F6 build problem

[wl336]

I just finished building a set of F6 amp (buffered version, with source degeneration resistor), since I have balanced front ends so I am using a LL1540 transformer to convert the balanced signal to unbalanced following the datasheet and directly connect the output to input of F6

The problem is, almost no sound, or more like the input is shunt to ground. So I did a little debugging.

1. I bypass the transformer and used a signal generator to drive it. It works, sounds alright and the scope gives me nice waveform and a gain around 15db

2. I left the amp power off and drive from transformer input and measured output(still hooked up to input to F6). It still looks fine, with an insertion loss around 0.6db which the transformer specifies as 0.5db.

3. Then I powered the amp on, then, the output from the transformer(input to F6) drops to almost 0. So I redid step 1 with various signal source resistance and calculated a input impedance around 10kohm, less than expected but still looks fine.

So now I am confused, could someone enlighten me what did I do wrong?

 

[Zen Mod]

1. you did not gave us enough data

 

[Nelson Pass]

Maybe the transformer windings are not phased properly.

😎

 

[wl336]

1. you did not gave us enough data

Could you please kindly tell me what data is needed😕

 

[Zen Mod]

pictures , for start,

my crystal ball still in repair facility

also - how you exactly connected that LL1540

 

[emmettr]

I just finished building my F6. Everything seems to be working! It sounds great, but one channel has a very faint hum that's only noticeable when no music is playing, and I have to put my ear right next to the speaker to hear it. The other channel is dead quiet. I've tried rerouting cables, shorting the input, and rotating the toroid, but there's been no impact. It's biased at 0.565 VDC and offset is 0.001 VDC. The heatsinks are should around 40 Degrees centigrade. The input wires are coax instead of twisted pair with a shield. All A/C and DC cables twisted and there's good separation except near the on-off switch. I'm using the Deluxe 4U chassis. One other point. The blue LED on the channel with the hum did not light up so I removed it, damaging it in the process without a replacement. So, there's no LED there at the moment, and the 15K resistor is still in place. Any thoughts on what to try next?

 

[Zen Mod]

which blue led?

ref. schematic please

 

[emmettr]

F6 Debug

The Blue LED power indicator on the F6 circuit board. On the schematic it is D1.

 

[emmettr]

It's the blue power indicator D1 on the F6 schematic.

 

[Zen Mod]

I don't know which boards and which nomenclature you're using

if you're sure that Blue LED is just indicator, you can live without it

pics can be useful .....

though, if all measurements are on spot, you can live without keeping your ear in bass cone

 

[emmettr]

The components I'm referring to are R13 and D1 on the F6 schematic in the F6 Component Assembly and Build Guide available in this forum. I'm going to leave it out for the time being. I'll take another pass at inspecting the board and solder joints.

 

[PRR]

.....The input wires are coax instead of twisted pair with a shield. All A/C and DC cables twisted and there's good separation except near the on-off switch....

The input change; is that suggested by someone who Knows? (Details matter here.)

What is the crowding at the on-off switch? (If that is switching Line AC it is an obvious hum source.)

 

[fabrice63]

Dont worry about that blue led it is an board mistake

for the hum , the measurement you should do is the ac noise at the speakers output ( both ) , depending on it you will see where to find it 😉

.

 

[emmettr]

F6 hum problem solved! I resoldered the power supply leads to the F6 circuit board and I relocated the speaker ground (black terminal) such that it now goes directly to the capacitor bank ground. The original orientation had the speaker negative returning to the F6 circuit board. No more hum! Thanks for the comments and input! Cheers!