Hello frieds,
My name is Nelson and this is my first thread in this forum, I´ve read a lot in the forum, awesome information about everything, i´m learning a lot. Recently i´m working on my very first vacuum tube preamplifier for balanced microphone, the schematic follows:
and the power supply schematic (the choke is 10H, not 100mH):
and finally, the construction:
The filaments are running with AC, for the V1 I used one 12AT7 and changed the Ra for 22k and Rk 560R for the first stage and in the second stage Ra 47k and Rk 1k, I´ve decided this values after studying the datasheet, the results for now are reasonably, the major problem is a HUM probably from the filaments, I´ve made an audio sample (in portuguese, I´m from Brazil, but this is irrelevant) and put in youtube in this link ruido preamp - YouTube , I did a center tap for the filaments with two 100R resistors, the hum was reduced, but not vanished, as the equipment will be used for recording if this hum isn´t under -65db that should be a problem, if anyone could help with some tips about the Ra and Rk values ou anything, I will be forever grateful.
In the input i´ve used a 1uF cap and a 1k grid resistor and the take away 160k resistor between the input transformer, those are the only mods I´ve did.
Sorry about any mistakes in my english, and thanks everybody!!!
My name is Nelson and this is my first thread in this forum, I´ve read a lot in the forum, awesome information about everything, i´m learning a lot. Recently i´m working on my very first vacuum tube preamplifier for balanced microphone, the schematic follows:
and the power supply schematic (the choke is 10H, not 100mH):
An externally hosted image should be here but it was not working when we last tested it.
and finally, the construction:
An externally hosted image should be here but it was not working when we last tested it.
The filaments are running with AC, for the V1 I used one 12AT7 and changed the Ra for 22k and Rk 560R for the first stage and in the second stage Ra 47k and Rk 1k, I´ve decided this values after studying the datasheet, the results for now are reasonably, the major problem is a HUM probably from the filaments, I´ve made an audio sample (in portuguese, I´m from Brazil, but this is irrelevant) and put in youtube in this link ruido preamp - YouTube , I did a center tap for the filaments with two 100R resistors, the hum was reduced, but not vanished, as the equipment will be used for recording if this hum isn´t under -65db that should be a problem, if anyone could help with some tips about the Ra and Rk values ou anything, I will be forever grateful.
In the input i´ve used a 1uF cap and a 1k grid resistor and the take away 160k resistor between the input transformer, those are the only mods I´ve did.
Sorry about any mistakes in my english, and thanks everybody!!!
It seems that the input transformer and power transformer are located too close to each other.
DC-heater supply is also best for low level amplifiers like this (and RIAAs).
DC-heater supply is also best for low level amplifiers like this (and RIAAs).
The output transformer or the power transformer should be rotated by 90 degrees to reduce magnetic coupling. Not knowing the lamination orientation of the input transformer you will need to experiment.I would also put an electrolytic capacitor across C2. E
Mount the power transformer on the rear panel, on the other side of the fuse holder.
It could be on the outside surface, if necessary.
Moving the fuse holder farther away from the audio transformers and XLR
connectors would also be good.
It could be on the outside surface, if necessary.
Moving the fuse holder farther away from the audio transformers and XLR
connectors would also be good.
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If this amp is indeed for microphones then soecial attention is needed to prevent noise and hum. No followers in a mic amp as these add noise and hum. Transformers are fine as they can reduce noise if used correctly. Look at pro mic amp designs and try to figure out why they did what they did. I use Siemens amps (V77) as these have a very good S/N ratio (120db). This is not bad for a tube preamp. But sure there are others that wil perform well.
Sometime, down the road, you will probably want P48 phantom power. Might as well include it now, while you're in the mood for modifying.
All good fortune,
Chris
All good fortune,
Chris
The power transformer actually is on the back panel, outside de box.
About C2 on the power supply, that should help, I´ll check if there are ripple on my scope. The noise mainly appears on 120hz (on an analizer in daw recording) and when I put mic switch off the noise went zero.
I really don´t know that followers could be source of noise and hum, redo all the output section will be a PITA but I´ll consider.
Thanks for all everyone!!!
Yeah, it´s on my plans +48v, about that, can I get these Vcc from the actual power supply (220vcc) and use some zener, or it´s better to get a dedicated power supply just for the +48v?
Worst case current draw is about 14mA. If you have that, plus the Zener's minimum current, available, you're ready to rock.
All good fortune,
Chris
All good fortune,
Chris
The C2 won't help. To reduce the 120Hz noise, split R2 into two about equal series parts,
and add another 220uF from their junction to ground. If you still have noise then,
probably it's a ground loop.
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C2 in the power supply schematic will help with ripple as I posted in post #5 and its been doing so in 90 % of of old tube radio designs/amplifier designs since the 1930,s so much so that I have a large collection of tube power supply design books by prominent designers of the last 90 years +.
Governments/military/commercial business --you name it used the same basic design on both continents and worldwide its so standard its written as a Standard --I have a long list of well know people of the past who will back this up.
Governments/military/commercial business --you name it used the same basic design on both continents and worldwide its so standard its written as a Standard --I have a long list of well know people of the past who will back this up.