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ECC82 based preamp

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SSassen

SSassen

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Joined 2005
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#1
I've built an ECC82 based preamp based on the below quoted schematic. It works well and serves as a line preamp with either 6dB or 12dB of gain (selectable). I was however wondering what topology was used, it isn't a SRPP or a TTSA, so what is it? And what can I do to further improve on it? I've already removed the C17 capacitor as all of my sources are DC free, hence it serves no real purpose.

11757.gif


Any suggestions or comments are most welcome.

Best regards,

Sander Sassen
http://www.hardwareanalysis.com
 
#3
You have an overly complex cathode follower. You don't need the coupling capacitor or the bias arrangement. Just connect the grid directly to the anode of the first valve and have a single 27k cathode load resistor. I assume that's a typo and that you haven't really loaded the output with a 5k6 resistor.
 
#7
Yes, that's it. You really don't need such a low value for R15 - all it has to do is to ensure that the output DC is at 0V. It's just possible that the cathode follower might oscillate at RF, and the standard cure is a 1k carbon resistor at the grid end between pin 7 and pin 1.
 
#11
The voltage at the cathode of the cathode follower is likely to be about 5V higher than the anode voltage of the common cathode valve. I doubt if it will sound any better, but it will certainly be simpler, and I like simplicity.
 
#12
Hi jane,

The preamp features a relay that mutes the output for about 3-seconds after you select the higher gain. It also does this when switching between input sources.

Actually, you might be familiar with this particular design, it is sold as the Velleman K8020 tube preamplifier kit, complete with a nice looking aluminum chassis. I bought the kit and assembled it and am now in the process of tweaking it a bit to unlock the full potential (or attempt to at least).

Best regards,

Sander Sassen
http://www.hardwareanalysis.com
 
#13
Alright, I promised to give some feedback regarding the modifications I made, so here it is. And excuse me beforehand for using hifi-journalist terminology, I wouldn't know how else to describe the subtle changes.

11760.jpg


There's clearly an improvement, more depth in the soundstage but also more punch overall, percussion has more punch and treble is more defined and balanced. It sounded a bit muddy before by comparison.

Thanks for all the helpful suggestions.

Best regards,

Sander Sassen
http://www.hardwareanalysis.com
 
#15
The gain switching arrangement drastically changes the operating point of the common cathode amplifier, the more linear operating point is going to be the 12dB position as the operating current is twice as high as in the lower gain position.

The ECC82 is not noted for the best linearity, and operating it at higher currents considerably improves its performance. IMHO it should operate at a minimum of 4 - 5mA for good linearity, although at line stage levels you can usually get away with less current.

See Morgan Jones Valve Amplifiers 3rd edition for more information on tube linearity.

Kevin
 
#16
Hi Kevin,

See Morgan Jones Valve Amplifiers 3rd edition for more information on tube linearity.
Click to expand...

I'm not familiar with that particular book, care to educate me (us?) on what he's got to say about tube linearity in the circuit I'm using? Or can you give me some pointers perhaps? For instance you mention operating current, how is this calculated?

Best regards,

Sander Sassen
http://www.hardwareanalysis.com
 
#17
Get the book, you won't be sorry.

One of the gems in it was a comparison between ECC82 and other similar medium mu tubes, tested under conditions that would maximize linearity. The ECC82 did not fare well, having about an order of magnitude more distortion than a 6SN7.
 
#18
Hi SY,

I ordered the book and it should be arriving here sometime next week. I was however wondering what could be done to further improve this circuit? From what I've learned in these past few days the resistor in the anode of the cathode follower isn't a necessity and could well be affecting its linear behaviour. Furthermore what would be a proper bias current for the first stage? I've also looked into using a different tube and the ECC88 seems to be the most suitable for a preamp. Any more suggestions you might be able to help with are most welcome.

Best regards,

Sander Sassen
http://www.hardwareanalysis.com
 
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