G7 finished!! But bad bass-response in omni-position

[Salossi]

I finally finished my G7, and from the first moment I powered it everything looks quite good. The sound is absolutely fine, no hum at all (in contrast to the other mic I built) and the noise is very low. Now I have to build a shock mount and do some mod´s on the rubber absorber the capsule is mounted on (it is too soft, the capsule is dithering a lot if the mic is moved).

There are 3 problems left, i have to solve:

- The main problem: The mic sounds very fat and good in both positions, cardioid and fig8. But when switched to omni-position, it sounds like a lo-cut being switched on, very thin. Almost no bass response in omni. First I wondered, if the backside of my capsule is bad, but then the sound of fig8 would also be affected, right??

- the phase is reversed, compared to all my other other mic´s. Of course this is not a big deal, but perhaps the layout of the Gyraf G7 is wrong? I wired the LL1538 like it´s mentioned in the G7-schematic. And I have a K67 capsule with a common backplate and separate terminals for both diaphragms (should be the same with the CK12). Has anybody already checked the phase of his G7??

- My plate-voltage is a little bit low... it measures only about 130V instead of 160V, so perhaps I´ll change one of the transformers and regulate the voltage. One of the reasons is my low mains voltage (only about 215 V instead of 230V). But I think it´s better to control this voltage. Anyway: the mic also runs on 130V, but probably with a lower output.

So, if you have any idea about my omny-sound-problem, please let me know!
And I will soon post some new photos of this mic...

Greets,
Sascha

 

[gyraf]

Are you sure that what you hear is not just the missing proximity-effect when switched to omni? This is normal, you know..

Jakob E.

 

[Greg]

Regarding the plate voltage...

There's a link on Jakob's site where someone had a similar problem. He changed the 10K resistor in the PSU to 5K, thus making his loaded PSU voltage closer to 160V.

I'm in the process of building a G7 and this is the route I'm planning to take.

 

[Samuel Groner]

Are you sure that what you hear is not just the missing proximity-effect when switched to omni? This is normal, you know...

I don't think that an onmi realised with two pressure gradient microphones doesn't show proximity effect. Neither does it show a bass response extended down to very low frequencies, I assume.

Samuel

 

[Salossi]

Yes, Samuel, that´s exactly my opinion.

But now I compare it to the behaviour of my C414 in different pattern-modes.

 

[Salossi]

...but in the end, jakob seems to be right...

I just compared it to the 414, it´s exactly the same effect on this mic! Proximity effect in cardioid and fig 8 - modes but NO proxi-efx in omni-mode :shock: :shock: :shock:

I was of exactly the same opinion like samuel was... Proximity effect appearing on pressure-gradient-capsules...

Could anybody explain the reason for the missing proximity effect in omni?? I mean, I understand the difference between pressure gradient capsules and omni "barometers" with no prox. efx. But why do combined capsules behave like this??

 

[gyraf]

I think there once was a good explanation of this in Klaus Heyne's mic lab over at ProSoundWeb. Might have been added to the microphone meta..?

Jakob E.

 

[Salossi]

Hey Greg,

Thanx for the idea of changing the resistor. I will try this tomorrow.

But if you´re building a new G7, I would recommend to stabilize the plate-voltage. In my other mic I did exactly this and it works fine. I used a small circuit of 2 transistors, one zener diode, one trimmer and 3 resistors for stabilizing. I think I´m going to regulate my G7´s Voltage as well.

 

[Salossi]

Here are some pics of the psu:

I love those Leybold´s :grin: :grin: :grin:

 

[ioaudio]

looks great! at least, the psu does-hope to see some pics soon
regarding the g7 psu: i dont think its necessary to regulate the ht. adjust the resistor while running the mic and be done. depending of which kind of transformers you chose for back to back, they act differently and will give varying results.

 

[Samuel Groner]

No proxi-efx in omni-mode.

That's indeed news for me--thanks!

Samuel

 

[SSLtech]

I'm looking for those connectors, the 12-pin ones that you've used for the mic cable etc. -Do you have any left over?

Thanks,
Keith

 

[Klaus Mogensen]

Your PSU looks extremely good Sallosi.

I finished my G7 last weekend and got about 130 V under load using a 4.7 kOhm resistor, so I will buy a 220V:12V transformer for my second PSU.
The sound is however very good and the output very hot. I can't compare it with other tube mic's, because this is my first.

Thanks a lot Jacob.

 

[Salossi]

Hey Klaus,
I think you´re right, a 12V transformer will be the better choice. First, I´m going to try it with a smaller resistor as well, and if this doesn´t work or begins to hum I will change the tranny. Then, in my opinion it´s better to stabilize the voltage, because the mains-voltage can vary a lot. I´m living in an old flat here, with sometimes about 210 or 215volts. If I use the mic somewhere else, the voltage may be up to 235 or perhaps 240 volts. That´s a big difference!
The sound of the mic is really very nice. I compared it to my other selfmade-tube mic (K89-capsule, 6s6-miniatur-triode and transformerless output) and the g7 (with K67 capsule) sounds marginally different, but both sound very cool! The g7 has much more output. I will post some audio-samples soon!
Keith, yes, indeed I do have some more of these connectors. But, sorry, I´m going to keep them, as I want to use them for building some more microphones in the future. Sorry for that.

 

[Salossi]

I just soldered another 10k R parallel to the other one to get 5k... Doesn´t change a lot... the voltage is very low... about 140V in front of the resistor and 135V after the resistor WITHOUT the mic connected. With the mic connected, the voltage is even lower, about 115 V in front and 110 V behind the resistor... I use my Unigor A43 for measurement, it has an Z input of 31,6 MOhm at it´s 300V setting, so this will not be the problem. The voltage is that low!

 

[Gus]

No current advailable, VA transformer rating might be too low. I am going to guess you have >300 ohms on the primary of the "backwards" stepup transformer. What is the 12 to 15vac rating 300ma?

I can't read the transformers in the picture can you post the specs?

Need both transformer ratings

I find I like the first one to have a min of 1.2amps on the secondary and the next one the same.

I don't like the back to back transformers. from memory when I built some power supplies like this testing testing under tube microphone type loads.

1.2amps at 12.6VAC to a 300ma 12.6VAC backwards maybe 120DC out. 1.2amp to 600ma maybe 140VDC out. You are limited by the VA rating and primary resistance.

I would use a 1.2amp to 1.2amp secondary rating transfomers with a voltage X2 diode cap setup to mult stage RCs for the B+

By reducing the R in the RC you are reducing the filtering not a good thing

Did you test the capsule in another known good microphone in omni?

 

[Salossi]

Hi Gus,

The first (step down) transformer is a 230V : 9V / 5VA type; the reversed (step up) tranny is a 230V : 15V / 3,2 VA type.

The capsule seems to be okay. The "bad" bass response was really the missing proximity effekt. I didn´t expect the missing of a proximity with pressure gradient capsules in omni-mode.

Have to go to bed now... its very late here in germany :roll:

 

[Gus]

Again I used two 120VAC to 12.6VAC at 1.2 amps for a supply I built(not for a G7).

I tried a 12.6VAC 300ma as the second transformer and it did not work well at all. 600ma might work OK

.3amps X 12.6VAC =3.78VA

1.2amps X 12.6VAC = 15.12VA

IMO the transformers need to be higher VA rated

 

[gyraf]

Using transformers "backwards" like we do gives some unpredictable results - behavior vary greatly from manufacturer to manufacturer, and from type to type.

The easiest way is probably by trial and error, if you want to get it completely right. The suggested transformer ratings are just what worked for me at the prototype stage..

Jakob E.

 

[Gus]

Jakob

I am wondering if the unpredictable results could be because the transformers you used were better than the ratings on them. Higher VA and lower winding R.

Others might be using ones with the same specs but at the min of the rating causing the problems.

I did some tests a few years ago with the back to back transfomers I bought a few from radio shack. 120VAC to 12.6VAC. 1.2 amps, 600ma and 300ma secondaries. The supply that worked the best was two 1.2 amp transfomers.

FWIW
Another tube microphone supply test was the Royer I found using a 2 amp 24VAC rated transformer gave better results lower noise better regulation. Honeybear microphone