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niss_man
diyAudio Member

Join Date: Sep 2008
Location: Brisbane
Quote:
 Originally Posted by Bigun Hi Terry, Let's look at those voltages you posted and see if we can narrow it down a bit more. The voltage across R10 is too high - clue 1. It says we have too much current flowing through it 8V/1k = 8mA - where is it going ? Well not through the base of Q8 (this wouldn't be normal for Q8 anyhow) since it would then appear through R11. The voltage across R11 allows me to calculate that we have only 0.2mA there. So it's going through Q7 (as it should) and then provides current for the LTP that sits on the negative side of the circuit. The current should then flow through R7 & R8 into Q3 & Q4. The voltage drop across R7 shows 14mV, so current = voltage / resistance = 14/22mA = 0.6mA which is about right. The voltage drop across R8 is 154mV, so the current is 7mA. Ah, now we know where all that current through R10 is going, it's going into the emitter of Q3. Q3 and Q4 measure the input signal (base of Q4) and the output signal (base Q3) and compare them - this is your feedback error amplifier. It's an LTP - Long Tail Pair. Invented by the Brit's during the war for radar it works very well. This design is a bit complex because it has two LTPs. This excessive current flow into the LTP will unbalance it and produce a dc-offset at the output. The other LTP, the upper one will try to fight this so it will reduce the amount of dc-offset but can't completely null it out. This is what you are seeing. But why is the current source flowing too much current in the first place. The finger points at the current source. This current source could be thought of as a simple feedback arrangement. Q8 monitors the current flow through R10 and throttles it back by modifying the voltage at Q7's base. So if Q8 isn't working properly we have a problem. Also, Q8 has D5 in it's emitter circuit which modifies how Q8 measures the current flow through R10. This is to allow the current to be further throttled back if the temperature of D5 increases. So if D5 isn't working properly we also have a problem. I'd look carefully at these parts, maybe replace D5 (a pitty as I notice you did a really neat job of wiring it up). It seems to me there is far too much voltage across D5 and I think this is where the issue lies. Try replacing D5, check it's not backwards or something, or that your version of the BD140 doesn't have a different pinout for some reason. By the way, since there is a feedback going on in the current source it is possible for the current source to oscillate and in my design I included a small 1nF capacitor between the base and collector of Q8 to slow it down.
This has got to be the best comment on these threads I have ever heard so far. Shows a great detail of knowledge of amplifiers and the process in which to fault find them properly. Thanks Bigun in spending your time to share this wealth of knowledge. Now I am off to print the schematics and follow your process to learn something as well.

Regards

Simon

Stuey
diyAudio Member

Join Date: Mar 2006
Location: Perth, Western Australia
Quote:
 Originally Posted by still4given I bought them for a seller named alweit on ebay.
Ahh, yes. He sold me some jfets once; good seller.

Bigun
diyAudio Member

Join Date: Jan 2009
Location: Waterloo, ON or Herefordshire
Quote:
 Originally Posted by niss_man Thanks Bigun in spending your time to share
It's great to see I was of help. I also like to learn what I can from the folk on this forum, it makes the hobby fun for me.
__________________
"The test of the machine is the satisfaction it gives you. There isn't any other test. If the machine produces tranquility it's right. If it disturbs you it's wrong until either the machine or your mind is changed." Robert M Pirsig.

still4given
diyAudio Member

Join Date: Oct 2004
Location: Victorville, CA
Quote:
 Originally Posted by AndrewT Still4, you are no further forward than when you started 9years ago. You have confused your self this time around by rushing to build 3 or 4 amplifiers at the same time. That rushing into multiple builds has incorporated avoidable mistakes. Slow down. Concentrate on one build and LEARN what is happening. It is no excuse to say it is "Greek" !
With all due respect, I am much further along than I was 9 years ago.
I was diagnosed with cancer last year. I will decide how slowly or quickly I choose to build amps. I learn something new with each build. I will not be taking any electronics courses so it may take me a little longer to learn it than those of you who did. If my ignorance bothers you, please ignore my posts.

Regards

pinnocchio
diyAudio Member

Join Date: Sep 2010
Location: Montreal
Quote:
 Originally Posted by still4given With all due respect, I am much further along than I was 9 years ago. I was diagnosed with cancer last year. I will decide how slowly or quickly I choose to build amps. I learn something new with each build. I will not be taking any electronics courses so it may take me a little longer to learn it than those of you who did. If my ignorance bothers you, please ignore my posts. Regards
The only important things in here is first get better and second have fun with what you're doing, after all it is a hobby.

All the best!
Do
__________________
PREAMPs : Bal AMB α10 + JC-80 | Bal DCB1 | Bal LDR3x | AMPs: AMB β24 | ExtremA | FO v1.4 | USSA5 | TSSA | Telos 390 Clone | NX Amp | FC-100
DACs : TPA Buffalo II + OPC's NTD1 I/V | JLsounds | Mirand AK4490 + Discrete Output Speakers : Troels Gravesen ATS-4 | Sagwee LT V3

 20th October 2013, 11:58 PM #306 AKSA   diyAudio Member   Join Date: Sep 2001 Location: Melbourne, Australia Terry, Get better. I presume you've been through the treatment, harrowing, but behind you now? Electronics should purely be enjoyable, no stress........ Hugh __________________ Aspen Amplifiers P/L (Australia) www.aksaonline.com
 21st October 2013, 03:21 AM #307 still4given   diyAudio Member     Join Date: Oct 2004 Location: Victorville, CA Thanks guys. Yes, went through the treatments. So far NED but left with neuropathy and no saliva. Big wake up call. The days we have are a gift and numbered. I plan to make the best of them. I do this for fun. I am learning as I go. It has been great meeting all you fine folks too. That is the best part of this hobby. Blessings, Terry
 21st October 2013, 09:58 AM #308 Stuey   diyAudio Member   Join Date: Mar 2006 Location: Perth, Western Australia Andrew, you are no further along in that time, in learning that everyone has different goals and works in different ways in this hobby. You must take this on board and stop lecturing on this forum. If you don't wish to help, please don't post, rather than telling people off.
 21st October 2013, 10:04 AM #309 TonyTecson   diyAudio Moderator     Join Date: May 2003 Location: Palatiw, Pasig City come on guys, DIY is supposed to be fun and that fun is diminished when one imposes on another, let this serve as a warning,,,,, __________________ planet10 needs your help: Let's help Ruth and Dave...http://www.diyaudio.com/forums/plane...ml#post5010547[B
Stuey
diyAudio Member

Join Date: Mar 2006
Location: Perth, Western Australia
Quote:
 Originally Posted by Bigun It's great to see I was of help. I also like to learn what I can from the folk on this forum, it makes the hobby fun for me.

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