T-network: the better feedback solution?

[mrskinny]

For gengcard- I tried this with my (unbuffered) inverted OPA549 amp.I used JoeR's parts values-10k+100r+10k and 10k on the + input. This does work, but I ended up with a huge amount of hiss thru the speakers.Don't know why it turned out this way-maybe the input buffer is nesessary for this mod? Went back to the 220k feedback with 220k on the + input. Sounds just as good and about 98% less noise!

[Tor M]

Originally posted by mrskinny
For gengcard- I tried this with my (unbuffered) inverted OPA549 amp.I used JoeR's parts values-10k+100r+10k and 10k on the + input. This does work, but I ended up with a huge amount of hiss thru the speakers.Don't know why it turned out this way-maybe the input buffer is nesessary for this mod? Went back to the 220k feedback with 220k on the + input. Sounds just as good and about 98% less noise!

Hi!
I think this ist way to much gain for the opa549.

OPA549 disappointing

Regards

Tor Martin

[Joe Rasmussen]

Originally posted by GregGC



Thanks Joe.
Because the I-offset is the problem, want would be the disadvantage then to still do what I proposed (DC gain of 5-10 and add AC gain with 47k FB res.) and fine-tune the resistor from +in to GND (not injecting voltage) to get the Output DC offset to zero. It sounds like the simplest way to solve the Offset issue. It'll create only a minor Z imbalance which shouldn't be a problem.

/Greg

That's exactly what we (at least four that I know off) were trying to avoid. Oh yes,I tried it.

The aim here was to keep the R & C the same and come up with a set of R values that will cope with a wide variety of offsets and yet (hopefully) have around or below 50mV DC offset.

The R value from (+) to gnd must match for best sound. If you use 47K in conjunction DC gain as low as you propose will not create a minor Z imbalance you suggest. It will be major. Indeed with 10K/100R/10K & 10K on (+) and 100 DC gain, to trim the DC offset may require the 10K to (+) to be trimmed as much as +/- 20%. If you use 47K base value and low DC gain, this will be even worse.

Also, please consider this: It's fine for us to come up with various esoteric one-off solutions, but there are those reading this who wouldn't be able to calculate AC gain of T-networks, let alone cope with major potential DC shifts it creates.

So: Rather than speculate I decided to spend a number my hours on the weekend, to come up with a VW solution... you know, the car for anybody who needs a car solution. This is more helpful than one-offs and maybe just as good. So we are not talking VW performance - just VW accessibility.

Sound wise it works and now anybody can try it.

Joe R.

[Joe Rasmussen]

Originally posted by Shoog
Hi there,
I have been following the posts about DC offset with interest. I noticed a higher DC component after the implementation of the T-network, but it dropped with a resistor to the non-inverting input. These measurements where made at idle. Am I to understand that the DC is created when the amp is been driven, and not when at idle. If so has anyone actually observed it as a problem ?? How much DC offset are we talking about ???

Help a poor soul to understand.

Shoog

Just measure the DC statically, no sound.

Can you post the values and the DC offset in mV. for both channels. This will be helpful feedback especially if you used the 10K based T-network. I have measured six 3875s with those values and so far below 50mV. I hope yours follow the same pattern.

BTW, The DC is always there, the AC is not. When AC settles (the sound dies down) the DC offset remains. The DC becomes the 0V reference for the AC. But the remaining DC will always push current through the Voice Coil of the driver, pulling the driver of centre (either slightly in or out - just like a battery does when applied to a speaker - try it), it also means the amp is wasting power that becomes expended as heat in both itself and the voice coil. So the DC needs to be kept low. Hope that helps.

Joe R.

[Joe Rasmussen]

Originally posted by mrskinny
For gengcard- I tried this with my (unbuffered) inverted OPA549 amp.I used JoeR's parts values-10k+100r+10k and 10k on the + input. This does work, but I ended up with a huge amount of hiss thru the speakers....

It's clearly unsuitable as hinted by others who read the spec sheet more carefully than I did.

Joe R.

[gengcard]

Hi mrskinny,

For gengcard- I tried this with my (unbuffered) inverted OPA549 amp.I used JoeR's parts values-10k+100r+10k and 10k on the + input. This does work, but I ended up with a huge amount of hiss thru the speakers.Don't know why it turned out this way-maybe the input buffer is nesessary for this mod? Went back to the 220k feedback with 220k on the + input. Sounds just as good and about 98% less noise!

O.K. give up OPA549 for T-network a moment. Let me start with LM3875 firstly.

[Terry_Demol]

Originally posted by mrskinny
For gengcard- I tried this with my (unbuffered) inverted OPA549 amp.I used JoeR's parts values-10k+100r+10k and 10k on the + input. This does work, but I ended up with a huge amount of hiss thru the speakers.Don't know why it turned out this way-maybe the input buffer is nesessary for this mod? Went back to the 220k feedback with 220k on the + input. Sounds just as good and about 98% less noise!

Mr skinny

WRT OPA549 noise:

Read the data sheet WRT IP voltage noise (70nV/rt Hz) and
current noise (1pA/rt Hz). Download one of the many application
notes from AD and work out the OP noise based on the IP
impedances and gain structure.

The current noise will interact with IP Z and be multiplied
by the gain of T network (100). Voltage noise will be RMS
summed to this also. At the end of it things get pretty
noisy just looking at some quick calcs.

It may also be oscillating, did you check this?


Cheers

Terry

[Terry_Demol]

Originally posted by Joe Rasmussen



The aim here was to keep the R & C the same and come up with a set of R values that will cope with a wide variety of offsets and yet (hopefully) have around or below 50mV DC offset.

The R value from (+) to gnd must match for best sound. If you use 47K in conjunction DC gain as low as you propose will not create a minor Z imbalance you suggest. It will be major. Indeed with 10K/100R/10K & 10K on (+) and 100 DC gain, to trim the DC offset may require the 10K to (+) to be trimmed as much as +/- 20%. If you use 47K base value and low DC gain, this will be even worse.

Joe R.

Joe,

Stop wasting time postulating on different gain
structures. What we really need to ascertain is how much
effect balancing the impedances to each IP makes *sonically*.
Usually it does improve sonics WRT opamps, especially
bipolar IP, however we don't know in this case.

My fully balanced / parrallel 3886 module is about 1/2 done.
It has exactly balanced everything, with a separate discrete
0 FB front end (with gain adj R).

The design is direct coupled IP to OP and hopefully offset
will not be a problem. Should be extremely linear but real proof
is in the listening so your JLTI is a good reference.

I'll give you a holler when it's done (hopefully by next week).

Unfortunately I will not get AP test set for a while so maybe
we can try your cleo.

Cheers,

Terry

[GregGC]

Originally posted by Joe Rasmussen


That's exactly what we (at least four that I know off) were trying to avoid. Oh yes,I tried it.

The aim here was to keep the R & C the same and come up with a set of R values that will cope with a wide variety of offsets and yet (hopefully) have around or below 50mV DC offset.

The R value from (+) to gnd must match for best sound. If you use 47K in conjunction DC gain as low as you propose will not create a minor Z imbalance you suggest. It will be major. Indeed with 10K/100R/10K & 10K on (+) and 100 DC gain, to trim the DC offset may require the 10K to (+) to be trimmed as much as +/- 20%. If you use 47K base value and low DC gain, this will be even worse.

Also, please consider this: It's fine for us to come up with various esoteric one-off solutions, but there are those reading this who wouldn't be able to calculate AC gain of T-networks, let alone cope with major potential DC shifts it creates.

So: Rather than speculate I decided to spend a number my hours on the weekend, to come up with a VW solution... you know, the car for anybody who needs a car solution. This is more helpful than one-offs and maybe just as good. So we are not talking VW performance - just VW accessibility.

Sound wise it works and now anybody can try it.

Joe R.

Thanks Joe!

Nice of you to give explanation. Sorry to waste your time.

/Greg

[Joe Rasmussen]

Originally posted by GregGC

Nice of you to give explanation. Sorry to waste your time.

/Greg

Hi Greg

I don't think of it as a waste of time, so don't worry - it gave an opportunity to present matters clearly. Those of us promoting T-networks would like as many as possible to try it, and now they can without trepidation.

Joe R.