Preamp Project: Samuel Groner's Shared Gain Preamp

[quote author="Samuel Groner"]

I still have some 50 Hz noise though...

Click to expand...

Can you post/PM/e-mail an audio sample (with 150 ohm resistor source at full gain)? I'd be interested to analyse it because just recently I started to collect my design ideas for "low hum" design.
[/quote]

I will do. Currently I am in England, and the preamp is in Edinburgh (!) at my parent's house, so I may need to wait until Christmas before I can start working on it again. IIRC the hum was only present at low gains....

Any word on this one, Roddy?

Just following along and curious how this project is shaping up.

[quote author="skipwave"]Any word on this one, Roddy?

Just following along and curious how this project is shaping up.[/quote]

It's coming along very slowly!! I am just ordering JLM Go-Betweens just now.

Other than that, I've hopefully fixed my oscillation problem and can PAT test it just to keep myself happy (and safe!). Looking forward to listening to all of its 80dBs of glory. :razz:

I can post photos of the current progress if you like - the frontpanel is rather tasty.

Any news? :wink:

It's 98% finished.

Just need to finish some slight mods which I have added to one channel to the remaining three, and I need to add the knobs!

I'm very busy just now as I'm in a training centre for the next six weeks...

One day, one day.

:grin: Thanks!

I finished this project a while ago and never posted any pictures. So here they are.

They have proven to be very good. Low noise with masses of gain (80dB). I found that they worked well on drums.

I have the specs written somewhere, but from memory the preamps are essentially +/-1dB between 5Hz and 20KHz. Without a Zoebel network, the transformers peak at 18KHz by 1.5dB within circuit with a 200 Ohm source. I can provide Zoebel values if anyone else is using Beyer 1:7 transformers. Distortion-wise, at 0dBu input, the Beyers give 0.015% THD @ 1KHz, 0.2% @ 100Hz, and 1.5% @50Hz. Just to give you an idea.

Thanks to anyone who gave me advice - especially obviously Samuel Groner and Keef who's PSU board I used, and Joe at JLM.

Roddy

Glad they turned out well--nice frontpanel!

Samuel

Hehe you got a spiffy distortion unit there with a 5534, eh?

You can spice it up by adding a switch that would change the 5-8pin caps from 33p, 100p, 330p, 1000p ... so you'll add a slew-rate distortion in the mix (or so I was told). :idea:

Or get the TLC2262 (don't get over +/-8V) or TLC272 and overdrive the bejeezus out of them

:green:

[quote author="rodabod"]I finished this project a while ago and never posted any pictures. So here they are.[/quote]
Nice :thumb: Tasty blue and all that Vero, looks good!

Cheers,

Peter

PS
I'll get back to you asap about you know what, don't know shipping yet.

what fixed the oscillations?
Jorge.

[quote author="3nity"]what fixed the oscillations?[/quote]

There were many causes which would probably take a long time to describe, but all of these issues have been described in this forum; all the information is here.

Essentially, grounding, shielding, opamp compensation and bandwidth limiting. Oh, and taming transformer resonance helped too.

Hey Rod i have the same Beyer transformers but no info on how to wire them!

Any info?

Thanks.

Hey 3nity,

Did you ask me about that, and I forgot to get back to you? Sorry man.

You can download the catalog with all info, discussed inthis thread:
http://www.groupdiy.com/index.php?topic=22099

Here's the pinout:

Good work. I used standard DIL sockets. Remember to tie pins 8+9 to ground.

:thumb:

Thanks i already have the breadboard with sockets!

Thank you.

Rod or Skipwave: Do i have to use a RL/CL load?

Thanx.

[quote author="3nity"]Rod or Skipwave: Do i have to use a RL/CL load?
[/quote]

Not necessarily. How flat a response fo you want? Would you like a little treble rise in the very high-end?

Unless someone has already calculated a combination for you, you'd need to calculated a Zoebel for the Beyer 1:5. I used a Neutrik test set and a 'scope for best frequency response and square wave response.