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Any 4S Universal Preamp 12a*7 builds here
Any 4S Universal Preamp 12a*7 builds here
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Old 16th January 2019, 01:57 PM   #11
Lingwendil is offline Lingwendil  United States
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Any 4S Universal Preamp 12a*7 builds here
^ replace that 12AU7 with a more linear tube like the 6CG7 and it may end up a pretty useful circuit.


I've never been a fan of the 4S builds at all, for all the above stated reasons. My main gripe with the idea is the often excessive gain, the high output impedance, and poor choice of tubes. I understand that the 12Ax series are popular, but they are a poor family of tubes for such an application without major design additions/changes.

If you want a simple preamp there are better designs.
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Old 16th January 2019, 02:00 PM   #12
eldarvanyar is offline eldarvanyar  United Kingdom
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Originally Posted by Eli Duttman View Post
As previously stated, the claim of universality is false. The 12AU7/ECC82 is non-linear. The more linear 6FQ7/6CG7 and 12BH7 usually work in 'U7 circuits, providing the requisite doubled heater current is available.

Again, I ask what your goals are. What level of drive signal does your power amplifier require? Many amps need zero gain in the "line" stage. Either a passive or buffered control center is an appropriate mate for such an amp. Amplifying the signal and then attenuating it, to adjust overall gain structure, has an adverse impact on S/N performance.
Hi Eli,

I have a Quad 606 power amp upgraded by Quad to 909 status last year.


Quad 606 Power Amplifier Specification
Power Output: 140 watts per channel (8 ohms)
Distortion: < 0.01%
Input Sensitivity: 500mV
Input Impedence: 20Kohm
Crosstalk: -100db (100hz), -85db (1 khz) and -65db (10khz)
Hum and Noise: 105db
Voltage Rating : 220-200V AC (110-120V via simple power board modification)
Dimensions (WxHxD): 321 x 140 x238 (mm)
Weight: 12kg

I also have a Quad 303 which was refurbished and upgraded by Audiolabs.

Specifications
Power output: 45 watts per channel into 8Ω (stereo)
Frequency response: 30Hz to 35kHz
Total harmonic distortion: 0.1%
Input sensitivity: 0.5V
Signal to noise ratio: 100dB
Speaker load impedance: 4Ω to 16Ω
Dimensions: 120 x 159 x 324mm
Weight: 8.2kg

I would like to use a valve preamp with these with a EAR 834P Phono MM clone.

I listen to all sorts of music and can stream from my server into a DAC although I am looking to get more into vinyl. Ido have a symphonic 40w pp integrated as well. I think it would be helpful for me to build something that has online support and encouragement

Does that help?
Thanks

Last edited by eldarvanyar; 16th January 2019 at 02:04 PM.
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Old 16th January 2019, 02:15 PM   #13
DF96 is offline DF96  England
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As you appear to need no gain, what role exactly do you envisage for your 'preamp'? Of course it is possible to design a preamp to add a little distortion, noise and restricted frequency response if that is what you require - but you can buy a cheap Chinese 'tube buffer' to do that.
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Old 16th January 2019, 02:19 PM   #14
Dinolobe is offline Dinolobe  Canada
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the original 4s preamp wont work with your quad 606. The output impedence of the 4s is too high for the 20k input impedence. Have a look at this thread:

DIY Audio Projects Forum • 4S Tube Preamp with 12AU7 Cathode-Follower Output Stage
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Old 16th January 2019, 02:36 PM   #15
Lingwendil is offline Lingwendil  United States
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Any 4S Universal Preamp 12a*7 builds here
Why do you need a preamp when the amplifier has 500mV input sensitivity? Especially if you plan to use it with an EAR834 type of phono pre, as they can ouput 30 volts easy. Are you looking for a buffer to drive the 20K input impedance of your Quad? It couldn't hurt if used with the EAR, as the 12AX7 used in them is a poor choice for a cathode follower, giving high output impedance into lower impedance loads.

I think you would want to stay away from any preamp with gain, unless it is very low (3 or 6X max maybe) otherwise you wll just be throwing it all away and increasing your distortion and noise floor.

I would stay away from the 4S design for such a build, and look to a basic cathode follower stage using a higher gm tube, or even something like the 6V6 line preamp from Salas- it can be built as a cathode follower (gain of ~1) or with gain (3~7 depending on choices) as needed for your application, with the cathode follower config being a good choice for you.
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Old 16th January 2019, 02:42 PM   #16
eldarvanyar is offline eldarvanyar  United Kingdom
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Quote:
Originally Posted by DF96 View Post
As you appear to need no gain, what role exactly do you envisage for your 'preamp'? Of course it is possible to design a preamp to add a little distortion, noise and restricted frequency response if that is what you require - but you can buy a cheap Chinese 'tube buffer' to do that.
Oh dear, this seems to be where I get my coat .....
I have spent hours on mouser drawing up a project list and some other sites. My Kemet Rifa 33uf 400v caps arrived today.

Would I be better off with a passive pre amp on the 606 then? If so what would you recommend for a potentiometer?

Where would I seek to build a valve preamp? Is it really only for a valve power amp? If I were to try and build a Williamson or Mullard type power amp what would you recommend then for a simple valve preamp and type of circuit?

Thanks
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Old 16th January 2019, 02:52 PM   #17
Eli Duttman is offline Eli Duttman  United States
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Quote:
Input Sensitivity: 500mV
Input Impedence: 20Kohm
Those numbers spell the requirements out. As a "standard" CDP exhibits a 2 VRMS O/P, "line" stage gain is both unnecessary and undesirable. The amp's 20 Kohm I/P impedance is fairly awkward. A 2 Kohm passive setup would be needed to satisfy the 1:10 driving/driven rule and 2 K is way too low a load for many (most?) signal sources. A buffered control center is (IMO) the proper choice. High transconductance (gm) triode cathode followers will exhibit a suitably low O/P impedance. JJ's ECC99 is a good candidate tube/valve.

All the online support needed can be obtained on this "board".
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Old 16th January 2019, 02:54 PM   #18
Lingwendil is offline Lingwendil  United States
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Any 4S Universal Preamp 12a*7 builds here
My suggestion-

6V6 line preamp cathode follower version

Quality stepped attenuator (or potentiometer) at the input, good quality capacitors, and a pair of big cathode resistors, very simple and reasonable design. Will work well with 15K and up loads, so should do well for you. A simple circuit that can be "hotrodded" as needed to your heart's content. Same thing with 6DJ8/ECC88, ECC99, 6N6P, etc types will also do very well. With the 6V6 the Zout is ~1.5K or so in cathode follower mode.
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Last edited by Lingwendil; 16th January 2019 at 03:00 PM.
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Old 16th January 2019, 02:55 PM   #19
Eli Duttman is offline Eli Duttman  United States
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My Kemet Rifa 33uf 400v caps arrived today.
Those parts will be fine in your PSU filter. A WVDC rating larger than absolutely necessary is not a problem.
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Old 18th January 2019, 01:57 PM   #20
Shawnstium is offline Shawnstium  United States
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The 4S preamp is the first tube circuit I built and it was a great learning experience. I did make some modification to the original circuit, for example I used my own power supply (135Vdc, regulated), 100k audio pot, added a input decoupling capacitor and DC heaters at 12V.

The "high gain" (albeit mine was probably less due to the lower supply voltage) was not a problem for driving a class D audio development board (MAX98400AEVKIT+), a B&K ST260 power amp and the Pass F4 power amp. I usually kept the volume at preamp at max and varied my source's volume.

Why is the volume control in the wrong spot and where should it be? If the restive volume control is placed on the input one would still have deal with high pass RC filter issues. In a perfect world I would use a variable transformer for volume control, but I have to live within a budget...

I plan to build the ECC802S SRPP Tube Preamplifier (DIY ECC802S (12AU7 / ECC82) Vacuum Tube SRPP Preamplifier) next to drive the Pass F4 power amp. Has anyone built this preamp with success?
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