Push-pull microphone preamp with UTC LS and Tamura transformers

[gridcurrent]

458a transformer notes

 

[Winston OBoogie]

Thanks for that!  Still haven't found the AES paper but, looking at the schematic, I see it uses a pair of parallel 6072's for low noise with cross-coupled anode-grid 3pF's to neutralize the input capacitance.  Bingo.   

 

[abbey road d enfer]

@aazaal and Winston,
I'm sorry you misunderstood my comment for denigration.
As I already wrote, the OP was a description of a design without any explanation as to why and what goal was pursued.
Members who have not achieved your level of understanding needed an explanation.
Winston, you provided it.

 

[Winston OBoogie]

@aazaal and Winston,
I'm sorry you misunderstood my comment for denigration.
As I already wrote, the OP was a description of a design without any explanation as to why and what goal was pursued.
Members who have not achieved your level of understanding needed an explanation.
Winston, you provided it.

Ah, then it may actually be ME who should say sorry my friend.  I understood completely that you wanted others to understand the goals or intentions.  I never in a gazillion years thought that you needed clarification.  Doh
I addressed you merely as a stepping off point from your questions, which were good and valid.  Probably should've been clearer in that
As I said, "I know you know" and lots more besides that I forgot to cover.

 

[abbey road d enfer]

Ah, then it may actually be ME who should say sorry my friend.  I understood completely that you wanted others to understand the goals or intentions.  I never in a gazillion years thought that you needed clarification.  Doh
I addressed you merely as a stepping off point from your questions, which were good and valid.  Probably should've been clearer in that
As I said, "I know you know" and lots more besides that I forgot to cover.

As always, language barrier and internet not carrying intention. 

 

[aazaa]

@aazaal and Winston,
I'm sorry you misunderstood my comment for denigration.

No worries, Abbey. We all appreciate you and your interventions!
Not a single moment I felt embarrassed.
It was very clear that - as a moderator - you were only looking to provoke reactions.

Well, you did very well!
You provoked me to tell my story. (I didn't want to repeat all the advantages CMRR... etc. as it didn't happen to impress anybody in other threads.)

But... it was so much fun to read the swirl of arguments given by a passionated Winston on fire.
When I wrote 'Wow, what overwhelming support' there was no intention against your provocative questions, surely not!
No, it was pure joy to see the flood of Winston's passionate defence.
So much passion... The tiniest reason seems enough to investigate full all possibilities of push-pull. Pure curiosity to see what will happen...

Also very nice to read other possible solutions for high roll off, suggested with such generosity!
Thanks Winston, Doug and Gridcurrent!


Paul

 

[aazaa]

I believe way back in time analag had posted some PP designs with output coupling more rarely seen, like dual parafeed with the output primary floating, maybe some other things too. 

Exactly what I did in the experiment UTC LS10/Luxman/UTC LS151.


Paul

 

[Winston OBoogie]

As a potential part for push-pull, choke-fed, parallel feed with floating output transformer, the UT26 from AMI looks good for not crazy money.  1280H end to end.

I wonder what the maximum output is as an output transformer too when push-pull fed, and from lower than 30K source.  No doubt it'd beat the +10dBu for 1% distortion spec that's listed, but by how much?

http://www.tab-funkenwerk.org/product/ut26/

 

[aazaa]

About the fear - omnipresent in this forum - of reaching 1mA unbalance in the primary of the output transformer:
(I didn't see any fear for tubes going bad in a SE amp...)

Suppose we have a decent transformer with good specifications, max unbalance 1mA.
Also having a good tube for V2, a tube with reasonably well-balanced triodes.

--------------------------------------
In my preamp:

B+ 248V
Tamura SMG-1000: Rdc of the windings used as primary: 679 and 677 Ohms, total 1356 Ohms.

ECC82: both cathodes are connected to a common resistor of 15K.
114V over this resistor.
Total current drawn by both halves: 114/15= 7.6 mA, or 3.8 mA each.
(only 3.8 mA in the primary...while 1mA unbalance is permitted!)

1/2 ECC82 - one triode - is limited to max 2.75VA anode dissipation.
in my amp: 248-114=134Vx3.8mA=0.509VA for each triode.
The tubes will have a long life!

--------------------------------------
I measured the unbalance in the primary of all my Philips tubes (8) in this circuit, nos and used. Connected without the trimpot of 1K.
Max unbalance between the anodes was 0.1V, most tubes measuring less. (Ok, good tubes, these Philips.)
0.1/1356=0.000073746A or unbalance of 0.0737mA. Far below the max allowed 1mA.

After trimming the 1K pot I measured <0.01V unbalance or <0.00737mA
Don't be afraid to see little DC fluctuations caused by changing temperatures and in my case also changing B+ .
(High tension is not stabilised).
Don't be tempted to do the trimming over and over. Conditions will never be twice the same. DC unbalance will fluctuate but is low enough after the first trim. Of course, with new tubes it is good to do a new adjustment after a run-in of about 10 à 20 hours.


This is the starting point.

As said, tube wear is low. We have a wide margin.
There is no necessity for frequent adjustment.

If you are in a hurry, adjustment will take 3 minutes and 7 seconds.
3 minutes for the tubes to warm up. 7 seconds for adjustment.

If you are relaxed, with plenty of time, it is good to use 10 à 15 minutes and 7 seconds.
10 à 15 minutes for a cup of coffee and maximum stabilization of the tube, 7 seconds for adjustment.

When after a long time tubes have reached the point where too much correction is needed for proper balancing, then at least one of the two halves is bad. It's time to replace the tube.

You can pull it out and ... use it in your SE-amp???


Paul


PS
By choosing 1K for the trimpot (instead of 220 Ohm) amplification of DC is reduced through local NFB.
Perfectionists can do even better by adding a resistor of 180 Ohm to both sides of the trimmer.
Then there is NFB across both cathodes caused by a resistance of approximately 500+180= 680 Ohms. Equal to the Rdc charge on the anodes. So, no more amplification of fluctuations in the tube...


Enough about all this. Time to take a few photos.

No...oh no...
Let's take a break first.
Time for my coffee!

 

[janos.hardi]

As a potential part for push-pull, choke-fed, parallel feed with floating output transformer, the UT26 from AMI looks good for not crazy money.  1280H end to end.

I wonder what the maximum output is as an output transformer too when push-pull fed, and from lower than 30K source.  No doubt it'd beat the +10dBu for 1% distortion spec that's listed, but by how much?

http://www.tab-funkenwerk.org/product/ut26/

Looks like they have an UTC-10 replica too which seems appropriate as a decent input transformer for PP operation. Probably it doesn't have the extended lows/highs like the LS series but the specs look really nice (plus shopping from a single source for i/o is great).

 

[aazaa]

I wonder what the maximum output is as an output transformer too when push-pull fed, and from lower than 30K source.  No doubt it'd beat the +10dBu for 1% distortion spec that's listed, but by how much?

http://www.tab-funkenwerk.org/product/ut26/

To know that Winston, the only solution I see is to buy one and take the time to try it out...


Paul

 

[Winston OBoogie]

To know that Winston, the only solution I see is to buy one and take the time to try it out...


Paul

For sure.  Looks like shipping for small quantities will be from about March.
The copy of the A24 they have shows similar specs to the A26 copy as far as distortion, and yet I know that the original A24 was better when push-pull driven rather than single ended.  I guess I was trying to extrapolate. 

Janos.hardi - yep the A10 copy seems like a good input transformer candidate for sure.

Be nice if you could get the in and out transformers in one go.

Hope your coffee break was good Aazaa, nice run through of the DC balancing  

 

[aazaa]

Hope your coffee break was good Aazaa, nice run through of the DC balancing

Thanks Winston, coffee was excellent!

Here are some pictures of my build.
(The pictured preamp seems too narrow, proportions are better in real world ...)

 

[aazaa]

Switches on the back are for powering phantom.

 

[aazaa]

The PSU

 

[aazaa]

The chokes on the PSU are Gardners.

 

[janos.hardi]

aazaa,

Stellar build! I like that you matched the style of the case to the transformers

J.

 

[Winston OBoogie]

Wow!  That coffee you had in the break must've been rocket fuel, stellar build is right!

 

[rock soderstrom]

 

[aazaa]

thanks Rock, Janos ... and yes Winston, you're right about the coffee!  ;-)

Paul