Langevin 4000series EQmake-up circuit mods.

[rafafredd]

I got my Langevin mic preamps working after fighting with ground loop noise for quite a long time:

http://hps.infolink.com.br/rafafredd/guts_small.jpg
http://hps.infolink.com.br/rafafredd/panel_small.jpg
http://hps.infolink.com.br/rafafredd/board_small.jpg
http://hps.infolink.com.br/rafafredd/AM4103schem.JPG

So, I´m working on my EQ make-up gain boards now that will fit on the two free slots that you can see on the pic. I´m trying to mod the original boards so that it can drive low impedance outputs. Here is what I had:

http://hps.infolink.com.br/rafafredd/MVC-315F.JPG

And what I have now after simulating some mods:

http://hps.infolink.com.br/rafafredd/4302r_max_gain.gif
http://hps.infolink.com.br/rafafredd/4302r_min_gain.gif

I added the output transistor pair and changed the emmiter resistor of the 2n3053 from 9.1k to 600R. Hope it will work fine like it´s simulating...

So, I´ll use the NYDave Passive EQ circuit before those boards.

The maximum input allowed without BAD clipping (on the simulation) is just 200mV. Would this be enought for a line level after attenuation of the EQ circuit?

Also, it can put, according to the simulation, almost +-10v on maximum gain setting with an input of 200mV!!! And almost +-4v with the same input, but gain set at minumum, as you can see on the simulation pics I´ve posted. And this with a load of 600ohms!

On the high gain setting I have a ratio of 9.5\0.2 = 47.5 = ~33db (I hope my math is right, as I hate to say ******** in here)

On the lower gain setting I have a ration of 3.8\0.2 = 19 = ~26db

So, I think it will work fine, if all the calculations is right. I just would like to hear some opinions about the mods in question before I go and solder all those extra components on my tiny boards. Really, any opinion is really welcome at this point. And I´m a little concerned if 200mV max input would be enought.

Also, I will let you know how this thing works out, as I think this a very interesting simple circuit. The mic pre sounds really nice, so this make up amp should also work nice...

Thanks a lot one more time!

 

[rafafredd]

Well, in fact I got a look at Dave´s EQ circuit again and I think the loss in this circuit to too little and I will end with more than 0.2v after the passive circuit. Maybe I´ll have to use another EQ circuit if I wanna use this make up gain...

 

[NewYorkDave]

Or put a 4:1 transformer in front of it. That would give you an additional 12dB of attenuation, and also ensure that the EQ is always driven from a very low impedance source, which it needs for best operation.

 

[rafafredd]

Thanks for the comment Dave.

I´ve changed the gain arrangement a liitle because the thing was not stable on the low band. Now I have a little less gain, what is good and it´s simulating very stable from 15Hz to 200Khz.

I just don´t know how good are those simulators.

Anyway, now I have an output of +-7.5v for an input of 350mv. More than 350mV will distort the thing...

Here is the updated circuit:

http://hps.infolink.com.br/rafafredd/4302r_max_gain.gif
http://hps.infolink.com.br/rafafredd/4302r_min_gain.gif

27dB at the maximum gain setting and 24.5dB at the minimum gain setting. :roll: It seems that the safe operating points of this thing is ridiculously short... Maybe not even worth putting a gain knob.

I´m still worried about the value of R5. Is it too litlle for the emitter resistor of a 2N3053? Do you see any possible problems with this value?

Anyway, I will start soldering tomorrow, so I will be testing it soon...

 

[NewYorkDave]

[quote author="rafafredd"]I just don´t know how good are those simulators.[/quote]

In my limited experience, not very good at all, especially when you're trying to simulate the circuit at the outer limits of its performance. Circuitmaker is a cool toy to kill time during lunch, but I advise against investing too much time in a simulated circuit without checking its performance in real life.

In the case of every single active circuit I've tried on Circuitmaker and then built in real life, Circuitmaker has lied to me to some extent, usually predicting better performance than the real circuit could deliver. Then there are times when certain circuit configurations, which work great in real life, will simulate poorly or will cause SPICE to crash. As an example, I have not been able to get CM to run a simulation on a vacuum tube cascode. It keeps throwing errors.

I'm not singling out Circuitmaker per se, but just using it as an example because that's the program I've used. I've heard similar complaints about other flavors of SPICE programs.

 

[rafafredd]

So, I ended with this:

http://hps.infolink.com.br/rafafredd/passive_EQ_SS_makeup.gif

Gyraf style passive section, with some frequency mods to fit my inductors, and the Langevin make up gain.

I used Gyrafs because in this case it´s better to have more loss, as my make up boards have plenty of gain, so that´s what I´ll do.

But the NYDave EQ circuit will go in another project very soon. And I´ll maybe use a tube make up for this one.

 

[rafafredd]

After I´ve read a PRR post saying that we want at least 14v for pro line level outputs, I got back at my EQ circuit and continued to tinker.

But the maximum output this thing gives me is a little more than +-10v (the operating point reads just 8.9v RMS).

Here is a pic:

http://hps.infolink.com.br/rafafredd/passive_makeup_test.gif

This is a 20Hz wave with the low at total boost.

So, does all this thing I´m working on just sucks and I should find another use for those boards as well as another board for this use... :roll: ?

I thought 8.9v RMS would be enought even to drive those old big ass compressors 600ohms inputs...

It seems that I was wrong.

 

[rafafredd]

got it...

+- 15v peak to peak at 600ohms load.

http://hps.infolink.com.br/rafafredd/passive_makeup_test2.gif

Now it´s time for soldering!

The little board with the output trasistors is done.

 

[sismofyt]

Looks nice rafafredd! :thumb:

Did you ever get a PSU up and running for those modules?

I used a simpler single darlington transistor emitter follower for beefin' up the output btw.. :grin:

 

[rafafredd]

Yes, the box is ready to receive the EQ. It already have two mic pre boards in there (working nicely), and two empty sockets for the Langevin makeup boards. All will run from the already installed +42v PSU. I will also install the stereo booster board I´ve done.

Have a look:

And this is my booster card:

Bad soldering, no?

I hope this thing works and I´ll have a 2 channel micpre/EQ in a 2U chassis based on those nasty Langevin boards... :green:
Well, I´ll put it all together and see how it works in real life.

 

[sismofyt]

The soldering looks fine :grin: The PSU looks small though :? ... remember the Langevin cards have no PSU decoupling whatsoever. I have soldered a 470yF + 100nF directly onto the print connectors since I don't use those chassis connectors...

Did you make boosters for the micpre cards too? They have a similar weeny output stage like the Hi-Z cards.. you could put a pot in between and overload the pre but not the outputstage for some colour.. :wink:

 

[rafafredd]

No, no need to boost the mic pre cards I´m using. Have a look:

http://hps.infolink.com.br/rafafredd/AM4103schem.JPG
http://hps.infolink.com.br/rafafredd/AM4103.jpg

It seems that they can drive 600ohms just fine.

Yes, I´ll try to include 470uF +100nF caps for each board. Thanks for the idea... I just thought that 2200uF would be enought for the four boards... That´s what I´m using now.

 

[sismofyt]

Is that 2200yF in the PSU? It depends on what design you've made. Bypassing each card is a very good idea.

I thought the Hi-Z cards were basicly micpre cards without a trannie, but they do look a bit different.

 

[rafafredd]

In fact the ones I´ve sent you were exactly like mine, but I´ve moded mine to drive 600ohms acording to the schem I posted.

Here is the schem for the unmoded boards:

http://hps.infolink.com.br/rafafredd/MVC-313F.JPG

The output impedance was really high, so I moded it according to the schem I posted above. Substituted the electro cap on the feedback for a film type (the big red one), added a bigger electro on the output (bypassed with film, but will put a better quality one there sometime) and added that 2n3053 "flying transistor" in there. Also, changed some resistor values and added NPO ceramic caps.

It looks funny now:

I´ll see if I can stuck a 220uF or at least 100uF cap on those boards for decoupling. It´s already very tight.

 

[rafafredd]

WAIT!

Now I remenbered that it´s a little suspicious when you say that a PSU is not big enought. It´s never big enought for you no?

:wink:

I love to see those SlowBlow mic pre pics with BIG CAPS! You are just crazy, but I like it!

:thumb:

 

[rafafredd]

I know that this one has been going very slow, but I would like to know what should I use for shileding the inductors.

I think aluminum won´t do it...

So, should I go brass, copper, iron or steel??? I hope there´s no need for mumetal.

here they are:

and here is the moded Gyraf Passive EQ mid section that I will use:

3.3nF - 20mH = 20kHz
6.8nF - 20mH = 13kHz
10nF - 70mH = 5.5kHz
22nF - 170mH = 2.5kHz
39nF - 290mH = 1.5kHz
150nF - 290mH = 800Hz

not sure about the frequencies yet. Maybe I´ll change some cap values to get better frequencies. Sugestions about usable frequencies are welcome, as this is my first DIY passive EQ.