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Old 10th August 2012, 01:58 AM   #91
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I was on a busyness trip today. I think i have a solution for a mirror that transfers bal- to un-bal, is very low distortion and is also a buffer. It has to be adjusted to your input stage though.

Last edited by Joachim Gerhard; 10th August 2012 at 02:22 AM.
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Old 10th August 2012, 02:21 AM   #92
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Here are two circuits for inspiration. They have funny names. The Balabala is a way to convert a balanced common base stage into single ended. The Starless Hulk has that Sampler Mirror.
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File Type: pdf Balabala 3.pdf (22.1 KB, 173 views)
File Type: pdf Starless Balnced Hulk 1.pdf (35.8 KB, 164 views)
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Old 10th August 2012, 06:48 AM   #93
zinsula is offline zinsula  Switzerland
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Quote:
Originally Posted by Joachim Gerhard View Post
[...]One of the major problems of BJt circuits.[...]
Joachim, are the base current in class A line level or phono circuits really a problem which affects sound negatively, or rather an academic one?
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Old 10th August 2012, 02:36 PM   #94
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Originally Posted by smms73 View Post
dirkwright
the distortion caused by a transformer increase as the ratio between the inductance and dcr in parallel with the impedance of the amplifier, if you have a transformer with zero ohms of dcr, the distortion will be also zero, silver will bring down dcr but will increase the costs. 140 dollars its a lot of money for that, but is your call.
I've mulled over the use of a transformer in my circuit for a while now. I think it's justified for this particular DAC. All of my effort went into dealing with the weirdness of the PCM1794a DAC. If I had a voltage output DAC, or some other DAC with a lot less interface issues, then I would not use a transformer.
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Old 10th August 2012, 02:44 PM   #95
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Zinsula, in my Paradise and Nobrainer phono stages i shunt the base current to ground with a big electrolytic. The distortion i get from optimized open loop class a circuits is around -100dB. With the idea presented here the distortion can get even lower. If that is audible i can not say. I can personally not hear better then 0.1% low order.
On the other hand if have experienced that improvements in a circuit can sound better, even when it is theoretically under the audible threshold. That may have to do with dynamically changing distortion like energy storage ( settling time ).
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Old 10th August 2012, 03:49 PM   #96
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Quote:
Originally Posted by Joachim Gerhard View Post
Here are two circuits for inspiration. They have funny names. The Balabala is a way to convert a balanced common base stage into single ended. The Starless Hulk has that Sampler Mirror.
I really like the Balabala. Thanks for sharing.
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Old 10th August 2012, 03:55 PM   #97
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No problem, i have not builded though.
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Old 10th August 2012, 04:55 PM   #98
smms73 is offline smms73  Portugal
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Joachim

I will use the bc327 (a base resistance of 30 ohms is low enough) in the prototype.

In circuit of post 49, ,i'm not yet convinced that the capacitor in the bases of the input transistors is necessary for lower noise, this bjt are already driven by a low impedance source, the r18 and r19 , the output impedance of the driver transistors (q5, Q2) is very low. what do you think ?

Last edited by smms73; 10th August 2012 at 05:00 PM.
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Old 10th August 2012, 04:58 PM   #99
smms73 is offline smms73  Portugal
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here is the circuit with the current mirror, but i don't like it very much.
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File Type: jpg diferencial.jpg (107.6 KB, 353 views)
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Old 10th August 2012, 05:28 PM   #100
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For noise you have to watch through what components the noise path goes until it reaches ground. In your latest schematic it enters R1 and R2 then goes through the emitter-base junction of Q1, Q4 and then through the emitter-base junction of Q2, Q5. Then there is the bias generator for Q2, Q5. Say we use Leds for bias and shunt them with a big electrolytic to ground we can ignore the noise of the bias generator. We are still left with the emitter-base junctions of Q2, Q5. Here i am only talking about the positive input. In the case of the non bypassed R9 and ignoring the effect of the idle ( it is quite high at around 10mA ) we can do the following noise calculation : 4.7 Ohm plus Rbee`of Q1 plus RbeeŽof Q2 ignoring the bias generator / 2 = 32.35 Ohm. Shunting the R9 resistor gives 4.7 Ohm plus 30 Ohm / 2 = 17.35 Ohm, an improvement of a bit less then 3dB plus more leeway to design the bias generator. Looking at that circuit balanced the noise will go up 3dB because the noise sources appear in series. Balanced is always noisier then single ended provided you use the same amount of input devices. This can of cause be solved by doubling the devices.
Why do you not like the mirror version ?
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