WIREGAIN-DC: PGA-2500 based preamplifier project for DIY

[Igor]

Hi Rochey, I allready bought 10 for prototyping...
Will need more in future.
Do you accept paypal?

And, regarding PGA2500...Yes, this IS an AWESOME chip.
Take into account, I said these words as absolute freak of
preamplifiers with ChaRacTeR
and COLOR....
Not sure I will ever prefer this preamp for rock recording or style where color needed
to my design called Albatross. But, for situation I need clean preamplifier, IT IS.

 

[Rossi]

Very cool project, Igor!

I have a Universal Audio DCS preamp, which, I suspect, is built around the same chip and has similar features. I bought it mainly for my work as a microphone reviewer, but I ended up using it for any kind of recording projects. It is a great piece of gear when you don't want additional color. It sounds very natural but not dull or lifeless. Plus, the remote feature is handy when you're recording yourself.

 

[iban]

I would be interesting in 4 units 

 

[Holger]

Cool project. I would like to build four channels...

 

[Pitop]

I m interested for two (or maybe for)

 

[tongenerator]

i would mybe go for 4 or 8

 

[promixe]

After looking at some datasheets and the schematic I have a couple of questions (mostly beginner electronics related =):

1) What is the reason for running OPA2604 at +\-24V (pretty much at the max allowed in the datasheet) vs. say +\-15 or 18V? Heat dissipation seems lower if you go with lower supply etc..

2) Looking at the schematic I can't figure out how the PAD and HPF work - where do the S1/S2 ends of HPF relay go? Shouldn't the PAD have a separate padding resistor for each side of the input instead of a single 340R one connecting the sides together? It's a DPDT relay, right?

3) Is R15 the "bridging resistor" setting the impedance seen by the mic? If not, what is the purpose of it there?

4) Can you drive the relays directly from PGA without switching transistors before them?

5) what does the trimpot at the output stage do?

Thanks!!!

 

[iban]

Any news?  :

 

[Andy Peters]

After looking at some datasheets and the schematic I have a couple of questions (mostly beginner electronics related =):

2) Looking at the schematic I can't figure out how the PAD and HPF work - where do the S1/S2 ends of HPF relay go? Shouldn't the PAD have a separate padding resistor for each side of the input instead of a single 340R one connecting the sides together? It's a DPDT relay, right?

Study the pad schematic again. When the relay is in the normal position, the 1k33 resistors are shorted. When the relay is in the pad position, the 1k33 resistors are unshorted and with the 340R, form the pad.

3) Is R15 the "bridging resistor" setting the impedance seen by the mic? If not, what is the purpose of it there?

Yes.

4) Can you drive the relays directly from PGA without switching transistors before them?

No. RTFDS for the relay and see what is mentioned for the coil resistance/current.

5) what does the trimpot at the output stage do?

It's a symmetry adjust.

-a

 

[Igor]

1) What is the reason for running OPA2604 at +\-24V (pretty much at the max allowed in the datasheet) vs. say +\-15 or 18V? Heat dissipation seems lower if you go with lower supply etc..

Personal taste
I like high voltage supply's. It can be runned at +/-18 V as well.
Working now on updated third version of PCB. 8 pre-amps will fit into 1RU.
People who heared this pre were surprised in best sense.
A bit of patience....
Hopefully there will be "PGA2500 Pre Built Thread" within 2 weeks

 

[pstcho]

very nice project man!

what about signal / noise ratio?

and fastness of the command when you push up the gain?

thank you

 

[Igor]

Hi!

what about signal / noise ratio?

See PGA2500 datasheet..
When everything is properly connected, 100% up to specs

and fastness of the command when you push up the gain?

Did not get you. You mean gain rising speed when 48V switched on/off?

 

[e.oelberg]

you can't ride the gain while recording, it has 1db steps and clicks

nicholas

 

[Igor]

Hi Nicholas!
Let me know if you had clicks experience with PGA-2500 based pre's. Me not
It is not good idea to ride the gain on any pre with gain switch in steps.
Sometimes 1db steps can be audible...gain controll if zcen used is absolutely
clickless except in cases you can hear 1 db steps when it switches
not at zero crossing with input of complex sound source.

 

[flaheu]

Igor,

I'm very interested in these preamps, I'm saving money to get some soon I hope.
Can you give more details about dimensions for the 3 boards when they are stuffed (control board, preamp, psu).
I'd like to put 4 or 8 in a 1He rack which is (internal) 415mm x 280mm x 40mm (height).
Details for the control board would be welcome for the front panel design.

Thanks in advance.

 

[Igor]

Hi, please see this drawing for measures.
http://i251.photobucket.com/albums/gg291/diy33609/pga2500_meas.gif
It is possible to print it and use as drill template, resolution is 400 dpi.
I took XLR measures on top drawing from Neutrik's .pdf and scaled, not very precision.
Done a bit quick...hopefully, all is clear. If not, mail me for .ps or .cdr file.
It is possible to fit 8 pre's in 1 RU rack, with external power supply.

 

[Tomer1]

So,
Is anyone enjoying this creation and care to talk about it?

 

[Igor]

About 30 of these pre's were assembled by people IIRC...
Anyone????? BUMP!!!!

 

[Rochey]

4) Can you drive the relays directly from PGA without switching transistors before them?

you need the switching transistor. The amount of current (and voltage) required to switch a relay is usually in the 5 - 12V (or move) at quite a few miliamps.

The digital logic outputs of the PGA2500 are 3.3V.

Cheers

/R

 

[Igor]

Rochey, the question was asked 2008-12-13
Yes, kits do have the transistors for driving relays.