Topic: interesting DIY preamp kit

[Nick's Picks]

I picked up two of these mono kits.
they look simple as all hell to assemble.  now, anyone care to recommend a different opamp or other parts?
looks like its a socket style system so you can pop them in/out w/o any soldering.

http://www.velleman.be/fr/en/product/view/?id=8897


I'm thinking two of them together in a dual mono setup.
I'll either design a step up VR for the 10v requirement, or I'll just use 18v.

any thoughts ?

[Church-Audio]

I picked up two of these mono kits.
they look simple as all hell to assemble.  now, anyone care to recommend a different opamp or other parts?
looks like its a socket style system so you can pop them in/out w/o any soldering.

http://www.velleman.be/fr/en/product/view/?id=8897


I'm thinking two of them together in a dual mono setup.
I'll either design a step up VR for the 10v requirement, or I'll just use 18v.

any thoughts ?

Yeah I recomend you dont waste your time with this circuit its crap and its noisey as all hell. It will not be suitable for any kind of quality recording. But you can always try it and see what you think.

[Nick's Picks]

its just another excuse to spend time in the basement burning my fingers.

[Church-Audio]

its just another excuse to spend time in the basement burning my fingers.

lol... man I know  I have not really worked until I burn something.. My personal favorite is my industrial glue gun that heats the glue up to 400f man when I get some of that on the top of my finger near my finger nail and it sticks to it and rips half the skin off I tell you it just makes my day!! 

[boojum]

its just another excuse to spend time in the basement burning my fingers.

You gotta be a masochist.  LOL

[Nick's Picks]

naaa...., I just like to tin my tips.
finger tips..that is.

[Eigenklang]

I like very basic and simple circuits, but i guess that is much too simple.

I guess, these small DIY kits are one of the reasons why some people think that all opamps sound crap 

[Nick's Picks]

luckily, the opamp supplied can be replaced...and I've all ready got the replacement picked out.
less gain, but much better.

[aegert]

So nick what will have to change in the components for the new opamp.. Are you adding or is there a filter circuit in it?

A

[aegert]

mshilarious

+T

[Church-Audio]

luckily, the opamp supplied can be replaced...and I've all ready got the replacement picked out.
less gain, but much better.

 

Gain in an opamp circuit is set by the resistors/pot in the feedback loop.  The opamp itself does have a maximum gain, dependent upon frequency, which is listed in the specs as "gain bandwidth" or GBW.  But any better opamp you go to will almost certainly have better GBW than the one in the kit.  There is also the issue of headroom, but that is mostly a function of the power supply voltage, and secondarily the ability of the opamp to be driven to its rails.

Apart from that, the only considerations are quality of the components (read: replace those caps) and especially the quality of the power supply.  If you intend to run this off a battery, you need to create a virtual ground reference.  That may be what a couple of those resistors in the circuit are doing, it's hard to tell.  Otherwise, you need to construct it.  Assuming your intended load is 1Kohm, a voltage divider with two 10Kohm resistors, and filter caps from + to virtual ground to -.

Next, you will probably want to add a small value cap in the feedback loop, something like 10-100pF.  Also, you should have filtration very close to the opamp's supply pins, I typically use a 0.1uF film cap.  I don't know if one of the electros on that board is a power supply filter or not.

If you want to get really fancy, a single channel opamp usually has a pin for DC offset adjustment . . . I typically use dual opamps and live without it.  A good opamp's offset should be small enough so as to not cause trouble.

After that, you could replace the trimpot with a more useful pot . . . something that mounts on whatever chassis you select.  The circuit will need to be shielded to be quiet.  I would add an impedance balanced output as well.

As for people that think all opamps sound like crap . . . I find it amusing that they think they can DIY a circuit with 4 transistors and beat a TI or AD chip that has 40 . . . I mean, sure there are people who can build an excellent 20 transistor discrete stage, but those usually aren't the people saying bad things about opamps . . . especially since such circuits are often discrete opamps.

Great advice only problem is by the time your done with all that you might as well get a breadboard and redesign the circuit My experience tells me he might also want to shorten the path of the feedback loop resistors and use metal film at least in the feedback loop. But again that involves a redesign of the pcb board.

Hey did you get my last email Jon?

[Nick's Picks]

ok, you guys are ruining this for me.


the chip replacement I have planned is the NJM5534
I think it will just drop into place ?

[rokpunk]

christ..that kit makes the crappy rolls preamp i took apart the other day look like a neve.

[Church-Audio]

ok, you guys are ruining this for me.


the chip replacement I have planned is the NJM5534
I think it will just drop into place ?

The 5534 is ok but try the opa227pa much better much less current. the 5534 is a single opamp derived from the 5532 the 5532 is a good opamp but its not a burr brown by any stretch. Also the slew rate on the 5534 is 13uS the opa is 2.3! much better this chip is also stable at low voltages.
http://search.digikey.com/scripts/DkSearch/dksus.dll?lang=en&site=CA&keywords=opa227pa&x=31&y=14

Edit I misunderstood the specs on the 5534 To read 13mV not 13V... in this case 13v Is a better slew rate then 2.3v but the 5534 does draw more current then the OPA227PA and to my ears I like the sound of the OPA much better...

Sorry for the confusion..

[Nick's Picks]

Thank you Mr. Church.