Small valve mixing console

[DaveP]

CB Boudio,

I wish my French was as good as your English.
I tried to make a very similar setup in my early 20's using parts of circuits from Mullard's famous book. The advantage was that Mullard had already designed the optimal operating points and voltages, but I failed because I had no scope to see what was going on! A useful tip is to assume that every tube will contribute say 20uV of noise so try to keep the level 10,000 times above that (S:N ratio 80dB). Note that Mullard only claimed around 60dB for most of their circuits so transformers will help lower the noise levels with very low inputs. Layout and earthing will be crucial to decent performance.
Good luck
best
DaveP

 

[C.B. - Boudio]

It is nigh on impossible to close the loop at dc using a single tube due to the large voltage difference between the output and input terminal

Your design closes the loop at dc via R15 direct from anode to grid which is liable to turn the tube hard on.

Thank you for pointing that out!

Maybe I'll look at the EQ section of the EAB VE12... I'm not sure.
Or just do away with an EQ section, but I'll need it too much I think - I'm not planning on doing only minimalist classical recordings, although I'd really love to do that too, some day.

[Please do give your opinions!] -- While I certainly do realize that my aging eyes are nowhere nearly as sharp as they once were during my much younger days, nonetheless.....the manner in which how I see things are connected-up here looks to me as though XLR Pins-2 and 3 are connected together!!! Am I not seeing things correctly? Are you creating an "unbalanced" circuit???.....

View attachment 145919

Oh I'm sorry, I drew the phase switch incorrectly - I guess your eyes could still be better than my younger ones!

@DaveP Do you still have any of your schematics from that period? I'd be very interested to see them! (and also, my French is worse than my English - I'm Flemish ;-) )

 

[DaveP]

@DaveP Do you still have any of your schematics from that period? I'd be very interested to see them!

I have the entire book but it's 50MB so I will just copy the relevant pages for you (attached)
Here is the Mixer.

 

[DaveP]

Here is the pre-amp section with Baxandall type tone control.
Have Fun!
best
DaveP

 

[MidnightArrakis]

I have the entire book but it's 50MB so I will just copy the relevant pages for you (attached)
Here is the Mixer.

[I have the entire book but it's 50MB] -- If you would either like and/or wouldn't mind.....I could provide you with direct access to a folder on one of my online web-storage sites where you could upload the entire 50MB of the book. Then, I could provide you with a link to share with everybody who would want to download it or you could privately share the link with only "@C.B. - Boudio".

Does this work for you? As an example, I have an online folder containing around 500MB of design data just on various aspects of "PCB Design Data, Guidelines, Standards & Techniques" that I occasionally share its link with, with some members on this forum.

/

 

[C.B. - Boudio]

I have the entire book but it's 50MB so I will just copy the relevant pages for you (attached)
Here is the Mixer.

I think I have the Philips version downloaded somewhere! It's drawn a bit differently and there are no instructions for the enclosures, but apart from that...
It's just such a bummer that all these preamps have such a low output voltage, about DIN line level it seems? The same seems to go for the EAB VE12, too. My goal is really to get +4dBu by an input of -66~-70 (and I don't mind 0,5 or even 1% distortion).
I can't see how the transformer input is done in your book, but it will be the same as the Philips, and it somehow made me wonder if I could adapt it to get some more gain out of my first PF86 (I know there it's grid leak which is not my case, but still); with 1M at the input, that stage should give up to 46 dB of gain (Rk 2,2k; Ra 220k; etc, as per the datasheet), but since it sees only 10k at g1, I thought of elevating one leg of the transformer, like this:

___. ._______( _ _ _ ) (this should represent g1)
. .3 | | E
. .3 | | E
___ 3 | | E _
. .|
. .|1M|
. .|
. .______
. .___
. ._

(my excuses for the laughable depiction)
In the Mullard/Philips book, a 10M resistor, bypassed by 5000 pF, is, however, put in the other leg, so it's not really comparable I think?
This was only a quick 'showerthought', and I can't seem to find something like this anywhere else.

 

[DaveP]

Midnight,
I am not entirely sure my copy is legal, but the permissions allow copying of some pages so I will leave at that, thanks for the offer.
best
DaveP

 

[wlinart]

That book is quite easy to find online, even sowter has parts of it somewhere on it's website.
Complete book
Sowter

 

[DaveP]

• That book is quite easy to find online, even sowter has parts of it somewhere on it's website.

The site where it used to be has gone down, it became labelled as"not secure"
Sowter only has the Mullard 5-10 circuit which is no use to CB.
best
DaveP

 

[DaveP]

CB,
To get 70dB of useable gain, there are other factors that need addressing. First is noise. You need something like a 1:10 input transformer (600:50k) on the input, this gives you a relatively noise free 20dB gain. You will also need ~ 20dB of NFB to reduce the noise to pro levels. Secondly the output Z needs to be low so that your gain will not be lost when connecting to the next stage.

This problem was solved back in the day, by an IPT as described above, then a gain stage EF86 which received NFB from the second stage followed by an OPT. The NFB also reduced distortion and lowered the output Z. These components would have given +20, +40, -20, +40, -10 =70.

You could simplify this to an IPT, EF86 then cathode follower, but this cannot have NFB between stages. You can put NFB from the anode of the EF86 to the grid, but this lowers the input Z.
best
DaveP

 

[wlinart]

The site where it used to be has gone down, it became labelled as"not secure"
Sowter only has the Mullard 5-10 circuit which is no use to CB.
best
DaveP

But i also posted the full link from worldradiohistory, which is still online.
Complete book

 

[DaveP]

All I get is "this site can't be reached"
best
DaveP

 

[wlinart]

All I get is "this site can't be reached"
best
DaveP

Strange.
Here are a few other links, all to the same book
Primary windings
Archive.org

 

[C.B. - Boudio]

I found the Philips version on http://www.introni.it/pdf/Rodenhuis - HiFi Circuits.pdf
It has the same basic schematics, with the addition of SRPP power amplifiers.

 

[DaveP]

@wlinart,
These links work, thank you
best
DaveP

 

[C.B. - Boudio]

Now with a conventional EQ, a carbon copy of the EAB (after all, that's one of the historic products closest to what I try to achieve, although not quite the same).

I'm still waiting with NFB for the first stage, also because of the lower transformer gain. I don't really have access to 1:10 transformers. I know this sounds like a great recipe for noise, but since the goal is to record to tape anyways (probably a Revox G36, or A77 at best; 55 and 62dB S/N respectively, according to specs), so for now, I'll be crossing my fingers to get away with it ;-)

As far as I can tell, I get about +12, +40, +46/-26, so 72dB, optimistically speaking. Looks good to me!

Please let me know if (and where) I might be in the wrong!

 

[MidnightArrakis]

Now with a conventional EQ, a carbon copy of the EAB (after all, that's one of the historic products closest to what I try to achieve, although not quite the same).

I'm still waiting with NFB for the first stage, also because of the lower transformer gain. I don't really have access to 1:10 transformers. I know this sounds like a great recipe for noise, but since the goal is to record to tape anyways (probably a Revox G36, or A77 at best; 55 and 62dB S/N respectively, according to specs), so for now, I'll be crossing my fingers to get away with it ;-)

As far as I can tell, I get about +12, +40, +46/-26, so 72dB, optimistically speaking. Looks good to me!

Please let me know if (and where) I might be in the wrong!

I just stumbled (OUCH!!!) across this and thought you might find it to be of some interest:



/

 

[ricardo]

Here is the pre-amp section with Baxandall type tone control.

This isn't a Baxandall tone control. Great Guru Baxandall dreamt up his circuit to avoid the evils of this type of circuit which is sometimes called a Tone Stack with log pots.

GG Baxandall's tone control uses linear pots in a virtual earth circuit and has much less THD bla bla than the primitive circuit shown in the Mullard book.

http://douglas-self.com/ampins/wwarchive/wwarchive.htm#baxtc

Be warned that you need to drive it from a LoZ source that is OK with LoZ loads cos GGB's circuit has very low input Z when giving loadsa boost.

There are slightly simpler versions which don't need his centre tapped pots that give slightly more useful control too.

 

[C.B. - Boudio]

Since you say a baxandall has to be driven from a low impedance source to a low impedance load, I'm a bit confused then by what they did here, with such high impedances.
(side question: isn't that a feed-forward between the first and second stage? I'm a bit confused by that as well)

 

[DaveP]

This looks like a very nice circuit.
That is a NFB tone control as used in many of the Mullard/ Philips in their project books. The low Z drive requirement does not apply.
That is not feed forward, it is a totally standard NFB circuit. You could do a lot worse than copy this schematic.
best
DaveP