ECC82 based preamp

SSassen · 16th February 2006, 06:56 AM

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.

Any suggestions or comments are most welcome.

Best regards,

Sander Sassen
http://www.hardwareanalysis.com

jane · 16th February 2006, 07:27 AM

Quote:

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.

EC8010 · 16th February 2006, 09:36 AM

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.

SSassen · 16th February 2006, 10:24 AM

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?

Best regards,

Sander Sassen
http://www.hardwareanalysis.com

EC8010 · 16th February 2006, 10:32 AM

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.

SSassen · 16th February 2006, 10:34 AM

Hi EC8010,

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

Best regards,

Sander Sassen
http://www.hardwareanalysis.com

EC8010 · 16th February 2006, 10:49 AM

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.

SSassen · 16th February 2006, 11:10 AM

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

jane · 16th February 2006, 11:15 AM

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?

SSassen · 16th February 2006, 11:19 AM

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.

Best regards,

Sander Sassen
http://www.hardwareanalysis.com

16th February 2006, 06:56 AM	#1
SSassen diyAudio Member Join Date: Mar 2005 Location: Edam, The Netherlands	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. Any suggestions or comments are most welcome. Best regards, Sander Sassen http://www.hardwareanalysis.com __________________ Acception without proper examination equals religion Condemnation without proper examination equals prejudice

16th February 2006, 09:36 AM	#3
EC8010 diyAudio Moderator Emeritus Join Date: Jan 2003 Location: Near London. UK	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. __________________ The loudspeaker: The only commercial Hi-Fi item where a disproportionate part of the budget isn't spent on the box. And the one where it would make a difference...

16th February 2006, 10:24 AM	#4
SSassen diyAudio Member Join Date: Mar 2005 Location: Edam, The Netherlands	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? Best regards, Sander Sassen http://www.hardwareanalysis.com __________________ Acception without proper examination equals religion Condemnation without proper examination equals prejudice

16th February 2006, 10:32 AM	#5
EC8010 diyAudio Moderator Emeritus Join Date: Jan 2003 Location: Near London. UK	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. __________________ The loudspeaker: The only commercial Hi-Fi item where a disproportionate part of the budget isn't spent on the box. And the one where it would make a difference...

16th February 2006, 10:34 AM	#6
SSassen diyAudio Member Join Date: Mar 2005 Location: Edam, The Netherlands	Hi EC8010, I've drafted up the changes you've proposed, would the following schematic accurately reflect them? Best regards, Sander Sassen http://www.hardwareanalysis.com __________________ Acception without proper examination equals religion Condemnation without proper examination equals prejudice


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16th February 2006, 10:49 AM	#7
EC8010 diyAudio Moderator Emeritus Join Date: Jan 2003 Location: Near London. UK	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. __________________ The loudspeaker: The only commercial Hi-Fi item where a disproportionate part of the budget isn't spent on the box. And the one where it would make a difference...

16th February 2006, 11:10 AM	#8
SSassen diyAudio Member Join Date: Mar 2005 Location: Edam, The Netherlands	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 __________________ Acception without proper examination equals religion Condemnation without proper examination equals prejudice

16th February 2006, 11:15 AM	#9
jane diyAudio Member Join Date: Jul 2004 Location: Norway	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? __________________ Life is hard - Then you die.

16th February 2006, 11:19 AM	#10
SSassen diyAudio Member Join Date: Mar 2005 Location: Edam, The Netherlands	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. Best regards, Sander Sassen http://www.hardwareanalysis.com __________________ Acception without proper examination equals religion Condemnation without proper examination equals prejudice

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