A clarification - without V- power supply, Vg relative to ground should measure about -12V, as the LM317 is set for 12V. So a full check of the LM317 circuit is needed.
Another check would be the voltage drop across R1, which should be 1.25V, the LM317 Vref. If the measured voltage is not approximately 1.25V, then the LM317 is probably bad. Or possibly bad diode D2.
Another check would be the voltage drop across R1, which should be 1.25V, the LM317 Vref. If the measured voltage is not approximately 1.25V, then the LM317 is probably bad. Or possibly bad diode D2.
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Without V- power and referenced to bias supply earth (negative):
Right Channel PCB
Vg max = 11.7V
Vg min = 8.6
Across R1 = 1.24V
Left Channel PCB
Vg max = 11.7V
Vg min = 0V
Across R1 = 1.25V
Voltage across D1 and D2 is same in both channels ( approx 9.4V and 1.25V respectively).
Values and orientation of bias supply components installed on PCB's were checked again and are as per schematic.
This is strange.
Right Channel PCB
Vg max = 11.7V
Vg min = 8.6
Across R1 = 1.24V
Left Channel PCB
Vg max = 11.7V
Vg min = 0V
Across R1 = 1.25V
Voltage across D1 and D2 is same in both channels ( approx 9.4V and 1.25V respectively).
Values and orientation of bias supply components installed on PCB's were checked again and are as per schematic.
This is strange.
Left channel bias supply voltages look correct.
Right channel bias supply maximum voltage is correct but minimum voltage is too high. Check resistors, compare to good channel, check solder joints - perhaps a solder bridge?
So if the bias supply is working but the voltage is not getting to the SIT gate, check resistors between bias supply and V- supply, namely R3, R9, R10. Check values and solder joints.
Right channel bias supply maximum voltage is correct but minimum voltage is too high. Check resistors, compare to good channel, check solder joints - perhaps a solder bridge?
So if the bias supply is working but the voltage is not getting to the SIT gate, check resistors between bias supply and V- supply, namely R3, R9, R10. Check values and solder joints.
I removed all components from right channel PCB, cleaned them and reinstalled on the same PCB. With the exception of D3 which showed a low forward voltage and was replaced, all components were within spec. After the rebuild, Vg max = -46.63V, Vg min = -36.81V, referenced to TP GND. From your previous comments, I assume this is the range where we need to be. But I still have no idea what the problem may have been. It is weird that the same components reinstalled on the same PCB produced significantly different results. I guess this is part of the mystique of DIY!!
Measurements of left channel PCB are similar (Vg max = -46.65V, Vg min = -36.83V). I now await delivery of the Hammond 193V choke this week so boards can be tested with this and the SIT in circuit. Hopefully all will be well.
Many thanks again for your assistance with this.
Measurements of left channel PCB are similar (Vg max = -46.65V, Vg min = -36.83V). I now await delivery of the Hammond 193V choke this week so boards can be tested with this and the SIT in circuit. Hopefully all will be well.
Many thanks again for your assistance with this.
Yes, perhaps a bad joint. They don't happen often but can be very difficult to find sometimes.
Re 195T5's, I'll probably continue to use them with the new PCB's in the negative supply source follower amp. They and the THF51S's have been doing excellent service for over a year in the positive supply version. I originally purchased them to use with the Inpol/MoFo (an Andrea Cuffoli design) but when I discovered SIT's and read about your choke follower designs realised they could be used in these designs, even though the inductance is lower than the 193V. To date I haven't noticed any roll off in low end response. In fact, quite the opposite; the bottom end of the choke loaded source follower is extraordinary in its depth and natural sound. I'm no expert but to my ears, this amp drives my speakers (Spendor A7) to perfection.
I ordered just one 193V essentially for testing purposes with the new amp and out of curiosity to see whether the extra inductance has an audible effect.
Re 195T5's, I'll probably continue to use them with the new PCB's in the negative supply source follower amp. They and the THF51S's have been doing excellent service for over a year in the positive supply version. I originally purchased them to use with the Inpol/MoFo (an Andrea Cuffoli design) but when I discovered SIT's and read about your choke follower designs realised they could be used in these designs, even though the inductance is lower than the 193V. To date I haven't noticed any roll off in low end response. In fact, quite the opposite; the bottom end of the choke loaded source follower is extraordinary in its depth and natural sound. I'm no expert but to my ears, this amp drives my speakers (Spendor A7) to perfection.
I ordered just one 193V essentially for testing purposes with the new amp and out of curiosity to see whether the extra inductance has an audible effect.
193V choke arrived today. Unfortunately, result is same as with 195T5. All good with SIT out of circuit and both V- and bias supply connected. However when V- and bias is applied with SIT connected, power supply shuts down. Trimmer position is maximum counterclockwise so SIT shouldn't be drawing current. I've tested with two THF51S and 2SK182ES (known to be good) with same results.
Speculation - could low S to D resistance (< 1 ohm) be an issue? Maybe the SMPS is seeing SIT as a dead short across its output and goes into protection mode? Odd thing is this has never occurred with V+ version. Its running same SMPS into a CLC with 15mF then into the SIT without any issues. Here its running into just 1mF (C7) which shouldn't cause problems. Also tried a soft start module with no effect.
Not sure where to go from here. Maybe linear PS instead of SMPS?
Speculation - could low S to D resistance (< 1 ohm) be an issue? Maybe the SMPS is seeing SIT as a dead short across its output and goes into protection mode? Odd thing is this has never occurred with V+ version. Its running same SMPS into a CLC with 15mF then into the SIT without any issues. Here its running into just 1mF (C7) which shouldn't cause problems. Also tried a soft start module with no effect.
Not sure where to go from here. Maybe linear PS instead of SMPS?
ZM, thanks for your comment.
Unfortunately, no channel is working. The SIT's I have tried (two THF51S and two 2SK182ES) are good. That is, they work fine in other source follower and Mu follower amps I've been playing with.
I'm now wondering whether there may be an adverse interaction occurring between the bias circuit and main V- voltage provided by the SMPS. If this is so, it only occurs when SIT is in circuit. The bias appears to work correctly when both bias and main V- supplies are present and SIT isn't. This is just a guess, I can't back it up with relevant data as it is beyond my limited DIY knowledge. Equally, it may be irrelevant what type of PS is used. But I think its interesting that others have successfully built this amp using a linear PS and I'm having problems using a SMPS.
Unfortunately, no channel is working. The SIT's I have tried (two THF51S and two 2SK182ES) are good. That is, they work fine in other source follower and Mu follower amps I've been playing with.
I'm now wondering whether there may be an adverse interaction occurring between the bias circuit and main V- voltage provided by the SMPS. If this is so, it only occurs when SIT is in circuit. The bias appears to work correctly when both bias and main V- supplies are present and SIT isn't. This is just a guess, I can't back it up with relevant data as it is beyond my limited DIY knowledge. Equally, it may be irrelevant what type of PS is used. But I think its interesting that others have successfully built this amp using a linear PS and I'm having problems using a SMPS.
Looking at the picture in post #719, the capacitor C7 at the board V- input appears to be quite large. Perhaps it is too large for the SMPS and is causing issues? What value is it? Remove C7 and see if that makes a difference. Edit: I read post #729 again and see that the capacitor is 1mF.
If that does not work, remove C8 and try again. C8 is in the bias voltage circuit for noise abatement but it does slow down the bias voltage from getting up to maximum voltage, so on start up, there is a momentary surge in current until the bias voltage stabilizes.
If that does not work, remove C8 and try again. C8 is in the bias voltage circuit for noise abatement but it does slow down the bias voltage from getting up to maximum voltage, so on start up, there is a momentary surge in current until the bias voltage stabilizes.
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Do not connect soft start to bias supply transformer. If possible, soft start V- but have bias supply start immediately.
Or start bias supply first, wait a few seconds, then start the SMPS.
Re: my post #734, leave C7 in place as you had previously used 15mF with no issues. But do try removing C8.
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monk55
I didn't use the CLC filter as the components are in existing V+ amp which I'm currently using. When I get the new PCB's running they will replace existing V+ amp bias supply and amp strip boards.
Re 1mF filter cap, its not present in V+ amp. As noted, I'm using a CLC filter on the SMPS output consisting of 10mF KZ, Hammond 159ZJ, 5mF KZ. It reduces ripple to a very low value (> 0.5mV). There's probably still residual high frequency hash, but it doesn't seem to get in the way of the music.
pinholer
I thought of trying this, but am hoping to solve the issue without adding components to Ben's elegantly simple design, if at all possible.
Ben
Your comment re no soft start for the bias supply is noted. I'll keep it separate from SMPS mains input. I'll remove C8 & see what happens.
Thanks.
I didn't use the CLC filter as the components are in existing V+ amp which I'm currently using. When I get the new PCB's running they will replace existing V+ amp bias supply and amp strip boards.
Re 1mF filter cap, its not present in V+ amp. As noted, I'm using a CLC filter on the SMPS output consisting of 10mF KZ, Hammond 159ZJ, 5mF KZ. It reduces ripple to a very low value (> 0.5mV). There's probably still residual high frequency hash, but it doesn't seem to get in the way of the music.
pinholer
I thought of trying this, but am hoping to solve the issue without adding components to Ben's elegantly simple design, if at all possible.
Ben
Your comment re no soft start for the bias supply is noted. I'll keep it separate from SMPS mains input. I'll remove C8 & see what happens.
Thanks.
With C8 removed, there is no change. That is, with bias on, when power is applied to the SMPS, it goes into hiccup (protection) mode. From what I've read about SMPS's, this can be indicative of a short across the output. Interestingly, I noticed that bias voltage is maximum (-11.70V referenced to bias supply ground) when the trimmer is turned fully clockwise despite it being oriented on the PCB to provide the opposite! No idea why this should be. From what I can see in photos of Ben's and monk55's PCB's, mine is oriented the same way. However I made sure that max bias voltage was present at SIT gate before applying V-. Also tried pinholer's suggestion and put a 1R resistor in series with the choke, with the same result (no change).
Any further comments or suggestions regarding what the problem might be would be appreciated.
Any further comments or suggestions regarding what the problem might be would be appreciated.
The bias voltage (at SIT gate) is negative because the floating bias voltage supply is in series with V-, with the bias voltage positive connected to V- and the bias supply negative connected to the SIT gate. So at maximum, relative to circuit ground, the voltage at SIT gate relative to ground should be about -11.7V when V- is not on. When V- is on, it should be about -46.5V relative to circuit ground.
When you measured -11.7V, was this at the SIT gate relative to circuit Ground with no V-? I do not understand what you measured when you said you measured -11.7V bias voltage referenced to bias supply ground.
So with the SMPS test, did you turn on the bias voltage, wait until you measured -11.7 voltage at the SIT gate relative to circuit ground, and then powered up the SMPS? Did you have a meter measuring V-? Did you have a meter across the choke to measure current? That information would be helpful for troubleshooting (Vg and V- relative to circuit ground, and Vdrop across choke readings as V- power is applied).
Do you have another power supply for testing? It does not have to be 36V. A 19V computer supply? Or a linear supply? It would be to test the circuit.
Post some clear pictures of the board and of your whole amplifier, showing all wiring.
When you measured -11.7V, was this at the SIT gate relative to circuit Ground with no V-? I do not understand what you measured when you said you measured -11.7V bias voltage referenced to bias supply ground.
So with the SMPS test, did you turn on the bias voltage, wait until you measured -11.7 voltage at the SIT gate relative to circuit ground, and then powered up the SMPS? Did you have a meter measuring V-? Did you have a meter across the choke to measure current? That information would be helpful for troubleshooting (Vg and V- relative to circuit ground, and Vdrop across choke readings as V- power is applied).
Do you have another power supply for testing? It does not have to be 36V. A 19V computer supply? Or a linear supply? It would be to test the circuit.
Post some clear pictures of the board and of your whole amplifier, showing all wiring.