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Old 11th March 2012, 04:30 PM   #21
Dimitris
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I m in the process of researching for a passive preamp for my setup and I am now working on a lightspeed variant with its output attached to a Lundahl 1592 1:1+1 transformer with one of its secondaries reversed so that I can get a proper balanced XLR output to drive my Electrocompaniet AW120 DBM (200k input impedance).

What worries me is that my sources may not have enough "juice" to drive the transformer after the passive attenuator, so I was looking for some kind of buffer to solve that problem for me and allow for some bigger interconnects after the transformers.

You think the B Board might be a good fit for my case?
Thanks in advance.
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Old 12th March 2012, 12:29 AM   #22
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Richard Murdey
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While still in the early stages of beta-testing, the B-board is cheap enough, and simple enough, that if you have space in your case to experiment I advise that you give it a try.

I expect it should do what you want, it's just at this point I can't guarantee it.
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Old 1st May 2012, 08:10 AM   #23
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Default B-board test mule

To test the B-board buffer modules, I stripped my Gainclone and transformed it into a buffered volume control. (Think of it as a preamplifier with no gain and no functionality other than a volume control.)

The volume control goes to 2.2uF* film coupling capacitors, which run to the B-boards with 100k resistor across the inputs.

Works fine, gain is a fraction low of course but its certainly quiet! Output offsets are about 50 mV.

Need to let it break in for a couple of days.

Unfortunately its going to be a bit difficult to make an evaluation, since the influence on the sound is going to be quite small I expect, and I don't have anything obvious to switch it out for to make a comparison.

* 0.47 uF is sufficient, I just didn't have that value on hand.
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File Type: png circuit.png (19.2 KB, 605 views)
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Last edited by rjm; 1st May 2012 at 08:12 AM.
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Old 4th May 2012, 12:08 AM   #24
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ltspice files for the B-board including the simulation for the Z-reg voltage regulator. For the 15 mA or so current drawn by the buffer, the Z-reg output ripple is about 30 microvolts.

PSRR of the buffer circuit is about -50 dB.
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File Type: png Z-reg2.png (6.5 KB, 39 views)
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Old 4th May 2012, 01:14 PM   #25
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Simple test, just to confirm what my ears already told me: a clean bill of health.

It's a unity gain line buffer with a regulated power supply. If its working properly, it's going to be essentially below the measurement threshold of my instruments. Indeed this is the case here.

With the volume at -inf, the output noise is below the measurement baseline of ~-115 dB. The peak at 28 Hz is an artifact of the test setup/instrumentation. There is no measurable hum or noise.

Applying a 100 Hz sine wave of 2.8 V amplitude (+6 dB), there is no measurable distortion above the digital sampling artifacts of the function generator which spread out evenly at around -90 dB for all higher harmonics.

(not shown) Above amplitude 5 V (about +12 dB) clipping distortion is evident.

Measurements taken with a National Instruments USB-6215 multi-function DAQ.
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Old 4th May 2012, 01:25 PM   #26
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So how does it sound?

Well, if I close my eyes, it is easy to imagine I'm listening to a high end pre-amplifier. It adds nothing but a slight sense of authority and a certain "lift", that micro-detail that seems to lend the music that little bit of extra realism. Perhaps. That's my impression, but I haven't done any direct comparison with the passive, unbuffered output of the attenuator yet.

I like it, anyway. "It" is subtle, perhaps very little, but whatever is there is, I'm convinced, a positive effect.
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Old 6th May 2012, 12:46 AM   #27
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Re-did the measurement at a slightly improved sensitivity setting. The measurement limit is -139 dB. The B-board output is perfectly clean, no ripple noise right down to this limit. (30/60/90 Hz peaks are measurement artifacts) The distortion signal of a -20 dB sinewave is also very clean, nothing visible right down to the digital noise of the function generator.

Long story short: a cannot "see" the B-board with my measurements, and, frankly, I can barely hear it either. It is transparent, while fulfilling it's role as a buffer.

Mission accomplished!
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Old 6th May 2012, 01:19 AM   #28
Dimitris
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I am still experimenting with my lightspeed clone, and it sounds a bit thin-ish.
The attenuator works like a ~8k pot.
Do you think the topology you describe above would work nicely with it?
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Old 7th May 2012, 02:52 AM   #29
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Yes, you should be able to connect the B-board/Sapphire directly to the output of the attenuator.
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Old 7th May 2012, 11:45 PM   #30
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To check a few things, I'm building a second set with an experimental BOM, BOMX below.
  1. Emitter resistors R3,4 lowered to 1k to increase the bias current.
  2. Emitter resistors R7,8 lowered to 1R, which raises the bias current in the output stage to more closely match the driver stage, at the expense of stability and increased likelihood of thermal runaway.
  3. Ceramic bypass caps added in parallel to the four electrolytics after the Z-reg. If there is any high frequency stability issues, this should quiet things down.
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