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45
diyAudio Member

Join Date: Dec 2008
Location: UK
Quote:
 Originally Posted by Richard Hi Guys, Hashimoto 20k:600 transformers to try. Rich
You will get no gain!
The step-down ratio of your transformer is approx. equal to the effective gain you will get from the 26.
If you want some gain using the Hashimoto you need a lower step-down ratio or add a step-up input transformer or you have to add another stage!

Cheers,
45

 4th November 2009, 02:13 AM #92 Richard   diyAudio Member   Join Date: Apr 2005 Location: Melbourne Hi 45, I have a couple of stepups to try, including a UTC KB143. I'm even considering the Silk STA 522A which does a + 6db . Can I influence the primary impedence by resistor loading the secondary? Rich out __________________ "A little learning is a dangerous thing"
 4th November 2009, 04:29 PM #93 CFT   diyAudio Member   Join Date: Jul 2006 Location: Hong Kong OK, getting a little bit more understanding now, please correct me if wrong... With CCS, what the tube sees is a load that comprises the inductance of the primary coil of the output tranny in parallel with the reflected impedance from the secondary..... Assuming the input impedance of the connected amp is 10k, if the ratio of the output tranny is 4:1, then the tube is essentially seeing the output tranny's inductance at primary in parallel with the reflected impedance of 160k. The rule of thumb is that this overall impedance should be no less than about 3 times the plate resistance. (for 26 it is 8kx3=24k) So I really don't care about the indicated impedance of the output tranny but it's turn ratio, right? : ) But to certain extend that the inductance of the tranny can't be too low, and the capacitance can't be high.... So I should be able to use a tranny with nice size of primary inductance, say connected as 600 ohm: 125 ohm for ccs-parafeeding the 26 tube, yielding an output impedance of about 0.5k (or 8k/16) and a gain close to mu/4 which is about 2, true?
45
diyAudio Member

Join Date: Dec 2008
Location: UK
Quote:
 Originally Posted by Richard Hi 45, I have a couple of stepups to try, including a UTC KB143. I'm even considering the Silk STA 522A which does a + 6db . Can I influence the primary impedence by resistor loading the secondary? Rich out
Yes you can.
I think that 20 Kohm is supposed to be both the source (i.e. the tube) internal resistance and primary load with open secondary. In fact the equivalent resistance is the parallel of the two: 10 Kohm. With 100H primary inductance you get 16 Hz at -3dB, as shown in the frequency response graphic!
If you use a set-up transformer make sure your source (CD player, phono amp or tuner) is able to drive heavy capacitive loads. The step-up transformer can be quite tricky.....

I think that using two direct-coupled 26's, the first resistor loaded and the second transformer loaded could work well if you don't have problems with microphonic (I do not recommend mounting the tube directly on the chassis, use a perforated board like this with 2-3 turrets: http://zsyabai.win.mofcom.gov.cn/www...0762711033.jpg ). You could get exceptionally low distortion at low levels because of harmonic cancellation ( this technique should work at its best because the two tubes are the "same" if you work a little bit on the plate voltages and anode currents). I mean something like the input/driver stage of the Sun Audio 2A3/300B amplifier where you have a direct coupling of the two halves of a 6SN7.
In your case the only difference would be the transformer in place of the plate resistor for the second tube. In this way, using your Hashimoto, you would get approx. 15 dB gain and low Zout.
You need more or less 300V supply.

Cheers,
45

Last edited by 45; 4th November 2009 at 05:11 PM.

kevinkr
diyAudio Moderator

Join Date: Sep 2004
Location: Boston, Massachusetts
Quote:
 Originally Posted by CFT OK, getting a little bit more understanding now, please correct me if wrong... So I really don't care about the indicated impedance of the output tranny but it's turn ratio, right? : ) But to certain extend that the inductance of the tranny can't be too low, and the capacitance can't be high.... So I should be able to use a tranny with nice size of primary inductance, say connected as 600 ohm: 125 ohm for ccs-parafeeding the 26 tube, yielding an output impedance of about 0.5k (or 8k/16) and a gain close to mu/4 which is about 2, true?

It's not quite that simple, the primary impedance of the transformer is important because it implies that there is enough primary inductance to maintain the frequency response within the defined limits under the stated conditions. A transformer with a primary Z of 600 ohms is not likely to have anywhere near the primary inductance of a transformer with a primary Z of 15K even though they might in fact have the same numerical transformation ratios.

Your assumption in the next paragraph is wrong, the transformation ratio scales as the square root of the impedance ratio, so although the impedance ratio is ~1/4 primary to secondary the voltage transformation ratio is going to be ~1/2. Your output impedance is going to a somewhat > 2K.

Gain will be about 1/2 of mu into an open circuit load, and could be substantially less if the load impedance is not at least 10X the reflected rp.

A 600 ohm transformer probably is not going to cut it, insufficient primary L will result in a substantial LF roll-off, worst case you may not have a useful response much below 300Hz, and even if you do this is an excellent recipe for increasing distortion as the frequency decreases. IMHO To guarantee really good performance the xl should be about 10X greater than the load impedance at the lowest frequency of interest. You need a transformer with a primary L of at least 150 - 200H to meet my criteria.
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kevinkr
diyAudio Moderator

Join Date: Sep 2004
Location: Boston, Massachusetts
Quote:
 Originally Posted by 45 Yes you can. If you use a set-up transformer make sure your source (CD player, phono amp or tuner) is able to drive heavy capacitive loads. The step-up transformer can be quite tricky..... Cheers, 45
45 has an excellent point here, if you apply a 1:2 step up transformer at the input you will effectively quadruple the capacitance seen by the stage driving it. This may not be much of a problem in practice if the source impedance and matching impedances are low - and assuming that the device in question can drive the transformer.

Some people place the gain control after the transformer and before the tube, this does have the advantage of reflecting only a % of the capacitance back to the source, but it does impose a variable rc roll-off dependent on the position of the volume control pot. (this is normally the case anyway)

Placing a pot (sometimes a constant impedance L pad) before the transformer is frequently done when the source and transformer are 500/600 ohm devices, but this is a bad practice with higher impedances because of the variable source impedance implied and the effect on frequency response as a function of pot position becomes a significant issue.

Using a 20K:600 ohm transformer will give you almost no gain into an open circuit and an appreciable gain loss into a matched load. Certainly with most digital sources there is no need for significant voltage gain in a line stage. I'm using 15K:600 ohm UTC HA-133 in my 26 pre-amp and get ~4dB of gain in this configuration, and this actually more than enough with my digital sources which all produce about 3Vrms FS. My phono stage is designed with almost 50dB of gain to provide sufficient signal level to this line stage.

You might want to read my Positive Feedback Online 26 DHT pre-amplifier article which you can access through my website. (I talk about some of these issues in the article.)
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"To argue with a person who has renounced the use of reason is like administering medicine to the dead." - Thomas Paine

 4th November 2009, 06:02 PM #97 CFT   diyAudio Member   Join Date: Jul 2006 Location: Hong Kong Thanks for pointing out the errors Kevin! : )
revintage
diyAudio Member

Join Date: May 2007
As I now plan a 26 pre with LL1660/10mA/4,5:1 I did a working simmodel of it. Ug ca -6V and Ia ca 8mA. Dmitrys 26-model follows the published curves perfect.

This one shows a gain of 5dB with a Zout of ca 500ohm. Due to the unbypassed 5.6ohm with 6VDC over it, this one takes well over 10V in before clipping. -3dB is well below 10Hz and 20Hz will be -0,6dB. 165H is a little on the low side and will cause some phase shift in the low end(ca 10degrees at 50Hz).

The simmed fourier-analysis shows extremely low distortion but shouldn´t be taken to seriously.

Would be fun to see an IRL analysis.

The right one is an idea of a prafeed with a voltagecontrolled gyrator instead of inductor/CCS. By playing with the two capacitors it can be made to have a very low Fc.
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 5th November 2009, 03:34 AM #99 vt4c diyAudio Member   Join Date: Jan 2001 Location: Malaysia Has any tried using 26 se with Lundahl LL1680?
kevinkr
diyAudio Moderator

Join Date: Sep 2004
Location: Boston, Massachusetts
Quote:
 Originally Posted by revintage The right one is an idea of a prafeed with a voltage controlled gyrator instead of inductor/CCS. By playing with the two capacitors it can be made to have a very low Fc.
I use a very similar approach based on voltage controlled gyrators to load the amplifier stages in my latest phono stage. All tube based though with 5687s used in the gyrator circuits. It works pretty well, not constant current of course, but there are instances where having entirely predictable plate voltages is a benefit - from an ac perspective it is possible to synthesize fairly high load resistances without the voltage drop associated with a real resistor. (I'm not actually simulating an inductor with this thing, but that is obviously possible.) In my phono stage in order to use a comparable resistive load I would have to have a >1kV supply versus the 240V supply I actually use.
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