NOT QUITE NEVER AGAIN, THE GROUP BUY DAC BUILD

[nige2000]

Coz all the regulators are in series we opted for plastic backed regs to save worrying about shorts on the heatsink

https://uk.farnell.com/stmicroelectroni ... dp/1087151

These super caps are an absolute pig to charge up. 360 farads
Maybe 20 min( have not timed it yet ) til full charge
Regs go to about 65c without heatsink
So we will be putting heatsink on these

That's a long charge time - are you sure the regs aren't being internally thermally limited?

i wouldnt have thought its hot enough for thermal, ill have to check the current output from the regs to be sure
to make sure the full current is moving

ive pushed regs to shutdown before they get hot proper hot, shut down then just remain warm with little output untill there turned off cooled and turned back on again

Only I've heard the version with zfoils. As I didn't manage to debugg it before home time sat.
Id say the soekris is sweating though

Against the tweaked Soekris, that's high praise

[abraxalito]

Dc offsets are circa +/- 2.75v on the differential outputs at 133ohm iv
Think it's either 800 to 1000mv output ac

I went over to the DS to confirm - its putting out 6.2mA at BPZ, the total swing is 7.8mA.

So I'd expect 800mV DC on those 135Rs and the swing is between 300mV and 1.3V so 370mV RMS. It'll need gain of X5.3 to get up to standard CD level (2VRMS).

Am I right in thinking if I introduce a negative supply I can use the same fet buffer on the negative output to drive a traffo without caps?

I can't quite visualize what you're suggesting here but using FETs comes with the downside of very wide variation of Vt (threshold voltage) and hence you're unlikely to get a low enough offset to drive a trafo without caps.

On the FET buffer driven from the I/V resistor there won't be any need for the input biassing circuits (Rs and Cs) - just connect the gate of the FET directly to the I/V resistor via a gate stopper resistor of 470R. You still need the output side components of course (CCS).

[nige2000]

Dc offsets are circa +/- 2.75v on the differential outputs at 133ohm iv
Think it's either 800 to 1000mv output ac

I went over to the DS to confirm - its putting out 6.2mA at BPZ, the total swing is 7.8mA.

So I'd expect 800mV DC on those 135Rs and the swing is between 300mV and 1.3V so 370mV RMS. It'll need gain of X5.3 to get up to standard CD level (2VRMS).

Am I right in thinking if I introduce a negative supply I can use the same fet buffer on the negative output to drive a traffo without caps?

I can't quite visualize what you're suggesting here but using FETs comes with the downside of very wide variation of Vt (threshold voltage) and hence you're unlikely to get a low enough offset to drive a trafo without caps.

On the FET buffer driven from the I/V resistor there won't be any need for the input biassing circuits (Rs and Cs) - just connect the gate of the FET directly to the I/V resistor via a gate stopper resistor of 470R. You still need the output side components of course (CCS).

definitely 2.72 on the iv resistors, the chips are driven out of spec though

from dddac
"Vcc 8 Volt
R ref 6,1KOhm (400ma)
R Load 270 Ohm, resulting in a Bias of 2,72 Volt
Current Output at zero signal ~ 10mA allowing headroom for full scale current (6,3mA)
Full scale voltage swings between 1,0V and 4,4V (4,5V is max before clipping)
Full scale voltage RMS therefore 1,2V RMS
d2 and d3 below 0,2%, which is very good for passive I/V"

[abraxalito]

He's saying that the BPZ current is 10mA, presumably by virtue of over-volting the chip and decreasing the Iref resistor (10k->6k1).

But if your I/V resistors are 135R rather than 270R you'll get half his offset voltage i.e. 1.35V not 2.72V.

[nige2000]

well its 135r between + output and gnd and anther 135r between - output and gnd
maybe thats the confusion?

that would make the filter u did wrong aswell?

[abraxalito]

I can't see how having two Rs (one on each output) to GND changes anything. Though his output voltage might be measured differentially between the two, that wouldn't change the offset, which would be zero measured between the two output pins.

[nige2000]

Ok I don't know why but dc offset is 2.7v and output is much higher than 300mv with the 135r in. Afair was same with dddac

[Sligolad]

New Aluminium Case Pack modeled and some parts dropped in to demonstrate available space.
Front Aluminium Panel measures 334mm x 83.6mm x 2.6mm thick.

A 10mm riser will give enough clearance to avoid cutting the extruded side panel for the SD Card.
If we add an inclined face then this would be added thickness to the riser.

I will look at preparing a 3D printed model over the coming days to see what we are looking at for machining an aluminium riser for mounting all front parts.

[nige2000]

looks great, loads of room maybe we can turn the dac board 90 degrees so the i2s input is facing the sd card player, and maybe we can mount the usb to i2s converter above the dac board somehow and have a usb extender cable to the rear panel? in favor of keeping the i2s cables as short as possible

[Fran]

Yep, it looks deadly.