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dac I/V convertion with very low distortion
dac I/V convertion with very low distortion
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Old 20th April 2019, 12:58 AM   #941
Terry Demol is offline Terry Demol  Australia
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Quote:
Originally Posted by keantoken View Post
It may be helpful for you to know that in the BC337/327 measurements I saw, the PNP has lower 1/F noise. I don't know what manufacturer it was though.
Here is some really excellent work on low noise BJT's.

Ultra low noise amplifiers
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Ultra low noise amplifiers

More info on 337/327, slightly conflicting with above

http://www.janascard.cz/PDF/Design%2...amplifiers.pdf

As you can see, there are some super low noise, medium current BJT's but they do have quite a bit of capacitance.

cheers, T
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Old 20th April 2019, 04:36 AM   #942
keantoken is offline keantoken  United States
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Ah yes, yet another knucklehead who who never thought it was important to mention who the manufacturer was.

Different manufacturers can use entirely different processes for the same transistor, and in practice it seems to be the case that you cannot measure the Rbb of one transistor and expect it to apply for different manufacturers.

Recently I did verify roughly in RMS output noise measurement the Rbb of Fairchild factory (now Onsemi) BC550C, to be around 900 ohms. The Onsemi factory was the best AFAIK, and it seem like the OnSemi factory produces the SMD parts, but I don't know that for sure.
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Old 20th April 2019, 10:01 AM   #943
jan.didden is offline jan.didden  Europe
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Originally Posted by smms73 View Post
Also thinking about adding a small relay and a 82ohm resistor at the output to bring the gain down 20db, this of course also make the noise at the output very low
Is this the i/v resistor you are switching? In that case, the s/n ratio does not change.

Jan
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Old 20th April 2019, 07:27 PM   #944
smms73 is offline smms73  Europe
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Originally Posted by kimschips View Post
Hello and thanks for your big effort, do the circuit work with a ES9018 Dac chip coupled for all 8 channels in parallel/ 16mA output?

Thanks
Hello,
no, it would have to be modified, I need the 8 individual channels of the es9018
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Old 20th April 2019, 07:31 PM   #945
smms73 is offline smms73  Europe
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Thanks Keantoken and Terry, I appreciate the links and information on this subject.
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Old 20th April 2019, 07:36 PM   #946
smms73 is offline smms73  Europe
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Quote:
Originally Posted by jan.didden View Post
Is this the i/v resistor you are switching? In that case, the s/n ratio does not change.

Jan
yes you're right of course, but I'm going to use the ES9018's digital volume, and since most of the time I listen to music at low volume, this switch is maybe useful.
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Old 21st April 2019, 03:34 AM   #947
Terry Demol is offline Terry Demol  Australia
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Quote:
Originally Posted by jan.didden View Post
Is this the i/v resistor you are switching? In that case, the s/n ratio does not change.

Jan
Yes it does - the amount varies.

There's a reason why big DR numbers on DAC data sheets are at large OP voltage swings. Double size of resistor = 1.414 x self noise.

Now, I know you will prolly point out that the self noise of the OP 'I-V' resistor is very small in the scheme of things -
it depends which DAC / OP current / size of resistor. When you get to crazy OP currents and DR numbers well over 130dB it can make a significant difference.

Just sayin'.
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Old 21st April 2019, 09:13 PM   #948
smms73 is offline smms73  Europe
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This week I've been thinking about alternatives to my I / V converter, because since I'm going to use 8 channels and balanced connections to the amplifier, that means I'll have to use 16 of these circuits.
A very interesting alternative is to use Jan Didden IV converter with a small change. just added a current source to increase the gain, the results are promising.
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File Type: png jan_iv_simple.png (7.8 KB, 107 views)
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Old 21st April 2019, 09:28 PM   #949
smms73 is offline smms73  Europe
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Default JD IV converter

This is the Fully Differential IV converter, it has low input impedance with 0.2 ohms in the audio band and 2ohms at 1Mhz and 3 ohms at 10Mhz.
Distortion 10KHz at full output is also very low with only 3th harmonic as the differential nature of the circuit cancels even harmonics.
The PSRR is also good, but in reality will be worst than this depending on the tolerance of the components.
Attached Images
File Type: png jan_iv_1.png (19.8 KB, 107 views)
File Type: png jan_impedance.png (30.2 KB, 105 views)
File Type: png jan_distortion_10Khz.png (15.6 KB, 99 views)
File Type: png psrr.png (32.2 KB, 99 views)
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Old 21st April 2019, 10:05 PM   #950
smms73 is offline smms73  Europe
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Even simpler, similar to Calvin's circuit in # 899
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File Type: png Calvin_1.png (15.8 KB, 61 views)
File Type: png calvin_impedance.png (37.3 KB, 62 views)
File Type: png Calvin_distortion.png (15.0 KB, 29 views)
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