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Old 19th July 2012, 02:15 PM   #1
disco is offline disco  Netherlands
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Default 27 line pre amplifier

After hijacking Rob's tread I'd like to give this one a try. As always comments and suggestions welcome, corrections too

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( schematic )

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( load lines )

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( power supply )

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( step response )
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Old 19th July 2012, 02:40 PM   #2
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Default layout of components

Sawing and glueing it together in a (usually too small) 32x21cm wooden box, looking for optimal placement. All is to come underneath... tight fit.

- home brewn toroid: 5V & 5Vac 3.5A each
- small 30VA 182V power transformer
- two 10.5H 100 ohm chokes
- Elna Cerafine 2x 220uF 350V (little bit overkill)
- 15uF motor run cap (yes, MKV B25832 )

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( layout 1)

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( dito )
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Old 19th July 2012, 03:06 PM   #3
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Looks like a fun project.

I've messed around a lot with the 27 some time ago, and I found that it really shines with a gyrator load.

Of course that would totally make the CCDA useless, but 15 euros worth of parts can make a really effective and simple regulator, which removes pretty much all worry about the PSU anyway.

In my experience the 27 doesn't have very good heater - cathode insulation, and I found it best to regulate the heater supply; it made an audible difference.

All in all it's a very good tube, if you can find a good condition mesh plate 27 it's the nicest sounding IDHT I've tried.
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Old 19th July 2012, 04:33 PM   #4
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Thanks, that will be fun next experimenting with heaters. On the other hand I feel once heaters are fixed the power supply can't stay behind. Comes time, comes .... something else. Look before one leaps over board etc..


Some numbers...

It would be nice to know the bandwith, as the pre amp should ideally not limit the output stage by a factor of 2 to 3. The latter being estimated at 30kHz would take the pre at around 75kHz. Alright, in goes a fresh battery to the old Casio.

Click the image to open in full size. Click the image to open in full size. Click the image to open in full size.

Where this current cancelling amp deviates from normal grounded cathode design is not apparent to me but I'll give it a try.

First there's the output impedance of the CD-player to the first stage. As I have no clue of its output impedance I'll guestimate 10k ohm. Cmiller= (6.5+1) x 3.3 = 25pF. Some 15pF for strays makes 40pF.
Cutt-off = 1/(2pi x 4.10e-11 x1.10e4 ) = 398kHz. That's a large number to count on one hand

Second there's the common grounded cathode stage.
rout= ra (the dynamic anode resistance) in parallel with RL (the 24 k ohm anode resister).
From the load line tanget I derive 200-60V over 16mA = 8.75 kohm. The result equaling 6412 ohm.

This 6K4 is looking in to the input capacitance of the cathode follower,
which is 3.3pF + (1-0.96)x3.5pF = 3.44pF. Allowing for some stray, say 7.5pF.
The cutt-off frequency = 1/(2pi x 7.5.10e-12 x 6.4.10e+3) = 3.3 megaHz. Wow, that must be a mistake

Last is the output impedance of the cathode follower at the combined capacitance presented by the interstage and the power amplifier. Because rout is lowish (round 1k5) this number is benign, meaning out of the way of the bottle neck of 398k.
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Old 19th July 2012, 04:59 PM   #5
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I'd put the power supply and pre-amp in separate boxes. (Hum pick up) Will give you much more space to work and tweak.. A metal box with a wood wrap would reduce external electro-static pick up issues. (The tube has this issue too and can be shielded as I do with the 26 for mitigation)

Most 27/227/327 (globes) are quite microphonic so be prepared to deal with getting a number of them to select for lower microphonics. Note that the very expensive and rare Arcturus Blue globe mesh plate with top nipple sound much worse IMVLE than the garden variety RCA UY-227. (Sadly they look soooo cool. ) I've used 27s in a few power amp and line stage designs, but actually prefer the 26/226/326.

In addition the filament to cathode insulation is not very good in the 27 so I would use a separate transformer to heat the cathode follower which should be referenced to a voltage just a few volts higher than that found on the follower cathode.

Other devices to consider include the 56 (2.5V) or 76 (6.3V)..

Something to note here is that the feedback is essentially positive which IMO is not a good thing on a number of fronts from DC stability to potential effects on linearity. Decades ago I experimented with essentially the same circuit using the 12AU7A, and there is a point at which it may oscillate with injudiciously chosen component values between the cathodes of the two tubes.
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Old 20th July 2012, 01:06 PM   #6
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The challenge is, Kevin, to make a good functioning piece of equipment to enjoy for a long time, both visually and auditory. Athough tastes may differ I enjoy the sight of those pretty, slim boxes hiding technique where possible. In my own country there's a tendency for building with RVS and exotic woods for those willing and able to produce a statement. In Italy and Japan too the same craftmanship has lead to beautiful expressions of good taste. It's far more satisfying than bolting it all down in an industrial box.

So, first I'll have a go with the one-in-all box, although not ideal. I bear in mind your remark w.r.t. a metal shield (against RF I guess?). Would the anode be prone to pick up stray fields of computers and light bulbs?

The 26 is a beautiful direct heated triode but I experienced a lot of fuzz heating it. This 27 try is for a 'petite'; easy to place and smooth to the ear.

You suggested the use of a seperate transformer for heating the CFs. My idea is to power them in series from a seperate winding of the heater trannie and raise its potential 120 volts. As I understand you correctly, it would be beneficial to go even 10 volts higher? The Cunningham C-327 datasheet (pre WW2) recommends no potential difference between heater and cathode. If this practice is not followed, the heater may be biased preferably negative, but allowably positive, with respect to the cathode by no more than 45 volts.

I'm not familiar with the ins and outs of the circuit as Mr. Broskie has a talent of loosing me somewhere in the story, not necessary his bad. Is the feedback positive because of the mutual cathode resistor?

It's good to experiment else we'd all be listening to ipods on earplugs.

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Old 28th July 2012, 05:55 PM   #7
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Got it in, almost all..

After sanding eight layers of blanc paint last week, I could not resist building it today.

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Tubes were selected from a large stack. I adjusted the AVO to my operating point (Va 125V, Vg 6,6V) and picked an equal pair of mesh plates for the gain stage. For the c.f. I matched two solid plates to identical current and transconductance as their g.s. brothers in crime.

The heaters are powered in series per pair (5V 1.75A). The measured voltage over them differs for 0.08 to 0.10V, hopefully this does not influence emission too much...

Initial test was performed with a quick setup using "candy laces" and without the volume pot in place. I could not tell the difference between source and pre (apart from the volume that needed to be turned down). No humm or audiable noise, so far so good for the "all in one build". I measured 3mV AC per channel with inputs shorted. PS ripple was a little higher than simulated at 6mV but not too bad

Next tidying up of the wires is necessary and some iron work has to be done for RCA connectors, selector switch and the volume control. Coming to speak of it, is the Light Dependent Resistor volume control still in esteem? Popularity of solutions seem to come and go like fashion

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Old 29th July 2012, 10:46 PM   #8
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Very cool, I see you have managed to fit the whole thing into that little box, you are a braver guy than me..

Have you had the opportunity to listen to it more extensively, and if so what do you think?

Is the 3mV a typo or the artifact of the measurement? If the hum at the output were really this high it should be very audible after being amplified by the power amplifier. The actual test will be with the volume control and I/O installed.

The anodes are subject to pick up from any ungrounded metallic object with AC on it within a certain proximity - this obviously should not be a problem with the cathode follower, but may be with the first stage.

I would not allow the filaments to be more than 12V positive with respect to the cathodes as filament to cathode insulation breakdown becomes more likely with the bigger delta. 45V seems very optimistic to me.

I like stepped attenuators and last time I checked there were some reasonably priced ones on eBay. The optical attenuator is another option of course. (One I keep meaning to try since I have a kit here for one.)
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Old 30th July 2012, 08:47 PM   #9
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Nope it's still on the test bench. Things don't look promissing because mains is dropping every 30 to 40 seconds a couple of volts for a second, interacting both with heaters and HV. Bummer, I need regulation. Back to the drawing board and next a larger coffin
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Old 31st July 2012, 01:09 AM   #10
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Hi Disco,
Sorry to hear that, I use regulated supplies with my transformer coupled 26 line stage for similar reasons.

I think this topology though is sensitive to minor variations in supply voltage and will actually make it much worse as I noted in a previous post due to positive feedback. First try removing the cathode feedback and see what that gets you..

Good luck, keep us posted!
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