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Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
23rd June 2014 I made new printed circuit boards, both for the power supply and amplifier. There was some changes to the power supply from the prototype. I added 150V stabiliser tubes for the 300V supply and the screen supply is also on the board.
Everything is installed in a enclosure from the Italian company HIFI2000.
25th November 2014
The first power up and test with signal generator as the amplifier is installed in its enclosure. There is some problems with hum that will have to be investigated.
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
8th June 2015
Further investigation of hum issues was conducted by waving a isolated 1000VDC rated screw driver around in proximity of different components while watching the secondary side of the output transformer on my oscilloscope.
I identified two vulnerable places where a great deal of noise could be induced through capacitive coupling and there is also sensitive to noise through induction from magnetic fields.
The first issue was a small part of the signal line in coaxial cable that was not shielded. Explanations are written on each screenshot from the oscilloscope. The first pictures show the output without any interference with the circuit. The second shows the effect of touching the isolation on the part of the signal line in cable that was not shielded.
The second issue was the coupling capacitor in the input circuit before the pre-amplifier. The yellow wave form with the highest amplitude show the induced noise by touching it as it was installed.
The two blue wave form screenshots show the test to locate the pin connected to the outer foil layer in the capacitor, the capacitor is simply connected to the signal and ground of the oscilloscope probe and squeezed around with your fingers. Switch the connections around to perform it at reverse polarity.
The wave form with the lowest amplitude tells us that the pin currently connected to the ground clip is the pin connected internally to the outer foil layer in the capacitor. This outer layer will also function as a shield in high impedance circuits and that pin should be connected to ground or the path with lowest impedance towards ground.
Now the question is, did it help? Measurable yes, on any audible hum performance heard with a human ear... not really :)
Update #2
All wave forms are from the secondary side of the output transformer.
The first oscilloscope screenshot shows a Fast Fourier Transform (FFT) analysis of the noise generated by the normal diodes for the 340VAC high voltage supply 1N5408 and 40VAC bias supply 1N4007.
The second oscilloscope screenshot shows the difference between normal diodes like 1N5408/1N4007 that have reverse recovery times around 2uS and fast diodes like MUR480/MUR420 that have reverse recovery times around 50nS is shown in the oscilloscope screenshot with yellow and green wave forms. The spike amplitude is around 15% less but the overall 50Hz hum at the positive half cycle is a little more prominent. Changing the diodes gave a difference in the sound from the switching spikes.
The third oscilloscope screenshot shows the much reduced noise levels after a ground loop formed from star ground point to signal input plug was removed and along with the much shorter switching spikes from the new fast diodes.
Audible it appeared like 90% of the hum disappeared. The greatest performance gain was however from removing a ground loop, the faster diodes did not have such a dramatic effect, it was hear able, but not on the magnitude of removing the ground loop.
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
The manual MOSFET linear regulation part of the power supply for the screens have also been scrapped and replaced by a resistor. It was stable and fine at operation, but every now and then it would short circuit on power up of the amplifier.
After playing around with the audio analyzer looped back to itself, I forgot I had it set for 6V output and hooked the amplifier up for a FR test. This resulted in OPT arcing over and I had to take it apart, no visible damages, but secondary side measures only half the DC resistance as the OPT I have for the other mono block, I will have to hi-pot test it to see if its really dead or can be saved with a little jiggling and varnish.
-----------
New measurements with a fresh OPT installed, the one that arced gives some spurious 0.5 to 1 Ohm DC resistance on the secondary side, it does not look like its good for anything but brief weighting.
Dummy load was a 8.6 Ohm 200 Watt resistor and thus the output power from the output level test gives some 70 Watt out at 0.5 V in. The other tests are done at 0.5 V input too.
I had the amplifier hooked up to my JBL 4333s for the first time and it is now obvious that there is a reason for the high thd+n measurements, there is a great deal of noise, still not sure which kind, but sounds like white and harmonic.
It plays pretty well, not too sure about bass yet, seems to vary a lot by the music, Metallica still has evil deep punch, whereas some other artists are flattened a bit.
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
A sad side note to the testing described above is that I had the audio analyzer looped to itself for testing and output voltage was set to the maximum 5V. I forgot about this setting and hooked the amplifier up to the audio analyzer and just as test began there was sparks flying from the output transformer and some smoke. The output transformer is damaged from internal arcing and I will have to buy a new one.
I changed the output transformer with the one I had for the 2nd amplifier. The output transformer was also shifted 90 degrees on two axis's in order to cancel any possible magnetic coupling to the power transformer. It did however not show any difference in measurements on the audio analyzer.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
I'm still learning the basics of this stuff, just about ready to put my power supply together, and due to pick up some parafeed chokes and a big OT this week, but is it worth adding some DC blocking capacitors to the OT? This would also limit saturation, if this is an issue. Otherwise, I'd try a larger OT. Also, how well is the output impedance matched to the speaker impedance?
Registered Member #135
Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735
Ash, you don't use DC blocking caps on the OT. The center tap feeds DC current to the plate of the tubes through the winding, a blocking cap between the plate lead and the transformer lead would "shut off" the tube.
Also the output is totally isolated from the primary, so there is no reason to have any DC blocking in the output portion of the circuit. If you wanted a Zobel network, that is a different story.
OPT's match fairly well because you are changing an impedance based on the turns ratio, which can be controlled quite well. the last transformer I measured with a HP vector impedance meter was right on, and this was a cheapo $25 5W OPT.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Hazmatt_(The Underdog) wrote ...
Ash, you don't use DC blocking caps on the OT. The center tap feeds DC current to the plate of the tubes through the winding, a blocking cap between the plate lead and the transformer lead would "shut off" the tube.
Also the output is totally isolated from the primary, so there is no reason to have any DC blocking in the output portion of the circuit. If you wanted a Zobel network, that is a different story.
OPT's match fairly well because you are changing an impedance based on the turns ratio, which can be controlled quite well. the last transformer I measured with a HP vector impedance meter was right on, and this was a cheapo $25 5W OPT.
Maybe a DC blocking cap was the wrong term to use. Maybe you are correct, I'm not that familiar with push-pull output valves, but surely all transformers work on AC?
I was just thinking you could limit saturation using capacitors, maybe it's not possible.
I do know a common mistake people make is not using a large enough output transformer, and then wondering why it doesn't sound as expected.
Also, matching the impedance of the output valves to the impedance of the speaker(s) isn't always as straightforward as it might first appear, from the research I've done.
I think I'll stick with single ended with parafeed for now
Registered Member #4104
Joined: Fri Sept 23 2011, 06:54PM
Location: Uk .
Posts: 122
Nice project ,
I do need to try and make a PP vlave amp oneday , I've only made single ended ones that never made it to a proper case apart from one for a hybrid cassette amp
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
Ash Small wrote ...
Also, matching the impedance of the output valves to the impedance of the speaker(s) isn't always as straightforward as it might first appear, from the research I've done.
I think I'll stick with single ended with parafeed for now
I made no further considerations about what type of speakers I have connected. 8 Ohm tapped output transformer is all I need and then find a output transformer with a primary impedance that matches the load impedance chosen for the output tubes.
TwirlyWhirly555 wrote ...
Nice project ,
I do need to try and make a PP vlave amp oneday , I've only made single ended ones that never made it to a proper case apart from one for a hybrid cassette amp
Thank you and just throw yourself into it, start out with a small EL84 pp amp, there are some good designs outthere and some well proven ones like:
My next tube amplifiers are going to be a couple of single end 250TH amplifiers, never tried a SE yet :) I might aswell design it and order the transformers needed now that I have to get new OPT for this 6P45S amplifier.
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100W 6P45S mono block tube amplifier
