Hi
I'm planning on building a 300B SET amp from the schematics here DIY 300B Single-Ended-Triode (SET) Hi-Fi Amplifier Project and have a few questions before I finalise my parts order.
Firstly, there is no AC line filter capacitor in the design. Would adding one decrease noise, or is it chasing inaudible benefits?
Secondly, the biasing is not adjustable. Would adding this be beneficial, and if so, what would be the best way to do so?
Finally, I am considering building the power supply in a separate enclosure to make the layout easier and maximise the distance between the mains transformer and the signal. The design uses DC for the heaters. I assume that there will be no problems running this alongside the B+ and ground in the umbilical, but would like to hear if it might be a problem needing shielding/distance.
Thanks in advance for your help. I'm still getting to grips with some of the electronics theory, but have found this forum extremely helpful.
I'm planning on building a 300B SET amp from the schematics here DIY 300B Single-Ended-Triode (SET) Hi-Fi Amplifier Project and have a few questions before I finalise my parts order.
Firstly, there is no AC line filter capacitor in the design. Would adding one decrease noise, or is it chasing inaudible benefits?
Secondly, the biasing is not adjustable. Would adding this be beneficial, and if so, what would be the best way to do so?
Finally, I am considering building the power supply in a separate enclosure to make the layout easier and maximise the distance between the mains transformer and the signal. The design uses DC for the heaters. I assume that there will be no problems running this alongside the B+ and ground in the umbilical, but would like to hear if it might be a problem needing shielding/distance.
Thanks in advance for your help. I'm still getting to grips with some of the electronics theory, but have found this forum extremely helpful.
Because the amp should have at least 6-7W power probably it is not like with lower power amp where one is likely to use the full headroom very often, especially if you use efficient speakers.
Anyway, to use fixed bias you will have change the voltage supply rail for the driver too. Assuming the bias is around -75V then your starting voltage supply would be about 355V instead of 430V and so that R7 is not suitable anymore. R7 should be reduced to 7.1K to get precisely the same 290V for the driver. It might be better to have one or more additional RC filters (with total R=7.1K) instead of just having R7 in series with 200H. 400V electrolytic caps are cheap..
If you get shielded transformers then it is just about sorting out good grounding. A shielded transformer should not cost much more. Not a fan of separate PSU when they are not necessary. I have the impression that the filament supply uses a separate transformer. If so the get a small AnTek shielded toroidal for the filaments and you won't need a separate PSU....
The idea of having film capacitor at the input of the anode supply voltage filter (i.e. C6 in those schematics) is good even if one uses solid state rectifiers and the reason is that electrolytic caps are possibly the best tanks/resevoirs but are not so good at filtering garbage...
Anyway, to use fixed bias you will have change the voltage supply rail for the driver too. Assuming the bias is around -75V then your starting voltage supply would be about 355V instead of 430V and so that R7 is not suitable anymore. R7 should be reduced to 7.1K to get precisely the same 290V for the driver. It might be better to have one or more additional RC filters (with total R=7.1K) instead of just having R7 in series with 200H. 400V electrolytic caps are cheap..
If you get shielded transformers then it is just about sorting out good grounding. A shielded transformer should not cost much more. Not a fan of separate PSU when they are not necessary. I have the impression that the filament supply uses a separate transformer. If so the get a small AnTek shielded toroidal for the filaments and you won't need a separate PSU....
The idea of having film capacitor at the input of the anode supply voltage filter (i.e. C6 in those schematics) is good even if one uses solid state rectifiers and the reason is that electrolytic caps are possibly the best tanks/resevoirs but are not so good at filtering garbage...
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In my neighborhood, I could see small amount of power line glitch at the speaker terminal through isolation transformers and filtering caps. A simple common mode choke at the AC line get rid of them. That said I have to try really hard to hear it at listening distant. IMHO, If you have to ask, you might as well look for one that support adjustable bias than try to modify this one. Self bias design such as this one are easier to bring up.
Any time you carry low voltage but high current power through some distant be mindful of resistant in the wire and "Connector". Depending on the brand of 300B, each take 1.2 to 2A. Should be OK, just be mindful of ohms law.
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