how to build a pre using S&B TX-102 transformers - Page 20

analog_sa · 24th March 2005, 02:04 PM

Quote:

Input impedance at 20kHz ist about 50K when configured as TVC

Sorry, i don't understand the above. Why at 20kHz? Surely at such a high frequency it's just a matter of transforming impedances, ie it will only depend upon the power amp input impedance and the position of the TVC.

krishu · 24th March 2005, 02:09 PM

Hello,

this information is just taken from the S&B website.

Cheers
Christian.

ThorstenL · 24th March 2005, 03:18 PM

Hi,

Quote:

Originally posted by krishu
there is a schematic in the attachment; could anyone please validate that I understood the difference between TVC and AVC correctly ...

Note: VTC/ATC-Switch open=VTC; closed=ATC

Yes, that seems all correct. If at the beginning you do not get any sound or very little in AVC connection you know you have the primary polarity wrong.

Quote:

Originally posted by krishu
As I understand the input impedance of an ATC is much lower than compared to TVC

You understand wrongly. The input impedance remains identical, discounting any parasitic DCR etc.

Quote:

Originally posted by krishu
How about the output impedance of the AVC compared to the TVC?

Same too....

MSG

Sayonara

ThorstenL · 24th March 2005, 03:32 PM

Konnichiwa,

Quote:

Originally posted by analog_sa
Sorry, i don't understand the above. Why at 20kHz? Surely at such a high frequency it's just a matter of transforming impedances, ie it will only depend upon the power amp input impedance and the position of the TVC.

You need to account for parasitic primary capacitance. It should be below 100pF, but JB would have to measure it, he is kinda busy right now. Now 100pFshould translate into around 80KOhm @ 20KHz.

Sayonara

analog_sa · 24th March 2005, 04:22 PM

Quote:

You need to account for parasitic primary capacitance.

Of course. Thanks.

ThorstenL · 24th March 2005, 04:56 PM

Konnichiwa,

Quote:

Originally posted by analog_sa
This i understand well. My question was in the context of determining the input impedance.

Yup, that is exactly what I am refering to.

Let us consider the TX-102 MKIII as having a primary inductance of 400H and a parasitic capacitance of 100pF. And let us also consider our load is a 100K//100pF Tube Amp with a 100pF shunt capacitance in the interconnecting cable.

Let us now consider the effective input impedance of our TX-102 at 0db, 6db and 20db attenuation.

Let us start with direct unity gain:

Zl for 400H @ 20Hz is around 50KOhm and 2.5MOhm @ 1KHz

Zc for 300pF (input capacitance of Amp, connecting cable and primary capacitance) @ 20KHz is around 26K and 530K @ 1KHz

All of this in parallel with 100K leads to an input impedance of around 81kOhm @ 1KHz and 33KOhm @ 20Hz and 21KOhm @ 20KHz.

For the 6db attenuation case:

Zl for 400H @ 20Hz is around 50KOhm and 2.5MOhm @ 1KHz

Zc for 150pF (input capacitance of Amp, connecting cable divided by 4 + 100pF primary capacitance) @ 20KHz is around 53KOhm and 1.06MOhm @ 1KHz

All of this in parallel with 400KOhm (transformed up 100K load) leads to an input impedance of around 260kOhm @ 1KHz and 44KOhm @ 20Hz and 47KOhm @ 20KHz.

For the 20db attenuation case:

Zl for 400H @ 20Hz is around 50KOhm and 2.5MOhm @ 1KHz

Zc for 102pF (input capacitance of Amp, connecting cable divided by 100 + 100pF primary capacitance) @ 20KHz is around 78K and 1.56MOhm @ 1KHz

All of this in parallel with 10MOhm (transformed up 100K load) leads to an input impedance of around 878kOhm @ 1KHz and 50KOhm @ 20Hz and 77.5KOhm @ 20KHz.

Obviously, in all cases the inductive/capacitive reactance is well above the specified 10K nominal impedance of the TX-102, but the differences at high and low frequencies are material. Just for fun the same calculation with a 10K//1nF IEC load, which was basically the "design normal" load for the TX-102....

Let us start with direct unity gain:

Zl for 400H @ 20Hz is around 50KOhm and 2.5MOhm @ 1KHz

Zc for 1100pF (IEC Load and primary capacitance) @ 20KHz is around 7.2K and 145K @ 1KHz

All of this in parallel with 10K leads to an input impedance of around 9.3kOhm @ 1KHz and 8.3KOhm @ 20Hz and 4.2KOhm @ 20KHz.

For the 6db attenuation case:

Zl for 400H @ 20Hz is around 50KOhm and 2.5MOhm @ 1KHz

Zc for 350pF (IEC Load capacitance / 4 and primary capacitance) @ 20KHz is around 53KOhm and 1.06MOhm @ 1KHz

All of this in parallel with 40KOhm (transformed up 10K load) leads to an input impedance of around 36kOhm @ 1KHz and 22KOhm @ 20Hz and 14.5KOhm @ 20KHz.

For the 20db attenuation case:

Zl for 400H @ 20Hz is around 50KOhm and 2.5MOhm @ 1KHz

Zc for 110pF (IEC Load capacitance / 100 and primary capacitance) @ 20KHz is around 72K and 1.45MOhm @ 1KHz

All of this in parallel with 1MOhm (transformed up 10K load) leads to an input impedance of around 479kOhm @ 1KHz and 48KOhm @ 20Hz and 67KOhm @ 20KHz.

Again, the parasitic elements actually belonging to the TX-102 MKIII are well out of contention compared to the nominal impedance of the TX-102.

In fact, based on inductive and capacitive reactance of the TX-102 MKIII one might even be reasonable to re-classify the TX-102 MKIII as having a 50KOhm effective impedance if we want to be comparable to a resistor type atenuator.

Sayonara

krishu · 24th March 2005, 04:59 PM

Quote:

Originally posted by Kuei Yang Wang
Hi,

You understand wrongly. The input impedance remains identical, discounting any parasitic DCR etc.

Sayonara

Hello,

I am a bit confused by this but won't mind as long as it does sound nice

I'll write you an email in German to make things more clear as this is impossible for me to write in English - sorry.

Why do Bent audio and the others not tell about the AVC option? Or did I miss it? What's the disadvantage?

Regards
Christian.

krishu · 24th March 2005, 06:27 PM

Hello again,

Kuei Yang Wang found my mistake ... the source does not "see" the input impedance of the power amp because I did not take the attenuation into my considerations...

Regards
Christian.

mikelm · 24th March 2005, 07:20 PM

Well, last night I tried the exact diagram that has been posted recently.

I think that Kuei may have been correct. The sound has a different charactor than before - don't get me wrong, it sounds really great but... it is more smooth again but still very detailed & refined - but.... I have been doing a lot of work on my CD player aswell so it's a bit hard to know what is causing what.

paralleling the coils seems to reduce the volume.

My original configuration was unusual. I was injecting the i/p signal into the secondary only, but two taps down from the top.
One end of half of the primary was connected to earth the other half was completely disconnected.
I have no idea what kind of bizarre effect this may have been causing.

Once I have listened to this for a few days I will try the primary in series with the secondary.

mike

mikelm · 24th March 2005, 07:31 PM

Kuei,

What is the difference between having a huge amount of the secondary out of cct by having the volume turned down low in a TVC configuration and having the input going straight to the secondary and the primary earthed one end and disconnected at the other end ?

Is is just a bit worse or is it a Cardinal Sin ???

mike

24th March 2005, 02:09 PM	#192
krishu diyAudio Member Join Date: Apr 2003 Location: .	Hello, this information is just taken from the S&B website. Cheers Christian. __________________ Visit my DIY page at www.krishu.de

24th March 2005, 06:27 PM	#198
krishu diyAudio Member Join Date: Apr 2003 Location: .	Hello again, Kuei Yang Wang found my mistake ... the source does not "see" the input impedance of the power amp because I did not take the attenuation into my considerations... Regards Christian. __________________ Visit my DIY page at www.krishu.de

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24th March 2005, 07:20 PM	#199
mikelm diyAudio Member Join Date: May 2003 Location: England	Well, last night I tried the exact diagram that has been posted recently. I think that Kuei may have been correct. The sound has a different charactor than before - don't get me wrong, it sounds really great but... it is more smooth again but still very detailed & refined - but.... I have been doing a lot of work on my CD player aswell so it's a bit hard to know what is causing what. paralleling the coils seems to reduce the volume. My original configuration was unusual. I was injecting the i/p signal into the secondary only, but two taps down from the top. One end of half of the primary was connected to earth the other half was completely disconnected. I have no idea what kind of bizarre effect this may have been causing. Once I have listened to this for a few days I will try the primary in series with the secondary. mike

24th March 2005, 07:31 PM	#200
mikelm diyAudio Member Join Date: May 2003 Location: England	Kuei, What is the difference between having a huge amount of the secondary out of cct by having the volume turned down low in a TVC configuration and having the input going straight to the secondary and the primary earthed one end and disconnected at the other end ? Is is just a bit worse or is it a Cardinal Sin ??? mike