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4S Universal Tube Preamp Project (12A*7)
http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=4628
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Author:  mwhouston [ 04 Jan 2017, 18:17 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

Because current drain is low you can use a step down tranni into a step down tranni backwards. So 240V to 24v then 24V to 240V. Use the first step down to power the filaments with a 12V reg.

Because you live in Australia Jaycar will have everything you need. One stop shop even a chassis of any type. See the post for the 4SU. My dual tranni PS is detailed there.

Author:  Chris65 [ 04 Jan 2017, 19:21 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

franck.forster wrote:
Hello, This look like a very nice project, I am new to tube building and to Australia, I am interested in building the 4S any suggestion for a good part supplier in Australia, mainly for transformer.


You can buy components from any major electronics supplier: RS Components, Mouser, Element14, Digikey.
They all offer free shipping to some degree, just different levels of order value required.
Mouser & Digikey stock Hammond transformers suitable for valve rectified circuits.

For my second 4S, I ordered an R-core transformer from Ebay with the same specifications as the Hammond 369EX.
Cost was US$50 including shipping - cheaper than two transformers from Jaycar, at least this side of the Tasman.

Author:  jdrouin [ 24 Jan 2017, 17:35 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

I'm considering building the 4S Universal Preamp and have a question about volume control.

The goal is to use the 4S as source selection and volume control for my whole system, which currently connects a phono preamp (450 ohm output impedance) or a CD Player directly to a Tubelab SE 300B power amp.

I don't need any more gain from the phono pre but would like a little more gain from the CD Player.

Do you think the originally spec'd 250K volume pot on the tube output is the way to go?

Here's the Tubelab SE schematic, incase it affects your suggestions. The amp currently has a 50K volume control in the position between the input jack and the 121K resistor that goes to ground before the grid of the input tube. I'd like to remove that volume control so that the 4S acts as sole volume control.

I ask about volume control on the 4S, because in another post in this thread, Chris65 recommended a 250K input resistor followed by a 50K volume pot, as follows:

Chris65 wrote:
Sure, like this. R4 is the 250KΩ resistor & R3 is a 50KΩ potentiometer. R5 is also 250KΩ.
Attachment:
Revised 4S Pot Input.jpg

Image

Not sure if it would be more advisable to do that.

Thanks!

Jeff

Author:  Suncalc [ 25 Jan 2017, 18:37 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

OK, I'm not quite sure I understand your desired topology. Are you saying that the 4S would go on the front of the tube amp and everything would run through the 4S via a selector switch?

jdrouin wrote:
Chris65 recommended a 250K input resistor followed by a 50K volume pot
What this is, is a fixed 16.6 dBv attenuator on the front end of the preamp. In most instances I don't think it's required.

If you use a 12AU7 in the 4S, you get about 20dBv of gain. However, a standard volume control is -20dBv at half volume. This means that a 4S running with a 12AU7 is an attenuator at less than half volume and a preamp at greater than half volume. This would cover both of your situations, a phono channel with good gain and a cd channel that needs gain.

The real question is what is the input capacitance of your power amp and cables? If it's low then leave the 250kΩ pot on the output. If it's high, put an appropriate pot on the input and a fixed 250kΩ resistor on the output.

Author:  jdrouin [ 25 Jan 2017, 21:21 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

Thank you for he helpful reply, Matt.

Half attenuator and half amplifier with a 12Au7 sounds perfect. I'll stick with the original design.

This is going to be a standalone 4S between the sources and the power amp.

I'm not sure about input and cable capacitance, as I haven't measured them and don't have the tools for it, but I think they're low. I'm going to make a pair of short interconnects (about 1 foot) between the 4S and power amp, using solid 18awg copper wires. There are no capacitors in the power amp between the input jacks and input tubes (5842).

Best,

Jeff

Author:  Suncalc [ 25 Jan 2017, 22:30 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

jdrouin wrote:
I'm not sure about input and cable capacitance, as I haven't measured them and don't have the tools for it, but I think they're low.
It's all about the first tube. The 5842 has a very high G-P capacitance (9µµf) and the G-P capacitance is 1.8 µµf. With a µ=43 the amplification factor could run in to the mid 30s and the CM into the 60µµf range. The 4S can have a relatively high output impedance at about 80% volume. Depending on the coupling impedances your high frequency response could be in jeopardy at some settings. Do yo have a schematic for the amp?

Author:  jdrouin [ 25 Jan 2017, 23:37 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

Yes, a .jpg and .pdf can be viewed here: http://tubelab.com/designs/tubelab-se/schematic/

Author:  invaderzim [ 26 Jan 2017, 13:11 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

I'm looking at using this preamp with a SS one that lists 10k for input impedance. It is the amp camp amp: http://www.firstwatt.com/pdf/art_amp_camp_1.pdf

are there any issues I should be aware of or tweaks I should make?

Thanks,
Trevor

Author:  mwhouston [ 26 Jan 2017, 16:29 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

invaderzim wrote:
I'm looking at using this preamp with a SS one that lists 10k for input impedance. It is the amp camp amp: http://www.firstwatt.com/pdf/art_amp_camp_1.pdf

are there any issues I should be aware of or tweaks I should make?

Thanks,
Trevor

Not amp camp and it will not work well with your SS power amp. The input impeadance is too low. There is another design of a two stage version where the second stage is a cathode follower. This will work much better. Or do as I do, add a Pass B1 to the output of any 4S build.

Author:  Suncalc [ 26 Jan 2017, 16:59 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

invaderzim wrote:
I'm looking at using this preamp with a SS one that lists 10k for input impedance. It is the amp camp amp: http://www.firstwatt.com/pdf/art_amp_camp_1.pdf
To drive a low impedance power amp, use this design instead.

http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=5186&start=1

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