Looks great Ben. I've been enjoying the first version of this amp (+Vcc) for a few months, now I can't wait to try the -Vcc PCB version. I'd greatly appreciate the Gerbers for amp and PS.
Many thanks for your work in designing the choke follower and Mu follower SIT amps and sharing them with the DIY community.
Many thanks for your work in designing the choke follower and Mu follower SIT amps and sharing them with the DIY community.
I have had a diyAudio member ask me about missing Gerber files for the left channel board. I am sure most of you know the answer but just to be sure, there is only one board and one Gerber file set for the amplifier board. The board is used for both channels. The "Right Channel" is on the front of the board. Flip the board over and the "Left Channel" is on the back of the board. Of course those are designations only and the boards may be used for whichever channel you choose. I have the "Left Channel" in my right channel monoblock and the "Right Channel" in my left channel monoblock.
Looking good. I have to get off my butt and write up some building tips.
But meanwhile, I see that you have copied the orientation of the trimmer in the bias supply. I purposely installed it that way so that at full counterclockwise adjustment, the bias voltage is maximum for no current flow. Clockwise adjustment decreases the bias voltage and starts/increases the current flow.
Building Tips
1. Latest schematic is here: Latest schematic
2. Trimmer RV1 orientation shown here: Left Channel Board - RV1 Orientation, Right Channel Board - RV1 Orientation
2. Capacitor C8: Do not use a higher value than the specified 470uF. In this case bigger is not better. Larger value increases the time it takes for the capacitor to charge and the time it takes for the bias to reach full voltage.
2. Floating bias power supply: Minimum voltage is 15VDC and maximum voltage is 22VDC. I used the 15VAC secondary winding of my Antek transformer for this, and rectified it with a bridge rectifier. The rectified but unfiltered voltage is then connected to the board. Capacitor C1 provides the filtering ahead of the voltage regulator. Note that 15VDC is possibly on the low side and on the verge on affecting regulation, so it is best not to go there. A minimum transformer secondary of 12VAC should be good.
4. Measure the amplifier load choke's DC resistance before connecting it the the board. The choke should have a low DCR so unless you have a meter that can accurately measure low resistance, you will need to rig up a circuit to accurately measure the resistance. If you are using a Hammond 193V, then their specified DCR of 1 Ohm is good enough for our use. I have measured my 193Vs and they are 1.0 Ohm.
3. For first power-up, a Dim Bulb Tester is highly recommended. Of course the power supply should be tested independently prior to connecting it to the amplifier board. Check and double check that the floating bias supply and main power supply voltage polarities are correct. Before turning on the power, confirm that the bias voltage trimmer is set to maximum counter-clockwise. Connect a meter set to DC Volts to measure the voltage drop across the amplifier load choke (use test points on board). Connect another meter set to DC Volts to measure voltage drop across the SIT (connect to T.P. Choke2 SIT S and Ground).
Upon power-up, the bulb should glow bright, and then fade to off. There should be no voltage across the choke and the voltage across the SIT should be the full power supply voltage. Now turn the trimmer clockwise. It may take a few turns but voltage should appear across the choke, and the voltage across the SIT should start to decrease. If the trimmer adjustment increases and decreases the voltage drop across the choke as it should, the it is time to turn the trimmer back to full counterclockwise, power down, remove the Dim Bulb Tester, and go for full power.
4. For full power-up check to make sure the trimmer is set to full counterclockwise. Again, multimeters should be connected to monitor the voltage across the choke and the SIT. With power on, there will be a momentary surge in current as the capacitors in the bias supply charges, but it should drop to zero within seconds. Then slowly turn the trimmer clockwise to start the current flowing. The target current is 2.5A, but the current should be brought up in stages, with time in between adjustment for the system to heat up. Keep the chassis cover on between adjustments.
5. Ohm's Law: You will need to use this to calculate the current through the choke and the SIT. If you are using the Hammond 193V with its 1 Ohm DCR, then no calculation is necessary.
Ohm's Law ; E = IR, where E is voltage in Volts, I is current in Amperes, and R is resistance in Ohms
For I when E and R are known, I = E/R
So for Hammond 193V with R=1 Ohm, I = E/1
6. To determine the resistance of a low DCR choke, connect a known higher value resistor in series with the choke and a DC power supply. Size the resistor for say 10mA and a 20VDC power supply. Resistor R=20v/0.010A= 2000 Ohm, P=0.010A x 0.010A x 2000R= 0.2W. A 1/2W resistor should work if connected for a only a short time. It will get hot.
Measure the voltage drop across the resistor and across the choke when powered up. Use Ohm's Law to calculate the current flow through the resistor, and then use the known current flow and voltage drop across the choke to calculate its DCR. Easy peasy.
But meanwhile, I see that you have copied the orientation of the trimmer in the bias supply. I purposely installed it that way so that at full counterclockwise adjustment, the bias voltage is maximum for no current flow. Clockwise adjustment decreases the bias voltage and starts/increases the current flow.
Building Tips
1. Latest schematic is here: Latest schematic
2. Trimmer RV1 orientation shown here: Left Channel Board - RV1 Orientation, Right Channel Board - RV1 Orientation
2. Capacitor C8: Do not use a higher value than the specified 470uF. In this case bigger is not better. Larger value increases the time it takes for the capacitor to charge and the time it takes for the bias to reach full voltage.
2. Floating bias power supply: Minimum voltage is 15VDC and maximum voltage is 22VDC. I used the 15VAC secondary winding of my Antek transformer for this, and rectified it with a bridge rectifier. The rectified but unfiltered voltage is then connected to the board. Capacitor C1 provides the filtering ahead of the voltage regulator. Note that 15VDC is possibly on the low side and on the verge on affecting regulation, so it is best not to go there. A minimum transformer secondary of 12VAC should be good.
4. Measure the amplifier load choke's DC resistance before connecting it the the board. The choke should have a low DCR so unless you have a meter that can accurately measure low resistance, you will need to rig up a circuit to accurately measure the resistance. If you are using a Hammond 193V, then their specified DCR of 1 Ohm is good enough for our use. I have measured my 193Vs and they are 1.0 Ohm.
3. For first power-up, a Dim Bulb Tester is highly recommended. Of course the power supply should be tested independently prior to connecting it to the amplifier board. Check and double check that the floating bias supply and main power supply voltage polarities are correct. Before turning on the power, confirm that the bias voltage trimmer is set to maximum counter-clockwise. Connect a meter set to DC Volts to measure the voltage drop across the amplifier load choke (use test points on board). Connect another meter set to DC Volts to measure voltage drop across the SIT (connect to T.P. Choke2 SIT S and Ground).
Upon power-up, the bulb should glow bright, and then fade to off. There should be no voltage across the choke and the voltage across the SIT should be the full power supply voltage. Now turn the trimmer clockwise. It may take a few turns but voltage should appear across the choke, and the voltage across the SIT should start to decrease. If the trimmer adjustment increases and decreases the voltage drop across the choke as it should, the it is time to turn the trimmer back to full counterclockwise, power down, remove the Dim Bulb Tester, and go for full power.
4. For full power-up check to make sure the trimmer is set to full counterclockwise. Again, multimeters should be connected to monitor the voltage across the choke and the SIT. With power on, there will be a momentary surge in current as the capacitors in the bias supply charges, but it should drop to zero within seconds. Then slowly turn the trimmer clockwise to start the current flowing. The target current is 2.5A, but the current should be brought up in stages, with time in between adjustment for the system to heat up. Keep the chassis cover on between adjustments.
5. Ohm's Law: You will need to use this to calculate the current through the choke and the SIT. If you are using the Hammond 193V with its 1 Ohm DCR, then no calculation is necessary.
Ohm's Law ; E = IR, where E is voltage in Volts, I is current in Amperes, and R is resistance in Ohms
For I when E and R are known, I = E/R
So for Hammond 193V with R=1 Ohm, I = E/1
6. To determine the resistance of a low DCR choke, connect a known higher value resistor in series with the choke and a DC power supply. Size the resistor for say 10mA and a 20VDC power supply. Resistor R=20v/0.010A= 2000 Ohm, P=0.010A x 0.010A x 2000R= 0.2W. A 1/2W resistor should work if connected for a only a short time. It will get hot.
Measure the voltage drop across the resistor and across the choke when powered up. Use Ohm's Law to calculate the current flow through the resistor, and then use the known current flow and voltage drop across the choke to calculate its DCR. Easy peasy.
Hi Ben,
Just confirming my understanding:
The ability of the voltage regulator to provide some current addresses the potential gate leakage issue - hence no need for an input buffer excepting where impedance of input source is too low"
is this accurate and is this potential gate current also why the value of R3 is not higher?
thanks
Just confirming my understanding:
The ability of the voltage regulator to provide some current addresses the potential gate leakage issue - hence no need for an input buffer excepting where impedance of input source is too low"
is this accurate and is this potential gate current also why the value of R3 is not higher?
thanks
observe issue of varying gate leakage in different way
simply take in account resistance from SIT gate to firm biasing voltage
if you have, say, 100K in between SIT gate and biasing generator, any eenyweeny gate current change would invoke significant change of biasing voltage at SIT Gate
if there is 10K in between SIT gate and biasing generator, any eenyweeny gate current change would invoke 10 times lesser change of biasing voltage at SIT Gate
if SIT gate is connected to buffer, which is effectively having Rout ( that plays here) in range of 20-50R, any eenyweeny gate current change would invoke practically nuttin' change of biasing voltage at SIT Gate
simply take in account resistance from SIT gate to firm biasing voltage
if you have, say, 100K in between SIT gate and biasing generator, any eenyweeny gate current change would invoke significant change of biasing voltage at SIT Gate
if there is 10K in between SIT gate and biasing generator, any eenyweeny gate current change would invoke 10 times lesser change of biasing voltage at SIT Gate
if SIT gate is connected to buffer, which is effectively having Rout ( that plays here) in range of 20-50R, any eenyweeny gate current change would invoke practically nuttin' change of biasing voltage at SIT Gate
No, the regulator does not address the problem of a SIT having high gate leakage current. If you have a SIT with abnormally high gate leakage current, the voltage regulator does not resolve the issue.
With this board, if high gate leakage current is an issue, possible fixes would be to reduce the value of R3, reduce Iq, replace the problematic SIT with a SIT with lower gate leakage current.
With this board, if high gate leakage current is an issue, possible fixes would be to reduce the value of R3, reduce Iq, replace the problematic SIT with a SIT with lower gate leakage current.
OK. Thanks Both.
so the primary concern with abnormal gate current is that it causes a voltage drop across R3 and so changes the bias seen by the device.
And if you put the bias on the input side of a buffer the current supplied by the buffer to the gate does not change the bias voltage.
In the situation where you have a device with significant gate current leakage, does a low output impedance source fix this too or are we talking about major current flow that absolutely requires a buffer?
so the primary concern with abnormal gate current is that it causes a voltage drop across R3 and so changes the bias seen by the device.
And if you put the bias on the input side of a buffer the current supplied by the buffer to the gate does not change the bias voltage.
In the situation where you have a device with significant gate current leakage, does a low output impedance source fix this too or are we talking about major current flow that absolutely requires a buffer?
What ZM said.
There are many ways to build an amplifier. I decided to go simple. The down side is that it may not work with some SITs. For a more fool proof design, ZM offers his Lazy Singing Bush, but more parts. ra7's design incorporates self bias, which is less susceptible to gate leakage current issues. So there are choices. Or you can roll your own with your choice of bias circuit and input buffer.
There are many ways to build an amplifier. I decided to go simple. The down side is that it may not work with some SITs. For a more fool proof design, ZM offers his Lazy Singing Bush, but more parts. ra7's design incorporates self bias, which is less susceptible to gate leakage current issues. So there are choices. Or you can roll your own with your choice of bias circuit and input buffer.
