QRV-07 DC problem

[alerma]

Hi,

I've just finished soldering my QRV-07, and I encountered very strange problem: after turning it on, a DC offset on the unloaded output (input is not connected) fluctuating around approx. 150-200 mV very slowly decreasing to normal DC value during ~ 5 min. After 5 min of warm up everyting gets stable on the level of 5-7 mV. Both channels expose this issue, but the amplitude of fluctuations is less in one of the channels. After warming it up I connect my phones and everything works amazingly! I have no idea what is wrong, maybe some cold joint... Could anyone advice a technique how to localize the problem?

TIA,
Alex

[peranders]

How much offset do you get from the opamp (input buffer)? Stable voltage?

I'll guess you have a bad soldering joint somewhere.

The voltage at the non-inverting input of the TPA6120? Should be stable and (12 uA * 500 ohms) max, = 6 mV The inverting input should differ mac 10 mV.

[alerma]

The offset from opamp is close to 0 mV. Not stable. The voltage at the non-inverting input of the TPA6120 is not stable also, but does not exceed 6 mV (3 mV in average).
How can I fully separate opamp and TPA to locate an origin of the problem? Can I desolder R6 and connect inverting input of TPA to ground via 1K0 resistor? Will it allow me at least to find out is it opamp or TPA?

Thanks,
Alerma

[peranders]

You can test the input buffer with headphones but add 300-1000 ohms in series to decrease the load. If it sounds good, the buffers are OK. The offset voltage will vary a bit due to temperature variations.

The gain of the TPA6120 is only 1 so if you measure the volatges around that circuit, you may be able to draw some conclusions. All resistors are 1 kohms? If you have a couple of mV's in the inputs, you should also have it at the output. When you have 200 mV at the output, try to measure around the IC before it gets stable. Du you have a good +- 12 V? at all supply pins?

If the opamps are OK the won't disturb and besides it's a bit hard to disconnect them. You can move R6 but wait with that. Start with measuring from the output and backwards.

[alerma]

seems it was a cold joint indeed. I've just resolderes all suspisious joints, cleaned it up and it works fine now. 6mV/9mV (left/right channels). It sounds very good also, very clear. I can't estimate it in full yet, as my current headphone is not top of the line.

You said your listening experience was very positive. What is your current rig (source, headphones)?
I'm thinking of buying DT990 600 ohms version. Will it work well with 600 ohm load?

Another silly technical question: what is a best safe place to connect a blue LED (20 mA) without disbalancing power supply?

[peranders]

6/9 mV is expected, less than 10 mV is good. I'm happy for you. Don't forget to write a small review if you are up to it.

The amp can drive any load, even 600 ohms!

My headphones are HD545, rather neutral sound and the source is a Denon DCD-1520 + my own DAC with a CS4328.

Connect the LED at the unregulated side or at the stabilized side but I adwise to run less current. Use a little as you can due to the small transformers. If you run it from the stabilized side connect it between + and - 12 volts, so the voltage drop will be equal.

I think you can manage with 5 mA but test this first. Don't have brighter light than necessary.

[alerma]

Thanks, I will, as soon as I get better headphones.  :roll: