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In the Forum: Melquiades Amplifier
In the Thread: Planning my DSET
Post Subject: First bass OPT prototype...Posted by anthony on: 7/28/2016
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 Romy the Cat wrote:

 
Output transformer. Here is what the thing become complicated. Of you drive ultra-low impedance with step down transformer then your secondary turns will decrease. They primary will have whatever they have for all case but secondary will go lover and lower as load impedance drop and might even become foil-made. That is for sure will make the transformer faster and better for transients BUT your need LF DSET transformer it means you need inductance. As you will be losing your turn on secondary you will be losing inductance and the only way to get it back would be to increase the core size.  You need to put your transformer maker on the task and see if you will be able to obtain a transformer core for half amper gap, no secondary turns and 5Hz at full power. That will be a small refrigerator size piece of iron and it might be very expensive. 


My transformer winder has completed the first prototype DSET bass OPT.  He was quite excited when he called me and used the word "phenonomenal" to describe its performance into a reflective 1R load.  At 4Hz it is down 0.5dB and at 10kHz and 20KHz it is down 1dB and 3dB respectively.  We were certainly not chasing the high frequencies but they seem to be there nonetheless...it is quite a full range OPT.  The tube load is 2kOhm, which is higher than intended, but it still puts out 9w at 5% distortion into 4Hz at 1R, virtually all 2nd harmonic.  The filter caps in the test circuit are only 680uF and they are visibly pulsing when seen on the oscilloscope so are struggling at that output but the 15kuF caps for the amplifier should improve the performance some more.  Lucas says the square wave at 1w is "almost perfect".

So, I have to make a decision regarding the direction for a second prototype, if indeed I should bother with a second at this stage. I think that 9w output into 1R will be fine athough if the plate is loaded more lightly then I may pickup another watt or more.  It is the 2kOhm plate load that concerns me...is it too high? Is it just right?  Who knows?  Shortening the primary or secondary is going to affect inductance but this may be overcome by going up not one, but two core sizes and using larger wire.  Then of course I could wire the speakers up so they load at 2R which would give more secondary wire, more inductance and the primary could be shortened to reduce the tube load to something more moderate.  

Could any of these things improve the sound?  Does the first prototype sound ok?  All questions with no real answers.  I am tempted to stay with the 2kOhm plate load as we have in the first prototype and see how well I can make it sound.

Anthony


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