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Project Specific Discussions => Preamplifiers => Topic started by: zebra50 on November 21, 2004, 04:30:02 PM

Title: G9-BBC variant - pix
Post by: zebra50 on November 21, 2004, 04:30:02 PM
Lots of G9 related stuff recently, so I thought I'd join in.

Having lost my first G9 to the studio forever, I put together second G9 for home use (if I'm lucky), based on leftovers from other projects, and bargain bin bits.

I got two lundahl 1530 xfos from canford audio (1:7) at discount price for the inputs, and used a pair of BBC LL228s for the output (a fiver each!). These are 2.8:1 ratio but work fine -  a little less output level than 2:1, obviously. I changed the 10K resistor across the secondary to 5K so that the output stage is still seeing the same load.

Some other changes - I was short on rotary switches and had no relays around so I skipped the whole phantom power and line level selection, and also omitted the phase switch. I'm usually using ribbons or tube mics, so I can live without phantom. HPF is just on-off; I picked a cap value somewhere between the two that Jakob uses - 15nF, I think.

Had some trouble with earth hum on this unit at first. Carful rotation of the two toroidal mains transformers was necessary. I got a great improvement by connecting the -ve side of the output transformers directly to ground using thick guage wire - that fixed it.

Some pictures showing the simpified front - haven't cleaned or labelled it yet  so you can still see my marking out for drilling. BIG chicken head knobs always sound better....


(http://www.omnipressor.com/Other/G9pix/G9b001.jpg)

Inside, you can see the transformers next to the PCB (one of Swedish Chef's - very good quality :wink: ), and the thick ground bus. It's not quite complete yet. Still need to nail down a couple of components.

(http://www.omnipressor.com/Other/G9pix/G9b002.jpg)

Cheesy picture of  the tubes lighting up..... :green:

(http://www.omnipressor.com/Other/G9pix/G9btubes.jpg)

 :thumb:  z50
Title: G9-BBC variant - pix
Post by: robomatique on November 21, 2004, 08:15:26 PM
That just looks great!!!   :razz:

Where did you get your tube sockets? I am looking for sockets in the UK that will fit swedish chef's boards. My sockets have flat pins that doesn't fit.

Thanks

Robert
Title: G9-BBC variant - pix
Post by: soundguy on November 21, 2004, 08:21:26 PM
wow, cool!

what kind of LED's did you use to light the tubes up with?

(chortle)

dave
Title: G9-BBC variant - pix
Post by: Smoke on November 22, 2004, 12:56:17 AM
I'm just curious, where is a good point to place LEd's for that purpose? I mean where on the track can you pick up the voltage, and would it change the sound in any way?
Title: G9-BBC variant - pix
Post by: zebra50 on November 22, 2004, 04:14:17 AM
:green:  :green:  :green:


(Seriously, in case anyone thinks we're not fooling around, it's just a tripod and a long exposure time - 25 seconds) on the camera, shot at night with the lights off).

I got the tube sockets from farnell. You have to push hard but they go in just fine - I don't think I had to widen the holes but if you do then you can just  twizzle a jewellers screwdriver round to widen the hole a little.

The on/off LED on the fron of the unit is hanging off the regulated 12V heater supply, with I think a 10K resistor in series to drop the voltage/limit the current. It won't affect the sound at all!

 :thumb:
Title: G9-BBC variant - pix
Post by: peter purpose on November 22, 2004, 04:17:09 AM
PHWOARR :sam:  :thumb:
Title: G9-BBC variant - pix
Post by: Swedish Chef on November 22, 2004, 06:26:16 AM
Subaru!!!!!!!!!!!!!!!!!!!!!!!!!!! :green:  :thumb:
Title: G9-BBC variant - pix
Post by: zebra50 on November 22, 2004, 06:43:51 AM
Quote from: "Swedish Chef"
Subaru!!!!!!!!!!!!!!!!!!!!!!!!!!! :green:  :thumb:


possibly more 'morris minor'  :cool:
Title: G9-BBC variant - pix
Post by: on November 23, 2004, 04:53:10 AM
ooohh!!!! very nice
One for my future (as I was one of the other people to buy those Canford cheapies...)
No subaru or Morris Minor - definately Inspector Morse Deep Red Jaguar territory

I wish I could ask decent questions - maybe in a few years until then I will just look and smile
Title: G9-BBC variant - pix
Post by: zebra50 on December 05, 2004, 06:26:52 AM
Just had a bit of fun checking out the HPF on this with a frequency spectrum analyser. I wanted to use a toggle switch rather than a rotary switch for the HPF so  I picked a 15nF cap - midway between the two in the original design (10 and 22 nF). To see what this did to my signal I used the pink noise source from the spectrum analyser. Here's a picture of what I got:

(http://www.omnipressor.com/Other/G9pix/BBCG9hpf.jpg)

You can just about read the scale on the left and bottom.

The signal is pretty much flat down to 160 Hz and then tails off when the HPF is in play. Overall, the frequency response of this pramp looks pretty good - some slight peaks and dips, but seems to be about ± 1 db from 30Hz up to 20KHz, and the noise source showed a bit of 'flicker' anyway without the G9 in line- these are 'snapshots'!

I'll do the same on my Lundahl-G9 and see how it compares, if I can ever get it back from the studio.

Not sure how scientific my method is, so let me know  -I'm sure you will.

z50
Title: G9-BBC variant - pix
Post by: flyingfader on December 05, 2004, 09:56:58 AM
Hi,
Here is a quick bench I did on my G9.
http://site.voila.fr/flyingfader/audio/G9/Bench_G9.pdf

Fly
Title: G9-BBC variant - pix
Post by: zebra50 on December 05, 2004, 10:58:58 AM
Hi Fly,

thanks for that. Can you explain what the top chart means? I can see those spikes at 1k, 2k , 3k 4k etc, but I don't know what I'm supposed to be seeing.

One think I did notice with my test is that the frequency response did not seem to change with gain position, which is reassuring.

Stewart
Title: G9-BBC variant - pix
Post by: romeojesus on December 06, 2004, 06:23:02 AM
nice work!

is that a kent audio spectrum analyzer??...

got mine on ebay and never found info about it on the internet..
Title: G9-BBC variant - pix
Post by: zebra50 on December 06, 2004, 06:47:44 AM
Hi,
The analyser is called 'Rocktorch'. I had never heard of them before, but I got this very cheap (along with an MBHO measurement mic) from a university music technology department who were having a clear out. It has an XLR for a calibrated mic on the front, and dynamic mic and line in inputs, A-weighting and sensitivity switching on the front, as well as attack and release settings. If it looks like yours, then i do have documentation I can share.
Stewart

(edited for dodgy spelling!)
Title: G9-BBC variant - pix
Post by: Mark Burnley on December 06, 2004, 10:00:12 AM
Aha,

Someone else who uses those ugly-but-cool BeeBeeCee trannies!

Great stuff Stewart!

 :guinness:

Mark
Title: G9-BBC variant - pix
Post by: zebra50 on December 06, 2004, 10:31:41 AM
Quote from: "Mark Burnley"

Someone else who uses those ugly-but-cool BeeBeeCee trannies!


Hi Mark,

I finally found a use for them - I'm pretty impressed, especially at the price I paid. Might see if I can grab some more before they get scarce or fashionable. To my ears (which should not be trusted) they are equal to the Lundahls, but I do still need to do some serious A:B testing.

I guess the real test will be how much it gets used. My 'Lundahl' G9, which I built last Christmas, gets used a lot, so it'll be really interesting to see how I feel about them both in another 6 or 12 months.

Stewart
Title: G9-BBC variant - pix
Post by: on December 06, 2004, 10:38:18 AM
I just bought another one today - why not at £3.50 each they are a rainy day item...
Title: G9-BBC variant - pix
Post by: on January 05, 2005, 08:51:31 AM
Canford has now sold out of them....
Title: G9-BBC variant - pix
Post by: zebra50 on January 05, 2005, 09:22:39 AM
:roll:

oops!

Rafa and I cleaned them out before Christmas.

Oh well.
Title: G9-BBC variant - pix
Post by: Tekay on January 05, 2005, 10:32:11 AM
Think I grabbed 10 before Rafa and Stewart  :cool:

Canford Audio is a nice company! Will buy all Sifam I need in the future from them much better price that I can get in Sweden! They even created a credit account for me. :grin:
Title: G9-BBC variant - pix
Post by: zebra50 on January 05, 2005, 10:36:40 AM
That explains why there were just six left when I went to place our order.

They must wonder why the trannies sat there for years without anyone buying them and then suddenly they get many orders in the space of a few days.
Title: G9-BBC variant - pix
Post by: on January 05, 2005, 10:48:31 AM
I managed to bag 2 before Stewart cleaned them out and they look very nice in my small but expanding transformer collection

BBC LL228 x 2
LL1530
LL5402
OEP A262A2E
Sowter 3195f

BTW - I am waiting for the EAO switches to drop further as well