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PostPosted: 11 Nov 2012, 07:35 
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After a lot of stuffing around tomorrow I should be able to finish this project. I had in mind to use the Hammond Al enclosure but the vertical 100uf caps were 2mm too high. Then I tried my local cast Al cases, same story.

So now for the last time a Twinnings Tea box tube preamp. I may never build another Tea Box preamp. I have built about six or so and one guy has two of them. But the boxes now (with tea) and heading toward $60 so time to stop. I'm building this one to compliment my new chip amp - SynergyII\Cu.

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PostPosted: 11 Nov 2012, 17:10 
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Everything ready for the final installation and wire-up. Since these images the cct. brd. has been mounted into the lid with the tube standing proud from the top. Hope to be listening to it in the next few hours. Because it is in a tea box I have named the preamp "4SUT".


Attachment:
4SUT_I.jpg
Attachment:
4SUT_II.jpg


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PostPosted: 12 Nov 2012, 02:25 
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I hate wire-warp wire, it is so hard to work with but I doubt whether any other hook-up wire is as sonically good. Just my opinion. So why do I hate it. For two hours I chased a broken wire in my newly finished 4SUTea (slight name change) preamp. The wire had broken down in the insulation but the insulation had adhered perfectly to the RCA. So when you pulled on the wire to see if there was a bad solder joint or it was broken it didn't come away. Only when I metered out the left channel did I realise the wire was broken. All good now.


I am currently using a new Mullard 12AX7 but later (after I calm down from a two hour broken wire hunt) I will replace the 12AX7 with a Tung-Sol 12AU7. The amp sound excellent by the way and with the high gain 12AX7 conducting the orchestra lot of gain and speed. The power amp is my new chip amp - SynergyII\Cu http://www.diyaudioprojects.com/Forum/viewtopic.php?f=10&t=4034#p33025 and though I love the big holographic sound stage I felt it need some tube warmth - thus the 4S Universal. It has done what I wanted it to do and on listening to the current big Bruckner (4) symphony I get a much better appreciation for the size of the recording venue. Bass has improved where I thought it excellent before it is fuller and rounder.

All housed in a Twinings Tea box: Bitumanised Al foil controls enclosure resonance and Russian paper in oil caps link the modulated HT to the real world. Silver solder is used throughout. Heavy duty high grade gold plated RCAs grace the back and all is powered by a 24V 1A plug pack. There are two switchable inputs, alps blue velvet pot and black chromed solid brass knob.

Checking some voltages: HT 250V+ and filament voltage 12.07V.

Attachment:
4SUTea_I.jpg
Attachment:
4SUTea_II.jpg
Attachment:
4SUTea_III.jpg


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PostPosted: 13 Nov 2012, 07:27 
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If I wanted to reduce the gain on the 4S Universal what would be the best way to do it? Rk is UN-bypassed.

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PostPosted: 13 Nov 2012, 10:55 
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With a 12AU7 in the 4S Universal (unbypassed cathode) the gain is approximately -9.9 or about 20dBv. This is the lowest gain configuration of the preamp. From this point you can either limit the input or simply turn down the volume. I could come up with a NFB version to get a lower gain but it would probably destroy the sound of the stage.

How low do you think you need to go?

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PostPosted: 13 Nov 2012, 15:51 
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Suncalc wrote:
With a 12AU7 in the 4S Universal (unbypassed cathode) the gain is approximately -9.9 or about 20dBv. This is the lowest gain configuration of the preamp. From this point you can either limit the input or simply turn down the volume. I could come up with a NFB version to get a lower gain but it would probably destroy the sound of the stage.

How low do you think you need to go?

I will try the 12AU7 it is the tube I would rather be using anyway I just had the 12AX7 on hand when I finished the preamp. There are some aspects of the 12AX7 I like over the 12AU7, it seems snappier, faster and more dynamic. The Mullard tubes have long plates compared to other 12AX7's I've seen.

My problem is the chip amp has a stack of gain and the Universal has a lot of again with the current tube. I can barely turn the volume to "2" when playing LPs. My phono stage appears to putout more than my CD player. Not so bad with the Oppo BDP-95. I'll try the 12AU7 tube and see where that goes.

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PostPosted: 13 Nov 2012, 20:29 
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I took some measurements and did some testing with the preamp. All test are done with a well run-in 12AU7. The replacement of the 12AX7 with 12AU7 really didn't drop the gain much.

HT+ = 250V
Plate to ground voltage Vp = Left chan: 85V and right chan: 80V
Cathode to ground voltage Vk = 2.5V left and right.
Input to the 12V regulator for heater voltage is 17V. The 100ohm limiting resistor could be increased to 120ohms. Filament current draw is about 170mA.

CRO tests all done feeding 33K load.
@ 2Vpp in and 2.83Vpp out -3db points are 10hz< -3db > 38.5khz - lower than I thought with pot about 12 o'clock.

With pot fully open upper -3db point rose to 53kHz. A 4.7Vpp input gave 27.8Vpp out. There was NO clipping even at this voltage swing (into 33K). I'm amazed at the Vout for this preamp. It may produce a greater voltage swing but my digital CRO can only take 30Vpp. On my calcs that is a 15db gain!

The pot at 12 o'clock both channels produced exactly the same level of output. Perfectly balanced.
The pot at about 1 o'clock and 2Vpp in one channel Xtalk was 10mV in the other channel. Looks good to me.
With both channels shorted (S\C) 6mV of noise was measured so maybe only 4mv of Xtalk once noise is deducted. Once the pot was fully open 47mV of noise. Pot fully closed both channels measured 5mV of noise.

How do these values look? Updated photos with name plate.
Attachment:
4SUTea_I_retroS.jpg
Attachment:
4SUTea_II_retroS.jpg


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PostPosted: 28 Dec 2012, 23:59 
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Had the preamp tested today with the following results:

0.5VRMS In and out unity
0.85%THD and THD+noise 0.79%
0.8V out 0.92%
There was a lot of low level 50hz and harmonics rubbish on the output not seen in the 6N7 and C3m preamp which under went similar tests. This maybe due to the PS and preamp cct. On the one cct. Brd. The noise was -90db down for a 0.5V signal so very low level.

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PostPosted: 31 Jan 2013, 16:12 
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Having completed three tube preamps in nearly as many weeks I have had the chance to compare them in two of my main systems. Previously I had BoZ style preamps in both systems. What I did find is with Luciano (807 power amp) at low volume levels Heavy Metal (6N7) preamp sounded excellent but once I upped the volume the sound got muddy and untidy. I'm using Menatzchim (C3M) in my main system with my Silver Dragon 300B amp - excellent.

I tried my new 4SUTea, 12AU7 single stage Universal preamp with the Luciano and Presto! a great sound at any volume. I have found the BoZ a little bass shy but now this new combo sounds really tops, loud, soft or in the middle. I think when a new piece of gear comes along you need to mix and match it with other gear to see what combos shines. I'm glad I did.

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PostPosted: 01 Feb 2013, 08:13 
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Very Cool! Thanks for the updates.

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