8 channels of AD797 build thread

[hazel]

Hello everyone,

I want to thank all contributors to this forum because i've learnt so much.

The only way i can add my grain of salt here is encouraging newbies like me.


THE PROJECT:

I recently build a 1U 8 channels of THAT 1512 with external PSU using these:

http://electronics.dantimax.dk/Kits/Preamps_-_poweramps/index.html, a standard application of 1512 datasheet

http://www.jlmaudio.com/shop/index.php?_a=viewProd&productId=8

http://www.jlmaudio.com/shop/index.php?_a=viewProd&productId=12

http://www.jlmaudio.com/shop/index.php?_a=viewProd&productId=118

Works like a charm, extremely transparent. I've done some recordings and overall quality is excellent. I even did a shootout with MOTU 828 MKII preamps, THAT 1512 and YAMAHA PM5D preamps. MOTU preamps were awful, lack of detail. YAMAHA was a little bit more crispy in a non-transparent way while 1512 was more clinical. The expression "just a wire with gain" comes to mind.

I'm still designing a definitive front panel so pics will be posted when finished.

Now i need 8 mic preamps for 16 track recordings (8 x THAT1512 and 8xAD797) and double balanced topology came to mind. I need something quickly to build, PCB / kit preferred so i found this:

http://diy-tubes.com/ad797-mic-preamp-kit.html

So build begins...

 

[hazel]

pic

 

[hazel]

well packed

 

[hazel]

let's start

 

[hazel]

5 hours later...

 

[hazel]

wiring almost done!

 

[hazel]

i recently received knobs...

 

[hazel]

another pic

 

[hazel]

and front panel from www.razorlab.co.uk, took the idea from this guy http://irfrench.tumblr.com/tagged/razorlab who also post here as irfrench

 

[hazel]

another front panel pic

 

[hazel]

infill with acrylic paint...

 

[hazel]

front panels after almost finished...

 

[hazel]

AD797 8 channels of gain!

 

[hazel]

THAT 1512 8 channels of more gain!

 

[keefaz]

Nice looking. You use acrylic panel? Is that solid enough?

 

[hazel]

Nice looking. You use acrylic panel? Is that solid enough?

Yes it's acrylic 3mm, solid beside rack ears but a little soft at the center so i decided to install stock aluminum panel behind, now it's solid and a lot cheaper than metal engraving. 1RU front panel 25 pounds, 35 euros

 

[ricardo]

You may find your THAT1512 preamps are up to 3dB quieter than your AD797 circuit, especially on condensor mikes.

But both are still pretty good.

 

[hazel]

You may find your THAT1512 preamps are up to 3dB quieter than your AD797 circuit, especially on condensor mikes.

Thanks for the advice Ricardo!

I've not compared them side by side, i'll make it as soon as possible.

Do you think there's space to upgrades or mods in AD797 circuit? i was thinking about http://www.sg-acoustics.ch/analogue_audio/microphone_preamplifiers/pdf/G_r1.pdf. Basically the same but with 1:2 input transformer. What do you think?

 

[ricardo]

Do you think there's space to upgrades or mods in AD797 circuit? i was thinking about http://www.sg-acoustics.ch/analogue_audio/microphone_preamplifiers/pdf/G_r1.pdf. Basically the same but with 1:2 input transformer. What do you think?

You might get at best 2.5dB improvement in noise.

If you are using a Coles 4038, this would be worth pursuing.

Though I said the measured difference is probably greatest between THAT1512 & your AD797 circuit on condensor mikes, these are also the least likely to complain cos they have high output.  It's the low source impedance that amplifies the difference.

The most critical would be a 50R nominal ribbon.  Low output & low resistance.  If you are not using that, i wouldn't worry.  You'll probably need a different transformer too as most mike input transformers assume 150-200R nominal.

 

[hazel]

If you are using a Coles 4038, this would be worth pursuing.

Though I said the measured difference is probably greatest between THAT1512 & your AD797 circuit on condensor mikes, these are also the least likely to complain cos they have high output.  It's the low source impedance that amplifies the difference.

The most critical would be a 50R nominal ribbon.  Low output & low resistance.  If you are not using that, i wouldn't worry.  You'll probably need a different transformer too as most mike input transformers assume 150-200R nominal.

I mainly record with Condenser mics on very loud sources (drums, orchestras, etc...) so do you think i don't need to worry about noise? sg-acoustics schematic won't make a major difference in sound in my AD797 circuit?

When needing a ribbon mic i use http://cloudmicrophones.com/cloud12/?page_id=227. 20dB of clean gain, plenty of headroom and adapted impedances