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Old 22nd October 2013, 06:27 AM   #1
Ceglar is online now Ceglar  Australia
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Default Q: Cascaded Transformers, S&B102+LL7903

Hi,

I'm hoping for some help in understanding the following situation.

If the inductive reactance of the S&B102mk3 primary is ~100kohm at 20Hz and drives a 1:4 step up transformer which is loaded by open grid to ground of a triode in parallel with 10M, I can see the reflected impedance across the stepup transformer will be 1/16th of 10M//open grid.

Can I assume open grid to ground of triode to be infinite and therefore calculate the 102mk3 to be loaded by 10M/16 = 625k ?

Working with worst case scenario, with zero attenuation and 1:1 TVC, is it Xl of the TVC that then fully determines the impedance that the capacitor works into?

With some attenuation at the TVC, say 4:1, the reflected impedance would be 625k x 16 = 10M.. but it is still the inductive reactance of the TVC primary that determines the impedance that the capacitor works into, it stays at 100kohm at 20Hz, and the 10M load has no bearing on Cx calculations, just re-affirms a suitably high load to the secondary?

If I understand this correctly (and that is the real question), a nice 0.22 - 0.47uF capacitor would suit well..


Any comment appreciated,
Shane
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Old 22nd October 2013, 08:33 AM   #2
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What you haven't accounted for is the high probability of resonances which will need damping. Almost all transformers have self resonance where the capacitive losses form a tank with the inductance. The easiest way to solve this is to load the secondary down with a resistor (often of a quite low value) which would make all your calculations mute.
The difficult way to damp the resonances is with a zobel - which has to be tuned for thew exact circuit the transformer is used in. This is a matter of trial and error.

In short - you can make what you plan work - but don't expect it to be as easy as the raw impedances would suggest.

Shoog
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Old 22nd October 2013, 09:21 AM   #3
DF96 is offline DF96  England
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A valve grid could have a DC/LF impedance in the M or 10's of M region, mainly resistive (due to grid current). The impedance at 20kHz could be much smaller, but capacitive due to Miller effect.

Transformers have leakage inductance and stray capacitance, so using them at impedances far removed from what they were designed for means they no longer act as simple wideband impedance transforming devices.
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Old 22nd October 2013, 11:17 AM   #4
Ceglar is online now Ceglar  Australia
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Thanks Shoog, I'm not so after square wave performance but rather good sound, I'm sure you know what I mean. I use an SE to PP interstage for the main amplifier - and it measures better but sounds worse when loaded down.

Thanks, DF96. Would you think the capacitive effects on BW might be limited somewhat when driven by ~200ohms?. - So far as impedances far from what they are designed for; I'm not fully realising the point that is being put forward: the TVC is driven and (presumably) loaded as it would like to be, and the 1:4 acts as an impedance matching device (voltage gain aside), driven low Z and loaded high Z - all within what might be considered within the means of what they were designed for. (??)
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Old 22nd October 2013, 01:15 PM   #5
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Hi!

The LL7903 needs low impedance drive to work best. I usually drive it with impedances well below 500 Ohms. The size of that coupling cap will surely play a role. If you have the parts in hand, try it and measure and adapt as necessary. The LL7903 will have some ringing if lightly loaded.

Best regards

Thomas
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Old 23rd October 2013, 02:30 AM   #6
Ceglar is online now Ceglar  Australia
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Quote:
Originally Posted by Vinylsavor View Post
Hi!

The LL7903 needs low impedance drive to work best. I usually drive it with impedances well below 500 Ohms. The size of that coupling cap will surely play a role. If you have the parts in hand, try it and measure and adapt as necessary. The LL7903 will have some ringing if lightly loaded.

Best regards

Thomas
Hi Thomas,

I was thinking the drive impedance to the LL7903 in this case would be 200 ohms with zero attenuation at the TVC, and less when attenuated. (say TVC at position 2:1, 200/4 = 50 ohm drive to LL7903)...

Your mention of the cap with regard to this has me thinking.. is the Xc of the cap at a given frequency added directly to the drive impedance, or does phase shift come in to play so that the total is not simply the sum of drive impedance + Xc ?.

Have you found the ringing to be a problem?, IIRC your schematics show the LL7903 with the secondary loaded in the same manner.


Regards,
Shane
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Old 23rd October 2013, 03:03 PM   #7
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When driving a stiff load from the MU output of a CCS it can help to add either a resistor or a CCS to ground from the MU output. The resistor or CCS increases the bias current in the main CCS. This makes more current available to drive the load, especially at high frequencies where the miller capacitance of the next stage becomes dominant.

The MU output of a CCS has a quite asymmetrical drive capability with large amounts of current available to drive loads in the positive direction but is limited to the bias current of the vacuum tube under the CCS when driving in the negative direction. The extra load on the MU output increases the drive capability when the signal swings in the negative direction.
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Old 23rd October 2013, 05:47 PM   #8
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Hi!

Quote:
Originally Posted by Ceglar View Post
is the Xc of the cap at a given frequency added directly to the drive impedance,
Yes


Quote:
Originally Posted by Ceglar View Post
or does phase shift come in to play so that the total is not simply the sum of drive impedance + Xc ?.
You will get some resonance peaks at low frequency depending on cap size.
Best to measure and see whats going on

Quote:
Originally Posted by Ceglar View Post
Have you found the ringing to be a problem?,
No, but some people mind the ringing

Best regards

Thomas
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Old 23rd October 2013, 05:59 PM   #9
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I find any ringing to be objectionable. It sounds exciting for a while but then listener fatigue sets in. The ringing I encounter is generally at around 60khz - which strongly effects audible frequencies.

Shoog
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Old 24th October 2013, 12:29 AM   #10
Ceglar is online now Ceglar  Australia
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Quote:
Originally Posted by Vinylsavor View Post

The LL7903 needs low impedance drive to work best. I usually drive it with impedances well below 500 Ohms.
At what frequency would you recommend the drive impedance be less than 500 ohm? The LL7903 and the main amplifier drive a loudspeaker which has a LF that rolls off at ~50Hz.

With 4.7uF, total drive then should be (338 + 200) 538 ohms at 100Hz. Of course the Xc increasing inversely proportional to frequency.


4.7uF
20Hz = 1693ohm
50Hz = 677 ohm
100Hz = 338 ohm

2.2uF
20Hz = 3617 ohm
50Hz = 1446 ohm
100Hz = 723 ohm

Quote:

You will get some resonance peaks at low frequency depending on cap size.
Best to measure and see whats going on
If the primary inductance of the S&B TX-102 is 400H at 20Hz, resonance with 2.2uF would be at approx 6Hz, and 3.6Hz with 4.7uF.

Neither resonance or LF pole seem to be a problem. It might be coming down to the increase the Xc has on drive impedance to the LL7903, in which case I might find needing to compromise between suitable drive impedance and the cost/size of the coupling capacitor.

Getting closer.

Regards,
Shane

Last edited by Ceglar; 24th October 2013 at 12:58 AM. Reason: .
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