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Old 27th November 2010, 10:25 AM   #1
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Default The Octal Preamplifier with Phono section

All,

since there were some concerns mentioned in the thread about my 6HS5 phonostage about the high voltage, here another preamp with more 'sane' B+ levels. This is a full function preamp. It uses a fairly conventional phono section with passive split RIAA networks. The line section is transformer coupled and uses transformer volume controls (Slagle AVCs in my case).

If on a budget the line section could also be built with RC coupling and a conventional resistive potentiometer for volume control and upgraded later on.

The motivation behind this preamp came up a few years ago, when I presented a SE power amp circuit based on cheaply available tubes in a german tube forum. The power amp got quite some interested and was built by several people. The wish for a preamp which would fit to it both in terms of sound quality and parts cost came up. The power amp in question is based on a 6CB5A in triode connection.

Several versions of this preamp have been built by now and their users are quite happy with the sound. I use one occasionally myself, although I also have several LCR based phono stages and several DHT line stages with tubes like 801A, 10Y, 26, etc. This preamp is also part of my commercial offering, both fully assembled and as a kit. The schematic is public domain and free for anybodoy to use.

Here is the circuit of the signal section:

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I used oil caps throughout, but if on a budget it can be done with electrolytics. Note the ultrapath cap from B+ of the line section to the cathode. This should be of best possible quality. If done with an RC coupled output, the ultrapath cap is not needed and the cathode resistor of the 6SN7 should be bypassed.

The sections of 6SL7 and 6SN7 are not paralleled. I used one tube shared between channels. If concerned with channel separation, both sections could be paralleled and one tube spent per channel. Also 6J5s could be used instead of a 6SN7.

The PSU schematic will follow.

Best regards

Thomas
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Old 28th November 2010, 08:36 PM   #2
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It looks great. Are the grids and plates of the 6N7 paralleled?
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Old 28th November 2010, 09:31 PM   #3
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Nice, but audiophools would not like it: 3 stages, cathode resistors, coupling caps...
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Old 28th November 2010, 09:50 PM   #4
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Quote:
Originally Posted by Palustris View Post
Are the grids and plates of the 6N7 paralleled?
Yes!
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Old 28th November 2010, 09:52 PM   #5
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Hi!

Quote:
Originally Posted by Wavebourn View Post
Nice, but audiophools would not like it: 3 stages, cathode resistors, coupling caps...


3 stages for a full function preamp is not that bad... especially with a transformer volume control and a low Zout of 200 Ohms (at full volume, less at lower volume settings).
Cathode resistors, yes, I prefer them over batteries. But no cathode bypass caps and no electrolytics...

Best regards

Thomas
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Old 28th November 2010, 09:55 PM   #6
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Hi!

Here is a PSU suitable for this preamp:

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Of course any other PSU with similar voltages can be used. The absolute voltage is not that critical. In the meantime I switched ot a slightly different arrangement myself, using a full wave bridge rectifier with 4 TV damper tubes for the B+

best regards

Thomas
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Old 29th November 2010, 12:55 AM   #7
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Hi, are you the same Thomas that published the 5687 toroidal parafeed preamp...10 years ago or so. I built a couple variations of that.
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Old 29th November 2010, 08:26 AM   #8
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Hi!

Quote:
Originally Posted by pedroskova View Post
Hi, are you the same Thomas that published the 5687 toroidal parafeed preamp...10 years ago or so.
No, that was not me. That was Manfred Huber I guess. A friend of mine

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Thomas
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Old 18th March 2011, 10:19 AM   #9
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Hi!

I summarized the development of this preamplifier in an article on my blog. If you are interested click here:

VinylSavor: The Octal Preamplifier Mk1

Best regards

Thomas
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Old 29th May 2011, 08:31 AM   #10
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Hi!

I posted the schematic of a new version of this preamp on my blog:

VinylSavor: The Octal Preamplifier Mk2: Circuit

Best regards

Thomas
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