at your EZ Tube Mixer, and it is interesting, but not exactly what I would like to build.
I used one or two neurones and i had an idea.

Tube compressors, called VariMU if I'm right are using a control signal to modify the gain of a valve stage right?
Why not making a preamp using this technique? The difference will be that the control signal will be fixed by the user's will to add or substract gain. Using a push pull configuration to eliminate second harmonics distortion.

Is it a stupid idea?

Cheers!


François

Hello everybody!

I started to think about my idea to start a schematic.

I can't give it to you because I have no access to any host right now, but I'll give it a try at home. Or maybe it is possible to host images on the forum, I don't know.
The input is normal with a pad and switchable phantom power. And finished with an input transformer.
And there is my idea: Two EF183 in push pull operation, with regulated power supplies, and a variable bias.

I couldn't draw any loadline because I couldn't find any curves that permit this (the Ia=f(Va) ones with the Vg curves on it), but I tried to start some calculation.

For the moment, I'll talk about one standalone EF183. The difference will be the load. well, not good enough to speak in English to explain with my words

For example: Load on the anode: 10k Ohms.
Anode voltage: 200V
Screen voltage: 80V
With this values, I found in the datasheet that the slope can varie from 1mA/V when Vg1=-4.1V to 16mA/V when Vg=-1.3V.
So in my exemple, I set the slope to 1mA/V, a signal of 1mV wille produce 1µA variation in anode current, so 10mV will appear there. If I set the slope to 16mA/V, with the same input signal, the output will now be 160mV.
So the gain is changing from 20 to 44dB by just changing this value.

The problem is that it is true only one small signals, like 10mV or so, because bigger signal on the input will start to make some distorsion due to the variation of polarisation. That's why the push pull configuration will be used. I will be able to use larger signals on the input. But with this configuration, I'll have to find or ask someone to build a really good transformer for the output of this first stage

What do you think about this idea? Is it useless to give it a try?

Best regards

François

Hello!!!

I found the option to upload an image on the forum... Sorry, not used to English options yet.
You'll find my idea for the first stage of my mic preamp with this message!

Tell me what do you think of it if you want!

Best regards

François

This is certainly a novel idea. You may need to add a decoupling capacitor to the gain pot because it has dc flowing through it. Pots and dc tend to causes scratching noises.

Cheers

Ian

Hello Ian,

Thanks for the reply
In fact, I think it is not possible to add some DC coupling capacitor to the pot, because it is working on DC. The gain pot is moving the bias voltage for the grid 1 of the tubes, so it has to let DC through it, and as it is wired on the middle point of the input transformer, it doesn't act on the signal (if I'm right).

But, you're maybe (surely ^^) right. This is just a first shot drawing, I'm going to try to find anothyer solution to modify the DC voltage on the grid 1 for my tubes. Or maybe I could try using a variable CCS on each cathode and leave the input transformer.

Maybe this could help to force the tubes working together.

I didn't think it could be a "novel idea", there are so much things that has been done yet...

Regards


François

Maybe a cap to ground will do it.

Hello DerEber,

What do you mean by a cap to ground? From the cursor of the potentiometer to ground?

Regards

François

Hey there!

I worked a little on my project this afternoon, and here is the full schematic I made.

I added a volume control in option on the input stage because I am not sure it could work like that.
The output stage is a copy of the Gyraf G9 output stage's SRPP, in differential mode, because it has the 18dB gain I need to complete my chain.

I just have a little question (again ): the output transformer LL5402 can't be used for my project because it says that it is for unbalanced operation. Could you give me an advice about the output transformer I could use please? I thought about the LL1539, but I have a last question about something.

The datasheets are talking about a max level in dBu. For example, 24dBu. Is it the maximum before destroying the transformer or the saturation point of this one?

See you all and don't hesitate to comment my schematics!


François

I couldn't attach both fils on the same post, so here is the second one (see above)

Cheers!


François

Personally I would not recommend the LL1539 for an output transformer for a tube device. The LL1539 is intended to be used in a feedback loop with a semiconductor output stage. You cannot do this (easily/effectively) with tubes.

Cheers

Ian

Hey there Ian

Thank you for the advice! Wich transformer would you use for this use?

François

Ashayar said:

Hello DerEber,

What do you mean by a cap to ground? From the cursor of the potentiometer to ground?

Regards

François

Click to expand...

Yes. Lowpass filter for the Pot-scratches.

Hello DerEber,

Thanks for the advice. Not a bad idea. This could smooth out the control voltage a little more.

No clue about a transformer I should use for the output of my first stage?

Regards


François

Ashayar said:

Hello DerEber,

What do you mean by a cap to ground? From the cursor of the potentiometer to ground?

Regards

François

Click to expand...

That is what I meant by a decoupling capacitor!

Cheers

Ian

Ah ok!
Sorry for the misunderstanding Ian , I think my English is not really good :'(


François

Ashayar said:

Ah ok!
Sorry for the misunderstanding Ian , I think my English is not really good :'(


François

Click to expand...

No need to apologise for your English. I learntFrench at school but never had much chance to use it. When I was at Neve in the 70s we went to an exhibition and a French company came to the stand to ask about the mixer on display. They spoke no English so I had to try to explain how it worked in French. It was very hard because at school they do not teach you the words for fader, gain, compressor, switch, echo, foldback and so on and so on. It took us a long time working together to find what they wanted and to tell him them we could provide. It was hard work but very enjoyable and we became good friends - and they bought the console.

Cheers

Ian

Thank you for your comprehension Ian

I looked at a lot of Lundahl transformers, and I think that the LL1585 could do the job

Hello everybody!

I think my schematic is going to be finished. There is a thing I thought:
Nothing tells me that the EF183 will work the same with this circuit. So, is ti a stupid idea to put a constant current sink driving both cathodes to force the current moving the same way but opposite in phase?
I just have to find a way to move the current setting and it would be fine
Another thing I have in my mind. Their is a problem with pentodes, called noise partition, so why not using a PNK circuit to rive the screen's?

Do you think the Lundahl l LL1585 would be fine as an output transformer for my double SRPP?

Last question but not least. I copied the SRPP from Gyraf's G9 project as it is. I know that it is supplied with 245V B+, but the information about the current is not precised. I tried to calculate it and I found 7.2mA for one branch. So my total consumption for the two SRPP's will be about 15mA at idle. Is this value correct for you?

If not, I'll have to find a way to design a circuit from scratch for this stage

That's all for the moment! The only modification I made until this post is deleting the switchs to use some relays for all switching movement. (Phantom power, phase, attenuation).

Have a nice day!!!


François