Author Topic: Line Level Tube Preamp  (Read 3640 times)

Offline Mars

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Line Level Tube Preamp
« on: June 18, 2020, 01:42:29 PM »
Hi All, this is a follow to the thread done in October 2013.  See original here https://www.avforums.co.za/index.php/topic,28051.0.html

So what more can be said about the design? If you have some patience, then let's look at some circuit ideas.
Geco's suggestion in the original thread is more or less where we are now:



I have been wondering about the wisdom of putting the volume right in front of the circuit. It has the advantage of only one capacitor in the signal path, but the volume pot could be seen as a difficult load to drive.

Another way of doing things would be to place the volume pot after the first triode, followed by the cathode follower. The first triode could arguably be a better driver for the volume pot, and the cathode follower gives a low output impedance. Something like this:



This way the output of the source drives a triode directly, and the volume pot is buffered from source and amplifier. Could this work?







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Offline DeonC

Re: Line Level Tube Preamp
« Reply #1 on: June 18, 2020, 01:52:45 PM »
I like. I will be curious to hear the answers. :rubhands:
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Offline gavinbirss

Re: Line Level Tube Preamp
« Reply #2 on: June 18, 2020, 01:56:06 PM »
Karel,

It could be disastrous when slider disconnects from volume pot?

it would have to be a very reliable pot.

Offline handsome

Re: Line Level Tube Preamp
« Reply #3 on: June 18, 2020, 04:26:18 PM »
The 'rule of thumb' is that input impedance should be at least 10x the output impedance of the device driving it plus your output impedance should not be so high that it causes a treble roll-off with the following device/stage's input capacitance, plus cable capacitance of course.

The output impedance of modern components is <1k (<100R most of the time) your volume pot is 100k so easy to drive. Input impedance of most modern devices is 10k, your volume pot is 10x that so easier to drive than most amp inputs. The output impedance of a grounded cathode 6SN7 with a bypassed cathode resistor is about 10k. That's a x100 to x1000 worse than driving directly from your CD player, streamer, what-have-you.

You could put your volume pot at the output of the circuit driven by the cathode follower, but then worst case output impedance (it varies with volume control position) is 25k. Although yo do get a noise advantage. But now you have lost the benefit of a low output impedance so not going to be great to drive long or capacitive cables. But your real problem here is your cathode follower; it now has a minute amount of bias - so you should grid bias it but this will add another capacitor to your signal path....

I would drop the value of the pot to 50k (small increase in bandwidth feeding the 6SN7 input capacitance) and leave it up front. Plus as Gavinbirss recommended run a 1Meg resistor from wiper to earth to mitigate any pot discontinuities (large thumps). Oh and unbypass the first 6SN7's cathode you don't need all that gain in a modern preamplifier.

Online 1200GXman

Re: Line Level Tube Preamp
« Reply #4 on: June 22, 2020, 08:18:07 AM »
When will this be available in kit form and what will price estimate be?
Sansui AU-101, Rotel RA935BX, Pioneer SA-6500 Mkii, Marantz PM53, DIY kit amplifier, GECO SRPP+ pre Amp

Offline Mars

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Re: Line Level Tube Preamp
« Reply #5 on: June 23, 2020, 12:03:49 PM »
handsome: Thanks for the informative and decisive input.

1200GXman: the kit is currently available. I have just sent out the first order (one without the enclosure) priced at R3900 + shipping. (With enclosure and logo R5 900).

I am taking the 76/6SN7 design by Berman from Sound Practices #13 as the guide for the moment.

Here's some pictures of the parts. This specific one is a once off with Parmeko transformers and choke!

Power transformer



Filament transformer



choke



3 input and one out pair of RCA



tag strips, pots, selector etc.



set of tubes. The RCA box contains a 5Y3GT rectifier



tube sockets



resistors

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Offline Mars

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Re: Line Level Tube Preamp
« Reply #6 on: June 23, 2020, 02:19:13 PM »
Here is my take on the power transformer for the preamp:





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Offline Mars

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Re: Line Level Tube Preamp
« Reply #7 on: July 06, 2020, 10:17:25 PM »
Schematic

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Offline Qualityten

Re: Line Level Tube Preamp
« Reply #8 on: July 07, 2020, 09:59:51 AM »
That first one with the Parmeko transformers will be quite something! Thanks as ever for sharing pics of your developing design and parts.

Offline Mars

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Re: Line Level Tube Preamp
« Reply #9 on: July 22, 2020, 11:42:08 AM »
For this project I am going to go with a 6J5G as the input tube, and a 6J5GT for the cathode follower.





The full wave rectifier will probably be a 5Y3G or GT
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Offline Mars

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Re: Line Level Tube Preamp
« Reply #10 on: December 19, 2020, 11:59:33 AM »
Hi All

An update on this project. The layout is per customer's wish, and the here is a photo of the unit before assembly:



The center tube is a 6X5G rectifier, and a parmeko 10H is going to be mounted between the mains transformer and the rectifier tube.

Then the two rear 6J5's are going to be the voltage amplifiers, and the front two the cathode followers.

Grounding is critical in preamp designs, so a central point will be determined. More on that later.

Working on the power transformer design, I realised that I would like to focus on really getting this part right, since much of the quality of the preamp is determined by the power supply. So some aims have emerged.

  • only one ring to rule them all - all of the heater and HT secondaries are going to be wound on one bobbin
  • the voltage amplifiers will get one secondary winding, and the cathode followers their own (correctly referenced to be inside of cathode/heater voltage specifications)
  • to limit long heater wires all over the chassis, each tube will get their own twisted pair 6.3V AC lead
  • secondary voltage referencing will be done at the PSU section using a center tap on the heater windings
  • each secondary will thus be wound bifiliar in such a way to fill the whole width of the bobbin
  • this will make it easy to run separate filament leads to each tube
  • the power transformer should run at low flux density to eliminate possible mechanical noise from the transformer itself
  • dual power supplies are used for the two preamp circuits, to eliminate crosstalk possibility
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Offline Mars

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Re: Line Level Tube Preamp
« Reply #11 on: February 03, 2021, 12:12:28 PM »
Hi Everyone,

I shot some videos of the build. Comments welcome.

https://youtu.be/7-id3pGhOIo

https://youtu.be/iKHrZ_RClK4

https://youtu.be/PiKLZTaw3Bg

Is there any way to embed a video in posts?
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Offline fredeb

Re: Line Level Tube Preamp
« Reply #12 on: February 06, 2021, 10:40:49 AM »
 :groovy:  Very interesting Karel , I think the videos are a good idea . It would be good to see more in future . Thank you .
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Offline Mars

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Re: Line Level Tube Preamp
« Reply #13 on: August 12, 2021, 06:54:10 PM »
For those interested: Here is a discussion of the 6J5 tube.

http://vinylsavor.blogspot.com/2013/03/tube-of-month-6j5.html
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Offline Jazzy

Re: Line Level Tube Preamp
« Reply #14 on: August 12, 2021, 09:47:16 PM »
Those tube of the month posts are priceless!
I have settled on the RCA branded 6J5 for their tonal balance but I still take avery opportunity to try different ones. Most curious about the G.E. ones that he strips in the post, also the 'G' coke bottles.
Which ones are you favorite?   
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