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Old 8th March 2012, 03:55 AM   #11
quan is offline quan  Australia
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Hi Richard ,just connected today- B boards to VSPS300i
V++ and V-- measures 35V, Dc off set measures 150mV and 240mV for L and R chanels respectively( same for in/out off set).
Is that too high?
Quan
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Old 8th March 2012, 06:03 AM   #12
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Richard,

Kit arrived safely.

Regards
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Old 8th March 2012, 06:19 AM   #13
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Richard Murdey
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@quan

Unless you have a DC-coupled amplifier, this offset, while slightly on the high side, is OK.

The VSPS output has an coupling capacitor, C3, so the DC impedance seen from the B-board input is high, equal to R7 or 75k.

If you connected a smaller resistor across the B-board inputs from IN+ to IN-, perhaps 10k, you would see the offset voltages very much reduced (<50 mV). Actually I'd recommend you do that if you can. Exact value isn't critical, roughly 10k to 22k.

The other option is a little more complicated: move C3 and R7 to the other side of the B-board. This is done by inserting the B-board in place of R6 (VSPS) or R8 (Phonoclone). The resistor is removed from the board, the inputs go to the pad connecting to the op amp output, the outputs to the pad connected to C3. See attached.

Richard
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File Type: png pcb-vsps300m.png (28.6 KB, 298 views)
File Type: png pcb-pc35f.png (30.5 KB, 292 views)
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Last edited by rjm; 8th March 2012 at 06:39 AM.
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Old 8th March 2012, 06:50 AM   #14
quan is offline quan  Australia
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Quote:
Originally Posted by rjm View Post
@quan

Unless you have a DC-coupled amplifier, this offset, while slightly on the high side, is OK.

The VSPS output has an coupling capacitor, C3, so the DC impedance seen from the B-board input is high, equal to R7 or 75k.

If you connected a smaller resistor across the B-board inputs from IN+ to IN-, perhaps 10k, you would see the offset voltages very much reduced (<50 mV). Actually I'd recommend you do that if you can. Exact value isn't critical, roughly 10k to 22k.

The other option is a little more complicated: move C3 and R7 to the other side of the B-board. This is done by inserting the B-board in place of R6 (VSPS) or R8 (Phonoclone). The resistor is removed from the board, the inputs go to the pad connecting to the op amp output, the outputs to the pad connected to C3. See attached.

Richard
Will do Richard
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Old 8th March 2012, 07:26 AM   #15
quan is offline quan  Australia
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Insert 10K resistors. New DC offsets- 39/45mV respectively.
Beautiful music at moment.
Initial impression- more air and lower noise floor level.
Thank you Richard.
BTW- do you think the resistor will be needed for the PC3 as well?
Quan
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Old 8th March 2012, 11:19 PM   #16
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I'm very glad to hear it's working now.

The short answer is "yes": 10k resistors across IN+ and IN- can be used for both phono stages to reduce the offset voltage.

It's the way the B-board circuit operates: the output offset is a function of the DC impedance (resistance) seen between IN+ and IN-.

If there is a blocking capacitor upstream, the B-board cannot "see" the low impedance output of the driving stage, and instead has to source/sink current from the resistor placed between IN+ and IN-.

(Full disclosure - I forgot about the influence on output offset voltage of the blocking capacitor, basing the performance instead on the Sapphire amp, where the buffer is driven directly by an op amp and the total output offset voltage is extremely low, less than 10 mV.)

Both the Phonoclone and VSPS normally have this blocking capacitor, so the output offset depends on whatever resistors to ground are placed after the blocking capacitor (C3). This is normally 75k for my phono stages. That's - a bit marginal. 10k is better, if only for peace of mind.
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Last edited by rjm; 8th March 2012 at 11:25 PM.
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Old 9th March 2012, 01:34 AM   #17
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Richard,

If one inserts the buffer in the Phonoclone circuit in place of R8, I assume that a 75k or less resistor needs to be added across the input of the buffer also, since the buffer no longer sees the phonoclone 75k at its input?

Martin
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Old 9th March 2012, 02:33 AM   #18
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No, that's not needed. When the circuit is powered, the buffer input sees the op amp output impedance of IC2 (phonoclone) on its inputs, this is basically 0 ohms. There is also a convoluted DC resistance to ground through the feedback resistor R6.
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Old 11th March 2012, 12:06 AM   #19
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Buffer installed (for the time being after the coupling cap of the Phonoclone). Works perfectly, about 160mV offset on one channel and 180 on the other without adding additional resistors on the input.

Verdict - very positive. With the Phonoclone I had been swapping out the 2nd opamp between the OPA27 and AD797 baing unsure which I prefered. The OPA27 was more mellow and listenable for a long session, but the AD797 seemed to carry a little more weight as well as detail and resolution, but at the expense of a certain edginess, that despite adding power pin decoupling never went away.

Now with the buffer added, the OPA27 setup gains the detail and weight of the AD797, without the edginess. I am sure that the easier driving of the buffer for the OPA27 is the reason, plus the buffers immunity to what it is driving.

I can thoroughly recommend the buffer!

I'll post pictures of the completed set up in a few days, the wiring is messy at the moment due to the addition of the buffers.
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Old 11th March 2012, 09:05 AM   #20
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Awesome!!!
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