Head-Fi.org › Forums › Misc.-Category Forums › DIY (Do-It-Yourself) Discussions › Millett "Starving Student" hybrid amp
New Posts  All Forums:Forum Nav:

Millett "Starving Student" hybrid amp - Page 420

post #6286 of 6780
Lol, I don't even have an enclosure yet I kind of over looked it, thanks for the tips I'll double check where my wires go. This might be a stupid idea but could I shield the wires with aluminum foil?
post #6287 of 6780

Silver tube shields will actually make the tubes run hotter since they'll reflect back the heat to the tube. Proper tube shields are coated in matte black paint, but they're really hard to find.

 

As to noise in your build Kim, I really wouldn't blame it on the design. My own build sits inside an ABS plastic enclosure and the only times it picks up noise are when I lay my cell phone directly next to it (then again my Woo Audio amp will also pick up noise that way) or if I put my netbook (while downloading a big file) right next to it.

 

Shamazo, the heater voltages in your build are a little strange. What values did you use for R2, R3, R8 and R10. Did you check the resistor value with your meter?

 

cheers!

post #6288 of 6780

yesterday i plugged a set of gr06 into mine, and with the volume knob all the way down, i actually picked up what sounded like an fm frequency. some random song i had never heard before was playing quietly, and when i turned the pot up, it would go away. tried it with my he-4 and it wouldnt pick it up. guess it was the high sensitivity of the iems. has anyone ever heard something like this happen?

post #6289 of 6780
Quote:
Originally Posted by the_equalizer View Post

Silver tube shields will actually make the tubes run hotter since they'll reflect back the heat to the tube. Proper tube shields are coated in matte black paint, but they're really hard to find.

 

As to noise in your build Kim, I really wouldn't blame it on the design. My own build sits inside an ABS plastic enclosure and the only times it picks up noise are when I lay my cell phone directly next to it (then again my Woo Audio amp will also pick up noise that way) or if I put my netbook (while downloading a big file) right next to it.

 

Shamazo, the heater voltages in your build are a little strange. What values did you use for R2, R3, R8 and R10. Did you check the resistor value with your meter?

 

cheers!

The resistor values SHOULD be 33kohm for 1/7 and 390 for 2/8, but i  get the feeling that i mixed up r2/8 with r4/10 which are 220kohms, ill edit this in a second when i check with a meter

 

EDIT r1/7 are 33 exactly for both, the other ones seem to keep going up as i measure them, do the caps affect my measurements? 

 

EDIT 2 according to resistor codes i used 390 kohm for r2/8, 220kohm for r4/10 and 2 1kohms in series for r 3/9.


Edited by shamazo - 11/5/12 at 9:24pm
post #6290 of 6780
Well now after I double checked everything I am sure is is interference, I had it sounding great when I had all the wire positioned in a certain wire, the bam I moved one and it started humming again. So I need to buy a case pronto to shield it. If it is internal interference how should I shield the wires?
Odd fact, most of the humming goes away if I touch the top of one of the caps.

EDIT remeasered the volts at pin4. It's at 19 volts way way to high. I think I have a short on r2/8 or 1/7 going to scrub it off with some alcohol and recheck it.
Edited by shamazo - 11/6/12 at 2:13am
post #6291 of 6780

Your resistor values seem certainly right, yet 19 V at the heater is not. I'll have to take out my meter tonight and check the voltage in my own 12AU7 build after running it for a while.

 

Indeed, measuring resistances in circuit is not always possible due to parallel current paths and capacitance. You need to unsolder one end and then measure.

 

 

Quote:
Originally Posted by kchapdaily View Post

yesterday i plugged a set of gr06 into mine, and with the volume knob all the way down, i actually picked up what sounded like an fm frequency. some random song i had never heard before was playing quietly, and when i turned the pot up, it would go away. tried it with my he-4 and it wouldnt pick it up. guess it was the high sensitivity of the iems. has anyone ever heard something like this happen?

 

It hasn't happened to me with the SSMH but it did happen from time to time with my electric guitar rig. Sometimes kicking in the overdrive pedal would make the radio sound really loud!

 

When I used to have my Etymotic ER-6i I could hear the noise floor of the SSMH. Not hum, but the pink (or is it white?) noise "SSSHHHH" sound coming out of the anp.

 

cheers!


Edited by the_equalizer - 11/6/12 at 4:58am
post #6292 of 6780

Whilst I still haven't got my ssmh entirely working, the voltages I measured are on this post:

 

http://www.head-fi.org/t/624475/yanssmh-yet-another-not-so-starving-student-millett-hybrid/15#post_8753199

 

They sound right but I still have permanent background noise; I'm putting it down to the power supply, because I'm not going to rebuild it again, I've already built it 3 times...

 

Hope you get yours sorted,

Chris

post #6293 of 6780

Hi there...

Next one alive and behaving quite good, no noise and only very very little hum.

 

 

Things left to do : order some slightly larger knob and add some paint.

Cheers

post #6294 of 6780

Nice one! I've seen people who use those heatsinks sand down the curved edges slightly to make them shiny, if that's the sorta thing that interests you :) 

post #6295 of 6780

Yeah, thought about that too. Looks pretty nice.
 

post #6296 of 6780

Last night I took out my meter and made some measurements in my SSMH 12AU7 build. I had some problems measuring the gate to ground voltage as it is a very high impedance circuit. My DMM would report only 13.71 and 13.62 volts on the gates with 11.82 and 11.71 volts at the tube heaters. The difference is less than two volts!

 

I had to bring out my old analog FET VOM with 10Mohm input impedance to get a more reliable measurement of ~16.1 and ~16.2 volts (the "~" is because it's an analog meter). Gate to source voltage was thus ~4.2 volts. Much better.

 

Shamazo, your measurements are definitely strange since they're not lower but much higher Might it be that you reversed R2 and R4?

 

I also measured the voltages at Q3 and Q4 in the constant current source plate loads. Q3 being my main concern since I knew it would bare the highest voltage drop of the two After the amp has run for a while I saw a voltage of 1.1 across Q4 while voltages of 23 and 24 volts showed up across each Q3.  I must have gotten really mixed up a few days ago when reviewing the datasheets for the BC327 since now that I recheck them I see Vceo listed as 45 V and Vces = 50 V. Since the only way the transistor would see more than 45 volts in regular operation would be that the tube became a short circuit (highly improbable, though not impossible),  I deem that the BC327 is just adequate for the application. Definitely go with the 2N5087 or BC556 if you want increased peace of mind.

 

By the way, measuring both the plate to ground voltate and the voltage across Q3 were also affected by my DMM's input impedance. I could only get reliable measurements with my FET VOM.

 

Finally, I noticed that the heater voltage was lowish so I measured the voltage drop across R13 and noticed it was too high. This was due to the higher current set by the CSS going through it. Since R13 sits at the top of the gate bias voltage divider, it was lowering the gate bias.

 

So I removed the CCS from that junction and connected them directly to the 48 V line from the power source. Since the CSS do a great job of decoupling the tube circuits from each other and from the output MOSFETs I think this configuration won't be a problem. Naturally, I'd definitely like to hear the opinion of someone with more experience building amplifiers with CCS plate loads. Another option would be to wire independent 'R13a' and 'R13b' for each CCS.

 

After moving the CCS directly to the 48V line the voltage at the MOSFET gates was now ~17 and the heater voltage went up to ~13.2. A little hot but quite alright. I did some listening tests and the amp worked quite alright. No hum and only by pushing the volume up real high was I able to hear the amp's noise floor through the Grado SR-80i

 

This showed something that I observed when I originally worked with the 12AU7 and 17EW8: I don't see a way to increase the MOSFET bias, that is to increase the voltage from gate to source in this amp. The voltage of the source seems to be "clamped" some 4.2 volts below the voltage set at the gate as when I increased the gate voltage the source voltage moved up correspondingly. Might this be because some sort of CCS-like behavior from the tube heaters? That by holding the drain-source current at ~150mA the source voltage is "clamped" below the gate voltage?

 

Shamazo: If you're seeing 19 volts at the heaters, what's the voltage at the MOSFET gates in your build?

 

Next I'll setup the amp and CCS loads to work with the 17EW8 and see how that measures.

 

cheers!

post #6297 of 6780
Quote:
Originally Posted by the_equalizer View Post

Last night I took out my meter and made some measurements in my SSMH 12AU7 build. I had some problems measuring the gate to ground voltage as it is a very high impedance circuit. My DMM would report only 13.71 and 13.62 volts on the gates with 11.82 and 11.71 volts at the tube heaters. The difference is less than two volts!

 

I had to bring out my old analog FET VOM with 10Mohm input impedance to get a more reliable measurement of ~16.1 and ~16.2 volts (the "~" is because it's an analog meter). Gate to source voltage was thus ~4.2 volts. Much better.

 

I also noticed these weird measurements with my cheap DMM. The only reliable way I can measure Vgs is to place my probes directly on the MOSFET pins. One probe at the gate, the other at the source. I didn't think of it as an impedance problem, this is good to know.

 

 

Shamazo, your measurements are definitely strange since they're not lower but much higher Might it be that you reversed R2 and R4?

 

I also measured the voltages at Q3 and Q4 in the constant current source plate loads. Q3 being my main concern since I knew it would bare the highest voltage drop of the two After the amp has run for a while I saw a voltage of 1.1 across Q4 while voltages of 23 and 24 volts showed up across each Q3.  I must have gotten really mixed up a few days ago when reviewing the datasheets for the BC327 since now that I recheck them I see Vceo listed as 45 V and Vces = 50 V. Since the only way the transistor would see more than 45 volts in regular operation would be that the tube became a short circuit (highly improbable, though not impossible),  I deem that the BC327 is just adequate for the application. Definitely go with the 2N5087 or BC556 if you want increased peace of mind.

 

By the way, measuring both the plate to ground voltate and the voltage across Q3 were also affected by my DMM's input impedance. I could only get reliable measurements with my FET VOM.

 

I compared some datasheets, and I concluded that the BC556 family may not be the best choice in an audio application. It's a very noisy transistor compared to both BC560 and 2N5087. Those last two are defined as "low noise", and their datasheet show a max noise of 2dB. In comparison, the BC556 has an typical noise of 2dB and a max of 10db. The 2N5087 is rated for 50V and is the cheaper of the bunch, so this is the one I'll get in my next parts order.

 

 

Finally, I noticed that the heater voltage was lowish so I measured the voltage drop across R13 and noticed it was too high. This was due to the higher current set by the CSS going through it. Since R13 sits at the top of the gate bias voltage divider, it was lowering the gate bias.

 

So I removed the CCS from that junction and connected them directly to the 48 V line from the power source. Since the CSS do a great job of decoupling the tube circuits from each other and from the output MOSFETs I think this configuration won't be a problem. Naturally, I'd definitely like to hear the opinion of someone with more experience building amplifiers with CCS plate loads. Another option would be to wire independent 'R13a' and 'R13b' for each CCS.

 

How did you connect the bypass capacitor without R13? I've been thinking about replacing R13 with a CCS for each channel, but since there's a decoupling cap in there it would make a CCS pointless, so I just don't see how we can do without R13. My build already has an R13 and a decoupling cap for each channel, so I'll give it a try as you recommend.

 

On a different note, why not simply lower the source current? Could you lower the amp's gain by lowering the CCS's current? That could potentially be a good idea, considering how troublesome the gain of the amp is. Starving the plates might not be such a good idea though.

 

 

After moving the CCS directly to the 48V line the voltage at the MOSFET gates was now ~17 and the heater voltage went up to ~13.2. A little hot but quite alright. I did some listening tests and the amp worked quite alright. No hum and only by pushing the volume up real high was I able to hear the amp's noise floor through the Grado SR-80i

 

This showed something that I observed when I originally worked with the 12AU7 and 17EW8: I don't see a way to increase the MOSFET bias, that is to increase the voltage from gate to source in this amp. The voltage of the source seems to be "clamped" some 4.2 volts below the voltage set at the gate as when I increased the gate voltage the source voltage moved up correspondingly. Might this be because some sort of CCS-like behavior from the tube heaters? That by holding the drain-source current at ~150mA the source voltage is "clamped" below the gate voltage?

 

4V is the gate to source threshold voltage. This means at this point the MOSFET already is fully ON. After this point, the source current is directly proportional to the gate voltage. Raising the voltage only raises the current trough the MOSFET, which by design also raise the current trough the tube heater. Since the heater is essentially a resistive load, pushing more current trough it will also raise the voltage drop across it. This is why it looks like the voltage at the gate is "clamped" 4V bellow the gate. The tube is not "holding" the source current at 150mA: The MOSFET is biased to allow only 150mA trough the tube. Then's it's just a question of how this current affects the voltages at different points of the circuit. The heaters of the 12AU7 are rated for 150mA at 12.6V. This can also be read the other way around: They will have 12.6V across them when 150mA goes trough them. If you raise the current, they will have a higher voltage cross them, and vice versa. Too little current, and they won't be hot enough for the tube to work properly: too much, and they'll simply blow up.

 

This is how I blew up a heater last week: I turned the amp ON without R4, so that Vgs was around 46V. This pushed an insane amount of current trough the heater, and since the drain of the MOSFET sits at 48V, the voltage across the heater was close to 48V before it blew.

 

I was wrong, see equalizer's post for the real explanation.

 

 

Shamazo: If you're seeing 19 volts at the heaters, what's the voltage at the MOSFET gates in your build?

 

Next I'll setup the amp and CCS loads to work with the 17EW8 and see how that measures.

 

cheers!

beerchug.gif


Edited by KimLaroux - 11/9/12 at 7:06pm
post #6298 of 6780
Quote:
Originally Posted by KimLaroux View Post

"On a different note, why not simply lower the source current? Could you lower the amp's gain by lowering the CCS's current? That could potentially be a good idea, considering how troublesome the gain of the amp is. Starving the plates might not be such a good idea though."

 

Once you have a CCS as a plate load Av~=mu. 

 

Without lots of mods you need to revert to resistors and load the tube down to waste some gain OR add a voltage divider. Neither is really a great option.

 

The best solution is to use a long tail pair, or redesign the amp around tubes with lower Mu. If your going to use an LTP you may as well just build one of Cavalli's amps that starts off with it as you will save endless headaches setting up the multiple power supplies that doing it well kind of needs. If your going with a lower Mu tube you simply have to redesign the amp around that. 

 

Quote:
Originally Posted by KimLaroux View Post

 

4V is the gate to source threshold voltage. This means at this point the MOSFET already is fully ON. After this point, the source current is directly proportional to the gate voltage. Raising the voltage only raises the current trough the MOSFET, which by design also raise the current trough the tube heater. Since the heater is essentially a resistive load, pushing more current trough it will also raise the voltage drop across it. This is why it looks like the voltage at the gate is "clamped" 4V bellow the gate. The tube is not "holding" the source current at 150mA: The MOSFET is biased to allow only 150mA trough the tube. Then's it's just a question of how this current affects the voltages at different points of the circuit. The heaters of the 12AU7 are rated for 150mA at 12.6V. This can also be read the other way around: They will have 12.6V across them when 150mA goes trough them. If you raise the current, they will have a higher voltage cross them, and vice versa. Too little current, and they won't be hot enough for the tube to work properly: too much, and they'll simply blow up.

 

This is how I blew up a heater last week: I turned the amp ON without R4, so that Vgs was around 46V. This pushed an insane amount of current trough the heater, and since the drain of the MOSFET sits at 48V, the voltage across the heater was close to 48V before it blew.

 

The mosfets are configured as simple source followers. The mosfet sets the source voltage and output current is conditional on the load.  

They are not current-controlled devices (ccs's) and should not be thought of as such. Ever. 

 

How could Vgs possibly be 46V when you said you put ~48V across the heater? Putting ~48V across the heater requires that the source be at 48V and the gate cant possibly be higher than the 48V rail...

Indeed, the Vgs curves only go to like 10V for the IRF510, and that sucking down 10A! What do the Vgs curves look like with your mosfet?

Vgs was still what it normally is, give or take a bit. Vg and Vs were no where near what they normally are which kind of screwed you up a lot. Lots of voltage, then the current came because of the voltage across a resistance... then poof. 

post #6299 of 6780
Quote:
Originally Posted by nikongod View Post

They are not current-controlled devices (ccs's) and should not be thought of as such. Ever. 

 

 

How about now? 

post #6300 of 6780
Quote:
Originally Posted by nikongod View Post

How about now? 

 

 

Nope. too soon

New Posts  All Forums:Forum Nav:
  Return Home
Head-Fi.org › Forums › Misc.-Category Forums › DIY (Do-It-Yourself) Discussions › Millett "Starving Student" hybrid amp