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MOSFET-MAX tube biasing issues (Solved, now about GrubDAC issues) - Page 2

post #16 of 55

I agree with jdkJake ... plus - did you confirm that you indeed have the matched quad set of Toshiba JFETs - the 2SK170/2SJ74's?  If not, they do NOT need to be reversed.

 

Please confirm whether you had the Toshiba JFETs or you just used the 2N5087/2N5088's for QB2 and QB3.

 

Also:

1. The "bridge" you mention is simply a scrape in the PCB's FRP.  It won't affect anything and didn't touch a trace as best I can see.

2. Let me repeat jdkJake's emphasis on  trimming the leads and cleaning things up.  It sure looks like a couple of those leads are touching/shorting, especially around the CCS sections (RA8, RA9, and the two Wima capacitors and 2N5087's for each channel to the outside of the tubes)..

3. I'm concerned with the burn marks on the small transistor pads and the power MOSFET pads in the middle.  It looks to me like those are not soldering burns and perhaps you fried a lot more when you had the buffer trimmers set to zero resistance.  I'm thinking the entire bank of small transistors may need to be replaced.

post #17 of 55
Thread Starter 

Wow, guys, I cleaned the pcb and trimmed some protruding leads on the back side, and for some reason, it magically works now! (I also replaced QA2 and QA3 yesterday but that didn't help) Anyway, the only things bothering me now are some hum (I imagine that gets better when it's in the case) and the push-down connectors, which I can't seem to work properly, do you have any tips on that? also, could the missing compensation caps be a contributing factor to the hum?

By the way, the burn marks are just burnt up flux from the desoldering braid, that stuff leaves horrible marks. And the bridge I was talking about is between the two grounded contacts of RB12R, but as I (and maybe you too, not sure I understood you correctly) said, it just follows the trace.
Edited by hitman47 - 9/28/11 at 6:55am
post #18 of 55
Quote:
Originally Posted by hitman47 View Post

Wow, guys, I cleaned the pcb and trimmed some protruding leads on the back side, and for some reason, it magically works now! (I also replaced QA2 and QA3 yesterday but that didn't help) Anyway, the only things bothering me now are some hum (I imagine that gets better when it's in the case) and the push-down connectors, which I can't seem to work properly, do you have any tips on that? also, could the missing compensation caps be a contributing factor to the hum?

By the way, the burn marks are just burnt up flux from the desoldering braid, that stuff leaves horrible marks. And the bridge I was talking about is between the two grounded contacts of RB12R, but as I (and maybe you too, not sure I understood you correctly) said, it just follows the trace.


There is no hum in the design.  If you hear it, it's either that you don't have the input signal wires correctly grounded (very likely if you're having trouble with the punch-down blocks), or you have a bad tube(s).

 

The punch down blocks should be straightforward, unless you're trying to use a much thicker gauge than 22 ga.  Simply strip the wire for an exposed length that's slightly longer than the width of the punch-down, and make certain that it straddles both ends of the punch-down block frame.  Then, push down on the punch.  It won't work if the insulation is jumbled up in the connection.
 

 

post #19 of 55
Thread Starter 

Hm, I guess I'll try again tomorrow. I got the punch down blocks to work fine today, so I don't think it's the grounding on the input wire, but perhaps the contact is still not optimal. It might also have been interference from my cellphone because I was using that as a source to try it out and when I got too close with it, the amp started acting up. Or maybe it's because the phone's DAC/Opamp suck. Anyway, thank you very much for your help (and your patience with a noob like me)!


Edited by hitman47 - 9/28/11 at 5:39pm
post #20 of 55



 

Quote:
Originally Posted by hitman47 View Post

Hm, I guess I'll try again tomorrow. I got the punch down blocks to work fine today, so I don't think it's the grounding on the input wire, but perhaps the contact is still not optimal. It might also have been interference from my cellphone because I was using that as a source to try it out and when I got too close with it, the amp started acting up. Or maybe it's because the phone's DAC/Opamp suck. Anyway, thank you very much for your help (and your patience with a noob like me)!


There are very few amps, if any, that can tolerate cell phones nearby.  That said, you may want to qualify your description of "hum."  Hum is specific and is 60Hz.  Sometimes people include ripple (120Hz) in that category, too, but both are power-related unless you've placed the amp directly on top/next to another electronic device's transformer.  Otherwise, you may be mistaking "hiss" with hum.  You can get hiss if you're using very efficient, low-impedance headphones and should probably put some output resistors in the circuit.

 

Note that I recommend 22R output resistors at a minimum on the MOSFET-MAX (RB14-L/R).  It seems the MOSFETs will oscillate with lower resistances or jumpers in the RB14 positions.

 

You might want to check that.

 


Edited by tomb - 9/28/11 at 5:46pm
post #21 of 55
Thread Starter 

No, it was definitely a very low, consistent hum. It might also be because I'm having to use a step down transformer which probably isn't shielded properly (got it from China) with the wall wart, since that only works on 110V and I live in Europe. It's rather close by so that might be the problem

post #22 of 55
Quote:
Originally Posted by hitman47 View Post

No, it was definitely a very low, consistent hum. It might also be because I'm having to use a step down transformer which probably isn't shielded properly (got it from China) with the wall wart, since that only works on 110V and I live in Europe. It's rather close by so that might be the problem

Ah-ha - yeah, the tubes themselves will pick up hum from an external source, much less the rest of it.  Try to keep the separation distance as far apart as possible.

 

 

post #23 of 55
Thread Starter 

Yes, that's what I'm planning on doing once I set it up at its final position. Right now I'm still working on the case. Do you happen to know a good (and cheap) source for 24VAC supplies that run on 220V?

post #24 of 55

Sorry, no - there's not much call for that over here in the US.  I've heard some other overseas MAX builders mention RS Components and Jaycar.

post #25 of 55
Thread Starter 

OK, it's not so important really. I noticed the hum was only there when I had no input connected (which means it wasn't really "hum" after all). But now I've noticed another small problem: I can't seem to get the power supply voltage above 25.5V. The trimmer's not busted since I can decrease the voltage OK, but no matter how much I turn the trimmer, I can't get it above 25.5. Is that just bad luck with my wallwart or the crap transformer I have or is there still some mistake I haven't fixed yet on the board? Also, when I flip the input source selector switch, nothing happens, isn't the sound supposed to turn off or does it only work when there's an input connected to both input terminals? By the way, I also found and installed the compensation caps, turns out I did order them after all. The sound from the amp is great, although I only have bad input sources (PC and cellphone) and no really appropriate headphones (running a pair of porta pros from it) to be able to properly judge it.

 

Edit: After listening to it again, I have found that there's two kinds of noise that (as far as I know) shouldn't be there: There's a very quiet background hum on both channels that doesn't change with volume or go away when I connect a source, and then there's some very diffuse  static-like noise when I don't have an input hooked up to the amp. When I don't have a source connected, it also produces noise when I touch the case or the volume pot, and I can regulate the volume of the noise with the volume pot. Sometimes, there are also some sounds in there that sound like the sound a truck makes when it's going backwards or something.

Edit: Seems the second noise is normal. The first definitely isn't though... I wonder what's causing this

 

Another update: The hum disappears when I turn the power supply down to around 23V, for some reason. I have no problem with this, as the amp sounds good to me, but I'm still curious as to why this is the case... Is there something wrong with the power supply section of my build?


Edited by hitman47 - 9/29/11 at 4:09pm
post #26 of 55



 

Quote:
Originally Posted by hitman47 View Post

OK, it's not so important really. I noticed the hum was only there when I had no input connected (which means it wasn't really "hum" after all). But now I've noticed another small problem: I can't seem to get the power supply voltage above 25.5V. The trimmer's not busted since I can decrease the voltage OK, but no matter how much I turn the trimmer, I can't get it above 25.5. Is that just bad luck with my wallwart or the crap transformer I have or is there still some mistake I haven't fixed yet on the board? Also, when I flip the input source selector switch, nothing happens, isn't the sound supposed to turn off or does it only work when there's an input connected to both input terminals? By the way, I also found and installed the compensation caps, turns out I did order them after all. The sound from the amp is great, although I only have bad input sources (PC and cellphone) and no really appropriate headphones (running a pair of porta pros from it) to be able to properly judge it.

 

Edit: After listening to it again, I have found that there's two kinds of noise that (as far as I know) shouldn't be there: There's a very quiet background hum on both channels that doesn't change with volume or go away when I connect a source, and then there's some very diffuse  static-like noise when I don't have an input hooked up to the amp. When I don't have a source connected, it also produces noise when I touch the case or the volume pot, and I can regulate the volume of the noise with the volume pot. Sometimes, there are also some sounds in there that sound like the sound a truck makes when it's going backwards or something.

Edit: Seems the second noise is normal. The first definitely isn't though... I wonder what's causing this

 

Another update: The hum disappears when I turn the power supply down to around 23V, for some reason. I have no problem with this, as the amp sounds good to me, but I'm still curious as to why this is the case... Is there something wrong with the power supply section of my build?

It's possible that you have the input wiring messed up.  There should definitely be a difference if flip the relay switch.  Noise may occur through crosstalk in the relays if the second set of input terminals are not connected to anything, you have a strong source connected to the other side and it's on, and then you switch to that side with no source connected.  As stated before, the background hum is most likely your step-down transformer.  Also, the Alps pot must be grounded, or you will get noise when you touch the pot, as you noticed.  Your earlier pics showed a ground wire connection, but I haven't seen anyone solder to the pot screw before.  (You should only screw the wire down at the pot.)  It may be that you have a bad connection.

 

As for your voltage, yes - step-down transformers used overseas are known for lowering the available voltage through the MAX.  What happens is that the step-down transformer steps all the way down to 110V, whereas the voltage in many US homes is all the way up to 120V.  The over-voltage and over-current rating of the walwart in the US allows the extra voltage and the setting at 27V (as opposed to 24V).  There's not much leeway, though.  You can only hope to get about 28V and still have regulation (in some cases).  So overseas with a step-down transformer starting out at 110V maximum, it's very difficult to get 27V on the MAX power supply.  The MOSFET-MAX is even more difficult because of the very large current bias in the buffer.  I had one customer who finally went to a step-down transformer that was rated for 4 or 5 Amps, I believe, before he could get 27V out of the MAX onboard power supply.  The real solution is to find a 24VAC walwart that takes 220V as its source.

 

Re-reading your post, it seems that your voltage supply is so low that you are actually pushing the MAX power supply (on the PCB) outside of its linear regulation.  The LM317 must burn at least a volt or two to provide regulation.  If you turn the adjustment up high enough, the voltage is simply bypassing the LM317 regulation.  That's why the noise goes away when you turn the voltage down.  You should be able to gauge this easily when adjusting - the trimmer will reach a point where further turning of the trimmer screw will not increase voltage.  From that point, you should turn it down at least 1 to 1-1/2V below that.  IOW, your voltage should be set to the highest whole number voltage setting where the voltage will still change if the screw is turned up or down.  I like to go another half to one volt down beyond that to be sure.  Otherwise, it's not in linear regulation and you have negated the use of the very quiet linear-regulated power supply on the PCB.

 

 


Edited by tomb - 9/30/11 at 12:11am
post #27 of 55
Thread Starter 

OK, thank you for that informative answer, I was guessing it was something among those lines. I guess the real question is if the lower voltage impairs the sound quality in any way? Because otherwise, I'll just leave it like this, but if I can still improve the quality of the amp I'd be willing to order a 220V 24V wallwart. And about the input, if I wire it up to the left terminal, there's an effect if I flip the switch. I'll have to try it with two inputs as soon as I get that grubDAC done (accidentally broke the PCB with that crap soldering iron and solder; lifted a pad, I'm gonna have to order a new PCB first). Also, touching the pot only produces a noise when there's no input connected, otherwise everything's dead quiet now that I have the slightly lower voltage set.

post #28 of 55



 

Quote:
Originally Posted by hitman47 View Post

OK, thank you for that informative answer, I was guessing it was something among those lines. I guess the real question is if the lower voltage impairs the sound quality in any way? Because otherwise, I'll just leave it like this, but if I can still improve the quality of the amp I'd be willing to order a 220V 24V wallwart. And about the input, if I wire it up to the left terminal, there's an effect if I flip the switch. I'll have to try it with two inputs as soon as I get that grubDAC done (accidentally broke the PCB with that crap soldering iron and solder; lifted a pad, I'm gonna have to order a new PCB first). Also, touching the pot only produces a noise when there's no input connected, otherwise everything's dead quiet now that I have the slightly lower voltage set.


It's quite possible you are connecting to the wrong set of inputs.  I forget which is which, but if the music switches off when you throw the switch, then you've got it correctly connected.  It won't make much difference if you connect two sources, the sources will switch when you throw the switch and it won't matter.

 

As far as lifting a trace, that shouldn't require you to order a new PCB.  Simply solder a wire to each end of the trace where it's still attached.  I've got boards with little resistor leads soldered instead of traces all over, because I did so much testing on them.  IOW, use a jumper to connect the ends of the traces that are still attached to the PCB.  It's really not too difficult.

 

As for losing performance with lower voltage, here's what happens: the supply voltage (set at V+) dictates the voltage swing that the tubes are able to use.  This dictates the dynamic range of the amp.  Now, the voltage swing is dependent on the impedance of the headphones you use.  High impedance headphones like Sennheiser HD580/600/650 will want to use that entire voltage swing (plus or minus 13.5V at 27VDC).  Lower impedance phones will not.  Lower gain tubes will not, either.  So, say for instance you are using Grados at 32ohms and 12FK6 tubes (low gain) and you set the voltage at 22VDC (being conservative to make sure you have linear regulation).  You would set the tube bias at 11V (not 13.5) and that would give you plus or minus 11V to swing.  The 12FK6 tubes would probably never swing that much with their gain and the Grados would never know the difference, anyway.

 

Unfortunately, the plates on the tubes perform better at higher voltages.  In fact, we "unofficially" recommend setting the bias at 15V on a MOSFET-MAX with 12FM6's, simply because that tube sounds better at that higher bias.  If you have 27V, that still gives you 12V on the plus side to swing, equivalent to many amps with a 24VDC power supply.  However, if you tried that at your 22VDC, you'd only have a top swing of 7V, which is worse than a portable amp with 2x9V batteries (18V).  IOW, the dynamic range of the amp would be less than a portable amp with two 9V batteries.

 

Now again, with low impedance phones you'd never notice this, IMHO.  With higher impedance phones, you would.  Bottom line, to get the best performance possible out of the amp, it's better to have the higher voltage than not.  Only you can decide whether it's worth the extra investment.
 

EDIT: I forgot to mention that MOSFETs compound the issue of low power supply voltage.  Unlike BJT's, MOSFETs actually have a turn-on voltage threshold.  This is the "Gate Threshold Voltage" I believe, and is a full 2.0V for the Z24/9Z34 MOSFETs.  So, you'd have to subtract that amount from the voltage swing available to the buffer once the signal left the tubes (clipping might occur, iow).


Edited by tomb - 9/30/11 at 7:12am
post #29 of 55
Thread Starter 

OK, thank you very much! I'll try to fix the grub PCB, but another problem is the crappy solder I used which, when heated, doesn't flow properly and gets all sticky and gel-like in consistence, even when confronted with the highest temperature my soldering station will do... I could try to use my heat gun, but I think that'll just end up snapping the PCB from the heat. I'll try it though. There's a power supply at RS-components for 11€, but last time I tried to order from there, it wouldn't recognize my credit card... I'll try again, let's hope this time it works

post #30 of 55



 

Quote:
Originally Posted by hitman47 View Post

OK, thank you very much! I'll try to fix the grub PCB, but another problem is the crappy solder I used which, when heated, doesn't flow properly and gets all sticky and gel-like in consistence, even when confronted with the highest temperature my soldering station will do... I could try to use my heat gun, but I think that'll just end up snapping the PCB from the heat. I'll try it though. There's a power supply at RS-components for 11€, but last time I tried to order from there, it wouldn't recognize my credit card... I'll try again, let's hope this time it works

OK - I didn't realize you were talking about the grubDAC PCB.  That's very small SMD and I don't think it's fruitful to attempt repairing one, especially when a new one is $3.  I thought you were talking about the MOSFET-MAX PCB, which is an entirely different story.

 

 


Edited by tomb - 10/1/11 at 7:14am
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