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Old 16th December 2012, 09:11 AM   #1
Feagil is offline Feagil  South Africa
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Hi everyone

I have a Lampizator 99% of the way complete as a Christmas present for my father. A Lampizator is a SRPP amplifier after a passive resistor I/V stage that replaces the usual NE5532 Opamps in my Rotel RCD-855. The one channel is working perfectly but he other has a relatively loud hissing sound coming from it. I can still hear music faintly underneath the hiss. If I swap inputs to the lampizator the channel with the hiss does change. Could this be a problem with the DAC?

Thanks for the help everyone.

V
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Old 16th December 2012, 11:11 AM   #2
Jaap is offline Jaap  Netherlands
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Yes, or something else
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Old 16th December 2012, 02:18 PM   #3
TheGimp is offline TheGimp  United States
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If the channel of the amp with the hiss does not change, then the channel has a problem.
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Old 16th December 2012, 05:26 PM   #4
Feagil is offline Feagil  South Africa
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Hi everyone,

Problem of the hiss is solved, I hadn't properly disconnected one of the resistors so that channel of the DAC was sharing its juice with the old circuit. Sadly a new problem has evolved. I have a relatively loud hum that sounds distinctly like AC mains. I have a star ground system and proximity of the valves to the trafos doesn't seem to affect the hum. I am using the power supply as per the diagram and I was under the impression the SRPP configuration would be very good at self regulating. All the voltage measurements meet the original design specifications (only multimeter sadly).

Any ideas on how to curb the hum? I will borrow an oscilloscope for my varsity tomorrow, perhaps it will reveal more. I have been testing with croc clips before I solder it in at the end, maybe the long leads are to blame.
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Old 18th December 2012, 08:12 AM   #5
Feagil is offline Feagil  South Africa
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Hi everyone

The modification is nearly complete however I still have one last problem. When pushed to about half the volume on the amplifier I can hear some low frequency hum and some high frequency buzzing on top of it. The buzz only really appeared after I soldered everything in to the CD player, wasnt present in the pretesting, croc clip phase. I have star grounded everything to the bolt as you can see in the image and my power supply is made of a CRC filter. The two caps are 100uF and the resistor is 1K. I also bypassed the first cap with a 0.1uF X2 type capacitor. Any suggestiong on quietening things down?

Click the image to open in full size.

Thanks again for the help.

V

Last edited by Feagil; 18th December 2012 at 08:19 AM. Reason: pic too big
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Old 18th December 2012, 08:14 AM   #6
Feagil is offline Feagil  South Africa
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Wow, sorry about the huge pic. Its my first pic post.
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Old 18th December 2012, 11:07 AM   #7
oshifis is offline oshifis  Hungary
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It could be HF oscillation. A 100R to 1k series resistor at each grid rigth on the socket might help.
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Old 18th December 2012, 11:30 AM   #8
Feagil is offline Feagil  South Africa
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The DAC I have is the TDA1541 which is a Iout DAC and so requires the resistance to ground to be small so a 1k resistor to ground from the grid will be paralleled with an existing 86 ohm resistor, thus not affecting much. If you mean a resistor in series then the problem is worse since the signal will have to trough the large resistor to get to ground. Something the DAC will not be happy with.

V
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Old 18th December 2012, 12:40 PM   #9
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A grid stopper is connected between DAC output and in series with tube grid. No difference from DAC perspective - the load is determined by i/v resistor, not the grid circuit. Important to place directly it on a tube socket pin.
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Old 18th December 2012, 01:41 PM   #10
wlowes is offline wlowes  Canada
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Lukas has a very good hum buster routine described on his site. If you follow it, you will likely loose the hum. If you have grounded everything properly following his perscription, try adding one more filter to your B+. Basically crcrc.
When complete it will be dead quiet with your ear on the speaker at normal listening volume.
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