QRV-08 with 12V rails

[jezz]

Has anyone built the QRV-08 with a 230V/2x24V transformer for use with 115V? If so, did you change any resistor values? I'd like to use the Myrra 44204 transformer, which should give me an unstable voltage of around 15V.

It looks like the amp sections don't need any resistor value changes, but the super regulators seem to need changes to the reference voltage dividers. It looks like R2 (from the LM431 datasheet) needs to get switched from 5K to 7.15K to get an appropriate Vref for 12V rails. However, I don't have a strong grasp on the super regulator design, so I'm hesitant to make any changes.

[peranders]

I know people have used 230/24 VAC. You should get enough voltage since those small transformers has a rather high raw voltage. In case 12 VDC is too high you can lower it to 11 V maybe. You won't hear any difference in sound level since 10 V peak is extremely high sound level unless you have 600 ohms headphones.

If you want to change voltage it's easiest to change the LM431. You can consult the data sheet of the reference. There you will find info about tuning the voltage but if you find it difficult, just ask me.

[jezz]

Thanks for the prompt reply. I'll give it a try and see what happens.

[jezz]

It seems to work fine. I ended up going with these because they had a higher power rating, closer to the BOM part. For anyone curious, TME's shipping to the US is around $10 and is very quick.

I'm getting +/-13.9V out of the super regulators, and all of the power rails seem fine. I haven't tuned the DC offset yet, but both channels sit at around -170mV.

There is one minor problem; the LEDs all light when I give it power, and then within a second they all go off. The BOM values for R38/39 seem too close to the LEDs forward current for my transformers. The LED has a forward current of 25mA; my unstable rails (after the LM317/337) are right at 15V, which gives around 15mA current across the LED. I haven't decided if I'm going to change the resistors yet. Do you think it's worth doing?

[peranders]

The LED's must be on, if not the regulator isn't regulating.

[jezz]

Bummer dude. I'll try dropping the output voltage to 12.5v by changing R46/47 to 7.15kohms. I think 2.5v should give it enough head room to stay in regulation.

[jezz]

I've been thinking about this. My operating points are: 16,0V before the preregulator, 14,6V after the preregulator, and 13,7V after the pass transistor/into the AD825. The output of the opamp is at 12,0V, which looks like it's probably saturated; per the spec, the AD825 will swing +/-13,3V given 15,0V rails (13,7V-1,7V=12,0V).

It looks like, instead of aiming for 12,5V Vout as I thought, I should aim for something lower. I don't mind this because most of my headphones are fairly low impedance so I'm never going to get that close to the rails anyway. I'm thinking 16,0V - 5,0V = 11,0V, so either a 6,98kOhm or 6,49kOhm resistor, which I have on order. Thoughts?

[peranders]

You must have at least 5 V between in and out. The real limit is 4.5 volts.

[jezz]

6,98kOhm it is then  :-).

[jezz]

6,98kOhm seems to work; the LEDs come on, momentarily dim, and then come back on. The LEDs on the negative rails end up slightly dimmer than the positive rails, but do remain lit. Do you think that's a problem?

It sounds wonderful. Or perhaps "sounds" is the wrong word; it sounds like a wire with gain and power. Thank you for the help and for designing such a lovely amp.

EDIT: Oddly, now I'm seeing 14,3V before the preregulator. Does this mean I need to set the output voltages even lower? I have some 5,5kOhm resistors which would drop me to 9,9V, or some 5kOhm resistors which would drop me even lower to 9,5V.

[jezz]

I dropped the PSU output voltage 0,9V. The strange startup behavior no longer occurs and the LEDs are all stable at the same brightness. I'm running at 10,4V or so.

Were I to do this again, I'd build with off board transformers. That said, I suspect the reduced power supply will never become a limitation given my headphones (none higher than 80ohm) and listening levels.

[peranders]

Good that it works now and maybe I should recommend using off board transformers in case you don't find 115 V pcb transformers.