Just for fun we measured the distortion spectrum from RCA input via Crossfeed, attenuator and then to the ZGF-D.
Attenuator set at usual litening level.
For me very nice spectrum.
Put a H2 null pot in appropriate place and null out 2H. Then it be much better.
THx-RNMarsh
Patrick
Just finished building the desktop version(ZGF-D) over the holidays. Much thanks to Patrick for providing the board and LH0033.
Build was simple as it consists of mainly ICs. The tricky part was to set the bias through fixed resistors. DC was only 5mV and 1 mV before trimming, largely due to well matched components.
Finally the sound. I am not really poetic at describing sound, but I shall try. My only headphone is the HD800s.
As some of you may know, I had a F5-HA and thus I have the chance to compare them. I find that ZGF-D seems to have more sparkle, little brighter and more musical sounding. Soundstage was a little wider, orchestra pieces were amazing on this amp. Bass is tighter and has more oomph. F5-HA seems to have a little more detail though.
I enjoy ZGF-D very much. To me, it is a little better than F5-HA, just slightly. I highly encourage others to build this.
Build was simple as it consists of mainly ICs. The tricky part was to set the bias through fixed resistors. DC was only 5mV and 1 mV before trimming, largely due to well matched components.
Finally the sound. I am not really poetic at describing sound, but I shall try. My only headphone is the HD800s.
As some of you may know, I had a F5-HA and thus I have the chance to compare them. I find that ZGF-D seems to have more sparkle, little brighter and more musical sounding. Soundstage was a little wider, orchestra pieces were amazing on this amp. Bass is tighter and has more oomph. F5-HA seems to have a little more detail though.
I enjoy ZGF-D very much. To me, it is a little better than F5-HA, just slightly. I highly encourage others to build this.
Glad that you do finish it after 2 years or so, and you like the results. 
The ZGF sets off as a project to use mainly ICs, with the exception of the power transsitors.
In that sense, it is not more complex to build than the more common IC+follower in a FB loop.
So it was intended as a simple project with a high success rate and little chance of failure in service.
But it uses the IC and the follower in different fashion.
It first uses the AD844 as a current conveyor to achieve voltage gain without negative loop feedback.
Because the resulting output impedance of the gain stage is high, it has to use a buffer with a JFET input, hence the LH0033.
But the 0033 does not have enough power for a desktop, so the power transistor Sziklai is included to boost the current driving capabilities.
But as the title suggests, it is ZGF (zero global feedback), in the same category as the DAO and the Pioneer Super Linear.
ZGF gain stage, followed by power buffer.
The F5-HA is different, in the sense that it is essentially a transconductance amplifier with voltage negative feedback (NFB).
As such, the amount of NFB is totally load impedance dependent.
The HD800s is one of those phones with quite a large impedance variations over frequency.
Thus, the amount of NFB varies accordingly, especially in complex music passages.
Since the F5-HA is equally capable of driving loads down to 32R, maybe you could experiment by adding a 50R resistor in parallel with the HD800S.
Make sure you use one with enough wattage and low distortion, e.g. 4x Dale RN60 200R in parallel.
In this aspect, the F5-HA is much happier with planar magnetics with constant impedance.
The ZGF-D has rather low Zout and is largely constant over the audio band.
Thus it is less susceptible than the F5-HA in that sense.
Time to also try out the portable version perhaps ?
Patrick
The ZGF sets off as a project to use mainly ICs, with the exception of the power transsitors.
In that sense, it is not more complex to build than the more common IC+follower in a FB loop.
So it was intended as a simple project with a high success rate and little chance of failure in service.
But it uses the IC and the follower in different fashion.
It first uses the AD844 as a current conveyor to achieve voltage gain without negative loop feedback.
Because the resulting output impedance of the gain stage is high, it has to use a buffer with a JFET input, hence the LH0033.
But the 0033 does not have enough power for a desktop, so the power transistor Sziklai is included to boost the current driving capabilities.
But as the title suggests, it is ZGF (zero global feedback), in the same category as the DAO and the Pioneer Super Linear.
ZGF gain stage, followed by power buffer.
The F5-HA is different, in the sense that it is essentially a transconductance amplifier with voltage negative feedback (NFB).
As such, the amount of NFB is totally load impedance dependent.
The HD800s is one of those phones with quite a large impedance variations over frequency.
Thus, the amount of NFB varies accordingly, especially in complex music passages.
Since the F5-HA is equally capable of driving loads down to 32R, maybe you could experiment by adding a 50R resistor in parallel with the HD800S.
Make sure you use one with enough wattage and low distortion, e.g. 4x Dale RN60 200R in parallel.
In this aspect, the F5-HA is much happier with planar magnetics with constant impedance.
The ZGF-D has rather low Zout and is largely constant over the audio band.
Thus it is less susceptible than the F5-HA in that sense.
Time to also try out the portable version perhaps ?
Patrick
Glad that you do finish it after 2 years or so, and you like the results.
Time to also try out the portable version perhaps ?
Patrick
This build is quite simple. Anyone with decent soldering skills and able to read schematics should be able to build this. The only slightly difficult part is adjusting bias with fixed SMD resistors. On hindsight, I should have used a trimpot to adjust before replacing with fixed resistors.
Even so, just with sticking to schematic will yield ~ 120mA, which is more than sufficient.
Portable version definitely on its way
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