New Gainclone with no gain: help requested

[SRMcGee]

Folks:

I have just completed an integrated Gainclone amp based on BrianGT's boards (LM3875). The amp turned out beautifully (pictures to come), but the amp's gain is very low. At maximum volume, music is politely audible -- two people standing 10' apart in the room could speak quietly and hear every word while the amp ran full blast.

I'm using the 330V Avel transformer sourced from PartsExpress, and am getting about 37.6 VDC (plus & minus) off of Brian's rectifier boards. The only things the incoming signal runs through are a Greyhill selector switch and a DACT 20k attenuator. The source is an old Proceed CD player and the speakers are Meadowlark Kestrels. Wires are all short (1 meter interconnects, 8' speaker cables).

Can anyone help me diagnose the problem? I gotta get more gain out of this amp!

Thanks,
Scott

[carlosfm]

It sounds like you made a mistake with the resistors.
Check out if you used the right resistor on the right place on the board.
First check: the resistor from signal input to ground.

[SRMcGee]

Carlos:

On Brian's amp board, there are only three resistors -- two 22.0k ohm resistors and a 220 ohm resistor. I installed the 22.0k resistors at points R1 and R2, and the 220 resistor at R3.

Was that wrong?

Thanks,
Scott

[SRMcGee]

Carlos:

Whoops, sorry -- there are four resistors: the 22.0k resistors went in at R1 and R2, the 680 ohm resistor went in at R3, and the 220 ohm resistor was installed at Rf.

Did I screw this up?

Thanks,
Scott

[joster]

Yes, you did. Did you read the .pdf?

[Greg Erskine]

Hi SRMcGee,

R1 = 220
R2 = 20k
R3 = 680
Rf = 20k

Looks like you have R1 and Rf are reversed but please double check with schematics.

These things have heaps of gain.

[carlosfm]

Check the instructions, I don't know the exact part ID, so it you post the schematic I can help.
Anyway, it really seams that you messed up with the resistors, as 220ohms for Rf is certainly not appropriate there.

[Peter Daniel]

Originally posted by grege
R1 = 220
R2 = 20k
R3 = 680
Rf = 20k

This is correct. You can use 22k instead of 20k, but it doesn't change things much.

[Nuuk]

Yes, you did. Did you read the .pdf?

May I spring to the defence of SRMcGee here?

By sheer coincidence, yesterday I assembled the BrianGT kit that I have here. Now, I am not really a complete newbie when it comes to building GC's! I've even made a couple of NIGC's before using P2P.

But I dutifully got out the pdf help file and read it through from start to finish. It's very good (and I even get a mention in dispatches ) but...

there is nothing that states clearly where each resistor should be soldered! Even I had to resort to the meter to check which solder pad was connected to what!

May I humbly suggest that the pictures showing the resistors in place (step 7) could have labels showing the values of each resistor, or that the circuit diagram could have the resistor numbers added, or preferably both?

This would improve an already excellent help guide!

[joster]

My reply sounded harsh and was not meant to be. The pdf does explain which values go where on page 5.