Preamp Project: Samuel Groner's Shared Gain Preamp

[rodabod]

[quote author="Samuel Groner"]
It's all about routing and grounding. Keep the input from the output, carefully check and re-route the ground pathes. Putting things into a grounded and shielded case usually helps a good deal as well. Increasing C4 a bit might help too. Do you have a proper PSU? If there's a lousy ground, such stuff is almost intractable.
[/quote]

Cheers, Samuel.

The PSU I'm using is the SSL9K PSU which I have tied the 0V (ie. the toroid's centre tap) to the star ground which is where the mains ground is too. The preamp's ground point goes to this star ground point once also. Does that sound ok?

It's not too easy looking at ground paths with the strip-board. It's not like using Orcad! I will see if I can improve it though...

I have a couple more 22pF caps, so I'll see if I add them in paralell to C4 helps at all.

Roddy

 

[Samuel Groner]

The PSU I'm using is the SSL9K PSU which I have tied the 0 V (i.e. the toroid's centre tap) to the star ground which is where the mains ground is too. The preamp's ground point goes to this star ground point once also. Does that sound OK?

That should work well.

Samuel

 

[rodabod]

Arrrgh! :evil:

I'm having trouble getting rid of this oscillation.

I think I will write an extra thread asking about combatting oscillation and see what everyone suggests....


Roddy

 

[SSLtech]

Hmmmm.

I missed this thread first time around...

That shared-gain topology is exactly the same schematic as the SSL SL571 preamp. -They use Jensen transformers, but otherwise it's the same.

I see you're using Veroboard. It's great stuff. -However it is rather prone to stray capacitance from all the parallel traces, and un-trimmed traces thend to work like antennae.

I personally recommend cutting ALL traces with a ¼" (6mm) drill bit after they make their last connection. Any excess just leads to trouble.

Shielding may also help, but veroboard is particularly problematic for this sort of work.

Also, are the other channel chips removed from their sockets? -I would while testing, otherwise they may be gibbering wildly at open-loop gain, or who-knows-what...

Keith

 

[rodabod]

Cheers, Keef,

[quote author="SSLtech"]
I see you're using Veroboard. It's great stuff. -However it is rather prone to stray capacitance from all the parallel traces, and un-trimmed traces thend to work like antennae.[/quote]

Shhhh! Don't say that!

I've been thinking veroboard might not be the best option, and have been convincing myself that it's ok. :razz:

I personally recommend cutting ALL traces with a ¼" (6mm) drill bit after they make their last connection. Any excess just leads to trouble.

That sounds like good advice. I have a veroboard hand-drill so that should be easy.

Also, are the other channel chips removed from their sockets?

Well, I've actually completely isolated each channel by drilling the strips between each channel, so that shouldn't be a problem hopefully.

 

[JLM Audio]

I cannot see any local power supply decoupling caps at all in the photo's which the OPA604 will not tolerate at all at high gain. You can just mount a 0.1uF cap between pin 4 and pin 7 or one 0.1uF from each power pin to ground but keep them very close to the opamp pins. Or do you have these under the Vero board?

A couple of 10uF to 100uF power rail storage caps locally on the Vero would also be good to keep crosstalk between pres down and will also help the 604.

If that doesn't fix it try a 220pF cap across the 100k input resistor.

But with 62dB opamp gain and a Beyer isn't there more than 80dB of gain at full gain. Which will be tricky on Vero like Keith mentioned

 

[SSLtech]

[quote author="rodabod"]

Also, are the other channel chips removed from their sockets?

Well, I've actually completely isolated each channel by drilling the strips between each channel, so that shouldn't be a problem hopefully.[/quote]
Hmmm... Looks like they could all "drinking from the same straw" when it comes to power (-if so, it WILL make a difference) and that was my concern.

As Joe says, you should locally decouple the power rails, very close to the ICs. I personally like disc ceramics, and often solder them directly onto the copper side.

I reckon if you do that it'll be fine. -I use Veroboard when I can (can't find it in the USA) and it works well, but I ALWAYS trim off unused lengths of track.

Also, once the project is working, I usually add a small amount of solder over the remaining tracks. One more problem with Veroboard is that -as the trace passes each hole- the conductive path is reduced to two VERY thin pieces of copper; one either side of the hole. -This is fine for a whilel but when oxidation sets in as the veroboard ages, I've had all sorts of trouble. The solder adds conductive material to the path, and also helps to 'clad' the copper and slows down the oxidation. My old veroboard designs work much better for a lot longer when I do this.

To summarise, with Veroboard, I:
1) Cut the trace as soon as it's done what it has to. Unnecessary excess length does the devil's work.
2) Clad it with a light coating of solder.
3) decouple locally (usually underneath with flat ceramic discs caps). This is actually more critical with veroboard, since the loads of holes (therefore loads of very thin conductive strip between the supply and the IC) makes it easier for the IC to locally modulate its supply voltage. -I learned that back in 1985 when I built a couple of mixers for a custom installation down in Ealing... They looked good on paper, but when I built them (up against a tight deadline!) I had full-scale oscillation everywhere!!! -The veroboard layout was very tight, so there were no holed left close to the chips for decoupling, and I laid them on the copper side. -Bingo-Fixo!

Keef

 

[rodabod]

Cheers, guys!

It's good to get advice from people who obviously had experience. I've only veroboard-ed about ten circuits or so, and I am learning more and more.

I'll write my results over at the opamp oscillation thread which I started.

Roddy

 

[Kit]

Hej, roddy.

Where did you buy those nice little beyer transformers?

Could use a few....

 

[rodabod]

Hi Kit,

I got the Beyers from a guy in Germany who must have bought as big job-lot of them.

What ratios are you looking for?

Roddy

 

[Kit]

Step up, microphone input.

1:4 will do just fine.

What are you using?

They dont have a website, do they? :?

 

[rodabod]

The guy doesn't have a website. I could maybe find his email address though.

I have two 1:5 Beyer transformers which I could consider selling or trading..... They are like these ones - DIL socket and probably have a centre-tap. They may also be bifilar wound (I can't remember).

 

[Sleeper]

Hi- I just found this circuit-looks pretty cool.
can you clarify something re: the schematic

so the input should be hooked up phase reversed/flipped from the transformer
and output- (r9,c6,r10) is hooked up direct to your xlr pin 3 ???

I'm sure that the 604 should be happy to drive a 1:1 transformer. Done it, but not with this circuit. Anyone tried it yet. It's a bit before I can get to this, but I'll have to try it.

Kelly

 

[Samuel Groner]

So the input should be hooked up phase reversed/flipped from the transformer and output- (R9, C6, R10) is hooked up direct to your XLR pin 3?

Yes and yes. This output topology is called "impedance balanced".

Samuel

 

[Sleeper]

hi and thanks.
not sure where this one's going yet... or when, but it looks great.
Nice one, thanks.
Kelly

 

[capnspoony]

im a bit confused.

how are you using an 18vdc supply when the schemo on the first page calls for +-24vdc to your op amps?

I'm interested in the solution to your problem.

Sorry if that question is newbie.. im still a newbie

-richie

 

[Samuel Groner]

How are you using a +/-18 V supply when the schemo on the first page calls for +/-24 V to your opamps?

Opamps have a minimum and maximum supply voltage. In between you can freely chose. Check a datasheet.

Samuel

 

[capnspoony]

thanks samuel

I don't have the data sheet to the opamps used in this piece but i'll keep that in mind for future projects

-richie

 

[rodabod]

Just a quick update, guys - I have been in a training centre for the last two months so my progress has been very slow.

The good news is, I now have the front-panel engraved in blue (no gold :sad: ) and I may have solved the oscillation problem. I still have some 50Hz noise though....

I now have access to Lindos and Neutrik audio test machines at work, so I will be able to test the device very accurately (including full spec measurements and response print-outs :grin: )

Roddy

 

[Samuel Groner]

I don't have the data sheet to the opamps used in this piece.

Google to the rescue... :wink:

I still have some 50 Hz noise though...

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.

Samuel