What is the value of V_G here? (resistive ground drive DC B1) - diyAudio
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Old 11th August 2014, 10:20 PM   #1
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Default What is the value of V_G here? (resistive ground drive DC B1)

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I want to predict the value of V_G on the linked schematic. Please help. I am a novice in DIY.

I am trying to build a very simple (fewer number of parts than in Mezmerize DCB1) DC coupled Pass B1 2 channel audio line level buffer with total 3 pairs of pair-matched 2SK170s. In the schematic, I simplified the 115/230VAC-to-18VDC power supply to one 18V battery. It may take a couple of minutes for the circuit to settle after turning the power on, but I can leave it constantly on.


Suppose the LOAD is a 10kOhm resistor.

If the voltage across the LOAD is 1V DC, then V_G is 0.5V DC, right?

If the voltage across the LOAD is 20Hz 1Vrms AC, then what is the value of V_G?

Last edited by ringing; 11th August 2014 at 10:57 PM.
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Old 11th August 2014, 10:37 PM   #2
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Actually there is a lot more circuit there than is needed.

You could easily take two 5K resistors in series from + to - and derive
a DC ground value where they join. Some capacitance to the rails will make
that impedance low at AC values, and since you are not using this to
amplify DC, it works like glue. If you are that worried about the DC
impedance, make the resistors a lower value. Two 1K resistors will each
dissipate about 1/10 watt and provide a 500 ohm impedance DC reference.

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Old 11th August 2014, 10:53 PM   #3
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Quote:
Originally Posted by Nelson Pass View Post
Actually there is a lot more circuit there than is needed.

You could easily take two 5K resistors in series from + to - and derive
a DC ground value where they join. Some capacitance to the rails will make
that impedance low at AC values, and since you are not using this to
amplify DC, it works like glue. If you are that worried about the DC
impedance, make the resistors a lower value. Two 1K resistors will each
dissipate about 1/10 watt and provide a 500 ohm impedance DC reference.

Thanks for your insight.

In reality, the capacitor values are not precise. Instead of two 22000uF capacitors, I am likely to have something like 26400uF on + side and 17600uF side. Will the two 1kOhm resisters you mentioned (without the active component - a pair of 2SK170s) make circuit settle to symmetric + and - voltage division?
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Old 12th August 2014, 03:23 AM   #4
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Yes. Any mismatch of the Jfets (as seen by DC at their output) will alter the
balanced a bit, but it really is slight. I have circuits using 20 ohm trimpots
between the Jfet Sources to null their offset, and I can run 12 sets of buffers
using 4.7K resistors as dividers. There is almost no difference, and the PSRR
of the buffers is pretty high for this sort of thing, and 1,000 uF seems to
be enough.

So the answer is: Don't worry about it, but if you want perfection trim your
DC offset on each buffer.



note: (complementary follower buffers - it's still the same)
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Old 12th August 2014, 03:25 AM   #5
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Also, you can get rid of all those diodes too.

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Old 12th August 2014, 03:31 AM   #6
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Location: Near Dallas Texas USA
What do you mean by V_G? The input to output offset of such a circuit depends on how closely the JFETs are matched. Such a circuit is used on many oscilloscopes, a dual JFET is commonly used.
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Old 12th August 2014, 04:13 AM   #7
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The diodes are to protect the buffer amp from surge when we connect or disconnect input our output cable. Bose 901 EQ has higher value input and output series resistors, but still has those diodes.

V_G is the voltage across the resistor R_G in the schematic.

We can completely remove R_G (R_G = 0 Ohm) and "drive the ground" directly with BUF634 or a high-input-impedance high-output-current op amp. (Due to high capacitance value in the schematic (22000uF), even the high current BUF634 will run at max capability for a while after turning on the power.) In this case, however, the audio signal current though the load, the amplifier connected to B1 output, flows also through a chip with negative feedback (BUF634 or an OP amp).

I did not like it, because I wanted to fully realize the "zero feedback" concept of Pass B1. So I chose a very high 5kOhm for R_G, instead of typical 0 Ohm, and use B1 instead of a high-current OP amp. This makes driving the ground easy, i.e. low current, and decoupled from the audio B1 at audio frequencies. A down side is that the ground drive is very slow.

To keep my B1 buffer constantly on, I want power consumption to be as small as possible. The B1 itself consumes small amount of energy, but there will be energy loss in the transformer. So I am going to use very small 115/230VAC-to-28VAC transformer. Perhaps 0.5W or 1W transformer. To prevent transformer overload during turn on, I will choose very slow turn on. You see the 1 Ohm series resistor to the power supply in the original Pass B1 schematic. I will use about 1 ~ 5 kOhm series resistor at that place to limit the current during turn on.

I do not follow the conventional wisdom of "oversized transformer for sound quality," since the power consumption of B1 does not fluctuate much with the music signal input.

Last edited by ringing; 12th August 2014 at 04:41 AM.
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Old 12th August 2014, 04:34 AM   #8
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As I understand from some posts here, the DC offset of Mezmerize DCB1 is also affected by the difference of | + supply voltage | and | - supply voltage |. So I want to divide the 18V in to precise half just as the Mezmerize + and - power supplies. Purchasing a 78L09 and a 79L09 does not seem to offer precise enough voltage division. You may get +9.3V from your 78L09 and -8.8V from your 79L09. With Mezmerize + and - power supplies, one can have +9.002V and -9.000V. I want similar precision from my ground drive power supply.

Last edited by ringing; 12th August 2014 at 04:48 AM.
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Old 12th August 2014, 05:33 AM   #9
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In my schematic, there are two voltage followers for audio and one voltage follower for ground driving.

Sorry that it is hard for me to understand that my schematic is like complementary follower buffers.

The ground drive voltage follower in my schematic is like the OP amp in unity gain configuration in Figure 7 in the following document.
http://www.ti.com/lit/an/sboa059/sboa059.pdf

The ground drive voltage follower in my schematic takes the role of BUF634 in Figure 6 in the following document.
http://www.ti.com/lit/ds/symlink/buf634.pdf

Last edited by ringing; 12th August 2014 at 05:41 AM.
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Old 12th August 2014, 01:45 PM   #10
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Quote:
Originally Posted by ringing View Post
You see the 1 Ohm series resistor to the power supply in the original Pass B1 schematic. I will use about 1 ~ 5 kOhm series resistor
I have to cancel this. Such a high value resistor does not work in DC supply due to voltage drop.
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