rhysh said:
Says who? This is DIY! Experiment and learn. To get a 50K max. output impedance you would need to use a 100K pot. Why so high? Use a 25K-50K pot then. What is the source impedance?
I made a simple passive attenuator in a small metal box with hardwired in and out connections (no RCA jacks, cables direct wired to the pot with RCA connectors). I made sure the output cables were short (about 20cm or 8") and low capacitance. It works fine and sounds very transparent. Others have made attenuators that mounted directly to the RCA input jacks (no output cables)
http://www.tweakaudio.com/Ultimate Attenuators.html
You still need to answer the question - "how much preamp gain do I *really* need for the loudest comfortable listening level?"
This is the whole premise of the B1 article. Try to build the preamp with that much gain, and no more, so that max level is achieved at max. rotation of the volume control. *Not* at 11'oclock, is more common.
A 50K signal over an interconnect is prone to interference with pretty much anything, it will pick up noise Very easliy, especially from AC lines. I have tried this in the past, the same with many others and got an undesirable result.
It may be DIY, but spending $300 on a attenuator project which may not produce desirable results is still a hole in my pocket, DIY or not.
The purpose of the B1 is to convert this into a 1K signal, which is much less prone to interference over an intrerconnect.
This will then be inputted into my preamp.
The purpose of the B1 in my case is to avoid as many interference problems as possible, not to be used as my 'preamp', just in the attenuator.
It may be DIY, but spending $300 on a attenuator project which may not produce desirable results is still a hole in my pocket, DIY or not.
The purpose of the B1 is to convert this into a 1K signal, which is much less prone to interference over an intrerconnect.
This will then be inputted into my preamp.
The purpose of the B1 in my case is to avoid as many interference problems as possible, not to be used as my 'preamp', just in the attenuator.
the maximum output impedance of an unbuffered attenuator is one quarter of the pot value plus the source output impedance.
50k pot and Rs=1k0 gives Max Zout=[50+1]/4=12k75
10k pot and Rs=1k0 gives Max Zout=2k75
100pF of input and cable capacitance will roll off the treble @ ~125kHz using a 50k pot.
If the power amp is fitted with 1nF RF filter then the treble is rolled off @ ~12kHz.
Unbuffered pots need very careful selection of component values and the listener must accept compromise.
50k pot and Rs=1k0 gives Max Zout=[50+1]/4=12k75
10k pot and Rs=1k0 gives Max Zout=2k75
100pF of input and cable capacitance will roll off the treble @ ~125kHz using a 50k pot.
If the power amp is fitted with 1nF RF filter then the treble is rolled off @ ~12kHz.
Unbuffered pots need very careful selection of component values and the listener must accept compromise.
I think this experience is not due to using a 50k passive attenuator, but to some other problem.rhysh said:
AndrewT said:
So do you think i would be fine, using a passive attenuator before my preamp with no buffer, if done properly? We are talking 1 - 2ft of interconnect.
AndrewT said:
The output impedance of my DAC (main source) is 3R. If i dont need to use the B1 after my attenuator then i wont bother.
BFNY said:
Sorry, this should read "To get a 50K max. output impedance you would need to use a 200K pot."
This would be way to high for a passive unbuffered attenuator.
but you attenuator has a high output impedance.rhysh said:
If you want Rs<500r then the maximum pot value would be 2k.
Can your source equipment drive 2k//cable capacitance?
My source will drive a normal 50k pot easily. What i am wondering is if i need the b1? Or could i just use a passive attntr.
try your passive 50k pot.
If you like what you hear you have saved a lot of time and money.
I would add a buffer to reduce the pot's 12k75 output impedance to something more sensible.
If you like what you hear you have saved a lot of time and money.
I would add a buffer to reduce the pot's 12k75 output impedance to something more sensible.
Balanced inputs on the B1
As suggested by Andrew I am posting this question - I have balanced outputs on my computer sound card and would like to use them into the B1 buffer I will be building - how can this be best achieved?
Andrew suggested two very closely matched B1s which is possible (I will be getting two sets of boards), but I would like to achieve this in one B1 if at all possible.
Alan
As suggested by Andrew I am posting this question - I have balanced outputs on my computer sound card and would like to use them into the B1 buffer I will be building - how can this be best achieved?
Andrew suggested two very closely matched B1s which is possible (I will be getting two sets of boards), but I would like to achieve this in one B1 if at all possible.
Alan
you could make a balanced line driver and balanced line receiver. Receiver in front of B1 and driver on the back end unless your amp has RCA inputs in which case you would simply use a balanced line receiver in front of the B1 and no driver, just straight to amp from there.
This way you only need one B1 board, but you would need one receiver for each channel. You can make a stereo BLR with a OPA2134.
Uriah
This way you only need one B1 board, but you would need one receiver for each channel. You can make a stereo BLR with a OPA2134.
Uriah
AndrewT said:
I will do just this. Do you not think there will be problems with interference or HF roll off?
I'm absolutely sure it will mess up the frequency response, but you seem intent on using an unbuffered 50k pot.
Would some 2SK117 work OK in this circuit?
From the manual: ....you can use substitutes having Idss 5-10mA and transconductance numbers from 5-30mSiemens
From the SK117 datasheet: Idss 1.2-14mA and Yfs 15mS
Am I reading that all wrong or should they work?
Fran
From the manual: ....you can use substitutes having Idss 5-10mA and transconductance numbers from 5-30mSiemens
From the SK117 datasheet: Idss 1.2-14mA and Yfs 15mS
Am I reading that all wrong or should they work?
Fran
if you already have them then it's worth trying.
sk170 are renowned for high Yfs, low pinch off voltage and low noise.
sk170 are renowned for high Yfs, low pinch off voltage and low noise.
AndrewT said:I'm absolutely sure it will mess up the frequency response, but you seem intent on using an unbuffered 50k pot.
Im happy to use a buffer if necessary, but it is another stage in the signal path. I can use a 10K pot if this would help anything?
R
rhysh said:
If you reffer to the potentiometer as the volume control of the Buffer,
I mounted a clone of the Buffer published on Nelson Pass website with 10k pot. For my earings, it work just as fine.
Here some pics...
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
