I remembered the mistake in the first BOM it had Q5 as a NPN instead of a PNP, but I didn't know that that Q3/Q4 was also wrong. my inexperience I trusted the published information and didn't make the connection that if Q5 was a PNP that Q3 should be also. I checked both channels and have ZTX450 for Q3, and ZTX550 for Q4 as per the BOM. Both R5 & R6 are 47.5R, and I was careful not to mix up the 221Rand the 2.21K resistors. It is odd that the left channel works though. In any event I'll order new Q3,4,5,6 and install them in the correct locations, hopefully that will do the trick.
Thanks for helping me out, much appreciated.
Paul
Hi Dennis,
For my own education how can you tell from the schematic that Q3 is a NPN. I am not formally trained on how to read electrical schematics so I'm not familiar with all of the nomenclature. I am analogous to a layman trying to read a piping and instrument drawing for a chemical plant, might get some of it, but a lot of important stuff is missed. The schematic only shows Q3 as the designation, for Q5 is does show a part designation which I can look up to learn is a PNP. If the arrow leaves the symbol does that mean it is a NPN and if it enters the symbol it is a PNP ?
Thanks
Paul
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Stuck on bias - can't get out!
Gentlemen,
First time writing to this forum. Before I begin my whining, I want to thank Jim for his outstanding build guide. And to the many other members for their contributions: many excellent questions and even better responses. And of course to Nelson for making this project possible. I have learned so much and have been having a blast. However,
Hoping to finish my build this weekend, I'm failing on the bias step before Vfet install. Monitoring R5 and R6 voltages, no change from 0 until P3 and P4 are half-turned (clockwise). Iterating from P3 to P4 until fully clockwise (while keeping T18 close to 0), I reach a max of +/- 0.39 V across R5 and R6 and am done. T18 is -0.5 V. Both boards yield the same result!
I've checked the resistances and voltages as instructed. All good. P1 and P2 set to get Vfet bias + 1 V. I thought the Mosfets Q5 and Q6 might be bad, so I switched them out for extras. No change. Perhaps my Jfets are bad or fake as others have been unable to achieve correct bias using fakes? But I got mine (Toshibas) from Spencer at FET Audio, a reputable provider.
What should I try next?
- David LePell
table { }td { padding-top: 1px; padding-right: 1px; padding-left: 1px; color: black; font-size: 12pt; font-weight: 400; font-style: normal; text-decoration: none; font-family: Calibri,sans-serif; vertical-align: bottom; border: medium none; white-space: nowrap; }.xl63 { text-align: center; }.xl64 { color: red; text-align: center; }.xl65 { font-weight: 700; text-decoration: underline; font-family: Calibri; text-align: center; }.xl66 { color: red; font-weight: 700; text-decoration: underline; font-family: Calibri; text-align: center; }.xl67 { text-decoration: underline; font-family: Calibri; }.xl68 { color: windowtext; text-decoration: underline; font-family: Calibri; } Vfet BOARD 1 Voltages* (no Vfets)
points Want Got points Want Got G-T6 28 27.9 G-T8 -28 -27.9 G-T4 -24 -23.97 G-T2 24 23.99 T6-T7 0 0 T8-T9 0 0 T6-T11 5 -5.1 T8-T13 5 5.17 G-T18 < a few +/- 0.11
G-T14 13 13.35 G-T15 -13 -13.4 G-T19 12 12.76 G-T20 -12 -11.93 G-T17 -10 -9.83 G-T16 10 10.54
Vfet BOARD 2 Voltages* (no Vfets)
points Want Got points Want Got G-T6 28 27.8 G-T8 -28 -27.8 G-T4 -24 -23.9 G-T2 24 24 T6-T7 0 0 T8-T9 0 0 T6-T11 5 -5.1 T8-T13 5 5.07 G-T18 < a few +/- 0.1
G-T14 13 13.16 G-T15 -13 -13.19 G-T19 12 12.68 G-T20 -12 -11.92 G-T17 -10 -9.82 G-T16 10 10.48
*VARIAC set to give +/-28 VDC from PSU
Gentlemen,
First time writing to this forum. Before I begin my whining, I want to thank Jim for his outstanding build guide. And to the many other members for their contributions: many excellent questions and even better responses. And of course to Nelson for making this project possible. I have learned so much and have been having a blast. However,
Hoping to finish my build this weekend, I'm failing on the bias step before Vfet install. Monitoring R5 and R6 voltages, no change from 0 until P3 and P4 are half-turned (clockwise). Iterating from P3 to P4 until fully clockwise (while keeping T18 close to 0), I reach a max of +/- 0.39 V across R5 and R6 and am done. T18 is -0.5 V. Both boards yield the same result!
I've checked the resistances and voltages as instructed. All good. P1 and P2 set to get Vfet bias + 1 V. I thought the Mosfets Q5 and Q6 might be bad, so I switched them out for extras. No change. Perhaps my Jfets are bad or fake as others have been unable to achieve correct bias using fakes? But I got mine (Toshibas) from Spencer at FET Audio, a reputable provider.
What should I try next?
- David LePell
table { }td { padding-top: 1px; padding-right: 1px; padding-left: 1px; color: black; font-size: 12pt; font-weight: 400; font-style: normal; text-decoration: none; font-family: Calibri,sans-serif; vertical-align: bottom; border: medium none; white-space: nowrap; }.xl63 { text-align: center; }.xl64 { color: red; text-align: center; }.xl65 { font-weight: 700; text-decoration: underline; font-family: Calibri; text-align: center; }.xl66 { color: red; font-weight: 700; text-decoration: underline; font-family: Calibri; text-align: center; }.xl67 { text-decoration: underline; font-family: Calibri; }.xl68 { color: windowtext; text-decoration: underline; font-family: Calibri; } Vfet BOARD 1 Voltages* (no Vfets)
points Want Got points Want Got G-T6 28 27.9 G-T8 -28 -27.9 G-T4 -24 -23.97 G-T2 24 23.99 T6-T7 0 0 T8-T9 0 0 T6-T11 5 -5.1 T8-T13 5 5.17 G-T18 < a few +/- 0.11
G-T14 13 13.35 G-T15 -13 -13.4 G-T19 12 12.76 G-T20 -12 -11.93 G-T17 -10 -9.83 G-T16 10 10.54
Vfet BOARD 2 Voltages* (no Vfets)
points Want Got points Want Got G-T6 28 27.8 G-T8 -28 -27.8 G-T4 -24 -23.9 G-T2 24 24 T6-T7 0 0 T8-T9 0 0 T6-T11 5 -5.1 T8-T13 5 5.07 G-T18 < a few +/- 0.1
G-T14 13 13.16 G-T15 -13 -13.19 G-T19 12 12.68 G-T20 -12 -11.92 G-T17 -10 -9.82 G-T16 10 10.48
*VARIAC set to give +/-28 VDC from PSU
Hi David,
Are you using Fairchild parts Q5 and Q6? My guess is that the combination of
R35,P3,Q1 (and similarly for R34,P4,Q2) is not giving enough voltage across
R35+P3 and R34+P4 to bias up the mosfets.
I suspect you will need to increase R34 and R35, perhaps to 500ohm or so
and then set P3 and P4 back to min value (fully counterclockwise) and try
biasing the front end again.
Hopefully others can comment as well.
Cheers,
Dennis
Are you using Fairchild parts Q5 and Q6? My guess is that the combination of
R35,P3,Q1 (and similarly for R34,P4,Q2) is not giving enough voltage across
R35+P3 and R34+P4 to bias up the mosfets.
I suspect you will need to increase R34 and R35, perhaps to 500ohm or so
and then set P3 and P4 back to min value (fully counterclockwise) and try
biasing the front end again.
Hopefully others can comment as well.
Cheers,
Dennis
Hi Dennis,
I've been obsessing on this issue for the past 48 hours so of course I did not wait for any other suggestions. I clipped in a 500 ohm resistor in R34 and R35 and the amp is sitting at +/- 1.25 V bias with 0.6 V on T18. And P3 and P4 are only at 7 o'clock! Will easily meet the +/- 1.5 V target. P1 and P2 slightly adjusted to their respective values.
Does this new value for R34/R35 require any other modifications to the circuit? Specifically does it have any affect on the Jfets and their operation?
Your suggestion seems to have hit the bullseye.
Thanks very much, and tonight's peaceful sleep thanks you! 
I've been obsessing on this issue for the past 48 hours so of course I did not wait for any other suggestions. I clipped in a 500 ohm resistor in R34 and R35 and the amp is sitting at +/- 1.25 V bias with 0.6 V on T18. And P3 and P4 are only at 7 o'clock! Will easily meet the +/- 1.5 V target. P1 and P2 slightly adjusted to their respective values.
Does this new value for R34/R35 require any other modifications to the circuit? Specifically does it have any affect on the Jfets and their operation?
Your suggestion seems to have hit the bullseye.
Thanks very much, and tonight's peaceful sleep thanks you! Bias update - success!
The iterative bias procedure went well, if a bit slowly. Sequentially I tested each amplifier after an hour of warm-up with a familiar and favorite CD. And I discovered qualities in the recording I had not heard before, and that speaker suddenly sounded fantastic. My first amplifier build: so far so excellent!
Thanks to Dennis Hui for pulling me out of a quagmire, and to the community for all the valuable information. I'll post a pic or two when I get it installed in my chassis.
- David
The iterative bias procedure went well, if a bit slowly. Sequentially I tested each amplifier after an hour of warm-up with a familiar and favorite CD. And I discovered qualities in the recording I had not heard before, and that speaker suddenly sounded fantastic. My first amplifier build: so far so excellent!
Thanks to Dennis Hui for pulling me out of a quagmire, and to the community for all the valuable information. I'll post a pic or two when I get it installed in my chassis.
- David
The iterative bias procedure went well, if a bit slowly. Sequentially I tested each amplifier after an hour of warm-up with a familiar and favorite CD. And I discovered qualities in the recording I had not heard before, and that speaker suddenly sounded fantastic. My first amplifier build: so far so excellent!
- David
Congratulations David! You're now a fearless amplifier builder.
Cheers,
Dennis