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Old 17-09-20, 09:47 PM
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Kei Kei is offline
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Join Date: Apr 2013
Location: South Wales
Posts: 562
After much testing, I came to the conclusion that although the amp sounded great, it had one issue; too much gain. The open loop gain was at 49.54dB and closed loop was 33.26dB. The problem this causes is that minute levels of noise can be amplified sufficiently to be come audible and you have less range on the pre amp volume control before it gets excessively loud.

The big issue I was seeing was noise caused by an HDMI ground loop in my marantz AV7701 pre amp. The noise is at a very low level where it only starts become apparent on 90dB efficient speakers with gain above about 25dB. (a not so pleasant sounding 50Hz hum) The best solution would have been to break the ground loop but unfortunately I only found one ground loop isolator for HDMI and it was nearly £800. (Basically an HDMI to LC fibre converter set) The easier solution was to find some way of simply dropping the gain level. The simplest but also most bodgtastic answer would have been to simply attenuate the input signal. This does work as my other solid state power amps all have gain control pots on them which actually don't adjust gain but simply attenuate the input signal. Turning these down makes the hum quieter and can make it disappear altogether.

Another simply approach to reduce the gain is to increase the level of negative feedback which subtracts from the open loop gain. More feedback results in lower closed loop gain. Unfortunately, I couldn't resort to this as I was pretty much at the limit with feedback as my stability margin was well tight.

The last option available was to remove the cathode bypass capacitor on the first valve which would drop the gain and also potentially allow for a little bit more feedback to be injected. The guy who helped to design and model all the tweaks to the original mullard 5-20 gave me a hand in making the changes to the design and then modelling it in LTspice.

It all looked good so I took the amp apart and made the necessary tweaks to the turret board layout.

On testing, it behaved similarly although there were some residual issues in the LF resulting in some slight oscillation. (anything lower than 13Hz above 0.5V in) The voltages across EF86 seemed a bit out of kilter which will always have a big effect on everything downstream. I reduced the dropping resistor for it's supply to raise the anode voltage which seemed to be just the ticket. No more oscillation at very low frequencies. All I get with very large infrasonic signals now is transformer core saturation which is without a doubt ugly and not pleasant sounding but it is not potentially amplifier destroying like oscillation can be.

10Hz clean @ 12.6W is incredibly good going for an output transformer.

10Hz @ technically 22W going by p-p values. The ugly look of core saturation.

10KHz square wave performance is pretty similar. The gain difference is obvious when you compare the input signal levels against the RMS value on the scope. I'm thinking of testing the input infrasonic filter to see if I can reduce the HF losses by reducing the 22K resistor to 10K.

These tweaks successfully dropped the gain from 49.54dB open loop and 33.26dB closed loop to 45.77dB open loop & 25.87dB closed loop. That also took feedback from 16.5dB to 19.9dB.

Under test, working very well. (still a prototype build)
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