Both channels behave the same way and both sound equally bad. I used 25 turn trimmers (so glad I didn't used single turn) and there is plenty of adjustment left either way. The pinouts are the same for both the ZTX and the 2Ns, this is something that I checked carefully before inserting.
I always clean up the underside of boards with a small bristle brush and meths after soldering and then inspect with a magnifying glass. I think it would be unlikely that both boards would have the same bad joint but I do appreciate any suggestions as such.
I have been using 3 meters (wish I had 4!) to monitor as many parameters as I could at the same time. As you say Wdecho, I wasn't too worried about the voltage offset at T18 (speaker output I could get to zero) it is the high level of distortion that really bothers me.
Thanks for your input so far, any help is welcome.
I always clean up the underside of boards with a small bristle brush and meths after soldering and then inspect with a magnifying glass. I think it would be unlikely that both boards would have the same bad joint but I do appreciate any suggestions as such.
I have been using 3 meters (wish I had 4!) to monitor as many parameters as I could at the same time. As you say Wdecho, I wasn't too worried about the voltage offset at T18 (speaker output I could get to zero) it is the high level of distortion that really bothers me.
Thanks for your input so far, any help is welcome.
You obviously have a common problem on both boards. I always suspect an active component if I am sure I have correct values of passive components. There are not that many active components minus the V-fets. You may want to change some out. I would probably change the J-fets last being they are more expensive. With proper test equipment you could narrow down the problem to a specific point but I like others do not use an oscilloscope even though I have one.
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Thanks wdecho, I measured the Jfets before installation and they seemed fine, with around 9mA Idss and around 25ish for transconductance. I'm unable to look at it again until tomorrow so hopefully by then someone else may have some thoughts.
I did check the voltages across R34/35 which measured around 2.1 volts which would give a current of around 9 mA flowing along the circuit containing the Jfets and the 2n5551/5401. This fits in with the Idss above.
I did check the voltages across R34/35 which measured around 2.1 volts which would give a current of around 9 mA flowing along the circuit containing the Jfets and the 2n5551/5401. This fits in with the Idss above.
Another thoughts.
I carefully examined 2SJ74BLs, which I bought recently, thought these are genuine.
I'm not so sure now.
I have some originals, bought some years ago, these are genuine ones, without a doubt.
Newer ones getting suspicious, because:
1. These has a number 7 with straight leg, while originals has curved leg.
2. My genuine parts has a diameter 3.9mm, while these new parts has only 3.6mm.
When I purchased these, I measured Idss, seemed O.K., with genuine looking shine surface and title.
I did not know then that straight-leg of 7 may be suspect.
I carefully examined 2SJ74BLs, which I bought recently, thought these are genuine.
I'm not so sure now.
I have some originals, bought some years ago, these are genuine ones, without a doubt.
Newer ones getting suspicious, because:
1. These has a number 7 with straight leg, while originals has curved leg.
2. My genuine parts has a diameter 3.9mm, while these new parts has only 3.6mm.
When I purchased these, I measured Idss, seemed O.K., with genuine looking shine surface and title.
I did not know then that straight-leg of 7 may be suspect.
My VFET kit and Supplemental Kit are in transit, so it is time to begin planning in detail
I would like to build mono-blocks in Mini Dissipante 3U 400mm case, which seems ok.
I would also like to build CLC, or possibly even LCLC, PSU for +28V and -28V rails. I have read through the docs, and skimmed back through the thread, but can't find details of total current draw per rail, either for stereo pair or per channel.
Could anybody who has already built one give me an idea of DC current draw?
Many thanks
I would like to build mono-blocks in Mini Dissipante 3U 400mm case, which seems ok.
I would also like to build CLC, or possibly even LCLC, PSU for +28V and -28V rails. I have read through the docs, and skimmed back through the thread, but can't find details of total current draw per rail, either for stereo pair or per channel.
Could anybody who has already built one give me an idea of DC current draw?
Many thanks
Well, I have got it!
Those 2SJ74BLs I have purchased recently are fakes!
With my really genuine parts, it is working like a charm!
I can get offset below 5mV at T18.
With using one multimeter only.
John, your JFETs are genuines?
Great
Thanks Nelson, much appreciated. I was doing all of the checking etc on our dining table but had to clear it all away yesterday as we had a dinner party last night (from which I am still feeling the after effects) but I will set it up again soon and report back. The voltages T6 to T7 and T8 to T9 were set at the required 1.5V but I will check the other 2 as well.
ColinA
Yes, that would have been a real talking point, unfortunately management had other ideas!
Nelson
Ok it's good to know that the 200mV T18 offset will not be a problem.
Here are the voltages that you requested plus a few others. The amp was allowed to warm up for about 1 hour before taking readings plus the voltages across R5 and R6 were adjusted to as near 1.5v as possible.
T7 to T10 = 3.93V
T9 to T12 = -3.67V
T6 to T11 = 6.7V
T8 to T13 = -6.72V
G to T2 = 24.3V
G to T6 = 26.5V
G to T8 =-26.8V
G to T15 = -12.8V
G to T17 = -8.55V
G to T19 = 12.1V
G to T16 = 10
G to T20 = -10.4
Both channels similar except for T18 offset which was 129mV for R ch and 206mV for L ch. Output offset was 17mV R ch and 6mV L ch.
I'm somewhat concerned about the apparent drop in the DC rail voltage to 26.5 as the AC secondary under load is still showing 22.6V which when multiplied by 1.414 gives 31.96V less the bridge diodes of say 1.2V should give around 30.75V with no load so somehow I am losing around 4V for each rail. The power supply is a CLCLC with hand wound toroid chokes so maybe there is an unacceptable voltage drop across these.
Any comments welcome.
Regards, John
Yes, that would have been a real talking point, unfortunately management had other ideas!
Nelson
Ok it's good to know that the 200mV T18 offset will not be a problem.
Here are the voltages that you requested plus a few others. The amp was allowed to warm up for about 1 hour before taking readings plus the voltages across R5 and R6 were adjusted to as near 1.5v as possible.
T7 to T10 = 3.93V
T9 to T12 = -3.67V
T6 to T11 = 6.7V
T8 to T13 = -6.72V
G to T2 = 24.3V
G to T6 = 26.5V
G to T8 =-26.8V
G to T15 = -12.8V
G to T17 = -8.55V
G to T19 = 12.1V
G to T16 = 10
G to T20 = -10.4
Both channels similar except for T18 offset which was 129mV for R ch and 206mV for L ch. Output offset was 17mV R ch and 6mV L ch.
I'm somewhat concerned about the apparent drop in the DC rail voltage to 26.5 as the AC secondary under load is still showing 22.6V which when multiplied by 1.414 gives 31.96V less the bridge diodes of say 1.2V should give around 30.75V with no load so somehow I am losing around 4V for each rail. The power supply is a CLCLC with hand wound toroid chokes so maybe there is an unacceptable voltage drop across these.
Any comments welcome.
Regards, John
Sorry, don't have a light bulb tester, whatever they are. I have already decided to change the power supply to the standard Firstwatt type by using the one from my F4.