Assistance on G7 stereo setup, please

Hi all DIY guys there!

This is my first post after nights reading the forum and learning from all you. I finished succesfully a pair of G7 with Peluso capsules (thanks a lot Gyraf!!). Great sound: all my firends were amazed after compare it with other expensive mics ':grin:'
But my question comes on after trying to modify the initial project to another more complicated: I'm mainly involved on stereo recording on location, so decided to transform the two G7 in a only one twin G7 m/s x/y Blumlein recording system.
Yeah tell me crazy, because although I'm learning fast, my knowledge on electronics is poor, and is not wise to undo a working project to start another!''
So I asssembled all in a big tube and kept the PS (I did only a PSU case for the two G7). The tube is 60mm diameter, the capsules are mounted 90º and -here I think I'm in trouble':?:'- the two tube are pretty close, only a few milimeters between them. I got the same great sound that I had with the separate G7 but also hum. The hum is gone when I disconnect one of them: I mean each one works perfect (no hum, no noise) without the other.
Is not a good idea to have the two EF86 so close? or had I to consider another tube setup -like the Debenham stereo microphone- with only one tube (maybe the 6072)? Being myself a newbie putting it all toghether was the easiest thing !

Any help or suggestion much appreciated

JuanLu

It's no problem to have the tubes working close to each other.

Have you checked the powersupply lines for hum? It could be that the heater or HT voltage gets loaded too much with both units running. Another possibility is to work on how the units are grounded to the mic body.

..If you have 90deg. fixed between capsules, the only sensible option is to use it as M/S..

Jakob E.

[quote author="gyraf"]It's no problem to have the tubes working close to each other.[/quote]

Great! I had to rebuild all otherwise

[quote author="gyraf"]Have you checked the powersupply lines for hum? It could be that the heater or HT voltage gets loaded too much with both units running. Another possibility is to work on how the units are grounded to the mic body.[/quote]

Here comes another newbie question:
hum in the powersupply lines is independent of the voltage I get with psu loaded? how I check the ps lines for hum?
I get 6.3 on the heater via the optional trimmer when the psu is loaded with both mics, but the HT voltage never gets beyond 139V although I included another 10k voltage drop resistor in paralell. I forgot to mention I use only two trafos and maybe it is too much load for them. Will I need to get doubled also the trafos?
The units share the 0v in the mic board and then I connected with the audio gnd to the mic chassis.

[quote author="gyraf"]..If you have 90deg. fixed between capsules, the only sensible option is to use it as M/S..[/quote]

I have also the x/y (two cardioids at 90º) option and the Blumlein (two eight figure angled 90º, very interesting approach in certain venues).

Thanks again.

JuanLu

hum in the powersupply lines is independent of the voltage I get with psu loaded?

Click to expand...

Yes, hum can be different even with same voltage. This because of poorer regulation under load. You could maybe experiment with larger reservoir capacitors for the heater voltage.

how I check the ps lines for hum?

Click to expand...

..check the power supply lines with an oscilloscope..

[quote author="gyraf"]
Yes, hum can be different even with same voltage. This because of poorer regulation under load. You could maybe experiment with larger reservoir capacitors for the heater voltage. [/quote]

I changed the 2200uF caps for 4700uF ones with no improvement=great sound but also great hum. :sad:

More symptoms: I have 6.1v at heater loaded but 11.8 unloaded... is this for normal? maybe an excesive load? I haven't see such differences on other posts.
When I switch off the ps I get (while the capacitors are charged) that I'm looking for :wink: the same sound wihout hum.. poor regulation?

Thanks again!

I had some hum in my G7 at first. I had the 220uf/25v cap near the output tx wrong way around. There's an error in the component layout picture no.3 and the '+' should be other way around.
Then I had to reinforce the ground-trace with solder to get rid of the very slight hum that was still left.

cheers,

Okko

Thanks for the advice Okko, but I did not use the official board for my G7s, I built them from scratch, so maybe I did errors (now triplechecking...).
Living yet with hum.

JuanLu

Juan,

it seems that there's too much variation between load and noload for your heater regulator. you better verify that the LM317 is working right.

Also, when not using the G7 PCB, you need to take extremely good care of how you run your grounding in the mic circuit to avoid groundloop hum..

Jakob E.

[quote author="gyraf"]Juan,

it seems that there's too much variation between load and noload for your heater regulator. you better verify that the LM317 is working right.

Also, when not using the G7 PCB, you need to take extremely good care of how you run your grounding in the mic circuit to avoid groundloop hum..

Jakob E.[/quote]

Hum is gone!

After your last post I rechecked the two LM317 and reconnected only une g7 after the other to ensure they were ok. Hum was present because I had raise too much the heater v with the trimmer. Then reconnected two mics and playing around with the trimmers I got (loaded) around 5.7 v each mic.
The two g7 are very quiet and giving a great sound! Maybe it can be improved, I have to test now the system in comparison with other mics.

I'm really happy :grin: :razz:

Thanks Jakob and all diyers!!

JuanLu

:thumb: