Circuit review

[neazoi]

Searching at the web for the ultimate preamplifier I found this circuit:

http://www.blackdahlia.com/html/tip_44.html

In my point of view it has many interesting things there. The type of tubes used, the local feedback part, the two stage single ended topology.

The first thing, I need an expert review of the circuit to determine if it is worth spending money to construct it.

The second thing I would like to ask is, will a tube electronically regulated power supply be better for this circuit?
I know they are much more expensive to construct but you have a very stable anode voltage. On the other hand I have heard that it is not as sweet as simple tube regulated power supplies because it does not allow fluctuations of the voltage (and thus some amount of distortion) in big rises of the music.


Thank you very much .

[freddymac406]

I built the preamp but I did use a solid state power supply.
I used a current source that supplys current to a 5 watt 200 volt zener diode on each channel.
I am very happy with the sound . My other tube reference is a lasuras basic that uses 12ax7s. It sounds less real and more in your face than the 6sn7 design.

I would like to post a picture of my preamp as it turned out great.

Any questions let me know.
Fred

[neazoi]

Yes of course! I would like to see your preamplifier!

So you say in general it is a good design right?

I have already collected carbon composite resistors for the signal path, musicaps capacitors and vintage rca smoked glass tubes.

I have found in radio amateurs handbook an electronically tube regulated psu that gives 0.03v ripple!! Although it uses 6 tubes it is perfectly accurate. It can be adjusted at 300-450v but I think I can easily bring the voltage down to 205v that is required, by connecting a resistor in series, since the current drawn from the preamplifier tubes in class-A1 is always constant.

I am waiting for your photo.
Thanks

[kevinkr]

Originally posted by neazoi
<snip>
It can be adjusted at 300-450v but I think I can easily bring the voltage down to 205v that is required, by connecting a resistor in series, since the current drawn from the preamplifier tubes in class-A1 is always constant.

I am waiting for your photo.
Thanks

Kind of defeats the purpose of a regulated supply which amongst other things also provides a very low source impedance at audio frequencies.
I'll post a schematic when I have a moment of a much simpler and much lower noise regulator which you can set to exactly 205V for your project.

Please note design provided here is for personal use only!

You can shunt R4 with a 1uF cap for a 6dB increase in closed loop margin which will further reduce output impedance and ripple. Note with careful construction you can get ripple under a couple of mVpp.

[neazoi]

Sir,
when you say "amongst other things also provides a very low source impedance at audio frequencies."
Is this a good thing? I mean , is the low impendance a desirable thing? (stupid question?)

I see that some things are similar to both designs. Mine has the ability of providing 350mA and it uses gas regulator tubes. It could be a supply of a small final amplifier. It also has a similar R4 as yours (pontentiometer) comming out from a 12au7 to adjust the output voltage. I was wondering if I could connect a series resistor after the regulator part in order to reduce the output voltage from 300 to 205?
The radio amateur handbook explains this technique in theory. As far as I have understood if the current load is constant (which happens in a class-A amplifier, since the amplifier draws the same current in all times?) a resistor could be connected in series to reduce the output voltage.
In each case testing in practice is the best technique...

For example in your design could I make such a connection if I want to have a lower voltage?

PS. Unfortunately the schematic is on a radio amateurs handbook and I need to scan it to post it here. This book also has other schematics of electronically regulated tube psu.

[kevinkr]

Connecting a resistor after the output of the regulator to drop the voltage totally defeats the purpose of having a regulator in the first place. It isn't the right way to go.

Having a low source impedance at audio frequencies is a very desirable attribute in a power supply, it prevents cross-talk between channels, and in designs where multiple stages are powered from the same supply greatly reduces the likelihood of low frequency instability.

I have variations that use gas tubes, but at these low voltages they don't perform as well as the zener based version. For best temperature stability you can use a 5W zener. If you need lower voltages in my design just replace the zener with a lower voltage type. If you need an adjustable voltage you can also replace R4 with a pot and series resistor. Use a very good quality pot and be aware that temperature stability will be affected by the quality of the pot you use.

The design I have posted with produce exactly the right voltage for the circuit you are trying to power, and will have a lot lower output noise and ripple. It is a variation on a design I used in my commercial products.

For a stereo line stage application this regulator has low enough noise and output impedance that no additional filtering/decoupling is necessary.

[neazoi]

Sir,
thank you for your prompt reply.
I understand the purpose of zenner and the pontentiometer in the circuit.
Basically the output voltage depends on these elements.
I am thinking of using a multi turn precision pontentiometer. Except the pontentiometer, I guess another way to lower the voltage in a circuit that uses gas regulator tubes is to connect more of them in series? Is that better than the resistance (bad) way I mentioned im my previous email?

And what about the pontentiometer you mention? If I connect a resistance the way you say will I be able to get a much lower voltage? Is there a limit in the value of resistance?

Thank you very much once again!

[neazoi]

In addition to the question another one in this.
I guess,in a circuit that uses regulator tubes, another way to lower the voltage except adding more regulator tubes is to lower the voltage of the transformer? If yes can this be done by adding one resistor in each plate of the main regulator tube (the one that is connected near the transformer e.g. a 5u4g)?

I appreciate for your help and apologise if flooding you with so many questions in my last two posts but I find the help provided in these furums extremely important!

Thanks once again

[neazoi]

I found on the web the cyclotron amplifier.
any comments?

http://circlotron.tripod.com/