Then that drawing is very confusing. I don't get that from what is shown. I thought the brackets depicted measuring across a resistor. And even then the numbers are way off. One more thing. I have 800mv at the base of the outputs. You show it should be double that. Won't lowering the bias bring it down even more? Maybe my devices are bad. Do you recall what the HFE of your BCxxx devices were?
Thanks, Terry
Hi Andrew,
I don't think the vbe multiplier is working properly. I changed the 220 to 1k1 (handy). I left the 2k pot for now. Bias was at 398mv across the degens with pot maxed. With bias set to 460mv the pot is still at 1k2. I also have a bit of noise. I see 120mvac on the output with the input shorted. If I plug in a speaker I hear a pretty decent hum. On the scope I see a sine wave. something is just not right. I checked all of the transistors and see between .6V and .63V vbe so it looks like they are at least turned on. I'll keep checking but not much joy so far.
To your knowledge, has anyone else beside you built this amp? The sx is in the title but until now the thread has mostly been about the nx.
Thanks, Terry
I don't think the vbe multiplier is working properly. I changed the 220 to 1k1 (handy). I left the 2k pot for now. Bias was at 398mv across the degens with pot maxed. With bias set to 460mv the pot is still at 1k2. I also have a bit of noise. I see 120mvac on the output with the input shorted. If I plug in a speaker I hear a pretty decent hum. On the scope I see a sine wave. something is just not right. I checked all of the transistors and see between .6V and .63V vbe so it looks like they are at least turned on. I'll keep checking but not much joy so far.
To your knowledge, has anyone else beside you built this amp? The sx is in the title but until now the thread has mostly been about the nx.
Thanks, Terry
It is correctly connected.
It is measuring the voltage drop across the OPS emitter degen resistors and then controlling the bias to that the OPS current is ~700mA (so total ~1.4A for two pairs of output devices)
Maybe I should have asked you before I proposed a change: was the problem with the 1k pot and the 220 Ohm resistor that you could not adjust it high enough?
(Yours will be the 4th build - I've built 3 - a proto and two fimal channels)
From 0V to each of the output bases you should measure about 800-900 mV (0.33 x 700mA + 0.6V vbe). So total from base of NPN to base of PNP ~1.7V
Maybe I was not clear on that - sorry.
The brackets indeed do show measurements across resistors where this is applicable.
It is ok than, for 0.7A OPS current.
R27+R26 voltage is ~0.46V. Add ~0.6V Q1be.
So, R6+R20 ~0.6V and on R21 should be ~0.46V.
That is, (R27+R26)=(R21*(0.6/0.46))= ~1.3k.
Little bigger actually, because of Q3 base current.
Even there is resistor 220ohm, 1k pot is not sufficient.
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He, he....... Terrybadjustmrnt should be my screen name.
I got it! It is playing lovely music.
I settled on a 2K trimmer and 1k1 for R20. A 1K pot would be maxed out and still maybe a little high. I'm not sure how you got away with the values you used. Maybe my 0R33 resistors measure different than yours. It is very difficult to get an accurate measurement of such low value resistors.
I've only had it playing for about 30 minutes so I can't tell how hot it will get but so far no problems. I'm using a 12-0-12vac transformer and getting about +-17Vdc rails. I've only listened to jazz so far but it is very clear and full. Good solid bass and clear highs. It sounds just as it should. Thanks for sticking with me and helping me get this working. It was worth it.
Blessings, Terry
Attachments
You're probably wondering why I'm asking this question since my PSU was already all working... I decided to build another PSU to go dual mono and test both setup.
Ciao!
Do
I've been listening to it all day. Beautiful. I have this huge 12-0, 12-0, 12-0, 12-0 1200VA transformer that I bought for the Aleph-X a few years ago. I've been looking for something good to use it on and I think this is it. It is giving me +-16.5V at full draw. I'm splitting the 4 outputs into two separate bridges feeding 4x22,000uF CRC filters. It is staying in Class A almost all the way to unity gain.
I recommend it!
I recommend it!
I think Alex made this-.see previous page for files. Otherwise,mi can only do it 2 weekends from now. Travelling on business.
Thanks , a lot ,also I suggest you to mount Q3 under PCB in contact with heatsink ......
Regards,Alex