T-network: the better feedback solution?

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#361
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.
 
#363
Re: Puzzling problem, experts needed

Sheldon said:
Ok, here's a puzzle for the experts. It's regarding the 4780 amp described a couple pages back - http://www.diyaudio.com/forums/showthread.php?postid=502637#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
Click to expand...

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
 
#366
Re: Re: Puzzling problem, experts needed

Sheldon said:


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
Click to expand...

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
 
#367
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. :angel:
 
#368
I thought T-network amplifier is as well as non inverting amplifier after my testing. I like its sound.
Click to expand...

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.
 
#369
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? 😕
 
#370
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. :angel:

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
 
#372
Testing...

Well... started listening test today to the LM1875 amp in conventional inverting topology.
Very very nice.
Back to the bench, change resistors to T-network, check DC, fine.
Back to the main system.
Power on.
Mmmm... there is a small degree of better treble precision.
I don't mean more detail, I mean precision.
But even if it's not night and day, it's an improvement and definitely this one is gonna end as an inverting T-network amp.😎

I'm also testing class-A bias, but that's a matter for Mike's thread, I will post my results right now here:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=28372&perpage=10&pagenumber=3
 
#373
Sawasdee Franz,

Now I have Panasonic 4.7uF BP AY series but I want a good cap value is 2.2uF for coupling to connect with R 22K that I will get the low cutoff frequency to be about 3.7 Hz.

Will you come here in soon? If you come here, please tell me, I will guide you some nice places in BKK.

Hi Nuuk,

Did you visit Pat pong?
 
#376
Gengcard

ROE must be Roederstein, a good caps brand. Just google for Roederstein.

Sorry, I don't have the time and money actually, to fly to Bangkok.

And I think, I immediately would pass by to Kho Samui / Kho Phang Ka to relax some weeks 😎

Regards
Franz
 
#378
Did you visit Pat pong?
Click to expand...

Many years ago (last time in 1983) but just to check it out for Franz you understand! 🙄

I don't think that I would want to visit Kho Samui now. I can remember it before the airport was built when there were no hotels outside of the town! 😎

And when I stayed on Haat Nai Haan (Phuket) there were only a few beach huts and no hotels for miles.
 
#379
I think it should be RDE capacitor certainly, I just found its picture in ebay website. It's so hard to find out the datasheet. I just want to know it is a non-polarized capacitor or not.

I have ever visited Kho Samui once a time. It's a good island in Thailand.
 
#380
I spent a very nice christmas in Chaweng Buri, Kho Samui.

An airconditioned bungalow nearby the beach! It still was great at this time. The airport is in fact one of the prettiest airports in the world, imho.

This was in the middle of the 90ies.

Back to caps:

ROE Logo

In this link, you will see the Roederstein Logo ROE.

Are you still insisting on RDE?

Franz
 
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