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Old 16th February 2006, 07:56 AM   #1
SSassen is offline SSassen  Netherlands
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Default ECC82 based preamp

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.

Click the image to open in full size.

Any suggestions or comments are most welcome.

Best regards,

Sander Sassen
http://www.hardwareanalysis.com
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Old 16th February 2006, 08:27 AM   #2
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Default Re: ECC82 based preamp

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Originally posted by SSassen
I was however wondering what topology was used, it isn't a SRPP or a TTSA, so what is it?
The first stage of V1 is common cathode the next is a (self biasing) cathode follower.
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Old 16th February 2006, 10:36 AM   #3
EC8010 is offline EC8010  United Kingdom
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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.
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Old 16th February 2006, 11:24 AM   #4
SSassen is offline SSassen  Netherlands
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Thanks EC8010,

So what you're saying is I can simply omit C18 (68nF) and remove R15 (5k6) or replace with a more sane value, ie. ~47K. But I don't follow your other suggestion, are you saying I can simply remove R10 (1M) and R16 (680) and connect R8 (27K) directly to pin 8 of the ECC82?

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Sander Sassen
http://www.hardwareanalysis.com
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Old 16th February 2006, 11:32 AM   #5
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Yes, connect R8 directly to pin 8, and put a link between pin 1 and pin 7. Your first valve's anode voltage is ideal for DC coupling to the cathode follower.

I would make R15 1M or more.
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Old 16th February 2006, 11:34 AM   #6
SSassen is offline SSassen  Netherlands
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Hi EC8010,

I've drafted up the changes you've proposed, would the following schematic accurately reflect them?

Click the image to open in full size.

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Sander Sassen
http://www.hardwareanalysis.com
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Old 16th February 2006, 11:49 AM   #7
EC8010 is offline EC8010  United Kingdom
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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.
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Old 16th February 2006, 12:10 PM   #8
SSassen is offline SSassen  Netherlands
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Hi EC8010,

I'll just put that 1K resistor between pin 1 and 7 for good measure, can't hurt and it'll reduce the changes of the cathode follower oscillating.

Thanks and best regards,

Sander Sassen
http://www.hardwareanalysis.com
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Old 16th February 2006, 12:15 PM   #9
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The attenuator network (R17) is a bit scary. When you switch between 0/+6dB this will cause quite a large pulse at the output. Maybe that is why the corner frequency (C21/R15) was quite high in the first place?
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Old 16th February 2006, 12:19 PM   #10
SSassen is offline SSassen  Netherlands
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Hi EC8010,

I guess the below schematic should then reflect all the changes, I'll get to work right now and implement them and post here to detail how it worked out in practice.

Click the image to open in full size.

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Sander Sassen
http://www.hardwareanalysis.com
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