Topic: another DIY preamp and some links

[Church-Audio]

thank's a lot Chris.

I don't think you need to go to 22uf

better to substitute them with 10 uF?

I don't like all the electrolytic caps in the circuit

could I substitute them with ceramic caps and would that improove the it's performance?

about the gain selector problem I had:
In the circuit drawing it shows the symbol of a simple resistor but as it should be adjustable I used a potentiometer instead. I connected line from between R1 and C1 to the Terminal 3 of the pot., terminal 2 went to the output, and I connected terminal 1 to ground. However it did not work propperly, therefore I tried all other possible connections but it never worked.
In the end I simply connected the 'output' of the circuit to the terminal 3 of the pot and terminal 2 became the new 'output'. ( and terminal 1 I connected to ground, though leaving it without connection also worked )

Why would the pot with the original wiring not work? or more imprtantly, is my 'work around' OK?

Not ceramic ceramic is for transistor radios not Audio polypropylene or metal film or stacked metal film but never ceramic. I may use monolithic in some circuits, but that is as low as I go. I also use Tantalum * I know some people think they suck* but I have seen tantalum in many top end preamps like Neve, so they must be good for something.  As far as the gain pot problem it sounds like you had the pot in backwards, term 1 should be N/C, term 2 should be going to r1 and term 3 should be going to c3. This sets the feedback loop resistor, I would also think that no connection would be unity gain try that and see if the circuit design is good it should be very stable with out a feedback loop resistor. You could use a rotary switch with resistors to set gain, use metal film and the more positions the better.

I like 0.5 db increments anything more is no good, many preamp makers use 1db I think that's not enough .5 db is much better. These rotary switches can be cheap in the surplus electronics market. I don't really like pots in the feedback loop unless they are very high quality, because more often then not they introduce more noise. This preamp circuit that your looking at is very simple. It should be a a breeze to build.

[kuuan]

thank's once again Chris.

decided it was time to learn about capacitors and found this site good:
http://www.uoguelph.ca/~antoon/gadgets/caps/caps.html
( more good info on this site, e.g. about opamps: http://www.uoguelph.ca/~antoon/gadgets/741/741.html,
resp. links to circuits on the main page: http://www.uoguelph.ca/~antoon/circ/circuits.htm )

term 1 should be N/C, term 2 should be going to r1 and term 3 should be going to c3.

thank's for looking that deeply into the circuit. I shall try this, but I with a new preamp that I am going to build, I do not want to open the first one I had succeeded fitting into this tiny box and which is working fine.

I don't really like pots in the feedback loop unless they are very high quality, because more often then not they introduce more noise. You could use a rotary switch with resistors to set gain, use metal film and the more positions the better. I like 0.5 db increments anything more is no good

Will try to find a rotary switch with 0.5 db increments.
Metal film for what? Does this refer to the resistors, or is there a metal film rotary switch?

This preamp circuit that your looking at is very simple. It should be a a breeze to build.

Well, I already had built one, you can see it here: http://i39.photobucket.com/albums/e190/kuuan/IMGA2584.jpg
however I used cheap materials bought in a local store in Thailand, but still I really like it's performance!
I shall build one more with better materials and incorporatring your suggestions, specially the rotary switch. ( those cheap pots I had bought sucked big time anyway, I had to buy 3 until I had gotten one which was not noisy when turning )

[Church-Audio]

thank's once again Chris.

decided it was time to learn about capacitors and found this site good:
http://www.uoguelph.ca/~antoon/gadgets/caps/caps.html
( more good info on this site, e.g. about opamps: http://www.uoguelph.ca/~antoon/gadgets/741/741.html,
resp. links to circuits on the main page: http://www.uoguelph.ca/~antoon/circ/circuits.htm )

term 1 should be N/C, term 2 should be going to r1 and term 3 should be going to c3.

thank's for looking that deeply into the circuit. I shall try this, but I with a new preamp that I am going to build, I do not want to open the first one I had succeeded fitting into this tiny box and which is working fine.

I don't really like pots in the feedback loop unless they are very high quality, because more often then not they introduce more noise. You could use a rotary switch with resistors to set gain, use metal film and the more positions the better. I like 0.5 db increments anything more is no good

Will try to find a rotary switch with 0.5 db increments.
Metal film for what? Does this refer to the resistors, or is there a metal film rotary switch?

This preamp circuit that your looking at is very simple. It should be a a breeze to build.

Well, I already had built one, you can see it here: http://i39.photobucket.com/albums/e190/kuuan/IMGA2584.jpg
however I used cheap materials bought in a local store in Thailand, but still I really like it's performance!
I shall build one more with better materials and incorporatring your suggestions, specially the rotary switch. ( those cheap pots I had bought sucked big time anyway, I had to buy 3 until I had gotten one which was not noisy when turning )

Metal film 1% or better 1 watt resistors even though you dont need 1 watt i like the fact that they will not break down over time.

[kuuan]

...but have built a couple of mics and was thinking about getting into some preamps next. Thanks! -Mike

Mike I am VERY interested in building mics!
Can you help me? How to do that? - Any advice, or introductory site you can point at?
( e.g. I had bought this very cheap Panasonic copy mic in China recently. It's a shotgun with a very long body. I'd love to use this body but repalce it's capsule and fet/amp but do not know WHERE should get WHICH parts for this job )

But, if you want to upgrade this yourself, you could add your own opamp there.  One obvious choice would be something like OPA2228.  This will improve the sound quite a bit.  There are lots of other opamps to try if you can't get that one.  Even the NJM2068 would be an improvement over what is in there now.

Try the OPA2227A as well thats a nice sounding opamp, I use this one for some of my preamps.

They did not have any of the three mentioned at the local store, bought a socalled OPA2604AP instead. Anybody knows about this one?

Metal film 1% or better 1 watt resistors even though you dont need 1 watt i like the fact that they will not break down over time.

thank's again for your valuable suggestions. The store did not have a rotary switch as you described, and I opted for <1% resistors, the 1 Watt res. looked too big for me as I want the amp to be small.

[kuuan]

I was very happy with this preamp because it is 'quiet' but I have been hoping for more gain.
Just now, very much delayed because of delivery problems, I received th SP-CMC4s I had bought at the yard sale a while ago.
And surprise, surprise: The level with them is much higher then with my AT822, no need for more gain, I am very happy with this preamp! ( talking the DIY preamp, not the AT )

[Church-Audio]

I was very happy with this preamp because it is 'quiet' but I have been hoping for more gain.
Just now, very much delayed because of delivery problems, I received th SP-CMC4s I had bought at the yard sale a while ago.
And surprise, surprise: The level with them is much higher then with my AT822, no need for more gain, I am very happy with this preamp!

Cool I will have to check it out you should put in a gain pot instead of the feedback resistor and see how that works out so you can change the gain of the feedback loop.

Chris Church

[kuuan]

Not ceramic ceramic is for transistor radios not Audio polypropylene or metal film or stacked metal film but never ceramic. I may use monolithic in some circuits, but that is as low as I go. I also use Tantalum...

so those are all different types of caps? and you recommend audio polypropylene or metal film or stacked metal film? or monolytic and tantalum?

... you should put in a gain pot instead of the feedback resistor and see how that works out so you can change the gain of the feedback loop.

You could use a rotary switch with resistors to set gain, use metal film and the more positions the better.

I would also think that no connection would be unity gain try that and see if the circuit design is good it should be very stable with out a feedback loop resistor.

Your truth finally dawned on me. time to get a metal film rotary switch ( if I can ) and above all connect it correctly.
Or could no connecting = unity, be my best option?


edit:
no metal film rotary switch to be found.

how about what they call 'Drahtpotentiometer' in german, translated 'wire-potentiometer' with 10 steps branded 'Vishay' which only can be had in mono.
 
the best stereo pot I have found is branded 'Alps', model number: RK27112 50KBX2CC

how about this:
substitute the 50k pot with a 470k resistor, and a 10k log pot in the out section ( somebody had suggested this )

[kuuan]

Cool I will have to check it out you should put in a gain pot instead of the feedback resistor and see how that works out so you can change the gain of the feedback loop.

Chris Church

Finnaly put the gain pot in correctly as you had explained. I can't make out any change of performance though.

Questions:
1: more gain:

• Can this circuit be modified to add more gain? another transistor gain loop?
  
• Or could I even simply put two of this circuit in series? 

2: Now I want to build a mic mixer:

• Can anyone recommend a circuit?
• If going mic > DIY preamp > line mixer could I also take a line mixer? E.g the one Chuck is trying to build, see: http://taperssection.com/index.php?topic=75397.0;all, - drawing attached.