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Old 24th December 2008, 05:20 PM   #1
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Default SE to Balanced tube preamp idea

I'm thinking of building a current-source biased LTP line stage, to convert from SE to balanced and drive my electrocompaniet clone amp. What are your experiences with regards to the sound of this topology, also keeping in mind that it has to be done on the cheap?
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Old 24th December 2008, 05:25 PM   #2
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If I could tag a similar question to the thread:

Are there good low power class A amp designs that have balanced input rather than using RCA with a phase splitter in the amp?

-David
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Old 30th December 2008, 08:12 PM   #3
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From simulation it seems that gain balance in the two outputs requires an almost ideal current source. Even a current source impedance of 20K, which in practice does not seem easily attainable, leads to an amplitude imbalance as high as 0.5dB. The JFET current source has an even lower impedance. Any thoughts would be greatly appreciated...
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Old 31st December 2008, 09:16 AM   #4
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20k is easily achieved - in fact, 20M is easily achieved. You need a couple of transistors to make a near-as-dammit perfect cascode CCS. It's been discussed before on this forum so a quick search should find it. What you will need, though, is a -15V (or so) quiet supply.
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Old 31st December 2008, 10:39 AM   #5
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Quote:
Originally posted by dw8083
If I could tag a similar question to the thread:

Are there good low power class A amp designs that have balanced input rather than using RCA with a phase splitter in the amp?

-David

Are you looking for a push-pull or single ended amp? With a balanced input, your phase inversion has been done for you, and that is quite nice. For low power single ended amplifiers, this doesn't offer you a whole lot of favors (I still tend to build in a transformer coupled XLR jack onto my SE amplifiers).
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Old 31st December 2008, 12:12 PM   #6
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Thanks EC8010. I too saw the need for a negative supply when I started simulating possible topologies.
The cascode CCS you suggest seems to work very well; only 0.03dB differential imbalance is produced. But out of pseudoscience or just biased intuition, I was thinking of not using bipolar transistors in this design. Mosfets and jfets connected as simple CSs do not yield the required impedance. I'm thinking of cascoding a jfet and a mosfet.
Cheers.
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Old 31st December 2008, 12:21 PM   #7
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I built such a preamp to drive the Pass F4 in balanced (The "ImPasse") and demo'd it at BAF '07. It can also be used to drive a balanced input of a power amp. The article was accepted by AudioXpress, but they've been sitting on it for nearly a year; we'll be posting it on diyAudio shortly. It will accept both single ended and balanced sources, and has a balanced output. Max gain of 20dB, with THD at under 0.05% at 20V out.
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Old 31st December 2008, 01:44 PM   #8
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Thanks SY, drop us a line in case we miss it!
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Old 31st December 2008, 01:46 PM   #9
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You won't miss it, trust me. And to correct my previous error, the gain is 20 (26dB), not 20dB.
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Old 31st December 2008, 04:49 PM   #10
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Quote:
Originally posted by audiowize



Are you looking for a push-pull or single ended amp? With a balanced input, your phase inversion has been done for you, and that is quite nice. For low power single ended amplifiers, this doesn't offer you a whole lot of favors (I still tend to build in a transformer coupled XLR jack onto my SE amplifiers).

I'm looking for a single ended balanced input amp. Although a good push-pull design would be OK. All my sources are balanced, seems a shame not to leverage them in balanced mode.

-David
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