Sjöström Audio DIY forum

Projects => Headphone amplifiers => Topic started by: Houghtkj on April 09, 2012, 08:16:36 PM

Title: My QRV08 Build
Post by: Houghtkj on April 09, 2012, 08:16:36 PM
Hi P-A     First off can I say many thanks for this amplifier! Not only does it look cool, it sounds even better.

Taken me a while to complete as I have been waiting for parts, plus I have had other commitments.
It still needs casing up but is working on the bench and sounds so good I don't want to stop listening!

I do have a couple of questions though if you don't mind:

1.   How do I calculate the amp gain if I adjust R14/E14? - I know the 100R gives me a gain of 4 - I
      would like a gain of 5 or 6.

2.   Output offset is good at about 0.15mV (both channels), but I am having great difficulty setting the
      input bias across R1 - The pot adjustment is super critical. Just the slightest touch makes a big
      difference. I have got it to about 0.5mV, but it's giving me nightmares worrying that it should be
      better.

3.   I will be feeding the QRV08 from the output of a DAC which has zero DC offset - I have chosen not to
      use C1/G1 and replaced with a 0 Ohm resistor. Is this wise?


Your feedback will be much appreciated.
Ken

Title: Re: My QRV08 Build
Post by: peranders on April 13, 2012, 09:56:26 PM
Title: Re: My QRV08 Build
Post by: Houghtkj on April 15, 2012, 07:15:48 PM
Hi P-A,      OK, thanks for your answers - I am OK with points 1 and 3, but I don't understand point 2.

Can you please explain the operation of the pot and setting the input bias in greater detail?

I don't intend using C1/G1, and my DAC output is via capacitors within the DAC. Does this mean I need
to correct the input bias using the pot?

Many thanks,
Ken
Title: Re: My QRV08 Build
Post by: peranders on April 15, 2012, 08:05:42 PM
Sorry, I didn't read your question carefully enough.

The amp needs an input bias which can be positive or negative and this is dependent of the current gain of the input transistors. You must always have a DC path from the input down to ground. The trimpot is for reducing the voltage caused by the input bias current and still have a rather high input impedance. The DC-servo can't handle too much offset so injecting some current at the input will make the life easier for the servo.

So, if you still have a AC coupled signal you must also have this trimpot,
Title: Re: My QRV08 Build
Post by: Houghtkj on April 16, 2012, 09:50:43 AM
OK P-A now I understand, many thanks.

I have found that the adjustment is VERY sensitive - perhaps a muti-turn pot would be better suited.

Thanks again for your help

Ken.
Title: Re: My QRV08 Build
Post by: peranders on April 16, 2012, 07:02:23 PM
The important thing is that the DC servo can adjust the output offset.
Title: Re: My QRV08 Build
Post by: Houghtkj on May 04, 2012, 05:54:52 PM
Hi again P-A,

I have just bought a pair of low impedance headphones (32 Ohms) to go with my existing HD600's (300 Ohms).
Both sets of cans work fine with J3 and J6 "IN" - Is there any problem with this as I assumed J3/J6 should
be "OUT" for low impedance and "IN" for high impedance phones?

Many thanks
Ken
Title: Re: My QRV08 Build
Post by: peranders on May 04, 2012, 08:43:10 PM
Most headphones are designed for 120 ohm source impedance and this creates a 1-2 dB bass lift. If you have 32 ohms cans you can rather easy blow them at high levels but the amp itself can handle 32 ohm loads.