• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Balanced tube preamp schematic or kit?

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
I'm sorry if this has been discussed (a lot) before, but please bear with me.

I'd like to try a tube preamplifier with my two mono bridged Abrahamsen V4.0UP power amplifiers. In bridged mode they have a 50KOhm input impedance balanced. (100KOhm with SE adapters) They also each have a 1000VA toroidial transformer and a 100.000uF capacitor bank. Bridged they should give about 240W each @8Ohms, I guess. I don't know if this matters, but anyway...

My current pre has 6dB gain (Balanced. Unity gain when used single ended) , and I wouldn't think I'll need much more than that, but a little more won't hurt either, I guess.

I'd need at least one or two balanced inputs and a balanced output, as well as two or three single ended inputs. Controls could be limited to just power, input selection and volume control.

Are there any kits or schematics available for a DIY preamplifier that fits this description? If it used tubes that are easily available, it'd be even better. It should sound "warm and tubey" of course.

...Oh. And the power transformer will have to have primaries fo the European 230V power grid

Any and all help and tips will be greatly appreciated!

Thanks a lot in advance!
 
Last edited:
Do you want it to be fully balanced end to end? If not, then you can just use some high quality audio transformers with any single ended tube preamp design or kit.

Preferably, yes. ...But I'd need 2-3 single ended inputs as well, and I guess I don't really know what the consequences of that are.

I guess i could just build single ended and use the XLR to RCA-adaptors that came with my power amps, but when my DAC has balanced outputs and my power amps have balanced inputs... Well, a balanced pre seems to make sense. (To me, at least. However, I'm a noob, so what do I know?) ;)
 
Preferably, yes. ...But I'd need 2-3 single ended inputs as well, and I guess I don't really know what the consequences of that are.

I guess i could just build single ended and use the XLR to RCA-adaptors that came with my power amps, but when my DAC has balanced outputs and my power amps have balanced inputs... Well, a balanced pre seems to make sense. (To me, at least. However, I'm a noob, so what do I know?) ;)

My personal preference would be to build something like this:
Power Supply Delay Finish PCB for DIY Tube Amplifier items in 8Audio store on eBay!
(no experience with his kits though!)
and use transformers for the balanced in and out. I like Jensen.

I agree that your situation warrants a preamp with balanced input and output. The single ended inputs can be connected directly to the single ended board, obviously.
 
Thanks! Appreciate it! I'll have a look at that. I'll probably need some help on the "use transformers for the balanced in and out" part as well. ;) I suppose that means it's easy to make an additional single ended output as well?

It's easy to use transformers if you are willing. Jensen has schematics and white papers under their "applications" section:
WELCOME TO JENSEN TRANSFORMERS, INC.

Sure, if the board is single ended, no problem.
 
How about a simple differential amplifier without any transformers like this.


An externally hosted image should be here but it was not working when we last tested it.


You can connect this as a SE or balanced input and output as you wish.
The low frequency response can be lowered to 20 Hz by replacing C2 and C3 with 220 nF if necessary.

Interesting. I must admit I'm not that familiar with these circuits and schematics, so: Do you have an URL or something to where I can read up a bit?

Thanks.
 
I'll take a closer look at it later today, an get my head around how it works.

There is no big magic in this circuit. These tubes are connected as a differential amplifier ( try to Google "differential amplifier" to get more info).

There are two inputs available to feed the balanced signal in.
If you feed non balanced, then just connect one input to ground.
Volume control can be arranged with dual 100 k pot at the inputs.
You can also add such input and output selectors as you prefer.

There are also two outputs and between them there is balanced signal output.
If you want to output only non balanced signal, you can choose one output only, inverted or non inverted.

This 6N6P tube has suitably low gain, good linearity (low THD) and low output impedance to feed 50 kohms load.

But do you need to see a simple power supply circuit for this or do you buy one from China ?
 
Last edited:
What is the output impedance of the differential gain stage? Gain?

dazed, FYI, the topology here is what BATBalanced Audio Technology use in all their preamps. If you parallel more tubes you'll get lower output impedance. Don't worry about the coupling capacitors they don't hurt as much as some people want us to believe. Use e.g. Mundorf Zn capacitors.

Thanks. I'll read up on the topology.
 
There is no big magic in this circuit. These tubes are connected as a differential amplifier ( try to Google "differential amplifier" to get more info).

I'll certainly do that.

There are two inputs available to feed the balanced signal in.
If you feed non balanced, then just connect one input to ground.
Volume control can be arranged with dual 100 k pot at the inputs.
You can also add such input and output selectors as you prefer.

There are also two outputs and between them there is balanced signal output.
If you want to output only non balanced signal, you can choose one output only, inverted or non inverted.

I understand. (I think) ;)

This 6N6P tube has suitably low gain, good linearity (low THD) and low output impedance to feed 50 kohms load.

Great. It's also readily available "everywhere", I assume.

But do you need to see a simple power supply circuit for this or do you buy one from China ?

Well.... I guess if I were to build an amp from scratch with p2p wiring, I'd try to do the same with the psu parts, but realistically; the easiest way for me to accomplish this project probably is buying PCBs (populated or not) and wiring them together with pots, switches and a power transformer and putting it all in a nice enclosure.

I've only ever built one small guitar tube amp p2p, which I have yet to dare plugging in to see if it works, and that wiring looks like an awful worm's nest. ;)

Getting populated PCBs for the amp and psu from China and then getting the transformer, switches and terminals locally sounds doable and is tempting, if it's likely that this method will yield good results.

...Or I could get just PCBs from China and get all the parts locally (or from mouser etc.) if there's reason to doubt the quality of the Chinese components. (?) Of course: There's always the option of upgrading the through hole components at a later time-

What do you guys think?
 
Last edited:
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.