With electronic construction complete the amplifier has been moved from the workbench to the listening room to get some initial listening impressions.
The speakers it's running into are floorstanding KEF C95s from around 1990. Efficiency is quoted at 90dB. These are sealed bandpass.
The amp has 3 line inputs and one phono. So the source used for listening tests was the Chromecast Audio and my turntable with its Audio-Technica AT11 MM cartridge.
First up - the line input. An easy test. Passed with flying colours. The measurements on the workbench were showing around 15 watts power into the 8Ohm dummy load before any sign of clipping, and the THD was reading less than 1% rising as the power level approached maximum. So expected behaviour, in other words.
The EL84s are biased to about 8 watts dissipation, which at the B+ of 365V means 22mA. The cathode shunt I am using is 10Ohm so that's a reading of 0.22V at each cathode test point.
EL84s with bias adjusters
The topology of this amp is that the first thing the line-level input signals encounter is the Cathode Follower which forms the first stage of the tone control. This means the input impedance is around half a meg. So a very easy load to drive. Most amplifiers route the input signal to the top of the volume control pot, so the input impedance is effectively the track resistance of that pot, usually 50 or 100K.
Hum levels are acceptable. There is a barely detectable hum if pressing the ear to the front of the speaker cabinet when no music is playing. However since I don't know anyone who would use the amplifier in this manner, I'm not going to allow myself to get too worried about it.
Sound quality assessed subjectively is clean, pleasantly detailed, no trace of any distortion or harshness in the treble, and with plenty of power to the bass. I would be happy having this sound quality as my daily driver.
The tone control behaved exactly as the previous build of this circuit, this is the second time I've built this one.
The phono stage however was not such a stellar performer.
The only problem with this one is an unacceptable level of hum. This is normal with phono stages and simply indicates a signal grounding problem somewhere.
Other than the hum, the RIAA stage sounded well. So I am confident some simple experimentation with the earthing will resolve the hum.
I tested the RIAA compliance on the workbench and found it to be within 1dB from 45Hz to 15KHz. This is acceptable.
One aspect of this build I am less happy with is the power transformer.
This is a Hammond 370FX and it's loaded to its rate current. Around 140mA DC load on the B+ and around 5A load on the 6.3v
However this transformer runs hot. After 30 mins run time it's noticeably warm and after 60 mins you can touch it but if you wrap your hand around it, you'll want to remove it after 3 or 4 seconds.
I found a few discussions on DIYAudio commenting about the heat output of these transformers, and to be honest it makes me reluctant to use them again, either that or I'll be sure to apply a generous de-rating margin next time.
Cosmetically I still have some work to do
The person who I am building the amplifier for has indicated his dismay at the power switch. Something with a chrome toggle is preferred. Finding something that will a) fit without fouling the adjacent EL84 socket and b) fit the existing 20mm hole, is going to be a challenge.
Secondly, on the right hand side, the hole in the front is for the remote control sensor. This needs to be permanently fixed in place internally and a piece of translucent white acrylic press-fitted into the hole.
Electronically I need also to arrange a mute circuit to avoid switching spikes going through to the speakers when toggling the speakers/headphones switch, and to avoid the open-circuit hum going through to the speakers when the switch is set to headphone (thus disconnecting the input to the preamp)
First order of business though: cure the ground-loop hum on the phono stage...