Just do it, as you want.
What have you experienced, it is quite normal.
I think, if T18 is about 0.5-1V, then you accept what Zen Mod said.
There is always a chance something goes wrong, when you measuring inside a working amp.
So, peace is better.
Congratulations!
Finished both channels. Everything fine. Offset at ~3mVdc. Psu Ripple is 0,00mVac
I'm listening one channel only in cheap 30 year old speakers and it sounds beautifully. The F5 in this combination sounds a little bit darker compared to the vfet. I am anxious to connect the dual monos to the FHmk3 fostex and listen to some records. I will post pictures
Thank you all
I'm listening one channel only in cheap 30 year old speakers and it sounds beautifully. The F5 in this combination sounds a little bit darker compared to the vfet. I am anxious to connect the dual monos to the FHmk3 fostex and listen to some records. I will post pictures
Thank you all
Zed Mod and all,
I've just finished my unregulated "Pass" style design PSU, with a suggested 22Vac transformer (measures unloaded 23.7Vac). However I'm getting 31Vac (with no load), which equates to a Vdc/Vac factor of 1.31 .
Under load the voltage should drop slightly.
Is the aimed for 28Vdc measured under load or out-of-circuit?
If the later, will I get away with 31Vdc without changing resistor values?
I can take a few turns off of the transformer if nessesary.
Thanks!
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I thought I will my query here as this is more a build thread and the other one discussion on the amp
My universal psu output voltage is +/-32.4v even with one amp board connected but without the vfets installed.
Vfets are not installed I have just started with the initial steps of voltage test points verification with all the parts soldered except the vfets on 1st amp board.
My VFETS are marked as J28-104 and K82-93 so added 1v and was able to set the voltage to 11.4v across G-T16 by adjusting P1 and 10.3v across G-T17 by adjusting P2.
Initially set both the P3/P4 at full counterclockwise position. After this started biasing the front end by looking at the test point of T6-T7 (R5) and T8-T9 (R6) adjusting with clockwise movements on P3 and P4 (in turns of 0.1v) to achieve 1.2v. But I was able to get only 0.940v across R5 and 1.067v across R6. The P3 and P4 trimmers (I am using single turn ones) are at full clockwise and no more turns left to increase the bias to 1.2v. Even after running the amp for more than 30/45mins the bias voltage remains the same.
Any pointers to how could I achieve 1.2v?
My universal psu output voltage is +/-32.4v even with one amp board connected but without the vfets installed.
Vfets are not installed I have just started with the initial steps of voltage test points verification with all the parts soldered except the vfets on 1st amp board.
My VFETS are marked as J28-104 and K82-93 so added 1v and was able to set the voltage to 11.4v across G-T16 by adjusting P1 and 10.3v across G-T17 by adjusting P2.
Initially set both the P3/P4 at full counterclockwise position. After this started biasing the front end by looking at the test point of T6-T7 (R5) and T8-T9 (R6) adjusting with clockwise movements on P3 and P4 (in turns of 0.1v) to achieve 1.2v. But I was able to get only 0.940v across R5 and 1.067v across R6. The P3 and P4 trimmers (I am using single turn ones) are at full clockwise and no more turns left to increase the bias to 1.2v. Even after running the amp for more than 30/45mins the bias voltage remains the same.
Any pointers to how could I achieve 1.2v?
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Hi Manniraj,
It sounds like the same issue as here:
Sony Vfet Illustrated build guide
Cheers,
Dennis
It sounds like the same issue as here:
Sony Vfet Illustrated build guide
Cheers,
Dennis
Thanks Dennis yes I am using the Fairchaild parts from Mouser and will change it to either 470 or 500 ohms on R34 and R35 and reset the bias from starting with full counterclockwise P3/P4
Thanks Dennis yes I am using the Fairchaild parts from Mouser and will change it to either 470 or 500 ohms on R34 and R35 and reset the bias from starting with full counterclockwise P3/P4
I found couple of Dale 470R resistors which I am planning to use on one of the boards. But for the other board I could only find Dale 562R which I am presuming might be too high or mismatch. May be I could use some generic 470R locally and I hope the generic ones should be fine
Based on the suggestion I replaced R34/35 with my available 562R resistors and one of the boards went well and got the required bias of 1.2v with 0.09v offset on T18 and ran an hour and increase the bias to 1.5v with 0.2v offset.
I did the same mod on the other board for replacement with 562R resistors. But now unable to get the required initial setup voltages after making sure that P3/P4 are fully counter clockwise turned. I suspected P3 single turn pot and replaced that with a multi turn Bourns 3296 of 500R but still its the same issue.
Below are my voltages.
G-T6 = +33.2V G-T8 = -33.2V
G-T4 = -23.8V G-T2 = +23.8V
T6-T7 = 0V T8-T9 = 0.39V
T6-T11 = 8.1V T8-T13 = 8.3V
G-T18 = 1V
G-T14 = +15.62V G-T15 = -15.62V
G-T19 = 20.6V G-T20 = 7.8V
G-T17 = 6.1V G-T16 = 17V
There is some problem with G-T19 and G-T16 with higher voltages on one of the sides compared to G-T20 and G-T17. I have checked all the resistors for the correct ohms and everything seems to be as per the schematic. Need any pointers on where exactly is the issue. I have even tried R34/35 with 221R and 1K but this does not seems to be the issue. I have also cross verified the correct P/N of Q5/Q6 including the ZTX450/550. The jfets are J74/J170 are as correctly mounted.
Also does the voltage shut reference Q11,12,13,14 could be issue with these unusual reference voltages?
I did the same mod on the other board for replacement with 562R resistors. But now unable to get the required initial setup voltages after making sure that P3/P4 are fully counter clockwise turned. I suspected P3 single turn pot and replaced that with a multi turn Bourns 3296 of 500R but still its the same issue.
Below are my voltages.
G-T6 = +33.2V G-T8 = -33.2V
G-T4 = -23.8V G-T2 = +23.8V
T6-T7 = 0V T8-T9 = 0.39V
T6-T11 = 8.1V T8-T13 = 8.3V
G-T18 = 1V
G-T14 = +15.62V G-T15 = -15.62V
G-T19 = 20.6V G-T20 = 7.8V
G-T17 = 6.1V G-T16 = 17V
There is some problem with G-T19 and G-T16 with higher voltages on one of the sides compared to G-T20 and G-T17. I have checked all the resistors for the correct ohms and everything seems to be as per the schematic. Need any pointers on where exactly is the issue. I have even tried R34/35 with 221R and 1K but this does not seems to be the issue. I have also cross verified the correct P/N of Q5/Q6 including the ZTX450/550. The jfets are J74/J170 are as correctly mounted.
Also does the voltage shut reference Q11,12,13,14 could be issue with these unusual reference voltages?
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Any pointers to the above issue and am I looking at the correct components for the voltage issues Q11,12,13,14?
Dennis you are right with the P1/P2 in center position the G-T19 and G-T20 are appx +/-2v range from the ideal voltages. So the voltages look fine.
I set the Vgs of J28 to 11.4 and K82 to -10.3 (based on the vfets marked voltages + 1V) as per the guide. When I try to set the bias on R5/R6 I have the following issues, R5 no voltage increase from 0.000V with the P3 turned full on (I suspected the single turn pot so replaced it with a 500R multi-turn pot where the turn screw comes on the left side to just indicate the direction). The R6 slowly increases from 0.000V to 0.254V with P4 fully turned on. By the way as I am using the Fairchaild jfets replaced R34/R35 with 562R which I had spares instead of the 221R as per the schematic. The 562R worked fine with the first amp board but the 2nd board it still unable to achieve 1.2V and then slowly increasing to 1.5V suggested bias voltage. Where could be the problem, do I need to increase the R34/35 to something higher resistance? Also not sure why R5 voltage does not change with the turn of the knob (this was the same case even with single turn pot).
Thanks
Also checked the shunt voltage reference regulators Q11,12,13,14 between the working board with the other and apart from 1 or 2 volts difference both the boards shunt voltages seems to be fine very close to each other. Today with both the amp boards connected to the universal PSU my DC output voltage remains the same around +/-32.5v which I think needs to drop to around +/-28v. I am using 0-22V 6.8A*2 dual secondaries transformer with 18000uf*8 with 4 x 0.47R 3W resistors per rail on the PSU for rectification. Is the high output dc voltage any concern as the max voltage for VFET board as per the schematic says +/-32v? Does the dc output voltage will drop with the VFETs installed which I am not sure?
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