Kuei Yang,
I don't think it's the HT psu... However my HT transformer is about
1 inch away from the 300b.
When I rebuild this amp I will re-arange the tubes, moving the 300b
and the input/driver an additional 3 inch's away from T1.
Any tips how I can test for magnetic or capacitive coupling?
I don't think it's the HT psu... However my HT transformer is about
1 inch away from the 300b.
When I rebuild this amp I will re-arange the tubes, moving the 300b
and the input/driver an additional 3 inch's away from T1.
Any tips how I can test for magnetic or capacitive coupling?
HI,
I have a pair of jelabs 300b mono blocks and a pair of 97.5 db Altec speakers. Using AC, I was able to get the hum down to about 5 - 6 mv. I moved to DC this past week, using a 6.3 supply and a voltage regulator and now have dead quite amps. Have not listened to them enough to tell if I like DC or not. Just know that the hum is gone and am very happy about that..
ck
I have a pair of jelabs 300b mono blocks and a pair of 97.5 db Altec speakers. Using AC, I was able to get the hum down to about 5 - 6 mv. I moved to DC this past week, using a 6.3 supply and a voltage regulator and now have dead quite amps. Have not listened to them enough to tell if I like DC or not. Just know that the hum is gone and am very happy about that..
ck
New guy here.
I built a SET 2A3 amp based on below
from http://www.angela.com/
It has low hum even for my 105db horn speaker but it’s audible when I’m 2 feet away from the speaker. The hum is also audible from the amp itself and found out that AC filament heater transformer is causing it. I’m worried about the hum affecting the input tube (microphonic). Then someone suggested that I should consider switching to DC filament heater (2.5V, 5 amp). My 2A3 are by Sovtek. Being a novice, I would like some help on DC supply schematics, parts list, and etc. I’ve seen premade units by Lamda, Apicon and Meanwell but 2.5 VDC at 5 amp rating is almost nonexistent or they are very expensive. What’s you take on this?
Thanks.
I built a SET 2A3 amp based on below
An externally hosted image should be here but it was not working when we last tested it.
from http://www.angela.com/
It has low hum even for my 105db horn speaker but it’s audible when I’m 2 feet away from the speaker. The hum is also audible from the amp itself and found out that AC filament heater transformer is causing it. I’m worried about the hum affecting the input tube (microphonic). Then someone suggested that I should consider switching to DC filament heater (2.5V, 5 amp). My 2A3 are by Sovtek. Being a novice, I would like some help on DC supply schematics, parts list, and etc. I’ve seen premade units by Lamda, Apicon and Meanwell but 2.5 VDC at 5 amp rating is almost nonexistent or they are very expensive. What’s you take on this?
Thanks.
cotdt said:
you wanna do WHAT?
well best of luck old chap - surely a touch LOUD?. meanwhile in another part of town: http://www.diyaudio.com/forums/showthread.php?s=&threadid=99253&perpage=10&pagenumber=1
thats my take on DC heating of the 300b. I'm working on it so it may well develop in parallel... (although my current squeeze is a big P-P project this keeps cropping up)
Andy
[edit] actually not 'my' take, most of the useful information comes from elsewhere
andrew_whitham said:
you wanna do WHAT?
meanwhile in another part of town: http://www.diyaudio.com/forums/showthread.php?s=&threadid=99253&perpage=10&pagenumber=1
thats my take on DC heating of the 300b. I'm working on it so it may well develop in parallel... (although my current squeeze is a big P-P project this keeps cropping up)
Andy
It doesn't have to be loud. I used 400W UcD modules to power headphones (with gain slightly reduced), and it worked wonderfully and was completely quiet. Sounded amazing. However, there is no replacement for the SET sound.
DC heating the 300B still seems to be noisy to me. Do you think it can reach the 0.1 mV hum goal?
I am currently running the single-ended triode-strapped EL84 (driven by 12AX7) and there is no hum at all with the right tubes. However, I want to move onto bigger better things, like the DHTs.
cotdt said:
Ok, well, I havent tried AC heaters on a DHT but I cant believe that they are quieter than a decent DC supply. the mechanisims for Humm creation in my mind dont allow it
balancing out of phase ends of a bit of wire that might be 20cm long? yowsers!you can see here I've really convinced myself that DC heaters are worth the bother, right?
For headphone amps I would stick (and do) to IDHT's. There is a Tubecad, "circuit of the month" that details a very interesting PSE amp with ECC88's and of course headwize has plenty of projects. but I think that they're all IDHT based. So far as Hum goes, I think that the most important part is in the actual HT psu.
I should qualify this with the fact I use the 32 ohm Grado's which are a bit of a headphone oddity
Incidentally I've heard respected types around here comment that a triode strapped EL84 is about as good as it gets. So I for one would be intersted in what you're using now. I reckon I might have all the big bits needed.
Andy
Evenharmonics said:
Several have claimed here that constant current is the way to go for DHT heaters. It's pretty easy to do if you have the voltage to work with. But with your supplies already set at 2.5V, Tentlabs (www.tentlabs.com) offers a nice, though not cheap, solution. I used them in my half watt 801 amp. It's dead quiet, even with headphones. BTW, the 6sn7 driver is AC heated.
http://www.diyaudio.com/forums/showthread.php?s=&threadid=72361&perpage=10&highlight=&pagenumber=7
cotdt said:
Yes, but you need a quiet supply, DC heaters for the DHT, careful wiring and care to avoid inductive coupling from or to the transformers. Note the Line component in some of the graphs. I had to put some shielding between the power transformer and the output transformer to eliminate it. I use two 5mm thick aluminum plates.
Sheldon
has anyone tried this idea from steve bench to cancel filament induced AC hum from DHTs?
http://members.aol.com/sbench/humbal.html
its a really simple idea. negative FW rectify the filament transf. output and inject the AC ripple from that back into the DHT's grid. You get a 120Hz component thats out-of-phase with the filament induced hum (which is 120Hz), so you just adjust it until it cancels.
I couldn't get it to work, though. it may have been an issue of the values of components used as I was just going with junk i had on hand. and I didn't have a scope around to see what was going on.
http://members.aol.com/sbench/humbal.html
its a really simple idea. negative FW rectify the filament transf. output and inject the AC ripple from that back into the DHT's grid. You get a 120Hz component thats out-of-phase with the filament induced hum (which is 120Hz), so you just adjust it until it cancels.
I couldn't get it to work, though. it may have been an issue of the values of components used as I was just going with junk i had on hand. and I didn't have a scope around to see what was going on.
Even for my EL84 indirectly heated triode, I get a 60Hz and 120Hz component at -90dB. It's barely audible with headphones, but I would still like to get rid of it completely for a perfectly dark background. Would DC heating do the trick? If using DC, should I still use 6.3V or lower it a little?
I found some products for DC power.Sheldon said:
http://store.qkits.com/category.cfm/PSU
I'm thinking of getting one of these.
http://store.qkits.com/moreinfo.cfm/CK239
http://store.qkits.com/moreinfo.cfm/FK809
http://www.meanwell.com/search/RS-25/default.htm (RS-25-5)
Is anyone familiar with them? Any "do"s and "don't"s?
Thanks.
The 120hz hum may be a product of noise on your B+ and/or on the heater rail. Rectified 60hz gives 120z sawtooth ripple. Most likely from the B+, especially given this is an SE amp.
Indirect heated tubes can have their heaters floated to reference voltage using a voltage divider, this will reduce noise coupling from the heater filament to the cathode. Beware of heater to cathode voltage insulation limits. The voltage across the heater should remain the same, when I say floating I mean its voltage relative to the ground, or cathode etc. Ei the heater is about thirty volts DC above ground with 6.3VAC across it.
Indirect heated tubes can have their heaters floated to reference voltage using a voltage divider, this will reduce noise coupling from the heater filament to the cathode. Beware of heater to cathode voltage insulation limits. The voltage across the heater should remain the same, when I say floating I mean its voltage relative to the ground, or cathode etc. Ei the heater is about thirty volts DC above ground with 6.3VAC across it.
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