DIY Simple Active Preamp

[Renegade]

Don't laugh I just threw it together to see if it worked.

This thread is way too long for me to look all the way through to see if someone beat me to it, but the 1st thing I thought when I saw this picture was "THIS GUY DONE BUILT HIMSELF A TUB AMP".

Couldn't resist,,,,,,,,,,,,,,,,

 

[Grainger49]

Funny, I built a couple of CNC Phono stages in the same meat containers.

 

[Judas Priest]

My Super Simple Preamp is done :thmbsp:

I still have to change a few things. I chose a relatively small case, so there isn´t much room inside. I ordered the wrong caps. The voltage rating is 400/630V and they are huge. Because of that, the pot doesn´t fit.

I also made a mistake with the switch. I cut a ragged hole where it was supposed to go, but it doesn´t fit there. The voltage regulator is in the way.

I am going to buy smaller caps and just omit the switch.

I am really pleased with the sound, though It just sounds clean and quiet; no noise and no hum. Just like it is supposed to be.

I am very happy I built one of these :thmbsp:

 

[Judas Priest]

And two more pics:

 

[nj pheonix]

Looking for a bit of guidance. I built board last night
(man this is a super simple design! I can't believe how small it is and how few components it has)
Back to topic. What is considered optimum operating voltage?
I'm not quite following the IDs on the pot connections
I get that CCW is counter clockwise and CW is Clockwise. W is the wiper
How does the CW and CCW correspond to the values as you adjust the pot?

 

[omishra]

We look at volume pot. When you close the volume pot counter clockwise wiper (central terminal) makes full contact to ccw pin. CCW corresponding to minimum volume. Its left side of central pin if you hold shaft pointing to you. CW is other side. For operating voltage, i m happy at 10V.

Sent from my E6 using Tapatalk

 

[nj pheonix]

Thanks so much. I hated the thought of powering up at max volume as opposed to min.

 

[Grainger49]

Just to be clear, look at the pot shaft with the lugs at the 7:00 to 5:00 position. The 7:00 lug is the CCW connection, the 5:00 lug is the CW connection. The center lug is the wiper of the pot, the variable voltage leg.

 

[omishra]

This is precise. Thank you.

Sent from my E6 using Tapatalk

 

[nj pheonix]

Thanks for sticking with me so far.
I temped out some RCA's and the pot.
I was just for testing purposes I was going to parallel the power from the PS from my CNC. IIRC, its + & -15vdc. Is that too high to feed this or should I be OK?

 

[nj pheonix]

OK,
impatience got the best of me. I fired it up and success. Nice and quiet. clear and detailed. Only problem so far is it has too much gain. With the stepped attenuator silence full CCW. first détente louder than I would usually listen (slightly) by second click its really loud. Glad I had pot wired the right way.
89db speakers and 3116 amp (in case you were curious).
This is the pot I used
http://www.ebay.com/itm/201382456954?_trksid=p2057872.m2749.l2649&ssPageName=STRK:MEBIDX:IT
what can I do to tweak the gain?

 

[Grainger49]

There are a number of ways to cure the hair trigger volume control. One is to put a 250k Ohm resistor in front of the volume input.

Or you can adjust the gain of the circuit. Looking at the schematic in the first post Rf is 100k and Ri is 22k. You can increase Ri or decrease Rf.

 

[sachu888]

You can also increase the value of two 8.2k resisters to 10k to decrease the gain.

Regards
Sachin

 

[nj pheonix]

Thanks so much again
Fortunately, resistors I have serious stock of. .
Grainger,
As I understand (or thought I understood at least) the 2 resistors are a ratio. Should I be thinking in the 10% range when I tweak or would it be a less or greater adjustment?
Is there a rule of thumb going one way or the other. It might seem obvious if you've played with these before. Again I appreciate all the help.

 

[Grainger49]

Sachin,

I have been looking at the schematic in the first post. It has no 8.2k resistor. What post is the most recent schematic in?

Phoenix,

Yes it is the ratio. Edit: The gain for the non-inverting input should be 1+ (Rf/Ri). As Omishra outlines below. I was thinking of a voltage divider, not the gain on an Op-Amp.

 

[nj pheonix]

OK
I don't have a copy of schematic of my board. I have the BOM
Part No Description
ECW-F2225JA CAP FILM 2.2UF 250VDC RADIAL
EEU-FC1E101S CAP ALUM 100UF 25V 20% RADIAL
K104Z15Y5VF5TL2 CAP CER 0.1UF 50V RADIAL
AD823ANZ IC OPAMP JFET R-R DUAL LN 8DIP
MRS25000C1002FCT00 RES MF 1/4W 10K OHM 1% AXIAL
MRS25000C8201FCT00 RES METAL FILM 8.20K OHM 1/4W 1%
9341129 (E14) RES METAL FILM 100K OHM 1/4W 1%
MF25 100R (E14) RES MF 1/4W 100 OHM 1% AXIAL
27ESB473MMF50NF POTENTIOMETER, D/GANG LOG 47K
ED90105-ND IC SOCKET 8PIN .300 SOLDER TAIL / Op Amp Sockets

If need be it wouldn't be tough to trace and draw.
First i changed 100k to 250k (based on post 456. It seemed to make little difference.
Then i changed 8.2k to 10k. I think that made a little difference.
Then i went back to read post #453. If that's right, i went the wrong way. the 100k should've gotten lower and/or the 22k should've went up.
Based on the fact that i don't have a 22k i'm assuming the 10k is Ri, which would explain the excessive gain (maybe).
Before i jump back in, any thoughts?

 

[omishra]

Ok this is for you. See that 10K and 8.2K. You can exchange its positions too.

 

[nj pheonix]

Thanks so much!
So if I have this straight, I'll put the 100k back where I started. I already have a 10k at the 8.2k position (as I said in #457). So I'm going to install 8.2k to current 10k position. At the risk of sounding dumb, this will reduce gain significantly?
Am I the first one to use this version and have a hair trigger gain issue?
For some reason I thought this was a well worn path already.
I won't get to play with this until tonight at the earliest.
Thanks for posting board and schematic. I'll be checking in to see if there are any other ideas before I make more changes. Thanks again

 

[omishra]

With default schematic as above, it is -
Rf = R27, R29 = 10K
Rg = R28, R30 = 8.2K

output = input x A, gain

gain, A = (1 + Rf/Rg) where Rf = 10K and Rg = 8.2K
= 1 + 10/8.2
= 1 + 1.21
= 2.21

output = input x 2.21 times

with swap Rf = 8.2K and Rg = 10K

Gain A = 1 + (8.2/10)
= 1 + 0.82
= 1.82

output = input x 1.82 times

Now you can experiment lowering Rf to something 4.7K or increasing Rg to 15K or higher.

 

[Grainger49]

omishra,

Thanks, I edited my post. I don't want incorrect information to linger here. We always called the resistor to ground Ri, but Rg makes sense here.

The resistance values you are giving are not what is in the first schematic. There must have been some changes in the 400+ posts.

I see them above this post. Thanks for the schematic.