Humner

Summing mixer/make up gain idea
« on: August 13, 2013, 02:14:56 AM »
I don't have a deep understanding of this, so hopefully this idea doesn't make me look silly  ;D

Could this work?

Passive summing 8 channels into 2 channels which go to a 300:2k7 Tamura transformer > unbalanced makeup gain circuit > 600:600 Tamura transformer to make it balanced again on the output.

Would this work? Could I use/modify a simple mosfet booster circuit -  http://www.muzique.com/schem/mosfet.htm - as make up gain?


ruffrecords

Re: Summing mixer/make up gain idea
« Reply #1 on: August 13, 2013, 05:07:40 AM »
Yes, this is a common approach. The 300:2K7 transformer gives a useful almost noiseless gain and provided the unbalanced gain make up has a good noise figure, enough gain and an adequate drive capability you should be able to strap 600:600 transformer across the output to balance it.

Unfortunately, the little FET booster you linked to does not meet all of these requirements.

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

Humner

Re: Summing mixer/make up gain idea
« Reply #2 on: August 13, 2013, 05:30:45 AM »
Thanks for getting back to me.

Probably best going with a 312 style circuit for the make up gain?

ruffrecords

Re: Summing mixer/make up gain idea
« Reply #3 on: August 13, 2013, 11:37:27 AM »
Thanks for getting back to me.

Probably best going with a 312 style circuit for the make up gain?

Pass. What's a 312?

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

joaquins

Re: Summing mixer/make up gain idea
« Reply #4 on: August 13, 2013, 04:20:39 PM »
Thanks for getting back to me.

Probably best going with a 312 style circuit for the make up gain?

Pass. What's a 312?

Cheers

Ian

Probably an API style gain stage, but 312 is a mic pre... He's probably thinking in a 325 line amp.

JS
If I don't know how it works, I prefer don't turn it on.

JohnRoberts

Re: Summing mixer/make up gain idea
« Reply #5 on: August 13, 2013, 06:01:01 PM »
I'm not familiar with the API model numbers but 8 input passive sum needs make up gain of at least 8x so if line input has that much gain trim it could work, mic pre would accommodate summing even more stems.

JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

joaquins

Re: Summing mixer/make up gain idea
« Reply #6 on: August 13, 2013, 11:43:35 PM »
325 is basically the 312 mic pre without the transformer, but he want to add a transformer so it's the mic pre... depends on how you draw it, transformer inside/outside...

JS
If I don't know how it works, I prefer don't turn it on.

Humner

Re: Summing mixer/make up gain idea
« Reply #7 on: August 14, 2013, 01:26:45 AM »
say I take the circuit of the Access 312 - http://www.groupdiy.com/index.php?topic=46362.0

I wouldn't need 48v, phase or other additions, just the main gain stage between the transformers.

ruffrecords

Re: Summing mixer/make up gain idea
« Reply #8 on: August 14, 2013, 04:30:25 AM »
I have to admit that because you said it was a passive summing mixer I ASSumed you have balanced inputs and that is the reason for the transformer. Even with balanced inputs you don't have to use a transformer. However, if you use a transformer designed for a 300 ohm input then you will have to slug the bus to match this impedance so you will need more make up gain than the 8X that John mentioned.

So, a little more detail about the application would be handy.

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

tv

Re: Summing mixer/make up gain idea
« Reply #9 on: August 14, 2013, 08:42:27 AM »
So ... what do you WANT?

An opamp based make-up for your transformer-based mixer?

Or do you want a deliciously crusty MOSFET gain stage that will considerably warm-up your precious tone - while also considerably warming up the surroundings? You know, pure Class-A. Almost like a real valve ...

Decisions, decisions, eh?

Look up "Bride of Zen" and "Moosefet". Google, of course.

Read, study, learn.


P.s. To get close to the "8x" makeup gain as JR suggested, you will have to mod the source and drain resistors values, f.e. 100R and 820R in BoZ.
If you sprinkle when you tinkle, please be neat and wipe the seat.


Humner

Re: Summing mixer/make up gain idea
« Reply #10 on: August 15, 2013, 04:16:10 AM »
I just noticed this thread - http://www.groupdiy.com/index.php?topic=52068.0

So I now want one of those boards(Ian I have sent you an email, hopefully you have some left) - and there is a nice guide to calculating resistor values.

Hi TV,

I'm just after a simple make up gain circuit to fit between 300:2k7 Input and 600:600 output transformers(just because I have them lying around).

Can be Opamp, DOA, fet, transistor based. Not fussed, just after a simple design. Thanks for those ideas, I will check them out. However looking at ian's db25 summing board thread, maybe going the Access 312 mic preamp design and "slugging" my buss channels might be the way to go.

I have to admit that because you said it was a passive summing mixer I ASSumed you have balanced inputs and that is the reason for the transformer. Even with balanced inputs you don't have to use a transformer. However, if you use a transformer designed for a 300 ohm input then you will have to slug the bus to match this impedance so you will need more make up gain than the 8X that John mentioned.

So, a little more detail about the application would be handy.

Cheers

Ian

Ian, that's right, you have assumed correctly. I'm going through your resistor values guide, and its very helpful(along with your post).
« Last Edit: August 15, 2013, 04:25:18 AM by Humner »

tv

Re: Summing mixer/make up gain idea
« Reply #11 on: August 15, 2013, 06:38:26 AM »
Ah-hah ... if so, I suggest that you go with an existing design, if possible available as a kit, with good support/troubleshooting docs.

Even though BoZ et al. are afaik also available in kit forms, I think those mosfets are a little hard to tame if you haven't tickled them before... especially if you plan to transformer-couple the damn thing.
If you sprinkle when you tinkle, please be neat and wipe the seat.


 

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