New Schematic Linestage Tube Preamp using EF86 & 12AX7

Edwind · 17th January 2012, 02:52 PM

This a new linestage preamp using EF86 & 12AX7.
What do you think?

kevinkr · 17th January 2012, 02:58 PM

Hi Edwind, Welcome to the forum!

In terms of your schematic I am wondering what the intended purpose would be? The gain for a line stage application will be very high. I've not done an analysis of this design, but generally gains of well under 20dB are the norm for modern designs, and in most cases less than 10dB is a reasonable choice.. (I would guess the gain here could be 60dB - in phono stage territory.)

The output impedance will be extremely high as well which will make it a poor line driver..

The input stage has limited signal handling capability, I suspect a couple of hundred mV would cause this design to clip as there is no way of dealing with the very high signal levels of digital sources (typically 2Vrms @ FS) and even most analog sources..

Filament circuit should not allow current to flow through chassis/circuit ground as this is an excellent recipe for hum.

DF96 · 17th January 2012, 03:15 PM

A guitar preamp? Certainly not a line stage. It seems designed to add noise and distortion.

Edwind · 17th January 2012, 04:13 PM

Hi, kevinkr annd DF96 (is it tube type?).
This are the gain for every tube that use: +22dB (12AT7), +16dB (12AU7), +21dB (12AX7).
This is low quality circuit, fun for beginners (low cost) and a little homeworks for experts

I already build this. Input directly from CD player.
There is a very little of hum, noise, and distortion (for my ear, may be no for others).
This circuit not like Quad II phono stage.
The output impedance still to high since it's use common cathode and may add a voltage follower to boost the output signal.
Many-many thanks for advices, it will give a positive feedback for next circuit.
By the way, will somebody upload the new scheme for upgraded version of this circuit?

SY · 17th January 2012, 04:31 PM

It looks like there's a 20dB attenuator in the front of the circuit. Followed by amplification. The logic here is not something I'm getting.

DF96 · 17th January 2012, 05:03 PM

Only in a guitar amp would someone think that ECC81,82 and 83 are directly interchangeable. Beginners can have much more fun with sensible circuits; cheaper too as you would not have to throw it away once you know better.

As a line stage this circuit is simply wrong. I will leave it to you to do the homework. It can't be upgraded, only replaced. Sorry to be blunt, but you asked what we think.

Edwind · 18th January 2012, 03:25 AM

It's ok, no need to sorry DF96.
You told the truth.
Thank you very much.

revintage · 18th January 2012, 08:20 AM

Hi Edwind,

As DF96 says the circuit is useless. But there is actually a solution for making it work:

1 Move the volumecontrol to the input
2 Triode-strap the EF86 Ra=47k Rk=820
3 Make the second stage a CF, DC connected to the first, and use ECC82.

The triodestrapped EF86 sounds great as lineamp.

Edwind · 18th January 2012, 03:19 PM

Hi, revintage/Lars,
Thanks for the solutions that yo give to me.
I'll try it.

kevinkr · 18th January 2012, 04:58 PM

Lars' solution is a good one..

17th January 2012, 02:52 PM	#1
Edwind diyAudio Member Join Date: Jan 2012	New Schematic Linestage Tube Preamp using EF86 & 12AX7 This a new linestage preamp using EF86 & 12AX7. What do you think?

17th January 2012, 02:58 PM	#2
kevinkr diyAudio Moderator Join Date: Sep 2004 Location: Boston, Massachusetts Blog Entries: 6	Hi Edwind, Welcome to the forum! In terms of your schematic I am wondering what the intended purpose would be? The gain for a line stage application will be very high. I've not done an analysis of this design, but generally gains of well under 20dB are the norm for modern designs, and in most cases less than 10dB is a reasonable choice.. (I would guess the gain here could be 60dB - in phono stage territory.) The output impedance will be extremely high as well which will make it a poor line driver.. The input stage has limited signal handling capability, I suspect a couple of hundred mV would cause this design to clip as there is no way of dealing with the very high signal levels of digital sources (typically 2Vrms @ FS) and even most analog sources.. Filament circuit should not allow current to flow through chassis/circuit ground as this is an excellent recipe for hum. __________________ "Extraordinary claims require extraordinary evidence." - Carl Sagan

17th January 2012, 04:31 PM	#5
SY diyAudio Moderator Join Date: Oct 2002 Location: Chicagoland Blog Entries: 1	It looks like there's a 20dB attenuator in the front of the circuit. Followed by amplification. The logic here is not something I'm getting. __________________ If there's a sucker born every minute, where do the rest of them come from?

18th January 2012, 04:58 PM	#10
kevinkr diyAudio Moderator Join Date: Sep 2004 Location: Boston, Massachusetts Blog Entries: 6	Lars' solution is a good one.. __________________ "Extraordinary claims require extraordinary evidence." - Carl Sagan

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17th January 2012, 03:15 PM	#3
DF96 diyAudio Member Join Date: May 2007	A guitar preamp? Certainly not a line stage. It seems designed to add noise and distortion.

17th January 2012, 04:13 PM	#4
Edwind diyAudio Member Join Date: Jan 2012	Hi, kevinkr annd DF96 (is it tube type?). This are the gain for every tube that use: +22dB (12AT7), +16dB (12AU7), +21dB (12AX7). This is low quality circuit, fun for beginners (low cost) and a little homeworks for experts I already build this. Input directly from CD player. There is a very little of hum, noise, and distortion (for my ear, may be no for others). This circuit not like Quad II phono stage. The output impedance still to high since it's use common cathode and may add a voltage follower to boost the output signal. Many-many thanks for advices, it will give a positive feedback for next circuit. By the way, will somebody upload the new scheme for upgraded version of this circuit?

17th January 2012, 05:03 PM	#6
DF96 diyAudio Member Join Date: May 2007	Only in a guitar amp would someone think that ECC81,82 and 83 are directly interchangeable. Beginners can have much more fun with sensible circuits; cheaper too as you would not have to throw it away once you know better. As a line stage this circuit is simply wrong. I will leave it to you to do the homework. It can't be upgraded, only replaced. Sorry to be blunt, but you asked what we think.

18th January 2012, 03:25 AM	#7
Edwind diyAudio Member Join Date: Jan 2012	It's ok, no need to sorry DF96. You told the truth. Thank you very much.

18th January 2012, 08:20 AM	#8
revintage diyAudio Member Join Date: May 2007	Hi Edwind, As DF96 says the circuit is useless. But there is actually a solution for making it work: 1 Move the volumecontrol to the input 2 Triode-strap the EF86 Ra=47k Rk=820 3 Make the second stage a CF, DC connected to the first, and use ECC82. The triodestrapped EF86 sounds great as lineamp.

18th January 2012, 03:19 PM	#9
Edwind diyAudio Member Join Date: Jan 2012	Hi, revintage/Lars, Thanks for the solutions that yo give to me. I'll try it.

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