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The Octal Preamplifier with Phono section

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Vinylsavor

Vinylsavor

Member
Joined 2002
vinylsavor.blogspot.com/
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
 
Vinylsavor

Vinylsavor

Member
Joined 2002
vinylsavor.blogspot.com/
Hi!

Wavebourn said:
Nice, but audiophools would not like it: 3 stages, cathode resistors, coupling caps... ;)
Click to expand...

;)

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
 
C

castellu

Member
Joined 2008
agenceroy.fr
Vinylsavor said:
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
Click to expand...
Hi Thomas,

congratulations for your design, and your blog...;)

The chokes used for the PS are all Lundahl ?

What are the capacitors used in the PS ? These are not polarized ?

I like the tube damper 6AX4, and the bridge graets look very interesting.

Thank for the information.
B r u n o
 
Vinylsavor

Vinylsavor

Member
Joined 2002
vinylsavor.blogspot.com/
Hi Bruno!

Thanks for you comments :)

castellu said:
The chokes used for the PS are all Lundahl ?
Click to expand...

Yes, of course other chokes could be used as well.

castellu said:
What are the capacitors used in the PS ? These are not polarized ?
Click to expand...

These are MP in oil. ASC X386S series. They are non polarized

Pretty much all the parts used are readily available from various sources as well as from me ;)


Best regards

Thomas
 
Vinylsavor

Vinylsavor

Member
Joined 2002
vinylsavor.blogspot.com/
Hi Bruno,

I have built quite a few of these in the meantime and also some people built the circuit and are happy with it.

You might have noticed that I came up with a more elaborate Mk2 version, using different tubes and LC coupling in the phono section.

I posted a minor circuit update on my blog a short while ago:

VinylSavor: The Octal Preamplifier Mk2: Circuit Update

It is just a bit different in the way the B+ is decoupled

Best regards

Thomaqs
 
soulmerchant

soulmerchant

Member
Joined 2006
very nice work Thomas! You know 6BY5 in a Graetz bridge is a favourite configuration of mine.

I recently aquired a whole stash of NOS boxed japanese IEC mullard 6DA4 damper diodes for about $2 a tube...

Currently doing 6AV5 PP with 6CG7 phase splitter and driver topology. all with some as yet unknown (and inexpensive) japan output transformers...

I will have to come visit you soon for some more of your superb mains transformers though!
 
dirkwright

dirkwright

Member
Joined 2011
I see a lot of similarities to what I believe is the best approach to preamplifiers. I like the transformer input for the phono. I did that 20 years ago, except that mine was a balanced line from the phono cartridge to the transformer, with the transformer providing step up and conversion to single ended. I like the step down on the output as well, except that these days I would make it push pull instead of single ended.
 
Vinylsavor

Vinylsavor

Member
Joined 2002
vinylsavor.blogspot.com/
M

massimo

Member
Joined 2002
Hi Thomas
Your lastest stripped down Octal Phono looks quite interesting.
I made a quick and dirty proto and I was a bit disappointed discovering a slightly low gain: I measured nearly 39dB at 1kHz.
Considering the high mu of the 6SF5 you didn't chose to squeeze all the juice having it working at 0.5-0.6mA
I was thinking to lower a bit Ra and Rk (and increase Ck) to 180K/1.8K (220uF), but before doing that, I would like to know your opinion, as you are a valuable designer and you most probably had a very good reason to chose the original operating point.

Massimo

P.S. the phono schematic is the one suggested, while for the power supply I'm using what I have handy: 300 VAC - 4 x SF4007 - 100uF - 20H (DCR 920) - 220uF - 870 ohms - 220 uF
 
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