AK4490 sings,but gain is a little high..

[Ryssen]

As I said the gain is a little on the hich side,can I use "I/V" resistor like ES9018 to lower the signal or will I fry the chip?

 

[abraxalito]

AK4490 is voltage out (on-chip switched capacitor filters) so no I/V resistor can be used.

Suggest a step-down transformer to reduce the output. Looks like you might already have trafos on your valve board, a tweak to the turns ratio would do the trick.

 

[Ryssen]

That is not trafos,that is caps with a chrome look.
Yes that would be an idea to use stepdowntrafos.

 

[abraxalito]

Ah right, I see now, not enough wires coming out They looked to be nicely screened trafos at first. So the best place to install step-down trafos would be right at the output.

 

[mk57]

I am no expert but i have a lampaztor amber diy output board for dac's and he uses resistors at the dac input to control gain on voltage dac's. It works well.

 

[Ryssen]

Do you have a schema of it?

 

[mk57]

No sorry it never came with a schematic. All i can tell you is the board has two inputs per side one is for voltage input the other is current input, just after the voltage inputs there is a resistor pad. He suggests trying different values to achieve the desired gain. Hope that helps

 

[Ryssen]

I have a pair of Lundahls LL1571 that can be used as 3,5:1 but they are "microphone trafos" I wonder if they can take the signal that comes out of the tube board,I don´t know what it can be maybe 10v?
And that they don´t spoil the sound quality..

 

[Redshift187]

The analog output voltage scales with the voltage of (VREFH - VREFL).

Vout(RMS) = (VREFH - VREFL)/5.

In other words, you get 1Vrms from each output when VREFH = 5V.
This gives 2Vrms when you sum the differential outputs.

If you want less output, just lower VREFHL and VREFHR and the output is scaled accordingly.

 

[Ryssen]

Hmm if I´m right this means that I would need another adjustable 5v supply and make some modification to to the pcb as the vfref is connected to VDD (analog 5V supply) allso.

 

[Ryssen]

I tried the 3,5:1 Lundahl trafos at the output,and the levels are better but I feel theres sommething missing in the music with the trafos.

I use my Squeezebox Duet and the built in volumecontrol and it´s working fine except when there´s no music playing there a little noise from the speakers,I guess I´ll have to accept that so that I can use it without trafos.

I think this DAC sound better then at least mine ES9018 with Unballancer outputstage....

 

[Ryssen]

I even tried an Alps Blue pot between the DAC and the amp,but still feel something is missing........

 

[ljufa]

What is the tube stage, preamp or buffer?

Sent from my KIW-L24 using Tapatalk

 

[Ryssen]

It is an unballancer:

 

[taisho_daniel]

Well, based on the image you provided, aside from the required 56r in the R-IV position for voltage output DAC setup, you could use up to 1k. I would imagine increasing the resistance from 56r would decrease the gain, you could experiment as to the value you'd enjoy.

On the other hand, if you didn't put a resistor at all, it says there that it will use a gain of 16x, which WOULD be too much.

 

[Ryssen]

I use 1k,and as I understand it I can´t use any lower with AK4490 (as it´s has voltage output)only with ES9018 and similar.

 

[ljufa]

Differential amp part of the unbalancer add unnecessary gain, if you can bypass second stage of unbalancer and use only BCF part you should have around 2vpp on the output.

 

[Ryssen]

Should I then connect the dac output to R18 and C6 ?
And disconnect them from R16 and R17?

 

[Spartacus]

Use a resistor attenuator at Vref to step down the 5 analog supply to, say 2.5V. Two 470 ohm resistors in series to ground, then a 1000uf cap decoupling the mid point which is taken to Vref and you have reduced gain, as well as a cleaner Vref.

 

[ljufa]

Should I then connect the dac output to R18 and C6 ?
And disconnect them from R16 and R17?

Due to DC offset on voltage dac output you must have two decoupling capacitors. So basically you need to add one more cap in series to R18 (same value as C6)

Everything is explained here.
BCF-2 & Aikido Single-Ended Output Stage