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Tube Buffer 6J1 / 5654 (6AK5)
Tube Buffer 6J1 / 5654 (6AK5)
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Old 4th August 2008, 04:11 AM   #51
aardvarkash10 is offline aardvarkash10  New Zealand
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Default no gain from this puppie

cathode follower, hence gain is probably negative. Haven't done the math - I'll leave that to someone better suited.
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Old 4th August 2008, 09:39 PM   #52
wa2ise is offline wa2ise  United States
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Quote:
Originally posted by milen007
anyone has any idea whats the gain in db for this tube buffer 6j1? thx
Cathode followers have a slight decrease in signal level voltage, but the reason cathode followers are used is that they increase the ability of the signal to drive heavier loads. If you have a SS amp with say 10K input impedance, and a source that has an output impedance of 1 megohm, and the signal voltage is 1V, the SS amp will see it as a 10mV signal. You get the effect as if the signal source is feeding a voltage divider resistor network of 1 megohm and 10K to ground, which will produce about 1% of the signal voltage. A cathode follower will reduce the signal's impedance to something that can drive 2/3V to 9/10V signal voltage into the SS amp.

A tube cathode follower can introduce some of that magic "tube flavor" aka 2nd harmonic about 45dB down into the music. The human ear likes a small amount of 2nd H, but really hates 3rd H. But be careful with the 2nd H, music with lots of instruments playing at the same time can make for "intermod", intermodulation distortion coming from the same mechanism that produced the -45dB 2nd H. Intermod makes for "muddy" sound, which is not pleasant.
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Old 8th September 2008, 12:17 PM   #53
milen007 is offline milen007  Indonesia
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Quote:
Originally posted by wa2ise


Cathode followers have a slight decrease in signal level voltage, but the reason cathode followers are used is that they increase the ability of the signal to drive heavier loads. If you have a SS amp with say 10K input impedance, and a source that has an output impedance of 1 megohm, and the signal voltage is 1V, the SS amp will see it as a 10mV signal. You get the effect as if the signal source is feeding a voltage divider resistor network of 1 megohm and 10K to ground, which will produce about 1% of the signal voltage. A cathode follower will reduce the signal's impedance to something that can drive 2/3V to 9/10V signal voltage into the SS amp.
Hi wa2ise

thanks for the thorough explaination.

i have another question, i am using usb dac driving this cathode follower then follow by a preamp.

my preamp input impedance is 27k ohm.

what my usb dac see the input impedance of the next stage it drive?

1. whats the input impedance of this tube buffer?
500k ohm input impedance as being stated in the 1st page?
2. or as wa2ise state the cathode follower will reduce the signal's impedance from 2/3v to 9/10 which is roughly 30%. hence i assume my preamp with 27k input impedance + 30% = roughly 36k?

thanks for the help. as i am tuning the ouput cap in my usb dac. i have tried change from 10uf to 4.7 uf. its just so superb. hence i assume it will get better with correct capacitor value.

below i attached the output caps table. courtesy of Doede
DDDAC. how to use the table? whats "Fc" on the table? whats the best suggested value of my ouput cap for usb dac (which is not DDDAC)? presume that all output cap for DAC can follow this table? or is this application dependent?

regards, Erwin
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Old 12th September 2008, 03:07 AM   #54
milen007 is offline milen007  Indonesia
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anyone know how much load this tube buffer draw? or how to measure it? 150mA enough? thx
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Old 12th September 2008, 06:56 AM   #55
athos56 is offline athos56  United States
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Should be way less then that for both tubes. Here's another question for the thread, I've seen cathode followers with and without the cap at the input. What is its purpose?
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Old 12th September 2008, 04:40 PM   #56
wa2ise is offline wa2ise  United States
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Quote:
Originally posted by milen007
My answers inserted in not bold font - wa2ise




I have another question, i am using usb dac driving this cathode follower then follow by a preamp.

my preamp input impedance is 27k ohm.

what my usb dac see the input impedance of the next stage it drive?

1. whats the input impedance of this tube buffer?
500k ohm input impedance as being stated in the 1st page?
yes
2. or as wa2ise state the cathode follower will reduce the signal's impedance from 2/3v to 9/10 which is roughly 30%. hence i assume my preamp with 27k input impedance + 30% = roughly 36k?
The 9/10 to 2/3 numbers refer to the resulting audio voltage output, as compared to the input audio voltage. In your case, I don't know what exactly the cathode follower output impedance of the tube you're using, but it is likely 1k or so. Which will deliver something like 80% of the audio voltage amplitude to your preamp with the 27K input impedance.

thanks for the help. as i am tuning the ouput cap in my usb dac. i have tried change from 10uf to 4.7 uf. its just so superb. hence i assume it will get better with correct capacitor value.
It's not that critical. It only effects the bass end of the audio spectrum. If it sounds great, I'd leave what you now have in there.

below i attached the output caps table. courtesy of Doede
DDDAC. how to use the table? whats "Fc" on the table? whats the best suggested value of my output cap for usb dac (which is not DDDAC)? presume that all output cap for DAC can follow this table? or is this application dependent?
Fc is the point in the audio spectrum where the amplitude is 3 dB down from the maximum. Remember that 1 dB drop is not really noticeable by the human ear. Far away from this Fc point, the responses look like straight lines, but near Fc it transitions to a curve. Where the straight lines would have crossed is the Fc point. It's "poles and zeros" in Bode polts, which can give electrical engineering students in sophomore year fits...
regards, Erwin
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Old 13th September 2008, 03:00 AM   #57
milen007 is offline milen007  Indonesia
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Quote:
Originally posted by wa2ise
The 9/10 to 2/3 numbers refer to the resulting audio voltage output, as compared to the input audio voltage. In your case, I don't know what exactly the cathode follower output impedance of the tube you're using, but it is likely 1k or so. Which will deliver something like 80% of the audio voltage amplitude to your preamp with the 27K input impedance.
If thats the case, whats the ideal input impedance of my amp to get better drive from the buffer? 30k to 50k?

will this added stage of tube buffer help with the impedance matching of each stages of the whole systems? or better of without them?

i am also wondering as athos56, whats the purpose of input caps of this cathode followers? as this is not likely as dc filtering since some also do not has input caps as athos56 claimed. thx in adv
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Old 13th September 2008, 06:28 AM   #58
coresta is offline coresta  France
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A lot lesser than 30k ! for a pentode Cf , the average source impedance is given by the formula : 1 / S (slope) . The 6AK5 gives an average value of 5mA/V so : 200ohms
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Old 13th September 2008, 06:36 AM   #59
milen007 is offline milen007  Indonesia
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Quote:
Originally posted by coresta
A lot lesser than 30k ! for a pentode Cf , the average source impedance is given by the formula : 1 / S (slope) . The 6AK5 gives an average value of 5mA/V so : 200ohms
Hi Coresta

mind elaborate? less than 30k input impedance? 6ak5 current draw is 5mA/V?
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Old 13th September 2008, 06:45 AM   #60
coresta is offline coresta  France
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no... a CF buffer is an gain amp where input impedance is high , very high , an unity gain (0,95 average) , and a low output impedance . At a certain working point of drawn current, given in data sheets , the slope reaches a certain value :this point is the S of the 1/S formula . The higher the slope , the lesser the Z ! So it'll be able to load any any preamp from 10k without any problem
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