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Old 9th July 2004, 04:13 PM   #1
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Default John Swenson's preamp! No more attenuators.

I'm so excited I just had to share it on diyAudio.com.

Originally posted over at AA.
http://www.audioasylum.com/forums/tu...ges/57358.html

Click the image to open in full size.

Here are the details on the BDT preamp I built last weekend. This design used Beam Deflection Tubes, which were originally designed as color subcarrier demodulators in color TVs. To my knowledge this is the first use as a straight forward linear amplifier.
I had been thinking about these tubes as amplifiers since I first saw them in an RCA tube manual 6 years ago, but never got around to getting it built (I bought most of the parts though). At VSAC last year Allen Wright's talk on differential amps got me thinking about these tubes again and I just had to see if they worked or not. Since I'm a bit low on cash these days the entire project was made from components in the parts bin so some of the values and details may not be optimal, it was what was on hand.

As you can see from the schematic the tubes are a little strange, it starts out as a normal pentode, but the plate is split in two and two extra electrods are added called deflectors, these work similar to the CRT tube in an oscilliscope. When the same voltage is applied to both deflectors the electron beam coming from the pentode is evenly split betwen the two plates. When there is a difference betwen them the beam is deflected so one of the plates gets more current and the other gets less.

The result is essentially a long tailed differential pair in one tube, with the added benefit that the current can be adjusted by a DC voltage on the grid. This mean that when used as a stereo preamp you don't need any attenuator (volume pot) either pot or switched resistors or TVC or any such device. The input goes directly to the tube and one pot controls the gain for both channels. Another interesting advantage is that there is NO miller capacitance on the deflectors!

My original design was going to use a good line output transformer running PP from the two plates but that was way too much money for this first project. I wanted to see if these tubes had any hope as audiophile tubes before I spent any money on the project. Latter on I'll try some transformers and see how THAT sounds. As is if bought from scratch the whole preamp would cost about $50!

A few notes on the current circuit: the 10 ohm cathode resistors are not really neccessary, they are just there so I could easily read the total current through the tube. The 2K plate resistora are a compromise between gain and output impedance, as it is now the gain is about 1, which works well for a lot of systems. Higher resistance gives more gain but also higher output impedance. The voltage to the volume pot is a battery because that was the cheapest way to get a really good 9-12V for the grid. This is probably the most critical voltage in the design, the B+ has quite good PSRR but this does not. Cathode bias would not work because you need to vary the cathode current over a wide range. I will probably wind up with a voltage doubler off the 6.3V running a 12V regulator of some type for the REAL version of this. As is I have to change the battery every 4 or 5 days of continuous playing.

The PS is a voltage doubler (thanks PJ for turning me on to them!) driving a CRC filter since I didn't have any chokes. But it works very well as is. PSUD says its less than 2mv PP ripple which coupled with the very good PSRR means a VERY quiet circuit.

The sound: WOW, AWESOME, AMAZING!!! I've never heard a preamp this good before and I wasn't even trying to make something really good yet. The number one impression is smooth, but yet it is wonderfully detailed, hughly dynamic, very "punchy" when the music is so. PRaT is very high, nothing seems to be emphasized, its all just there. Its hard to describe because it just lets the music through.

Next up a 3W power amp using BDTs, but that will take some transformers so I have to wait till I have a little more money in hand.

John S.
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Old 9th July 2004, 04:40 PM   #2
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No more pot's, switched attenuators or TVC'S !

Gotta admire it!
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Old 10th July 2004, 12:28 AM   #3
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Hi Bas. I'm not going to say that I completely understand how it works yet but the idea is interesting. The beauty here is that the input and ouput impedances won't change with the gain. Or will they?
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Old 10th July 2004, 12:38 AM   #4
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Hi G,

I'm a dumbass. You know..essentially a cut and paste amp builder. So I can't pretend I understand how it works exactly either..... But I generally know a good thing when I see it. ;-)

And the absence of a volume pot in the direct signal path...and just one in the circuit appeals tremendously ...for a 6 channel Home Theatre preamp I have been dreaming of.

The low or non-existent miller capacitance is also a big advantage I think.

Quote:
The beauty here is that the input and ouput impedances
won't change with the gain. Or will they?
I think you may be right...that is another thing I never thought of.

But on the subject of impedance...the output impedance is quite high I think...I read an old post of John asking about a tranny of 40k primary impedance.

But I am going ahead and build it...to see for myself... :-)

Cheers,
Bas
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Old 10th July 2004, 12:40 AM   #5
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I think the impedance will change. But it's a great idea.
We in the UK, and possibly Europe too, didn't have the "privelige" of valve chroma demodulators, as colour TV came later, and transistors were in vogue by then. Therefore most of us will never have come across these devices.
It's going to take a few days thought to get them straight in my mind
Thanks to Bas for "porting" this one, and especially to John Swenson
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Old 10th July 2004, 12:49 AM   #6
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Hi,

Quote:
The beauty here is that the input and ouput impedances won't change with the gain. Or will they?
With a "regular" preamp the tube doesn't care what the setting of the volume control is for as long as it's not too extreme.

Output impedance of the stage won't change with volume setting either.

Quote:
The low or none existand miller capacitance is also a big advantage I think.
Not really, Miller capacitance only becomes troublesome with high mu tubes or with highly capactive Cag.
For a preamp it's nothing to worry about though.

Quote:
But on the subject of impedance...the output impedance is quite high I think...
It's bound to be since we're talking beam tetrodes and plate followers here.

Quote:
But I am going ahead and build it...to see for myself... :-)
Please do, Bas. Let us know what you think of it....
Just don't let'em beam ya up, hey.

Cheers,
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Old 10th July 2004, 01:07 AM   #7
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Another advantage is that nobody shows much interest in TV tubes; as with a few exceptions they are of no use to us ampbuilders; this means that they are very cheap to buy!!!!
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Old 10th July 2004, 01:44 AM   #8
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Bas,

Howv about some pics. A balanced version might be an idea.
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Old 10th July 2004, 02:03 AM   #9
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I would definitely be interested in a schematic of a balanced version.

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Old 10th July 2004, 02:51 AM   #10
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A balanced O/P is easy. Just add a coupling cap to the anode currently without 1.

Allowing for balanced I/P is easy too. The deflector that currently is directly grounded gets a 140 KOhm resistor between it and ground. Apply the balanced I/P signal to the deflector plates.

I'm in agreement with the earlier remarks about using buffering voltage followers, especially if the value of the load resistors is increased. Increasing the value of the load resistors raises both the gain and the O/P impedance.
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