Probably "most" of your noise problem.
Separate windings to heat the rectifier and DC for the signal tubes is pretty standard for a preamp AFAIK. jeff
I've used the 6CA4 before with filament voltage shared between the rectifier and other tubes as you show, with no hum, although not with a phono preamp.
You should try ground referencing or elevating the heaters as shown here:
The Valve Wizard
That may resolve your problem.
By the way, I used to live in Lawrenceville and have been to Loganville many times.
You should try ground referencing or elevating the heaters as shown here:
The Valve Wizard
That may resolve your problem.
By the way, I used to live in Lawrenceville and have been to Loganville many times.
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It is possible to have AC heating in a phono section, but the challenge is DAUNTING. The use of tubes with hum bucking, spiral wound heaters, is mandatory and, as has been mentioned, the heater supply must be biased off B+. Suitable tubes are the 6EU7 and the 7025. In current production, the Sovtek 12AX7LPS is a genuine 7025 equivalent.
Constructing a regulated 12 VDC supply for the phono section 'X7 heaters is easy enough. A "Rat Shack" 273-1366 filament trafo, a MBR20100CT twin Schottky diode, a big 25 WVDC 'lytic, and a 7812 3 terminal regulator are what you need.
BTW, regulating phono section B+ is a good idea. The little TO92 case LR8 is great for the job. Use 1 LR8 in each channel.
Constructing a regulated 12 VDC supply for the phono section 'X7 heaters is easy enough. A "Rat Shack" 273-1366 filament trafo, a MBR20100CT twin Schottky diode, a big 25 WVDC 'lytic, and a 7812 3 terminal regulator are what you need.
BTW, regulating phono section B+ is a good idea. The little TO92 case LR8 is great for the job. Use 1 LR8 in each channel.
If the valves share a heater winding then the rectifier must be first on the chain, not last as you show it. This reduces the AC current around the first stage. Use thick wire to the rectifier, then thinner wire from there so you can twist it more tightly.
You must have a DC reference for the heaters; what voltage it is is less important than the fact that it exists.
Show us a picture. AC heater wiring is one of the few areas of audio where a picture tells us more than a circuit diagram.
You must have a DC reference for the heaters; what voltage it is is less important than the fact that it exists.
Show us a picture. AC heater wiring is one of the few areas of audio where a picture tells us more than a circuit diagram.
For phono I prefer dc heaters raised about 45volt above ground . This works based on RCA, and others data from the 1930s . At the time it was ac and raised voltage. I have seen an unused bias tap as a reference voltage be very quite . Care must be taken with heaters since the signal voltage is so vey small .
OK all,
This is the current state of my pre-amp. Now that I know it will work to my satisfaction, I will rebuild it to make it look more presentable. I may even try making a PC board version. I took the 6CA4 out and put a pair of 1N4007's in.
The background hum did not change. I then put the 100 ohm resistors on the heater lines. That made a major improvement. I took the 1N4007's out and put the 6CA4 back in, it was still quiet. Just as quiet as with the 1N4007's. I tried putting the heaters on a DC power supply, the hum was way too high so I went back to AC on the heaters.
I WELCOME all comments as to component values and mods to this circuit.
Thanks to all
This is the current state of my pre-amp. Now that I know it will work to my satisfaction, I will rebuild it to make it look more presentable. I may even try making a PC board version. I took the 6CA4 out and put a pair of 1N4007's in.
The background hum did not change. I then put the 100 ohm resistors on the heater lines. That made a major improvement. I took the 1N4007's out and put the 6CA4 back in, it was still quiet. Just as quiet as with the 1N4007's. I tried putting the heaters on a DC power supply, the hum was way too high so I went back to AC on the heaters.
I WELCOME all comments as to component values and mods to this circuit.
Thanks to all
Attachments
You aren't using distributed star grounding from the photo - that should be first and foremost the main change to make.
The spigot of each 12AX7 should be made as the local star ground.
You should use a tuned humdinger, with a 100 to 200 ohm pot, instead of a fixed humdinger.
Pedantically, you should have a separate twisted pair heater cable to each 12AX7.
Are you using a shorting RCA plug on each channel when you check for hum level?
The spigot of each 12AX7 should be made as the local star ground.
You should use a tuned humdinger, with a 100 to 200 ohm pot, instead of a fixed humdinger.
Pedantically, you should have a separate twisted pair heater cable to each 12AX7.
Are you using a shorting RCA plug on each channel when you check for hum level?
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Your heater wire should be laying flat against the chassis, not flying in the air above the components, sockets and other wiring. Have you had a look at the sticky at the top of this forum?
Yes, you won't want to hear this now, but AC heater wiring should be the first to go in as near to the chassis as possible. Then signal wiring can be left floating in the air.
All cables carrying AC should be at least loosely twisted with a cable carrying the same current the other way. You appear to have no twisting on the heater secondary or the mains primary.
The loop containing charging pulses should also be kept as small as possible. The sort of wiring you have done could be got away with in a power amp or a radio, but not a phono preamp.
All cables carrying AC should be at least loosely twisted with a cable carrying the same current the other way. You appear to have no twisting on the heater secondary or the mains primary.
The loop containing charging pulses should also be kept as small as possible. The sort of wiring you have done could be got away with in a power amp or a radio, but not a phono preamp.
Hello everyone,
I think I have all the bugs worked out.
I rewired the heaters for 12 vdc and laid the twisted pair next to the chassis.
The amps current state is such a mess (from all the mods) that I will rebuild it completely. At that time I will take care to get better lead dress and proper star grounding.
Thank you for all your help and suggestions. This was truly a learning process.
The attachment shows the noise level when I lift the cartridge off the record.
I know, it's not a sophisticated test but I should do for now.
Oh yea, it sounds very nice.
I think I have all the bugs worked out.
I rewired the heaters for 12 vdc and laid the twisted pair next to the chassis.
The amps current state is such a mess (from all the mods) that I will rebuild it completely. At that time I will take care to get better lead dress and proper star grounding.
Thank you for all your help and suggestions. This was truly a learning process.
The attachment shows the noise level when I lift the cartridge off the record.
I know, it's not a sophisticated test but I should do for now.
Oh yea, it sounds very nice.
Attachments
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