Hamptone Tube Mic Preamp Output Transformer Selection

I've got a question about building a Hamptone tube mic pre as per his TapeOp article from years ago.  I'm unsure about the output transformer I need.  It uses a totem pole output stage and calls for a 4:1 step down output transformer.  I think the Jensen JT-10K-61-1M is recommended.  But I'm a) cheap and b) curious about other options.

Do I have to have a center tap for this application?  And if so, could somebody give me a simple explanation of why?  The schematic shows one, but I was wondering if I could just wire up something like the Carnhill 10k:600R offerings from AML.  Or Edcor XSMs or similar.  It would save me bundles of money over the Jensens, which are really pricey.

The schematic for the circuit is available here:
https://web.archive.org/web/20030408091534/http://www.hamptone.com/figure1.htm

More info:
https://web.archive.org/web/20030407232915/http://www.hamptone.com/mic-pre_kit.htm

Thanks

I don't see a centre tap on the output transformer in the circuit you posted. I do see a connection ti the transformer core. Any Carnhill or Edcor 10K:600 transformer should be OK.

Cheers

Ian

Xsm works well

It is a nice preamp schematic, very straightforward.
Could either sound great or ugly depending on building quality.
Most critical components are:

• input and output x-formers
• C2, C4, C5
• C1, C3

in this order.

And, of course, the quality of the power supply will also have a big influence on sound.

Go for cheap components and you will get cheap sound,
Go for all the quality you can afford if you wanna get quality sound.
It's that simple.
You know what you are looking for.

For the output you could look for some vintage x-former like UTC A26 or Stancor WF-36
primary windings in parallel for both of them.
Good sound, but they are getting rarer and rather expensive this days.
I am not sure you will save on Jensens.

I also happen to make some great outputs for the same use.
Gorgeous sound but rather pricey,
as you should expect from high quality gear.

Most critical component is C8. Do not use it.

Ciro Marzio said:

Go for cheap components and you will get cheap sound,
Go for all the quality you can afford if you wanna get quality sound.

Click to expand...

The vast majority of components available from reputable suppliers like Mouser/Digiey/Farnell are high quality. There is no need or point in buying expensive exotic components because they will sound no better.

In this particular design, where there is no negative feedback, the sound will mostly be determined by the tubes, not the passive components that surround them.

Cheers

Ian

ruffrecords said:

The vast majority of components available from reputable suppliers like Mouser/Digiey/Farnell are high quality. There is no need or point in buying expensive exotic components because they will sound no better.

In this particular design, where there is no negative feedback, the sound will mostly be determined by the tubes, not the passive components that surround them.

Cheers

Ian

Click to expand...

Ian,
I talked about high quality vs. poor quality,
exotic vs. not exotic is not of concern in audio design.
Unfortunately, producing high quality items is usually more expensive than the contrary.

Also about your assertion that tubes are the only important component, I respectfully disagree.
I was of your same advice when i was a beginner in tube gear design and building, but not now.

This change is based on a few decades of experience in tube audio design and audio transformer design,
and countless hours of testing both in the lab and by listening.

IMHO it is exactly when there is no negative feedback,
that you hear difference between good quality and poor quality components the most.

If you do not hear that difference maybe it is because the quality of irons, either in PS or on signal path, is so poor,
that it screws everything up from the very beginning.
IMHO irons are the most critical passive component in audio circuits

I know this schematic intimately, and have learned of its many shortcomings.

Throw away C8. It causes low pass filtering.

I suggest using the Jensen "Cascode Follower" (actually a White follower) schematic in place of the Hamptone schematic. The Hamptone values are incorrect and cause greater distortion. I fought this one for way too long.

You should use no less than a 4.7uF output coupling cap. Higher is better, up until about 60uF.

The Edcor 4:1 rings badly in the high frequencies and needs a Zobel to calm it down. The Jensen 4:1 rings sub 10 Hz and requires some secondary loading. 680 ohms does the trick.

I worked on this very thing for about five hours last night!