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
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.
- Gain = R15/R14 + 1 Decrease the R14. Let R15 be.
- Input bias is necessary only when you have a AC connected input in order for the DC servo to be able to work.
- Can't say but. I might work for you.
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?
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,
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
The important thing is that the DC servo can adjust the output offset.
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?
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.