T-network: the better feedback solution?

[ashok]

And the amp chip likes to "see" low impedance at the input, something that a pot doesn't have.

Hi Carlos,
In a IGC configuration the chip will see the input resistor plus the output impedance of the previous stage. So the impedance that the chip see's can't go down much. Maybe 10K to say 47K or so, based on what I see in other posts.

Does this affect the sound ? Any explanation for this ? Or is the chip succeptible to oscillation with high Zin.
Thanks,
Ashok.

[Joe Rasmussen]

Originally posted by Terry_Demol

OK, so you have done direct comparison and balancing impedances is better?

Yes, and others too. It does seem that bipolar inputs need proper balancing, and not just DC. Removing by-pass cap in earlier JLTi config confirmed that and I now agree what I thought was an assertion of yours earlier on. But when you combine that with Lower Z T-network, another level again is reached.

... I'm not convinced that if properly compensated and balanced that x45 is optimum. I will start low (x15) and work up. There is much to do as yet and much is unknown at this stage. We will see.

Agree, but interested if you come to same conclusion independently..

WRT CLIA

CLIO

What is the baseline or, sig gen distortion level and spectrum?

At 0dBu, putting Clio into a loop, the THD is not all that good @ 0.097% - 2nd harmonic hugely dominent. But for some reason, at -10 dBu and increasing sample size and adjust sample freq, it is possible to get as low as 0.017% with a noise floor -120dB, again relative to 0dBu.

Joe R.

[carlosfm]

Originally posted by ashok
Hi Carlos,
In a IGC configuration the chip will see the input resistor plus the output impedance of the previous stage. So the impedance that the chip see's can't go down much. Maybe 10K to say 47K or so, based on what I see in other posts.

Does this affect the sound ? Any explanation for this ? Or is the chip succeptible to oscillation with high Zin.
Thanks,
Ashok.

Hi Ashok

Remember, on an IGC the input resistor is actually part of the feedback loop, and also sets the gain of the amp.
If the previous stage is active it will have constant (and also ideally low) output impedace, much better than a pot.
If that previous active stage is on a separated box some distance away (like a standalone pre) the feedback loop will be huge, and the amp may even pick up noise from the interconnect (acting as an antenna).
The IGC works better with an input buffer, even on a power amp being driven by a standalone pre.
It works without an input buffer, but there are some issues.
With an input buffer it also sounds better.
It may seam strange for some.
But everytime I use a good active stage between a pot and a power amp I have big improvements.
Ultra-minimalism is only good in theory.

[ashok]

Hi Carlos,
Thanks for your reply.
If the previous stage is reduced to a voltage source and output impedance (or R source) , the next stage (inverting) will add this Zo ( or Rs) to the Rin of the Inverting stage and treat it as a larger resistor . While it does affect the gain , it only provides an input current ( theoretically) to the Inv stage , independant of the following stage.
The -ve input is at a virtual ground ( 0 Volts).
So actually the chip is now receiving an input current that depends only on the series impedance between the -ve input and the voltage source.
It's possible that the chip also has poor common mode performance which is why it sounds better in inverting mode.
Wonder if anyone has checked the spectrum in both modes with identical gain.
I have also found a good buffer between the power stage and signal source helps. It generally seems to sound better to the ear.
Cheers,
Ashok.

[Upupa Epops]

This is distortion of 4780 at 32 W / 8 Ohm, 6 kHz, noninverting mode ( spikes at 15625 Hz is relict of TV, at 19 kHz is pilot signal of FM tuner ) - resistors are normal 0207, any Caddocks .

[Upupa Epops]

Now at 12.5 W / 8 Ohm.

[carlosfm]

Originally posted by ashok
It's possible that the chip also has poor common mode performance which is why it sounds better in inverting mode.

Generically speaking, op-amps have better THD in inverting mode.

[ThorstenL]

Konnichiwa,

Originally posted by carlosfm
Ultra-minimalism is only good in theory.

Actually, it works absolutely stonking in reality too. But you got to do it right...

Sayonara

[carlosfm]

Originally posted by Kuei Yang Wang
Konnichiwa,
Actually, it works absolutely stonking in reality too. But you got to do it right...

Sayonara

Hi Kuei,

As a "wire with gain" is still impossible, please tell me how to avoid an active stage (comparing to a passive pre with a pot ) and have similar results soundwise.
My post was related to this matter, it was not a generic assumption.

[ThorstenL]

Konnichiwa,

Originally posted by carlosfm
As a "wire with gain" is still impossible,

I find it eminently possible. Simply wind the wire around a Mu-Metal Core.

Originally posted by carlosfm
please tell me how to avoid an active stage (comparing to a passive pre with a pot ) and have similar results soundwise.

I can't tell how to have similar results (nor would want such), but I could tell you how to get better ones...

Sayonara