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  #11  
Old 17-09-20, 09:47 PM
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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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  #12  
Old 17-10-20, 06:45 PM
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I've started construction of a Mk IV amp which shrinks the overall package down. The Mk III underneath is the same size aluminium sheet, 19"x10.5" down to 15"x9.5". Even though the overall chassis size has been reduced, I have been able to increase the separation of the output valves in order to improve thermals.


So far I only had time to get the piece cut down to size and mark the holes with a punch. Tomorrow I will be drilling the holes out.
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  #13  
Old 17-10-20, 10:21 PM
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I've now deconstructed the first amp I built so that I can rebuild it as the Mk4. It's surprising just how quickly I took the whole thing to bits. 1.5 hours and it was done. I swear it took me over a week to build it.


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  #14  
Old 18-10-20, 11:50 PM
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More progress today. All holes now drilled and countersunk. I quickly found that I had undersized the holes for the cable grommets by 2mm so need to sort that tomorrow.


I've cut the turret boards down so that the cathode bias circuit is now placed under the output transformer. I need to buy some shorter nylon standoffs for it.


Test fitted the transformers to get an idea of the size and distances. The output transformer does sit a bit closer to the valves than I'd like but it's not extremely close so shouldn't pose a problem.
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  #15  
Old 19-10-20, 07:59 AM
Warchild Warchild is offline
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Jeez that thing is a mass monster
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  #16  
Old 19-10-20, 11:11 PM
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Quote:
Originally Posted by Warchild View Post
Jeez that thing is a mass monster
It is a tad large and heavy. To get a stereo pair, it'll weigh around 25Kg.

Sorted the turret boards today.


I also decided that since the finish on the aluminium was a bit marred, I would sand it smooth. I'm now trying to decide whether to go all out and polish it up or try to get a smooth brushed finish. Currently part way through as it's taken more effort than I expected.
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  #17  
Old 20-10-20, 09:16 AM
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I don't understand much of it but looks like an awesome effort!
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  #18  
Old 21-10-20, 10:45 PM
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Much sanding and polishing later...




I'm also about 90% of the way through building the boards. A few capacitors, the zeners and the veroboards are left to do.
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  #19  
Old 23-10-20, 08:26 PM
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I finished up the turret boards yesterday after building the veroboards for the CCS and the input filter.


I found some more inner tube which I've cut to the shape of the transformer base which acts as a vibration damper. I did the same on the Mk III build and it seems to work reasonably well. This time I had sufficient to cut one piece to do the whole base where I did the 4 mounting points previously. All the bits and pieces are now fitted to the plate.


The polished plate definitely looks better than the anodized version. I've noticed a few small niggles with the layout that I've corrected for the final version. The rectifier valve is a bit close to the choke and the position of the phase splitter valve is closer to the input valve than it needs to be. Neither is a big issue though and shouldn't pose any problem. The only thing I need to make now is the chassis framework to hold the plate in place.
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  #20  
Old 25-10-20, 06:33 PM
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I've now made up some side panels. The sides are actually recycled from the MK II wooden build as this happened to be the same width. Sadly they come with some holes from their previous use.

I just need to create a front and rear panel to complete this version. (plus obviously finishing off the wiring.)

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