T-network: the better feedback solution?

[Upupa Epops]

Franz, do you mean, that by inverting mode is on + input only DC bias current without any fluctuations ? If both inputs are working by the same conditions ( AC and DC ), theoreticaly, but practicaly also, is in this case distortion lowest, 'cos all is balanced.

[carlosfm]

Originally posted by Franz G
Carlos:
Noise and distortion are not the same thing, as you surely know!

Franz, distortion you can measure, but do a favour to yourself: try a 220k resistor from NI to ground and tell me you you listen to noise or not.
Hiss.
LOUD AND CLEAR.

[Sheldon]

Originally posted by Sheldon
Ok, here's a puzzle for the experts. It's regarding the 4780 amp described a couple pages back - http://www.diyaudio.com/forums/showt...637#post502637

I changed the buffer to a low value resistor for the LP (110R, 30nf, 20k input) and was doing some listening tests. I noticed that the right channel was a little louder, so I started doing some checking. I disconnected the speakers and checked the offset, which was about -30mv on the right (relative to ground) and about +30mv on the left. But if I turn the amp off and then turn it back on, I get -21v (yes volts) on the right, but 30mv on the left (same as before). This doesn't happen with a load connected, and if I turn it on without a load and briefly short the output, the offset drops back to -30mv and stays there. If I let it run at -21v (no load), the amp heats as if I were driving it at fairly hard.

It gets more odd. If I ground the output of the buffer and then turn the amp on, the offset is -30mv. If I turn the amp on, then ground the buffer, the offset starts at -21v, then drops to -30mv when I ground. And here's the really odd part: If I switch the buffer outputs left to right, I get the low offset on both channels. So, only when the right buffer is connected to the right channel do I get a no load -21v offset. BTW, the supply voltage is about +/- 24.

Some sort of oscillation?

Sheldon

Well, another bit of info, and a solution. But I still don't understand it.

The new info is that the problem does not occur if the buffer is not connected when the main ps is turned on (remember the buffer is always on, ala Joe R's tube buffer). If I turn on the ps and then connect the buffer, it acts normally. The offset is even a little lower. If I mute the amp during turn on, no problem. I had thought it unnecessary, at least for the turn on thump. Not so, at least with this chip. Some sort of weird latch up (one one channel only and only with one of the buffers) if the buffer is on while the amp turns on.

Back to see what I can do with the NE555 and a relay.

Sheldon

[Franz G]

Franz, distortion you can measure, but do a favour to yourself: try a 220k resistor from NI to ground and tell me you you listen to noise or not.
Hiss.
LOUD AND CLEAR.

Whats the matter, Carlos? I dont want noise, so why should I try 220k?

Franz

[carlosfm]

Originally posted by Franz G
Whats the matter, Carlos? I dont want noise, so why should I try 220k?

Franz

Franz, I was just explaining you that the higher the value of that resistor, the more noise you have.
Of course you don't want that.
You can use 22k, still a low value.
And you will have matched impedances.

[Sheldon]

Originally posted by Sheldon


Well, another bit of info, and a solution. But I still don't understand it.

The new info is that the problem does not occur if the buffer is not connected when the main ps is turned on (remember the buffer is always on, ala Joe R's tube buffer). If I turn on the ps and then connect the buffer, it acts normally. The offset is even a little lower. If I mute the amp during turn on, no problem. I had thought it unnecessary, at least for the turn on thump. Not so, at least with this chip. Some sort of weird latch up (one one channel only and only with one of the buffers) if the buffer is on while the amp turns on.

Back to see what I can do with the NE555 and a relay.

Sheldon

Final note. The delay worked (simple 9k mutes to ground with 100uf cap. to ground, gives about 1 sec delay). However, if I hook up a preamp and turn up the gain to a little loud position, the offending channel latches up to -21v. Not so good for speakers. The only conclusion is a defective channel on the chip. Time for a replacement.

Sheldon

[gengcard]

From my last assumption, it's correct. After I removed the volume out and made the amplifier to be audio power amplifier, it can now sing a song. without any noise. I thought T-network amplifier is as well as non inverting amplifier after my testing. I like its sound.

[Franz G]

I thought T-network amplifier is as well as non inverting amplifier after my testing. I like its sound.

Great! I like this statement.

Do you have a cool room? Then I could come "ti nai krungtheep" to listen to your setup !

Sawadee
Franz

P.S.
My youngest daughter, 6 years old, is speaking swiss german and thai.

[gengcard]

Sawasdee Krub,

"Ti Nai Krungthep" is mean "Where is Bangkok?" but for correct word, you should say " Krungthep Yoo Ti Nai".

I don't have a cool room in my home but at the future I will.

I planned to modify my amplifier by changing some R values and some capacitors. I will change some R values like Carlos because Gaincard input impedance is 22k, however, I must calculate new three feedback resistors again to match with the new input impedance. So I must change the DC coupling capacitor from 4.7uF to maybe 2.2uF. Will it be good idea?

[Franz G]

I am a farang and with

"Ti Nai Krungthep" is mean "Where is Bangkok?"

I wanted to say: I could come to Bangkok and visit you to check your gainclone quality.

Lowering the value from the coupling cap will always be a good idea, (at least, as it is easier to buy good quality caps with lower values) as far as you dont have a too high rolloff frequency from the resulting HPF together with the following input Z.

Regards
Franz