DIY Tube Preamp

[fdegrove]

Hi,

This what your Harbuch costs:

$112.15 as EI or $77 as toroid.

But has it the same specs as the Hammond one?

The more secondaries the more it is going to cost since the winding machine has to be stopped and so on.
That's labour and you end up paying for it.

The Hammond are fine but not exceptional IMO,the good thing is they have a range that is particularly suited for projects such as yours.

Ciao,

[Brett]

Originally posted by Joel
I'm not quite sure why you are so opposed to the designs that Frank and I have offered him. This is supposed to be a simple, affordable project to get the guy started in DIY. You have now successfully clouded the entire discussion to the point where he and many others will now doubt that these are "good designs". Congratualtions.

Joel, just like you, I'm entitled to my opinion, and I expressed it. I've also built and used many different topologies and know IME what works well. I find your comment to be very disingenuous: I said <i>"My suggestion would be to breadboard one of these on a simple chassis and listen for a while (the offer of the 6N1Ps still is there). Most of the passives can be reused as can the iron, so the experimental cost will only be a few dollars. Later try something like ......"</i>. Maybe not everyone wants to stick with what you find appealling. Please also keep your sarcasm in check.

<b>Also,
1) I did not suggest that anyone use an input impedance of 20k. My schematic shows an input impedance of 100k - which is just peachy for a cd player to work into.</b>

I never suggested you did. My reference to 20k was in regards to questioning what sort of input Z his power amp might be. Many SS amps are less than this. Please ensure you take my comments in context in future, it makes you look less of a fool.

<b>(2)A "headphone amp" IS a preamp. I said you could use it as one.</b>

You used a mention of your headphone preamp as a justification for needing the increased gain in your previous reply to me. Hence my response. Also, as there is no requirement to drive an <i>extremely</i> low load for a line stage, ie a set of cans, why would you want to add a piece of sonic pig misery like a CF?

<b>3) I didn't mention that the CF loses gain because I assumed everyone knew that.</b>

Why would you assume that? The original poster is a newcomer, and neither you or I know what he knows. So I explained my reasoning for more than you.

<b>4) I hardly think the 20 or so gain of the 12AY7 is going to add 30db of noise! This is ludicrous.</b>

Comment in context please. With a stage gain of 20x or 26dB (none of which is needed), which you want to attenuate on the input to the stage, it can certainly increase the <i>output</i> SNR by a theoretical 26dB, possibly more depending on implementation and the setting of the volume control. The 30dB was a quick mental calculation and I was maybe off by a couple of dB. the 12AY7 isn't a particularly quiet tube IME.
I think I see though. Your reference to the 12AY7 indicates quite clearly that you are not upset that I choose something different to you, but that I am critiquing your design. I would not ordinarily suggest using a 12AY7 in preference to a 6SN7 which has better linearity. I couldn't care less what <i>you</i> listen to and like. But if I have tried it (numerous times) and found it wanting, I'm going to post it if I feel like it. There are much better sounding and measuring circuits out there than 2 stage CFs.

<b>4) If you have a better alternative for a newcomer to build, please offer it.</b>

I already have. No budget was mentioned, so I assumed it was modest, and suggested trying the SRPP and AF. Then try a CCS or choke loaded 12B4A. Neither of those are complex, the CL design being just as simple as an AF and has other advantages too. Kits are even available for the Bottlehead C4S (a type of CCS), and I could veroboard one in 15mins for about $A20 in parts.

I read the original poster's intent was to learn, as well as having a functional line stage. Just building an AF, or what you have and leaving it at that, will only have some educational value in construction and soldering. There are better circuits than an AF, SRPP and <b>definitely</b> a CF (though Kimmel suggests a pentode CF is excellent, and I can see why, I haven't tried it myself) for a linestage.

[Brett]

Originally posted by FBLEAGH
Thanks for the intersting discussion guys.

are hammond power xformers supposed to be really good of something ??

302AX 300-0-300 200mA 2.5v 2A, 5v 3A, 6.3v 6A
$250.50AUD + GST

why such a big difference between that and my custom one ?

The difference in price is due mainly to import duties, shipping and an Australian agent adding their markup. I have used Harbuch transformers many times in various industrial applications and found them of fine quality. Two of my vintage pieces run off a Harbuch step down trans. The only advantage to you purchasing the Hammond would be in having a lighter wallet to carry around, and the difference in cost between the two will pay for most of the passives and tubes probably.

Cheers

[Joel]

Brett, if you could refrain from openly calling me a "fool" in your posts in the future, I'd appreciate it. I'm sure I don't need to remind you about the forum rules.

You are certainly entitled to your opinion, and I think you already expressed that. Yet, once again, you offer no evidence to support any of your statements. The whole "cathode followers are bad" line is so old and tired. Where is the proof? Why exactly is building a kit of a CCS better for learning than any other circuit??

I'm not saying everyone should build an rc-coupled CF amp. But if you are going to offer criticism, please use facts so we can learn. If the circuit I posted is 3db down at 15kHz, I want to know! But, I suspect you are not capable of providing any real information to educate others - only vague, diffusing commentary on whatever's currently fashionable.

[Joel]

Originally posted by fdegrove
Not a mistake but if you want to use this circuit as a linestage preamp wouldn't it be wise to add a second smaller value coupling cap...

Hi Frank,
You mean across the ouput cap? I don't bypass electrolytics with film caps - it doesn't add anything I can measure, and it doesn't effect the sound, to my ears.

Loading down a lineoutput with a small value resistor is going to take down the input impedance of the amplifier offering a severe load.

I have to disagree. The input impedance of the power amp is fixed - at least in any amp I've seen. Changing any value in the preamp output will not change the fact that it must devlop it's voltage across the grid resistor in the amp's input.
The 2.7k is there to bleed off any stored charge in the big cap, as a protection.
If the follower is faced with developing its voltage across 3k or the impedence of the next tube's grid (500K? 1M?), which do you think it will chose? You can see how the small resistor becomes irrelevant.

Joel

[fdegrove]

Hi Joel,

You mean across the ouput cap? I don't bypass electrolytics with film caps - it doesn't add anything I can measure, and it doesn't effect the sound, to my ears.

No,that is not what I meant.
Having a second otput at the CF for specific use as a lineout using a smaller film or PIO cap in such a way that you do not have to couple through a large electrolytic.

If you consider the imput impedance of a tube at 100K a 5 micro F
cap should be adequate if you shunt that with a 100K.

Ciao,

[Joel]

Frank,
Yes, if you have no intention of driving phones, you can use a much smaller film cap at the output. If you think this makes an audible difference, you should do it.
I just don't hear it.

[planet10]

Originally posted by FBLEAGH
re hammond power xformers supposed to be really good of something ??

Think of Hammond as "Transformers R Us" -- they have a huge range and are relatively inexpensive in North America where they are made.

Anything heavy is best sourced locally.

dave

[dice45]

Peter,
not intending to interfere with you here, "... But I cannot be a judge here... " just stuffing you

Joel,

Originally posted by Joel
[B]... The whole "cathode followers are bad" line is so old and tired. Where is the proof? ...

my scanner is not yet up and running, otherwise i would post the page from the guru book:
Valley & Wallman, Vacuum Tube Amplifers,
Chapter 11: direct coupled amplifiers, p483, figure 11.21: Curve and circuits "Linearity of a cathode follower"
Bottom line: a CF ought to work into an infinite resisitance -- or atleast into one as high as possible. As the CF has to run on a certain quiescent current, you need either a huge load resistor to a huge negative supply voltage or a CCS.
The curves tell that clearly.
My practical experience: in most cases a CF with small load resistor sounds like crap. No urban legend but experience and worse, backupped by theory of the guru book.

DTopic: Joel, i tried to reach you twice today via email. Why do my emails bounce? I expect members to have valid email adresses. Not only during registration. Ok?

[Gunders]

If you guys want to read end maybe learn more about buffer stages like the cathode follower etc I would recommend the October 1999 issue of Tube Cad.

http://www.tubecad.com/october99/