It's been over 10 years since I built my first (and only) amplifier - the original zen. I decided it was time that I got back into it so I thought I'd have a go at the zv9. I have built the amp with a mix of veroboard and ptop wiring (to the mosfets and jfets) and am powering it (for the moment) with a 48V, 350W SMPS. The PS is separate from the amplifier and delivers 40V DC (I detuned the supply voltage to the lowest possible) via an umbilical cord. I thought this setup would probably introduce some noise but might still work.
I am working on one channel at a time and have run into some weird problems. When I try to set the bias and adjust P1 to give 20V (mid PS voltage) at the drain of Q2 it gets to either 15V or 25V (depending from which side I am approaching from) and then jumps to about 35V before settling back down to the 15V or 25V - I cannot get to 20V. I thought maybe the mosfets (IRFP240) were cactus so I tested for voltage drop across the series gate resistors (R12 and R15). I read that there shouldn't be any current passing through these and there wasn't - can someone confirm I interpreted that advice correctly? I don't have an oscilloscope so I decided to connect an old junk speaker to the output and see what happened. There is a weird noise like you get when trying to tune an old style radio. I tried to play some music through it, it works but the volume is very low even with the source cranked up. Both transistors get hot but Q3 is definitely hotter than Q2. Sorry that's about all I have observed so far.
Hopefully someone can point me in the right direction as I am at a loss to what is going on. I am open to buying an oscilloscope but I don't want to spend a fortune and have no idea what make/model would serve my needs. Any advice there would be welcome as well. Thanks.
Dan.
I am working on one channel at a time and have run into some weird problems. When I try to set the bias and adjust P1 to give 20V (mid PS voltage) at the drain of Q2 it gets to either 15V or 25V (depending from which side I am approaching from) and then jumps to about 35V before settling back down to the 15V or 25V - I cannot get to 20V. I thought maybe the mosfets (IRFP240) were cactus so I tested for voltage drop across the series gate resistors (R12 and R15). I read that there shouldn't be any current passing through these and there wasn't - can someone confirm I interpreted that advice correctly? I don't have an oscilloscope so I decided to connect an old junk speaker to the output and see what happened. There is a weird noise like you get when trying to tune an old style radio. I tried to play some music through it, it works but the volume is very low even with the source cranked up. Both transistors get hot but Q3 is definitely hotter than Q2. Sorry that's about all I have observed so far.
Hopefully someone can point me in the right direction as I am at a loss to what is going on. I am open to buying an oscilloscope but I don't want to spend a fortune and have no idea what make/model would serve my needs. Any advice there would be welcome as well. Thanks.
Dan.
most likely culprit is SMPS - do ypu have cap bank at amp's side ?
can you try your actual build with PSU of your Zen amp ?
can you try your actual build with PSU of your Zen amp ?
No, no cap bank. The large value caps I have on hand are too low a voltage to use with this PS (35V). I have a 19V laptop PS that I can use (it's 4A so maybe just one channel) and then I can throw 35,000uF or more at it.
I figured that I could at least get a workable amp using a SMPS following the lead of the ACA #1 which uses a SMPS sans large smoothing caps. Unfortunately I can't use the zen amp PS as I dismantled it many years ago when the kids started crawling about. I'll try the laptop PS and report back.
Can anyone comment on the use of a SMPS and perhaps how it sounds with the ACA #1? Thanks.
I figured that I could at least get a workable amp using a SMPS following the lead of the ACA #1 which uses a SMPS sans large smoothing caps. Unfortunately I can't use the zen amp PS as I dismantled it many years ago when the kids started crawling about. I'll try the laptop PS and report back.
Can anyone comment on the use of a SMPS and perhaps how it sounds with the ACA #1? Thanks.
Sounds fine, but the SMPS has to have a large current reserve and its
output needs to be filtered for noise.
😎
output needs to be filtered for noise.
😎
The SMPS has over 7A @ 48V so I figured it should be able to supply 2 channels @ up to 2A each. When you say - filtered for noise - do you mean capacitors or something more sophisticated? I will try 19V (120W) supply with large capacitors (35,000uF - 35V) and see if that works. If you could let me know what size capacitors I can get away with I will buy some 50V caps for the larger supply. Many thanks.
Success! Well, sort of. The weird noise is gone and it now plays nice music at a good level. However, with a 50k pot (p1, 25k not available) turned all the way up (I.e. To 50k) I get 14.5V at the drain of Q2. I don't really understand the circuit well enough to figure this out but I suspect that I would need a larger pot to achieve 10V. I won't try for this as I really want to get the 40V PS working. I used 35,000uF for the filter caps in this test but I'll try something much smaller (maybe 1000uF) when I repeat with the 40V PS. Thanks for helping.
- Status
- Not open for further replies.