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Old 18th August 2013, 05:20 PM   #161
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I finally got around to finishing this, but I'm having problems. Does anyone have any info regarding test points and what voltage I should be expecting where? yes.. I'm a newbie and still trying to get my head around reading schematics. Any help would be greatly appreciated
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Old 20th August 2013, 12:30 AM   #162
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Hello Russ,

I've read through this thread and also absorbed everything on your website. Still, there are a few things about your Retro preamp that are unclear to me.

First, I can't work out the difference between the low Z (impedance?) and high Z versions. Do these refer to the input impedances? Which would be suitable for an MC cartridge?

My MC cartridge is an Ortofon Cadenza Red. Its output at 1KHz is 0.45 mV. Can your Retro preamp be adjusted to 60 dB without too much compromise? Is it really as simple as inserting a 75R resistor in parallel with R13?

You say you use your Joshua Tree attenuator at the output of the Retro. What is the value of the attenuator? 50K? I can find no information on your site. What is the output impedance with the attenuator in place? Does your Joshua Tree attenuator present a constant impedance to the output opamp on the Retro?

Finally, I note from reading the datasheet of the OPA1632 that the output common-mode voltage pin (VOCM) can be used to set the output common-mode voltage. From the schematic it appears you are applying a voltage from a low impedance source (the outputs of servo OPA1642). What then is the output voltage of the Retro at this setting? The maximum? Is the maximum 2.5V?

My sincere apologies for the blizzard of questions. I hope you will humour me.
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Old 27th August 2013, 03:16 AM   #163
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Low Z is the best possible solution for MC cartridges, but the High Z circuit also works quite well.

Joshua tree is a somewhat odd bird in that its input impedance changes. But it will always be > 2K if memory serves correctly.

The output common mode is GND. The output swing will depend on the gain of he amp and the cartridge you choose.

There are two aspects of the servo at play in the design, one differential and one common mode. In the end the goal is to bring both toward zero (limited by part tolerance)
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Old 27th August 2013, 02:17 PM   #164
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Quote:
Originally Posted by Russ White View Post
Low Z is the best possible solution for MC cartridges, but the High Z circuit also works quite well.

Joshua tree is a somewhat odd bird in that its input impedance changes. But it will always be > 2K if memory serves correctly.

The output common mode is GND. The output swing will depend on the gain of he amp and the cartridge you choose.

There are two aspects of the servo at play in the design, one differential and one common mode. In the end the goal is to bring both toward zero (limited by part tolerance)
I have read that an I/V converter is best for MC cartridges, and that in this case, it should have a balanced input to reduce noise. Maybe one of the I/V converters for a DAC could be adapted for use with a MC cartridge?
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Old 27th August 2013, 02:45 PM   #165
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The low Z retro is *precisely* an I/V converter.
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Old 27th August 2013, 02:57 PM   #166
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Thank you, Russ, for your answers.
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Old 27th August 2013, 04:18 PM   #167
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Quote:
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The low Z retro is *precisely* an I/V converter.
OK, I have not studied your circuit so my apologies.
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Old 27th August 2013, 04:20 PM   #168
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No problem! No apology required. It's a good point to bring out. It is why the design is something I really like. I was actually listening to some vinyl last past few days as I finish up some PCBs. Brian and I were just discussing producing the low Z variant.
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Old 27th August 2013, 04:36 PM   #169
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I've gone through the thread again and now understand the difference between the low Z and high Z versions. The low Z, which seems to have not been developed beyond an initial prototype, uses an OPA1632 rather than the FET-input OPA1242.

I realise now I've ordered the high-Z boards, capable of a maximum of 40 dB, and my MC cart can manage only a middling output.

Time to get me a Shure V15!
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Old 27th August 2013, 04:58 PM   #170
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The high Z version gain can be increased if needed.

I have the low Z version done - we just have never made it available for sale. It's hard work for Brian to maintain a large set of kits. So we just have to prioritize things.

Cheers!
Russ
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