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.

[keith]

Also the slew rate on the 5534 is 13uS the opa is 2.3! much better this chip is also stable at low voltages.

You seem to be saying that a lower slew rate is better.  The 5534 is around 13V/uS, the OPA227 is 2.3V/uS.  Are you saying lower is better?  Please explain.

[Church-Audio]

Also the slew rate on the 5534 is 13uS the opa is 2.3! much better this chip is also stable at low voltages.

You seem to be saying that a lower slew rate is better.  The 5534 is around 13V/uS, the OPA227 is 2.3V/uS.  Are you saying lower is better?  Please explain.

\


Man I think I misread the spec sheet I thought the 5534 has 13mV not 13v Sorry in this case the slew rate of 13 volts trumps 2.3 volts I stand corrected.. Dam I still think the burr brown sounds better.. 

As we all know the higher the voltage = better performance / reaction time with the input signal..

[keith]

Man I think I misread the spec sheet I thought the 5534 has 13mV not 13v Sorry in this case the slew rate of 13 volts trumps 2.3 volts I stand corrected.. Dam I still think the burr brown sounds better.. 

Obviously slew rate is not the only measure of how an opamp will sound... 

[Church-Audio]

Man I think I misread the spec sheet I thought the 5534 has 13mV not 13v Sorry in this case the slew rate of 13 volts trumps 2.3 volts I stand corrected.. Dam I still think the burr brown sounds better.. 

Obviously slew rate is not the only measure of how an opamp will sound... 

Thats true what opamp do you like?

Chris

[keith]

Obviously slew rate is not the only measure of how an opamp will sound... 

Thats true what opamp do you like?

I do like the Burr Brown opamps.  I used an OPA37 in a design a number of years ago - I think the OPA227/
OPA228 are newer cousins of that one.  I'd be inclined to try an OPA228 where I had the OPA37 before.

The OPA228 is the decompensated version of the OPA227, so it isn't unity gain stable.  If the gain is 5 or higher of the circuit the 228 would probably be better, IMO.  Less than 5 the 227 is appropriate.

[Church-Audio]

Obviously slew rate is not the only measure of how an opamp will sound... 

Thats true what opamp do you like?

I do like the Burr Brown opamps.  I used an OPA37 in a design a number of years ago - I think the OPA227/
OPA228 are newer cousins of that one.  I'd be inclined to try an OPA228 where I had the OPA37 before.

The OPA228 is the decompensated version of the OPA227, so it isn't unity gain stable.  If the gain is 5 or higher of the circuit the 228 would probably be better, IMO.  Less than 5 the 227 is appropriate.

Interesting how about supply voltage and current draw? always factors when we are talking about battery life.. I use a OPA 2227PA and a ic based rail splitter design for my 9100. It works quite well.. But I am always looking for lower current and more gain with less noise.. Right now I am about -99db at +20 db of gain from 20hz to 80khz.
I am thinking of putting a cap in the feedback loop and bocking off anything about 40k... maybe squeeze more gain out of the amp that way.. I dunno its hard I am not a real electronics guru. I built things mostly out of trial and error luckily I know guys like Sparkie and a few other talented guys who have helped me with some of my designs. In electronics when your trying to build a preamp its not as simple as it seems... For me anyway.

Chris

[Church-Audio]

One thing to keep in mind with slew rate is how fast do you need to slew?  And that question has to be answered by the application.  If you are building a high-headroom pro-level (+4dBu) preamp, then you need more slew rate than for a few extra dB in front of a -10dBV portable recorder.  Actually, you'd need something like 8+ times the slew rate.

On the other hand, you have issues like current consumption and noise.  But again, what is your application?  Most people here are using condenser mics with a reasonably hot output.  If you are using small diaphragm condensers, especially the weensy ones, those by their nature will have a higher noise level than nearly any quality opamp you'd stick them into.  And if you are powering it with a battery, you'd be more concerned with current than noise.  If you are using unbalanced microphones, you might have enough induced noise to not even care.

I am using OPA2277 in a new mic I'm working on that will end up with total current ~2.5mA.  If you look at that chip, it isn't the quietest, it isn't the fastest, has a low GBW, but at 0.8mA/channel, it's incredibly efficient.  The slew rate is just fine when you consider the capsule's FET can't swing much more than 2V peak to peak before it clips, and the circuit is unity gain.  Note that slew rate is measured in microseconds; a 20kHz signal takes 25 microseconds (somebody check that math please) to go from peak to peak.

If I were building a general-purpose pro-level preamp, I usually use OPA2134.  OPA2277 uses a little less current, with less noise, but slower.  There are even sexier ones . . . OPA211, OPA627, and we still haven't left TI's range; there are a few other well-regarded chip makers out there.

But you could drop any of those in a particular design and not hear any difference, if the application did not test the limits of each chip's strength and weaknesses.


On the other hand, you might drop in OPA627 to a circuit that was designed with a 5534, and it might oscillate horribly . . .

That is one thing I have learned just because its pin for pin compatible does not mean it will work in your circuit.. but thats part of the fun of designing.. Its hard for guys like me and Jon because if your like me your so busy building you dont have as much time as you would like to try new things.. Thats the hard part for me anyway..



I would like to thank you for your contributions here!

Chris